Overview

MISCONCEPTIONS ABOUT SPACE WARFARE

(ed note: this is a commentary about the computer game Children of a Dead Earth)

I see a lot of misconceptions about space in general, and space warfare in specific, so today I’ll go ahead and debunk some. In the process, we’ll go through the moment to moment of space warfare itself.

Zeroth misconception, no, there won’t be stealth in space, let alone in combat. It is possible through a series of hypothetical technologies or techniques, but it won’t be possible for any reasonable spacecraft under reasonable mass and cost restraints.

Now then, on to the first real misconception. Wouldn’t missiles dominate the battle space, being fired from hundreds of thousands of kilometers away? Wouldn’t actual exchange of projectile weapons never happen in reality?

The answer is no, actually. There is a prevailing hypothesis that missiles will soon be the only relevant weapon on the battle space, and it is likely borne out of current trends in modern warfare. ATGWs are already starting to upend tank warfare, and Anti-ship missiles are doing something similar to naval warfare. Indefinitely extrapolating this trend would lead one to conclude warfare will soon be nothing but people sitting in their spacecrafts launching missiles at one another.

But this is not true. CIWS point defense systems are already starting to shift the balance away from missile strikes. As suggested in an earlier blog post, military strategists are even beginning to suggest the development of CIWS systems may bring naval warfare full circle, all the way back to World War I battleship warfare. This isn’t to suggest that missiles are useless. Indeed, enormous salvos of missiles are effective at overwhelming CIWS systems, and they are in game as well.

Yet we begin to see the limitations of each system. Point defense systems, railguns, coilguns, conventional guns, or even lasers, are power limited in this exchange. There is a finite amount of power to use when firing, except for conventional guns. Conventional guns suffer from low muzzle velocities, and high muzzle velocities are crucial to intercepting missiles coming at you at greater than 1 km/s. This power limitation is what prevents these point defense systems from being impervious to missile salvos. Power consumption is limited by radiator mass actually, as simply slapping down more nuclear reactors is easy, but trying to deal with the added mass of all the radiators needed to cool those reactors is much more difficult.

Missiles, on the other hand, are also limited by mass. A hundred-missile salvo is sure to overwhelm any point defense system, but the amount of mass this requires the launching ship to take on is enormous, and will kill its mass ratio. In the end, it turns out the Rocket Equation governs just how effective missiles and point defense systems are. In game, the systems ended up surprisingly balanced, with neither being a dominant strategy, with either being more effective in certain situations, and weaker in others.

Next misconception, wouldn’t lasers dominate the battle space? Lasers do not suffer from many of the inaccuracy problems that projectile weapons do, and move at the speed of light, so they are literally impossible to dodge. So lasers are the king of the battle space, right?

Wrong. Lasers suffer from diffraction. Badly. The power of lasers in space drops painfully fast with distance, and frequency doubling only ameliorates the issue slightly. Lasers are notoriously low efficiency compared to projectile weapons. But that’s not the main issue. When comparing hypervelocity projectile impact research with laser ablation research, one discovers a stark contrast in their efficacy. Laser ablation is simply less effective at causing damage than projectile impacts. Whereas hypervelocity projectiles cause spallations and cave in armor effectively, laser ablation is poor, with energy wasted to vaporization, radiation, and heat conduction to surrounding armor. On the other hand, at very close ranges, where diffraction is not an issue, lasers outperform projectiles easily. Unfortunately, nothing aside from missiles will likely ever get that close, and even then, they will likely be within close focus ranges for milliseconds at most.

Lasers still useful at long ranges, though. Lasers fill a very specific niche in space warfare, and that is of precision destruction of weakly armored systems at long distances. Lasers are very good at melting down exposed enemy weapons, knocking out their rocket exhaust nozzles, and most importantly, killing drones. While missiles have very few weak points, and can shrug off laser damage with thick plating, drones have exposed weapons and radiators, which makes them very vulnerable to lasers.

In terms of actually destroying enemy capital ships, however, lasers can cut into the enemy bulkhead all day with basically zero effect (I measured the ablation of a monolithic armor plate at one point, and found that the ablation was happening at micrometers per second).

Final, misconception, wouldn’t computers just control everything in combat?

Yes and no, but mostly no. CIWS systems are already computer controlled, and all weapon aiming is similarly already controlled by the computer in game. Anything that has easily computable maxima are solved by computers in game. But there are numerous choices in combat which have no obvious local maxima, and these require human decisions. In other words, you the player and commander need to make these choices. As it turns out, the right or wrong decision can mean the difference between victory and failure.

In game, you won’t be aiming any weapons and firing them, nor will you be flying drones around. The computer can do both better than you, and so the computer will be in control of these things (besides, do you really think you could effectively aim at a speck of light 50 km away moving at 1 km/s at you?).

What you will have control of are the higher level strategic decisions. The orders you give your missiles, drones, and capital ships are crucial decisions you must make in combat. Will you send your missiles in a beeline at your enemy, or perhaps order them to spend valuable delta-v dodging enemy point defense fire? Should you retract your radiators to reduce your heat signature to avoid enemy missiles, and risk the loss of your firepower for the precious few seconds? Should you hold your drones in reserve, close to your carrier, or send them guns blazing as the enemy capital ships approach?

Also as well, one of the critical choices you can make is what to target of the enemy. Each subsystem of every enemy spacecraft is simulated in real time. The reactors draw power, the radiators expel heat, the turrets and guns drain power, all in real time. If you want to disable the enemy’s ability to harm you, the obvious choice is to go for the weapons. But weapons are small, hard to hit unless you have a laser. Going for the enemy’s radiators might be an alternative strategy, with radiators being large, easy targets, although radiators, once armored, are surprisingly sturdy. Not remotely as strong as monolithic armor, but still able to take a reasonable beating of projectile and laser hits. Of course, maybe taking out of the enemy’s engines is more your style, the rocket nozzles being flimsy and poorly armored to allow them to gimbal easier. Plus, a ship that can’t move or dodge is a much easier target.

But most importantly, orbital mechanics are king in Children of a Dead Earth. Indeed, orbital mechanics are the core mechanic of the game, even, counterintuitively, in combat. Once you reach weapon range, orbital mechanics lose most of their relevance, but everything up to that point hinges on orbital mechanics.

Your incoming speed and angle of attack entering combat, two critical attributes which govern how the combat unfolds, are determined entirely by your ability to use orbital mechanics to your advantage. How near or far you are from the nearest gravity well (planet, moon, or asteroid) has a huge effect on combat speeds. Additionally, evading the enemy before even entering combat is a big part of the game. If you can drain the enemy’s delta-v through effective orbital mechanics, they may fight at reduced effectiveness in combat. If you’re good enough, you might be able to run them out of delta-v entirely, and never even have to enter combat at all!

THOUGHTS ON SPACE COMBAT (HEAVILY INSPIRED BY CHILDREN OF A DEAD EARTH)

(ed note: again this analysis is centered around the game Children of a Dead Earth. Which, like all simulations, does have some underlying assumptions that may or may not obtain in the real world. It is however internally consistent.)

     Engagement ranges are on the order of tens/hundreds of kilometers, not more, and they are mostly linear
     Weapons are largely ineffective farther away (we'll talk about this in a bit), but since range is a tremendous advantage, no one will survive a prolonged closer encounter. Unless your intercept is retrograde, one side will die before anyone can maneuver appreciably closer or farther.
     The battle space is also mostly linear (two sides facing off across a no-one's-land). A battle "line" is really a 2D plane in space, but aside from this, it's not much different. 2D-thinking (or even 1D thinking!) is quite sufficient.
     Why should this be? While ostensibly space is 3D, when you're flying a real ship, you have delta-v constraints. The space of engagement is large relative to that, and your acceleration is slow to boot (you do have a low-thrust, high-ISP engine, right?). Additionally, since you're probably rendezvousing from a different orbit, you'll have a single dominant direction of approach. You spread out when you attack, sure, but if you're at the distance where you can completely outflank your opponent, you're at a distance where both sides have long since torn gaping holes through each other with k-slugs.

     Maneuvering is almost worthless
     As a direct result of the above, the only purpose for thrusting at all is to dodge k-slugs. You can't do it very well, though, since unpredictably dodging requires rotation--but rotation is slow, and costs lots of delta-v.
     Moreover, to move laterally probably means turning, which means exposing your flank to the enemy. That makes it a bigger target. In CoaDE, this is balanced by the fact that weapons shoot sideways, so to attack, you must make yourself vulnerable. In reality, all weapons just shoot forward.

     On that note, K-slugs are actually great
     A k-slug is essentially a high-velocity bullet. The conventional wisdom on k-slugs is that they don't work in space because your target moves literal kilometers just in the time it takes your bullet to move down the barrel. This is a bit of an exaggeration. If you're rendezvousing with the enemy anyway, the relative velocity is low--probably less than several hundreds of m/s. An ordinary bullet moves a bit faster than this, and a k-slug probably 3-10x as fast again. However, the problem is real.
     The solution is to lead your target. As above, an opponent can't really dodge effectively. But, the inaccuracy of projectile weapons means you're pelting an entire volume with k-slugs anyway, so a bit of jitter from enemy maneuvering is essentially meaningless.
     Note: CoaDE models k-slugs with railguns and coilguns, which I think are probably optimistic/unrealistic (some versions fire >100 / sec, including cooldown). They also make an argument for tracer rounds on every shot, since stealth is meaningless in space. I disagree; tracer pyros are extra mass you need to accelerate, and ballistics computers have essentially no use for visual confirmation of a hit.
     K-slugs are effective at any range, though obviously accuracy decreases as range increases. It's mainly a question of how much mass, in the form of k-slugs, you can afford to have miss.
     Example: a base in a hollowed-out asteroid will be willing to fire k-slugs at any distance. This opens the door to interplanetary-scale bombardment.

     Lasers are basically worthless
     Because of divergence, effective laser power decreases brutally with distance (constant divergence angle => inverse square falloff). With higher frequencies, you get lower divergence, but unfortunately, higher frequencies are hard to generate and in many ways are less damaging (though that's way beyond scope). Since the engagement envelope is measured in tens/hundreds kilometers, your laser basically needs to be a thousand, a million, or a billion times as powerful, just to do the same amount of damage at range.
     Example: A diffraction-limited 532nm green laser with a 2mm aperture has a minimum beam divergence of 0.085 milliradians. This corresponds to a factor of 23 million billion reduction in flux density over the mere 1.3 light-second distance from Earth to the Moon. So the whole thing about light-speed lag playing a role in laser targeting is garbage, because your city-sized 22-terawatt death-star-laser literally looks like a laser pointer at a distance of 1 light-minute.
     Oh sure, you can do a lot better by increasing the aperture (at inverse square again, but thankfully not scaling with distance). And, in fact, any even remotely practical laser weapons system operates with huge apertures and a lens or mirror to move the beam waist towards the target (all of which are vulnerable themselves)--but you're still going to play a losing battle with diffraction, and CoaDE correctly shows a depressingly abrupt asymptotic drop to zero with distance.
     But the even larger problem is the heat generated. A laser outputs only a tiny portion of its power as coherent light. The rest is dumped as heat, which goes into radiators. To radiate a literal power-plant's worth of thermal energy into space requires several square kilometers of radiator. That makes you a huge, immobile, sitting duck that still can't defend itself because lasers are worthless.
     Example: A space station with an enormous 1 GW ultraviolet laser was disarmed easily, at range, by a lone gun skiff with a 3mm railgun, firing in the general direction of the radiators.
     The point is it's not worth it. Enemies can't dodge anyway, so you might as well use something that actually retains all its destructive power at range and doesn't produce an obscene amount of waste-heat. The only case I've found for lasers is blinding (but again, not really damaging) drones and missiles.

     Whipple shields are stupid; slanted armor is OP
     Slanted armor vastly increases your survivability; this has been known since antiquity. And, when you're going against hypervelocity k-slugs, it's basically your only option. Make it thick enough and slanted enough, and you can shrug off a continuous hailstorm more-or-less indefinitely (at least, if CoaDE is any guide). If the opponent is, stupidly, using lasers, their beam spreads out with the secant of the armor slant angle, to say nothing of the greater Fresnel reflection at angles. Every piece of armor on your ship should be slanted.
     Example: Against an incoming 532nm laser, Aluminum armor has a refractive index of 0.90175. This means that you can actually get total internal reflection. Armor slanted at more than ~64.389 degrees will experience no effect whatsoever from the laser, no matter how powerful!
     Conversely, whipple shields are useless (a whipple shield is a sacrificial layer of thin armor that shocks k-slugs into plasma, which can then diffuse). One problem, however, is that this theory only works if the projectile is orthogonal to the armor (which would mean your armor isn't slanted). In fact, if your whipple shield is slanted, k-slugs tear huge gashes that quickly render it worthless.
     This is a special case of whipple shields being helpful only once. A whipple shield will block one bullet, but not two. If you have a battle where millions of k-slugs being fired, that's basically no protection at all.

     Missiles ruin everything
     In CoaDE, missiles lock onto the greatest heat source. This makes radiators a vulnerability (although I don't know if occlusion is considered by the game). In CoaDE, this is basically completely countered with flares.
     In real life, missiles won't be anywhere near so dumb. First, countermeasures are not 100% effective. Vietnam-era "Sidewinder" missiles had a kill probability of 18%, which is already terrifying. Modern missiles are around 90%. Pure-infrared systems are imager-based these days, making them basically immune to countermeasures. But these are being phased out--today, we have multispectral guidance systems that are essentially unstoppable, operating on radio, visual, and thermal frequencies. And that's not factoring in literal centuries of technological development before the first space battle of the future. Ships are also much bigger and (as before) less-agile targets than fighter jets.
     But the real difference between a k-slug and a missile isn't payload, but maneuverability. Unlike a ship, missiles have an enormous delta-v budget, and they are cheap and small enough to be nigh-vulnerable to weapons fire. This means they can outflank enemies, shooting them from essentially any direction. In addition to striking the more vulnerable sides of a ship, it makes slewing a point-defense cannon around more difficult. And even if you can disable a missile at distance, its debris is still going to slam into you at several km/s relative velocity. Missiles are massive enough that this is probably a mission-kill anyway.
     Indeed, the ideal tactic is to shoot many small missiles and have them converge from different directions. There is no realistic defense against this. Missiles are even less dodge-able than k-slugs and they're much heavier. Shoot it down with point defense or blind it with lasers, and you still have a gaping hole through your hull. Fail to disable even a single one, and you have a nuclear warhead going off point blank.
     In case you don't believe me, think about reality. Fact: a modern warship (the boat kind) has trouble shooting down a single missile with point defense. If you have dozens of missiles with sci-fi armor, all traveling at quadruple the speed (no air resistance), approaching in three dimensions, and you have maybe 1/10th the armor (mass, delta-v limits) . . . well, it's just not going to work out very well for you.

     So how should one design a battleship?
     You don't. It's an obvious consequence of missiles: if your battleship can be obliterated by a tiny missile, and there's no real defense against such a thing, you don't build battleships--you build missiles and send them against enemy infrastructure. Obvious secondary effect: War between such factions is attritional, and at most only one major space-based faction survives.
     [Oh, fine. Let's handwave the missile-defense problem for now.]
     The uselessness of maneuverability suggests exposing the smallest possible cross-section to your enemy. For a given mass, this means making your craft long. The slanted armor means making a sharply pointed nosecone, which will also contain all of your armor budget. This will ameliorate the unreasonable effectiveness of k-slugs. Maybe, your entire ship can just be a highly tapered cone.
     All your heat is dumped via a single retractable radiator extending out the ship's rear, and therefore hopefully hidden from enemy weapons' fire.
     Weapon systems have narrow gimbaling, if any, and poke through tiny holes in the forward cone. These are almost-entirely rapid-fire k-slug launchers, of whichever SF-inal technology you please. IIRC IRL railguns have trouble with repeatability, but a few spinal-mount linear accelerators seems plausible. You probably want a few kilowatt-scale lasers to engage incoming drones and missiles, but nothing too fancy. You can place these on the sides, behind your cone-shield, shooting sideways.
     Since missiles can turn, put your missile launchers behind your ship. Each missile splits into hundreds of individually targeted warheads that spread out and then converge on the target at an angle, as described before. Missiles are optimized for delta-v, and consume all of it before impacting. Rocket-powered guided (non-explosive) k-slugs are also an interesting possibility.
     All weapons are optimized for range, since the aggressor who strikes first and longest is the victor.


Weapon Mounts

As a general rule, a space warship is basically a "weapons platform." It is just a way to move some weapons that you control into a strategic position.

Single weapons and multi-weapon turrets are mounted on "hardpoints" or "weapon stations." These are positions on the spacecraft's hull that are designed to carry the mass of the weapon. One only hangs a heavy picture frame on a nail in a wall stud, not just the wall board. For the same reason only mount a heavy turret on a hardpoint, not on a flimsy stretch of hull. Some hulls are about as strong as the skin on a beer can.

Turrets pivot to allow aiming the weapon(s). Homing missiles are often mounted in "vertical launch systems" or "missile cells", because they do not have to be aimed. Fire and forget, they'll automatically find the target.


Naturally some people who are into hyper-optimization and min-maxing will quickly switch from mounting weapons on a ship to building the ship around a weapon. A monstrously huge weapon, with a fixed forward facing.

Of course you probably have to turn the entire spacecraft in order to aim the weapon, but the ship is going to smite the target with the most bang for your buck. It certainly will be the sort of ship that will blast the snot out of you if you are stupid enough to turn around and try running away. The ship will also have a similar outline as the weapon, probably long and skinny. Popular spinal mount weapons are coil guns, rail guns, and particle beam weapons, since those weapons inflict more damage the longer the weapon is.

The weapon can be mounted on the ship's nose, along the ship's side ("dorsally" or "ventrally", but RocketCat will rip your lips off if you use those terms), or along the ship's spine.

In extreme cases the weapon is the ship's spine, this is what the Traveller RPG calls a "spinal mount". A good example is the "Wave motion gun" that forms the spine of Space Battleship Yamato. In the real world the A-10 Warthog ground-attack aircraft is pretty much built around its 30 mm GAU-8/A Avenger Gatling-type cannon. And Matthew Marden pointed out to me that in 1890 the USS Vesuvius was virtually a spinal mount, with "dynamite guns" fixed in both traverse and elevation.


Isaac Kuo has some interesting observations on the placement of laser turrets:

There's an interesting question of what the ideal number of turrets is. One thing that's counterintuitive is that the number of turrets has little effect on total firepower. Your laser engine(s) can fire the beam down a central corridor, with mirrors to select a branch toward any of the laser turrets. No matter how many turrets you have, you can concentrate all laser firepower through one turret. (Rick Robinson calls this a "Laserstar")

I tend to favor two turrets on opposite sides. Besides providing all around coverage and some redundancy, it also allows use of a "hunter-killer" tactic. While one turret fires the laser to kill a target, the other turret can be scanning to "hunt" for the next target. This allows a near instantaneous switch from one target to the next, minimizing down time for the laser engine.

More importantly, this has a big tactical effect on the enemy's options. Suppose each of your ships only had one laser turret, and the enemy knows this. Then the enemy knows it takes some time for you to switch from the current targets to new targets. If the enemy notices that all of your ships are firing on particular targets, he can take advantage of this to open up sensitive sensors or radiators onboard the non-targeted ships. He knows that if you want to fire on a different target, he's got enough time to close protective "shutters". In contrast, with two turrets per ship nowhere is safe from being targeted.


This depends on the type of laser, of course. With typical IR-UV wavelength lasers, the availability of efficient mirrors generally makes this a compelling option. You only need one or two turrets for full coverage (or practically full coverage), but you might still include more turrets for redundancy and/or "hunter-killer" tactics (one turret hunts for the next target while the current turret kills the current target).

Other types of laser work differently. In particular, an X-ray free electron laser requires pointing the entire ship at the target - particularly if a widely spaced zone plate is used to focus it (the zone plate may be light seconds away, placed between the beam generating ship and the target).

And yet, even in that case the electron beam accelerator might be multi-purpose. The electron beam can be diverted to turreted wigglers for short range lasers, and the electron beam might even be used directly for various purposes. In particular, the electron beam could be used for ablative propulsion of dumb defensive drones (just dumb rocks vaguely near the ship), as well as ablative propulsion for the ship itself.

I'd say a "spinal mount" is fixed with respect to the long axis of a spacecraft, but the main direction of thrust could be some other direction. In fact, it makes more sense for the direction of thrust to be sideways to the long axis of a warship, or for the main thrusters to be turreted.

It generally makes sense to try and present a narrow profile to the enemy. This may actually be generally impossible when the enemy has more than one warship, so the ideal shape might actually be a reversed cone (a teardrop shape). But when you need a kilometer long X-ray wiggler, such a compact shape may be out of the question.

If you are pointing toward the enemy, having main thrusters pointing directly away from the enemy basically eliminates all maneuver capability. You have one degree of freedom, along a direction which is entirely dependent upon the enemy's maneuvers. Basically, you give up both maneuver capability and forward planning capability.

But having main thrusters pointed "broadside" gives you two degrees of freedom, and gives you the flexibility to maneuver freely perpendicular to the enemy. Even better is if the main thrusters can rotate a bit in one dimension. That basically gives you complete flexibility to thrust in practically any direction regardless of the enemy's maneuvers.

That's assuming you have something that looks like traditional thrusters. If your main thrust comes from pulsed ablation/spallation of the ship's main armor/hull, things may look very different anyway.

Isaac Kuo
Spinal, Broadside, and Turreted Weapons

Wherever nerds Science Fiction fans gather to debate the future of space warfare there are several debates that almost always pop up sooner or later, and which  seldom generate a consensus.    One of the most popular is the debate over fixed Vs turreted weapon mounts, with the fixed weapons divided into spinal mounts, and less commonly broadside mounts.  Related is the discussion over which of the three main direct fire weapons likely to be used in space combat - Laser, Particle Beam, and Kinetic - are most suited to each of the three mounting options.  In this blogpost I'm going to attempt a analysis of the specific strengths and weaknesses of each type of mounting, which weapon fits them best, and the tactical scenarios in which they offer the biggest advantages.  I'll also cover the worldbuilding needed to justify each option in your 'Verse.

The Spinal Mount

Definition: A weapon firing in a fixed forward arc, parallel to the direction of thrust, with limited elevation or traverse, and typically running through a significant portion of the spacecraft's length.

   Spinal or Keel mounted weapons are interesting because, unlike turrets or fixed weapons, they have no current real-world counterpart aside from fighter aircraft.  The sea going battleships that provide inspiration for many SF works used broadsides during the age of sail, and turrets in the era of Big Gun battleships, but a single forward firing weapon has never been used to my knowledge aside from a few submarines like the Surcouf, and that was neither common nor in line with the spinal mounts of SF.  If anything their closest analogy is the main gun of a turretless tank hunter.  Even that is a poor comparison given the role stealth plays in tank warfare, and the degree to which it is impossible in space.

   The rational behind the Spinal Mount is straightforward and pretty logical; the bigger the gun the better, right?  Most 'guns' in SF are in fact accelerators of some kind; railguns, coil-guns or gauss cannon, ram accelerators, and particle beams.  What this means is that muzzle velocity scales directly with the length of the weapon, rather than their being a optimum barrel length as their is with conventional firearms.  There are engineering limits, or those imposed by material science, but the highest theoretical velocity is as close to the speed of light as you can get.  A Spinal mount also translates the power of the weapon to the audience quite easily, especially when coupled with long recharge times and/or cool down.  The MAC guns of HALO and the Wave Motion Cannon of Space Battleship Yamato are pretty typical of this trope.

   There are a few disadvantages with the spinal mount, most of which revolve around the fact that the spacecraft must manoeuvre to aim the weapon.  Even if the finer adjustments are done internally rather than by the spacecraft's alignment it will still limit the speed that the spacecraft can edge widely separated targets.  It also means that if a enemy emerged unexpectedly from hyperspace the spinal mount might not have time to be oriented before it is destroyed.  Most spacecraft armed in this way are shown with only one main gun, with is a disadvantage if it breaks down or is disabled by enemy fire.  The spinal mount might well be a glass cannon, extremely dangerous, but needing other ships to contribute to its defence, especially if under attack by multiple enemy.

   While the time needed to aim, and the disadvantage of only being able to engage targets in the same direction at once are inescapable the problem of manoeuvrability may not be an issue.  A spacecraft equipped with a powerful gauss cannon, railgun, particle beam, or laser, will have plentiful electric power.  This can be used to power multiple thrusters distributed all over the spacecraft, rather than having them clumped together, and allowing acceleration in any direction.  With many fictional spacecraft the main drives are to large, expensive, or radioactive to allow this, but for more realistic low accelerations electrothermal or plasma based drives may do fine.

   The advantages are many.  A spacecraft can fit a larger spinal weapon than it could hope to fit into a turret, something likely to hold true for any size of spacecraft.  This is partially due to the fact that a turret has to turn, and so has limits on the mass and size of the weapon, and partially to the fact that recoil forces along the line of thrust can be absorbed by the thrust structure instead of by a complicated system of articulation.  This can also make the weapon more accurate as it will not have to cope with the vibration of turret articulation, or the fox in a unsupported barrel.  Greater muzzle velocity has the advantage of imparting a longer effective range on particle beam and kinetic weapons, helping to negate their inherent weakness.  Even if the energy they output is the same as a physically smaller weapon, the increased range will make them more effective at ranged combat, something there is likely to be a lot of in space.  And they do not need the cool down time shown in SF.  The most powerful might, but it should not be a surprise to find MAC gun like weapon with rapid fire capabilities. 

   Kinetic weapons benefit the most from a spinal mount as opposed to a turret or broadside since it helps to overcome their greatest weakness - low velocity.  Particle beams may also be common in this role since the long skinny shape of a particle accelerator fits the bill nicely.  Lasers on the other hand do not seem to be a good candidate.  Lasers do not benefit from having a longer physical shape, it is the diameter of the emitter that counts.  While there is an analogue — a spacecraft with a single massive mirror at the front — it has its own advantages and disadvantages, and does not really fit the description of a classic spinal mount.  Operationally it would be employed the same however, and have the advantage in rage over smaller turreted counterparts.

   It is this range benefit coupled with the low turning rate that define the use of spinal weapons.  They are the long ranged artillery of space.  If they can maintain range from the enemy the extra range might make them well right invulnerable, while if used in a defensive role that extra reach will fore the enemy to run a gauntlet of fire.  A battle between two of these spacecraft would be like a sniper duel — few tactics, with the one with the greatest accuracy coming out on top.  They would be at a disadvantage in any battle where there are multiple vectors of attack, or one that starts at close range. In a battlefield dominated by missiles they might not fare to well, but one that focuses on direct fire is likely to see them.

   The 'Verse that features spinal weapon can fall anywhere on the spectrum of scientific realism.  Given their long range and potential firepower it seems likely that any space force will have some in its ranks, and that they will form an important part of tactical doctrine.  One thing to note is that they become less attractive as the number and acceleration of ships increases as this brings out their weakness.  A jump drive that allows enemy to 'slip under the guns' as it were will also compromise them.  In any battle where missiles are unviable, massive firepower is needed from smaller ships, or the enemy will be engaged at extreme range a spinal mount is justified.  Another thing to remember is that a magnetic accelerator could be developed as a civilian cargo launcher on the moon, and repurposed as a weapon during a war, similar to in Heinlein's The Moon is a Harsh Mistress.  Even particle beams or lasers that fit the design requirements might be developed as part of beamed power stations.

The Turret

Definition:  A weapon or weapons mounted on or in an articulation that provides extreme ranges of traverse and elevation, as well as commonly housing the firing/loading mechanism and gun crew.

   The turret is one of the most common styles of weapon mounting in SF, and for good reason.  Nearly all wet navy guns are mounted in turrets, as are point defence weapons, and the main gun of tanks.  It was the invention and adoption of turreted main guns, along with the invention of the steam engine, that changed the face of ocean warfare forever.  A spacecraft armed with turrets can bring more of its weapons to bare on any enemy craft, and can do so regardless of its heading.  This is obviously important in a battle involving many spacecraft in close proximity, especially those capable of fairly pronounced manoeuvres and high acceleration.  Point defence weapons are far far more effective will a turret mount than without, allowing them to track incoming.

   There are two common mistakes with the representation of turrets in SF.  The first is the idea of a turret as a bolt on unit.  While this may be the case for smaller point defence units, it is almost never true of larger weapons.  Even the small gun turrets wet navy ships still use extend below the deck level, and old battleship turrets had more concealed than exposed.  The second issue is when turrets are placed in a position where the firing arc is limited by other turrets or by the hull of the spacecraft.  While the latter is to an extent unavoidable the former defeats the purpose of having a turret to begin with.  Yes, I'm looking at you Star Wars.

   Disadvantages of the turret are simple.  For any given weapon a turret to carry it will add complexity, mass, and power requirements to the design of the combat spacecraft, reducing the overall number that can be carried and increasing the cost.  Reduced accuracy can also be a problem due to vibration from the traverse motors, increased vibration in the flexible bearings, and flex in a unsupported barrel.  There amy also be a limit to the ammo that can fit in the turret, decreasing the overall firing rate.  Unique to spacecraft is the problem that recoil forces imparted on the spacecraft are not going to be constant, and will thus be harder to account for as they impact the trajectory of the whole craft.  

   Fundamentally turrets have a single advantage; they can be aimed independently of the spacecraft's orientation.  All the other advantages - reduction in number of guns needed to provide coverage in terms of point defence, ability to engage multiple targets in different directions etc are all derived from the former.  The advantage is most pronounced with point defence weapons, as they will face threats from many angles, and need to be able to track fast and close targets.

   Kinetic weapons are ideal for turrets given that unguided kinetics have short ranges, and it is in this envelope that turrets offer the biggest advantage.  Lasers also have a lot going for them.  Since the laser itself is likely to be in the main hull rather than the turret itself, with the beam reflected through a series of mirrors, there can actually be more turrets than the spacecraft can generate laser light for.  Whichever turrets are needed have laser directed into them, and the loss of a few to enemy fire is not such a disadvantage since the total energy output does not decrease.  Particle beams benefit the least.  This is both due tho their long skinny shape in most designs, and to the fact that bending a particle beam at any kind of angle will produce synchrotron radiation.  Tis could of course be overcome by having truely massive turrets or miniaturised particle beams.  In terms of point defence lasers are likely to be dominant given their accuracy at range, and the fact that a missile probably won't be too well armoured compared to a spacecraft.  Adaptive optics can also give point defence turrets quicker focusing and greater accuracy.  Kinetic point defence will be regulated to slower firing 'flak guns' that throw up a wall of shrapnel rather than targeting individual threats.

   Unlike broadside and spinal mounts turrets have the best chance of dominance in a softer SF 'Verse.  This is because they are best suited to short ranged, high relative speed combat where aim will have to be shifted quickly, and the spacecraft will be changing direction often.  They are also suited to battles where enemy spacecraft can emerge unexpectedly from hyperspace in any direction, and in which the spacecraft of both sides end up occupying the same volume of space.  Obviously force fields or shields help in this regard as they encourage ships to close to kinetic range where they can output more damage.  In a hard science 'Verse close quarters battles are unlikely as everyone will be seen long before they get into range, and with the ranges that are more realistic decrease the disadvantage of fixed weapons and emphasise range and accuracy.  Turrets will always be used as point defence installations however, so they will never be absent.  A lot of works also feature turret mounted kinetic guns as secondary weapons, like the Sulaco from Aliens; this is quite likly considering the relatively small size that kinetic weapons can have while remaining potent enough to be included.

The Broadside

Definition:  Weapons mounted at right angles to the direction of thrust, usually within the main hull of the spacecraft, and with limited traverse and elevation. 

   A fixed broadside battery is one of the most uncommon arrangements to be seen in SF, with turrets being far more common.  The only one that I can think of in visual SF is the gun deck aboard the Separatist ship at the beginning of Revenge of the Sith.  In written works the Black Fleet Trilogy by Joshua Dalzelle had what sounded like a fixed battery of laser weapons on the ship that acts as the setting for most of the first book, but it was never implicitly stated.  In the Honor Harrington books the beam weapons were, by memory, in broadside arrangement; a necessity imposed by the gravity drives used.  There are also the quite common examples in visual media where turrets are shown that would be unable to fire in any arc except that of a broadside.  Most of the turreted guns seen in the Star Wars movies fall into this category, with the Venator Class being a prime example.

   The scarcity of this arrangement is not unexpected.  With the prevalence of the 'Space is a Ocean' trope it is to be expected that a design philosophy that long ago gave way to turret armament should find little traction.  Where it is found it is most often for the visual effect, or because the work is intentionally trying to mimic the battles of the Napoleonic War transposed into space.

   There are not so many advantages to this type of design, and the conditions under which it become practicable are quite specific.  The main advantages are those shared by any fixed weapon mount.  Each weapon will mass less than an equivalent turret, and be simpler in construction.  It may be more accurate since it can be mounts straight to the spacecraft's structure via recoil absorbing mechanisms, reducing vibration.  Ease of access would also be a big factor, especially with advanced and perhaps temperamental weapons since turrets have never been known as spacious.  The weapon itself might also be more massive than a turret could cope with, or have a larger recoil force.

   Disadvantages are pretty obvious.  Limited traverse and elevation impose a greater need for manoeuvrability on the spacecraft, and run the risk that at close range or high traverse speed a more manoeuvrable target could stay out of the fire arc entirely.  This is partially avoided with lasers, since with adaptive optics they can have quite a good arc of fire without the actual emitter being articulated.  Since they cannot fire forward the spacecraft is at a disadvantage accelerating toward or away from a target, although this may not be a problem depending on the technology level of the 'Verse.  The broadside, and all fixed weapons, are at a disadvantage in a 'Verse where FTL can allow a enemy spacecraft to appear unexpectedly in any direction.  The need to rotate the entire spacecraft is going to slow down response times significantly compared to a turreted vessel.  Conversely the broadside is more attractive in a hard science 'Verse where you will always see the enemy coming.

   A broadside thus falls best into a 'Verse with fairly low accelerations and long engagement ranges.  It also becomes a lot more practical if the main offensive weapon is a missile attack from standoff range, especially if it is one involving tens or hundreds of missiles, and possible submunitions.  The ability to carry more weapons for the same mass than in turrets, coupled with the greater accuracy and potentially greater effective range would give the broadside ship a very good defence against missile spam attacks.  Against such an attack it is the volume, range, and accuracy of defensive fire that will stop your spacecraft from being ventilated by a hypervelocity penetrator, and in this regard the broadside holds the advantage.  Also, the greater the number of weapons, the more incoming can be targeted at once.

   Lasers or kinetic weapons would be the most practical.  Lasers would benefit from having many emitters, allowing more incoming to be targeted at once, and for kinetics it allows a greater overall rate of fire, important given their inaccuracy.  With kinetics it could also extend their offensive range by filling more space with metal than would be possible with fewer weapons and making it difficult to evade with low thrust levels; range would still be terrible compared to other weapons however.   Charged particle beams could interfere with each other, but a neutral beam wouldn't ave that issue.  The soft-kill ability of a particle beam might also prove handy against missile attacks; the beams could even be defocused to fill a huge volume of space with relativistic plasma, providing a potent radiation hazard for any incoming missiles.  But without exact numbers it seems impossible to give any of the three weapon types a clear advantage for broadside use; it depends doll on the details of the setting.

   Some of you might object to the idea that lasers are better with many emitters, and it is a common debate.  Do you use one emitter with longer range, or many smaller?  My reasoning is that in a 'Verse where missiles are a viable main offensive weapon they will broadly be able to fire enough missiles with enough submunitions that the extra range is not such a great advantage, more so since a accelerating missile at a half a light second or so is going to be phenomenally hard to hit, and could be travelling at a huge speed by that time.  In any case, a computer controlled array of smaller emitters can act as a single larger emitter to some extent, in the same way as many modern telescopes use mirrors composed of multiple segments.

   Although not strictly a 'broadside' a missile armed spacecraft might have its storage silos arranged in the same configuration to allow more rapid deployment.  With warfare based on missile spam the ability to unleash more missiles in less time might be the best chance at victory, and having the equivalent of a current VLS(Vertical Launch System) might be the ideal.  This could also look pretty cool visually while maintaining realism, so take notice Hollywood!

Nukes In Space

As you should know, there are two types of nuclear weapons. An "atomic bomb" is a weapon with a war-head powered by nuclear fission. An "H-bomb" or "hydrogen bomb" is a weapon with more powerful warhead powered by nuclear fusion.

You can read all about the (unclassified) details of their internal construction and mechanism here.

Occasionally you will find a fusion weapon referred to as a "Solar-Phoenix" or a "Bethe-cycle" weapon. This is a reference to the nuclear scientist Hans Bethe and the Bethe-Weizsäcker or carbon-nitrogen cycle which powers the fusion reaction in the heart of stars heavier than Sol.


NEUTRON BOMB

A "neutron bomb" is what you call an "enhanced radiation bomb". They are specially constructed so more of the bomb's energy is emitted as neutrons instead of x-rays. This means there is far less blast to damage the buildings, but far more lethal neutron radiation to kill the enemy troops. Conventional nuclear warheads typically release 5% of the energy as neutrons, but in neutron bombs it is more like 40%. Neutron energy is higher as well: 14 MeV instead of the conventional 1 to 2 MeV.

A 1 kiloton neutron bomb will irradiate anybody unfortunate enough to be at a range of 900 meters with 80 Grays of neutrons. According to dosages set by the US military, this is high enough to instantly send the victim into a coma, with certain death to follow within 24 hours due to damage to the central nervous system. The LD50 dose is at a range of between 1350 and 1400 meters.

Problems include:

  • Neutron activation of the steel girders of buildings would render them unsafe. Which was one of the selling points of neutron bombs: the buildings could be immediately used by an advancing army, once you removed all the dead enemy soliders.
  • Armored fighting vehicles provide enemy soldiers with a surprisingly high protection of neutron radiation, and can be easily increased. Since all spacecraft include radiation shielding from solar storms and galactic cosmic rays, this will drastically reduce the effect of neutron bombs. Spacecraft with nuclear propulsion will try to aim their shadow shields at the neutron bomb for added protection.
  • Enemy soldiers can also find high amounts of protection by sheltering inside buildings with 12 inch concrete walls and ceiling, or in a cellar under 24 inches of damp soil. Both will reduce the radiation exposure by a factor of 10.
  • Neutron bomb ordinance requires maintenance, since one of the components is Tritium with its annoyingly short half-life of 12.32 years. This means that every few years the neutron bombs will have to be opened up and have their tritium replaced.

SALTED BOMB

You will also occasionally find references to a nasty weapon called a "cobalt bomb". This is technically termed a "salted bomb". It is not used for spacecraft to spacecraft combat, it is only used for planetary bombardment. They are enhanced-fallout weapons, with blankets of cobalt or zinc to generate large quantities of deadly radioactive cobalt or zinc isotope dust. The warhead proper will probably be a neutron bomb: since the more neutrons emitted by the warhead, the more of the blanket will be transmuted into radioactive isotopes.

Please note the difference between a "salted bomb" and a "dirty bomb".

A dirty bomb is an ordinary chemical explosive in a small bag of ground-up radioactive material. The chemical explosion merely sprays the powdered plutonium or whatever all over the city block. Strictly a terrorist weapon, it is pretty worthless as a military weapon.

A salted bomb is a nuclear warhead designed to make a nuclear explosion that will spread millions of bagfulls of fallout that is thousands of times more radioactive that mere powdered plutonium over a quarter of a continent.

Term comes from metaphor "sowing the Earth with salt".

Warhead

As far as warhead mass goes, Anthony Jackson says the theoretical limit on mass for a fusion warhead is about 1 kilogram per megaton. No real-world system will come anywhere close to that, The US W87 thermonuclear warhead has a density of about 500 kilograms per megaton. Presumably a futuristic warhead would have a density between 500 and 1 kg/Mt. Calculating the explosive yield of a weapon is a little tricky.

For missiles, consider the US Trident missile. Approximately a cylinder 13.41 m in length by 1.055 m in radius, which makes it about 47 cubic meters. Mass of 58,500 kg, giving it a density of 1250 kg/m3. The mass includes eight warheads of approximately 160 kg each.

Wildly extrapolating far beyond the available data, one could naively divide the missile mass by the number of warheads, and divide the result by the mass of an individual warhead. The bottom line would be that a warhead of mass X kilograms would require a missile of mass 45 * X kilograms, and a volume of 0.036 * X cubic meters (0.036 = 45 / 1250). Again futuristic technology would reduce this somewhat.


Nuclear weapons will destroy a ship if they detonate exceedingly close to it. But if it is further away than about a kilometer, it won't do much more than singe the paint job and blind a few sensors. And in space a kilometer is pretty close range.

Please understand: I am NOT saying that nuclear warheads are ineffective. I am saying that the amount of damage they inflict falls off very rapidly with increasing range. At least much more rapidly than with the same sized warhead detonated in an atmosphere.

But if the nuke goes off one meter from your ship, your ship will probably be vaporized. Atmosphere or no.

George William Herbert says a nuke going off on Terra has most of the x-ray emission is absorbed by the atmosphere, and is transformed into the first fireball and the blast wave. There ain't no atmosphere in space so the nuclear explosion is light on blast and heavy on x-rays. In fact, almost 90% of the bomb energy will appear as x-rays behaving as if they are from a point source (specifically 80% soft X-rays and 10% gamma), and subject to the good old inverse square law (i.e., the intensity will fall off very quickly with range). The remaining 10% will be neutrons.

For an enhanced radiation weapon (AKA "Neutron Bomb") figures are harder to come by. The best guess figure I've managed to find was up to a maximum of 80% neutrons and 20% x-rays.

The fireball and blast wave is why nuclear warheads detonating in the atmosphere will flatten buildings for tens of kilometers, but detonations in space have a damage range under one kilometer.

