Introduction

With all this frightfulness flying at your ship, you'd want some kind of defense, besides just hoping they'll miss. Go to The Tough Guide to the Known Galaxy and read the entry "SHIELD".

As mentioned before, advances in effectiveness of weapon lethality and defensive protection are mainly focused on the targeting problem. That is, the weapons are generally already powerful enough for a one-hit kill. So the room for improvement lies in increasing the probability that the weapon actually hits the target.

And the room for improvement on the defensive side is to decrease the probability of a hit.

Weapons can be improved two ways: increase the precision of each shot (precision of fire), or keep the same precision but increase the number of shots fired (volume of fire).

Precision of fire is governed by

  1. the location of the target when the weapons fire arrives
  2. the flight path of the weapons fire given characteristic of the shot and the environment though which the shot passes
  3. the weapon's aiming precision

Volume of fire is governed by

  1. the weapon's rate of fire
  2. the lethality of a given shot

A defense can interfere with the [a] location of the target by evasive maneuvers.

There isn't really a way to interfere with [b] the characteristics of a shot, short of inserting a saboteur into the crew of the firing ship (in science fiction there are sometimes technobabble "nuclear damper fields" that prevent nuclear warheads from exploding). A defense can interfere with the environment through which the shot passes by such things as jamming the weapon's homing frequencies or clouds of anti-laser sand (which may work in the Traveller universe, but not in reality).

There isn't really a way to directly interfere with [c] the weapon's aiming precision (again short of a saboteur), though one can indirectly do so by decreasing the target's signature, increasing the range or jamming the firing ship's targeting sensors and degrade their targeting solution (in science fiction is the infamous "cloaking device").

Finally, while one cannot do much about the [d] weapon's rate of fire, the [e] lethality of a given shot can be effectively reduced by rendering harmless shots that actually hit. This is done by armor and point defense (in science fiction there are "force screens" and "deflector shields").

Defensive Systems

The innermost of a starship’s defensive systems is its armor. The primary armor is a multilayer (“honeycomb”) system over the core hull, composed of multiple vacuum-separated layers of refractory cerametals, sapphiroids, and artificially dense metal nanocomposites, strapped together via flexible, shock-absorbing forms. Atop this, a thick sprayed-on layer of foamed-composite ablative armor (whose vaporized form is designed to scatter incoming energy weapon fire) provides additional protection.

To provide thermal protection, each of these layers is threaded through with a mesh of thermally superconducting material, preventing heat input from lasers or other energy weapons from creating localized “hot spots”. This mesh spreads out external heat inputs, and ultimately dumps them into tanks of “thermal goo”, an artificial substance of very high specific heat capacity. Under normal circumstances, this heat is disposed of via the ship’s radiative striping and external radiators, but if necessary, the thermal goo can be vented to space, taking its heat (and, unfortunately, its heat capacity) with it.

Outside the armor, starship defenses come in three more layers:

First and innermost, the kinetic barriers. These are not a single, all-encompassing bubble; rather, they are a grid of plates of gravitic force, instantiated as needed to intercept incoming material objects. (They cannot shield against massless radiation.) They don’t attempt to directly retard incoming projectiles; rather, their job is to “slap them aside”, imposing enough sideways vector on them to generate a miss.

(ed note: kinetic barriers are science fictional)

Outside that, the defense drones: a military starship at general quarters surrounds itself with a “cloud” of small defense drones, serving multiple purposes: as electronic warfare platforms to obscure its signature; as participants in the kinetic-barrier generation and point-defense grid; as additional sensors; and ultimately, as sacrificial platforms capable of physically intercepting incoming projectiles or autonomous kill vehicles (AKVs) before they reach the ship itself.

Outermost is the point-defense zone guarded by the point-defense laser grid, extending substantially outward from the ship itself. Composed of phased-array plasma lasers which can be generated across large regions of the starship’s hull, the point-defense grid is used to vaporize incoming projectiles (or to use partial vaporization to decelerate incoming projectiles for the kinetic barriers and armor to deal with more effectively) and to force AKVs operating nearby – which have relatively little heat-dissipation capacity – into thermal shutdown.

The point-defense laser grid can also be used as an offensive weapon against any other starships unwise enough to stray into its range, but few captains are stupid enough to bring their starship into another ship’s point-defense zone.

The final defensive system that any starship has is drunkwalking: when at any alert state higher than peacetime cruising, every military starship engages in a pseudo-random “drunk walk” of vector changes around its station-keeping point or base course. This ensures that the starship is almost impossible to achieve a firing solution upon from a distance, since its movement since your most current observation of the target is unknown, and further increases the difficulty of achieving a solid firing solution in close.

(Of course, this depends greatly upon the quality of your drunkwalk algorithms and that they have been kept secure from the opposing force, which again underscores the importance of information warfare in the modern battlespace. A starship whose base course is identifiable and whose drunkwalk algorithms are known is a sitting duck even in the outer engagement envelope!)

Defeat

Unlike starship armor, neither the point-defense laser grid nor the kinetic barriers are subject to direct attrition; if subjected to low-volume or low-power incoming fire, either or both could continue to destroy or repel it essentially forever.

In order to defeat these defensive systems, it is necessary to swamp them; to concentrate incoming fire to the point at which the defensive systems are unable to handle it all simultaneously. At this point, attrition may take effect as kinetic effectors and laser emitters are destroyed, but more importantly, it generates heat.

Heat is the primary limitation on combat endurance. Maneuvering burns, the use of high-energy equipment such as the point-defense grid, the kinetic barriers, and so forth, as well as the ship’s normal operation, all produce heat. In combat – when the ability to radiate heat is limited, usually to radiative striping and small (and exhaustable, if the starship is forced to maneuver) droplet radiators alone – military starships generate heat more rapidly than they can radiate it to space. As heat increases beyond the critical point, the efficiency of onboard equipment begins to fall (processor error rates rise, for example, and tactical officers must conserve their remaining heat capacity), some equipment goes into thermal shutdown, and the crew spaces become increasingly uninhabitable.

While some starships in any major space battle are destroyed physically, reduced to hulks, the majority of starships are defeated by either heat-induced equipment failure, or by being forced to surrender and deploy radiators lest their crew literally cook.

Evasive Maneuvers

The first rule of fighting is: Don't get hit.

If you can complicate your opponent's firing solution enough (i.e., dodge enough so all the shots miss), you do not need all that heavy bulky armor. Of course, if a shot does hit, you are up doo-doo pulsar without a gravity generator.

With fighter aircraft: weapon speeds, aircraft speeds, and target ranges are such that the main targeting problem is the large angular changes the undertaken by the target (you cannot slew the gun around quick enough). With spacecraft, however, the problem is light-speed lag and weapon lag. Light-speed lag means if your target is at a range of one light-second, you are seeing it where it was located one second ago, not where it is now. Weapon lag means you have to lead your target so that your plodding weapon shot will intercept it (the technical term is "deflection").

The attacker can try to minimize this by reducing the range, or using homing weapons that guide themselves to the target. The defender will try to open the range, and use various counter-measures to confuse the weapon's guidance system.

Naturally, if the target moves at a constant velocity with no course changes, light-speed lag and weapon lag cease to be a problem. Attackers love a target wtih a perfectly predictable course. Therefore, a target that wishes to live had better dodge and jink as much as possible.

Light speed lag and weapon lag will put an upper limit on the maximum probablity that an unguided weapon will strike the target. If you dare, you can calculate it with an equation I cobbled together all by myself (which means you had better double-check it first as I have been known to make childish mistakes in algebra).

H = Cm / (0.7854 * a2 * ((Dm / 299,792,458) + (Dm / Wv))4)

where:

  • H = maximum percent chance to hit target given light-speed lag (0.0 - 1.0 with 1.0 = 100%)
  • Cm = target ship's mean cross section (m2, for a purely convex object this is approximately 1/4 of the surface area)
  • a = target's acceleration (m/s, where 9.81 = 1 g)
  • Dm = range to target (m)
  • Wv = weapon velocity (m/s)
  • 299,792,458 = meters in one light-second
  • 0.7854 = π / 4

Cm is the average target cross section. The target will be trying to orient itself so it presents the minimum possible cross section to the attacker, but the requirements of its propulsion system and other factors will interfere.

Since laser weapons travel at lightspeed (Wv = 299,792,458), for them the formula simplifies to:

H = Cm / (0.7854 * a2 * ((Dm + Dm) / 299,792,458)4)

Please note that this equation does not work if the target's acceleration is zero (since dividing by zero is mathematically undefined). In that case the target's official status is Sitting Duck and H = 1.0 or 100%. Neither does the equation work if the range is zero, in which the target's official status is At Point Blank Range or Eating The Gun Muzzle, and again H = 1.0 (Thanks to Eric Henry for pointing this out). Just remember that H cannot go over 1.0 and you'll be fine.

How was this equation derived? (just wait until you get a load of my assumptions...) Well, if H is chance to hit, a is acceleration in m/s, Dm is range in meters, and Cm is target's mean cross section:

Scircle = π * Rcircle2

where:

  • Scircle = surface area of a circle
  • Rcircle = radius of the circle
  • π = Pi = 3.14159...

H = Cm / (π * displacement2)

where:

  • displacement = maximum distance perpendicular to line of fire that the target can move in time between a shot being fired and the shot arriving at target

In other words, take the cross section surface area of the target, divide it by the surface area of the circle the target can move to, and you have your maximum hit chance. e.g., if the target has a surface area of 1, and it can displace anywhere into a circle of surface area 3, then the maximum hit chance is 1/3.

d = 0.5 * a * t2

where:

  • d = distance (m)
  • a = acceleration (m/s2)
  • t = duration of acceleration (s)

which is the classic acceleration equation, assuming a starting velocity of zero. We can assume zero because all we care about is the change in the target's current velocity, that is, the jinking

Now, to use acceleration equation to calculate displacement:

t = (Dm / 299,792,458) + (Dm / Wv)

where:

  • t = time it takes light from target to travel to targeting sensors plus time it takes weapon to travel to target (s)
  • t = time target has to jink before weapon arrives
  • Dm = range to target (m)
  • Wv = weapon velocity (m/s)
  • 299,792,458 = meters in one light-second
  • Dm / 299,792,458 = time it takes light from target to travel to targeting sensors (s)
  • Dm / Wv = time it takes weapon to travel to target (s)

d = 0.5 * a * t2

replace t with jink time

displacement = 0.5 * a * ((Dm / 299,792,458) + (Dm / Wv))2

Inserting displacement equation into hit chance equation and simplifying:

H = Cm / (π * displacement2)

H = Cm / (π * (0.5 * a * ((Dm / 299,792,458) + (Dm / Wv))2)2)

H = Cm / (π * 0.25 * a2 * ((Dm / 299,792,458) + (Dm / Wv))4)

H = Cm / (0.7854 * a2 * ((Dm / 299,792,458) + (Dm / Wv))4)

Note this equation only calculates the percentage chance of missing due to light-speed lag. There are many other factors that can contribute to a miss. However, most of these are not under control of the target.

Run Silent Run Drunk

Deep recon and forward scouting is the job of the recon destroyers. And contrary to “Running Cold”, it’s not a particularly stealthy job. For all the boffins dream about stealth starships and talk airily about basement universes and domain walls and dimensional transcendence, I’ve never seen one. And no, that’s not a joke.

The aim is not to avoid being seen. They can see you, bright, hot, and clear. (It’s not all bad — this also means that you can see them.) The aim is to be seen sailing through the target area fast and high and out of the way — beyond intercept range and outside their engagement envelope – so they can’t touch you. Except for exchanging the usual bluster.

Not that that stops people from taking a pot-shot or two at you anyway on general principles. So you jink, jink, jink and trust to light-lag! But all that drunkwalking cuts deep into your delta-v reserve for evading and running, which is why they give you a whole library of variable-power drunkwalk algorithms, from a pro-forma wobble on the reaction wheels up through the affectionately named Torpedo Tango, Missile Minuet, Warhead Waltz, Firing Solution Foxtrot, and so forth, right on up to the good old Hellfire Hop. Choose carefully, ‘cause you might need whatever you burn now later. And if you’re really worried, you can fire up the kinetic barriers to military power — if you wouldn’t rather keep that energy to go into thrust, and if you don’t mind being provocative by shining the EM signature of a battle-ready warship all over the system. Any misjudgment at this point may result in a salvo or two of unanticipated k-slugs ripping big holes in your ‘can.

This is why every recon captain I ever served under had an ulcer and the temperament of a grouchy bear.