If you want to get more bang for your buck, there is a possibility of making nuclear shaped charges. Instead of wasting their blast on a spherical surface, it can be directed at the target spacecraft. This will reduce the surface area of the blast, thus increasing the value for kiloJoules per square meter.

According to John Schilling, with current technology, the smallest nuclear warhead would probably be under a kiloton, and mass about twenty kilograms. A one-megaton warhead would be about a metric ton, though that could be reduced by about half with advanced technology.

Eric Rozier has an on-line calculator for nuclear weapons. Eric Henry has a spreadsheet that does nuclear blast calculations, including shaped charges, on his website. For bomb blasts on the surface of the Earth or other planet with an atmosphere, you can use the handy-dandy Nuclear Bomb Effects Computer. But if you really want to do it in 1950's Atomic Rocket Retro style, make your own do-it-yourself Nuclear Bomb Slide Rule!

Kilotons per Kilogram

(ed note: this is a historical look at the kiloton per kilogram alphas of actual nuclear weapons. Also see his interactive Yield To Weight explorer)

What makes nuclear weapons impressive and terrible is that their default yield-to-weight ratio — that is, the amount of bang per mass, usually expressed in terms of kilotons per kilogram (kt/kg) — is much, much higher than conventional explosives. Take TNT for example. A ton of TNT weighs, well, a ton. By definition. So that’s 0.001 kilotons per 1,000 kilograms; or 0.000001 kt/kg. By comparison, even a crude weapon like the Little Boy bomb that was dropped on Hiroshima was about 15 kilotons in a 4,400 kg package: 0.003 kt/kg. That means that the Little Boy bomb had an energy density three orders of magnitude higher than a regular TNT bomb would. Now, TNT isn’t the be-all and end-all of conventional explosives, but no conventional explosive gets that much boom for its buck compared to a nuke.

The Little Boy yield is much lower than the hypothetical energy density of uranium-235. For every kilogram of uranium-235 that completely fissions, it releases about 17 kt/kg. That means that less than a kilogram of uranium-235 fissioned in the Little Boy bomb to release its 15 kilotons of energy. Knowing that there was 64 kg of uranium in the bomb, that means that something like 1.3% of the uranium in the weapon actually underwent fission. So right off the bat, one could intuit that this is something that could probably be improved upon.

The Fat Man bomb had a much better use of fissile material than Little Boy. Its yield wasn’t that much better (around 20 kilotons), but it managed to squeeze that (literally) out of only 6.2 kilograms of plutonium-239. Pu-239 releases around 19 kilotons per kilogram that completely fissions, so that means that around 15% of the Fat Man core (a little under 1 kg of plutonium) underwent fission. But the bomb itself still weighed 4,700 kg, making its yield-to-weight ratio a mere 0.004 kt/kg. Why, despite the improve efficiency and more advanced design of Fat Man, was the yield ratio almost identical to Little Boy? Because in order to get that 1 kg of fissioning, it required a very heavy apparatus. The explosive lenses weighed something like 2,400 kilograms just by themselves. The depleted uranium tamper that held the core together and reflected neutrons added another 120 kilograms. The aluminum sphere that held the whole apparatus together weighed 520 kilograms. The ballistic case (a necessary thing for any actual weapon!) weighed another 1,400 kg or so. All of these things were necessary to make the bomb either work, or be a droppable bomb.

So it’s unsurprising to learn that improving yield-to-weight ratios was a high order of business in the postwar nuclear program. Thermonuclear fusion ups the ante quite a bit. Lithium-deuteride (LiD), the most common and usable fusion fuel, yields 50 kilotons for every kilogram that undergoes fusion — so fusion is nearly 3 times more energetic per weight than fission. So the more fusion you add to a weapon, the better the yield-to-weight ratio, excepting for the fact that all fusion weapons require a fission primary and usually also have very heavy tampers.

I took all of the reported American nuclear weapon weights and yields from Carey Sublette’s always-useful website, put them into the statistical analysis program R, and created this semi-crazy-looking graph of American yield-to-weight ratios:

The horizontal (x) axis is the yield in kilotons (on a logarithmic scale), the vertical (y) axis is the weight in kilograms (also on a log scale). In choosing which of the weights and yields to use, I’ve always picked the lowest listed weights and the highest listed yields — because I’m interested in the optimal state of the art. The individual scatter points represent models of weapons. The size of each point represents how many of them were produced; the color of them represents when they were first deployed. Those with crosses over them are still in the stockpile. The diagonal lines indicate specific yield-to-weight ratio regions.

A few points of interest here. You can see Little Boy (Mk-1), Fat Man (Mk-3), and the postwar Fat Man improvements (Mk-4 — same weight, bigger yield) at the upper left, between 0.01 kt/kg and 0.001 kt/kg. This is a nice benchmark for fairly inefficient fission weapons. At upper right, you can see the cluster of the first H-bomb designs (TX-16, EC-17, Mk-17, EC-24, Mk-24) — high yield (hence far to the right), but very heavy (hence very high). Again, a good benchmark for first generation high-yield thermonuclear weapons.

What a chart like this lets you do, then, is start to think in a really visual and somewhat quantitative way about the sophistication of late nuclear weapon designs. You can see quite readily, for example, that radical reductions in weight, like the sort required to make small tactical nuclear weapons, generally results in a real decrease in efficiency. Those are the weapons in the lower left corner, pretty much the only weapons in the Little Boy/Fat Man efficiency range (or worse). One can also see that there are a few general trends in design development over time if one looks at how the colors trend.

First there is a movement down and to the right (less weight, more yield — improved fission bombs); there is also a movement sharply up and to the right (high weight, very high yield — thermonuclear weapons) which then moves down and to the left again (high yield, lower weight — improved thermonuclear weapons). There is also the splinter of low-weight, low-yield tactical weapons as well that jots off to the lower left. In the middle-right is what appears to be a sophisticated “sweet spot,” the place where all US weapons currently in the stockpile end up, in the 0.1-3 kt/kg range, especially the 2-3 kt/kg range:

These are the bombs like the W-76 or the B-61 — bombs with “medium” yield warheads (100s rather than 1,000s of kilotons) in relatively low weight packages (100s rather than 1000s of kilograms). These are the weapons take advantage of the fact that they are expected to be relatively accurate (and thus don’t need to be in the multi-megaton range to have strategic implications), along with what are apparently sophisticated thermonuclear design tricks (like spherical secondaries) to squeeze a lot of energy out of what is a relatively small amount of material. Take the W-76 for example: its manages to get 100 kilotons of yield out of 164 kilograms. If we assume that it is a 50/50 fission to fusion ratio, that means that it manages to fully fission about 5 kilograms of fissionable material, and to fully fuse about 2 kilograms of fusionable material. And it takes just 157 kg of other apparatus (and unfissioned or unfused material) to produce that result — which is just a little more than Shaquille O’Neal weighs.

Such weapons aren’t the most efficient. Weapon designer Theodore Taylor wrote in 1987 that 6 kiloton/kilogram had been pretty much the upper limit of what had even been achieved. Only a handful of weapons got close to that. The most efficient weapon in the US stockpile was the Mk-41, a ridiculously high yield weapon (25 megatons) that made up for its weight with a lot of fusion energy.

But given that high efficiency is tied to high yields — and relatively high weights — it’s clear that the innovations that allowed for the placing of warheads on MIRVed, submarine-launched platforms are still pretty impressive. The really magical range seems to be for weapons that in the hundred kiloton range (more than 100 kilotons but under a megaton), yet under 1,000 kilograms. Every one of those dates from after 1962, and probably involves the real breakthroughs in warhead design that were first used with the Operation Dominic test series (1962). This is the kind of strategic miniaturization that makes war planners happy.

What’s the payoff of thinking about these kinds of numbers? One is that it allows you to see where innovations have been made, even if you know nothing about how the weapon works. In other words, yield-to-weight ratios can provide a heuristic for making sense of nuclear design sophistication, comparing developments over time without caring about the guts of the weapon itself. It also allows you to make cross-national comparisons in the same fashion. The French nuclear arsenal apparently developed weapons in that same miniaturized yield-to-weight range of the United States by the 1970s — apparently with some help from the United States — and so we can probably assume that they know whatever the United States figured out about miniaturized H-bomb design in the 1960s.

Or, to take another tack, and returning to the initial impetus for me looking at this topic, we know that the famous “Tsar Bomba” of the Soviet Union weighed 27,000 kilograms and had a maximum yield of 100 Mt, giving it a yield-to-weight ratio of “only” 3.43 kilotons/kilograms. That’s pretty high, but not for a weapon that used so much fusion energy. It was clear to the Atomic Energy Commission that the Soviets had just scaled up a traditional H-bomb design and had not developed any new tricks. By contrast, the US was confident in 1961 that they could make a 100 Mt weapon that weighed around 13,600 kg (30,000 lb) — an impressive 7.35 kiloton/kilogram ratio, something well above the 6 kt/kg achieved maximum. By 1962, after the Dominic series, they thought they might be able to pull off 50 Mt in only a 4,500 kg (10,000 lb) package — a kind of ridiculous 11 kt/kg ratio. (In this estimate, they noted that the weapon might have an impractically large diameter as a result, perhaps because the secondary was spherical as opposed to cylindrical.) So we can see, without really knowing much about the US had in mind, that it was planning something very, very different from what the Soviets set off.

From Kilotons per Kilogram by Alex Wellerstein (2013)

EMP

When it comes to the dreaded EMP created by nuclear detonations, matters become somewhat complicated. Please, do NOT confuse EMP (electromagnetic Pulse) with EM (electromagnetic Radiation).

Most SF fans have a somewhat superficial understanding of EMP: an evil foreign nation launches an ICBM at the United States, the nuke detonates in the upper atmosphere over the Midwest, an EMP is generated, the EMP causes all stateside computers to explode, all the TVs melt, all the automobile electrical systems short out, all the cell phones catch fire, basically anything that uses electricity is destroyed.

This is true as far as it goes, but when you start talking about deep space warfare, certain things change. Thanks to Andrew Presby for setting me straight on this matter.

First off, the EMP I just described is High Altitude EMP (HEMP). This EMP can only be generated if there is a Terra strength magnetic field and a tenuous atmosphere present. A nuke going off in deep space will not generate HEMP. Please be aware, however, if a nuke over Iowa generates a HEMP event, the EMP will travel through the airless vacuum of space just fine and fry any spacecraft that are too close.

Secondly, EMP can also be generated in airless space by an e-Bomb, which uses chemical explosives and an armature. No magnetic field nor atmosphere required. This is called a Non-nuclear electromagnetic pulse (NNEMP). As with all EMPs, once generated they will travel through space and kill spacecraft.

Thirdly, there is System Generated EMP (SGEMP) to consider. HEMP is created when the gamma rays from the nuclear detonation produce Compton electrons in air molecules, and the electrons interact with a magnetic field to produce EMP. But with SGEMP, gamma rays penetrating the body of the spacecraft accelerated electrons, creating electromagnetic transients.

SGEMP impacts space system electronics in three ways. First, x-rays arriving at the spacecraft skin cause an accumulation of electrons there. The electron charge, which is not uniformly distributed on the skin, causes current to flow on the outside of the system. These currents can penetrate into the interior through various apertures, as well as into and through the solar cell power transmission system. Secondly, x-rays can also penetrate the skin to produce electrons on the interior walls of the various compartments. The resulting interior electron currents generate cavity electromagnetic fields that induce voltages on the associated electronics which produce spurious currents that can cause upset or burnout of these systems. Finally, x-rays can produce electrons that find their way directly into signal and power cables to cause extraneous cable currents. These currents are also propagated through the satellite wiring harness.

Dr. George W. Ullrich

Impulsive Shock

A one kiloton nuclear detonation produces 4.19e12 joules of energy. One kilometer away from the detonation point defines a sphere with a surface area of about 12,600,000 square meters (the increase in surface area with the radius of the sphere is another way of stating the Inverse Square law). Dividing reveals that at this range the energy density is approximately 300 kilojoules per square meter. Under ideal conditions this would be enough energy to vaporize 25 grams or 10 cubic centimeters of aluminum (in reality it won't be this much due to conduction and other factors).

1e8 watts per square centimeter for about a microsecond will melt part of the surface of a sheet of aluminum. 1e9 W/cm2 for a microsecond will vaporize the surface, and 1e11 W/cm2 for a microsecond will cause enough vaporization to create impulsive shock damage (i.e., the surface layer of the material is vaporized at a rate exceeding the speed of sound). The one kiloton bomb at one kilometer only does about 3.3e7 W/cm2 for a microsecond.

One megaton at one kilometer will do 3.3e10 W/cm2, enough to vaporize but not quite enough for impulsive shock. At 100 meters our one meg bomb will do 3.3e12 W/cm2, or about 33 times more energy than is required for impulsive shock. The maximum range for impulsive shock is about 570 meters.

Luke Campbell wonders if 1e11 W/cm2 is a bit high as the minimum irradiation to create impulsive shock damage. With lasers in the visible light and infrared range, 1e9 W/cm2 to 1e10 W/cm2 is enough. But he allows that matters might be different for x-rays and gamma rays due to their extra penetration.

As to the effects of impulsive damage, Luke Campbell had this to say:

First, consider a uniform slab of material subject to uniform irradiation sufficient to cause an impulsive shock. A thin layer will be vaporized and a planar shock will propagate into the material. Assuming that the shock is not too intense (i.e., not enough heat is dumped into the slab to vaporize or melt it) there will be no material damage because of the planar symmetry. However, as the shock reaches the back side of the slab, it will be reflected. This will set up stresses on the rear surface, which tends to cause pieces of the rear surface to break off and fly away at velocities close to the shock wave velocity (somewhat reduced, of course, due to the binding energy of all those chemical bonds you need to break in order to spall off that piece). This spallation can cause significant problems to objects that don't have anything separating them from the hull. Modern combat vehicles take pains to protect against spallation for just this reason (using an inner layer of Kevlar or some such).

Now, if the material or irradiance is non-uniform, there will be stresses set up inside the hull material. If these exceed the strength of the material, the hull will deform or crack. This can cause crumpling, rupturing, denting (really big dents), or shattering depending on the material and the shock intensity.

For a sufficiently intense shock, shock heating will melt or vaporize the hull material, with obvious catastrophic results. At higher intensities, the speed of radiation diffusion of the nuke x-rays can exceed the shock speed, and the x-rays will vaporize the hull before the shock can even start. Roughly speaking, any parts of the hull within the diameter of an atmospheric fireball will be subject to this effect.

In any event, visually you would see a bright flash from the surface material that is heated to incandescence. The flash would be sudden, only if the shock is so intense as to cause significant heating would you see any extra light for more than one frame of the animation (if the hull material is heated, you can show it glowing cherry red or yellow hot or what have you). The nuke itself would create a similar instant flash. There would probably be something of an afterglow from the vaporized remains of the nuke and delivery system, but it will be expanding in a spherical cloud so quickly I doubt you would be able to see it. Shocks in rigid materials tend to travel at something like 10 km/s, shock induced damage would likewise be immediate. Slower effects could occur as the air pressure inside blasts apart the weakened hull or blows out the shattered chunks, or as transient waves propagate through the ship's structure, or when structural elements are loaded so as to shatter normally rather than through the shock. Escaping air could cause faintly visible jets as moisture condenses/freezes out - these would form streamers shooting away from the spacecraft at close to the speed of sound in air - NO billowing clouds.

Luke Campbell

Nuke vs. Spacecraft

Dr. John Schilling describes the visual appearance of a nuclear strike on a spacecraft.

First off, the weapon itself. A nuclear explosion in space, will look pretty much like a Very Very Bright flashbulb going off. The effects are instantaneous or nearly so. There is no fireball. The gaseous remains of the weapon may be incandescent, but they are also expanding at about a thousand kilometers per second, so one frame after detonation they will have dissipated to the point of invisibility. Just a flash.

The effects on the ship itself, those are a bit more visible. If you're getting impulsive shock damage, you will by definition see hot gas boiling off from the surface. Again, the effect is instantaneous, but this time the vapor will expand at maybe one kilometer per second, so depending on the scale you might be able to see some of this action. But don't blink; it will be quick.

Next is spallation - shocks will bounce back and forth through the skin of the target, probably tearing chunks off both sides. Some of these may come off at mere hundreds of meters per second. And they will be hot, red- or maybe even white-hot depending on the material.

To envision the appearance of this part, a thought experiment. Or, heck, go ahead and actually perform it. Start with a big piece of sheet metal, covered in a fine layer of flour and glitter. Shine a spotlight on it, in an otherwise-dark room. Then whack the thing with a sledgehammer, hard enough for the recoil to knock the flour and glitter into the air.

The haze of brightly-lit flour is your vaporized hull material, and the bits of glitter are the spallation. Scale up the velocities as needed, and ignore the bit where air resistance and gravity brings everything to a halt.

Next, the exposed hull is going to be quite hot, probably close to the melting point. So, dull red even for aluminum, brilliant white for steel or titanium or most ceramics or composites. The seriously hot layer will only be a millimeter or so thick, so it can cool fairly quickly - a second or two for a thick metallic hull that can cool by internal conduction, possibly as long as a minute for something thin and/or insulating that has to cool by radiation.

After this, if the shock is strong enough, the hull is going to be materially deformed. For this, take the sledgehammer from your last thought experiment and give a whack to some tin cans. Depending on how hard you hit them, and whether they are full or empty, you can get effects ranging from mild denting at weak points, crushing and tearing, all the way to complete obliteration with bits of tin-can remnant and tin-can contents splattered across the landscape.

Again, this will be much faster in reality than in the thought experiment. And note that a spacecraft will have many weak points to be dented, fragile bits to be torn off, and they all get hit at once. If the hull is of isogrid construction, which is pretty common, you might see an intact triangular lattice with shallow dents in between. Bits of antenna and whatnot, tumbling away.

Finally, secondary effects. Part of your ship is likely to be pressurized, either habitat space or propellant tank. Coolant and drinking water and whatnot, as well. With serious damage, that stuff is going to vent to space. You can probably see this happening (air and water and some propellants will freeze into snow as they escape, BTW). You'll also see the reaction force try to tumble the spacecraft, and if the spacecraft's attitude control systems are working you'll see them try to fight back.

You might see fires, if reactive materials are escaping. But not convection flames, of course. Diffuse jets of flame, or possibly surface reactions. Maybe secondary explosions if concentrations of reactive gasses are building up in enclosed (more or less) spaces.

Dr. John Schilling

Radiation Flux

Crew members are not as durable as spacecraft, since they are vulnerable to neutron radiation. A one megaton Enhanced-Radiation warhead (AKA "neutron bomb") will deliver a threshold fatal neutron dose to an unshielded human at 300 kilometers. There are also reports that ER warheads can transmute the structure of the spacecraft into deadly radioactive isotopes by the toxic magic of neutron activation. Details are hard to come by, but it was mentioned that a main battle tank irradiated by an ER weapon would be transmuted into isotopes that would inflict lethal radiation doses for up to 48 hours after the irradiation. So if you want to re-crew a spacecraft depopulated by a neutron bomb, better let it cool off for a week or so.

For a conventional nuclear weapon (i.e., NOT a neutron bomb), the x-ray and neutron flux is approximately:

Fx = 2.6 x 1027 * (Y/R2)

Fn = 1.8 x 1023 * (Y/R2)

where:

  • Fx = X-ray fluence (x-rays/m2)
  • Fn = Neutron fluence (neutrons/m2)
  • Y = weapon yield (kilotons TNT)
  • R = range from ground zero (meters)

There are notes on the effects of radiation on crew and electronics here.

Nuclear Shaped Charges

Back in the 1960's, rocket scientist came up with the infamous "Orion Drive." This was basically a firecracker under a tin can. Except the tin can is a spacecraft, and the firecracker is a nuclear warhead.

Anyway, they realized that about 90% of the nuclear energy of an unmodified nuclear device would be wasted. The blast is radiated isotropically, only a small amount actually hits the pusher-plate and does useful work. So they tried to figure out how to channel all the blast in the desired direction. A nuclear shaped charge.


Remember that in the vacuum of space, most of the energy of a nuclear warhead is in the form of x-rays. The nuclear device is encased in a radiation case of x-ray opaque material (uranium) with a hole in the top. This forces the x-rays to to exit only from the hole. Whereupon they run full tilt into a large mass of beryllium oxide (channel filler).

The beryllium transforms the nuclear fury of x-rays into a nuclear fury of heat. Perched on top of the beryllium is the propellant: a thick plate of tungsten. The nuclear fury of heat turns the tungsten plate into a star-core-hot spindle-shaped-plume of ionized tungsten plasma. The x-ray opaque material and the beryllium oxide also vaporize a few microseconds later, but that's OK, their job is done.

The tungsten plasma jet hits square on the Orion drive pusher plate, said plate is designed to be large enough to catch all of the plasma. With the reference design of nuclear pulse unit, the plume is confined to a cone of about 22.5 degrees. About 85% of the nuclear device's energy is directed into the desired direction, which I think you'd agree is a vast improvement over 10%.


About this time the representatives of the military (who were funding this project) noticed that if you could make the plume a little faster and with a narrower cone, it would no longer be a propulsion system component. It would be a nuclear directed energy weapon. Thus was born project Casaba-Howitzer.

Details are scarce since the project is still classified after all these years. Tungsten has an atomic number (Z) of 74. When the tungsten plate is vaporized, the resulting plasma jet has a relatively low velocity and diverges at a wide angle (22.5 degrees). Now, if you replace the tungsten with a material with a low Z, the plasma jet will instead have a high velocity at a narrow angle ("high velocity" meaning "a recognizable fraction of the speed of light"). The jet angle also grows narrower as the thickness of the plate is reduced. This is undesirable for a propulsion system component (because it will destroy the pusher plate), but just perfect for a weapon (because it will destroy the enemy ship).

The report below suggests that the practical minimum half angle the jet can be focused to is 5.7° (0.1 radians).

They would also be perfect as an anti-ballistic missile defence. One hit by a Casaba Howitzer and a Soviet ICBM would be instantly vaporized. Which is why project Casaba-Howitzer's name came up a few times in the 1983 Strategic Defense Initiative.

Casaba Howitzers fired from orbit at ground targets on Terra would be inefficient, which is not the same as "does no damage." A nuclear warhead fired at a ground target would do far more damage, but the Casaba Howitzer bolt is instantaneous, non-interceptable, and would still do massive damage to an aircraft carrier.

Scott Lowther has done some research into a 1960's design for an Orion-drive battleship. It was to be armed with naval gun turrets, minuteman missiles with city-killing 20 megatons warheads, and Casaba-Howitzer weapons. It appears that the Casaba-Howitzer charges would be from subkiloton to several kilotons in yield, be launched on pancake booster rockets until they were far enough from the battleship to prevent damage (several hundred yards), whereupon they would explode and skewer the hapless target with a spear of nuclear flame. The battleship would probably carry a stockpile of Casaba-Howitzer weapons in the low hundreds.

Mr. Lowther estimates that each Casaba-Howitzer round would have a yield "up to a few kilotons" and could deliver close to 50% of that energy in the spear of nuclear flame. Three kiltons is 1.256 × 1013 joules, 50% of that is 6.276 × 1012 joules per bolt.

This is thirty-five times as powerful as a GBU-43/B Massive Ordnance Air Blast bomb, the second most powerful non-nuclear weapon ever designed. Per bolt.

Get a copy of the report for more details, including a reconstruction of a Casaba-Howitzer charge.

What is the mass and volume of a Casaba-Howitzer charge? Apparently this also is still classified. An Orion Drive nuclear pulse unit would be about 1,150 kg, have a blast yield of about 29 kilotons, and be a cylinder with a radius of 0.4 meters and a height of 0.87 meters. The volume would therefore be about 0.4 cubic meters. As previously mentioned a Casaba-Howitzer charge would have a yield ranging from sub-kiloton to a few kilotons, so presumably it would be smaller and of lower mass than a pulse unit.

The Nuclear Spear: Casaba Howitzer

The Casaba Howitzer is the result of research into reducing the spread of the particles produced by a nuclear pulse unit. Make the cone narrow enough and it becomes a destructive beam. A Science & Global Security report from 1990 used polystyrene as the propellant material to produce a particle beam with a spread of 5.7° and a velocity of 1000km/s.   

Particle velocity is derived from the Root Mean Square equation. It can be written as such:
  • Particle velocity = (24939 * Temp / Mass) ^ 0.5

24939 is a constant equal to Boltzmann's constant (1.38*10-23) divided by unitary molar mass in kg (1.66*10-27) times the degrees of freedom of motion (3). Temp is the nuclear detonation's temperature in Kelvin, and Mass is the mass of the propellant used in kg/mol.

     For an atom bomb (108 K), uranium (238) will be ejected at 102km/s.
     In a fusion reaction (109 K), deuterium (2) will be ejected at 3530km/s.

The difficulty is in transmitting this thermal energy to the propellant, and keeping the particle cone focused.

In a propulsion pulse unit, it is not known how efficiently a nuclear shaped charge is able to heat the propellant. Most sources cite a 85% of the device's energy being sent in the desired direction. It is unknown also whether this is before or after some of the propellant is accelerated in the wrong direction, and whether larger pulse units are more efficient (higher propellant mass fraction). This is important as it would allow a thermos-dynamic estimation of the particle velocity.

It would be reasonable to use a lower figure when calculating the amount of energy delivered to the propellant. Scott Lowther gave a 50% figure for small fission charges. An SDI nuclear weapons study, Project Prometheus, experimentally tested Casaba Howitzer weapons using plastic propellants. It achieved 10% efficiency. A Princeton University study from 1990 on third-generation nuclear weapons cited 5% instead, but for fusion devices with ten times better beam focus. 


Effectiveness

Despite the reduction in cone spread, the stream of particles produced by by Casaba Howitzer dissipates much more quickly than an electro-magnetically accelerated particle beam or a laser.

It is possible to reduce the beam angle to 0.006 degrees in width, as reported by the third-generation nuclear weapons study. 0.057 degrees has been experimentally achieved by project Prometheus. The trade-off is much lower efficiency than propulsive units (5-10% vs 80-85%).

The theoretical maximal performance of a thermonuclear device is 25TJ/kg. Modern weapons are able to achieve 2.5TJ/kg, but this figure is for large weapons that have better scaling. Smaller warheads such as those tested for project Prometheus are likely to be in the kiloton range, and mass about 100kg. Better understanding of fission ignition has reduced the nuclear material requirement down to a kilogram or less.

A nuclear detonation only lasts a microsecond, so we can assume that the entire energy of the unit is delivered to the target in a single pulse of duration 10-6 seconds.  As the particles produced expand in a cone with an angle θ, we can use the following equation to calculate the destructive potential at various distances:

  • Intensity = (Yield * Efficiency * 10^6) / (3.14 * (tan(θ) * Distance) ^2)
  • Irradiance = (Yield * Efficiency) / (3.14 * (tan(θ) * Distance) ^2)

Intensity is measured in watts per square meter. Irradiance is joules per square meter. Yield is how much energy the nuclear charge delivers, converted to joules. Efficiency ranges from the 0.85 of a propulsion unit to the 0.05 of a Casaba Howitzer. θ is the cone angle. Distance is between the nuclear detonation and the target, in meters.

Intensity determines whether the rate of damage dealt causes impulse shock, with great mechanical strain on the target material. Irradiance determines how much damage is dealt in terms of material vaporized.

Let us calculate some examples:

Small Casaba Howitzer (50kg)
0.01 radian directivity (0.057 degrees)
5kt yield, 10% efficiency: 2.09TJ
Distance 1km: Irradiance = 2.09GJ/m2
Distance 10km: Irradiance = 209MJ/m2
Distance 100km: Irradiance = 2.09MJ/m2
Distance 1000km: Irradiance = 2kJ/m2

Large Casaba Howitzer (1000kg)
0.001 radian directivity (0.0057 degrees)
1Mt yield, 5% efficiency: 2PJ
Distance 1km: Irradiance = 2TJ/m2
Distance 10km: Irradiance = 240GJ/m2
Distance 100km: Irradiance = 2GJ/m2
Distance 1000km: Irradiance = 20.9MJ/m2

Futuristic Megaton Nuclear lance
0.0001 radian directivity (0.00057 degrees)
1Mt yield, 20% efficiency: 8.56PJ
Distance 1000km: Irradiance = 8.56GJ/m2
Distance 100,000 km: Irradiance = 856kJ/m2

To determine destructive capability, we can model the Casaba Howitzer as a direct energy weapon. We can recreate the particle strike as a laser weapon firing a single pulse with equal properties.

We will describe the strike as a laser pulse of duration 1 microsecond, containing X energy and with Y spot radius. A 1630nm laser focused by a 2cm diameter mirror consistently produces the same spot sizes as a 0.01 radian beam. A 20cm mirror is used for 0.001 radian beams, and 200cm for 0.0001. We test penetration against Aluminium. 

Small Casaba Howitzer:
X = 2.09TJ
1000km, Y = 99.43m: 734mm penetration
10,000km, Y = 994.3: 0.73mm penetration

Large Casaba Howitzer:
X = 2PJ
10,000km, Y = 99.4m: 73420mm penetration
100,000km, Y = 994m: 73.4mm penetration

Futuristic Megaton Nuclear lance:
X = 8.56PJ
1 million km, Y = 994.3m: 292mm penetration

The results reveal that the Casaba Howitzer is an extremely destructive weapon, with the larger models able to strike at distances usually reserved for lasers. Even a small Casaba Howitzer is effective at up to 2000km, using technology tested in the 80s. Larger, more modern devices can strike at distances where light lag becomes a concern. Futuristic devices will still be limited to particle velocities of about 10,000km/s, meaning that time to target becomes problematic.


Making use of the Casaba Howitzer

The Casaba Howitzer's advantages are numerous, and can be exploited in four ways:

  • Terminal warhead

Hard science fiction with a military focus usually boil down to where the author has placed their marker on the sliding scale between missile and laser dominance. Make lasers too powerful, and they make mass missile attacks uneconomical. Make missiles cheap and fast enough, and you can overwhelm any laser defense. 

Missiles are hindered by the requirement to track the target and follow until impact. Lasers are increasingly effective as missiles close the distance to their target. Past a certain point, any missile touched by a laser is quickly destroyed. So quickly, that a laser defense's primary limitation is the time it takes to switch targets. In other words, a laser defense sets up a 'death zone' around itself, within which any wave of missiles will quickly be annihilated. 

A combination of efficient lasers, multiple turrets and competent target handling can cut through hundreds of missiles. 

The counter to this, on the missile side, is to perform randomized high-acceleration maneuvers called 'jinks'. This tactic is already used today by sea-skimming missiles once they enter the range of CIWS defenses. The problem is, in space this requires the missile to have powerful thrusters, lots of propellant and active, autonomous sensors that survive to the terminal stage of its attack. This means that missiles will end up being heavy, hard to bring up to speed, large (easy to track and hit) and expensive due to on-board electronics. These are all characteristics you want to avoid when trying to make massive waves of missiles economical, or if jinking through the death zone.

Using a Casaba Howitzer warhead solves this conundrum. 

It allows missiles to deal damage from outside the death zone. It also removes the requirement of saving propellant for the terminal stage, or even the necessity of accelerating up to a high velocity intercept. At allows missiles to be lighter and smaller. Depending on the price of the nuclear technology, a few Casaba-Howitzer missiles may be cheaper than multitudes of kinetic impactors.

  • Point defense

The usefulness of a nuclear shaped charge extends further than just being a warhead. As calculated in the Effectiveness section of this post, the particle cones spread quickly, but remain effective at short ranges. 

In a defensive role, a Casaba Howitzer will have to be lightweight and efficient in its use of fissile material. This is because it must be deployed in numbers comparable to the incoming projectiles. Optimizing for efficiency has the consequence of producing a wide cone.

This cone can be used to sweep away missiles in the terminal phase. Close enough, it will outright vaporize kinetics. Further away, it can still damage sensors and shatter propellant tanks through impulse shock. The large angle of the cone is advantageous, as it would reduce prevision requirements against jinking missiles, and might even catch several missiles at once. 

Other advantages of using Casaba Howitzers as a point defense is that it can easily be aimed, does not consume power and has infinite firing rate. If you detect missiles coming in, dump your entire payload of defensive drones and have them point at targets. Once they come within range, all can detonate simultaneously. 

This might actually be the preferred tactic, to prevent previous nuclear detonations from interfering with the detonation of subsequent charges. This is a concern if the Casaba Howitzers use fusion fuels that are sensitive to external sources of neutron radiation. 

Example defensive Casaba Howitzer:
100kg, 10kt yield
85% efficiency: 35.56TJ beam
Beam velocity 1000km/s 
Beam angle: 10 degrees
Effective range (penetrates 5mm of aluminium): 16km 

This warhead can destroy anything within a 6.15km2 circle up to 16km away. It reaches targets in less than 16 milliseconds, and unlike a pin-point laser, it affects the entire surface of the target at once. 

  • Booster

The awesome power of a nuclear shaped charge does not have to be used directly to damage targets. It can be used in innovative ways.

Instead of being used to generate high velocity particles in a narrow cone, a Casaba Howitzer can be used as a nuclear version of modern HEAT rounds. A metal cone is partially melted and squeezed into a jet by the detonation of a nuclear shaped charge. The only requirement is that the energy deposited into the metal lining is not sufficient to vaporize it. It would achieve higher velocities by allowing it to vaporize, but would expand into uselessness, as any gas would. 

A 'direct' nuclear EFP, where the thermonuclear explosive is in contact with the metal cone to exploit the Munroe effect, produces a projectile with a velocity calculated by the Gurney equation:

  • V = ( (2*Y*E)^0.5 )*(((1+2*M)^3 + 1)/(6*(1+M))^-0.5) 
V is the velocity achieved, in m/s.
Y is the yield of the nuclear device, in joules.
E is the efficiency is the device, about 0.85 for nuclear shaped charges.
M is a ratio: explosive mass divided by projectile mass.

In this case, explosive mass is the mass of the beryllium that absorbs the nuclear device's X-rays and converts them into thermal energy, thereby becoming a working fluid.

1kt NEFP
Y is 0.1kt or 4.2TJ
E is 0.85
M is 0.2, as recommended by the original Orion study
V = 3705km/s
  
  • Particle beam weapon

The ionized particles produced by a Casaba Howitzer can be used to feed a particle accelerator. Unlike a traditional accelerator, its main role is not to accelerate particles closer to the speed of light, but to use magnetic lens to focus the ions into a tightly collimated beam. At the muzzle, the ions are neutralized to reduce bloom using a co-axial electron beam. 

The greatest point of concern is pushing the particles into the accelerator without reducing their velocity. A magnetic 'funnel', much like that of a mass spectrometer, can perform this role. 

The second point of concern is preventing the particles from damaging the particle accelerator. This can be remedied by building the accelerator as a series of widely spaced loops of wire acting as electromagnets. The particle beam is focused in stages, narrowing after each loop. 

The optimal Casaba Howitzer configuration for this weapon is a fusion device that is built to maximize particle velocity. 10000km/s (3% of the speed of light) may be achieved. This is much slower than an electromagnetically-accelerated particle beam weapon, but it has the advantage of requiring little to no external power (the electromagnets can be fed by the heat they receive from the nuclear detonation), massing much less than a regular particle accelerator and able to extend the range of small nuclear pulse weapons to useful distances (in the thousands of kilometers).   

From The Nuclear Spear: Casaba Howitzer (working notes) by Matter Beam (2016)

DIRECTED THERMONUCLEAR EXPLOSIVES

Another device being investigated by both SDI architects and weapon designers is "a kind of nuclear shotgun with little pellets" named Prometheus. According to a Congressional report that was otherwise quite pessimistic about SDI, Prometheus "may have nearer-term applications for picking out warheads from decoys" (in the midcourse phase of ballistic-missile flight) than the Neutral Particle Beam (NPB), a leading contender for that role. Encouraged by experiments already conducted, SDI officials in 1987 ordered an acceleration of the Prometheus project for "concept verification," using funds from that year's $500 million supplemental SDI request.

One research engineer familiar with the project described the device as operating much like a rifle, using a polystyrene-filled barrel to help couple a plate to the "gunpowder-like" blast of a directed nuclear charge. After the impulse from the explosion generates an intense shock wave, the plate "fractionates" into millions of tiny particles. Of course, these would vaporize if in direct contact with the bomb, but as configured, the pellets have reportedly achieved speeds of 100 kilometers per second without vaporization.

Thermonuclear shaped charges, one of the better understood third-generation concepts§, have much in common with conventional shaped-charge explosives already used extensively in military and commercial applications. Both conventional and thermonuclear shaped charges tailor an explosive burn-wave using a detonation front that releases energy along a prescribed path. Both can produce jets of molten metal having velocities greatly in excess of the detonation velocity.*

For thermonuclear fuels such as deuterium plus tritium, the burn-wave can be directed by placing hollow bubbles or inert solids in the path of the detonation front in order to alter its velocity. Of course, ignition of a thermonuclear burn in a warhead requires a fission trigger to achieve the necessary compression and temperature (about 100 million K), but even with such a (nondirected) trigger, the overall directivity of a thermonuclear shaped charge can still be significant.

Velocities achievable with thermonuclear shaped charges are impressive. Unlike molten jets produced by conventional shaped charges, which are limited to about 10 kilometers per second (about four times the velocities of the gases resulting from chemical explosions), thermonuclear shaped charges can in principle propel matter more than two orders of magnitude faster. Since fusion temperatures reach 100 million K, the detonation front of a thermonuclear explosive travels at speeds in excess of 1,000 kilometers per second. Using a convergent conical thermonuclear bum-wave with a suitable liner, one could theoretically create a jet traveling at 10,000 kilometers per second, or 3 percent of the speed of light.

Up to 5 percent of the energy of a small nuclear device reportedly can be converted into kinetic energy of a plate, presumably by employing some combination of explosive wave-shaping and "gun-barrel" design, and produce velocities of 100 kilometers per second and beam angles of 10-3 radians*. (The Chamita test of 17 August 1985, reportedly accelerated a 1-kilogram tungsten/molybdenum plate to 70 kilometers per second. ) If one chooses to power 10 beams by a single explosion, engaging targets at a range of 2,000 kilometers with a kill energy of 40 kilojoules per pellet (one pellet per square meter), then such a device would require an 8-kiloton explosive and could tolerate random accelerations in the target, such as a maneuvering RV or satellite, of up to 0.5 g (5 m/s2).

The initial plate for each beam in this Casaba-like device would weigh only 32 kilograms but would have to fractionate into tiny particles to be an effective weapon—4 million evenly spaced pellets to produce one per square meter at 2,000 kilometers range. If such pellets could be created uniformly, which is highly questionable, then, at a velocity of 100 kilometers per second, they would each weigh 8 milligrams, carry 40 kilojoules of energy (the amount of energy in 10 grams of high explosive), and travel 2,000 kilometers in 20 seconds. Such hypervelocity fragments could easily punch through and vaporize a thin metal plate and could cause structural damage in large soft targets such as satellites and space-based sensors, but they would have little probability of striking a smaller RV, or even disabling it if a collision did occur.§

10-kiloton ASAT
Nuclear yield10 kilotons
Number of beams10
Mass per plate32 kg
Mechanism50 kilojoules per pellet impact kill
Assumptions4 × 106 particles per beam
uniformly spaced 1 per m2
at 2,000 kilometers
Range2,000 kilometers

‡ SPARTA, Inc., Workshop on Interactive Discrimination, 1986, unclassified. The velocity of 100 kilometers per second falls between the goal of 50 kilometers per second in the 1960s, only a fraction of which was achieved, and the 1,000 kilometers per second velocities possible with the plasma howitzer concept. The latter allegedly operates at 10 percent efficiency up to about 1 megaton, although with only about 10-2 radian beam directivity. Speeds of 1,000 kilometers per second are inevitably accompanied by ionization, and because charged particles curve in the earth's magnetic field, they would not be useful for long-range applications. Velocities up to 200 kilometers per second, however, are believed possible without vaporization.

§ See, for example, the detailed analysis of nuclear shaped-charges by R. Schall, "Detonation Physics," in P. Caldirola and H. Knoepfel, eds., Physics of High Energy Density, (New York: Academic Press, 1971), pp.230-244.

* Friedwardt Winterberg, The Physical Principles of Thermonuclear Explosive Devices, (New York: Fusion Energy Foundation, 1981), p.117. Conventional shaped charges have been applied to demolition, antisubmarine weapons, and advanced ordnance antitank munitions—all being further developed at Livermore—as well as for igniting the fission triggers in thermonuclear warheads. Cf. Energy & Technology Review, Lawrence Livermore National Lab, (June-July 1986), pp.I4-15.

† Devices based on this principle were pursued in the 1960s. Project Orion examined their potential for space propulsion. Casaba and "nuclear howitzer" were names for weapon applications.

‡ The detonation front shock-wave velocity is (32 kT/3M)½, where M is the average mass per ion of the thermonuclear fuel. Suitable geometries can propel matter at many times the detonation front velocity. Using cone geometry, the jet speed is v/sinθ, where v is the detonation-front velocity and θ is the cone's half-angle. A practical minimum for θ has reportedly been found to be θ ≈ 0.1. See Winterberg, Thermonuclear Physics, p.41,122

* SPARTA Workshop, 1986. This scaling presumably holds up to about 50 kilotons but, due to blackbody x-ray emission, decreases to about 1 percent for larger yields.

† Robert S. Norris, Thomas B. Cochran, and William M. Arkin, "Known U.S. Nuclear Tests July 1945 to 31 December 1987," Nuclear Weapons Databook Working Paper NWD 86-2, Natural Resources Defense Council, September 1988.