- Senior Chief Viviré Galicios, Imperial Navy (unpublished memoir)


Eric Tolle

I think some ideas in this article are applicable to hard-SF space tactics. The goal that is, of not trying to keep from being detected, but of having a high enough delta-V that one cannot be intercepted. It would depend on the offensive and defensive weaponry of course, but if weapons are comparatively short range it might be a viable tactic.

Of course I think scouting would depend more on long-range observation and masquerading as civilian traffic, ut still, I wonder if a edge case can be made for scout vessels?


Alistair Young

Well, that's more or less the intent (granted, the kinetic barriers are more sort of "firmish SF", but...), so thank you kindly.

In-setting, effective weapons range isn't limited by the weapons, but by the ability of your fire control to generate a hit, which light-lag usually keeps down to no more than about a light-second against any target that isn't obligingly moving under constant single-vector acceleration. The outer engagement envelope is larger, one or two light-minutes, but firing in that is mostly to force your opponent to expend point-defense resources (including delta-v) and generate heat, rather than in serious hope of generating a hit.

(Hence the drunkwalking, since I doubt very much that any practical ship design could let you avoid interception by a k-slug — which can have a millionth or less of your mass and still mission-kill your ship. If it hits.)

On the other note, light-lag tends to be a problem with extracting tactical intelligence from long-range observation, as the situation tends to have changed unrecognizably by the time the information hits long (by space standards) range. I do agree, though, that a lot of recon is going to be done by masquerading as civilian traffic and other presumptively harmless things (comets, say, comets are always good)...

...but around secure bases and in times of, ah, interstellar tension, civilian traffic isn't going to be allowed near those places you really want to take a look at. And that's when these chaps go in. :)

Moment of Inertia

Roger M. Wilcox (creator of the indispensable Internet Stellar Database) spotted a major flaw with the above equation. In it, I assume that the target can instantly dodge in any desired direction. Bad assumption. Most rockets can only accelerate in the opposite direction of the main engine's thrust plume. True, a rocket can turn using its attitude control system so the plume is aimed in any desired direction, but this takes time. And it takes longer if the rocket is long and skinny like a pencil, instead of short and fat like an orange.

I was perusing your page wherein you calculate the probability for missing an accelerating target with a light-speed weapon at a distance of 1 light-second.

You carefully prefaced your equations with "Just wait'll you get a load of my assumptions" — but there's one additional assumption you were making that I think you might not be aware of. You assumed that the target's acceleration (a) could be applied in ANY randomly-chosen direction with equal ease.

This implies that the target is able to point its engine in any direction instantly, or nearly instantly. I did some calculations and discovered that it's much harder for a sizable spacecraft to rotate along its pitch or yaw axis than I thought.

Consider a modestly-sized 100 meter long spacecraft with a mass of 1000 tonnes, with a great big torch engine at the back capable of producing 20 million Newtons of thrust (enough for 2g of acceleration) and small attitude thrusters pointing sideways at its nose and tail. These attitude thrusters are what the spacecraft uses to rotate. We'll assume that the spacecraft is roughly rod shaped with its mass uniformly distributed along its length, so that its center of mass is at the 50 meter mark.

Let's say this spacecraft wants to start rotating. We want to apply a VERY MODEST angular acceleration of 1 radian per second squared — that is, after firing its attitude thrusters for 1 second continuously, its angular velocity will be 1 radian per second (it'll take 3.14 seconds to face the opposite direction at this angular speed). We fire the attitude thruster on one side of the nose, and simultaneously fire the attitude thruster on the opposite side of the tail.

How hard will each those attitude thrusters have to push?

For a rod-shaped object, the Moment of Inertia. (I) is 1/12*M*L2. Here, L = 100 meters, so I is 833 * 1,000,000 kg = 833 million. Our angular acceleration is 1 rad/s2. Thus, the total amount of TORQUE we need to apply to the spacecraft is 833 million meter-Newtons. Each of the 2 attitude thrusters will have to provide half this torque, or 416 mega-meter-Newtons each. Since each thruster is situated 50 meters from the center of mass, each will have to push with a FORCE of 416/50 = 8.33 million Newtons.

In other words, each of the ATTITUDE THRUSTERS has to produce enough thrust to accelerate the ENTIRE SPACECRAFT at 0.83 g !! The thrusters themselves would have to be torch drives!

And this is JUST to produce a very modest 1 radian/sec2 angular acceleration.

If you want to be able to point your nose in any direction in only, say, half a second, you'd need at least 12 radians/sec2 of acceleration — 24 rad/s2 if you wanted to angularly accelerate through half this angle and then angularly decelerate through the other half.

Oh, and the amount of attitude thrust force required works out to being proportional to your spacecraft's length as well as its mass. A 200 meter long 1000-tonne spacecraft would require 16.6 million Newtons from each thruster for 1 rad/s2 of angular acceleration. Note that I haven't increased the spacecraft's MASS there, JUST its length. A 200 meter long 2000-tonne spacecraft would require 33 million Newtons from each thruster.

As a side note, if a 100-meter long spacecraft WERE rotating at 1 radian/sec, everything in its nose and tail section would be pinned to the outer wall by a centripetal acceleration of 5g.

And if you keep making your spacecraft longer from nose-to-engines, there'll come a point where you can actually jink more rapidly by thrusting SIDEWAYS with your attitude thrusters than you will by rotating and using your main engine. (In my example, a 100 meter long spacecraft requires 0.83 g from its nose thruster and another 0.83g from its tail thruster to get 1 rad/s2 of angular acceleration. If you point both of those thrusters in the same direction, though, you'd get 1.66 g of acceleration sideways, which is almost as much as the 2g that its main engine can provide! You'd still have to ROLL the spacecraft to position those side thrusters onto the correct side, but rolling a rod-shaped spacecraft requires much less torque than pitching or yawing.)

Roger M. Wilcox

I too had no idea such huge amounts of torque were required (though I did know about the moment of inertia problem, having read Sir Arthur C. Clarke's short story "Hide and Seek"). Since torch drive attitude jets are highly unlikely, this drastically reduces the angle the ship's nose can be changed by, and thus drastically reduces the area the ship can dodge into. Instead of a sphere, it will be reduced to a cone sharing its long axis with the ship. The angular width of the cone will depend upon the target's moment of inertia, the torque produced by the target's attitude system, and the time it takes the weapon to fly to the target.

If your rocket exhaust is cool enough that material objects can actually survive being inserted into the exhaust plume, a possible solution to the dilemma is Cascade Vanes. So instead of making your attitude jets as big as your main engine, you actually use your main engine as an attitude jet. However, an exhaust that cool is probably way below torch rocket levels.

(ed note: Excerpt from "Hide and Seek". Secret agent K-15 is being chased by enemy ship Doradus. If K-15 can survive a few hours, friendly spacecraft will come to his rescue. So to buy time, K-15 bails out of his ship in a spacesuit, and goes to ground on the tiny Martian moon of Phobos. The commander of the Doradus is most displeased.)

To the layman, knowing nothing of the finer details of astronautics, the plan would have seemed quite suicidal. The Doradus was armed with the latest in ultra-scientific weapons: moreover, the twenty kilometers which separated her from her prey represented less than a second’s flight at maximum speed. But Commander Smith knew better, and was already feeling rather unhappy. He realized, only too well, that of all the machines of transport man has ever invented, a cruiser of space is far and away the least maneuverable. It was a simple fact that K-15 could make half a dozen circuits of his little world while her commander was persuading the Doradus to make even one.

There is no need to go into technical details, but those who are still unconvinced might like to consider these elementary facts. A rocket-driven spaceship can, obviously, only accelerate along its major axis-that is, "forward." Any deviation from a straight course demands a physical turning of the ship, so that the motors can blast in another direction. Everyone knows that this is done by internal gyros or tangential steering jets, but very few people know just how long this simple maneuver takes. The average cruiser, fully fueled, has a mass of two or three thousand tons, which does not make for rapid footwork. But things are even worse than this, for it isn’t the mass, but the moment of inertia that matters here — and since a cruiser is a long, thin object, its moment of inertia is slightly colossal. The sad fact remains (though it is seldom mentioned by astronautical engineers) that it takes a good ten minutes to rotate a spaceship through 180 degrees, with gyros of any reasonable size. Control jets aren’t much quicker, and in any case their use is restricted because the rotation they produce is permanent and they are liable to leave the ship spinning like a slow-motion pinwheel, to the annoyance of all inside.

In the ordinary way, these disadvantages are not very grave. One has millions of kilometers and hundreds of hours in which to deal with such minor matters as a change in the ship’s orientation. It is definitely against the rules to move in ten-kilometer radius circles, and the commander of the Doradus felt distinctly aggrieved, K-15 wasn’t playing fair.

"Hide and Seek" by Sir. Arthur C. Clarke

Compartmentalization

If the pressurized habitable section of your warship was one single area, a hull breech would depressurize the entire ship (I was going to recount the ancient joke about "why is a virgin like a balloon", but luckily good sense intervened). A prudent warship design would use air tight bulkheads to divide the interior of the pressurized section into separate areas. This comes under the heading of "not keeping all your eggs in one basket". The keyword is redundancy.

For the same reason, you'd want back-up life-support systems, power plants, control rooms, and other vital components. And these duplicate systems should be located in widely separated parts of the ship. Otherwise a single lucky enemy weapon shot could take both of them out.

Even in the non-pressurized section, bulkheads can help contain destructive effects of hostile weapons fire. So an explosive warhead, with any luck, will merely damage the interior of one compartment, instead of gutting the entire interior of the ship.

Mark Temple says:

Recently at one of the RPG boards I visit, a discussion about "armor belts" and the durability of space warships has cropped up. This got me thinking about compartments and how they'd be an integral part of a ships survival.

Modern naval vessels are divided up into compartments to make them more survivable. Compare a naval frigate to a main battle tank. A tank is basically one compartment. Breach its (very thick) armor and you wreck the tank, since the hit will usually kill the entire crew and/or destroy the internal systems.

A frigate however has multiple compartments. Breach the hull of the frigate and while you might wreck one compartment, the entire ship will still float and will often still be able to fight. You have to wreck many compartments, or very specific compartments, in order to mission-kill the ship.

It seems to me this vital part of naval design would not be overlooked in space warship design. Beyond the obvious benefits of making it easier to control atmospheric leaks, a space warship built with many compartments that can be isolated would gains a structural benefit in combat.

Now, compartments would be worthless if one hit could completely disable vital systems like life support or command-and-control. Thus all these systems would be distributed all across the ship, with multiple redundancies. Thus if you lose a compartment with life support systems, you have others to fall back on. Having the main CiC compartment destroyed will not totally eliminate your ability to control the ship. This is standard for real world navy ships. Engine systems, command rooms (bridges, CiC's, etc.) would have secondary locations kept manned in battle in case the main compartments for them are destroyed.

This is also why those compartments would be buried as deep inside the ship as possible. No sense in making things easy for your enemy. True, on modern wet navey warships bridges are still mainly at the highest point of the ship, but that's mainly to facilitate visual tracking and identification. In space, you cannot see the enemy with the naked eye anyway, so you might as well put your command centers where the enemy has to destroy the entire ship to get at it.

In science fiction movies and television, we have never really seen all of these features at once. Ironically Star Trek managed to get the distributed systems part correct, we eventually even saw that Federation starships had "battle bridges" to provide emergency control should the main bridge be damaged. But Star Trek has utterly failed to put the bridge in defended positions, or show proper compartments in their designs (As David Gerrold noted, that silly bridge perched on the saucer top of the Starship Enterprise would have been shot off a long time ago). Apparently they rely on their handwavium deflector shields to do the job, which is great until you run out of power.

(New) Battlestar Galactica came pretty close, though. The ships systems are distributed, the ship itself compartmentalized, and it has a bridge buried deep in the hull. We just never see redundant engine rooms or command centers, which is probably more of a failing of the script writers than of design.

In novels we see this idea used properly, though. The Honorverse novels showcase the benefits of compartmentalization in a very obvious and graphic form, in nearly every novel.

Mark Temple

"There are six main classes of fighting machines. The great battleships are first, weighing in the neighborhood of one million five hundred thousand tons. A battleship is almost indestructible. Even when blown completely in two, it Is exceedingly dangerous, as it maintains maneuverability and fighting power... "

...Thirty great battleships formed the front, against twenty-nine whole Tefflan battleships, but there were no less than eleven half ships in action, and each of these was fully half as deadly as a full battleship...

...A battle between battleships of space is not like a sea battle, for the battleship of space never sinks, and every portion is capable of fighting until every man within is killed; a battle between space battleships is to the death of every individual...

From The Mightiest Machine by John W. Campbell jr. (1934)

Armor

Armor is a shell of strong material encasing and protecting your tinfoil spacecraft. Unfortunately as a general rule, armor is quite massive, so it really cuts into your payload allowance.