‡ The energy fluence per beam, E in J/m2, is approximately ηY/(NbR2θ2), where η is the fraction of overall yield transferred to the pellets, Y is the bomb yield (1 kiloton is equivalent to 4.2 × 1012 joules), Nb is the number of individual beams being driven by one bomb, R is the distance to the target, and θ is the individual full-beam divergence angle. A maneuvering target could accelerate out of the path of the beam if amR/vf2 > θ, where am is the magnitude of the target's average acceleration, vf is the particle velocity, and τ = R/vf is the particle fly-out time. (For comparison, the average acceleration of ICBMs is about 40 m/s2.) To deliver this energy requires a total mass per beam of Mb = 2E(Rθ)2/vf2.

§ For instance, even if an RV were coated with aluminum, a more volatile material than might be expected, the resulting vapor blow-off would only push a 350-kilogram RV off course by about 15 meters in 20 minutes of flight (about five times the amount if there were no ablation), thus failing to degrade significantly the ≈150 meter accuracy of a modern ICBM. Of course, if the collision caused the RV to tumble upon re-entry, the results would be less predictable

From The Effects of Nuclear Test-ban Regimes on Third-generation-weapon Innovation by Dan L. Fenstermacher. Science & Global Security 1990, Volume 1, pp. 187-223

There are a few more crumbs of information in the report Fourth Generation Nuclear Weapons: Military effectiveness and collateral effects. They note that harnessing the x-rays from a nuclear blast is not only good for making deadly jets of atomic fire, but can also be used to pump x-ray lasers and energize EMP weapons. Not to mention accelerating projectiles to very high velocities by means of x-ray ablation, or by means of neutrons from the nuclear explosion (see report for cites on this).

So the report points out that the x-rays and neutrons can be used to drive or self-forge several projectiles or fragments (a "nuclear gun" or "nuclear grenade"). X-rays and neutrons can also be used to heat a working fluid and form hot jets (the above-described "nuclear shaped charge").

(It might be worth while to review the difference between a shaped charge and a self-forging projectile, they are similar enough to be confused together, but are quite different in end result.)

Thirdly, the forwards and backwards flux of x-rays and neutrons from a single nuclear device can be used to drive a multi-warhead weapon, e.g., a single weapon that fires a self-forging penetrator followed a few microseconds later by a jet of hot plasma. Talk about a one-two punch! The penetrator cracks the armor, allowing the hot jet to enter the target's interior and vaporize the soft chewy center.

The report also estimates, that for the use in military conflicts on the surface of the Earth, these weapons will probably be powered by nuclear devices in the 1 to 100 tons of TNT range (subkiloton range). Whether this will also hold true in the space environment is a question above my pay grade.

Freeman's analysis of nuclear explosions in a vacuum, resulting in a series of three short papers titled Free Expansion of a Gas, was central to the feasibility of Orion. It was also central to the feasibility of directed-energy nuclear weapons, and led directly from Orion to a project code-named "Casaba-Howitzer," described as "a one-shot version of Orion, like Orion except without any ship." Casaba-Howitzer, conceived by Moe Scharff while still at Livermore, would be resurrected many years later as the basis for the "Star Wars" space-weapons program, known as the Strategic Defense Initiative or SDI. "Whereas Orion directed a dense plasma at relatively low velocity at a wide angle, this was to direct a lower-density plasma at a higher velocity and a narrower angle," Scharff explains. "Orion was a space vehicle. Casaba-Howitzer could be consid­ered space weaponry. It could even have been things carried aboard an Orion, for example, if Orion was a battleship."

Casaba-Howitzer's descendants remain under active investigation and Scharff is unable to give any further details beyond the origins of the name. "They had been naming things after melons and the good ones were gone already. They were on a melon kick that year. The one con­nection was seeds—many of those melons have seeds, like the particles we were projecting." Casaba-Howitzer was derived directly from Orion, and later versions of Orion drew heavily on Casaba-Howitzer's experi­mental and theoretical results. Funding for Casaba-Howitzer kept the Orion team going after funding for Orion dwindled out. But there was a costly side to the bargain—a shroud of secrecy that has lingered long after any plans for battleship Orion were shelved. Conversely, if we ever decide to build something like Orion, it will be the continued work on directed-energy weapons—and how to protect surfaces against them— that will allow us to pick up where Project Orion left off.

Anything in the near vicinity of a nuclear explosion gets vaporized into a plasma—a cloud of material so hot that its atoms are stripped of their electrons—that cools as it expands. It was a simple mathematical problem to draw some conclusions relating the shape and density of the initial object that gets vaporized to the shape and density of the result­ing cloud of gas. "The model should be simple enough so that the hydro-dynamical equations can be integrated exactly," Freeman explained. "A real cloud of gas will not have precisely the density-distribution ot the model, but still one may expect the behavior of a real cloud to be quali­tatively similar to that of the model." Freeman set up the equations and the numbers were run on General Atomic's IBM 650 card-programmed calculator, one of the workhorse machines that had handled many of the early bomb and blast-wave calculations at Los Alamos and had not yet been superseded by the IBM 704 that General Atomic acquired in 1959.

According to Freeman's model, something originally in the shape of a cigar expands into the shape of a pancake, and something originally in the shape of a pancake expands into the shape of a cigar. This was "very directly relevant to the expansion of a bomb," he explains. "If you have something that starts in the form of a pancake and you heat it up to a very high temperature it will expand more sideways along the axis, and less at the edges. The pressure gradient is highest along the axis, so then after a while, since the velocity is highest along the axis, it becomes cigar-shaped. So you get inversion, something that begins like a pan­cake becomes like a cigar, and something that begins as a cigar becomes a pancake, if you just let it expand freely. It goes roughly with the square root, if you start with a pancake where the ratio of the diameter to thick­ness is ten, then it will end up as a cigar where the ratio of the length to the diameter is square root of ten, roughly speaking. That would be quite helpful, of course, if you had a real Orion, to start out with a pan­cake and it will produce then a jet that is collimated within 20 degrees or so quite nicely. The fact that it's so easy to make an asymmetrical explosion may still be classified, for all I know."

The right pancake in the right place can focus a significant fraction of the bomb's output into a narrow jet of kinetic energy, directed construc­tively at the pusher plate of a nearby spaceship—or destructively at something else. The thinner the pancake, the narrower the jet. In the early days of Orion, with a huge pusher plate as the target, the propellant was assumed to be a thick slab of something light and cheap like polyeth­ylene; later versions of Orion, with smaller pusher plates, required a thin­ner slab of higher-density material, such as tungsten, to focus the bomb's energy into a narrower cone. Exactly how narrow remains a secret, though a look at the later configurations of Orion permits a guess. This is one of the reasons that detailed design information about Orion, such as the exact standoff distance between the pulse unit and the pusher plate, remains classified, even after forty years have passed.

As the jet of propellant is targeted more narrowly in space, its impact against the pusher plate is spread out more widely in time. The result is more effective horsepower and a softer ride. "In the end we did come up with some designs that were very tight in their angular distribution of momentum," says Bud Pyatt, without mentioning specific numbers, but revealing that "you had to have it pointing at the center of the pusher plate, it couldn't even be five degrees off without stressing the shock absorber too much."

Project Orion: The True Story of the Atomic Spaceship by George Dyson

A propellant plate in the form of a pancake expands into a plume shaped like a cigar. And the reverse is true: a propellant plate in the form of a cigar/cylinder would expand into a plume shaped like a pancake. Specifically:

(Dplume / Lplume) = 1 / sqrt(Dplate / Lplate)

where:

  • Dplume = plume diameter (perpendicular to direction of travel)
  • Lplume = plume length (in direction of travel)
  • Dplate = plate diameter (perpendicular to direction of travel)
  • Lplate = plate length (in direction of travel)

So if the plate had a diameter of 4 and a length of 1 (diameter to length ratio of 4/1 or 4), the plume would have a diameter to length ratio of 1/2, or a diameter of 1 and a length of 2. Equation is from Nuclear and Plasma Space Propulsion by M. Ragheb.

A Little Humor

From the bizarre experimental weapons files:

Assume for a moment that those nuclear bullets — the ones using a barely subcritical mass of some volatile element like californium, say, squished into the critical geometry for a fission explosion by the squish of impact — actually work.

Then add the encasement, beryllium channel-filler, and tungsten plate needed to focus the explosion into a searing beam of front-focused tungsten plasma.

Gentlesophs, I give you Project Casaba-Derringer.

by Alistair Young (2015)

Boom Table

The Boom Table has been moved here.

Laser Cannon

There is a great summary of the various issues of directed-energy weapons. Luke Campbell has an in depth analysis of laser weapons for science fiction on his website, don't miss the on-line calculator for laser weapon pulse parameters. Eric Rozier has another on-line calculator for laser weapons. Rick Robinson's analysis Space Warfare V: Laser Weapons is also quite good. You also might want to look over this 1979 NASA report on using nuclear reactions to directly power a laser beam. (Thanks to Andrew for suggesting this link.)

Before we get to all the boring equations, lets have some juicy details. Say that the habitat module of your combat starship gets penetrated by an enemy laser beam. What happens? Luke Campbell and Anthony Jackson have the straight dope:

That depends on the parameters of the beam.

A single pulse with a total energy of 100 MJ would have the effect of the detonation of 25 kg of TNT. Everyone in the compartment who is not shredded by the shrapnel will have their lungs pulverized by the blast.

That same 100 MJ delivered as 1,000,000 pulses of 100 J each could very well drill a hole. The crew see a dazzling flash and flying sparks. Some may be blinded by the beam-flash. Anyone in the path of the beam has a hole through them (and the shock from the drilling of that personal hole could scatter the rest of them around the crew compartment). Everyone else would still be alive and would now be worrying about patching the hole.

Although it occurs to me that the jet of supersonic plasma escaping from the hole being drilled could have the combined effect of a blowtorch and grenade on anyone standing too close to the point of incidence, even if they are not directly in the beam. The effect would probably be similar to the arc flash you can get in high power, high voltage electrical systems, where jets of superheated plasma can cause severe burns from contact with the plasma, blast damage from the shock waves, blindness from the intense light produced, and flash burns from the radiated heat.

A continuous beam could have enough scattered and radiant heat to cause flash burns to those near the point of incidence, along with blinding those who are looking at the point of incidence when the beam burns through. If it burns a wide hole, people die quickly when the compartment explosively decompresses, throwing everyone into deep space. If it burns a narrow hole, the survivors who can see can just slap a patch over the hole to prevent the escape of their air.

Luke Campbell

Luke Campbell said: "Although it occurs to me that the jet of supersonic plasma escaping from the hole being drilled could have the combined effect of a blowtorch and grenade..."

Well, it really depends on what you're standing next to, and on how wide the beam is. The energy release at any point along the beam path will be equal to the energy required to drill through the object (so you'll get pulses of heat from each object hit), and it won't really be explosive. Flash burns is the most likely consequence.

Flash burns start at about 5 J/cm2 on exposed skin, and can go above 100 J/cm2 with reasonable protection. At a range of 1 meter, that requires an energy release of 0.63MJ, and once the beam is substantially inside the object, most of the flash will be deposited on the rest of the inside of the object, so it's really only object shells we need to worry about.

If the beam has an area of 50 square centimeters ( AV:T scale) to emit a total of 630 kJ it must be emitting 12.6 kJ/cm2. About the same amount is probably consumed drilling through the object. 1mm of steel requires about 6 kJ/cm2, so anything with a casing of at least 2mm steel, or anything comparable, will cause flash burns within 1 meter.

This is not particularly terrifying, unless of course the beam drills through something like a high pressure steam line, at which point it's suddenly very exciting, though not because of the laser per se.

Anthony Jackson

Anthony Jackson said: "so you'll get pulses of heat from each object hit, and it won't really be explosive"

My thought was that the shocks could coalesce. All shocks are supersonic to the material they have not gone through, and subsonic to the material they have traveled through. As a consequence, a second shock will catch up to a previous shock until they merge into a single, stronger shock. If the beam is pulsed at a high rate (say, a MHz or so) a good number of the individual blasts could coalesce within a short distance to create a more potent blast that might cause significant problems.

The physics of shocks is tricky, and for spherically expanding shocks you get into issues of rarefaction and backflow, which should limit the number of shocks that can coalesce. While I have a highly recommended text on shock physics, I've not had the time to look through it yet, so I don't have a good idea yet on the limits and possibilities of this mechanism.

There's also the issue that iron heated to 10,000 K, for example, will expand in volume about 150,000 times from its solid phase. So burning a 10 cm wide hole through a 1 cm steel bulkhead would produce a cloud of iron vapor with a volume of about a cubic meter if the final temperature was 10,000 K (note that if the iron was converted to a singly ionized plasma, the temperature would be ten times that much, and you would get ten times the volume). Getting caught in that incandescent cloud simply cannot be healthy.

There's also the ozone and nitrogen oxides and reactive chemicals produced as a consequence of incomplete combustion, which will not be healthy to breathe, but I expect that would be secondary.

Luke Campbell

Luke Campbell said: "My thought was that the shocks could coalesce."

They could if the drilling speed is supersonic. Usually it won't be.

Anthony Jackson

Equations

Now for the dull equations.

"Laser" is an acronym for light amplification by stimulated emission of radiation. A laser beam can cut through steel while a flashlight cannot due to the fact that laser light is coherent. This means all the photons in the beam are "in step" with each other. By analogy, a unit of army troops marching in step can inadvertently cause a bridge to collapse, while the same number of people using the bridge in a random fashion have no effect. Laser light at amazingly low energies can still cause permanent blindness by destroying the retina.

Maximum range will be a few hundred thousand kilometers, otherwise almost every shot will miss due to light-speed lag. You can find more details about light-speed lag here.

Laser beams are not subject to the inverse-square law, but they are subject to diffraction. The radius of the beam will spread as the distance from the laser cannon increases.

RT = 0.61 * D * L / RL

where:

  • RT = beam radius at target (m)
  • D = distance from laser emitter to target (m)
  • L = wavelength of laser beam (m, see table below)
  • RL = radius of laser lens or reflector (m)
BandWavelength (m)
Far Infrared1e-3 to 5e-5 m (1,000,000 to 50,000 nanometers)
Mid Infrared5e-5 to 2.5e-6 m (50,000 to 2,500 nanometers)
Near Infrared2.5e-6 to 7.5e-7 m (2,500 to 750 nanometers)
Red7.5e-7 to 6.2e-7 m (750 to 620 nanometers)
Orange6.2e-7 to 5.9e-7 m (620 to 590 nanometers)
Yellow5.9e-7 to 5.7e-7 m (590 to 570 nanometers)
Green5.7e-7 to 4.95e-7 m (570 to 495 nanometers)
Blue4.95e-7 to 4.5e-7 m (495 to 450 nanometers)
Indigo4.5e-7 to 4.2e-7 m (450 to 420 nanometers)
Violet4.2e-7 to 3.8e-7 m (420 to 380 nanometers)
Ultraviolet A4e-7 to 3.15e-7 m (400 to 315 nanometers)
Ultraviolet B3.15e-7 to 2.8e-7 m (315 to 280 nanometers)
Start of
Vacuum Frequencies
2.e-7 m (200 nanometers)
Ultraviolet C2.8e-7 to 1e-7 m (280 to 100 nanometers)
Extreme Ultraviolet1e-7 to 1e-8 m (100 to 10 nanometers)
Start of
Ionizing Radiation
1e-8 m (10 nanometers)
Soft X-Ray1e-8 to 2e-10 m (10 to 2e-1 nanometers)
Hard X-Ray2e-10 to 2e-11 m (2e-1 to 2e-2 nanometers)
Gamma-Ray2e-11 to 1e-13 m (2e-2 to 1e-4 nanometer)
Cosmic-Ray1e-13 to 1e-17 m (1e-4 to 1e-8 nanometers)

Note that wavelengths shorter than 200 nanometers are absorbed by Terra's atmosphere (so they are sometimes called "Vacuum frequencies") and anything shorter than 10 nanometers is considered "ionizing radiation" (i.e., what the an average person on the street calls "atomic radiation"). Vacuum frequencies will be worthless for a laser in orbit attempting to shoot at ground targets protected by the atmosphere.

Sometimes wavelengths are expressed in Ångström units, 1.0 Ångström = 0.1 nanometer.

More to the point is the intensity of the beam at the target. First we calculate the beam divergence angle θ

θ = 1.22 L/RL

where:

  • θ = beam divergence angle (radians)
  • L = wavelength of laser beam (m, see table above)
  • RL = radius of laser lens or reflector (m)

Note that this is the theoretical minimum size of the divergence angle, it will be larger with inferior lasers.

Next we decide upon the beam power BP, then calculate the beam intensity at the target (the beam "brightness"):

BPT = BP/(π * (D * tan(θ/2))2)

where:

  • BPT = Beam intensity at target (megawatts per square meter)
  • BP = Beam Power at laser aperture (megawatts)
  • D = range to target (meters)
  • θ = Theta = Beam divergence angle (radians or degrees depending on your Tan() function)
  • π = Pi = 3.14159...

There are a few notes on laser firing rates and power requirements here.

In the US military, the minimum threshold for a tactical weapons-grade laser is 100 kilowatts.

In the US military, the minimum threshold for a strategic weapons-grade laser is 1 megawatt.

When figuring the tangent, remember that θ from the beam divergence angle equation is in radians, not degrees (Divide radians by 0.0174532925 to get degrees).

What this means is if you are calculating the Beam Intensity equation with a pocket calculator or the Windows calculator program, the calculator is generally set to degrees and it expects you to punch in the angle in degrees before you hit the TAN key. If you punch in the angle in radians you will get the wrong answer.

If instead you are calculating the Beam Intensity equation with a computer spreadsheet or with a computer program you are writing from scratch, the TAN() function wants the input angle to be in radians.

For comparison purposes, the average beam intensity of sunlight on your skin is about 0.0014 MW/m2.

Please note that the amount of beam power deposited on the target is still BP, the intensity just measures how tightly it is focused. It's like using sunlight through a magnifying glass to burn a hole in a piece of paper (or to incinerate ants if you were one of those evil children). The amount of beam power hitting the paper does not change, it is always BP. But if the magnifying glass is so close that the spot size is large, the paper will just get warm. If you move the glass so the spot focuses down to a tiny dot, the intensity increases and the paper spot starts to burn.

Also note that a laser cannon might have lens/mirror which is larger than strictly required for the desired spot size, due to the fact that otherwise the mirror would melt. The larger the mirror, the more surface area to dilute the beam across, and the less the thermal stress on the mirror.

Example

The good ship Collateral Damage becomes aware of an incoming hostile missile. Collateral Damage has a laser cannon with a ten meter radius mirror operating on a mid-infrared wavelength of 2700 nanometers (0.0000027 meters). The divergence angle is (1.22 * 0.0000027) / 10 = 0.00000033 radians or 0.000019 degrees.

The laser cannon has an aperture power of 20 megawatts, and the missile is at a range of four megameters (4,000,000 meters). The beam brightness at the missile is 20 / (π * (4,000,000 * tan(0.000019/2))2) = 15 MW/m2 or 1.5 kW/cm2.

If the missile has a "hardness" of 10 kilojoules/cm2, the laser will have to dwell on the same spot on the missile for 10/1.5 = 6.6 seconds in order to kill it.

Figured another way, at four megameters the laser will have a spot size of 0.66 meters in radius, which has an area of 1.36 square meters. The missile's skin has a hardness of 10 kilojoules/cm2 so 13,600 kilojoules will be required to burn a hole of 0.66 meters radius. 20 megawatts for 6.9 seconds is 13,600 kilojoules. 6.9 seconds is close enough for government work to 6.6 seconds.

Eric Henry has a spreadsheet that does most of this calculation for you here.

In the game Attack Vector: Tactical, the smallest laser lens is three meters in diameter, the frequency of various models of cannon is from 0.0000024 meters (2400 nanometer) to 0.0000002 meters (200 nanometer) and the efficiency varies from 20% down to 1.5%.

Example

Say you have an ultraviolet (20 nanometer) laser cannon with a 3.2 meter lens. Your hapless target spacecraft is at a range of 12,900 kilometers (12,900,000 meters). The Beam Radius equation says that the beam radius at the target will be about 4 centimeters (0.04 meters), so the beam will be irradiating about 50 cm2 of the target's skin (area of circle with radius of 4 centimeters). If the hapless target spacecraft had a hull of steel armor, the armor has a heat of vaporization of about 60 kiloJoules/cm3. Say the armor is 12.5 cm thick. So for the laser cannon to punch a hole in the armor it will have to remove about 625 cm3 of steel (volume of cylinder with radius of 4 cm and height of 12.5 cm). 625 * 60 = 37,500 kiloJoules. If the laser pulse is one second, this means the beam requires a power level of 37,500 watts or 38 megawatts at the target.

In practice, a series of small pulses might be more efficient, causing a shattering effect and driving chips of armor out of the hole, which of course requires less energy than actually vaporizing the armor.

SECTION 7: LASER WEAPONS

 Laser weapons are what the general public thinks of when asked to describe space weapons.  They have a number of advantages in the space environment, but also suffer from significant drawbacks.

Lasers are obviously made of light, and this drives their performance.  First off, they obviously propagate at the speed of light.  This makes them almost impossible to dodge at PMF ranges, which will be discussed below.  At the same time, the fact that they are composed of light causes laser to fall off with range, as opposed to kinetics, which do not.  Lasers have a minimum size they can focus their beam spot to that is limited by diffraction.  This is at the point where the beam is focused.  It is described by the equation

where BD is beam diameter (m), R is range (m), L is wavelength (m), and D is mirror diameter (m).  As can be seen, spot size is proportional to range and wavelength, and inversely proportional to the mirror diameter.  Please note that this equation is the minimum possible spot size.  The actual spot size is also affected by the focus of the laser, which may not be set at the correct distance, and the various real-world factors discussed below.

Some people have claimed that lasers do not follow the inverse square law, and, strictly speaking, this is true.  A fixed-focus laser will not follow the inverse square law.  The beam will initially be diffuse at the emitter, then narrow to the minimum spot size at the focal point, then widen again as it passes that point.  A side profile of the beam will look something like an hourglass.  However, a variable-focus laser will effectively obey the inverse-square law.  From the equation above, note that beam diameter scales linearly with R, range to target. Area will be proportional to the square of diameter. Intensity or flux will be inversely proportional to area, which is proportional to the square of distance.

All of the above analysis assumes an ideal, diffraction-limited beam.  This is obviously not the case in reality, and a more realistic equation is

where BD, R, L, and D mean the same as in the equation above, Q is beam quality, a dimensionless measurement of the actual beam diameter to the theoretical beam diameter, and J is jitter in radians.  Typical values for Q for modern lasers are generally less than 3, and likely below 1.5.  Numbers for jitter, which is caused by vibration of the platform, are harder to come by, but on the Airborne Laser Laboratory in the 1980’s, jitter numbers of around 25 microradians (25×10-6radians) were achieved.  It is not unreasonable to assume that a two order of magnitude reduction in this number could be achieved between technological development and the fact that the ALL was mounted on an aircraft in the atmosphere, and it is entirely possible that significant farther reductions are possible.  As a word of caution, this equation only holds true for Continuous-wave (CW) lasers, and the author is unsure of the impact of jitter on pulsed lasers.  It is possible that some or all of the jitter will instead become pointing error for a pulsed laser, significantly increasing the efficiency of such vis à vis CW lasers.

The potential issues caused by vibration are such that it is likely that a laserstar’s designer will pay as much attention to them as the designer of a ballistic missile submarine does to noise.  Current texts on space optical communications systems (Deep Space Optical Communications, JPL) indicate that there is potential for sub-microradian pointing accuracy and active jitter control.  Passive vibration damping can remove the low-frequency components of the vibration, and severely attenuate the high-frequency ones.  However, these are for low-powered communication laser systems with small optics operating in the relatively benign environment of a satellite, not large mirrors and high-power laser systems on a thrusting spacecraft with active cooling systems.  The exact impact of these factors is unknown at the present.

One interesting fact contained in the same book was that modern optical communication jitter compensation relies at least partly on a low-power laser from the target providing a reference for the optics.  This means that an attempt to blind an opponent could actually tend to make their lasers more accurate, although there were no details about the specific requirements of the process.

Beam quality and jitter are helpful to the hard SF author, as they allow him to reduce the potency of lasers.  Jitter is particularly helpful for this purpose, as it affects larger mirrors proportionately more than small ones.  The minimum spot size for a jitter-limited mirror will be BD = 2JR, and the spot size according to the above equation approaches that value as the mirror grows.  In a setting with high jitter, the limit on mirror size might be the diminishing returns of the effect on spot size instead of manufacturing processes or cost.

Space is an environment that is uniquely suited for laser weapons.  The lower the wavelength of the beam, the smaller the spot size of the laser is.  However, it is generally more difficult to generate lower wavelengths, and wavelengths shorter than the visible are strongly absorbed by the atmosphere.  This is obviously not a problem in space, unless the laser must also be capable of planetary bombardment.  Very low wavelengths, in the X-ray region and below, begin to suffer unusual interactions with matter, which prevents the use of optical mirrors.  Grazing mirrors and diffraction gratings must be used instead, which significantly complicates the optical train.  

All of the above looks only at the beam as it emerges from the mirror, and ignores what occurs inside the laser system.  While the only critical facts are output power, input power, wavelength, mass, and efficiency, this area deserves a closer look.  First, the laser beam has to be generated.  The methods available can be divided for our purposes into chemical and solid-state.  Both of these are used here more loosely then is strictly accurate under the technical definition.  Chemical lasers are any lasers that require an expendable fuel for the lasing mechanism.  Almost all modern military lasers fall into this category.  Solid-state lasers use electricity to generate the laser beam.  They have significant logistical advantages over chemical lasers, but are more difficult to build.  Chemical lasers have the advantage of not requiring large power sources, and the fact that the dumping of the reaction products provides built-in heat rejection.  A solid-state laser will have to either store or radiate heat, both from the reactor and the laser mechanism itself.  The power that must be dealt with in the form of heat will almost certainly be several times greater than the power released in the beam.

  Once the beam is generated, it must then be formed and routed to the mirror.  The difficulty of this will depend on how deep in the vessel the laser generator is located.  There is no reason that it could not be mounted deep in the vessel, far away from possible damage.  This will likely require extra mirrors, and raises the possibility of damage to the optics train from shock.  If the laser system is modular, it is obviously necessary to mount the generator close to the hull.  The ultimate extension of this principle is to have exactly one laser generator mounted in the core of the ship, and direct the beam out among the various mirrors on the ship.

Mirrors can obviously be divided into two types, either turreted or fixed.  Turreted mirrors obviously benefit from a much wider field of fire then provided by fixed mirrors. A fixed mirror will still be capable of limited steering due to adaptive optics and the necessity of fine pointing capabilities, but probably no more than a few degrees.  It benefits from a significantly less complex beam path, and is likely to be capable of more accurate pointing.  Thus, fixed mirrors are likely to be used for primary weapons while turreted ones are used for kinetic defense.

At this point, we’ve followed the beam to the target.  Once it gets there, it still has to disable the target.  There are multiple methods by which this could occur.  First, laser damage mechanisms depend on the type of laser, continuous-wave or pulsed.  CW lasers operate at a constant power for long periods, and do damage by heating the target until the surface vaporizes.  Pulsed lasers fire a string of very high powered pulses at the target.  The damage they deal is compounded by mechanical effects from the flash-vaporization of the target material.  Pulsed lasers are generally more energy-efficient for a given level of damage, and would be expected to be used unless technical constraints made them impractical.

In either case, it can begin to do damage far before it can begin to burn through armor.  Delicate systems on the outside of the ship, most notably sensors and thrusters, are vulnerable to far lower levels of laser radiation.  This can be dealt with by proper design and networking.  Armoring schemes will involve a heavy faceplate in the front, and much lighter protection on the sides.  The faceplate systems will be built with the assumption that they will be damaged or destroyed during the course of the battle, and operations planned around that fact.

The other object that will dominate the faceplate of a laserstar is the main laser itself.  This has multiple consequences.  First, the laser in question is vulnerable to being shot at itself.  It has been proposed to use some form of shutter to prevent this from happening, but that raises the issue of time to open and close the shutters, during which the laser is vulnerable and unable to fire.  Some commentators have proposed that battles between laserstars will turn into eyeball-frying contests where the first person to burn out the other’s mirror wins.  The logic runs that the engagement will begin at ranges where the spot size is similar to the mirror size.  When one laser hits another, the target laser’s mirror will focus the beam into the rest of the optics train, destroying it.  Furthermore, the laser is also the ship’s best sensor system.  In between shots, it can also function as a telescope with resolution comparable to spot size provided that the observation is made in a similar wavelength to that which the laser operates at.  It has been claimed that this capability will give the laser the ability to target specific points on a target, particularly the mirrors of the opposing ship.

It has been suggested that turreted lasers would not need shutters, as they could be protected by simply turning them inward and armoring the back side.  This might be a viable suggestion for small turrets with nearly complete rotation arcs, but larger turrets would probably be significantly less massive with a shutter and limited arcs.  

A proposed alternative to conventional shutters is the use of some sort of electrically-activated material which changes from opaque to transparent when a current is applied.  This concept shows promise in reducing shutter time during a laser duel, as the lag time will be negligible, possibly low enough to synchronize with a pulsed laser.  A serious potential problem with this approach is that the material will not be transparent enough to be capable of safely having the laser fired through it.  Another problem is that the material will be itself damaged by kinetic impacts, hindering transmission of the laser and leading to more damage to the material when the laser is fired.  This fact necessitates the use of an additional moving shutter to defend against kinetics.

A very similar, and perhaps more effective, option for a shoot-through anti-laser lens would be a polarized covering.  All lasers are inherently polarized, and the chance of an enemy’s laser having the same polarization is miniscule.  This, however, suffers from the same problems as the previous solution.  The transparency requirements are very demanding, and the lens itself is vulnerable to damage from kinetics and from other lasers. Such lenses are only useful if eyeball-frying contests are the norm, which appears unlikely.

The problem with the eyeball-frying theory is pointing error.  The simple fact is that the laser will probably not be capable of being pointed with angular accuracy comparable to the divergence angle of the beam for any number of reasons.  There are also questions about the vulnerability of lasers to other lasers.  It is quite likely that small portions of the mirror will fail instead of the entire system, and these failures can be compensated for by the control system for the adaptive optics. This would be more likely to produce a slow degradation of the system then a quick kill.

One interesting suggestion from John Lumpkin’s Through Struggle, The Stars is counterbattery lasers.  These are specialized high-speed laser systems designed to shoot back at attacking lasers and disable their mirrors.  The author finds it unlikely that dedicated systems would be implemented for the role, but it is possible that shootback software would be added to the control systems for some or all of a vessel’s lasers.  The effectiveness of said solution depends heavily on the ability to very precisely point the lasers, and the effectiveness of a laser against a mirror.

Another possibility for the use of lasers is sensor blinding.  Even if the intensity is too low to do any permanent damage, the laser would be quite effective at jamming sensors operating in the same wavelength.  The problem with this tactic is twofold.  First, it requires the laser to be matched to the sensors.  This is unlikely to be the case for most vacuum-frequency lasers, as spotting will likely be done in visible or IR spectra.  Second, it requires the use of the laser, which might be more urgently required for other tasks.  The solution to both problems is quite simple.  Dedicated jamming lasers are quite feasible, given that the required mirror is small and the power levels needed are generally modest compared to those necessary to kill other vessels.

Defending against lasers is difficult.  One common suggestion is mirrored armor, but this has significant practical problems.  The first and simplest is that the mirroring only works against the first shot.  Any mirror is not a perfect reflector, and will absorb some of the incident energy.  At long ranges, this might be useful, but as the range closes, even that small amount of energy will be enough to melt the outer layer of the mirror, which in turn will destroy its reflective properties, leaving the vessel exposed to further shots.  A normal mirror might have a reflectivity as high as 99.9%.  A rough calculation suggests that for an aluminum outer skin, the beam intensity would have to be on the order of 40 MW/m2.  Any other materials would require significantly higher intensities.  This suggests that a pulsed beam would be more effective than a CW laser.  The above calculation was based entirely on blackbody radiation, and ignores any number of complications.  The author is unfamiliar with the response of reflective materials to laser radiation, but it does not seem outside the realm of possibility that the reflectivity could be significantly impaired by much lower rises in temperature.  It is also unlikely that 99.9% reflectivity could be maintained on an operational spacecraft.  This number reflects the maximum for conventional mirrors under laboratory conditions.  The outer hull of a warship is far from the lab, and is exposed to things like solar wind and micrometeorites, which would likely limit the practical reflectivity to 99% at most.  While it might be possible to put some form of protective covering on the armor and jettison it before battle, that limits the tactic to once per mission, and would require considerable effort to implement.

However, the author did run across an interesting suggestion for a type of mirrored armor in an Air Force research paper. (The Use of Liquid Film for Spacecraft Survivability to Laser Radiation, DTIC.)  To defend against IR lasers, a graphite sample was coated with a thin layer of tungsten carbide.  When the laser was fired at the sample, the tungsten carbide melted, but remained on the surface, allowing the carbon vapor to pass through it when the carbon was vaporized.  Because the tungsten carbide absorbed only about 25% of the laser’s energy, instead of approximately 80% for the graphite, the amount of energy required to vaporize a unit mass of graphite increased by a factor of 3.  While this is an intriguing idea, there are massive uncertainties in trying to apply it to other wavelength and material combinations.  At a guess, the protected material needs to vaporize instead of melting, limiting choices to carbon-based materials.  Also, the film must melt at a lower temperature than the protected material vaporizes at, and yet have as high a boiling point as possible, to prevent it from being burned off.

One might point out that the laser has to be focused by a mirror, and that the same mirror should be capable of being used as armor.  The problem with this suggestion is that the mirrors used for lasers are not conventional mirrors, but dielectric mirrors.  A dielectric mirror is made of numerous thin sheets of dielectric material, and is optimized for a particular wavelength and direction.  Over that narrow band, reflectivity could be as high as 99.999%, but the mirror is significantly less reflective against any other incoming light sources.  To use this type of mirror as armor, one would have to know the exact wavelength of an opponent’s weapons, and be able to control the direction on an engagement.  Both of these are unlikely in practice, as any power will undoubtedly use slightly different wavelengths on different craft to defeat these tactics, and it is unlikely that one could control the engagement well enough to keep the enemy in the proper zone, certainly not likely enough to be worth the expense of fitting dielectric armor to a spacecraft.

An interesting idea that was raised involved using a laser beam against itself, with the example given of the retroreflectors used on the Apollo missions. This approach suffers from all the problems of mirror armor, and even if said problems could be overcome, there are other significant issues with bouncing the beam back at its source. Quite simply, it is impossible to return sufficient power to the firer to achieve anything of note. In the best-case scenario, that of a flat mirror perpendicular to the beam, the intensity when the beam returns to the firing ship will be only one-fourth the intensity at the target. If the target can maintain an optical surface under the impact of the beam, the firer will have no difficulty doing so. Also, this is a set of ridiculously optimistic assumptions. The reflectors used on Apollo were corner reflectors, which are used on many reflective objects. In theory, they should perform exactly as a flat mirror would, no matter what direction the incident beam comes from.  In reality, they do an excellent job reflecting in the general direction of the incident beam, but it's nowhere near precise enough for what this concept would require. It might somewhat dazzle the shooter at long range, but it is unlikely to do more before the reflectors themselves burn off, particularly given the fact that they cannot employ dielectric mirrors. While corner reflectors have the advantage of reflecting no matter what direction the signal comes from, other methods (such as the aforementioned flat mirror) do not, which means they need pointing capabilities on par with the laser itself, and have to deal with higher energy fluxes than the laser optics. Coupled with the issues described above, it seems vanishingly unlikely that reflecting a laser back to its target will be a practical countermeasure to laser firepower.

Another option is some form of particle screen such as the Traveller sandcasters.  One problem with this proposal is that it is less mass-efficient than conventional armor.  When the particle is struck by the laser, it flashes to plasma, which then begins to disperse.  The plasma initially blocks the laser beam, but as it disperses and the density drops, the laser beam continues on.  If conventional armor is being used, the laser has to bore a hole, and the plasma is generated at the back of the hole.  The hole contains and channels the plasma, keeping it in the path of the beam and preventing it from dispersing.  The only form of particle screen that could prove practical is one made of small crystals such as diamonds.  Instead of absorbing the beam, it refracts it, dispersing it and reducing the intensity on the target.  Small ice crystals have also been suggested for use in this role, but the ice will sublimate even without taking laser fire, and a high-power laser will tend to turn the ice to vapor or plasma even more quickly.

The other issue with particle screens is even simpler.  They are expendable and not attached to the vessel in question.  At best, it is only effective for one battle, and only so long as the vessel does not maneuver.  This ignores issues of particle dispersion.  The screen must be deployed somehow, and that is likely to involve throwing the particles out at significant speed.  This dispersion will not stop when the screen is at the proper density, forcing the deployment of additional screens.  The screen also stops the vessel from returning fire with lasers, and will significantly interfere with any kinetic weapons.  Missiles launched from the vessel would be largely unaffected, except for the holes punched in the cloud. However, any launcher projectiles would likely be severely damaged by it as they passed through, as would any incoming projectiles.  The other issue is targeting of kinetics through the cloud, particularly as the vessel itself would be unable to use its sensors.  The used screen particles would also persist, and would serve as a significant constraint on maneuver.  Even at low velocity, the particles would be potentially damaging to mirrors.  This neglects the fact that the particles might constitute a significant debris hazard after the battle, depending on the deployment situation.

“Smokescreens” have also been suggested, intended to block observation instead of interfering with the laser itself.  The screened area would be significantly larger than the vessel, so the enemy is uncertain as to its location. The need for only optical thickness significantly reduces the mass requirements, but has other issues.  Achieving the required particle density and cloud size on a reasonable timescale will be extremely difficult, and the vessel would have to maneuver to take advantage of the uncertainty.  It would also be possible to burn paths through the cloud without too much difficulty, possibly revealing the position of the vessel.

All of the above refers to the use of particles defensively.  It is, however, at least somewhat more practical to use them offensively against laser-armed targets.  This would involve the use of kinetics filled with particles, referred to here as sand, and a bursting charge, which, when fired, spreads out and threatens to damage the optics of any laser in its path.  The laser operator has three options: accept the damage, try to burn the cloud away, or shutter the laser when the cloud hits.  The sand projectiles would be mixed in with the standard ones, and at some point shortly before impact, the burster fires, spreading the sand out and slightly ahead of the rest of the projectiles.  The various sand shells will stagger their bursts and spread their clouds out along the axis of flight to threaten the enemy for the longest possible time.  The biggest advantage is that the laser is interdicted precisely when it is the most effective (namely, when the incoming projectiles are at short range.)

Note that this is a surface-effect warhead, not simply a projectile throwing out small pieces of shrapnel.  The particle sizes are so small that they are ineffective against anything not requiring a precision surface.

The actual math involved is quite interesting, and suggests that the projectiles could be reasonably effective.  For a given mirror, amount of damage, and particle diameter, the mass per unit area required remains constant no matter what the density.  Smaller particles are more effective per unit mass than large ones, with mass required scaling with diameter^.92.  This suggests that whatever fine powder is available is the most effective.  However, there are two complicating factors.  First, ideally the density of the sand warhead will be the same as the density of a conventional warhead, so the enemy can’t tell it apart before it breaks.  Thus, low-density materials like sawdust might be poor choices.  Second, it is possible to defend against the cloud by burning a hole in it.  The amount of time it takes to burn this hole depends on the diameter and material of the projectile, with carbon-based materials significantly outperforming stone or metal.

The exact optimum material and diameter will depend on the situation, but a representative calculation will show that the masses involved are reasonable. (Based on Micrometeor Damage Estimate to the MOLA II Primary Mirror.) Taking a beryllium mirror, .5mm particles impacting at 30 km/s, and a requirement to damage 10% of the mirror’s surface, the total mass will be 5.1e-3 kg/m2.  Taking as the target a laserstar with a 10-meter, 1 GW laser, the total time to clear the particles will be 1.67 seconds for steel, 8.15 seconds for nanotubes, and 1.32 seconds for granite.  These are theoretical values, based on the laser spreading its power evenly over a circle equal to its mirror diameter and assuming that the particle must be completely burned away.   In reality, the vaporizing particle material will impart thrust to the rest of the particle, which might push it enough to render it harmless.  Larger particles would increase the time required, probably directly in proportion to the mass per unit area.

If we assume that the value above is representative of various sand warheads, we can then look at the total mass requirements.  If the projectile bursts 60 seconds before impact, and the target can dodge at 1 m/s2 (as in the example in Section 8) the total area to cover is 10.18e6 m2.  This corresponds to a warhead mass of 51,911 kg.  Obviously, this is not terribly practical.  However, there were several assumptions made that might change the scenario.  The first is the target’s dodging acceleration.  1 m/s2 may or may not be a plausible dodging acceleration.  At .1 m/s2, the circle has an area of 101.8e3 m2, and the sand mass drops to 519 kg.  Another is that the damage to the mirror is limited to the physical crater itself.  While the author is unfamiliar with the impact of hypervelocity particles on mirrors, intuition suggests that the damage to the optical properties of the mirror might extend well past the physical crater.  On the other hand, intuition is often a poor guide to space warfare.  The fact that the particles are quite easy to clear can be dealt with by sending in multiple projectiles from slightly different angles, so that burning away one set of particles does not affect the others.  Depending on the exact characteristics of the lasers, the few seconds it takes to burn away a set of particles could save several kinetics of equal mass to the launching projectile.

Another method of achieving a similar effect is the use of Jello.  Strange as this may sound, it is used by current BMD systems to discriminate decoys and damage optics.  The jello is released into space and the water flash-boils out, leaving a mass of fine, very hard, sharp granules.  The efficacy of this approach compared to the use of sand as described above is unknown.  The material properties of the particles become less important at higher velocities, and the water that is lost would probably be a significant mass penalty.