Basically, the energy requirement to damage a surface is measured in joules/cm2. If you exceed that value, you do damage, otherwise you fail. Keep in mind that a Joule is the same thing as a watt-second.

There are three ways that weapon energy damages a surface: thermal kill, impulse kill, and drilling.

Thermal kill destroys a surface by superheating it. Impulse kill destroys a surface by thermal shock. In the calculations for the SDI, the amount to thermal kill a flimsy Soviet missile is about 1 to 10 kilojoules/cm2 (100 MJ/m2) deposited over a period of a second. The same energy deposited over a millionth of a second is required for an impulse kill. Since the laser beam tends to be meters wide, the beam energy is in the hundreds of megaJoules.

However, neither thermal kill nor impulse kill works very well with armor. So we use the third method: drilling. The amount of energy required to drill through an object is within a factor of 2 or so of the heat of vaporization of that object. There are also two other limits: the maximum aspect ratio of the hole is usually less than 50:1, and the actual drilling speed, for efficient drilling, is limited to about 1 meter per second (depending on the material).

Therefore, the best anti-laser armor will be that material with the highest vaporization energy for its mass. The best candidate is some form of carbon, at 29.6 kilojoules/gram. You do not want a form that is soft or easily powdered, or the vapor action under laser impact will blow out flakes of armor, allowing the laser to penetrate much faster. Steel has a higher vaporization energy, but it masses more as well.

Under laboratory conditions, if an armor layer was 5 g/cm2 of carbon, burning through a 1 cm2 (1.12 cm diameter) spot of armor would take about 148 kilojoules and 20 milliseconds. An AV:T laser cannon with 50 megaJoules could burn through 330 such armor layers in a few seconds, under laboratory conditions (i.e., enough layers to burn through the entire ship the long way).

However, under combat conditions there is no way one could focus the laser down that tiny and keep it on the same spot on the target ship for multiple seconds.

It would be better to use a beam focused down to a larger 10 cm2 spot (11.2 cm diameter). Granted the beam power required to penetrate jumps from 148 kilojoules to 15 megaJoules, but now if we have an uncertainty in the target's velocity of up to 5 meters per second it doesn't matter.

Of course, if price is no object, you can do better than carbon. Boron has a vaporization energy of 45.3 kilojoules/gram and is only slightly denser than carbon. Expensive, though.

In a 1984 paper on strategic missile defense, it suggested that your average ICBM would require about 10 kilojoules/cm2 to kill it. This would rise to 20 to 30 kilojoules/cm2 with ablative armor, and it would be tripled if the ICBM was spinning on its long axis since the laser couldn't dwell on the same spot 100% of the time.

As a side note, a Whipple shield is very effective at stopping hypervelocity weapons. With kinetic weapons at closing velocities in excess of 10 km/sec, you're getting into the realm where armor is less important than blow-through. For armor, you want something that will resist being turned into a plasma for as long as is possible, followed by gaps made of vacuum to make it a Whipple shield.

Anti-radiation armor is discussed here.

Armoring Laser Optics

(ed note: the context is how to armor your laser cannon optics so that the enemy cannot destroy them with pin-point laser strikes)

Isaac Kuo

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.


Ray McVay:

COOL.

This way, when one side gets it's ass kicked, rotating armored grids can be part of the next generation of warship!


Isaac Kuo

Yes, perhaps, but anti-laser armor protection in general seems like a pretty obvious idea once lasers become powerful enough to require armor protection. I wouldn't think it would be something that requires an ass kicking to realize should be done.

However, if you want to justify laser warship with no anti-laser armor...

You might posit that up until the time of your setting, missiles have been overwhelmingly dominant over weapons lasers. As such, warships are designed purely with missile combat in mind. It's not that weapons lasers don't exist, it's just that no one bothers to put them on spacecraft because missiles are superior weapons.

In this situation, it might make sense to design a laser armed spacecraft designed to fight only missiles because there are lots of enemy missiles and no enemy lasers. It's like how the first tanks weren't designed to fight enemy tanks because enemy tanks didn't exist yet.

Then, when the enemy sees them and reacts by slapping some laser on their own spacecraft, the first guys are like, "Derp, we didn't think that would happen."

(No really, people can be that stupid.)

Aaarggh...this scenario really hurts my head. It just doesn't seem to make sense from the start. Weapons lasers seem to be inevitably useful thing to have around. If the POTUS wants to zap terrorists from orbital drones, a near future laser will be able to do it with precision but a missile would take minutes to cross the distance required. Orbital lasers could zap hostage taking pirates like a team of snipers, while orbital missile launchers just...can't. There's just no way missiles will ever be better than lasers for this, and it's hard to imagine a future where that isn't one of the first things mankind does with space weaponry.


Isaac Kuo

Getting back to an earlier question — the level of armor protection on the laser turrets vs other systems is a matter of design priorities and the level of the threat. If the armor required for all around armor protection is reasonably low mass, than the obvious thing is to provide all around armor protection.

But if offensive weaponry gets strong enough to make that impossible, you have to prioritize things. Historically, the nature of the fuses on increasingly powerful armor piercing explosive shells led to the concept of "all or nothing" armor. It wasn't possible to provide all around protection and still float, so the idea was to concentrate armor around the most critical systems while hopefully providing so little armor on the other systems that shells would pass fully before detonating (causing far less damage than if it detonated within the ship).

The nature of tank armor vs weaponry led to directional concentration of armor--concentrating armor on the front and sides, to protect in a frontal arc. The engine would provide some protection against attacks from the rear (albeit it would still be a mission kill, but the crew would stand a decent chance of survival).

With hypothetical space warships, I see two very different sorts of threats--beam weaponry and missiles. You plainly don't want excessively powerful beam weaponry, or it would dominate utterly (never runs out of ammo, reaches the target more quickly, can be used for things like zapping terrorists). That's okay, because near term practical beam weaponry might be limited to 1MW or less anyway...good enough to take out missiles with some dwell time, but perhaps with minimal armor penetration capability. You can tweak things upward from there to provide whatever level of armor penetration is appropriate to balance against other stuff.

The precision of lasers is such that the enemy can likely specifically target any spot on the target vehicle. This strongly suggests that all around protection is called for. You can dramatically improve the level of armor protection with the use of rotating armor...this prevents concentrating on a single spot until it penetrates. The laser basically can't ever penetrate unless it can slice through all at once.

You probably don't want to redesign all your artwork and ships and design to take rotating armor into consideration. A less radical solution is to assume some sort of multi-layer spaced armor filled with some sort of open mesh foam. As bits of foam get melted/vaporized, nearby foam expands to fill in the gap. Ideally, most of the melted/vaporized material gets absorbed by the mesh, so armor loss is minimal. I think you're already assuming a foam armor layer, so that works out.

The bottom line is that any particular enemy laser have a pretty sharp transition from being entirely ineffective at a particular range to being extremely effective at a slightly lower range. If you do anything other than even protection for all components, then you'll end up with a situation where the less protected components are practically guaranteed to be wiped out as soon as the range closes to that transition range. It might make sense to completely write off living quarters, drop tanks, and missile racks. But lasers, power, radiators, and propulsion probably need the same level of protection all around.

(Yes, radiators can be armored, and should be. Schemes to fold away radiators are a false economy.)


Ray McVay:

I totally agree about the armored radiators — assuming quartz for anti-debris armor on civilian craft, and diamonoid for military.

I LOVE the idea of rotating armor, or other active measures. The virtue of the setting I'm developing is that it pre-supposes technological innovation once an interplanetary war begins. This means that I can design different generations of spacecraft as the arms race heats up.


Isaac Kuo

I'm not sure anti-debris armor makes sense. I mean, I like making military radiators out of quartz or diamond because they're basically invulnerable to visible wavelength lasers (lasers mostly harmlessly pass through them, causing ignorable local heating of coolant). But for civilian radiators, debris impacts should be rare enough that it generally makes more sense to simply patch leaks than prevent them. Current space radiators are based on a combination of conduction and circulation, being mostly solid conductive panels with small armored coolant pipes. The small armored coolant pipes try to present a small target to random debris--trying to prevent any leak in the first place. But once you have robotic drones to autonomously detect and patch leaks, then it's more efficient to simply have transparent radiator pipes so the coolant directly radiates heat to space (no conduction step necessary). The repair drones might be shaped like dumb-bells. The two spherical ends isolate a small higher pressure section. If the drone detects a drop in pressure, it knows it's passing by a leak, so it sprays some transparent patch glue.

Defense against missiles is a bit different than defending against random space debris, since it's going to be incomparably nastier in damage potential, and you can see it coming. Because of the momentary nature of the threat, it makes sense to use some sort of "shutters"...simply turtle up entirely for the moment the missiles arrive. Who cares if you're momentarily unable to fire lasers or radiate any heat?

Personally, I prefer these "shutters" to be low mass defensive anti-missile drones a small distance from the ship, rather than armor integrated with the hull. These form a temporary wall between the incoming missiles and the ship. It's not necessary to stop the missiles entirely, it's just necessary to break them up enough so the remaining debris will mostly miss the ship. (Another layer or two of these drones will stop what small fraction of the debris is headed for the ship.)

Hull armor, in contrast, has to stop the incoming missile dead in its tracks--redirecting the explosive force back into the hemisphere from whence it came.

Isaac Kuo in a Google+ thread

Sandcasters

And you can forget about laser defenses like Traveller style Sandcasters. These fire clouds of magic "prismatic" dust that provide protection from hostile laser fire. In reality they would not work. There is no way that they can project a cloud dense enough to do any good.

In Frank Chadwick's starship combat game Star Cruiser, there are anti-laser fields.

Screens are not mysterious force fields that prevent enemy weapons from penetrating. Instead they are electromagnetic fields which hold reflective particles in suspension. When a laser hits the screen, the particles reflect a portion of the laser light and then vaporize, absorbing the rest of the laser's energy. Although some energy will penetrate the screen, often the screen absorbs or reflects enough energy that the remainder is insufficient to damage the ship.

From Star Cruiser by Frank Chadwick

However, the gang at rec.arts.sf.science are skeptical:

I don't remember that thread but the idea intrigues me. Why would a levitating cloud of metallic particles be any better at protecting a ship than the same metal used to make ordinary hull plating?

It sounds ike you are just wasting energy on maintaining armor with more holes in it than conventional armor. On the other hand there may be heat dissipation issues with conventional armor. On the third hand if you have a magnetically shaped armor you could concentrate the cloud on the side you are being attacked from so you don't have to create thick armor on all sides. This could cut the weight in half or more - but levitating plates instead of a cloud would seem better suited for the task.

Michael Grosberg

Seems to me it might even be worse. If you're talking about insanely powerful laser beams, when they hit the particles they'll just turn them into projectiles that will hit the ship. It doesn't seem to me like you could plausibly get a "shield" of magnetically-levitated particles in such a way that would give you any kind of real coverage -- especially if you're positing it being used in defense against superpowerful laser beams. The beams just knock the particles out of the way and fire straight through.

I would think it's because every little metallic particle would be exposed to the beam only a short time. Then more would fill in. Like having your hull plates jump in front of any hole. Sort of. That presumes particles circulating around in this levitating cloud.

: Seems to me it might even be worse.

Well... yes, there is that. Much easier to vaporize each particle, though it might be quite hard to get a particle to actually recoil and hit the ship. Hmmm. Anyways... yes, I suspect it wouldn't really work well, and you'd have to levitation a large, large: mass of particles.

Seems like most of the particles would hit the ship. To serve their purpose, after all, they are have to be between the beam and the ship. Whether that would be really dangerous to the ship depends on how thick the "shield" is and how big each particle is.

If we're granting superpowerful laser beams, it seems to me that the energy required to displace or even vaporize these particles will be much smaller than the amount of energy in the beam, which suggests, as you say, that such a "shield" won't be of much use unless it's very thick. At some point, it seems to me you're just better off having armor; you have to carry around the extra mass anyway. But without attaching numbers it's hard to be sure.

Put simply, a layer of sand is no more effective at stopping a laser beam than a similar areal density of monolithic armor (in fact, it's a bit less effective due to structural issues); you can simply shoot holes in a cloud of sand, just like you can shoot holes in armor. As such, why spend X tons of your mass budget on temporary armor when you can just spend the same X tons on permanent armor?