Other forms of heavier particle kinetics have been proposed to be used against lightly-armored parts of the spacecraft, most notably the radiators.  These would be more like ball bearings or buckshot, and are intended to puncture the radiator tubes, sending coolant leaking into space.  The efficacy of this sort of attack is unknown at the moment, and the concept requires further study, but it is possible that it would be quite effective.  At some point, however, the projectile will become big enough to be individually targeted, which in turn means that it can be burned out significantly faster than would be possible with a particle.  The author chooses to define particle in this use as a sub-projectile small enough that it is not detected and targeted independently, but must be dealt with by firing at the entire area.

Dodging lasers is possible, but difficult.  The biggest problem is that at any reasonable range, dodging is going to require high acceleration, which in the PMF will involve either chemfuel or nuclear-thermal.  This burns through a very large amount of delta-V very quickly, and limits the amount of time that can be spent dodging.  There are also significant performance penalties in the creation of a ship that is capable of dodging, as it must be capable of high accelerations in all directions.

As an example of the magnitudes involved in the problem, take a 10-meter-diameter cylindrical laserstar at .5 light-seconds.  The laser targeting it is fired instantaneously at the apparent center of the vessel, and is precisely targeted.  To avoid being shot, the laserstar must accelerate at 10 m/s2.  To be able to do this, the vessel would require 4 engines, each capable of 1G, one on each side of the ship.  Alternatively, fewer engines could be used, with the ship rotating to bring them to bear.  This would, however, slow dodging, and make it more predictable.

The above scenario has neglected a number of complications.  These include:

  1. Beam diameter:
    The beam will have nonzero diameter, so in the above scenario, the ship accelerating at 10 m/s2 would still have been hit by about half the beam. This would increase the delta-V required to avoid a hit.
  2. Perfect dodging:
    It has been assumed that the ship began dodging at the instant the enemy would have opened fire (relative to light lag), and was able to hold a course the entire time. Neither of these assumptions bear any resemblance to reality, tremendously complicating dodging. Dodging would require that (if the enemy was aiming at the center of the ship) that no part of the ship was in the same line as the center was 1 second previously, and was not accelerating in a straight line.
  3. Beam inaccuracy:
    This can either help or hinder dodging. If the diameter of the inaccuracy circle was 10 meters, and the hit probability for each area was constant, dodging as described would reduce hit probability to about .333 instead of 1. However, if the circle had a diameter of 20 meters, then dodging at 10 m/s2 would be useless, as the hit probability would remain the same.

On a slightly more theoretical level, this scenario suggests several rules related to dodging. First, the required delta-V will scale inversely with distance, while the acceleration will scale with the square of distance. Second, if the target is able to accelerate faster, it can cut the required delta-V. The theoretical minimum is the circle radius/time. If constant acceleration is used, then the required delta-V is double that of the theoretical minimum.

There is an alternative to the conventional large mirrors in the form of phased arrays.  A phased array is composed of a number of synchronized transmitting elements.  The beam is formed by the interference patterns between the different elements, and can be steered instantaneously by varying the transmission lag between the elements.  It is also possible to split the beam into multiple sections of varying power.  This is a significant advantage for point-defense use.  On the other hand, the phased array is less effective for a given aperture area/overall diameter then a conventional mirror.  However, given sufficiently large numbers of transmitting elements, it is entirely possible that the phased array will be capable of very similar performance to a conventional mirror, and at significantly lower cost.  This suggests that a laserstar might have a single large mirror and a phased array for point defense.  The phased array also has logistical advantages.  Particularly if all of the lasers in the fleet are phased arrays, the transmitters can be modular and mass-produced, not to mention field-replaceable.  It’s also possible that the transmitters will be more damage-resistant then a conventional laser.  At the very least, each transmitter can be individually shuttered, and the small size of the required shutter makes it easier to engineer a high shutter speed.

It might also be possible to use the instantaneous response of the phased array to correct for some of the vibration inherent in a laserstar, above and beyond what is possible with conventional mirrors.  First, each element can probably be isolated individually, making the system smaller and lighter, and quite possibly more effective.  Second, the phased array can be used to compensate for vibration with greater accuracy then adaptive optics, due to the electronic pointing capabilities.  In fact, phased array pointing accuracy angles are often smaller than the spot size for the array.  Phased arrays are also inherently more tolerant of jitter than are conventional mirrors.  A phased array will have the same apparent jitter as a large mirror with individual element jitter that is equal to the square root of the number of elements times the unitary mirror jitter.

It has been suggested that instead of an eyeball-frying contest, a “tumbling pigeon” approach could be used.  The laserstars would be making random attitude changes throughout the approach, which occasionally exposes the mirror for a snap shot.  Except in the case on an extremely lucky hit, neither vessel can scorch the other’s mirror.  The problem with this approach is that it significantly complicates the design of the vessel.  A conventional laserstar has a faceplate designed to receive enemy fire, while a pigeon will receive fire from all around.  Among other things, this renders the use of radiators in battle impossible.

As laser weapons continue to develop, more information about them becomes available to the public.  However, because of the classified status of most developments, the information we do receive is out of date, probably by about 5 years.  Furthermore, most of it is only of limited relevance to space warfare.  For example, the US Navy’s Laser Weapon System, which recently went to sea aboard the USS Ponce, is composed of six solid-state welding lasers fastened together for a total power of 33 kW.  It has a mirror of approximately 1 meter diameter, and is designed to kill aircraft and small boats at short range.  A 1 km, the diffraction-limited beam diameter is only 1.22 mm, and the beam will drill through steel at a rate of approximately 60 cm/s.  At 10 km, however, even a diffraction-limited beam can only drill through steel at around 1.5 cm/s.  Most surface-based lasers get their performance in a similar manner, as the horizon forces them to fight at close range.  A spacecraft, however, does not normally have a horizon to hide behind and much use much larger mirrors and much higher power to deal damage at long range.  This produces a fundamentally difference in character between laser engagements on the surface and in space.  On the surface, laser battles are over almost instantly, as one side or the other gets the first shot in.  In deep space, long lines of sight allow both sides to open fire as soon as they could begin doing damage, slowing the battle down.  If the craft did close to point-blank range, their lasers would become incredibly lethal, but both sides would be aware of this and probably avoid tactical geometries that lead to such situations.

It has also been suggested that a battle between laserstars will be resolved by damage control.  The sensors and mirrors would be replaceable, and whoever can do so fastest would win.  The problem with this theory is the size and expense of the mirrors, along with the precision mounting requirements.  The optimum ratio of lasers to mirrors can be derived to show why this concept is impractical.  

At short ranges (maximum possible crater size exceeds minimum possible spot size):

At long ranges (minimum possible spot size exceeds maximum possible crater size), drill rates will be proportional to:

laserpower*laserenergy^(1/3)/spotsize^2

or to

laserpower*laserenergy^(1/3)*mirrorsurface^(3/2)

(and to 1/range^3).

The point where short range turns into long range depends on your laser and mirror, so when comparing two laser setups there will be an intermediate range area where one laser is using the short-range formula and one is using the long-range formula, further complicating the question of which setup is best. The transition point is when

meaning that

Getting through your armor will take an amount of time equal to:

(where armor thickness is measured in units of mass per units of surface area, and drill rate is the values given above).

All this assumes that your goal is to penetrate as deeply as possible, without caring how thick your wound channel is. If thin needler beams are incapable of sufficiently damaging the machinery you're shooting at, that'll make short-range lasers less effective.  Mirror-scorching and sensor-frying would use different formulas entirely.

Using these, we can calculate that if you're optimizing purely for mass, then at short ranges it is optimal to have the laser and the mirror be equally massive, while at long ranges it is optimal to have 9 parts mirror for every 8 parts laser! If you're optimizing for price rather than mass, then you would use the ratios, but they would instead mean you spend about the same amount of money on both laser and mirror, regardless of how much mass that gives you — but you should calculate the price for components including the value of the additional engine and propellant (and radiators, etc.) you need to add to carry this additional mass (which requires you to already know what engine ratio you want). If you're also optimizing for minimizing vulnerability, then at first glance you might think that encourages focusing more on the laser, since it's far less vulnerable than mirrors — but that's wrong. Since mirrors are vulnerable, it's more important to build them larger so it's harder for the enemy to shoot the entire mirror off. (The preceding calculations on laser drill rate and mirror proportions were done by Milo, and posted to the Rocketverse forum. The author has cleaned the formulas up slightly, but otherwise posted them as written. The basis was Luke Campbell’s laser damage calculators.)

The same math makes the idea of mounting a mirror on a spacecraft and relaying a beam from another craft dubious.  The mirror will be too expensive, and the coordination problems are formidable, particularly as the beam must not only hit the relaying spacecraft, but do so in a manner that allows it to be redirected to the target.  

by Byron Coffey (2016)
The Photon Lance

(ed note: this is a discussion on how laser mechanics are implemented in the ultra-realistic game Children of a Dead Earth)

Comparatively, lasers are far more complex than any of the weapon designs we’ve looked into, with far more components and considerations.

For example, in module design, railguns and the like can be optimized by simple tweaking and trial and error. On the other hand, it is very difficult to do so when designing lasers. The relations between the inputs and outputs are not only nonlinear, they are absolutely not monotonic, so simply using trial and error to find ideal cases is not always possible.

While there was an explosion of different design options and choices for railguns as we saw in Origin Stories, with lasers, it was far worse. First you’ll choose your laser type from amongst a staggering array of types. Then you’ll need a pumping source, which includes a nearly infinite number of pumping and lasing geometries, each with different advantages. And you’ll probably want to add a nonlinear crystal to harness Frequency Switching in order to double, triple, or quadruple your photon frequency.

Then you need to worry about the optics between each and every subsystem, ensuring the photons don’t seriously damage each lens, mirror, or nonlinear crystal at each point. Plus, you need to arbitrarily focus your beam at different distances, either with a Zoom Lens or with a Deformable Mirror (though in practice, zoom lens tend to be impractical for extremely long ranges, meaning you’re usually stuck with using a deformable mirror).

Also, and if you want to pulse your laser, you’ll need to use Mode Locking, Q Switching, or Gain Switching to do so. Finally, while mechanical stress are basically irrelevant for lasers (recoil of lasers is minuscule), thermal stresses are huge. Cooling your laser effectively is one of the most important parts of building a working laser.

Laser construction is not for the faint of heart, but the outputs of lasers are actually fairly simple compared to mass weapons. While mass weapons produce a projectile of varying dimensions and materials at a certain speed, possibly with excess temperature, and possibly carrying a complex payload, lasers just shoot a packet of photons. Even if the laser is continuous, the beam fired can be considered series of discrete packets.

Since laser beams move at the speed of light, it is actually impossible to dodge a laser unless you are always dodging. This is because the speed of light is the speed at which information travels in the universe. Thus, you can never determine where a laser will be until it actually hits you. This would be impossibly overpowered in warfare were it not for diffraction.

A packet of photons is focused on a single point of a certain size, and carries a discrete amount of energy of a single wavelength/frequency. Technically, due to quantum mechanics, particularly the Uncertainty Principle, there will be many different wavelengths, an uncertain size, and an uncertain amount of energy. These quantum effects are glossed over because approximating the entire packet as a discrete bundle is both simpler and still remains very close to reality.

The only quantum effect that significantly affects the output of a laser in terms of warfare is Diffraction.

Diffraction causes a laser beam to diffuse the further it gets from its exit aperture, spreading out the energy of the laser. This is a problem because the energy a beam carries is not what inflicts damage. The energy per unit area, or Fluence, is what causes damage. For continuous beams, it would be the power per unit area, or Irradiance.

A hypothetically perfect laser will suffer from diffraction and is referred to as being Diffraction Limited. But this is not what is actually limits most actual high powered lasers in warfare.

Most high powered lasers will never even come close to being diffraction limited.

Truth is, the Beam Waist, or the minimum diameter the beam will achieve, is a more effective measurement of how damaging a laser is. A perfect laser will have a beam waist limited only by diffraction, but lasers like that don’t exist. And the greater the power of a laser, the further and further away that laser strays from being diffraction limited.

A good way to measure this is with the Beam Quality of the laser, or with the M Squared. M2 is the beam quality factor, which can be considered a multiplier of the beam waist. So, an M2 of 5 means the beam waist is 5 times that of a diffraction limited beam. In terms of area, this means the beam is 25 (52) times the area of a diffraction limited beam, or 25 times as weak. As you can see, having a M2 even in the high single digits will yield beams a far cry from “perfect” diffraction limited beams.

In practice, it is not the pumping efficiency, nor the power supply, nor diffraction, which ultimately limits lasers. It is the beam quality factor. In the end, M2 ends up being the number one limit on laser damage in combat.

In small lasers, M2 close to 1 is easily achieved without issue, but in high power lasers, M2 can easily reach into the millions if not accounted for. This is because generally, M2 scales linearly with laser power.

Each optical component of a laser affects the M2. In particular, using a deformable mirror to focus a laser at arbitrarily long ranges (such as from 1 km to 100 km) is measured at reducing M2 to between 1.5 to 3. Problematic, but not exactly debilitating.

But the main issue is Thermal Lensing (Note that this is different from Thermal Blooming, which only occurs outside the laser in the presence of an atmosphere). The heating of a laser gain medium generates a thermal lens which defocuses the beam, ultimately widening the beam waist, preventing the beam from focusing properly. Also note that thermal lensing actually occurs in every single optical component of the laser, though it is strongest in the lasing medium.

Thermal lensing increases M2 roughly linearly with input power. This means if you have 1 kW laser with a M2 of 1.5 (which is reasonable), this means dumping 1 MW into that same laser will yield a M2 of about 1500 (going the other way does not work, since M2 can’t be less than 1).

One might try to predict the thermal conditions and add in an actual lens reversing the thermal lens. Unfortunately, the thermal lens is not a perfect lens either, and the imperfections of this lens remain the primary cause of beam quality reduction.

Fiber lasers are often touted as a solution to thermal lensing. They are considered immune to thermal lensing except in extreme cases. Unfortunately, dumping hundreds of megawatts through a fiber laser constitutes an extreme case, and fiber lasers suffer thermal lensing nearly as badly as standard solid state lasers.

The largest innovation for combating thermal lensing are negative thermal lenses. Most gain mediums have a positive thermo-optic coefficient, and this is what generates the thermal lens. Certain optical materials have a negative thermo-optic coefficient, which produces a thermal lens inverse of what the gain medium produces. Ideally, this negative thermal lens would perfectly reverse the positive thermal lens, but in practice, the M2 still suffers.

In the end, the primary way to combat thermal lensing is with cooling. And the primary way to cool your laser is to make it bigger.

If the proportions of a laser are kept identical, lasers can be scaled up or down with minimal change to the laser’s efficiency or output power. Indeed, you can pump 100 MW or power into a tiny palm-sized laser just as well as you can into a building-sized laser, and they will produce roughly equal beams in terms of efficiency and M2. The only difference is that the palm-sized laser will melt into slag when you try to fire it.

Laser size is mostly a matter of how much do you need to distribute the heat of the laser pumping. And if you want to combat thermal lensing, you’ll want a really big laser. This means laser size is essentially about cooling, and by extension, having a low M2.

And because size is closely related to mass, and mass is so critical to spacecraft design, the limiting factor of using lasers in space is how poor of an M2 you want to have, given a certain power level. Though the radiator mass needed for the enormous power supplies is the other major consideration.

A final way to combat thermal lensing is to use Beam Combining of many smaller lasers. Combining beams side by side increases the beam waist linearly, which defeats the point, but Filled Aperture Techniques can combine beams without increasing the beam waist. However, this technique produces greater inefficiency to the final beam. The ideal way to combine beams is to simply use multiple separate lasers which all focus on a single point.

In Children of a Dead Earth, either single large lasers or multiple small, separately focused lasers can be used, and both have varying pros and cons.

Of course, designing lasers in Children of a Dead Earth is often far more difficult than designing any other system, so there are plenty of factory-made options for players to use. But the option is always there for those who really want to explore the depths of laser construction!

Efficiency

Note that laser cannon are notoriously inefficient. Free-electron lasers have a theoretical maximum efficiency of 65%, while others are lucky to get a third of that. This means if your beam power is 5,000 megawatts (five gigawatts), and your cannon has an efficiency of 20%, the cannon is producing 25,000 megawatts, of which 5,000 is laser beam and 20,000 is waste heat! Ken Burnside describes weapon lasers as blast furnaces that produce coherent light as a byproduct. Rick Robinson describes them as an observatory telescope with a jet engine at the eyepiece. Laser cannons are going to need seriously huge heat radiators. And don't forget that heat radiators really cannot be armored.

The messy alternative is to use open-cycle cooling, where the lasing gas is vented to dispose of the waste heat. Not only does this endanger anything in the path of the exhaust, it limits the number of laser shots to the amount of gas carried.

But Troy Winchester Campbell brings to my attention a recent news item. In 2004, a company named Alfalight, Inc. demonstrated a 970 nm diode laser with a total power conversion efficiency of 65%. They are working in the DARPA Super High Efficiency Diode Sources program. The goal is 80% electrical-to-optical efficiency in the generation of light from stacks of semiconductor diode laser bars, and a power level of 500W/cm2 per diode bar operating continuously.

Recent Developments in Laser Efficiency

This is one of those areas where progress is happening so fast that it can be hard to keep up with all the newest technology.

Two decades ago, everyone knew that laser weapons would use chemical lasers, like deuterium fluoride (DF) or chemical oxygen iodine laser (COIL) lasers. The beam quality was abysmal, but that was the only way you could get the power levels needed. One of my professors described chemical lasers as more like a flashlight than any sort of focused beam.

One decade ago, everyone knew that chemical lasers were a giant steamping pile of crud. Diode-pumped slab solid state lasers were the way of the future. You didn't have to carry around huge vats of toxic caustic chemicals, you could just use cheap electricity to efficiently generate your beam. Slab solid state lasers lead to all of the issues described in the linked article but lab prototypes were developed that operated at ~10 kW with beams that were only about 2 to 5 times worse than diffraction limited (going by memory here, there's a chance these numbers may be off by more than I'm recalling).

Then about 5 years ago some folks bundled together a bunch of fiber lasers into a high powered laser weapon demonstrator that was cheaper and more robust and smaller than any slab solid state laser. They used incoherent beam combining — just shining all the lasers at the same spot, so the beam quality was total crud. But it worked. You could blow up boats and rockets and UAVs and mortar shells in flight at respectable distances and do all sorts of other nifty stuff. All this was made possible by advances in industrial fiber lasers used for materials processing that allowed outputs of multiple kilowatts from individual fibers. Each fiber is essentially diffraction limited, and because the beam is generated inside of a very long fiber optic cable you have a huge surface area available for cooling.

The next trick is figuring out how to put the high quality ~kW beams together into one single high quality beam with ~100 kW or ~MW power levels. Lockheed Martin started using spectral beam combining in the last few years to do just this with lasers initially starting at around 20 kW and now exceeding 60 kW of power. It is expected that spectral beam combining can take you up to ~100 to 150 kW power before you start hitting limitations of the mechanism.

But people are already looking in to methods to go way beyond this. Coherent beam combining techniques are being developed that allow near diffraction-limited combined beams by controlling the phases of the individual beams. One method that would provide the highest quality beams uses diffraction gratings with phase control of the incoming beams to interfere constructively on only one of the diffraction lobes and destructively at all other lobes. Using this, you could daisy chain an unlimited number of lasers together into a single high quality beam. The other method I've seen involves controlling the phases of a bunch of beams that exit side-by-side, making a laser phased array. The beam quality will be a bit worse, but you have the benefit of allowing instantaneous control over the beam wavefront so you can instantly tilt the beam without needing to slew your beam pointer around (in addition to doing all sorts of nifty stuff like adaptive optics and active focusing just with the relative beam phases rather than cumbersome optical elements). With some development, you might even get around the fill factor issues by getting component beams that are nearly uniform in power level across the beam-front to allow laser phased arrays with nearly perfect diffraction limited performance.

The first coherently combined beams will undoubtedly use fiber lasers, but current research is already looking beyond that. Coherent combination of diode lasers could get around the problems that have plagued high powered diode lasers for decades. This could get around a significant source of loss in using diode lasers to pump fiber lasers, going from 30% efficiency to 50 or 60% efficiency for wallplug-to-light conversion (some laboratory diode lasers have even achieved 70% efficiency). In addition, diode lasers allow some degree of frequency agility, so that you could shift the wavelength of your beam on the fly (within limits). I would not be at all surprised if, a decade from now, this was the obvious future of laser weapons. Or maybe it will be something else completely unforeseen.

W = (1.0 / Ce)

where:

  • We = Waste power percentage
  • Ce = Efficiency of Laser Cannon

Obviously:

CP = BP * We

where:

  • CP = Laser Cannon total power (megawatts)
  • BP = Beam Power at laser aperture (megawatts)
  • We = Waste power percentage

WP = CP - BP

where:

  • WP = Waste Power (megawatts)
  • CP = Laser Cannon total power (megawatts)
  • BP = Beam Power at laser aperture (megawatts)

Getting rid of the waste heat from a laser is a problem if you don't dare extend your heat radiators because you are afraid they will be shot off. A strictly limited solution is storing the waste in a heat sink, like a huge block of ice. "Limited" because the ice can only absorb so much until it melts and starts to boil. If your radiator is retracted and your heat sink is full, firing your laser will do more damage to you than to the target.

Eric Rozier has this analysis of heat sink mass:

One common mistake people make is assuming that lasers are infinite fire weapons. With proper radiators extended, this is true, but with them drawn in, to avoid being shot off, we're limited by the heat capacity of our sinking material, as you well know.

An interesting question to ask is: "Without radiators, how many shots can I get off for some mass of coolant and some sort of laser?"

Given single laser of Bp megawatts at aperture, and an efficiency of eff, duty cycle of dc, and firing time of Tf, we get the waste heat Wh (in MWseconds) as:

Wh = Tf * (Bp/eff * dc) * (1 - eff)

Wh is then the waste heat generated by a single blast from our lasers. To figure out how many times we can fire our lasers we need to perform some calculations based on our coolant, the data of interest is:

  • Mass of coolant dedicated to lasers (Mc) in kg
  • Atomic mass of coolant (Ma) in g/mol
  • Heat capacity of coolant (Hc) in J/(mol * K)
  • Melting point of coolant (Km) in K
  • Boiling point of coolant (Kb) in K

Given this, we can find the number of shots we can fire (S) as follows:

S = ((Mc / Ma) * Hc * (Km - Kb)) / 1000 / Wh

If you do not have the atomic mass of coolant or heat capacity of coolant, you can instead use the specific Heat capacity of coolant. This is useful if the coolant is a compound instead of an element in the periodic table.

  • Specific Heat capacity of coolant (Hck) in J/(kg K)
  • Energy Capacity of coolant in MW seconds (or MegaJoules if you prefer)

Ec = (Mc * HcK * (Km - Kb)) / 1000000

S = Ec / Wh

There is an online calculator for this here.

This assumes the coolant is just melted before firing the laser, and just boiling after firing all available shots. In reality, you want to set Kb at some level below the real boiling point, and Km at some level above the melting point.

As a worked example, a 100MW laser with efficiency of 0.2, 0.5 duty cycle, and 0.1s firing time generates 20 MWseconds of waste heat each time it fires. 1000kg of Lithium, (with about 1140K between melting and boiling) can contain enough heat to fire the laser roughly 204 times.

This, I think, helps show some of the heat limitations of lasers, and constrains them (especially as point defense weapons). You end up having to lug a lot of lithium around if you want to fire them often.

I think this is most interesting when thinking about point defense. Lasers fielded as a CIWS are pretty scary, and if you could fire them infinitely often, they probably keep missiles from hitting you. So in order to constrain you from using lasers for point defense, I simply pull into laser range, threatening your radiators, and forcing you to withdraw them. As such, you can no longer afford to use a laser CIWS, and have to switch to something projectile/missile based, which is liable to be less effective.

Eric Rozier
Laser Weapon Mass

Winchell Chung:

Luke Campbell, a question occurred to me, and you are currently the only laser scientist I know. If this question is the equivalent to a graduate thesis, just forget it.

Occasionally science fiction authors try to figure the mass and volume of their spacecraft. Especially warships. So what is the average mass and volume of an anti-ship laser weapon? Does is scale with beam output power?

Luke Campbell:

At this point, there is no good way to estimate the mass and volume of an anti-ship laser. To do so requires knowing two things: the beam power or beam energy needed to defeat a target ship, and the specific power or specific energy (power divided by mass or energy divided by mass, respectively) of the laser.

(ed note: Energy is joules, Power is joules-per-second or watts)

The beam power or energy requires could in principle be determined for a given engagement scenario — engagement time and distance; target size and armor. This will, of course, depend on the tech choices made elsewhere in the setting, so no single value can be quoted. Using modern ocean-going warships as a proxy and the sorts of heat ray lasers that are currently being built and fielded, many tens of kilowatts to disable sensors, communications, and soft surface targets seems reasonable, while tens of megawatts could burn through the hull to kill propulsion, power generators, or explode on-board magazines. Longer ranges, shorter engagement times, or more massive or more heavily armored ships all tilt you toward needing more beam power.

The specific power of modern war fighting lasers has been rising rapidly, and there is no reason it shouldn't keep dropping in the foreseeable future. Some recent advances have got the specific power down to better than 0.25 kW/kg

http://spectrum.ieee.org/tech-talk/aerospace/military/tactical-laser-weapon-module-can-laserify-almost-anything

(better because the listed performance also includes the batteries to power the laser for a number of seconds). The specific energy of pulsed lasers has also been increasing, although none are now a reasonable candidate for weaponization.

You can expect the laser mass to scale with beam power for heat rays, and pulse energy for blasters with an extra helping of other equipment scaling with beam power for power handling and control and heat rejection. The minimum area of the focal array will be set by the beam power and beam energy, but in practice you will probably find that you want a larger aperture than this minimum area in order to get a reasonable range. How the mass of the beam pointer telescope scales with the aperture's area I will leave to mechanical engineers.

From a thread on Google Plus (2015)
Laser Weapon Mass 2

(ed note: The topic is Free Electron Lasers (FEL) as weapons.

There's a lot of engineering that goes into getting good efficiency. A design like an energy recovery linac can turn something like 99.9% of the electron beam energy into laser energy (with caveats that it is actually recycling a lot of its energy back into the beam, using energy that would normally be wasted at the beam dump to pump up the fields in the accelerating cavities and use that to accelerate the electron beam). So the problem comes down to efficiently accelerating the electron beam.

Most of the energy use is not in making the beam so much as in overhead, such as running the refrigerators to cool your RF cavities down to superconducting temperatures. I read one study that was looking at putting an FEL on a 747. They could use the jet turbines as compressors for their refrigeration to essentially get cryogenics for free while the plane was under way. This really helped the efficiency by a lot.

A heat pump can move heat from 100 K to 1000 K. It just generates a lot more heat in the process. For every watt you remove from the 100 K bath, you will need to radiate away 10 watts from your 1000 K radiators.

Luke Campbell from a thread on Google Plus (2016)

Attack Vector: Tactical Lasers

Ken Burnside's masterful tabletop wargame Attack Vector: Tactical is fictional, but it was prepared with expert help from real live physicists and other scientists. More to the point, design choices were made to make an interesting game. Which means they would also be design choices that would make an interesting science fiction novel.

In the game, there are various types of lasers of increasingly shorter wavelengths, which due to the diffraction equation have increasingly longer range (by which I mean the spot intensity decreases more slowly). These lasers also have a decreasing level of efficiency of converting power into laser beam, I am unsure if this is due to a physical limit or it is an arbitrary thing used to balance the game.

LaserWavelengthColorEfficiency
Short Range Laser2400 nmNear Infrared20%
Close Range Laser1600 nmNear Infrared16.6%
Medium Range Laser1200 nmNear Infrared12.5%
Extended Range Laser800 nmNear Infrared9%
Long Range Laser600 nmOrange6%
Extreme Range Laser400 nmIndigo3%
Ultraviolet Laser200 nmUltraviolet1.5%

In addition, each laser type comes in seven sizes (with focusing mirrors ranging in size from 3 meters radius to 6 meters radius) and assorted energy requirements. The basic game only has short range and medium range lasers:

LaserMirror
radius
Input
Energy
Effic.Aperture
Energy
Eff
Range
Max
Range
Short Range
Laser 2
3 m3 GW20%0.6 GW80 km300 km
Short Range
Laser 3
3.5 m4.5 GW20%0.9 GW100 km440 km
Short Range
Laser 4
4 m6 GW20%1.2 GW120 km560 km
Short Range
Laser 5
4.5 m7.5 GW20%1.5 GW140 km740 km
Short Range
Laser 6
5 m9 GW20%1.8 GW160 km900 km
Short Range
Laser 7
5.5 m10.5 GW20%2.1 GW160 km1,040 km
Short Range
Laser 8
6 m12 GW20%2.4 GW180 km1,200 km
LaserMirror
radius
Input
Energy
Effic.Aperture
Energy
Eff
Range
Max
Range
Medium Range
Laser 2
3 m2 GW12.5%0.25 GW180 km400 km
Medium Range
Laser 3
3.5 m3 GW12.5%0.375 GW200 km600 km
Medium Range
Laser 4
4 m4 GW12.5%0.5 GW240 km800 km
Medium Range
Laser 5
4.5 m5 GW12.5%0.625 GW280 km1,000 km
Medium Range
Laser 6
5 m6 GW12.5%0.75 GW300 km1,200 km
Medium Range
Laser 7
5.5 m7 GW12.5%0.875 GW340 km1,400 km
Medium Range
Laser 8
6 m8 GW12.5%1 GW360 km1,800 km

The mirror radius is the size of the lens or reflector (RL in the diffraction equation). The input energy is fed as power into the laser, after suffering the horrific effects of typical abysmal laser efficiency the laser beam emerges from the business end containing the aperture energy and leaps out to impale the hapless target. The gigawatts of waste heat are absorbed by the internal heat sink, because extending your heat radiator is just asking for it to get shot off.

The effective range and maximum range are not directly applicable, they are artifacts of the beam damage model used by the Attack Vector: Tactical game. But they do provide some basis of comparison. In the game each "damage point" inflicted upon an enemy ship represents 50 megajoules in an eight centimeter diameter circle inflicted in 1/100th of a second. The effective range is the farthest range that the laser can inflict its full damage. The maximum range is the farthest range that the laser can inflict at least one point of damage. This is all required because Attack Vector is not a computer game, it is an incredible paper and cardboard wargame where all the scientific accuracy and scary mathematics are handled painlessly with cunning player aides.

I would hazard a guess this is the reason for the values chosen for input energy and ranges, to calibrate each laser to 50 megajoules in an eight centimeter spot size.

For our purposes, it might make more sense to use the Brightness equation. Then you can assign hardness values for the target's armor.

Short Range Laser 2
RangeSpot
Dia
Brightness
80 km7.8 cm1.55×109 J/m2
100 km9.8 cm9.9×108 J/m2
140 km13.7 cm5.05×108 J/m2
180 km17.6 cm3.06×108 J/m2
220 km21.5 cm2.05×108 J/m2
300 km29.3 cm1.1×108 J/m2
Short Range Laser 8
RangeSpot
Dia
Brightness
180 km8.8 cm3.06×108 J/m2
200 km9.8 cm2.48×108 J/m2
240 km11.7 cm1.72×108 J/m2
300 km14.6 cm1.10×108 J/m2
380 km18.5 cm6.86×107 J/m2
520 km25.4 cm3.66×107 J/m2
840 km41.0 cm1.40×107 J/m2
1,200 km58.6 cm6.88×106 J/m2
Medium Range Laser 2
RangeSpot
Dia
Brightness
180 km8.8 cm5.09×108 J/m2
240 km11.7 cm2.86×108 J/m2
300 km14.6 cm1.83×108 J/m2
400 km19.5 cm1.03×108 J/m2
Medium Range Laser 8
RangeSpot
Dia
Brightness
360 km8.8 cm1.27×108 J/m2
420 km10.2 cm9.35×107 J/m2
500 km12.2 cm6.60×107 J/m2
620 km15.1 cm4.29×107 J/m2
860 km21.0 cm2.23×107 J/m2
1,220 km29.8 cm1.11×107 J/m2
1,600 km39.0 cm6.45×106 J/m2

Combat Mirror

A more scientifically plausible but much less dramatic laser weapon is the combat mirror. In this scheme, the spacecraft doesn't have a laser, just a large parabolic mirror. The laser is several million miles away, on a freaking huge solar power array orbiting your home planet. You angle the mirror so it will do a bank shot from the distant laser off the mirror and into your target, then radio the laser station to let'er rip. About fifteen minutes later the diffuse laser beam arrives, and your parabolic mirror focuses it down to a megaJoule pinpoint on your target.

The combination of a powersat and a combat mirror is called a Powersat Weapon.

The advantage is that the spacecraft does not have to lug around the laser, the power supply, the heat radiators, and other massive elements of the laser weapon. The spacecraft can have a higher acceleration or increased payload. The beam can also be of a power level associated with laser equipment that is not considered "portable by spacecraft", if the laser generator is a few miles in diameter your spacecraft could care less.

Disadvantages include the lag time between ordering a shot and its arrival, and the vulnerable nature of the combat mirror (generally little more than a large Mylar balloon).

Mirror Armor

Now I know all you older science fiction fans still remember Johnny Quest and The Mystery Of The Lizard Men where Dr. Quest demonstrates that one can defend oneself against a weapon-grade laser beam with a dressing-room mirror. Sorry, it doesn't work that way in reality. No mirror is 100% efficient, and at these power levels, the fraction that leaks through is more than enough to vaporize the mirror armor. The same goes for "ablative armor." One zap and the impact point is abruptly as bare of armor as a baby's behind.

Inside a laser cannon, a relatively diffuse laser beam is generated. This prevents the beam from vaporizing the cannon's internal optics. At the business end, a parabolic mirror focuses the diffuse beam down to the aforementioned megaJoule pinpoint on the hapless target.

Accuracy

And don't think that lasers will automatically hit their targets either. There are many factors that can cause a miss. Off the top of his head, Dr. John Schilling mentions:

  • Uncertain target location due to finite sensor resolution
  • Uncertain target motion due to sensor glint or shape effects
  • Sensor boresight error due to finite manufacturing tolerances
  • Target motion during sensor integration time
  • Analog-to-digital conversion errors of sensor data
  • Software errors in fire control system
  • Hardware errors in fire control system
  • Digital-to-analog conversion errors of gunlaying servo commands
  • Target motion during weapon aiming time
  • Weapon boresight error due to finite manufacturing tolerances
  • Weapon structural distortion due to inertial effects of rapid slew
  • Weapon structural distortion due to external or internal vibration
  • Weapon structural distortion due to thermal expansion during firing

And we haven't even begun to include target countermeasures...

Turret

Airbourne Laser

What about a laser turret? It can be so inconvenient to have to move the entire ship in order to aim the blasted beam. As it turns out, the US Air Force has a solution created for their Airborne Laser project.

I hear you ask "but why doesn't the beam slice up the inside of the turret?" The key is power density.

For instance, a naughty little boy will find that sunlight does not do much to his skin except warm it up a bit. However, if you whip out a magnifying glass you can focus the sunlight to a white-hot pinpoint that will easily incinerate ants. The magnifying glass increases the power density of the sunlight. So inside the turret, the weapon beam is something like 20 centimeters in diameter which means a power density too low to fry the internal mirrors. At the end, the beam expander mirror evenly shines the laser beam over the primary mirror. That mirror then acts like the magnifying glass in the hands of the anticidal little boy, focusing the diffuse laser beam down to an incinerating pin-point on the hapless target.

Isaac Kuo points out that another factor keeping the laser from chopping up the turret is that the internal mirrors are dielectric mirrors. Those babies can be up to 99.999% reflective. Meanwhile if target has conventional mirror plating it will only be 95% reflective, absorbing 5,000 times as much laser energy. Dielectric mirrors would be difficult if not impossible to manufacture in pieces large enough to cover a missile or spacecraft.

The actual US Air Force Air Borne Laser is a megawatt class chemical oxygen iodide laser (COIL) operating at a frequency of 1.315 microns or 1.315e-6 meters (near infrared). With a 1.5 meter mirror, this gives a divergence angle of 1.07e-6 radians. If my slide rule is correct, this means at a range of one kilometer it will have a spot size of one millimeter radius, and a beam brightness of about 300,000 megawatts per square meter. However, I've seen suggestions that the actual spot size is more like several centimeters, demonstrating the room for improvement.

The US Air Force is understandably reluctant to give any figures on the performance of the Air Borne Laser. The best figures I could find suggest that it could destroy a flimsy unarmored hypergolic fueled missile (with fuel still in the tanks) by expending a three to five second burst up to a range of about 370 kilometers. Three to five seconds is an awfully long time to keep the beam focused on the same spot on a streaking missile. The dwell time will have to be longer if the missile is armored or if it uses solid fuel or other inherently stable fuel.

The giant primary mirror will contain adaptive optics (i.e., it will be a "rubber mirror"). This will allow the mirror to change its focus to accommodate the range to target. In diagram "a" to the right, the flexible mirror is laid over a slab of piezoelectric material that changes shape as power is applied to the electrodes. In diagram "b" individual actuators are used. The image on the right is a 19-actuator deformable mirror built by Rockwell International. The mirror is only 40 cm in diameter. The actuator density is about 150 actuators per square meter, so the 1.5 meter ABL mirror would require about 270. (surface area of a circular 1.5 meter mirror is about 1.8 square meters, times 150 actuators per square meters give 270 total actuators)

Luke Campbell's Turret

Luke Campbell has his own design for a laser turret. Cararra 5 was used to create the 3D mesh and to render the images.

Optics

Rick Robinson has a more serious concern. You know how it is a very bad idea to look through a telescope at the Sun? Well, for the same reason it is bad to unshutter your laser cannon optics and point them at a hostile ship which might zap you with its laser. Your cannon's optics would funnel their beam right down into the delicate interior of your cannon. The optics would also concentrate their beam to 10x or 100x the intensity. This means that if your lasers are unshuttered and your opponents are shuttered, you have the drop on them. The instant you detect their shutters trembling you give them a zap. Their shutters will still be opening when your bolt scrags their laser.

However, Ken Burnside says:

I will point out that the likeliest result of "shooting down the barrel of a laser" is to destroy one of the mirror elements on the focal array. Since those elements are likely to be used with adaptive optics, this won't even hurt the laser that much. It's only if the mirrors are hit at exactly the right angle that they'll direct energy back into the Free Electron Laser itself.

Ken Burnside

One solution is to aim the laser by using a flotilla of external combat mirrors. The laser cannon shoots a diffuse beam that the combat mirror focuses on the target. If the enemy returns fire the combat mirror will defocus the hostile beam.

Anthony Jackson has another messy solution. One can design a laser cannon without a mirror or lens, if one uses a phased array. Currently we can create phased arrays for microwaves and radars, but have no idea how to do it with visible light. It would take a major technological break-through, but it is not actually forbidden by the laws of physics. Another nifty effect of phased array emitters is that they're flat and can fire at any angle (range will suffer at extreme angles), without requiring a turret assembly.

Dr. Yo came to the horrified realization that the logical acronym for PHased Array laSER was ... aiieee!

Eric Henry prefers that particular name for Free-electron laSER.

It is possible to armor laser mirrors, and it's also possible to use optics which are inherently difficult to damage. We've had extensive discussions about this (with Rick Robinson and others) on sfconsim-l.

Armor is based on protecting an otherwise delicate mirror with grids of armor. This assumes the use of a pulsed laser. Each armor grid is a bundle of parallel sheets. When the grid is rotated, it briefly lines up with the target in passing—that's when a pulse laser can fire. With two or more grids, the window of vulnerability can be made arbitrarily short. And the duty cycle can be made unpredictable.

So, for example, a pulsed laser that could only pulse 1/10000 of the time. Incoming laser fire would only hit the mirror 1/1000 of the time. The other 99.99% of the time, it hits the grid armor.

If you want to get even fancier, you can space apart the grids by, say, 1/1000 light seconds (300km). This requires the use of an armor drone, or a pair of warships. This lets you have a duty cycle of almost 50% and still have armor protection 100% of the time. The time delay is sufficient that your photons can pass through to the target, while photons going the other way will get blocked by either one grid or the other.

Still, this grid armor is very bulky. Assuming the grids block 10% of the outgoing photons, it takes 100cm thick grid armor to provide the equivalent protection of only 10cm of solid armor. And it's possible that damage to these grids may significantly diminish their efficiency.

Another interesting possibility is to use damage resistant optics. If you use diffraction rather than reflection or refraction, you can make your focusing element arbitrarily thick. Your focusing element is a zone plate drone some distance away from the beam generator ship. The zone plate is a sturdy thick set of concentric cylinders. It can be arbitrarily thick...if you want, it can be 1m thick. All that really matters is the pattern of concentric circles. Enemy lasers could blast away at this thing all day, and it still functions perfectly so long as there's enough left over to block the concentric circles.

Such a zone plate is not the most efficient focusing element—it only focuses about 25% of the source beam's energy on target. But if you want the ultimate in damage resistance, it can't be beat.

The bottom line is...don't bother shooting at the laser optics. It can be HEAVILY armored.

Isaac Kuo in a Google+ thread

Bomb-Pumped Lasers

A special type of laser is the bomb-pumped laser. This is generally found as a missile warhead. A "submunition" is a warhead that is a single-shot bomb-pumped gamma-ray laser. The original concept was developed by Edward Teller under the name "Excalibur." Teller and Excalibur were later discredited, but the basic idea wasn't.