In addition, a cloud of sand

  1. needs to be somewhat larger than the ship it shields (reducing areal density, and thus armor value)
  2. cannot maneuver if the parent ship maneuvers (so if you deploy sand, you're stuck in your current position)
  3. without some form of containment will simply disperse in a time frame that's comparable to the deployment time (if the cloud can cover the entire ship in 10 seconds, after 20 seconds it will have expanded to twice the size of the ship, reducing protection by a factor of 4. You can improve this time somewhat by using multiple projectors)
Anthony Jackson

Creative Measuring Units

In his novel The Wellstone, Wil McCarthy proposes a unit called the TW or "train wreck". It is measure of impulsive acceleration (i.e., from a crash or explosion) equal to an inertial acceleration of 40 g. A human being can survive a 1 TW impulse lasting no more than a couple of seconds, while a 2 TW impulse of longer than a second is typically fatal. In The Hitch-Hiker's Guide To The Galaxy, Douglas Adams creates a tongue-in-cheek unit called the "hurt", with spacecraft weapons rated in "mega-hurts". Har-har.

Force Fields

Force shields, deflectors, energy screens, they are all handwavium science fictional armor composed of some form of energy instead of dull boring matter.

Names include force fields, force shields, force screens, energy screens, energy shields, deflectors, deflector shields, ray-screens, repulsor screens, and many more.

Since they are imaginary their abilities and properties are only limited by the imagination of the science fiction author. Some stop bullets but let laser beams through, some stop lasers but are useless against bullets, some stop both. They can be invisible, mirrored like chrome, or glowing in various colors and levels of brilliance. They are occasionally used in a creative fashion, e.g., "structural integrity fields" are force fields interpenetrating a spaceship's metal frame in order to make it stronger than is possible with mere matter. Or gravity shields that allow one to ignore the gravity of a nearby planet.

In other words the author has to be real careful in order to avoid unintended consequences.


forcefield

[first use unknown; also "force field"]

General SFnal term for a hypothetical technology which can interact with any material object in the ways that magnetic fields interact with magnetized ones. A forcefield may be deployed as a shield around a spaceship or other object (a force shield). Other applications include the tractor beam, the pressor beam and antigrav.

force shield

[dates to early space opera, c. 1930]

The concept goes back to E.E. "Doc" Smith's Skylark stories, a classic space opera series originally launched in 1928; those books referred to force screens. By the time of Asimov's Foundation in the 1940s the idea had been sufficiently naturalized that his world featured personal force shields as a defense against the blaster.

In "Doc" Smith's and most later versions, a force shield has no thickness and has resistence proportional to the power of the generator. It is possible to overload a shield by throwing more energy at it than the generator can handle, or to wait for the exhaustion of the generator's power sources.

From An SF Glossary by Eric S. Raymond (2006)
Force Field

In sf Terminology — unlike Physics, where it has a different meaning — a force field (sometimes a force shield or energy screen) is usually an invisible protective sphere or wall of force. It first seems to have been used in this sf sense in E E "Doc" Smith's Spacehounds of IPC (July-September 1931 Amazing; 1947). Throughout the 1930s and 1940s the force field performed sterling service, notably in Smith's Skylark and Lensman series, where force fields under attack routinely glow red and orange and then all the way up through the spectrum until they reach violet and black and break down. Isaac Asimov's "Not Final!" (October 1941 Astounding) uses force-field research as a vehicle for the message that absolute statements about scientific possibility tend to be unwise.

(ed note: Spoiler Alert - in Not Final, force fields were proved to be impossible because they only last a fraction of a second. Until some joker figured you can do an end run around the problem by activating the field stroboscopically.)

Force fields are also a sovereign remedy against Rays, Death Rays and usually bullets as well — though not against "space-axes" in E E Smith's First Lensman (1950) or against swords in Charles L Harness's Flight into Yesterday (May 1949 Startling; exp 1953; vt The Paradox Men 1955 dos; rev 1984). In these books the efficacy of the shield is directly proportional to the cube of the velocity (Smith) or to the momentum (Harness) of the object it resists. This property of force fields gives Harness a good excuse to introduce swordplay, where the momentum involved is relatively small, into a technologically advanced society — an example of Medieval Futurism that other writers were not slow to follow, most notably Frank Herbert with his personal "shields" and knife-fighting in Dune (fixup 1965).


The eponymous device in Poul Anderson's Shield (June-July 1962 Fantastic Stories of the Imagination; 1963) can recharge its batteries by soaking up the kinetic energy of the bullets it stops. But these are comparatively late examples, when the concept was sufficiently familiar in sf to allow Parody and sophisticated variations. Still later force fields are often simply accepted as a given, as in Star Trek with its shields or in Iain M Banks's Culture stories.


It is the essence of the traditional sf force field that by a kind of judo it converts the energy of an attacking force and repels it back on itself. Few writers, however, were able to give — or concerned to try to give — a convincing rationale for forces being conveniently able to curve themselves around an object and to take on some of the properties of hard, resistant matter. A well-ground mirror might more plausibly carry out the same function, at least against death rays. Indeed, Colin Kapp's "The Pen and the Dark" (in New Writings in SF 8, anth 1966, ed John Carnell) features an Alien defensive wall which is essentially a 3D mirror, opposing each physical assault with a precisely similar reflection. James Blish nevertheless made an interesting attempt, using analogies from radar technology, to justify a kind of standing-wave force field placed around New York by some unspecified hostile power in "The Box" (April 1949 Thrilling Wonder). The true rationale for the force field and for its close relations, the Tractor Beam (which pulls objects towards the beam projector) and the Pressor Beam (which pushes them away), is that — like Faster Than Light travel — they help tell stories.

TV Tropes: Deflector Shields

Also called "Force Fields", these are invisible (or, if the budget allows it, barely-visible) energy barriers placed around objects to protect them from harm.

Most common around space ships, but also seen around bases and — very occasionally — individual people.

How much protection they offer is usually proportional to their power. This also makes for yet another reason you are boned if the energy runs out. They may also be subject to Phlebotinum Overload if they get hit by too many Energy Weapons.

An energy barrier can zap or repel anyone who touches it, or can simply behave like an impenetrable wall. It's often represented in the form of a Beehive Barrier or another Hard Light construct. In practice, this is somewhat less scientifically feasible than Energy Weapons, but not by much, at least in the way it is usually depicted — a strong electromagnetic field really can be used to deflect particle beam weapons or railgun/coilgun slugs. A better example is the Earth's magnetic field which safeguards us from charged particles in the solar wind. NASA and ESA are actually researching how to reproduce the effect on future interplanetary spaceships, to protect Mars travellers from cosmic radiation.

Shields may be handled as a single egg-shell or as several independent barriers covering different areas of the ship. The latter encourages certain maneuvers, such as making sure your shielded side is always facing the enemy.