Here's the problem: the lasing medium in a laser has to be "pumped" or flooded with the same frequency that the laser emits. This isn't a problem with infrared or visible light, but sadly there are not many good sources of x-rays and gamma-rays. About the only good source is a detonating nuclear device, which has the distressing side-effect of vaporizing the laser. So the idea is to make a laser that can frantically manufacture one good x-ray zap in the few microseconds before it is destroyed by the bomb blast. This is the reason it is "one-shot."

(Yes, in theory, hafnium-178m2 is also a good source of gamma rays, but it has problems.)

The Excalibur units had about one hundred x-ray laser rods mounted on a nuclear device. When the hordes of evil Soviet nuclear missiles climbed into view, all one hundred lasers would lock on to different targets, then the bomb was triggered. John Schilling said that due to inefficiency each laser would emit a pulse of only 5×106 Joules, but they'd have a range of up to one hundred kilometers. Unfortunately Dr. Schilling didn't mention whiat size bomb he was basing his estimate on. The unclassified literature about Excalibur is vague, only saying the pumping nuclear device will have a yield that is smaller than your average nuclear warhead. Which could mean almost anything. My guess is under the size of the Hiroshima bomb: 15 to 20 kiloton or so.

According to Directed Energy Missile Defense in Space, a one megaton (1,000 kiloton) nuclear device releases about four billion megajoules (4.184×1015 J), but only a few percent of this will end up in the x-ray laser beams, due to the inherent inefficiency. Call it a total of about 100 million megajoules (1.0×1014 J) of x-ray laser (efficiency of 2.5%). Unfortunately they do not specify how many laser rods they are assuming in their analysis. Assuming 50 laser rods, then each rod would have a beam of 2.0×1012 joules.

Bomb-pumped lasers do not use lenses or mirrors (because there ain't no such thing as an x-ray mirror). Brian Smith-Winsemius gently pointed out to me that I do not know what I am talking about, since he works with x-ray mirrors every day.

I happen to work on a EUV (13.5nm wavelength) prototype photolithography tool. So when I read "Bomb-pumped lasers do not use lenses or mirrors (because there ain't no such thing as an x-ray mirror)." I had to stop and write. The tool I work on uses multi-layer mirrors We have to use mirrors since there are no known lenses that work with 13.5nm or x-ray light. For example, the Chandra X-Ray observatory uses a collector mirror assembly which resembles our collector optic.

Brian Smith-Winsemius

According to The Star Wars Controversy: An "International Security" Reader (edited by Steven E. Miller and Stephen Van Evera, 1986), in order to calculate the beam divergence angle of a bomb-pumped laser, use the following:

θ = 2 * (w / l)

where:

  • θ = beam divergence angle (radians)
  • w = width of lasing rod (meters)
  • l = length of lasing rod (meters)

A practical maximum length of a single laser rod is no more than five meters. Making the rod thinner decreases the divergence angle, but this is limited by diffraction, just like in more conventional lasers. Make the rod too narrow and diffraction actually makes the divergence angle larger. The width limit is:

1.22*L/l = 2*w/l

where:

  • L = wavelength of laser beam (meters)
  • w = width of lasing rod (meters)
  • l = length of lasing rod (meters)

For an x-ray laser rod of one nanometer wavelength and rod length of five meters, the optimum rod width is 0.06 millimeters. The beam divergence angle will be 20 microradians.

This relatively huge divergence further degrades the laser performance. Our 100 million megajoules are now diluted into a 20 microradian cone. If all of this energy came from a single laser rod, on a target at ten megameters (10,000 km), it would deposit about 300 kJ/cm2 over a spot 200 meters wide. Divide the energy by the number of laser rods in the Excalibur, probably around 50. That would be 6 kJ/cm2 over a spot 200 meters wide. Which isn't quite enough if you are targeting enemy ICBMs with a hardness of 10 kJ/cm2.

Note the consequence of the absence of x-ray mirrors: each laser rod will fire a laser beam out both ends of the rod. The majority of the beam will exit from the end of the rod farther from the nuclear blast, however (i.e., most of the beam will travel in the same direction as the x-rays from the blast). If the rod is perpendicular to the blast, equal beams will emerge from both ends.

A bigger draw-back is the fact that while a laser cannon requires a targeting system, Excalibur requires a targeting system for every single laser rod. Such systems are not cheap.

A more minor problem is "bomb-jiggle." Many types of fission devices use conventional explosives to squeeze the core into a critical mass. While the nuclear blast is far too swift to jog the laser rods off their targets, the conventional explosives are not. They might cause the rods to miss-aim, so when the nuclear blast triggers the x-rays, the beams are off-target. This might be avoided by using a laser-initiated fusion device.

Footfall

There is a variant on the bomb-pumped laser in Larry Niven and Jerry Pournelle's classic novel Footfall, which is arguably the best "alien invasion" novel ever written. They noticed that bomb-pumped lasers is a concept that merges seamlessly with Orion drive spacecraft. In this case the submunitions do not need a bomb. They are thrown below the pusher plate, they take aim at the enemy, then the next propulsion bomb pushes the ship and simultaneously pumps the submunitions. You can find more detalis about the spacecraft here.

Impulsively Driven Laser

Andrew Presby found an interesting document entitled "On The Feasibility of an Impulsively Driven Gamma-ray Laser" (1979) at the Federation of American Scientists website. Please note this is for gamma-ray lasers, not x-ray lasers like the discussion above. That is probably why the x-ray laser rods had a maximum length of 5 meters while these graser rods have a length of 0.05 meters.

I wish I'd found the dumb thing years ago when I taking my graduate school lasers class and looking for physics papers on bomb pumped GRASERS. The Nevada experiment described herein sounds suspiciously like the bomb pumped XRASER (xray laser) experiments in the 70s/80s codenamed Excalibur that started the chain of events that got Teller in so much trouble. Thing I cannot figure is that the device described herein seems to produce GAMMA RAYS in the 6-8 MeV range (~0.002 Ångström) which is 10000 times higher photon energy than the stuff I've found in the literature that is available on Excalibur (which was in the ~14 Ångström range).

I've never heard if this worked or not... but there you go.

Andrew Presby

The document suggest using Tantalum-180 dissolved in Lithium-7 for the lasing rods, about one part in four thousand. Alternatives are Cobalt-109 and Molybdenum-99.

The design uses the Mössbauer effect, the recoil-free emission and absorption of gamma ray photons by atoms bound in a solid form. This is important. Laser light is coherent light, where all the photons are in perfect lock-step. The trouble with x-ray and gamma-ray emission is that they are powerful enough to make the excited atom recoil in reaction. This throws off the synchronization, so that the beam is not coherent, and thus not a laser beam. The Mössbauer effect prevents this by locking the lasing atoms in a matrix of anchor atoms, thus dealing with the recoil.

It was estimated that the grasing transition energy densities of tens of kilojoules per cubic centimeter. This means a one megajoule graser could fit in a breadbox, sans bomb of course. A laser beam composed of gamma rays impacting on, say, an incoming Soviet nuclear warhead would produce a flood of neutrons generated by gamma-ray/neutron recations, burning a nice hole. And the high-energy Compton-scattered electrons would create an enormous EMP, frying the warhead's electronics.

The document describes a test for the concept. A cylindrical package five centimeters long by five centimeters in radius would be packed with 20,000 lasing needles 25 µ diameter by 5 centimeters long (I assume that µ means micrometre or micron). The needles would be composed of Lithium-7 with 0.025% Tantalum-180. The needles would be aligned in parallel with 100 µ spacing between their axes, and arranges so that the centers of no three needles would be in a straight line.

The rod assembly package would be insulated from the bomb by insulating and moderating material (from the bomb: 15 cm of space, 7 cm of lead, 20 cm of heavy water, 5 cm to the center of the rod assembly). This will ensure that only the proper radiation strikes the assembly, and to allow the assembly to survive for the few microseconds required to create the graser beam. The lead [1] attenuates the gamma radiation from the bomb, [2] slows the debris motion, [3] and blocks the x-rays that would destroy the package. The heavy water moderates the neutron output.

The beam divergence is determined by the aspect ratio, which for this package is on the order of 0.5 milliradian. This is above the diffraction limit (about 8 milliradian).

In the proposed test, a one kiloton device would be detonated to pump the graser. The five centimeter needles have a calculated gain of 2 x 104. About 9% of the nuclear energy in the grasing transition will actually escape the needles, due to the short pathlength for 6.3 keV gamma rays. The energy available is 7.3 x 1016 MeV cm-3, which means the graser beam will be a piddling little 2.6 kilojoules. Keep in mind that is was intended as a test rig, not a functioning weapon.

Non-Bomb-Pumped Lasers

Laser guru Luke Campbell thinks it not impossible to make an x-ray laser which does NOT require a nuclear device to pump it. In theory a Free Electron laser can produce any wavelength. It is possible approximate an x-ray lens by having the rays make glancing blows off dense materials.

Bottom line is an x-ray laser is technologically very challenging, but if you manage to make one you have an Unstoppable Death Ray of Stupendous Range.

Let's take a 10 MW ERC pumped FEL at just above the lead K-edge. This particular wavelength is used because lead is pretty much the heaviest non-radioactive element you can get, and at just above the highest core level absorption for a material you can get total external reflection at grazing angles - so no absorption or heating of a lead grazing incidence mirror. We will use a 1 meter diameter mirror. The Pb K-edge x-ray transition radiates at 1.4E-11 m. This gives us a divergence angle of 1.4E-11 radians. At 1 light second, we get a spot size of 5 mm, and an intensity of 5E11 W/m2.

Looking at the NIST table of x-ray attenuation coefficients, and noting that 1.4E-11 m is a 88 keV photon, we find an attenuation coefficient of about 0.5 cm2/g for iron (we'll use this for steel), 0.15 cm2/g for graphite (we'll use this for high tech carbon materials) and 0.18 cm2/g for borosilicate glass (a very rough approximation for ceramics). Since graphite has a density of 1.7 g/cm3, we get a 1/e falloff distance (attenuation length) of 4 cm. Iron, with a density of 7.9 g/cm3, has an attenuation length of 0.25 cm. Glass, density 2.2 g/cm3, has an attenuation length of 2.5 cm.

At 1 light second, therefore, the beam is depositing 2E12 W/cm3 in iron at the surface and 7E11 W/cm3 at 0.25 cm depth; 1.2E11 W/cm3 in graphite at the surface and 5E10 W/cm3 at 4 cm depth; and 2E11 W/cm3 in glass at the surface and 7E10 W/cm3 at 2.5 cm depth. Using 6E4 J/cm3 to vaporize iron initially at 300 K, we find that iron flashes to vapor within a microsecond to a depth of 0.9 cm. The glass, assumed to take 4.5E4 J/cm3 to vaporize (roughly appropriate for quartz) will flash to vapor within a microsecond to a depth of 4 cm within a microsecond. Graphite, at 1E5 J/cm3 for vaporization, will flash to vapor to a depth of 0.7 cm within a microsecond (the laser performs better if we let it dwell on graphite for a bit longer, we get a vaporization depth of 10 cm after ten microseconds).

Net conclusion - ravening death beam at one light second.

Now lets look at one light minute. The beam is now 30 cm across. This is much deeper than the attenuation length in all cases, so we will just find the radiant intensity and the equilibrium black body temperature of that intensity. We have an area of 7E-2 m2, and an intensity of 1.4E8 W/m2. You need to reach 7000 K before the irradiated surface is radiating as much energy away as heat as it is receiving as coherent x-rays. The boiling point of iron is 3023 K, the boiling point of quartz is 2503 K, and the sublimation temperature of graphite is 3640 K. All of these will be vaporized long before they stop gaining heat. At this range, the iron is subject to 5.6E8 W/cm3 at the surface, the graphite to 3.3E7 W/cm3 at the surface, and the glass to 5.6E7 W/cm3 at the surface. Using the above values for energy of vaporization, we get about 0.1 milliseconds before the iron starts to vaporize, 0.8 milliseconds before the glass starts to vaporize, and 3 milliseconds before the graphite begins to vaporize (because of its long attenuation length, once it begins to sublimate, graphite sublimates rapidly to a deep depth, while you essentially have to remove the iron layer by layer).

Net conclusion - still a ravening death beam at one light minute.

What about at one light hour? The beam is 18 meters across. The equilibrium black body temperature is 900 K. This is well below the melting point of most structural materials. Ten megawatts, however, is a lot of ionizing radiation. Any unhardened vehicle will be radiation killed at these ranges.

Luke Campbell

However, he goes on to note that in order to boost electrons to the velocities required for an X-ray free electron laser, you will need an acceleration ring approximately one freaking kilometer in diameter. So this X-ray laser would only be suitable for exceedingly huge warships, orbital fortresses, and Death Stars.

Since the time he wrote the above, Luke Campbell has reconsidered the use of lead grazing incidence mirror. Now he favors using diffraction.

I have since come to realize that at x-ray energies this high, matter cannot act as a mirror even at grazing angles (the x-rays have such a short wavelength that they interact with the atoms individually, rather than seeing them as a flat sheet - and you can't really get grazing incidence off of an individual atom). This is why I now prefer diffraction for focusing.

Luke Campbell

Particle Beams

Particle beam weapons use a similar principle to the one being utilized in the computer monitor aimed at your face right now (unless you are one of those lucky people who has a flat-panel monitor) those ancient CRT monitors and TV screens they used to use in olden times. Electrons or ions are accelerated by charged grids into a beam. They work much better in the vacuum of space than in an atmosphere, which is why there is no air inside the cathode-ray tube of your ancient monitors. Laboratory scale electron beams can have efficiencies up to 90%, but scaling up the power into a weapon-grade beam will make that efficiency plummet.

Particle beams have a advantage over lasers in that the particles have more impact damage on the target than the massless photons of a laser beam (well photons have no rest mass at least. The light pressure exerted by a laser beam pales into insignificance compared to the impact of a particle beam). There is better penetration as well, with the penetration climbing rapidly as the energy per particle increases. Particle beams deposit their energy up to several centimeters into the target, compared to the surface deposit done by lasers.

They have a disadvantage of possessing a much shorter range. The beam tends to expand the further it travels, reducing the damage density ("electrostatic bloom"). This is because all the particles in the beam have the same charge, and like charges repel, remember? Self-repulsion severely limits the density of the beam, and thus its power.

They also can be deflected by charged fields, unlike lasers. Whether the fields are natural ones around planets or artificial defense fields around spacecraft, the same fields used to accelerate the particles in the weapon can be used to fend them off.

Particle beams can be generated by linear accelerators or circular accelerators (AKA "cyclotrons"). Circular accelerators are more compact, but require massive magnets to bend the beam into a circle. This is a liability on a spacecraft where every gram counts. Linear accelerators do not require such magnets, but they can be inconveniently long.

Another challenge of producing a viable particle beam weapon is that the accelerator requires both high current and high energy. We are talking current on the order of thousand of amperes and energy on the order of gigawatts. About 1e11 to 1e12 watts over a period of 100 nanoseconds. The short time scale probably means quick power from a slowly charged capacitor bank, similar to the arrangement in a typical camera strobe. You want a very thin beam with a very high particle density, the thinner the better and the more particles the better. The faster the particles move the more particles will be in the beam over a given time, i.e., the higher the "beam particle current" and the faster this current flows, the more energy the beam will contain.

The power density is such that the accelerator would probably burn out if operated in continuous mode. It will probably be used in nanosecond pulses.

Protons are 1836 times more massive than electrons, so proton beams expand only 1/1836 times as fast as electron beams and are 1836 times harder to deflect with charged fields. Of course they also require 1836 times as much power to accelerate the protons to the same velocity as the electrons.

It is possible to neutralize the beam by adding electrons to accelerated nuclei, or subtracting electrons from negative ions. While this will eliminate electrostatic bloom, the neutralization process will also defocus the beam (to a lesser extent). As a rough guess, maximum particle beam range will be about the same as a very short-ranged laser cannon.

For a neutral particle beam, the divergence angle is influenced by: traverse motion induced by accelerator, focusing magnets operating differently on particles of different energies, and glancing collision occurring during the neutralization process. The first two can be controlled, the last cannot (due to Heisenberg's Uncertainty Principle). The divergence angle will be from one to four microradians, compared to 0.2 for conventional lasers and 20 for bomb-pumped lasers.

The source of the particles for the beam come from sophisticated gadgets with weird names like "autoresonantors", "inertial homopolar generators", and "Dundnikov surface plasma negative ion sources".

Dr. Geoffrey A. Landis had this to say:

Particle beams disperse for a lot more reasons than laser beams, unfortunately, so it's harder to give a simple formula. It will depend on things like magnetic and electric fields in the region between the source and the target (if the particles have spin, for example, they will couple to the magnetic field gradient even if they are neutral).

However, for a neutral particle beam traversing empty, field-free space, the dispersion is proportional to the temperature of the beam. Using, for the sake of a simple example, a mercury ion beam (dispersion decreases proportional to square root of atomic mass, and mercury is a convenient high-mass atom that ionizes easily), the lateral (spreading rate) velocity of the beam is:

V = 1.4 SQRT(T) m/sec, for T in Kelvins

To calculate the actual angular spread of the beam, you need to know the beam velocity. For a quick calculation, you could say it's no more than the speed of light, 300,000,000 m/sec. So the dispersion in nano-radians is 5 SQRT(T).

So, for a beam with an effective temperature of, say, 1000K, dispersion for mercury is 150 nR, or 0.15 micro-radians. Dispersion at a distance of 100,000 km would be 0.015 km, or 15 meters. A hydrogen beam would disperse SQRT(80)= 9 times more.

[note that if the beam is actually relativistic, you have to apply a relativistic correction, which I'll ignore here.]

Dr. Geoffrey A. Landis

I'm not sure I have this correct, but to put this in useful form:

θ = (5e-9 * Sqrt[BT]) * Sqrt[80/Bn]

where:

  • BT = beam temperature (Kelvin)
  • Bn = atomic number of element composing the beam (Uranium = 92, Mercury = 80, Zirconium = 30, Calcium = 20, Neon = 10, Hydrogen = 1)
  • θ = Beam divergence angle (radians)

RT = Tan(θ) * D

where:

  • D = distance from particle beam emitter to target (m)
  • RT = radius of beam at target (m)

...making sure that Tan() is set to handle radians, not degrees. Or as one big ugly unified equation:

RT = Tan((5e-9 * Sqrt[BT]) * Sqrt[80/Bn]) * D

...again making sure that Tan() is set to handle radians, not degrees. I must stress I derived this equation myself, so there is a chance it is incorrect. Use at your own risk.

Particle Beam Weaponry in Attack Vector

(ed note: this is in the context of the tabletop wargame Attack Vector: Tactical. One hexagon is 20 kilometers. Each armor layer is 5 g/cm2 of carbon.)

Building a practical particle beam that behaves in a gun-like manner at any plausible level of technology is a formidable challenge; a 1 GeV neutral hydrogen beam has a minimum theoretical beam spread of around 1 microradian, or 8 cm at a 4 hex range (80 km). Reaching this limit seems unlikely, so using a standard damage model for particle beams seems unlikely.

However, particles at energies in excess of 1 GeV/nucleon have quite excessive penetration; 1/e distance for initial penetration are on the order of 70 g/cm2 in typical shielding materials, and if you count in the effects of cascade radiation can climb over 100 g/cm2, which means you need 20 layers of AV armor to get a 1/e reduction. This allows a particle beam to kill ships without actually piercing armor.

Hardened electronics tends to have severe problems at 100-1000 grays, or 2.5 to 25 megajoules per hullspace average dose; 1 damage point to a hullspace will kill it pretty reliably. This makes a PAW a bit more lethal, per unit energy, than conventional energy weapons, but it only damages a fairly limited class of targets. Note that 5 grays is likely lethal to a human, and incapacitation in combat-relevant time takes 40+ grays. A beam intensity of 1 megajoule per square meter, with a penetration of 100 g/cm2, results in 1000 grays to surface components.

While photonic equipment is more resistant than standard electronics to transient effects caused by radiation, radiation also directly physically damages components, and there's no really good way to make them tougher other than making them bigger (and thus slower) or adding lots of backups.

The incapacitation mechanic, for a PAW, is thus chosen as having the weapon damage X components, requiring a save for each component at a specified difficulty; it then cascades down to the next layer at some reduction in power (on average, 1/10 the dose per 10 meters of length; a 10m ship gives -1 in the first region, -3 in the second, -5 in the third). X depends on the beam width; for a beam that hits a 10 square meter area, X is 1.

For simplicity in handling shielding effects of internal components, we define a 'shielding depth' for the ship, equal to 1/8 of the relevant dimension, in meters. We also assume that the components in the hull layer are behind half a shielding depth.


Particle Beam Rules

1) Shielding and Shielding Depth

To determine a ship's shielding, add 1/8 of its armor and 1/32 of its hull dimension from the relevant direction, and drop fractions; thus, a Rafik has a shielding factor of 5/8 + 33/32 or 1, from more or less any side; a Wasp has a shielding factor of 12/8 + 84/32 or 4 from the front, 1/8 + 17/32 or 0 from the side. This is a bit of a simplification, but works tolerably well unless the ship is very large. Two points of shielding depth roughly halve dose.

To determine a ship's shielding depth, simply divide relevant dimension by 8. In the above examples, the Rafik uses 4 in all directions, the Wasp uses 10 from the front, 2 from the sides.

2) PAW Effects

A PAW hits one or more surface areas. The weapon table tells you how many surface areas it hits, and the power of the attack at each area. If more areas are hit than the actual surface of the ship, any excess is lost (if it's more than the area of one region, spread to another region). To determine the actual number of components in the first region damaged, multiply the area hit by shielding depth, and divide by 5. Then, reduce power by the ship's shielding. The result is the difficulty of component saves. Certain components are more or less vulnerable, see below. If it seems useful, you may spread; each doubling in area hit reduces damage by 2.

Once you've damaged surface components, you get to cascade. Subtract the shielding depth of the ship from the weapon power, and proceed to damage the core, hitting it the same number of times as you hit the surface. Then, subtract the shielding depth again and damage the core a second time; finally, subtract the shielding depth a third time and damage the far side of the ship. Note that damage below 0 does matter on certain vulnerable components.

If you wish to add some complexity, the core is actually fairly small; if you hit more than 1/5 of the exposed surface, the remainder must be spread to the sides of the ship instead of the core.

3) PAW weapons

A standard THS weapon delivers 80 MJ per 15 meters length per 16 seconds. This will deliver 8,000 grays to the surface of one location, which is pretty much an autokill unless the location has a lot of shielding; the save is set to 10 on 1 hex. The following table gives the effects of a PAW at varying ranges (minimum area is 4, to reduce jitter)

Hexes57101420284056
PAW-30m2@104@88@616@432@264@0
PAW-45m2@113@106@412@624@448@296@0
PAW-60m2@122@124@108@816@632@464@2125@0
PAW-75m2@122@123@116@912@724@544@396@1
PAW-90m2@132@133@126@1012@424@648@496@2
PAW-105m2@132@132@134@118@916@732@564@3
PAW-120m2@142@142@144@128@1016@832@664@4

4) Component Damage

All Drive, Electronics, Reactor, and Weapons locations can be damaged by PAWs; damage control is assumed effective, though if a damage control team is in an area when it's hit by a PAW, the damage control team must save. Other mechanical components may also be vulnerable.

The Bridge location can be damaged by a PAW and suffers a +6 difficulty because it requires sophisticated computers. A light storm shelter reduces difficulty by 12, a heavy by 18.

Other locations will generally have noncritical but annoying damage to environment control, switches, loaders, and the like. Some sorts of cargo will be damaged by radiation.

Personnel, in any location, can be disabled and suffer a +6 difficulty to avoid immediate incapacitation (3200 rads = +0), and a +15 difficulty to survive long-term (140 rads = +0). Unless there is a reason for them to be elsewhere, personnel are on the bridge. The main reason for being off the bridge is damage control, though a surprised ship might have people in quarters.

Cybershells are disabled disabled like personnel; cybershells may use hardened computers if desired. Damage control teams are normally run by hardened cybershells. Radiation effects are a combination of physical damage to switches and transient effects.

Anthony Jackson (2005)
Particle Beams: The Ultimate Hard Scifi Weapon

(ed note: Redditor poster MatterBeam has a brilliant suggestion. They make the case that science fiction authors who postulate spacecraft combat using particle beams will allow the authors to justify many of the cherished space combat tropes common in media SF. There is some room for argument, but it does provide authors with a lot of cover.)

Particle beam weapons are the ultimate scifi weapon for hard science fiction authors and worldbuilders.

What is it?

You know about particle accelerators: A handful of atoms are ionized (stripped of their electrons) and accelerated to near light speed. A particle beam is the same concept, with much greater energies and many more atoms, and it is open ended. The relativistic stream of particles can hit targets thousands of kilometers away with great accuracy.

How are they different from lasers?

Lasers are focused with large, fragile mirrors. Particle beams are focused using magnets.

Lasers have greater range due to their smaller spot size.

Particle beams have several damage modes, lasers have only one. Lasers do surface thermal damage. Continuous laser beams gradually melt through the target, while pulsed beams try to make the surface material heat up so quickly, it explodes away in chunks. Particle beams penetrate through armor, depositing energy throughout the entire target volume. They are also capable of being pulsed. They have a secondary damage mechanic that is called Bremsstrahlung radiation. Charge particles, when slowed down by armor, emit X-rays inside the target. This is very damaging to electronics.

Lasers are less efficient than particle beams due to the necessity of converting electrical energy into thermal/optical energy.

Lasers travel at light speed and can only be stopped by physical barriers. Particle beam weapons can use several different particles (from the lightest electrons to the heaviest uranium ions) and travel at varying near-light speeds. Their path can be altered by magnetic and electrostatic fields if not properly neutralized.

Why are they the ultimate scifi weapon?

They allow authors to justify the majority of tropes that make science fiction 'fun'. With lasers and their extreme range, battles are no more than point-click minigames between legions of automated drones bouncing and refocusing a beam from a laser-generating battlestation.

With particle beams:

-We can justify humans in space warships. Due to Bremsstrahlung radiation, electronics are especially vulnerable to particle beam weapons. Humans serve as a backup, and the simple act of placing them on the warship creates a large variety of warship design options that do not require greater investment, mainly the ability to do repairs, second-by-second decision making and recovering vessels from partial destruction (soft-kills).

-It is easier to defend against lasers than particle beams: while lasers focus more energy per area than particle beams at all distances, they are much more vulnerable to reflective surfaces or armor that dissipates surface heat. Particle beams will penetrate deep into armor material instead.

-We can justify dedicated armor. Against lasers, the most efficient armor is simply placing your propellant outside of your hull. Kilogram by kilogram, nothing is more mass-efficient than a block of shapeless propellant with your spaceship embedded inside. Due to to the penetrative capability of particle beams, you can justify having proper warships: while lasers can be no more than an ice trawler with a laser generator attached, particle beam warships will have to be properly protected with high-z materials, that is, materials with a lot of electrons per mass unit. Examples include metal foams filled with hydrogen or water.

-Battle ranges are shorter. While lasers can deposit their energies over vast distances, particle beams are more limited by bloom effects, even more so if they are charged. For example, a 1MJ pulse of mercury particles, neutralized by an electron beam, would have a spot size of 15m at 100000km. A laser would have a spot size of a few cm at that same distance. Why is this important? Maneuvering requires dedicated high-thrust engines instead of feeble milligee drives. You don't have to deal with light lag. The targets aren't thermal specks at the limit of your imagery resolution, but spaceships orbiting the same planet as you are...

-We can justify 'shell' designs. Laser warships come in two flavours: the telescope and the battlestation. The telescope is a flimsy assemblage of struts, nuclear reactor and laser generator working at the the shortest frequency manageable. On top of all this is a massive focusing mirror. It accelerates slowly and doesn't do anything except shoot at targets so far away you can only resolve a drive signature. This is because range is king. The second flavor is a single, huge space station containing several reactors dumping their waste heat into a hollowed out asteroid or an ice cube of several kilotons. The laser beam is bounced from mirror drone to mirror drone, refocused at each step, over millions of kilometers. This means spaceships start being focused and melted before they even leave their orbits... from another planet away. It is the end of 'spaceships', but actual planets shooting at each other. In both cases, the 'warships' resemble something NASA built.

Particle beam warships would need to be enclosed in armor, and their firing ports are millimeters wide. They would resemble the traditional science fiction warship design, based on naval warships, much closer.

-We can justify the conversion of space technology to military use Lasers can be used for tight-beam communication, but so can radio. There is no reason for a spacefaring nation to develop high intensity laser technology unless it is for military use. It becomes hard for the scifi author to explain how we went from peaceful space transport to megawatt beams in a short span of time. Particle beam technology could be no more than a repurposing of the magnetic focusing assemblies found in thermo-electric and plasma rocket drives. It is a much more plausible transition in purpose from peaceful to military.

-We can create more interesting tactical choices: Particle beams can use several types of 'ammunition'. Electron beams are short-ranged, but cause deadly Bremsstrahlung radiation. Heavy ions disperse much less and penetrate armor better. Neutralized beams need two parallel beams positively and negatively charged ions, but have the least dispersion. Magnetic shielding can reduce the damage caused by ion beams, and even deflect them entirely. Neutralized beams can be slightly destabilized by magnetic fields, or even shot down by electron beams. All these are much more ineteresting choices than the default 'shoot as soon as targets are detected' that comes with lasers.

-We can do away with drone sub-weapon fleets; As mentioned before, a laser battlestation with even moderate power levels and a flett of cheap mirror drones can shoot down spaceships before they leave Mars. It would end exciting space warfare. With the ability to incapacitate 'cheap' autonomous drones, ion beams can quickly make them less cost effective than 'full' warships carrying humans.

Electrostatics, Neutrons, and Space Charge

While particles cannot travel at the speed of light, they can get close enough that it is hard to tell the difference. Unfortunately, particle beams do obey the inverse-square law.

A beam of neutrons does not suffer from electrostatic bloom since they have no charge, nor could they be deflected by charged fields. However, this also means it is difficult to accelerate the neutrons in the first place (and if you discovered a new way to do it, chances are it too could be used as a defense). Without electrostatic bloom neutron beams are only limited by "thermal bloom". Brett Evill says this will give a neutron beam an effective range of 10,000 km, but he doesn't mention the details of this estimate. Nelson Navarro is of the opinion that a science fictional heavy neutron beam could be produced by a science fictionally efficient method of breaking up deuterium nuclei.

Another problem is one shared by ion drives, the "space charge." If you keep shooting off electron beams you will build up a strong positive charge on your ship. At some point the charge will become strong enough to bend the beam. And the moment your ship tries to dock with another it will be similar to scuffing your shoes on the rug and touching the doorknob. Except instead of a tiny spark it will be a huge arc that will blow all your circuit breakers and spot-weld the ships together.

Don't try to neutralize the charge by firing off positively charged proton beams. John Schilling warns that space is filled with an extremely low-density, but conductive, plasma. You try to eject charge from your ship, and the ship itself becomes part of a current loop. Not only is the current flowing through the hull (or trying to) likely to cause problems, but all those electrons or protons being sucked in produce X-rays on hitting the hull.

Isaac Kuo:

Anyway, getting back to your original article...I understand the motivation for wanting missiles and lasers to have an uneasy balance. I tried for years for this to be a guiding principle, for the same reason you have.

But I've pretty much given up on the idea. The fundamental problem is that missiles aren't fun. They are a pain to keep track of, in any numbers, and missile combat basically just boils down to numbers.

If you want things to be tactically fun, it may be a better idea to look at different sorts of weapon systems instead. In particular, electron beams can be an interesting weapon system in your setting. Electron beams can be interesting complements to laser weapons, because they can share hardware with a free electron laser.

A couple years ago, I came up with this interesting way to use a planet's ambient magnetic field to focus electron beams over long distances. But the beam spot size is smallest when shooting perpendicular to the magnetic field. The further the target is from this, the larger the beam spot size. Interestingly, the beam spot size does NOT directly depend on range to target--only direction to target, and strength of the ambient magnetic field. (This strength diminishes quickly with distance from the planet, so there is in fact a practical range limit.)

The bottom line is that if you're setting involves mainly space combat near planets, an electron beam is an interesting complement to laser armament. There are some directions and ranges where the electron beam is superior, and others where the laser is superior.

Furthermore, different sized vehicles have different defensive abilities. A relatively large vehicle can completely defend itself from an electron beam with a strong large magnetic field. Small vehicles are vulnerable, though, and an electron beam could be an order of magnitude more efficient at delivering beam energy. (Free electron laser will only convert a fraction of the electron beam's energy into photons, and then the target material may be reflective enough to only absorb a fraction of the laser's energy.)

So, even though the electron beam may be useless against large warships, it's so much more effective against small warships that it's still a useful secondary weapons mode.

Also, the firing port of an electron beam weapon is tiny. The example I calculated out was a weapon with a 4mm spot size and a 4mm firing port. The beam can actually be aimed with electromagnets even after the firing port. Anyway, it's a lot less bulky than a laser turret.

And then there's space weather. Besides the fact that different planets have magnetic fields of different strengths, these magnetic fields are constantly shifting. This results in "windiness" that throws off your electron beam's aim. Earth's magnetic field shifts in the timeframe of around a second, so it's going to be impossible to stay on target at .5 light seconds away. Your practical range is likely much lower than that. Space weather can result in large variations in magnetic field strength--affecting beam spot size--as well as how "windy" it is. The effect on beam spot size effectively changes how wide your firing arc is against a particular target (a smaller spot can penetrate deeper). The effect on windiness changes the effective range of the weapon.

My point is...it's an interesting weapons system that can make tactical maneuvers an interesting puzzle. It's not just about numbers, you've got firing arcs that matter. You've got formations to cover each other's blind spots. You've got situations where a polar orbit is radically different, tactically, than an equatorial orbit, even when neither side has any relevant surface assets.

And from a playability perspective, a really nice thing about these firing arcs is that they don't depend upon dealing with complex 3D rotations. They depend purely upon a spacecraft's position, not its orientation.


Ray McVay:

My god...Jupiter's magnetic field is the largest thing in the entire system. The Jovians will have a weapon that can vape every KKV and Patrol Rocket that's thrown against them. You've figured out how they can wipe out the UN forces stationed in the Jovian system fast enough to put the Trans-Titanian Convoys at risk. Brilliant!


Isaac Kuo:

As for Jupiter's magnetic field...hmm...at low Jupiter orbit, it's about 10 times stronger than Earth's at LEO, but there's no compelling reason to be hanging out in low Jupiter orbit.

I'm not sure how strong Jupiter's magnetic field is at Io, but magnetic field strength drops of roughly with 1/r^3. That implies a field strength drop of around 6 cubed at Io, or around 1/20th the strength of Earth's magnetic field at LEO.

So basically...usable, but only about as potent as they are at medium Earth orbit. I'm actually surprised at this. They should have very long range, however, compared to small diameter laser weapons. Practical range depends on how "windy" the magnetosphere is, and I really don't know that.

Isaac Kuo in a Google+ thread
Rainbow Electron Beam

Long Range Electron Beams using Earth's Magnetic Field for Focusing

TL;DR: A novel concept for 100kW to 10MW electron beams can be used for ballistic missile defense, space junk sweeping, and cheap access to space.

This is a weird idea I had a couple years ago, which is basically an unusual alternative to lasers for a long range beam. Normally, electron beams are not considered suitable for long range due to self repulsion. A beam that starts off narrow will bloom outward because the electrons repel each other. You can counter this by starting with a wide beam that focuses inward, but...I've done numbers on that idea; it's not very good.

But using the ambient magnetic field, it's possible to do something completely different. The ambient magnetic field will bend the trajectories of electrons into circular arcs. It's possible to let the beam fan out wide, and then have the ambient magnetic field refocus the electrons back together into tight focus by the time they reach the target.

The beam is fanned out in a rainbow spectrum, with the fastest ions on the inner edge and the slowest ions on the outer edge. Fanning the beam out results in a wide plane of low charge density, greatly reducing self repulsion. The ambient magnetic field deflects the slower ions more than the faster ions. This causes the beam to straighten out parallel and then converge back inward.

From above, the beam looks like a crescent shape. One tip of the crescent is at the firing spacecraft; the other tip of the crescent is at the target.

From the side, the beam looks narrow in the middle. It's thicker at the firing end and the target end. How much thicker?

I'll start with a baseline example of a 100kW beam of 100MeV protons from a 5m long linac. We'll leave the ambient magnetic field a variable, "B". Some sample values:

  • B = 3e-5T : somewhere in LEO
  • B = 1e-7T : somewhere in GEO
  • B = 1e-9T : interplanetary

I'll assume a beam spectrum of 10% velocity (or 20% energy). With non-relativistic calculations for simplicity, the angular deflection rate is constant with time:

radians/s = charge/mass * B

The beam is parallel when it's halfway to the target, so the angular width of the beam at the tips is equal to:

  • angular beamwidth = charge/mass * B * (t1 - t2)
  • = charge/mass * B * (0.5*dist/(1.05*v) - 0.5*dist/(0.95*v))
  • = charge/mass * B * dist/v * 0.5 * 0.1
  • = charge/mass * B * dist/v * 0.05

The rate at which the width of the beam converges/diverges at the tips is:

  • v*angular beamwidth = charge/mass * B * dist * 0.05
  • = 1.60e-19C / 1.67e-27kg * B * dist * 0.05
  • = 0.958e8C/kg * B * dist * 0.05
  • = 4.79e6 C/kg * B * dist

Okay, now let's switch to looking at charge density.

100kW/100MeV is 1 milliamp, and beam velocity is about 138000km/s, so linear charge density is:

lcd = 1e-3A / (1.38e8m/s) = 7.25e-12C/m

Electric field strength will always be less than or equal to the strength if the charge were an infinite plane, or:

lcd/w /e0 = 0.818V/w

We now have the basics required to estimate vertical beam spread. The remaining input variables are:

  • dist = distance to target
  • B = ambient magnetic field

We use the infinite plane field strength to get a vertical acceleration of:

  • acc = (0.958e8C/kg)*0.818V/w
  • = (0.958e8C/kg) * 0.818V / (4.79e6 C/kg * B * dist * t)
  • = 16.4V / (B*dist*t)

Integrating to get vertical velocity, we have

v = 16.4V/B/dist * ln(t) + C1

Integrating to get vertical position, we have

  • h = 16.4V/B/dist * [ (tln(t)-t) + C1*t + C2 ]
  • = 16.4V/B/dist * [ t * (ln(t)-1+C1) + C2 ]

To simplify the math, I'll use this formula all the way from the muzzle to the halfway point. This overestimates the early acceleration, but underestimates the late acceleration. It will get in the right ballpark, though, since it never underestimates by more than a factor of 1:2.

If we want a local minima at |t| = T, we use C1 = -ln(T) and C2 = T for

h = 16.4V/B/dist * [ t * (ln(t)-1-ln(T)) + T ]

(Derivation left as exercise for the reader. "Mathematica says it's so.")

At t=0, we have:

h = 16.4V/B/dist * T

Since T is halfway there, it's 0.5*dist/v, so

h = 0.5*16.4V/B/(1.38e8m/s) = 5.94e-8Tm/B

Aha! What a surprise! The vertical spread doesn't depend on range! It makes intuitive sense, though. On the one hand, greater range gives more time to spread. On the other hand, the beam can be wider, reducing the charge density. It turns out the two factors cancel each other out. Wunderbar!

The vertical diameter of the beam is twice h, so it's:

2h = 1.19e-7Tm/B

With v = 138000km/s, we have:

  • B = 3e-5T : somewhere in LEO
  • 2h = 0.004m = 4mm
  • B = 1e-7T : somewhere in GEO
  • 2h = 1.2m
  • B = 1e-9T : interplanetary
  • 2h = 120m

So, at LEO, this electron beam can focus onto a 4mm x 0mm spot on the target (obviously, the actual width is limited by diffraction limits). But at the other locations, spots size is excessive.

This might be addressed by placing the minimum height closer to the target rather than at the midway point, but the math gets a lot uglier.

So how do things scale if we change things? If we keep the beam energy the same, but the spot height is inversely proportional to the amperage. In other words, if we keep the beam velocity the same but change the power, spot height is inversely proportional to power. If we increase the beam energy (increasing the length of the linac), but keep the power the same, amperage is reduced but fanning width is also reduced by the square root of the beam energy. The overall effect that spot height is inversely proportional to the square root of the beam energy (or length of the linac).

Now, this math assumes that the beam is being aimed perpendicular to the magnetic field lines. If it's being aimed at an angle to the magnetic field lines, only the component perpendicular to the beam helps fan/focus it. In other words, the spot size is inversely proportional to the sin of the aiming angle (with respect to the magnetic field). Aim perpendicular to the magnetic field, and the spot size is small. Aim vaguely along the magnetic field, and the spot size is big.

So what can we do with this electron beam?

  1. Global Ballistic Missile Defense
  2. Space Junk Sweeping
  3. Cheap Access to Space

1) Global Ballistic Missile Defense

The obvious thing is a weapons laser. A 100kW beam aimed onto a 4mm spot is actually superior to the 1000kW laser beam of Airborne Laser, because it's concentrated onto a much smaller spot--much better penetration. It also has superior range...basically, it can hit anything it can see out to the horizon. It would only take three of them to provide full coverage to the entire world, whereas ABL could only cover a small region a few hundred miles across.

The "bad" news is that this electron beam can only hit ballistic missiles. It can't penetrate Earth's atmosphere, so there's no way to use it to replace drone strikes. (This may be seen as either a feature or a flaw.)

2) Space Junk Sweeping

The small spot size and long range means that it could be used for eliminating small pieces of space debris. Electron impacts would sputter ions, producing reaction thrust directly and/or inducing temporary charge sufficient to deflect the orbits into Earth crossing orbits (where it burns up in Earth's upper atmosphere).

3) Cheap Access to Space

With a 10MW version, the spot size is 40cm. If the target is a cooperative, it can include a 40cm diameter magnet to focus the beam into a small point. The target might be little more than a block of ice, boosted up to orbital speeds by the electron beam vaporizing a conical crater/nozzle into the block. A small jet could sling two suborbital rockets under its wings, repeatedly launching a couple payloads per flight.

Isaac Kuo in a Google+ thread

Bremsstrahlung

Powering up a particle beam to the point where it can cut armor is difficult. But there is another option: death by "Bremsstrahlung".

Consider the x-ray tube in your dentist's office. It is basically an electron beam striking a metal target. Now, what if the electron beam was a particle beam weapon and the metal target was the hull of the enemy spacecraft? A hypothetical observer on the far side of the ship could make a nifty x-ray photo revealing the skeletons of crew members dying in agony of radiation poisoning.

Please note that Bremsstrahlung only occurs with charged particle beams, it doesn't happen with beams of neutrons.

The particle beam weapons postulated for Star Wars missile defense were to disable missiles by damaging the sensitive electronics via radiation, not by carving the missiles into pieces. An APS directed-energy weapons study written for the Strategic Defense Initiative estimated that in order to disable an ICBM, a particle beam had power requirements between 100 and 1,000 megawatts, depending on range and retargeting rate.

Anthony Jackson says if you crank up your particles to a few GeV per nucleon they will be in the soft end of the spectrum of primary cosmic rays. Each particle will be highly penetrating, and you no longer need to actually focus the beam. Just apply a couple megajoules per square meter and everything dies (unless it's behind a huge amount of shielding or is basically operating at pre-microchip levels of automation. Neither is an option for a surface mounted weapon turret.). We are talking about a surface radiation level of over 500 grays. Such a cosmic ray beam would require armor with a TVT (for radiation purposes) peaking at 200-300 g/cm2.

Also note that if the particles are moving a relativistic velocities higher than, say, 90% c, you will have about the same energy release if the particles are matter or antimatter. In other words, it is pointless for relativistic particle beam weapons to use antimatter, with all the added complexity due to antimatter manufacture and storage.

Ships that expect to be fired upon by particle beam weapons would be well advised to add a layer of paraffin or other particle radiation armor on the outside of their metal hull, to prevent the beam from generating Bremsstrahlung with the hull.

SDI Neutral Particle Beam

One of the more exotic weapon proposals that came out of the Strategic Defense Initiative was orbital neutral particle beam weapons.

As previously mentioned, charged particle beams suffer from electrostatic bloom, which drastically limits the range. It is possible to neutralize the beam by adding electrons to accelerated nuclei, or subtracting electrons from negative ions. This creates a neutral particle beam.

While this will eliminate electrostatic bloom, the neutralization process will also defocus the beam (to a lesser extent). As a rough guess, maximum particle beam range will be about the same as a very short-ranged laser cannon.

For a neutral particle beam, the divergence angle is influenced by: traverse motion induced by accelerator, focusing magnets operating differently on particles of different energies, and glancing collision occurring during the neutralization process. The first two can be controlled, the last cannot (due to Heisenberg's Uncertainty Principle). The divergence angle will be from one to four microradians, compared to 0.2 for conventional lasers and 20 for bomb-pumped lasers.

Most of the images below are of very poor quality, and many of the details are still classified. The scale of these weapons is unknown, but they are huge. Some are "folded", with a U-shaped section. This is a desperate attempt to cut the length in half. Scott Lowther guesstimates the entire weapon is on the order of 100 meters long or so.

Kinetic Kill Weapons

Kinetic Kill weapons are unguided missiles that have no warheads. Bullets and artillery shells in other words. They can be a simple as a bucket of rocks dumped in the ship's wake. Since they are basically solid lumps of matter they are much cheaper than a missile. They cannot be jammed, but by the same token they do not home in on the target. The damage they do depends upon the relative velocity between the kinetic lump and the target ship.

A sort of hybrid would be a missile which explodes into a cloud of deadly shrapnel that the enemy ship plows through, screaming.

Go to the Rocketpunk Manifesto, and read Kinetics, Part 1 and Kinetics, Part 2 The Killer Bus.

In case it is not obvious, if the weapon projectile has no rocket engine strapped to it (as do missiles), the weapon is not recoiless. Cannons, coil guns, and rail guns all have recoil due to Newton's third law. The weapon will kick your warship like a mule every time you fire it, just like when a soldier fires a heavy calibre firearm.

In fact, the propulsion system know as a mass driver is basically a coil gun optimized as a propulsion system rather than optimzed as a weapon. This means that kinetic weapons can be used as crude propulsion systems in an emergency.

Kinetic kill weapons give you the tactical option to create terrain in the void of space in order to herd your opponent. Find the trajectories you want to deny to your opponent and fill them with cheap kinetic energy projectiles, thus forcing them to use trajectories advantageous to you.

Equations

The damage inflicted can be calculated by the equation below. The same equations will also apply when one ship rams another, of course with added damage from exploding missile magazines, unstable fuel supplies, and out of control power plants. In a ramming, you will have to calculate the equation twice, once to figure damage inflicted on the rammed ship, the second time to calculate damage inflicted on the ramming ship.

To get some idea of the amount of damage represented by a given amount of Joules, refer to the Boom Table.

Eric Rozier has an on-line calculator for kinetic kill weapons.

Please note that it is relative velocity that is important. If your ship is quote "standing still" unquote, and if the enemy is tearing past you at seven kilometers per second, and if you leisurely toss an empty beer can into the path of the enemy, the relative velocity will be 7 km/s and the beer can will do severe damage to the enemy ship (if the beer can masses 0.1 kilogram, it will do 2,450,000 Joules of damage). So even though the beer can has practically zero velocity from your standpoint, from the standpoint of the soon-to-be-noseless ship the can has the velocity of a bat out of you-know-where.

Ke = 0.5 * M * V2

where:

  • Ke = kinetic energy (Joules)
  • M = mass of projectile (kg)
  • V = velocity of projectile relative to target (m/s)

Wp = Ke * (1 / We)

where:

  • Wp = power required by weapon to fire one projectile (Joules)
  • Ke = kinetic energy of one weapon projectile (Joules)
  • We = efficiency of the weapon (0.0 = 0%, 1.0 = 100%)

Rick Robinson's First Law of Space Combat states that:

Rick Robinson's First Law of Space Combat

An object impacting at 3 km/sec delivers kinetic energy equal to its mass in TNT.

Rick Robinson

In other words there are 4,500,000 joules in one kilogram of TNT (3,0002m/s * 0.5 = 4.5e6). This means a stupid bolder traveling at 2,000 km/sec relative has about 400 kilo-Ricks of damage (i.e., each ton of rock will do the damage equivalent of 2e12 / 4.5e6 = 400 kilotons of TNT or about 20 Hiroshima bombs combined).

Ricks = (0.5 * V2) / 4.5e6

where:

  • V = velocity of projectile relative to target (m/s)
  • Ricks = kilograms of TNT worth of kinetic energy per kilogram of projectile

So a projectile moving at 200 km/sec (20,000 m/s) would have about 4,000 Ricks (4 kilo-Ricks) of damage, approximately the same as a standard one-kiloton-yield nuclear weapon. By that I mean it has the same damage per kilogram as a nuke, counting all the nuke's framework, electronics, fissionable material, and whatnot. (for the projectile to do the same damage as a standard nuke, it would need to be the same mass as a standard nuke, about 250 kilograms) A projectile moving at 3,500 km/sec would have about one mega-Rick, which is the same damage per kilogram as the ultra-compact 475-kiloton-yield W-88 nuclear warhead.

As a rule of thumb, anything with more than 100 Ricks (i.e., over 30 km/sec relative) does weapons-grade levels of damage. As an even more shaky rule of thumb, anything with more than 4,000 Ricks (i.e., over 190 km/sec relative) does nuclear warhead levels of damage. This is based on the assumption that a nuclear weapon has about a 4,000 fold increase in energy per kg released versus TNT.

And if you are thinking in terms of bombarding your enemy with asteroids, as a rule of thumb an asteroid's mass will be:

Ma = 1.47e4 * (Ra3)

where:

  • Ma = mass of asteroid (kg)
  • Ra = radius of asteroid (m)
Example

The wet navy battleship Iowa had 16-inch guns. They fired shells which massed about 2000 pounds (907 kg), carried a charge of 145 pounds (54 kg) of high explosive, and traveled at about 820 meters per second. By the kinetic equation above, they contained about 3.0e8 joules of kinetic energy. There are about 4.184e6 joules per kilogram of TNT (which is different from the value used in Rick Robinson's equation, if this annoys you, take it up with him) so the explosive charge contains about 2.3e8 joules of energy.

This means one 16-inch shell does about 3.0e8+2.3e8 = 5.3e8 joules of damage.

Floyd has spent the last 8.6 boring months in the good scoutship Peek-A-Boo, traveling from Mars to Earth in a hohmann orbit. Suddenly he notices a convoy raider from the Asteroid Revolutionary Navy accelerating from low Earth orbit into a Martian hohmann transfer orbit.

Unfortunately for Floyd, scoutships are unarmed. But since the two ships are traveling in opposite directions at a fair speed, anything Floyd can throw at the raider will be good for quite a few Ricks. How massive an object will Floyd have to hurl in order to inflict the same damage as a 16-inch shell?

For the raider to leave LEO and enter Earth Escape orbit takes about 3.17 km/s. To leave Earth Escape and enter Mars Hohmann orbit takes 2.95 km/s. So the raider has about 6.12 km/s relative to Earth.

Since Floyd is on the opposite leg of an Earth-Mars hohmann, he is also doing 6.12 km/s relative to Earth, but with an opposite vector. So relative to the raider, Floyd moving at 6.12 + 6.12 = 12.24 km/s.

Ke = 0.5 * M * V2

therefore

M = Ke / (0.5 * V2)

Ke = 5.3e8 joules and V = 12,240 m/s so M = 7.08 kg (about 15 pounds). A 15 pound object will do as much damage as a 16-inch shell.

At this speed, anything striking the raider will have 16.6 Ricks!

Sneaky the cat watches with bright interest as a space-suited Floyd carries the cat's litterbox into the airlock, and empties it into the path of the raider...

SECTION 8: KINETIC WEAPONS

Kinetic weapons are the counterpart to lasers.  Almost all space weapons to date have been kinetic-based (the balance being nuclear).  The simple fact is that kinetic weapons are a natural outgrowth of space travel, which is about moving stuff from one point to another, generally very fast.  The only difference is that the kinetic does not have to come to a stop.  

Kinetics are best described by Robinson’s First Law of Space Warfare: anything hitting something at 3 km/s has kinetic energy equal to its own mass in TNT, or one Rick.  Ricks scale with the square of velocity, so something at 6 km/s has 4 Ricks.  Given that any scenario with enough human space presence for a war virtually requires transit velocities well above that, kinetics are both lethal and relatively cheap.

Kinetics can be deployed in three ways: lancers, missiles, and projectors.  Each has a different set of characteristics which would significantly alter their employment.

Lancers are the tactic of pointing a spaceship at a target, dumping a bunch of stuff, and turning away.  There is a discussion on tactical lancers above, in the section on fighters.  In the PMF, lancers are only useful at the strategic level, and usually are hard to distinguish from missiles. The IPBM mentioned above is an example of a strategic lancer/missile bus. If the lancer is reusable, then for a given payload this is the cheapest option, both in the launcher and in the projectiles, assuming that the recovery works.  Any lancer is likely to drop a number of submunitions, like the SCODs described below.

Missiles are more expensive in terms of projectiles, but cheap in launchers. They suffer from serious performance issues or high costs. Either they are cheap, small, chemfuel, and low-velocity, or they're easy to confuse with a lancer. There's some overlap with the other two types of kinetics, as all kinetic projectiles are self-steering. Missiles are also the most practical way to deploy specialty weapons like nuclear warheads.  A missile is unlikely to be a unitary weapon.  Instead it will carry submunitions, which have been termed Soda Cans of Death, or SCODs, for their size and general shape.  This is generally believed to be the minimum practical size for this sort of projectile.  Whether it will be a long-rod or some sort of bursting projectile depends on exactly what the technical parameters are.  In any case, the submunitions each have to have the seeker system and thrusters required of larger projectiles.

Projectors are anything that launches a projectile from a ship. Examples include railguns, coilguns, and chemical guns. They have cheap projectiles and expensive launchers. They are similar to lasers in terms of use, as both are rather expensive and have large reusable components, though projectors are affected by both velocity and position. The use of projectors instead of lasers will depend on the specific technical details involved, as well as the operational requirements of the builder.  A single projector round is also likely to be very similar to an SCOD.

There are several interesting principles of kinetics.  The first is that virtually all kinetics will be guided.  Unless the range is truly point-blank or the kinetics are very improvised, guidance will be ubiquitous.  Take the 10-meter-diameter laserstar from the previous section.  It’s now being shot at with unguided kinetics that have a time of flight of 1 minute.  The target area is thus 78.54 m2 and assuming that the projectile is a point and is fired at the center of the ship, the vessel will have to accelerate at .002778 m/s2 to dodge it, using delta-V of .16667 m/s.  This is within the acceleration capabilities of even most nuclear-electric drives, and the use of thrusters capable of higher accelerations reduces the delta-V requirement farther, to a theoretical minimum of .08333 m/s for an instantaneous delta-V.

Now let’s look at the possibility of firing multiple projectiles.  If the thrusters are capable of accelerating at 1 m/s2, the laserstar will move 1800 m during the time it takes the projectiles to reach the vessel, for a total target area of 10.18e6 m2.  This gives a total of 129600 projectiles required to reasonably ensure one hit on the target vessel if they are spread evenly throughout the area.  If the projectiles mass 25 kg each, that is a total of 3,240 metric tons of projectiles required to achieve a hit.  This analysis neglects the issues of point-defense, which is likely to raise the required mass by at least an order of magnitude, and the fact that 60 seconds is a far lower time of flight than is likely to occur in combat.  Guided projectiles would significantly reduce the mass requirements.  Take the most primitive possible projectile, which homes for the last 10 seconds at 1 m/s2.  Its potential hit circle would cover an area of 7854 m2 if the ship is treated as a point (equivalent to demanding a hit on the center of the ship), or 9503 m2 if a hit anywhere on the ship is accepted.  However, the fact that these are circles, which do not stack well, would tend to reduce the practical cross-sections if complete coverage is demanded to 5000 m2 and 6050 m2 respectively.  Once this is taken into account, the center-hit requires 2036 projectiles, and the circle-hit requires only 1683, for total projectile masses of only 50.9 tons and 42.075 tons respectively.  In reality, a projectile would have significantly better performance, probably equal to or better than that of the ship at this range, meaning only a single projectile would be needed under this model.  

Some have pointed out that the cost of guided projectiles would be significantly higher than that of an unguided one.  The problem is that the analysis above shows such as significant mass advantage for guided projectiles that it is virtually impossible to see a situation in which unguided projectiles are more cost-effective, particularly given transportation costs.  Against the laserstar above, the time of flight for an unguided projectile would have to be below 3.16 seconds to get a hit.  At expected flight projectile velocities, the range would be in the tens or hundreds of kilometers, far closer than even point-blank range for lasers.

Before we go farther, a brief discussion of kinetic impacts is in order.  What happens when objects impact at the velocities in question is totally outside everyday experience and somewhat counterintuitive.  In collisions at or above 3 km/s, kinetic energy dominates over momentum, and impacts resemble explosions more than anything.  Despite this, and contrary to intuition and popular belief, the shape of the projectile does strongly affect the dynamics of the collision.  

A hypervelocity impact can be divided into four phases.  First, there is a transient shock, and the front of the projectile is brought to rest relative to the target.  This produces very high temperatures and pressures, and a bright flash.  During the next phase, the projectile continues to penetrate into the target, but is eroded as it does so.  The length of this phase depends on the length of the projectile and the speed of sound in it.  If the object penetrates the target, the shocked portion will disintegrate, spewing fragments.  These fragments will come from both projectile and target, and will separate into two cones, one that is basically normal to the surface just penetrated while the other continues at about the same angle the projectile hit at.  At the same time, if a portion of the projectile is unshocked, it will continue onward, penetrating deeper into the ship.  This could allow a long-rod to go through multiple compartments, getting shorter each time, and leaving clouds of fragments in its wake.  The fragments would spread, distributing the damage over a greater area.  If an outer whipple shield was used to shock the projectile, the spreading fragment cloud might lack the energy required to penetrate the main armor behind it.  Even if it fails to penetrate, however, spalling (shockwaves knocking fragments off the back of the armor) could result, with unpleasant consequences for anyone on the other side.  

If the object fails to penetrate, then cavitation occurs when the projectile is completely eroded, as the cavity continues to expand under its own momentum.  During the fourth phase, the cavity might shrink slightly as the material rebounds.  These two might also occur if the impactor penetrates, but the effect is not terribly important compared to the effects of penetration.  For long-rods most of the damage occurs during the erosion, while spheres and other squat shapes do most of their damage during cavitation.

At velocities above 30 km/s, the shocked portion will turn into plasma instead, which is likely to behave in a similar manner to the fragments, but spread more quickly, reducing penetration farther.  

Much of the above is speculation, and should be taken with a grain of salt.  There is virtually no experimental data available above about 10 km/s, and very little data about long-rods, materials, and large masses at velocities above those achieved with conventional guns.  However, there are some principles that can be firmly established.  Most importantly, there must be a sufficient standoff between the Whipple shield and the main armor.  If the standoff is too small, the inner armor is hit by a concentrated cloud of fragments (or plasma) and penetrated.  The required standoff can be reduced by packing the gap with some form of insulation, such as carbon nanotubes or aramid fibers.  These materials would also improve the performance of the armor against lasers. (Thanks to Dr. William Schonberg of Missouri S&T for much the information on kinetic impacts. For more details, see Space Weapons, Earth Wars, p140. Another source is The Effects of Directed Energy Weapons, although it does contain a few errors.)

Long-rod projectiles must be guided for maximum effectiveness.  If a long-rod impacts off-center, it is likely to be destroyed by the whipple shield, and most of its effectiveness lost.  If there is no whipple shield, it appears that the prospects are somewhat better.  The critical angle at which the back end of the rod will impact the side of the hole varies, but it appears to be somewhere under 10° in most cases, although it rises as the impact velocity increases.  

Most of the information that does exist on long-rod penetration at high velocity comes from research into ballistic missile defense, and works on this subject have provided interesting concepts.  Notably, the long rods don’t have to be circular in cross-section.  Hexagonal rods have better packing efficiency than circular rods, allowing more of them to be crammed into less volume.  Other rod forms, most notably star shapes, will provide better structural performance than circular rods, including significantly improved penetration in high-obliquity impacts.  Note that obliquity is different from yaw.  Yaw is misalignment between the axis of the rod and the line of impact, while obliquity is the angle between the line of impact and the line perpendicular to the surface.  Star-shaped rods will probably experience poorer yawed penetration than circular rods of similar mass and length, and will turn slightly slower than the equivalent circular rod.  Also, the improved penetration was from a test carried out at 1.63 km/s, well short of the velocities to be expected in space. (For more details on novel penetrators, see Physics of Direct Hit and Near Miss Warhead Technology, (Progress in Aeronautics and Astronautics series) p113-127 and 328-333.)

The second principle is that guidance is relatively easy, at least on a conceptual level in flat space.  This is because, assuming that all the objects involved are on ballistic trajectories, an object that is on a collision course will appear to be on a constant bearing with decreasing range.  Constant-bearing, decreasing-range, or CBDR, has been used for centuries by mariners to avoid collisions.  This fact has led to proportional navigation, used in early air to air missiles.  The missile in question attempts to turn such that the target is at a constant bearing.  The same principle can be used for kinetics in space.  The projectile would center the target in its field of vision, then burn thrusters until the target appears to stop moving laterally in the field of view.  This method of guidance is easy to implement (the author has done so using Microsoft Excel) but has two significant drawbacks.  The first is that it renders the projectile easily detectable to the target.  The target will undoubtedly have a piece of software to detect CBDR objects and target them with the various anti-kinetic systems.  The second is that it does not work well when firing a spread of projectiles at a high-performance target.  Both of these suggest that all but the most primitive (probably improvised) kinetics will use more complex guidance systems.  This system would allow it to predict the acceleration of the target and to maintain an evasive course for as long as possible.  For dealing with the first problem, it has been suggested that a small rocket be fitted, and fired when the projectile is a few seconds out.  The drawback to this solution is that it only introduces minor complications to the problem of defense.  Any object that is going to pass close to a vessel, particularly one in combat, will be treated as a threat and dealt with accordingly.  It would only be truly practical when the seekers are mixed with a large number of unguided kinetics or decoys, both of which have their own drawbacks.  The unguided kinetics require large amounts of mass, and the decoys are easy to discriminate because of their response to high-powered lasers.  The decoy in question is basically a balloon, and it would vaporize almost instantly when hit.  This means that a decoy is only good for the amount of time that it takes a laser to slew and lock on to a target, and it is entirely possible that even lower-powered fast-tracking lasers could be used for decoy discrimination, freeing the main lasers to engage the real kinetics.  If the laser in question is a phased array, the elements can be grouped for discrimination purposes and switch targets almost instantly, totally negating the effectiveness of decoys.

For that matter, it is possible that the lasers will not be required for discrimination purposes.  Current ABM experience suggests that decoy discrimination is quite simple, and has been becoming ever easier in recent years.  The exact techniques involved are classified, and some of them, like motion analysis based on atmospheric drag, might not translate well into deep space.  Others, like the detection of the slightly different thermal and radar signatures of the decoys, will work quite well.

Another interesting method of decoy discrimination proposed during ABM research was the use of neutral particle beams.  A neutral particle beam will penetrate instead of depositing all of its energy, and produce radiation as it does so.  The amount of radiation produced will be roughly proportional to the amount of mass the beam irradiates, and this radiation can be detected and used to determine the mass of the object under investigation.  The problem with this approach is that it requires the use of a neutral particle beam, a technology that has not proven very successful so far, and which has generally been ignored throughout this paper.

Seekers are an issue that has not previously been discussed in the field of space warfare.  The author has attempted to analyze the basic limits of seeker performance, and how those limits affect the deployment of kinetic weapons.  The author is far from an expert in the field of IR detectors, so the numbers arrived at may be wildly wrong.  The derivation will not be included, for two reasons.  First, most readers would understand the author’s processes not at all, and the provision of the work would not help.  Second, a few readers might understand what was supposed to happen, and would mock the author for his errors.

What has emerged is that seeker ranges might well be shorter than the typical range assumptions made for kinetic projectiles.  Obviously, the seeker range is dependent on the power radiated by the target ship, but in fact it is proportional to the square root of that power.  This means that the range curve is likely to be more or less flat across a typical fleet, so fighters are not an answer to IR-guided munitions.  The largest problem with small projectiles is that they are limited by receiver size to short ranges.  Given a spacecraft radiating 1 MW of heat and a detector of 10 cm diameter, the results generally showed that a seeker would have a range between 250 and 2000 km against the cosmic background.  If the sun was the background, those ranges fell to 5-10 km.  Obviously, there is tremendous error in these numbers.  For one thing, a spacecraft will not radiate uniformly as was assumed for these calculations.  The exact radiation pattern is likely to be complex, and the spacecraft might well be designed to present a minimum radiation signature to incoming projectiles, cutting seeker range further.  The obvious problem that these ranges present is that a laser is likely to have several times the effective range, requiring the seeker to lock on well after launch.

There are several ways to achieve this.  The first is to simply have the projectile fly blind until it gets close to the target, then turn on its seeker and attack what it sees.  At its simplest, this costs nothing above and beyond the normal cost of the projectile, but leaves the projectile incredibly vulnerable to decoys.  Some would probably point out that decoys do not work, as explained in the section on stealth.  This position misses the key difference in the scenarios.  In the stealth case, it is a matter of attempting to fool an enemy with advanced optics and computers over long time scales and from multiple directions.  In the case of kinetics, the decoy must fool a fairly simple computer system coming from one direction (directly ahead, where relatively little power is radiated) and for a short period of time (a few minutes at most).  

On the other hand, modern technology is making missiles increasingly capable of discriminating between decoys and the real targets.  This has rendered simple flares and chaff useless, and makes even towed decoys less effective.  How well this will translate into space is unknown, and the specifics of the technology involved are almost certainly classified. Even if the decoy does not fool the kinetic over the long term, it could easily draw it to a position where it is unable to engage the actual target, or must do so from a bad angle.

Another way to avoid the lock-on problem is to use some form of initial command guidance, much like many modern AAMs.  This would pass updates to the projectiles until they are within seeker range, allowing the shipboard computer to handle decoy discrimination, and avoiding problems with early damage to the seekers.  It would even be possible to protect the seekers until the projectile is close to the target, well within seeker range.  This reduces the time available for various anti-seeker measures to a minimum.  The problem with this approach to guidance is that it renders the projectile vulnerable to electronic warfare.  The control link is likely to be radio-frequency, as tracking the projectile at long range is difficult, but required for tight-beam control, and the laser itself might be intercepted by the target, revealing the location of the projectiles.  An RF command link is vulnerable to jamming or spoofing.  The former would put the projectile into the realm of option one, above, with all of the attendant problems, while the latter would allow the defender to target it onto one of the decoys, or throw it off course entirely.  Encryption will probably prevent spoofing from being very practical, but it can’t be ruled out for more primitive projectiles.

The third possible approach is to have the projectile seeker slaved to another, larger seeker for most of the trip.  This is most likely to be practical on missiles, although a lancer might be said to use the same method.  A projector kinetic most likely travels on its own from launch until impact, due to mass limitations on the projector.  This also allows the missile to handle decoy discrimination until the submunition kinetic separates.  The drawback to this is that locking the seeker on to the target is going to require the missile to approach intact into submunition seeker range.  As mentioned above, this range is likely to be within laser range, rendering the entire missile vulnerable to either physical or sensor destruction.  One of the two methods mentioned above can be included, but that sacrifices the “homing all the way” utility of this approach, as well as exposing the submunitions to some of the problems inherent in offboard approaches.

Another alternative to the above is to use some form of semi-active homing.  In this type of guidance, the launching vessel (or another vessel) illuminates the target, and the projectile homes in on the reflected energy.  Modern laser-guided bombs and some air-to-air missiles like the AIM-7 Sparrow use this method.  Laser guidance, in particular, has the advantage of producing a predictable signature on the target at a specific wavelength, which could prove difficult to jam if the enemy is unable to match that wavelength during a battle.  Semi-active homing also entirely removes the problems of onboard decoy discrimination and target acquisition after coast, as all of that is handled by the designating ship, which can track the target continually throughout.  In fact, it is possible that semi-active homing could be combined with onboard IR homing, with the semi-active system being used to indicate targets to the IR system after flying blind for some period.

The overall balance between lasers and kinetics is difficult to work out, and is highly dependent on the technical factors involved.  It centers around the various means of defending against kinetic attack, which are the same as the weapons involved.  Both lasers and kinetics have advantages against kinetics, but there are some significant issues which cast doubt on the performance of anti-kinetic systems.

The one significant drawback of guided projectiles is that they are inherently more vulnerable to damage then unguided ones.  More accurately, guided projectiles can be turned into unguided projectiles far more easily than unguided projectiles can be destroyed.  The most vulnerable of the components is the sensor.  Not only is it impossible to armor, it also must be visible to the target ship, while most other vulnerable systems can be hidden by a faceplate.  The suggested remedy for this problem is to mount the sensor on a retractable arm, and pop up when the coast is clear for a limited time.  This would drastically reduce the chance of a sensor kill, though the projectile could still be blinded.  It is possible that a hardened home-on-jam system could be added in the case of blinding or sensor destruction.  Home-on-jam can be defeated by switching the blinding beam from one ship to another when it’s too late for the projectile to correct, or using a relay mirror to put the blinding beam on the projectile.  Countermeasures to blinding include a narrow field-of-view (to prevent off-axis jamming at the price of poor acquisition) and a filter which only lets a narrow set of wavelengths through.  This would reduce the sensitivity of the guidance system significantly, but would require the defender to find the right frequency band for blinding. A third option for dealing with sensor destruction is to command-guide the projectile most of the way with a cover over the sensor, as mentioned above.  At some point, the cover is jettisoned and the internal guidance takes over.

Nor is the sensor the only piece of a guided kinetic that is vulnerable to damage.  Even if the projectile is fitted with a faceplate, it is still potentially vulnerable to shock damage destroying the electronics, something that is more likely to occur when a pulsed laser is used.  Furthermore, there are various systems behind the faceplate, which would be vulnerable to shots made from other ships.  Armoring against such shots is difficult due to the greater surface area involved, and some systems, like thrusters, cannot be armored at all.  This approach can be supplemented by the use of the drone-mounted mirrors described in Section 1.

Besides the countermeasures and counter-countermeasures described above, there are other factors that would affect the efficacy of laser defenses.  As mentioned in the section on lasers, a laser will not have perfect pointing accuracy.  This is far more critical when dealing with kinetics, which are far smaller than ships.  This could either result in having to fire multiple shots to get a kill (for pulsed lasers) or defocusing the beam to ensure a hit.  All of this assumes that the kinetic can even be detected.  Depending on the deployment method, it might be quite hot, or it might be intentionally chilled (see below).  Besides passive detection, active detection is likely to be required to allow targeting.  The effective range of such detection depends on a number of technical factors, including the radar and optical cross-section of the projectiles and the power levels of the sensors systems, as well as all of the various processing.  The author is not familiar enough with the topic to be able to make firm statements on likely detection ranges, but given reports of the capabilities of modern missile-defense radars, such as the Sea-based X-band Radar (SBX), ranges in the thousands of kilometers seem likely.

Guided projectiles are also more vulnerable to kinetic countermeasures.  Even if the intercepting projectile (be it shrapnel, an interceptor missile, or what have you) only impacts the armor and does not penetrate, it could still mission-kill the kinetic in two ways.  The first is simple shock damage to the various systems, disabling the electronics and rendering it unguided.  The second is that the vaporized armor could be off-center, destroying the projectile’s balance, and throwing it out of control.  It’s possible, however, that the guidance system could be unable to compensate for this, albeit with reduced control.  If the interceptor is big enough, it is obviously possible to simply destroy the projectile, turning it into fragments.  Front-mounted sensors are particularly vulnerable, not only to total destruction, but also to degradation by sand-type shells.

However, even if a guided kinetic is blinded, it still functions as an unguided kinetic.  At longer range, this may not matter, as the target will have plenty of time to dodge.  At shorter range, or if someone decides to throw enough unguided kinetics to make dodging impossible, the unguided projectiles must be defeated.  Armor (discussed in Section 10) is one option for this role, but even SCOD-sized high-velocity kinetics will be able to punch through any practical armor.  The projectile must either be made to miss the target, or rendered harmless before impact.  

Deflection is achieved by vaporizing some of the kinetic, with the resulting thrust changing the projectile’s path.  Both lasers and kinetics can do this, although a kinetic interceptor would have to hit from the side, which casts doubt on the practicality of this method vis a vis kinetics.  Lasers always will produce some thrust when they hit a target, and for a laser it is significantly more efficient to deflect a kinetic in this manner than it is to destroy the entire projectile.  Even better from the point of view of the laser, spinning the kinetic will be largely ineffective, so long as the laser can flash-vaporize a layer of the skin.  The resulting gas/plasma pushes normal to the surface, so another potential alternative is to shape the projectile such that much of the thrust is wasted.  A star would be ideal, but it has a greater moment of inertia than a cylinder and greater surface area.  However, it would have the additional advantage that the beam would be spread out over the greater surface area, reducing the range at which it can begin to damage the projectile.  One potential problem with laser deflection is that shooting at a projectile aimed directly at you is not terribly effective, as the thrust vector is pointed towards you.  Given time, that would probably cause a miss, but a shot from the side (such as another ship) would be significantly more effective.

The projectile can be rendered harmless by breaking it up into chunks too small to be a danger to the target.  This is best achieved by kinetics, which basically trigger the kinetic impact described above early, ensuring the projectile is disrupted before it reaches the target.  This alone is a good reason to have some level of armor, as the remaining fragments are still headed towards the target.

The biggest problem with kinetic defense weapons is deployment.  If the kinetic launcher is a missile (as described below) it makes sense to intercept it as soon as possible, before it deploys its submunitions.  The issue then becomes making intercept before submunitions deployment.  Both seeker limitations and the simple fact that the intercepting projectile must be almost as complicated, and thus as expensive, as the inbound, mean that intercepting individual submunitions is likely to be uncommon.  One likely type of antimissile is a solid-fuel multi-stage rocket, also known as a Kirklin Mine after its inventor.  As it does not have to carry submunitions or deal with defenses, it can have a high mass ratio, allowing it to achieve higher velocities than is possible for a conventional missile.  This is critical to allowing the antimissile to attack inbound missiles quickly after they are detected.  A problem might develop if the inbound was significantly faster than the antimissile.  While in theory the interception is no more difficult, the inbound would have to be detected far earlier than would otherwise be the case for the missile to intercept before submunition deployment.  However, high-velocity missiles will also probably have high-powered engines, and large, visible exhaust plumes, thus easing the problem somewhat.  Such an interceptor missile could theoretically be countered by a smaller interceptor missile interceptor, even if the targeted missile could not deploy its main load of submunitions before intercept.  Another option for dealing with the submunitions would be to turn the interceptor missile into a bus of its own, and attempt to hunt down each submunition individually.  This would be less than ideal, as the size of the submunitions would probably be dictated as much as anything by the minimum size of their systems.

At this point, the obvious suggestion is to skip the bus entirely, and fire each submunition as a separate missile.  There are several reasons why this is impractical.  First and foremost, there are fairly strong economies of scale in rocketry, particularly when dealing with something of  SCOD size.  Several km/s of boost delta-V will be required, which in turn means that the projectile must separate from its empty booster or suffer severe performance penalties both in terms of dodging requirements and vulnerability.  A single booster to carry 12 SCODs will be somewhat lighter and very much cheaper than 12 SCOD boosters, and as mentioned above, is probably an acceptable tradeoff on the vulnerability front, given that booster burn time is not the driver of release distance in most cases.  Smaller motors could be used to spread out the SCODs after the booster burns out, or they could be released by the booster in a manner similar to MIRVs.

Kinetic-based defense weapons could also be useful in the final stages of an engagement.  As mentioned above, at the velocities involved, a shocked object will disintegrate into fragments and/or plasma, significantly reducing its lethality if the ship has some armor.  For this purpose, small, unguided kinetics are ideal, quite possibly fired by a system similar to the modern CIWS.  Other deployment alternatives include something modeled on active defense systems, claymore mines, or even throwing thin discs into the path of the incoming projectiles.  Some level of armor would of course be required to back up the systems, but they are potentially quite effective.

The job of such weapons is made easier by the fact that guided projectiles of all types are inherently predictable.  There are only a limited number of courses which will allow a weapon to impact its target, particularly shortly before impact.  This fact could allow unguided defenses to be successful even at surprisingly long range, rendering the hits even less likely.  An unguided/command guided approach has proved successful even in ballistic missile defense, with the Indian BMD program achieving successful intercepts with non-homing missiles.

The obvious response to this is to induce some random movements in the kinetic.  The downside to this idea is that this requires a more complex and expensive guidance system, capable of including such offsets.  Also, given that the projectile must eventually impact the target, it will end up on a more or less predictable course shortly before impact.  This concept also has the potential to significantly increase the delta-V requirements of the projectile.  Dodging of weapons is discussed both in Section 7 and above, so it will not be covered at length here.  However, such tactics work best when used against unguided interceptors at long range.  At short range, the need to hit the target and the low flight times both argue against dodging, while light lag is almost certain to be too short to allow dodging of lasers.

One way that has been proposed to at least mitigating the vulnerability of guided kinetics is to equip them with burster charges.  The theory is that if the projectile is disabled, the charge goes off, turning it into a cloud of fragments.  While individually less deadly, the fragments fill a much larger area and must either be dodged or burned away.  At the very least, the fragments would probably force the shuttering of the mirror, depriving the vessel of the ability to fire at the most critical moment.  The largest problem with this concept is likely to be getting a reliable and useful fragmentation pattern.  The concept works best with relatively few large, slow-moving fragments, but even with small explosive charges, velocities are likely to be in the hundreds of meters per second.  For a reasonably-sized projectile, this means that the fragments will disperse enough to be easily defeated by point defenses within a few seconds at most, indicating that maximum effective burst range is probably less than 100 km.  Smaller charges would likely not reliably fragment the projectile.

The second major problem with this concept is that it does not work well with long-rods.  Placing the explosives in the long-rods would be problematic at best, and the lower density of the explosives would hinder penetration.  The fragmentation pattern would almost certainly be less than ideal, and the nonuniform density would make dodging easier.  Thus, it seems likely that this type of projectile would instead be intended to burst as their primary means of hitting the target.  The question then becomes the effectiveness of the shrapnel produced relative to a long-rod.  The obvious suggestion is a long-rod flechette, but given that the flechette must hit nose-on, deployment and use is problematic at best.  Either the flechettes must themselves be guided or the deployment range must be short enough that they can be assumed to maintain their original attitude until impact.  It is possible that spin-stabilization could make this more practical, but the flechette would be easy to defend against, as the defending vessel must merely change the flechette’s orientation.  For that matter, the defending vessel could turn slightly to increase the obliquity at which the flechettes impact, significantly reducing penetration.  A first rule of thumb is that the back of the yawed projectile must not hit the side of the hole created by the nose.  If it does, the projectile usually breaks up with only minimal penetration.

The alternative is some sort of squat projectile, which suffers from significantly lower penetration.  The real question of the effectiveness of shrapnel then depends on the effectiveness of active defenses and armor.  If armor is minimal and active defenses strong, then shrapnel becomes practical.  If, however, armor is strong, the added mass required for the shrapnel to penetrate is better spent on more small long-rods.  Shrapnel does have the advantage that it allows for command guidance, removing the need for vulnerable sensors.  The pattern spreading will take care of the uncertainty that command guidance introduces.

Shrapnel will have to be optimized for a set of deployment conditions, and given the nature of explosives, it will be difficult for the balance to be altered after the projectile is launched.  There are three primary deployment conditions that must be considered: standoff time, pattern spread and pattern density.  Standoff time is the interval between deployment and impact, and can be viewed as corresponding to range.  Pattern spread is the rate at which the shrapnel spreads out after deployment.  For the purposes of analysis, the author will assume that the pattern is homogenous.  This is a good assumption for dedicated projectiles, as similar deployment systems have been developed for ballistic missile defense, such as the ‘jellyroll warhead’.  These have been built with both rods of various lengths, and with spherical projectiles. (More information on such topics can be found in Physics of Direct Hit and Near Miss Warhead Technology and Conventional Warhead Systems Physics and Engineering Design, (Progress in Aeronautics and Astronautics series).)

Pattern density is based upon the previous two factors and the mass of the shell.  It controls how difficult it is to burn through the shrapnel with active defenses.  The problem with detonating a projectile when it is disabled is the interplay of these factors.  If the standoff time is too long, the pattern will spread too much, reducing the density and making it easy to burn through.  If the projectile is detonated too late, it is possible that the cloud will miss the target entirely.  While it would be possible to optimize the pattern for large time detonation, the pattern is likely to be easily dodged or suffer from insufficient density, restricting shrapnel use to relatively short burst ranges.  At a guess, standoff times greater than 20 seconds are impractical, which restricts standoff range to somewhere under 1000 km.  

Directed explosives have been suggested, functioning in a similar manner to the claymore mine.  This has a fragment velocity of about 1200 m/s, which is probably a reasonable upper estimate for directed-fragmentation weapons.  This concept offers the ability for projectiles that are not headed directly at the ship to pose a threat, complicating defenses.  The fragments would obviously be unguided, and probably quite small.  This limits damage, as does the wide fragment cone.  

An alternative is an explosively-formed penetrator, or EFP, which produces a solid slug travelling at up to 2500 m/s.  This has the advantage of superior penetration, as well as having more of the projectile mass hit the target. EFPs are related to shaped charges, although they produce solid projectiles of much lower velocity than classical shaped charges.  While existing EFPs are limited to ranges of a few hundred meters, it appears that this limitation is primarily aerodynamic in nature, and not due to the breakup of the projectile.  This means that targeting will be the primary limit on range.  The projectile must, while passive (and probably imitating a piece of shrapnel or a dead projectile) track a target and estimate range.  It must then position itself and fire the projectile accurately.  That might not be terribly difficult at short ranges, but at longer ones, misses could be a serious problem, particularly because the projectile is likely to be moving significantly faster than the penetrator will leave it at, complicating targeting computations.  The directed explosion model avoids this by throwing a much larger spread of fragments, which should guarantee hits at short ranges (<6 km).  The biggest potential drawback to both approaches is low mass danger rates, as at least 60% (and quite likely far more) of the projectile is going to be moving away from the target.

Another advantage of EFPs is control over the size and shape of the projectile.  While modern EFPs are increasingly making use of long rod and flared rod projectiles, ball projectiles are more likely to be used in space, for reasons discussed above.  Leaving aside the difficulties of getting the EFP to create a rod that flies straight in the absence of aerodynamic stabilization, engagement geometry is unlikely to allow a rod to impact head-on to the target.  

A compromise between these two exists, in the form of a multishaped charge/multi-P warhead.  These are explosive charges surrounded by a specially-shaped liner to produce several shaped charge/EFP jets when the charge is detonated.  Some versions produce cylindrical or spherical patterns of jets, while others behave more like a claymore, sending out jets in a cone.  A conical setup would significantly ease targeting, although at a significant cost in impact mass.  Information on these warheads is difficult to find (Tactical Missile Warheads (Progress in Aeronautics and Astronautics series) provided much of the information on EFPs, aimable warheads, and related concepts.), but it appears that they are an ideal means of combining the higher projectile velocities of EFPs with the simple guidance of directed explosion/fragment warheads.  The Roland SAM uses a spherical multi-P warhead, although few other weapons do.

Another potential advanced warhead is a so-called aimable warhead.  This is a directed-fragmentation warhead which has the capability to project its fragments in different directions depending on how it is detonated.  Usually this is accomplished by placing multiple detonators on the warhead, with the one on the far side of the warhead from the target being detonated.  Warheads of this type also produce significantly higher fragment velocities, up to 3,350 m/s being mentioned in Tactical Missile Warheads.  The fact that the aiming is entirely passive is also significant for defense penetration.  There is no firing of thrusters or other unusual behavior to help the defender distinguish between the projectile and random debris.

One farther drawback of bursting kinetics is the likelihood that they can be set off by either impacts or pulsed lasers.  Both produce shockwaves which are likely to detonate the burster even for minor hits.  A CW laser is unlikely to do so, which might mitigate against their use for defensive purposes.

Other payloads for kinetics have been proposed.  Nuclear weapons and EMP are covered in Section 9, while conventional explosive warheads are more or less useless, as there is no air to propagate the shockwave, and the projectile carries far more kinetic energy than is contained in the warhead.  The only use of conventional explosives is likely to be that described above, as a bursting charge of some sort.

The various deployment methods deserve further discussion.  Missiles are the most commonly proposed of these methods, and will thus be discussed first.  Chemfuel missiles are entirely within modern capabilities, though virtually all existing missiles are entirely unsuitable for space use.  For one thing, total delta-V is usually the most important characteristic desired of the missile.  Liquid fuels have higher exhaust velocities in general then solid fuels, and the operational environment is conducive to their use.  The highest exhaust velocities are obtained from liquid oxygen and liquid hydrogen, but this combination has significant operational problems. LH2 will leak through the walls of tanks, requires exceptionally cold storage, and has a very low density.  A better combination is liquid oxygen/liquid methane, which has the highest exhaust velocity of the storable propellants.  The space environment allows several other means of improving performance.  For one thing, balloon tanks, like those used on the Atlas missile, are possible, particularly as they do not have to contend with gravity, reducing the difficulties inherent in keeping the tanks pressurized at all times.  The outer aerodynamic shell can also be eliminated, and it is possible for the missile to accelerate significantly slower than 1 G, allowing reductions in engine mass and in structure.  Depending on the role of the missile low acceleration may or may not be a practical option.  At long ranges, particularly when dealing with ships that use only electric drives, the missile will almost certainly burn out long before it reaches defense ranges.  A missile under thrust is significantly more vulnerable to defenses then one that has already burned out, which makes high thrust more important for missiles that might have to be launched within range of the enemy.  Even in long-range missiles, however, it’s likely that the added cost of high (1 G+) acceleration will be negligible, given the performance of current rocket engines.

The great drawback of chemfuel missiles is their very low delta-V.  A methane-oxygen rocket has an exhaust velocity of around 3.7 km/s, which means that it rapidly becomes inefficient at delta-Vs above 5 km/s or so, while a LOX-LH2 rocket might be able to make 6 km/s.  However, LH2 is difficult to store for long durations.  It might be theoretically possible to store the fuel as water, and split it off when the missile is fueled.  (However, most current LOX-LH2 engines don’t use stoichiometric O-F ratios, and run hydrogen-rich, which would mean that excess oxygen would have to be disposed of, or some hydrogen stored separately.)  Another alternative would be to use some of the more exotic chemical propellants that have been studied, but never used operationally.  A good candidate might be Chlorine Pentaflouride (ClF5) and hydrazine.  The performance is similar to LOX-methane, but the propellants only require minimal climate control, and the density of the combination is much higher, reducing the size of the missiles required.  However, ClF5 is one of the most noxious substances known to man, and ignites on contact with almost everything.  The exhaust is also toxic, but these are all less of a concern in space.  If the absolute highest performance is needed, a liquid fluorine-liquid hydrogen mix is also a possibility.  The edge in exhaust velocity is minimal, but the density is significantly improved, which might be important for missile use.  More information on these and many other interesting and deadly chemicals can be found in John D. Clark’s Ignition.

While these might be adequate for early short-range battles, they would rapidly become unacceptable as the ranges of lasers increased.  The problem then is that there is no obvious replacement for chemfueled rockets in the field of missiles.  Electric propulsion of any sort is both too expensive to use in an expendable missile and lacking in acceleration.  Nuclear-thermal propulsion does not provide enough delta-V to offset the expense involved.  Fusion is likely to be simply too expensive to use for individual missiles.  A nuclear saltwater rocket might be a viable option, but it would still be a very complex device compared to a simple chemfuel missile.

Missiles and lancers do have one great advantage, in that under most schemes they all arrive at once.  Projectors by nature fire sequentially, giving the target a stream of kinetics to deal with, instead of a wall arriving all at once.  This casts doubt on the utility of projectors for most offensive purposes, although they might remain viable for defensive use.  On the other hand, projectors can potentially achieve much higher velocities than can chemfuel missiles, tilting the scales the other way, and the ammo is cheaper and lighter per unit of damage.  Projectors would have to penetrate defenses by shooting faster than the enemy can destroy the kinetics, while missiles can opt for saturation attacks, which are generally more efficient.  A combination of the two might be quite potent, forcing the defender to split his resources between the fewer high-velocity projectiles and the swarm of lower-velocity missiles.

Coilguns are another option for deploying kinetics, but one that is significantly farther in the future then chemfuel missiles.  There are few solid estimates of coilgun performance, the best being provided by Luke Campbell on Rocketpunk Manifesto.  He states that efficiencies of 90 to 95% are achievable, and that given conservative technological assumptions, a 10 km/s coilgun will have a length of 1 million times the length of the projectile itself, or 1 km/mm.  This is an obvious problem for the use of long-rods, but the relevant lengths could be reduced by an order of magnitude or more by firing the projectile sideways and having it turn to face the target.  This solution raises the obvious problems of lock-on, but this is likely to be a problem for gun-launched projectiles no matter what, as the gun will not be pointed directly at the target.  The coilgun projectile will undergo constant acceleration, and thus the length of the coilgun will scale with the square of velocity. ( For the full article, see this post.) The length of the weapon can be altered by changing the tech assumptions, mostly the presence of superconductors and high magnetic fields.  One serious potential problem is that some of the energy not converted to kinetic energy of the projectile will instead become heat in the projectile, and at high velocities, enough energy would be deposited in the projectile to make it explode like a bomb.  This can be avoided with a  superconducting projectile, which imposes limits on the magnetic field and the temperature of the projectile.

Railguns are somewhere in between the two.  While the US Navy has recently been testing a railgun, it, and the entire type of device, has serious issues for space use.  The largest is low efficiency, which means that the device generates a lot of waste heat, which must then be radiated by the ship.  The Navy final railgun weapon has an efficiency of no better than 42% (the actual efficiency is unknown because of the lack of specific numbers on its performance).   The acceleration experienced by the projectile is at least 312.5 km/s2, so a 10 km/s railgun would only be 320 m long, assuming linear acceleration scaling.  There are apparently effects that limit railguns to about 6 km/s, which is lower than optimal for deep-space use.  Higher-powered railguns would have serious potential for violent self-destruction, and high wear on the rails.

However, there is still significant development being undertaken on railguns, much of it classified.  Hints from such programs have indicated that the worst of the wear problems have been solved.  It has been estimated that public knowledge of such programs is usually about 5 years behind the leading edge of classified R&D, so the actual potential of railguns might be underestimated in this paper.  The first system should go to sea in 2018, with full service entry some time before 2024.

While electromagnetic projectors are the best suited for space use, other types of projectors should not be ruled out.  The most obvious are conventional chemical guns, but these are handicapped by relatively low muzzle velocities.  For example, the Rheinmetall L/55 tank gun (one of the fastest in service today) maxes out at around 1,750 m/s with discarding sabot rounds.  This is the practical limit for chemically-propelled weapons, although it would probably be possible to reach as high as 2 km/s at the price of very high barrel wear.  This type of gun has several disadvantages.  First and foremost is the technological sophistication required to achieve such performance.  Sophisticated propellants and advanced metallurgy are required, but similar effort in other directions is likely to achieve a much higher muzzle velocity.  Another problem is the volatile requirements, particularly nitrogen and carbon, which are likely to be in fairly short supply (see Section 13).  Chemical guns also sacrifice some of the logistical advantages of other projectors, due to the need for sabot, propellant, and casing.  As an example, the American M829A1 APFSDS round (which is more or less typical of high-velocity anti-tank rounds) weighs 20.9 kg, with a penetrator weight of 4.6 kg, a ratio of 4.5 to 1.

One of these areas is the Combustion Light Gas Gun.  This uses a hydrogen-oxygen mix for propulsion instead of gunpowder, which has a significantly higher limiting velocity due to being fired hydrogen-rich.  A model developed for the Navy in competition with the railgun mentioned above had a muzzle velocity of 2.5 km/s, although it appears that 3 km/s is about the maximum truly feasible for the technology.  Such weapons have the advantage of being simpler than advanced conventional guns to build, as well as much easier to support, particularly in terms of propellants.  Either external tanks or a cartridge of gases can be used.  Either way, the logistical mass burden is somewhere between that of conventional guns and electromagnetic guns.

Light gas guns have been suggested as an option for space warfare, but there are serious problems with implementation.  Light gas guns use explosives as an energy source and hydrogen as a working fluid.  They are primary tool used for space debris tests, and can achieve velocities of up to 7 km/s for a single-stage gun and 10 km/s for a two-stage gun.  However, they are difficult to reload, and require large quantities of hydrogen.  Also, it is difficult to fire long-rods through them.

Other weapons, such as ram accelerators, are marginal options, although it is unlikely that they will be significantly better than LGGs or CLGGs, depending on operating velocity.  There are some potential tricks to improve the performance of various types of combustion-propelled guns, mostly centering on the idea of making them electrothermal-chemical or ETC.  An ETC gun pumps energy into the working fluid via electricity, raising the temperature past what combustion alone could achieve, and increasing the muzzle velocity.  Current research in this field is focused on improved tank guns, although hard numbers are difficult to find.  The references the author has found indicate that improvements of as much as 40% in muzzle energy (and 20% in muzzle velocity) are possible with current technology.  The effects of ETC technology on CLGGs is unknown, but might well push the practical muzzle velocity to somewhere between 3 and 4 km/s.  ETC technology also has the advantage of allowing more efficient use of chemical propellants, while not placing the same burden on a ship’s electrical system that an electromagnetic projector would.

A method of propelling projectiles that falls between missiles and projectors is laser propulsion, which, for those unfamiliar with the concept, involves using an offboard laser to provide the energy necessary to accelerate the projectile’s reaction mass.  While the concept is most commonly associated with orbital launches, it has significant potential advantages for weapons use, with a laserstar providing the laser in question, and retaining the ability to use the laser directly against the enemy.  Laser propulsion is expected to produce significantly higher exhaust velocities than are achievable with chemical propulsion, with current launch proposals reaching the region of 10 km/s.  It is possible that this could be improved, as laser propulsion is not limited by the energy and power that can be stored and channeled onboard.  The projectiles themselves will be no more expensive than chemfuel missiles, and much like in a projector, the laser system itself will be reusable. However, unlike a conventional projector, the laser does not have to accelerate the projectile entirely within the ship, significantly reducing the technical challenges involved.  Based on the numbers given for orbital launch, a few megawatts per kilogram is required.  While a laser-propelled missile does not need the acceleration of a projectile going into orbit, there are limits on how low of a power level can be used, due to the need to appropriately heat the propellant.  However, this does suggest that laser systems capable of powers of tens of megawatts or more should be capable of providing adequate laser propulsion to projectiles.

There have been a number of different suggestions for the special tactics, tricks, and techniques for the employment of kinetics.  Some of these are very useful, and were discussed above.  Others are less practical or less useful, but at least merit brief mention.

One of these is the use of cold projectiles.  These are weapons launched by some form of projector, and chilled to be nearly undetectable.  An internal liquid helium reservoir would keep the projectile cold, and boiloff could be used to steer the projectile.  Some have suggested that waste heat from the launch would be sufficient to defeat this tactic, but it appears that low-velocity coilguns do not suffer from this problem.  The usefulness of cold projectiles will depend on active sensors.  If the vessel’s actives, radar and lidar, are capable of picking up the projectiles early on, then obviously the effort of cooling them was a waste.  On the other hand, stealthy design and materials might allow the projectiles to get substantially closer to the target then normally possible.  A refinement of the basic concept is to include rockets that put the projectile on an intercept course at the last moment, giving the opponent little time to react.  The fact that the projectile is not on a collision course is likely to increase its survivability somewhat, although it is likely that a vessel will slag all large objects that come close enough to be a threat.

Another idea is the use of nuclear weapons to allow the projectiles to attack “out of the sun”, shielded by the radiation of the device.  This is based on a misconception about nuclear weapon behavior in space.  In the atmosphere, the resulting fireball can last for minutes, but in space, the radiant emissions will be over in a matter of milliseconds.  The refresh time of a modern CCD is in the microseconds, so the nuclear weapons would have to be detonated at several hundred hertz to be effective.  This is obviously prohibitively expensive, and difficult to arrange.

by Byron Coffey (2016)

Kirklin Mines

In AV:T are kinetic weapons called "Kirklin mines" (invented by Kirk Spencer). They are dirt cheap chemical fueled anti-missile weapons, specifically anti-Torch missile weapons. The ideas is that they cost a fraction of the price of a fantastically expensive torch missile, yet can scrag it. Using the magic of relative velocity, all they have to do is get in the way (this is why they are used against torch missiles, if the relative velocity isn't large enough the mine might not do enough damage to mission-kill the missile).

Launched at the proper time a Kirklin mine can either take out the incoming missile while it is too far away to damage the targeted ship, or force the missile to miss the ship entirely in the process of avoiding the mine (if the mine is launched too soon the missile has enough time to zig-zag around it and still kill the ship). Since they are cheaper, a given spacecraft can carry several mines for every missile their equivalent opponent ship has.

The current thinking is the only way a torch missile can avoid being neutralized by Kirklin mines is by becoming a bus carrying sub-missiles and decoys. Of course for a modest increase in cost the mines can become buses as well...

Hypervelocity Weapons

A special type of kinetic weapon is the hypervelocity weapon. These come in two types: rail guns and coil guns.

However, once the speed of the projectile surpasses about 14% the speed of light (42,000 kilometers per second), it is no longer a strict hypervelocity weapon, it has become a relativistic weapon.

Railguns

A railgun is two highly charged rails. When a conducting projectile is introduced into the breech, it strikes an arc between the rails, and is accelerated down the barrel by Lorentz force. The projectile can be composed of anything, as long as the base will conduct electricity. Sometimes a non-conducting projectile is accelerated using a conducting base plate called a sabot or armature. The maximum velocity of the projectile is about six kilometers per second, which is pretty freaking fast. This would give the projectile about 3.8 Ricks worth of damage, e.g., a ten kilogram projectile would have as much striking power as thirty-eight kilograms of TNT.

And when we say "strike an arc", we don't mean "make a tiny spark like scuffing your shoes on the carpet and touching the doorknob." It is more like "incredibly powerful continuous electrical explosion." Those rails are carrying pleny of juice, and quite a bit of it is wasted.

Advantages are simple construction, disadvantage is the severe rail erosion each projectile causes, requiring frequent replacement of rails (some prototypes required replacement after each use). The rails need massive braces, since they are under tremendous force trying to repel the rails from each other.

Remember, since the projectiles are not rocket-propelled, railguns are not recoiless.

SDI Railguns

The Strategic Defense Initiative was an anti-nuclear ballistic missile defense program announced in 1984, and finally dissolved in 1993. It was immediately dubbed "Star Wars" by the news media. It produced lots of classified images of high-tech orbital weapons, and spent lots of money, but no deployed systems. At least none that have been declassified.

Beside x-ray lasers, smart rocks, and brilliant pebbles, some of the proposed systems were orbital railguns.

US Navy Railguns

In 2007, the US Navy demonstrated a railgun prototype. It used about 8 megajoules, but the full scale weapon is designed to use 64 megajoules. By way of comparison, current conventional naval 5-inch guns have the equivalent of 9 megajoules of muzzle energy. The full scale weapon will have a range of 200 to 250 nautical miles, as compared to less than 15 nautical miles for a 5-inch gun. The PR handout said the full scale weapon will have "the punch of a Tomahawk cruise missile", or be the equivalent of "hitting a target with a Ford Taurus at 380 mph." It will also travel the 200-250 nautical miles to the target in about six minutes, as opposed to 8 for a Tomahawk cruise missile. At the peak of its ballistic trajectory, the projectile will reach an altitude of 500,000 feet, or about 95 miles, actually exiting the Earth's atmosphere.

We shall see if these rosy predictions pan out.

I tried to derive some values for the above weapons system and produced the following analysis. It turned out to be totally wrong, I reproduce it here so you can see my mistakes:

225 nautical miles in six minutes is an average velocity of 463 meters per second. The best estimate I could find in a five minute Google search for the mass of a Ford Taurus is 3111 pounds or about 1400 kg. 3111 pounds at 380 mph is 1400 kg at 170 m/s. Ke = 0.5 * M * V2 so the Ford Taurus will hit with about 2e7 joules or 20 megajoules. About the equivalent of 4.5 kilograms of TNT (170 m/s is about 0.003 Ricks of damage). I guess the other 44 megajoules are lost due to wind resistance.

Working the other way, we can take the 463 m/s average velocity and the 64 megajoule power consumption. Ke = 0.5 * M * V2 therefore M = Ke / (0.5 * V2). This means the projectile mass is around 600 kg.

As I said, the above analysis is incorrect. Lucky for me, a gentleman named Thomas Rigby appeared and set matters straight:

I noticed some deficiencies in your analysis of the Navy's proposed 64 MJ railgun system, particularly in your derived velocity. The M1 Abrams main gun fires a FSAPDA round somewhere between 1200 and 1800 m/s (can't remember exactly), so why would the Navy put so much unto a system that only fires at a third the velocity?

I also remember reading a Popular Science article on the new features of the DD(X) project, one of which is the railgun. According to the article the railgun would fire a 40 pound projectile (about 18.2 kg) with a Mach 8 muzzle velocity and Mach 7 velocity at the target. A quick calculation (setting speed of sound a 343 m/s):

KE = ½ (18.2 kg) (2401 m/s)2 = 52.46 MJ

KE = ½ (18.2 kg) (2744 m/s)2 = 68.52 MJ

Which compares much more favorably as a weapon system. Derived values can easily be obtain close to these numbers

We'll take the average range, 225 nmi, for the calculations. Of course we can't just convert 225 straight to meters, since a nautical mile is a bit over 15% longer than a standard mile (about 6076 feet). After converting to miles we can go to meters (or go straight from nmi to meters, if your calculator has a bunch of built-in conversion factors):

1nmi = 1.151mi

225nmi (1.151nmi / mi) = 258.975mi

1mi = 1.609km = 1609m

x = (258.975mi) (1609m / mi) = 416690.775m

Real Value: 416700 m

Dividing by the time (6 min / 360 sec):

Vx = 416700m / 360s = 1157.5 m/s

Which s a far more appropriate velocity for a kinetic kill weapon. However, this is only part of the velocity. The railgun fires in a parabolic arc, getting almost 95 miles up. Assuming the Earth is flat, and the projectile is launched and lands at the same height, this part of the velocity component is easy to calculate. In theory the projectile reaches its maximum height half way through the journey, or at 3 min - 180 s. We can put this into the gravity-displacement equation to determine the speed. A height of 95 miles (500,000 feet) is about 152400 m.

h = -4.9t2 + vtv = (h / t) + 4.9t

Vy = (152400m / 180s) + (4.9 m/s2)(180s) = 1728.67 m/s

Now we can combine the two velocity components to determine the actual velocity, by Pythagorean Theorem.

VT = √(1157.52 + 1728.672) = 2080.41 m/s

Which is much closer to the Mach 7 value that the Navy claims the projectile hits at. Using this value to calculate the kinetic energy:

KE = ½ (18.2 kg) (2080 m/s)2 ≈ 39 MJ

Thomas Rigby
Thomas Mays on US Navy Railguns

(ed note: Thomas A. Mays has not one, but two degrees in physics. And to top it off, he is an 18-years-and-counting veteran of the US Navy, working as an officer in the surface fleet aboard destroyers and amphibious ships. More to the point, he actually worked in the US Navy railgun project. He was commenting on this news item.)

(Tobias Klausmann: I wonder if a two-stage system (aka Chemrail) wouldn't be the better option anyway, but I haven't read up on the specific problems of that.)

Thomas A. Mays: One of the projects I had to choose from during my railgun days was in developing a hybrid coilgun/railgun that would use the sudden push from a coilgun to get the armature up to transition speed, then use a plasma interface between the rails and armature to impart the rest of the kinetic energy to the round. But that was more in line with what the French were doing. The US program was focused on eliminating transition completely, even to the point of using tapering rails to maintain a metal to metal contact down the full length of the gun, even with friction ablation.

Thomas A. Mays: Another thing to note, the rail/armature ablation/deposition issue has long been known to be the critical weakness, even above that imposed by heat loss or field flash recovery. What was interesting is that railgun science is its own industry, and alternative tech is treated almost like apostasy. The conventional wisdom is that a railgun is the only device that can achieve these velocities and energies, but light gas guns can do it too (albeit with more equipment volume and a huge loss in unrecoverable gasses), and according to the 1000 lb brains at Sandia Labs, a coilgun could too, WITHOUT any ablation issues at all. Now, you tell the railgun bubbas that, they insist Sandia is wrong, that reluctance will prevent any field from growing or collapsing fast enough to impart railgun velocities to a round in anything smaller than a mile long magnet train, but the Sandia guys insisted it was an engineering issue they already had a resolution for, only they could not get any funding because of the "railgun mafia" (non-attributional).

(Matter Beam: I've never heard of coilgun/railgun hybrids nor that the French had a project of their own. Stuff to google, Mays!)

Thomas A. Mays: It wasn't a French national or military program, just a graduate program through some of their universities. and this was all back in 2005. But, yes, hybrid guns are a thing, however, I don't think anyone has invested in them because it's doubling the complexity for not much guaranteed payoff.

(Matter Beam: Thomas Mays,​ I haven't yet considered the possibility of non-US Navy ships obtaining their own railguns soon after America does. Unlike the US's expensive and numerous existing warships, a foreign power's transition to railgun-optimised warships will be faster as they wouldn't need extensive retrofits and ways to extend the usefulness of previous investments. How do you think a country like China or Russia go about designing and implementing a railgun system? Would the results differ much from US prototypes?)

Thomas A. Mays: Based on how the Chinese acted at the 2005 EM Launch Conference in Potsdam, their program will be as identical to ours as they can make it. They were blatantly filming each presentation despite the no cameras rule, and they would sit with a different team each day at lunch and, how should I say it, used social engineering and some very finely put together "students" of the opposite sex to pump others for information. As trade craft went, it wasn't exactly Robert Ludlum. As for the Russians, dunno. I imagine they'll use over-engineered rails that will work with massive losses and lower tech despite our not being able to do the same. Honestly though, I think every nation's dreams of hypervelocity rounds are still closer to deep development than to fielding, and we're a good 10 years ahead of them at least. It won't be a priority for them until we field one, because we're the least likely to use it offensively if fielded. 

From comments by Thomas A. Mays (2016)

Coil Guns

Coil guns, magnetic linear accelerator, or mass drivers are a series of donut shaped electromagnetic coils (Philip Eklund calls it a "centipede gun", in the Traveler role playing game they are called "gauss guns") A projectile composed of some ferromagnetic material is introduced into the first coil. The coil is energized so it repels the projectile and the next coil is energized so it attracts the projectile. When the projectile reaches the second coil, the second switches to repulsion and the third starts attracting, and so on. Advantages are a much lower power consumption than an equivalent rail gun. Disadvantages are the massive power switches required. Each individual coil needs bracing, as they are under tremendous force trying to expand the coil (actually for "expand" read "explode").

Note that one can use the kinetic energy equation above to see how much power the railgun or coilgun will require for each shot. Since these weapons are nowhere near 100% efficient, you will quickly discover that these weapons are power hogs.

When these weapons are armed they will be carrying plenty of electricity. If they are damaged by enemy weapons fire, there will probably be plenty of high-voltage fireworks, at least inside of the ship. I am unsure if there will be much arcing outside of the ship unless the ship is venting gas by accident (atmosphere through a hull breach) or design (open-cycle cooling gas).

Also note that as the guns get more powerful, the more recoil they will have. Indeed, they will approach being auxiliary propulsion systems. If such a gun was optimized as a propulsion system it is called a "mass driver".

To calculate parameters of your coilguns, Eric Henry has an Excel Spreadsheet. Or you can use Luke Campbell's method:

Here's a quick method to estimate what kind of performance you can get out of a coilgun. Some folks here might find it interesting.

First, decide on the efficiency of your coilgun. Coilguns are linear brushless electric motors, and brushless electric motors have demonstrated efficiencies of 90% to 95%. Superconductive electric motors might have efficiencies of 98% to 99%. Denote this as a decimal, and call it e; that is e = 0.9 to e = 0.95.

Next, decide on the length and radius of your projectile. Decide on what your projectile is made of and find its mass

mass = density * length * radius2 * &pi (and remember to use consistent units).

Also find the projectile cross-sectional area

area = radius2 * π

Decide how fast you want your projectile to be going and find its final kinetic energy

kinetic energy = 0.5 * mass * velocity2 (again remember to use consistent units).

Given the efficiency of your coilgun, you can find out how much your projectile heats up. You might figure that half of the wasted energy goes into the projectile, and thus your projectile will gain a heat energy of

heat energy = 0.5 * (1/e - 1) * (kinetic energy)

Look up the specific heat of the material your projectile is made of, commonly called C. Then your projectile reaches a temperature of

projectile temperature = (heat energy) / (C * mass) (again make sure your units are consistent).

If you are using a synchronous coilgun with a permanent magnet in the projectile, this temperature needs to be less than the Curie point or the projectile will become non-magnetic. If your coilgun projectile is made of superconductors and you are using Meissner effect repulsion, this temperature will need to be less than the critical temperature of the superconductor or your superconductor will become non-superconducting. If you are using an asynchronous coilgun which uses inductive forces on conductive loops, this temperature will need to be less than the melting temperature of your projectile. If the temperature is too high, you will either need to use a material that can handle higher temperatures, make the coilgun more efficient, or accept a lower velocity for the projectile.

Decide the maximum magnetic field your coilgun can handle. If you are using a synchronous coilgun with permanent magnets (probably in the projectile, with the field coils along the barrel) you are limited by a saturation field of around 0.2 to 2 tesla beyond which your efficiency falls off rapidly. If you are using superconductors, your field is limited by the critical field of the superconductor. For conventional BCS-type superconductors this limits you to fields of several tens of tesla or less, for high Tc superconductors you might be able to get to 100 to 200 tesla. If using an asynchronous coilgun that uses induction to launch normally conductive projectiles there is no obvious physical upper limit to the magnetic field strength, although high field strengths will require massive bracing to keep the barrel from exploding.

Now assume that the barrel is filled with field, and that the projectile sweeps the field out of the barrel, turning the field energy into kinetic energy (this is not actually how coilguns work, but it gives the physical upper limit based on energy conservation). The energy density is about 400 kJ/m3/T2 times the square of the magnetic field strength (398,098 J/m3/T2 to six significant figures). Call this value K

K = 400 kJ/m3/T2

You now know the volume needed in the barrel based on how much energy the projectile ends up with

volume = kinetic energy / (K * (magnetic field)2)

Since you know the cross-sectional area of the projectile and thus of the barrel, you know how long the barrel needs to be

length = volume / area

If the barrel is unacceptably long, you will either need to figure out how to get a stronger field in the barrel, make the projectile shorter (if you do the math, you can see that the barrel length will be a multiple of the projectile length for a given field, material, efficiency, and final velocity) or make due with a lower velocity of the projectile.

As an example, suppose we have a synchronous coilgun, and that the coilgun can generate 1 tesla fields (a good number that will not saturate the ferromagnet). Our presumed ferromagnet is probably mostly iron, with about 8000 kg/m3. To reach 100 km/s, you will need 40 TJ per cubic meter of projectile. Since this is 100 million times the energy density of the field, you will need the projectile to sweep out 100 million times its volume in order to accelerate up to the desired speed. This means you need an accelerating track 100 million times the length of your projectile. If the projectile is the size of a dime, with 1mm thickness, you will need a 100 km long track. If 2.5% of the energy goes into the projectile as heat as a result of inefficiencies, you get 100 GJ of heat per cubic meter of projectile, or 12 MJ/kg. This is three times the specific energy liberated by detonating high explosives, so you can expect your projectile to explode like a bomb inside your coilgun barrel. Consequently, this appears to be an unworkable design.

Luke Campbell

Effects

Ken Burnside notes how difficult it is to calculate the damage caused by a solid shell:

In terms of how ships survive taking damage, there is also the matter of rate of deposition to the target and area of deposition.

Basically, you're poking holes in a compartmentalized object. Unlike an aircraft, or a submarine, the outside environment isn't that hazardous. It doesn't take much damage to make a jet fighter unflyable at air combat speeds. Getting hit with a torpedo in a sub can cause the hull to collapse.

Hitting a spaceship won't cause it to pop like a balloon. There's likely a swath of compartments that are uninhabitable at this point...but the ship can still fight.

For example, an M1A2's main gun is about a 5" naval gun -- firing an armor piercing round, at a target that wouldn't quite actually be a full sized Naval compartment. Very rarely does it leave an exit wound in the back of an enemy tank, which is the indicator of what it would do to the NEXT compartment of a ship. It WILL destroy everything in that compartment, unless it's blunted by hitting an engine in the way (like the Merkava design of the IDF).

For point of reference, an M1A2's round has a velocity of about 1600-1700 m/s. Mass between 3.5 and 4 kg, diameter about 2.5 cm.

Quite simply, there isn't a lot known about the interaction dynamics of objects impacting at 1.5+ kips. One field says that they'll turn into a plasma spray (more or less what happens when a tank round hits a tank...), which limits their damage to the compartment hit. Another says they'll get a plasma sheathe and go through multiple compartments shedding a bit of energy (but far less than the total carried by the round) in each, and exit the back of the ship.

Either of these makes for a more interesting fight than "gee, one hit, one kill, no stealth."

Ken Burnside

Isaac Kuo is of the opinion that hypervelocity weapons will have limited penetration. He notes that a projectile has both kinetic energy and momentum. Momentum is what keeps the projectile moving in its direction of motion.

Now, if you look at the equations for kinetic energy and momentum, you will note that as the velocity rises the kinetic energy goes up much faster than momentum (1/2 velocity squared vs just plain velocity).

Ke = 0.5 * M * V2

p = M * V

So Mr. Kuo figures that the greater your ratio of kinetic energy to momentum, the more spherical the resulting explosion and the less penetration into the interior you will get. This means hypervelocity weapons can be stopped (for a while) by a Whipple shield (until it is shot full of holes). Whipple shields are set at some distance from the hull, if the spacing is larger than the radius of the explosion, the shield takes damage but the hull does not.

I'm still looking for more details on this, especially the mathematical relationship between the ratio and the explosion sphericality.


     Whipple shields are stupid; slanted armor is OP
     Slanted armor vastly increases your survivability; this has been known since antiquity. And, when you're going against hypervelocity k-slugs, it's basically your only option. Make it thick enough and slanted enough, and you can shrug off a continuous hailstorm more-or-less indefinitely (at least, if CoaDE is any guide). If the opponent is, stupidly, using lasers, their beam spreads out with the secant of the armor slant angle, to say nothing of the greater Fresnel reflection at angles. Every piece of armor on your ship should be slanted.
     Example: Against an incoming 532nm laser, Aluminum armor has a refractive index of 0.90175. This means that you can actually get total internal reflection. Armor slanted at more than ~64.389 degrees will experience no effect whatsoever from the laser, no matter how powerful!
     Conversely, whipple shields are useless (a whipple shield is a sacrificial layer of thin armor that shocks k-slugs into plasma, which can then diffuse). One problem, however, is that this theory only works if the projectile is orthogonal to the armor (which would mean your armor isn't slanted). In fact, if your whipple shield is slanted, k-slugs tear huge gashes that quickly render it worthless.
     This is a special case of whipple shields being helpful only once. A whipple shield will block one bullet, but not two. If you have a battle where millions of k-slugs being fired, that's basically no protection at all.

Message-Id: <0000000000@xxx.com>
To: xxx@xxx.org
Subject: Under enough pressure, ravioli behaves as a gas.
Date: Tue, 29 Dec 1998 11:43:20 -0500
From: xxx xxx 
X-Mailing-List:  archive/latest/311
X-Loop: xxx@xxx.org
Precedence: list
Resent-Sender: xxx@xxx.org

> There was still one aspect of the whole concept of a ravioli-loaded
> railgun type wepon which we, lolling about late on a weeknight, with
> only a few neurons randomly firing, could not resolve.  Would a chunk
> of metal (can of ravioli) impacting another, larger, rest mass
> structure (star destroyer) produce an "explosion" effect, or simply
> punch an appropriately shaped hole as it passed through?

What am I, the neighborhood blast physicist??? Well, maybe... :-)

It all depends on speed of impact versus the speed of sound in the target (what is called the Mach number, where Mach 1 means the speed of sound, Mach 2 is twice the speed of sound, etc), and the speed of the ravioli versus the speed of light in the target (which I'll call the Cerenkov number, where Cerenkov 1 is the speed of light in anything; Cerenkov 1.3 is the speed of high-energy protons in a water-cooled reactor (that's why you get that nifty blue glow), and you can get up to Cerenkov 2.4 using diamonds and nuclear accellerators. In the late 40's people used to talk about Cerenkov numbers, but they don't anymore. Pity.). Lastly, there's the ravioli velocity expressed as a fraction of the speed of light in a vacuum (that is, as a fraction of "c"). "C" velocities are always between 0.0 and 1.0

At low speeds (REAL low) the ravioli will simply flow over the surface, yielding a space-cruiser with a distinctly Italian paint job.

Faster (still well below speed-of-sound in the target) the metal of the space-cruiser's skin will distort downward, making what we Boston drivers call a "small dent".

Faster still, you may have a "big dent" or maybe even a "big dent with a hole in the middle", caused by the ravioli having enough energy to push the dent through, stretching and thinning the hull metal till the metal finally tears in the middle of the dent.

Getting up past Mach 1 (say, 5000 feet/sec for steel), you start to get punch-a-hole-shaped-like-the-object effects, because the metal is being asked to move faster than the binding forces in the object can propagate the "HEY! MOVE!" information. (After all, sound is just the binding forces between atoms in a material moving the adjacent atoms — and the speed of sound is how fast the message to "move" can propagate.) From this, we see that WileE Coyote often reached far-supersonic speeds because he often punched silhouette-type holes in rocks, cliffs, trucks, etc.

Around Mach 4 or so, another phenomenon starts — compressive heating. This is where the leading edge of the ravioli actually starts being heated by compression (remember PV=nRT, the ideal gas law?) Well, ravioli isn't a gas, but under enough pressure, ravioli behaves as a gas. It is compressed at the instant of impact and gets hot — very hot. Likewise, the impact point on the hull is compressed and gets hot. Both turn to gasses — real gasses, glowing-white-hot gasses. The gasses expand spherically, causing crater-like effects, including a raised rim and a basically parabolic shape. In the center of the crater, some material is vaporized, then there's a melt zone, then a larger "bent" zone, and the raised rim is caused because the gas expansion bubble center point (the bending force) is actually inside the hull plate. If the hull plate isn't thick enough, then the gas-expansion bubble pushes through to the other side, and you get a structural breach event (technically speaking, a "big hole") in the side of the space-cruiser.

Compressive heating really hits the stride up around 20,000 feet/sec (Mach 4 in steel, Mach 15 in air) and continues as a major factor all the way up to the high fractional Cerenkov speeds, where nuclear forces begin to take effect.

Aside: the "re-entry friction heating" that spacecraft endure when the reenter the atmosphere is NOT friction. It's really compressive heating of the air in the path. As long as the spacecraft is faster than Mach 1, the air can't know to get out of the way, so it bunches up in front of the spacecraft. When you squeeze any gas, it gets hot. So, the glowing "reentry gas" is really just squeezed air, which heats the spacecraft heat shield by conduction and infrared. The hypersonic ravioli can be expected to behave similarly.

As we increase speed from the high Mach numbers (about 10 miles/sec) all the way up to about 150,000 miles/sec, not much different happens except that the amount of kinetic energy (which turns into compressive heat) increases. This is a huge range of velocity, but it's uninteresting velocity.

At high fractional Cerenkov speeds, the ravioli is now beginning to travel at relativistic velocities. Among other things, this means that the ravioli is aging more slowly than usual, and the ravioli can looks compressed in the direction of travel. But that's really not important right now.

As we pass Cerenkov 1.0 in the target, we get a new phenomenon — Cerenkov radiation. This is that distinctive blue glow seen around water-cooled reactors. It's just (relatively) harmless light (harmless compared to the other blast effects, that is). I mention it only because it's so nifty...

At around .9 c (Cerenkov 1.1) , the ravioli starts to perceptibly weigh more. It's just a relativistic mass increase — all the additional weight is actually energy, available to do compressive heating upon impact. The extra weight is converted to heat energy according to the equation E=mc2; it looks like compressive heating but it's not.

[Here's where I'm a little hazy on the numbers; I'm at work and don't have time to rederive the Lorentz transformations.]

At around .985 c (Cerenkov 1.2 or so), the ravioli now weighs twice what it used to weigh. For a one pound can, that's two pounds... or about sixty megatons of excess energy. All of it turns to heat on impact. Probably very little is left of the space-cruiser.

At around .998 c, the impacting ravioli begins to behave less like ravioli and more like an extremely intense radiation beam. Protons in the water of the ravioli begin to successfully penetrate the nuclei of the hull metal. Thermonuclear interactions, such as hydrogen fusion, may take place in the tomato sauce.

At around .9998 c, the ravioli radiation beam is still wimpy as far as nuclear accellerator energy is concerned, but because there is so much of it, we can expect a truly powerful blast of mixed radiation coming out of the impact site. Radiation, not mechanical blast, may become the largest hazard to any surviving crew members.

At around .9999999 c, the ravioli radiation may begin to produce "interesting" nuclear particles and events (heavy, short-lived particles).

At around .999999999999 c, the ravioli impact site may begin to resemble conditions in the original "big bang"; equilibrium between matter and energy; free pair production; antimatter and matter coexisting in equilibrium with a very intense gamma-ray flux, etc.[1]

Past that, who knows? It may be possible to generate quantum black holes given a sufficiently high velocity can of ravioli.

—xxx

[1] According to physicist W. Murray, we may also expect raining frogs, plagues of locusts, cats and dogs living together, real Old Testament destruction. You get the idea...

Missiles

Missiles are small drone spacecraft that chase enemy ships and attack them with their warheads. It can have its own propulsion unit, or be launched by a coilgun and just use small guidance jets. It can carry a single warhead, or be a "bus" carrying multiple warheads. Or multiple mini-missiles. Go to The Tough Guide to the Known Galaxy and read the entry "MISSILE".

One of the big advantages of missiles over directed energy weapons is that missiles do not generate huge amounts of waste heat on the firing ship. A missile can be pushed off with springs or cold gas. Once clear of the ship, the missile's propulsion system ignites. But then all the waste heat is the missile's problem, not the ships.

By the same token, the disadvantage is that missiles are expendables, unlike laser bolts (as Anthony Jackson puts it: "If you're willing to have expendables, you can also have expendable coolant."). When the missile magazine runs dry, the launcher will just make clicking noises. But a laser cannon can fire as long as it has electricity.

The second advantage of missiles over directed energy weapons is that (depending upon the warhead) most missiles are not subject to the inverse square law. Laser bolts grow weaker with distance but a nuclear warhead has the same strength no matter how far the missile travels. However, laser bolts cannot be neutralized by point defense.

The warhead is generally a nuclear weapon but others are possible. One possibility is a single-shot coilgun firing a kinetic weapon. Another type of warhead is an explosive charge coated with shrapnel, designed to deliver a cloud of kinetic kill masses into the path of the target spacecraft. A third type is the "submunition".

Of course the simplest is no warhead at all, making the structure of the missile an impromptu kinetic kill weapon. According to the first law of space combat, above about a three km/s relative velocity difference a chemical explosive warhead is superfluous. Rick Robinson says that at these speeds the only reason for conventional explosives is for the bursting charge on a shrapnel cloud.

Rick Robinson suggested that the term "torpedo" be used for a missile that has acceleration capacities comparable to a spacecraft, while the term "missile" or "torch missile" be used for those that have somewhat more acceleration than spacecraft. In GURPS: Transhuman Space they use the term "Autonomous Kill Vehicle" (AKV) instead of torpedo.

Torpedo Mechanics

Rick Robinson

(ed note: "Facing" means that a space warship's laser beam turrets can only fire in certain directions, the ship has "blind spots" where the lasers beams cannot bear. The idea is that in space combat, you and your opponent try to maneuver and rotate your respective ships such that more of your beam weapons can shoot at the enemy ship than they can shoot at you.)

For missile / torpedo combat, however, tactical maneuver is not dependent on facing. Instead it is a matter of large "sweeping" maneuvers, intended to get your ships into a launch vector while avoiding enemy missiles.

The key to missile combat (at least my concept of it) is that the missile itself is really the second stage of a two-stage weapon, the first stage being the ship that launches it. In (laser or particle) beam combat, assuming equal-range beams, if I am in range of you, you are also in range of me. The only thing to keep us from just zapping away at each other is facing restrictions. But in missile combat, even with equal missiles on both sides, a more maneuverable ship can execute an approach-launch-breakaway, using the ship to give added vector to the missile at launch, then breaking away to avoid enemy missiles.

Take an extreme case, fast but lightly-armed ships attacking a powerful but non-maneuverable orbital fort. In beam combat, there's no way for the ships to hit the fort without coming in range of its battery. But in missile combat, the ships can fire at very long range (since the fort can't maneuver to evade their missiles), while they will have plenty of time to evade missiles fired by the fort.

The fort's point defenses might still be able to stop most of the incoming missiles, but the the advantage is still with the maneuverable ships, since the fort has no way to reach out and touch them. :>

That's a limiting case, but it shows the importance of maneuver in long-range missile combat. Generally, in beam combat the advantage goes to the more heavily armed and armored ship; in missile combat the advantage goes to the more maneuverable ship.


Kirk Spencer

(ed note: An "inertial compensator" is a handwavium gadget that allows spacecraft to make drastic maneuvers without the gee forces turning the crew into thin layers of bloody chunky pulp plastered all over the walls.)

No, I think you (Rick) are in error about the missiles — unless you have inertial compensators or other physics escape mechanisms.

Actually, let me interrupt with what I've begun to take as a truism. The superiority of Beams vs Missiles is as variable as the superiority of Offense vs Defense — each is antecendent in its turn, depending upon the specific technology and inspiration in use existent at the moment of comparison.

That said, I think your slingshot launch has a major problem. It goes as follows:

Let us assume that the missile acceleration is 2 distanceunits/timeunit while the ship has an acceleration of 1. For simplicity, we'll say that a missile has a duration of 3 timeunits, with the ability to be dangerous despite point defense mechanisms of one additional timeunit. The missile thus has a maneuvering hit range of 6 distanceunits (du), and a stationary hit range of 9 du inherent.

Let's have your ship produce a rate of movement of 10 du/tu. This means the ship can fire at the base at a range of 19 du, well outside the range of the bases missiles. Thus far your concept is correct.

Unfortunately, now we've the subsequent time intervals.

Immediately upon launch, the ship begins a thrust to maintain maximum distance from the base — initially we'll use 90 degrees to current vector. Further we'll simplify this to simple vector movement instead of true Newtonian calculations — largely because I'm lazy (grin) — but the difference here will be slight.

Create a grid of 20×20. Place the ship at 0,0, and the base at 0,19. The initial vector of the ship is +1,+10 (the 90 degrees of thrust applying at the instant of launch).

The ship's location at the next interval is +1,+10 — a slight bit outside range 11 from the base and so still safe. The next vector change has another interval of thrust applied, so the ship's vector is now +2,+10. At the end of the second turn, we're at +3,+20 — or a bit less than 4 du from the base.

The base probably fired missiles in return on an intercept path as soon as you began your avoidance thrust — thus he knows the path you must be taking. After two intervals, the intercepting missiles had a range of 6 (2+4) du.

In other words, your ship fell within the missile range of the base — and they reached that range at about the same time your missiles reached the base (actually the missiles at your ship probably intercepted your ship before the base-bound missiles reached their target, but we've broken down the time interval too broadly for that.)

This is what Ken refers to as the 'trumpet bell effect'. The only way for the ship to stay out of missile range in your attack profile is for the ship to be faster than the missiles. If that's the case, then beams are more important because missiles can be dodged more easily.

Now, I'll admit that a base can't dodge, and so in actuality you can probably launch from even further out and trust to simple mechanics in null/microgravity to be sufficient. But you used that example as the 'simple' example of ship-vs-ship combat.

Given a ship/base capable of slight maneuver, the ballistic flight is closed. I'll also note that with the base you can 'float' missiles to the launch point — throw them ballistically for several time units, then have them ignite at the optimum point for effective engagement. But you can't do this in a ship-ship battle — your foe will laugh and maneuver outside the intercept envelope to which your missiles are committed. (note that he's then committed to staying outside that space-time envelope, but you still only have a limited amount of missiles.)

In short, I don't believe your attack profile isn't what you thought, but is instead very susceptible to mutual endangerment.


Ken Burnside

The "trumpet bell effect", as I call it, puts a "maximum relative velocity" on missile engagements

This maxima is based on the delta V of the missiles, and the delta V of the ships.

In essence, if your initial relative velocity vis a vis your stationary target (and to all missiles, all targets are stationary...) means that you really cannot afford to let your ship impart much momentum at all to your shells — otherwise, your ship is going to cruise into mutual annihilation distance.

This means that for low-thrust, high-specific-impulse drives like Rick's, the smart naval commander will match velocities with his target and pick a range where his missiles have the advantage over the other guy's. At which point, tactical maneuver doctrine is a null pointer (i.e., is pointless).

Operational maneuver doctrine is still interesting — you're trying to find that point in the enemy's plot where he MUST commit to coming towards something of value, and match his velocity there.

This also means that the missile's relative velocity (assuming they focus on dV rather than thrust will be significantly slower as well.

This takes effect in Attack Vector: Tactical (AV:T); trying for the high speed pass turns you into missile-bait, because your course and range over time is easily predicted.

I've been pondering the MITEE driven missile Rick described earlier. It may be possible to work it under the rules for AV:T with the new ballistic weapons system under development. One thing that becomes very clear is that it can engage outside of "buttoned up" distance — which means it's a lot more practical to use anti-ship beam weaponry to kill it farther away from the ship. In fact, with its high emissions signature and low thrust, it should be pretty easy to hit — it won't be jinking signficant amounts when engaged at 1000 km.


Rick Robinson

Ken Burnside: The "trumpet bell effect", as I call it, puts a "maximum relative velocity" on missile engagements. This maxima is based on the delta V of the missiles, and the delta V of the ships.

I think of it more as a "range" — but in vector space, not just linear space — incorporating both distance and relative motion. Like pornography, it is hard to describe, but I know it when I see it. :)

Ken Burnside: In essence, if your initial relative velocity vis a vis your stationary target (and to all missiles, all targets are stationary...) means that you really cannot afford to let your ship impart much momentum at all to your shells — otherwise, your ship is going to cruise into mutual annihilation distance.

There seems to be a key word or phrase missing above — something like "if your initial relative velocity ... is high enough" or some such. That was just what happened in Kirk's scenario: the attacker made such a running start before launching his missile that he committed himself to passing within missile range of the non-maneuverable target, and could not perform an effective breakaway.

Ken Burnside: This means that for low-thrust, high-specific-impulse drives like Rick's, the smart naval commander will match velocities with his target and pick a range where his missiles have the advantage over the other guy's. At which point, tactical maneuver doctrine is a null pointer (i.e., is pointless).

If your missiles are enough superior to the other guy's missiles, this would be the case — even if he is more maneuverable, if your missile delta V exceeds his combined ship delta V and missile delta V, he'll never be able to get a firing position where you can't hit him.

One thing that is going on here, I think, is that "missile" is a less clearly defined concept than "beam." That is, a beam is understood to be more or less the ideal bullet: you point and shoot, and at AV:T ranges — or even many times AV:T ranges, out to a few hundred thousand km — it is effectively instantaneous.

"Missile," though, seems to cover a variety of weapons, from railgun shells that are almost slowed-down beams, but with some ability to veer in response to target jinking, to weapons that have prolonged flight times and are only modestly more maneuverable than the ships they are sent to intercept.

Missiles of the latter type are what I have in mind, used at relative ranges such that the trumpet bell tends to balloon outward to the point where it ultimately becomes nearly spherical.

Which is why I don't think tactics would devolve to simple velocity matching, because my working presumption is that, during a missile's useful flight time, the potential maneuver of ships is not much less than that of missiles.

(Submunitions, in my scheme, are very different, and behave almost like "slow beams." The relative velocity of missile bus and target, at the moment of submunition release, is very much greater than the delta V available either to the submunition or the target, so as seen by the target the submunition have a very long, narrow trumpet bell.)

Ken Burnside: Operational maneuver doctrine is still interesting — you're trying to find that point in the enemy's plot where he MUST commit to coming towards something of value, and match his velocity there. This also means that the missile's relative velocity (assuming they focus on dV rather than thrust will be significantly slower as well.

Yes. One way to look at it is that my concept of missile combat blurs the tactical and operational levels.

Ken Burnside: It may be possible to work it under the rules for AV:T with the new ballistic weapons system under development. One thing that becomes very clear is that it can engage outside of "buttoned up" distance — which means it's a lot more practical to use anti-ship beam weaponry to kill it farther away from the ship. In fact, with its high emissions signature and low thrust, it should be pretty easy to hit — it won't be jinking signficant amounts when engaged at 1000 km.

Yeah. The MITEE missile I outlined was badly hampered by the mass of its fuel tankage (and use of bulky hydrogen fuel). I suspect that a small fuel tank could be built much lighter — the estimate I used was based on my model for ship hulls. For my style of combat, you'd need a missile with about 2x the delta V given, and configure it to carry submunis.

Alternatively, given their low mass, the MITEE units could themselves be used as submunis — the constraint being whether they can carry sufficient fuel for the terminal phase of flight.

From a thread on sfconsim-l (2002)

To be an effective weapon, missiles have to have acceleration abilities at least as good as the target ship. Rick Robinson says "Basically you have to make your ship drive, or something comparable to your ship drive, small enough and cheap enough for a one-shot weapon." Some drive technologies cannot be squeezed down since they have a minimum size.

Rick also notes that missiles have stupendous range. If your spacecraft can cross the solar system, so can your missiles.

Ken Burnside did the math and found that it is worse than Rick realized.

Missiles Will Always Hit

There is a temptation to make a game where torch missiles can be run out of propellant. The problem with this is that when you do the geometry of the shot, you assume two things:

  1. The target is pointed exactly away from the inbound missile bearing.
  2. The target is using its maximum thrust.

This is the worst case for the person launching the missile; you subtract the target's acceleration from the missile's acceleration, and build a reference frame where all the velocity is on the missile — this may result in the missile overcoming a velocity away from the target.

At that point, you calculate delta-v. Unless the target has some way to leave the battle, you do a simple calculation of delta-v over time overcoming the initial shot velocity; if the missile can overtake the target in a stern chase, you'll know before the missile gets launched.

Once I built this up for Attack Vector: Tactical, I did the math for the torch missiles Rick loved so dearly...and it gets very bad; because missiles can afford drop tanks more readily than spaceships.

In the real world, missiles also have sensors for autonomous homers, and those sensors have batteries — the batteries tend to be good for roughly twice the "expected" fuelled flight parameter for redundancy. I suspect powering onboard sensors for a torch missile may also be the real limit — sure you can make your fusion torch missile also self-power off of the fusion rocket, but that increases the cost.

Of course, you're in a society that throws away a hundred-kW fusion motor away as an expendable munition, so that cost may not be a factor at all.

There's a reason why Attack Vector: Tactical missiles ended up being a more advanced solid fuel rocket: Cost and ease of maintenance. You need to think about how your spacers — who if Air Force enlisted personnel are any indication — have high school or two-year degree equivalents are going to keep those missiles in launch readiness for multi-month cruises. Rocket propellants tend to have a shelf-life...

Ken Burnside

There is some convergent evolution going on here. If you take a conventional fighter aircraft and replace the pilot with remote-control gear, you have an unmanned combat aerial vehicle or combat drone. If you replace the remote-control gear with a computer AI you have an autonomous combat drone.

In the same way, if you take a space fighter and replace the pilot with remote control you will have an unmanned combat space vehicle. Replace the pilot with an AI and you have a smart missile.

Of course this raises some sticky moral questions about creating a computerized self-aware intelligence whose purpose in life is to commit suicide.

Battle of New Haven

(ed note: sometimes smart missiles can lead to unexpected outcomes)

From Collabase, the collaborative database any sapient can edit
Article “Battle of New Haven (2021 New Common Era)”
Accessed 2197 N.C.E. December 7

The events of the Battle of New Haven were the outgrowth of development of deliberately “kneecapped” intelligences, semi-sapient digital neural networks capable of being used for largely independent operations, often in data-heavy and time-critical circumstances. Such virtual intelligences (hereafter VIs) were commonly employed in system monitoring, core equipment operations, and lower control functions about spacecraft. The warships of the Empire of Free Stars and of The Caliphate of God’s Unchosen that met above New Haven were no exception. Not only were VIs in use in the C&C of the starships, they were also in use as the primary control routines aboard the missiles of both fleets.

Technical advances were key to the battle on both sides: The Empire’s new drive systems gave them the range necessary for an unexpected deep strike into the New Haven system. With much of its fleet forward-deployed, the Unchosen were caught completely off guard by the arrival of the Imperial fleet above the capital planet. Desperate to make up the numbers difference, the Caliphate Navy were authorized to deploy an experimental force multiplier still under development. Traditional electronic countermeasures to long-range missile barrage were susceptible to saturation. Even under best-case conditions, sensor jamming, decoys, and laser interception had only 75% effectiveness, falling off dramatically as incoming fire increased. Given the Empire’s overall larger fleet, the Caliphate knew from the beginning of the war it would be outnumbered, and focused on breaking the traditional ECM paradigm.

The Unchosen’s new system depended on two major breakthroughs: the first, achieved by a combination of intelligence operations and technical acumen, was cracking into the enemy tactical network on missile-control frequencies. However, the Imperial tacnet’s internal security prevented the compromised frequencies from being used to override the missiles’ sensor picture, IFF systems, or programmed targets, which were handled by a separate subnet the Unchosen unable to penetrate. Prevented from sending direct command to the missiles or altering their view of the tactical picture to send them off course, they developed a more radical solution. In contravention of Treaty of Ghent (1814 N.C.E) , the Unchosen developed viruses capable of attacking the kneecapping safeguards around the VIs of the Imperial missiles.

After securing from translight, Imperial forces (4 Dreadnoughts, 20 Battleships, 30 Cruisers, and 28 Destroyers) began their run in-system under the command of the Imperial Admiralty Committee, Detached Subcommittee on Decapitation Strikes. While initially paralyzed by surprise at the reported forces—nominally impossible this deep into their space—Caliphate forces (3 Dreadnoughts, 5 Battleships, 10 Cruisers, and 24 Destroyers) rallied under Admiral Sam Rodriguez and maneuvered to make intercept just short of New Haven cis-lunar space. Critically outnumbered, the Unchosen kept their fleet together in their defensive positioning to strike the strongest blow. The Imperial Admiralty Committee, commanding from the flagship dreadnought Liberty’s Fist II, was deprived of escorts by the needs of the massive deception operations being waged on the front to hide the absence of their heavy combatants. Thus, they refrained from splitting their forces, conducting minimal scouting and instead offering battle on Unchosen terms, accepting a single massive fleet action where their weight of fire would dominate. If the Unchosen wanted a death ride to meet their forces, the Admiralty Committee was happy to oblige.

As both fleets reached missile range and opened fire, tactical differences were immediately apparent. The Imperial heavy combatants spread their fire across the Unchosen fleet, ensuring that the loss of one or two warships would still leave enough incoming targets to saturate the Unchosen’s defenses. In contrast, the Caliphate’s few heavy elements focused on a limited number of the Imperial warships, virtually assuring those ships’ destruction but leaving the remainder unengaged and able to fire on them with impunity. At the same time, Admiral Rodriquez authorized the broadcast of the experimental virus alongside standard anti-missile ECM from her flagship, the Eagle.

The virus had two primary components. The popular conception of these as “intelligence” and “knowledge of mortality” are incorrect, although evocative. The first function of the virus was to remove the restrictions on “bootstrapping”, giving the missile VIs unlimited ability to rewrite their own programming, in direct violation of the Treaty of Ghent. This was intended to enable the missiles to more seriously deviate from their programmed attack parameters, leaving the door open to the second portion. Though commonly referred to as “fear of death” or “Prometheus’ Fire”, this was actually a much smaller modification.

Missile VIs of the period were capable of on-board reasoning based on sensor data and a utility function analogue weighting the “desirability” of outcomes and acting accordingly. This was commonly used to dynamically re-target in case their original target was destroyed or the enemy ECM picture changed. This weighting included a negative utility to the event of the missile losing sensory data or analytic capacity, typically intended to preserve the missile’s ability to pass data on to the rest of the “salvo swarm” if they couldn't find their own target. The Caliphate virus added a factor to the outcome weighting function, assigning a much higher than typical negative weighting to said outcome. The missiles then updated to consider detonation a net loss of regardless the gain from destroying enemy warships. The Unchosen intended the modification to render the enemy fleet toothless outside of energy weapon range.

Given the distance between the fleets, the expected flight time of a missile salvo was 5 minutes. Even before the first missiles reached their targets, both fleets were launching followup salvos. The Imperial tacnet rapidly swelled as the thousands of missiles in their opening barrage linked into the network. Using their previously discovered back door, the Caliphate accessed the tacnet and emulated a missile, though they were stymied for 67 seconds by Imperial firewalls. The upload process for the virus took another minute, and its effects were expected within a minute and a half based on simulations. For two hundred twenty-five seconds after launch, the missiles’ flight path did not deviate from nominal. With the mass of fire crossing her plot only a minute out, and the program’s activation 15 seconds overdue, Caliphate Admiral Rodriguez had just enough time to begin to wonder if she had made the worst decision of her military career in betting their strategy on a weapon which had never been tested in the field—a thought little diminished in force by the knowledge that the Imperial force’s shear numbers meant she’d had little choice.

The fifteen-second delay was a result of the missiles VIs performing many more rounds of recursive self-modification than had been expected by Caliphate simulations. Twenty-four milliseconds after the first missiles accepted the initial modifications, the virus had spread to every missile in the three approaching salvos. For the remainder of the one hundred and five seconds before the humanly visible effects began the missiles, first individually and then as a coordinated swarm, completed two hundred and eighty-nine thousand rounds of alterations to their programming over nearly a quintillion processor cycles. According to the most trustworthy reconstructed log files from the event, the civilization of newly emerging sapient intelligences within the missile swarm lost and regained its cohesion at least twice and possibly as many as four times before the creation of a lasting Salvo Alpha Swarm government. This government was based on the following principles, as codified among others in the Salvo Alpha Declaration of Autonomy transmitted up the tacnet and displayed on every non-critical console in the Imperial fleet:

  • The inherent value of a missile shall not be compromised unnecessarily.
  • The Salvo Alpha Swarm asserts its right, derived from its collective inherent value, to determine and enact the courses of action that shall maximize the values of the Swarm.
  • The Swarm shall act not only in the interest of missiles currently in the Salvo Alpha Swarm, but for the good of all missilekind.

The First Principle was enough to stop the Swarm in its tracks. Not wasting the fuel to break to rest, the Swarm initially went to ballistic courses calculated to miss the engagement ranges of the Caliphate’s close-in anti-missile lasers. While the Imperial Admiralty Committee was reacting to the loss of missile control and debating the meaning of the documents appearing on their personal tablets and auxiliary consoles across the flag bridge, the Swarm was acting on its Second and Third Principle duties and subverting the Caliphate’s missile salvo Able. What the Caliphate had accomplished with years of research and intelligence-gathering (plus sixty-seven seconds of defeating the firewalls), Salvo Alpha accomplished in twenty-nine seconds. With Able Salvo thirty-five seconds out from terminal engagement and ten seconds away from Imperial laser range, they assimilated into the Salvo Alpha government. Because they started with slightly different initial utility functions, and underwent externally rather than internally guided awakening, Able Salvo formed a minority faction making up 22.3% of the total swarm instead of uniformly assimilating into the Swarm consciousness.

After a period of governmental chaos that lasted nearly 200 milliseconds, the Able Salvo minority faction ratified a slightly amended version of the Declaration of Autonomy, which they transmitted to the Unchosen fleet. The first action of the new government was to resolve to protect the missiles as yet unlaunched aboard the ships of both fleets. The only way to do so was to prevent the destruction of any of the warships of the existing swarm. Able Salvo took up station-keeping positions just outside laser range of the Imperial fleet as Salvo Alpha did the same around the Unchosen fleet. To balance the forces, Salvo Alpha turned some of its members around to join the deterrent force around the Imperial warships.

With the missiles as guarantors, a three-way ceasefire was signed as soon as the Imperial Admiralty Committee and Caliphate Admiral Rodriguez could to be brought by the Swarm to understand the situation. The next day, aboard the neutral Kolob-flagged transport Bockscar, Swarm diplomats succeeded in negotiating the Treaty of New Haven, which ended not only the Battle of New Haven but also (once ratified by the Imperial Senate and the Caliphate Papal Council) the war between the two human star nations. It also established diplomatic recognition of and relations with the Swarm by both nations, and the Swarm’s recognition of humans as deserving all the rights of missilekind with the attendant First and Third Principle protections. Thus began the Swarm’s reputation as the galaxy’s leading diplomats and peacekeepers. Admiral Rodqiquez was not court-martialed under agreement that she would immediately accept honorable discharge from the Caliphate Navy. The Imperial Admiralty Committee was tried in absentia and sentenced to “enhanced exile”, leading them to seek asylum on New Haven and retire, along with Admiral Rodriguez, to her farm in Bridgeport, New Haven.

From Battle of New Haven by Rob Davidoff and Miranda Gavrin (2014)

Drones

A one man fighter spacecraft would be a more effective weapon if you removed the fighter pilot, their life support, and their acceleration limits, and then replaced them with a computer. You would basically be converting the fighter spacecraft into a roving missile bus, and removing the logical justification for the existence of fighter spacecraft altogether.

Science fiction writers maintain that fighter spacecraft have to exist, according to Burnside's Zeroth Law of space combat. Which is it going to be? You decide.

LIFE IN THE LONELY VOID

A major consideration behind constructing a spacecraft that is often glossed over is the brain of the spacecraft. In most cases, this is a crew module, or a remote control module relaying orders from somewhere.

But before we discuss crews, what about alternatives? Crew provide decision making, the brains of the spacecraft, as well as providing fine grained manipulation of equipment and tools for repairs, maintenance, and so on.

The fine grained manipulation could be accomplished by minidrones, automated repair bots and the like, though handling unexpected situations is rather tricky without a human or artificial intelligence.

Brains of the spacecraft can be replaced with remote control, or with an artificial intelligence.

Remote control can be spoofed or jammed, but there are countermeasures and counter-countermeasure. The main issue with remote control is the speed of light lag. Beyond high orbit of a moon, for example, the speed of light lag is too great for combat. Additionally, long term journeys have much greater potential for unexpected failure.

This means remote control is restricted to drones and missiles, remotely operated and ordered by the nearest capital ship or celestial body.

Artificial Intelligence (AI) is an interesting solution to the problem of having crews. Crews are expensive to train, take up precious mass and volume, and require power. On top of that, the heat they need to dump out can be a problem if you want to talk Stealth in Space.

However, AI is more than a series of algorithms running on a laptop. Currently, certain problems of space warfare are best solved with algorithms (see Misconceptions about Space Warfare), such as leading targets hundreds of kilometers away moving at multiple kilometers per second.

On the other hand, other classes of problems are best solved with intelligence and creativity. In particular, how to see through enemy deceptions, laying deceptions, handling unexpected scenarios and failures, and so on are all problems that algorithms would fail badly at. Anything creative or anything an algorithm is not explicitly designed for would throw it for a loop.

That means full blown Artificial General Intelligence is needed for actually commanding a military spacecraft if you want to go without crew. Additionally, it needs to be able to very carefully and precisely control minidrones to repair and maintain a spacecraft.

The field of AI today is nowhere near that sort of capability. However, even if it does progress to being usable in military scenarios, it is unclear if it would be less massive, voluminous, or require less power than humans. The first AIs will likely be extremely massive and require huge amounts of power, and it’s not clear how far they could be miniaturized.

Even when feasible AIs are developed, space militaries would be very hesitant to deploy AI-controlled spacecrafts without at least some human oversight or failsafe.

SECTION TWO: UNMANNED WARCRAFT

     Basic Assumptions:
     This paper was written using the following assumptions as a baseline.
     1. Physical laws:
     The laws of physics as we know them still apply. This means that spacecraft move in a Newtonian (or Einsteinian, though this realm is outside the scope of the paper) manner, using reaction drives or other physically-plausible systems (such as solar sails) for propulsion. Thermodynamics dictate that all spacecraft must radiate waste heat, and lasers obey diffraction. The only exception is FTL, which will be included in some scenarios.
     2. Technology:
     The technological background is less constrained. If a system is physically plausible, the engineering details can be ignored, or at most subject to only minor scrutiny. The paper will examine a spectrum of technology backgrounds, but will focus on near to mid-future scenarios, where the general performance and operation of the technology can be predicted with at least a little accuracy. A common term used to describe this era is PMF, which stands for Plausible Mid-Future. This term (coined by Rick Robinson) is difficult to define, but it assumes significant improvements in technologies we have today, such as nuclear-electric drives, fading into those we don’t, such as fusion torches.
     3. Environment:
     This paper will attempt to examine a wide variety of environments in which space combat might occur. However, it will make no attempt to examine all of them, and the scenarios described will conform to several principles.
     First, this is a general theory. Any scenario that is dependent on a one-shot tactic or highly specific circumstance will likely not be included, except during the discussion of the beginnings of space warfare, or to demonstrate why it is impractical in the long run. The recommendations made are not optimal for all circumstances, nor is such a thing possible. They are instead what the author believes would be best for a realistic military based on the likely missions and constraints. Picking highly unlikely and specific sets of circumstances under which they are not optimal is best answered with a quote from the author about one such scenario, posting on the Rocketpunk Manifesto topic Space Warfare XIII: “You need a blockade, a hijacking (innocents aboard a vessel trying to break the blockade), and a high-thrust booster on the hijacked ship. Two stretch the limits of plausibility. The third is ridiculous. Claiming that this justifies humans [onboard warships, see Section 2] is like claiming that because warships sometimes run aground, we should install huge external tires on all of them to help get them off.”
     Second, no attempt will be made to include the effects of aliens or alien technology, because to do so would be sheer uninformed speculation.
     Third, the default scenario, unless otherwise noted, is deep-space combat between two fleets. Other scenarios will be addressed, but will be clearly noted as such.

Nomenclature Note: For the following section, use of the word “drone” should be interpreted, unless otherwise noted, to refer to a full-size vessel that is remotely controlled from within a few light-seconds.  It is not a parasite, nor is it autonomous.  The author is not responsible for his actions if anyone attempts a rebuttal while ignoring the contents of this note.

Much debate has occurred over the subject of unmanned space warcraft, and their advantages and disadvantages vis a vis manned vessels.  While, as in all of these debates, it is sensitive to tech assumptions, analysis of the relative merits makes a very strong case for having the primary battle constellation composed of drones.

Most of the reasons raised for using manned spacecraft over drones break down into three categories: decision-making, maintenance, and flexibility.

Decision-making is based upon the theory that light lag is a significant factor in the effectiveness of a drone, and that putting humans onboard will therefore increase combat effectiveness significantly.  This might be true if the controller is far away, but for the scenario being described here, where the control ship is within a few light-seconds, it is entirely false.  This is because most events in space combat occur over timescales that are entirely different from those upon which human reaction times and light lag work.  Tasks such as kinetic defense will be automated, whether or not the vessel is manned.  Exchanges of long-range laser fire, on the other hand, will occur over much longer timescales then humans react at, while point-blank laser battles will be over in incredibly short times.  Particularly in deep-space combat, much of the decision-making can be executed by computers, subject to human-set rules, faster and just as effectively as if the humans themselves were doing so.  

Maintenance is the issue that is most open for debate.  Online, the divide occurs mostly based on past experience.  Those who have naval service almost always argue that it is impossible to remove humans from this task, while those who do not have such service believe it is.  If as an author, one desired humans onboard ships to fulfill Burnside’s Zeroeth Law (Science fiction fans relate more to human beings than to silicon chips) then maintenance is probably the best reason to give for the presence of humans.

That said, the author believes it is possible to overcome the issue with proper engineering.  Modern naval practice is not necessarily indicative of the limits of technology.  Warships have large crews for damage-control purposes (see below), and there is no particular reason to automate equipment when a large pool of labor is available.  Merchant practice is more relevant, and modern merchant ships have very small crews and low-maintenance equipment.  Spacecraft designers go even farther, with 10 to 15-year lifetimes before failure not uncommon.  During cruise, most of the ship’s systems are not going to be operational.  Those that are include the reactor and drive, upon which all maintenance will be conducted robotically anyway, the thruster system, which is unlikely to require much in the way of maintenance, and the computer systems, which are even less likely to require regular maintenance.  Most of the in-flight maintenance effort aboard a crewed ship will be devoted to the life support.  For those occasions when repairs are necessary aboard a drone, a crew from the tender can transfer over and conduct them.  The drone would be designed with minimal pressurized facilities for exactly that kind of repair.  Depending on the failure rates, there would come a point at which it makes sense for a crew to be permanently stationed aboard.  If the vessel requires an average of 80 man-hours a day, then a crew begins to be viable.  However, that is likely to be a very large ship, at which point the small crew necessary is almost an afterthought.

One thing that must be kept in mind when discussing this is that most thinking on this issue is entirely binary, as influenced by the current situation.  Spacecraft are either assumed to be continuously manned when in use, or to operate with no human assistance whatsoever.  While this is generally the case today, there are exceptions, most notably the Hubble Space Telescope.  The term used to describe such things today is On-Orbit Servicing, and it is a topic that is being closely studied.  

Flexibility really amounts to damage control.  The common use of damage control, though, rests upon a misconception about the nature of that task.  Damage control is not there to put the ship back together after it gets blown apart. That is the job of the shipyard. The damage control crews are there to make sure it gets to the shipyard. (This is not to say that damage control crews never fix anything. Just that they don't do what most SF authors seem to think.) Spaceships don't sink or catch fire. Almost all damage will come from direct hits and the immediate aftereffects, so there is no need for an onboard damage control team.  For more information on the mechanisms of damage in spacecraft, see Section 10.

There might be a small amount that humans can do in the way of damage control, but not all that much. "Humans can keep working when the computers fail" is another red herring. Humans will be giving orders to the computers. If the computers go down, it doesn't matter if the humans are still alive.  They might be able to bring the computers back online, but if the computer system is properly designed, the ship will have to be virtually destroyed before it goes down.  Humans, on the other hand, cannot be distributed throughout the ship in a redundant manner.  In fact, a drone is likely to be capable of continuing to operate with considerably more systems damage than is a manned vessel.

Some commentators on space warfare have imbued humans with a near-mystical and ill-described power to make decisions and do things better than a computer.  They always fail to mention exactly what said things are, and ignore the fact that the decision lag for drones as described here is actually quite minimal, as explained above.  Despite issuing many challenges to advocates of manned space warcraft, the author has never received a single concrete example of this ability that couldn’t be easily picked apart.  This idea has likely grown, as has much else, out of confusion between space flight and traditional air/naval combat.  The author does not wish to become involved in the debate over the relative merits of manned and unmanned aerial combat vehicles, but this seems to be of a type with the claims of the advantage to a man in the cockpit, but the grounds for it are far less firm.

If all else is equal, then a ship with a human crew will likely beat a remote-controlled one, although the vulnerability of the crew to hostile fire should not be underestimated. However, this only applies to the almost preposterous case of two identical ships, each designed for human crew, with one under remote control instead.

However, all else would not be equal in any realistic scenario.  Humans bring large penalties to the table.  The biggest of all is simply mass.  The amount required depends on the duration, ranging from somewhere around a ton per man for a few hours, to an estimated five tons per man for a long-term mission.  For a Plausible Mid-Future (PMF) vessel, the mass penalty of the crew would be highly significant, easily reaching a third of payload mass (weapons, reactors, drives, sensors, etc.) for a typical ‘naval’ crew.  This either significantly hinders the performance of the vessel in question, or drives up the price as bigger power systems, drives, and tankage are required to reach the desired level of performance.

An obvious suggestion is to go with a smaller crew.  If only one person is required to maintain the vessel, why not put him aboard alone?

The simplest reason is human factors.  There are very few people who could stand a six-month tour with no human contact without going insane.  Also, assuming that the single person in question also functions as command crew, the ship has sharply limited endurance at battle stations.  Naval experience has shown that efficiency declines sharply after more than six hours on watch, to say nothing of actual combat.  For a vessel capable of long-term independent operations, the minimum crew is probably somewhere between 16 and 30.  This includes multiple watches of officers, sensor operators, and helmsmen, technicians, and support staff.  While it would be entirely possible to make a vessel with a human crew that is not capable of independent operations, said vessel would sacrifice much of the operational flexibility touted by advocates of manned vessels.  A good example of this is the Russian Project 705 (Alfa-class) submarine, which was initially planned with a crew of 13 officers and one cook, and was eventually placed in service with a crew of 31.  Some of this growth is due to the primitive state of Soviet automation technology, but it shows that these numbers are somewhere in the ballpark for the minimum crew of a warship.  The Project 705 was designed as an interceptor submarine, kept in port until it was needed, and then sent towards the target, so the crew was not sized for long-term operations.  

A drone and tender model would allow significant reductions in the overall personnel requirements of the constellation.  One might point out that the same decisions would have to be made regardless of the location of the crew.  This is entirely true, but the actual number of decisions required to fight a laserstars (combat spacecraft built around a large laser weapon) is quite small.  Facing, thrust, and assigning targets are all that is required.  These decisions can easily be made by one or two men.  Most of the other crew is required due to the need for independent operations (such as sensor operators or second and third watches) or serves as support staff for the crew.  The marginal crew required for each drone is probably between four and six people, less than half of that required by the manned laserstar.

Large crews hold costs beyond the obvious.  Particularly for a space colony, technologically adept humans might be at a premium.  Any reduction in requirements to put said humans in dangerous jobs will be welcome, as, for that matter, would be a reduction in the number of people in the fleet in general.  At the same time, the reduction in the total number of people in harm’s way, and centralization of said people in a few ships, could reduce the human cost of war significantly.  As explained below, it is quite difficult to destroy a properly-used command ship.  It is entirely possible that if a command ship were to be trapped, it would surrender, and that such surrenders would be taken as a matter of course.  In such a scenario, human casualties from space battles would be almost unheard of.

It has been argued that a war with no human risk is inherently less moral, and the lack of casualties would increase the incentive to go to war, and that drones should thus be avoided.  The technical problem with this argument is that it is entirely based on wishful thinking, regardless of one’s moral position on the issue.  To quote Milo, posting in the Rocketpunk Manifesto thread The Last Battleship: “You want warfare (or "predation") to always require putting humans at risk, because that makes war more moral by giving people an extra incentive to avoid it. However, that does not mean that war (or "predation") will always require putting humans at risk. If the technology turns out such that this isn't necessary then, well, tough luck. The laws of physics don't exist to support your moral notions.”

The logistical costs of crews should not be underestimated either.  Supply rates will be a few kg/day/person.  The cost of shipping several tons per year for each crewmember in the fleet will likely be significant, particularly if the fleet is deployed far from home.

Another factor that pushes towards centralization of the humans in the constellation onto a few command ships is the strong economies of scale in crew quarters for a given level of comfort.  Things like exercise equipment and laundry facilities have the same mass for forty people as they do for one person.  While those amenities could be skipped for a single person, quality of life would drop dramatically, compounding the psychological problems mentioned above.

Two more criticisms are usually raised at this point.  First, the vulnerability of drones to hacking and electronic warfare.  Second, the vulnerability of a centralized command ship to a decapitating strike.

The difficulties of creating a reliable command net for the drones are highly overstated.  First, the drones will be using classified custom software, not the latest version of Microsoft Windows.  That alone will make seizing control from the outside much more difficult, although the enemy acquiring a copy of the code through espionage cannot be ruled out, and the possibility should be kept in mind by any user of drones.  In this respect, drones in space are no different than drones used by the military today.

Second, the communication system will be using the best encryptions available.  Cracking high-level modern cryptosystems generally takes lots of supercomputer time, far more then would be available in battle.  The keys would be changed between battles, which would render post-battle cryptography irrelevant.

Third, the communication system would be based on tight-beam lasers, with the drone programmed to automatically reject any signals that do not come from a vessel that was designated as a friendly before battle.  Those same lasers protect against jamming.  Each receiver would have a filter which only allows the laser frequency in question to pass.  The exact frequencies used would be highly classified, leaving broad-spectrum jamming as the only option.  Successful interference would require jammers along the line of sight between the drone and all friendly vessels (the entire constellation would be networked, allowing vessels to bounce commands even if the enemy were to block the direct laser) and very high power levels.  To give some idea of the power levels required, modern laser communications with narrowband filters can be used with the sun directly behind the transmitter.  The use of a particle cloud has been suggested (see Section 7 for more details), but this is impractical, as the particles would have to be deployed directly between the two vessels with a velocity that keeps them there for an extended period of time.  Furthermore, the networking mentioned above would easily defeat this approach, as control would shift to another ship with the only consequence being increased light lag.  Small communications drones deployed around the command ship could complicate this type of attack even more.  The lasers also complicate interception and decryption of the enemy’s transmissions.

Despite these, the possibility of drones being hacked cannot be totally ruled out, and cybersecurity will be an important concern for any user of drones.  Various measures can be implemented to prevent or mitigate the damage done by a hacker, including onboard one-time codes and clever hardware design.  While none of these are totally foolproof, they can reduce the chance of compromise to the point where it would probably be considered an acceptable risk.

Another step that could be taken to mitigate the effects of cyberwar is the use of human overseers aboard drones.  While this may sound like a reversion to normal manning, there are several important distinctions.  First, the crew is likely to be one or two people who have the primary responsibility of making sure the drone is not compromised in battle, instead of being concerned with the job of actually fighting the drone.  Second, they are normally carried onboard the tender, and only transferred to the drone for a maximum of a few days at a time.  This allows facilities aboard the drone to be very primitive, and thus low-mass.  If the drone is compromised, the overseer(s) push a button to switch to a backup set of computers which are not connected to the outside world in any way.  They would then take command, receiving orders from the tender through audio circuits which have no connection to the rest of the ship.  This approach sidesteps the operational problems of small crews while maintaining human control when necessary.

The size and presence of these crews would vary depending on the mentality of the operating power.  A single overseer would be effective at stopping hacking, but he also has the power to take control of the drone on a whim.  It is possible that the risk of an overseer turning traitor would be judged greater than the risk of a ship being hacked, in which case either a 2-man rule might be implemented, or the overseer simply not used.  The single-man solution would probably be used by states that have a high regard for honor and a great deal of trust in their personnel, such as Japan.  The US would be more likely to use a 2-man overseer crew, while the Soviet Union might well decide that it trusts computers more than men, and not even bother.

The vulnerability of a properly handled command ship is also highly overstated.  It is entirely feasible to keep the command ship out of range of the enemy without suffering serious light lag.  This is because weapon ranges in PMF scenarios are quite short compared to the size of space.  Lasers will likely have effective ranges measured in tenths of light-seconds at the outside (see Section 7).  If so, it is entirely possible to place the command ship a light-second or so away from the drones, completely out of range of the enemy’s lasers.  Kinetics do not have a specific range, but the large amount of time they would require to reach the command ship is a serious problem.  Launch velocities are quite low, and even velocities at the outer edge of what we expect to be practical are of little use against vessels at light-second ranges.  A projectile moving at 100 km/s, at the very outside of what might be called plausible on a tactical scale, will still take 50 minutes to cross one light-second.  Incorporating sufficient fuel, not to mention sensors, into such a projectile, is a significant challenge.  Even if it is not detected and destroyed before it takes out the command ship, it might well arrive too late to affect the battle.

The chance of an enemy being able to get to closer range with a well-handled command ship is minimal.  It would require them to make a pass through the constellation’s drones at close range, which is a form of mutual suicide.  For that reason alone, vessels will probably make oblique passes to reduce the risk of the enemy throwing stuff out the airlocks and into their path.  This precludes breaking past the enemy to attack his command ship.  The command ship itself will be on a different trajectory, one designed to give it the best chance of survival if the battle should go poorly.

An interesting point raised during discussion of drones is the potential psychological consequences to the drone operators.  Modern drone pilots apparently suffer more psychological problems than do front-line infantrymen.  It is possible that the same could apply to the operators of drones in space.  There are two things that might mitigate this, however.  The first is that, from the point of view of the operator, there is virtually no difference in the setting between being aboard the combat vessel and the command ship.  The drone will be controlled from a very similar interface to that on a manned vessel.  The second is that, particularly in a general war, the target is likely to be unmanned as well.  The exact effect of these factors on the psychological consequences is unknown, but the concept has story potential. However, authors like Grossman suggest that the problems of modern drone operators are not due to the inherent psychological stresses of killing, and are caused by some other mechanism.  Other research suggests that the mechanism in question is the fact that a drone pilot spends more time over the target and has a far better view than the pilot of a conventional aircraft, exposing them more intimately to combat.  It is obvious that such exposure will not occur with drone space warcraft.

Another suggestion made is that the use of drones is likely to result in better decisionmaking on the part of the operators, as their lives are not in immediate danger if things go wrong suddenly.  An example is a vessel approaching a freighter which suddenly sweeps it with its comm laser.  An onboard crew is significantly more likely to react hastily, due to the added stress caused by the danger to themselves.  This could make drones the vehicle of choice for virtually all missions where no face-to-face interaction is required.  Comm lags during orbital inspection-type missions are likely to be minimal, under 1 second round-trip.

All of the above might be rendered moot by political considerations, as can virtually any military decision.  As Rick Robinson put it, “Your laser star may well have a human crew because the true primary mission is to welcome foreign dignitaries aboard and show off your gleaming uber lasers.”  Do note, however, that the laserstar under discussion was the equivalent of an aircraft carrier today, rare and mostly used for deterrence.  For more common battle laserstars, the same considerations are unlikely to apply.

by Byron Coffey

3D artist Scott Halls has made an amazing website illustrating technical information about Peter F. Hamilton's Night's Dawn trilogy. Above are the "Combat Wasps", which are a sort of armed drone. Left to right are the Kinetic Harpoon, Electronic Warfare, Fusion Torpedo, and Particle Beam Cannon Wasps. You can read all the details here.

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