Shields are popular in fiction because it allows the Cool Ship to participate in battles without the inconvenience of having to spend the rest of the episode making repairs to physical armour and systems. In older or lower-budget movies and TV, they also eliminate the need to show battle damage on the ship; e.g., having your Ensign call out "Shields down to twenty percent!" can be a lot cheaper than showing a gash blasted in your ship's armor. The downside is that creators often have to resort to Explosive Instrumentation to provide combat casualties on a shielded ship. In somewhat-harder science fiction, shields are useful to overcome the seemingly overwhelming attacker's advantage—cities on planets can't dodge, so if there isn't some way to defend against space-launched missiles and kinetic projectiles, wars are going to be short and boring.

In Video Games, deflector shields have a special use. They are effectively a way to justify regenerating hit points, but only for a part of a unit's health. Shields get to regenerate, but if there is something beneath them, like armor, the armor doesn't regenerate. Also, for many games where a target can take Subsystem Damage, that won't start until you penetrate the deflector shields.

Compare and contrast Containment Field and Reinforce Field. See also Some Kind of Force Field.

(ed note: see link below for lots of examples from TV, movies, and novels)

From the Deflector Shields entry at TV Tropes

In the real world, defensive force fields do not exist. But if they did it would make things so much easier.


There are a couple of remotely possible real-world "force fields". Researchers have been experimenting with using magnetic and electrostatic fields to ward off particle radiation. More on the fringe are plasma windows, which could defend against microwaves and particle radiation. But they have a long way to go before they can stop weapon-grade particle beam weapons.

Plasma windows can separate pressurized areas from unpressurized areas with a sort of force-shield "door". Air cannot pass through the plasma window into the unpressurized region, even under a pressure differential of up to nine atmospheres. So if a radiation bean generator requires vacuum but the item being irradiated is in an atmosphere, the plasma window works nicely.

It is currently used for electron beam welding. The beam generator is inside a vacuum chamber, the electron beam passes through the plasma window and welds the metal sheets on the workbench in the shirt-sleeve environment. In theory one could use this as a quick-pass airlock or hangar-bay door on a spacecraft.

Plasma windows have a bright glow, the color depends upon what gas the plasma is using. Argon is violet, nitrogen is orange.

The drawback of plasma windows is that they are power hogs. For a round window they need 8 kilowatts per centimeter of window diameter. Other than that there is no limit on diameter.


But there isn't anything in the real world like E.E."Doc" Smith's electromagnetic radiation stopping "ray-screens", nor his matter stopping "repellor screens."

Considerations

As always when dealing with rubber science, the smart move is to nail down the ground rules for the item in question, think out all the logical consequences and implications, and stick to them.

If the force field blocks incoming laser fire, will it block your outgoing fire as well? In Isaac Asimov's "Black Friar of the Flame", a ship has to drop its field entirely in order to fire its weapons. This lead to chain reactions, ship A drops and fires, then it is hit by ship B who drops and fires, who is hit by ship C who drops and fires... In Larry Niven and Jerry Pournelle's The Mote in God's Eye, the Langston Field can have temporary holes opened to allow egress of your laser fire. In other novels, the field is on stroboscopically, that is, it flickers. It will be on, say, 80% of the time, and off for 20%. If your lasers flicker in synch with your field, 100% of their energy will penetrate. But since your opponent's lasers will probably not be in synch, only 20% of their energy will penetrate. However, if your opponent manages to match your synch rate, you'll be clobbered.

Does the force field block matter only (e.g., kinetic weapons), energy only (e.g., lasers), or both? Doc Smith had separate types of force fields for each ("repellors" and "ray-screens"), while the Langston Field would absorb both the kinetic energy of projectiles as well as the electromagnetic energy of lasers. The fields in "Black Friar of the Flame" only block energy, so the good guys get a bright idea from the Battle of Salamis.

Is the field a bubble around the ship, or flat planes that can be positioned? There was that throwaway line in the movie Star Wars, where Red Leader tells the Red Squadron X-Wing pilots to angle their deflector shields "double-front". Presumably this means rotating the rear shields to face forwards, so there is double the protection forwards and zero protection aft.

How fast can the field be charged up? The usual model is that energy is fed into the field, and each incoming shot reduces the energy in the field ("Deflector shields are down to 40%, Captain"). When the field energy reaches zero, the field goes down and the incoming weapons fire impacts directly on the ship. For dramatic reasons, it is desirable to have the rate of shield charging to be a fraction of the rate of shield reduction. Otherwise ship's shields will never go down.

Does the field obey the law of Conservation of Momentum? Say your force field generator is located in the Engineering deck. You put the force field around the ship, then quite by accident the ship crashes into an asteroid. One would expect that as the field hit the asteroid, the shock of impact would be transmitted to the field generator. You might wind up with the generator plowing through the hull and out the rear of the ship.

In Poul Anderson's novel Shield, the field has a sharp gradient on the outside, and a more gradual one on the inside. This means if you were running and collided with the shield it would feel like hitting a brick wall. But if you were inside the shield it would feel like hitting a mound of feather pillows.

Unintended Consequences of Force Fields

   Force-fields - or shields, screens, deflectors, etc. - have been a recurrent trope in SF since the creation of the genre; scarcely has there been a mainstream novel or movie that does not feature them to some extent.  Like many other SF magitech devices force-fields often shape the story and 'Verse in which it is set; without planetary defence shields the Rebel Alliance on Hoth would have had little chance against the might of the empire.  Star Wars is not alone, the universe of Dune, Star Trek, and countless others use the technology in a unique way.  Usually the factors and applications considered revolve around the primary usage of forcefields in SF, defensive measures for military vessels.  In this article I will look at a few, most of which have admittedly little impact overall on the 'Verse in which they are used, but which help to anchor the story and the readers attention firmly in the future world.

   For the sake of simplicity the forcefields in this blogpost are assumed to have the following characteristics; can be projected in a variety of geometrical shapes, have mechanical strength, repel kinetic and radiant energy.  It is also assumed that either the forcefield is invisible when in matter-repelling mode, or it can be tuned to block or admit certain frequencies of light.
  • Airlocks:  this has been seen in both Star Trek and Star Wars, and might be one of the least stupid of the non-defensive ways that forcefields have been used.  The entrance to the hangar bay has physical doors, a forcefield that allows slow moving shuttles to push through is activated when required.  As air molecules move quite fast, they cannot penetrate the field, and so pressurisation is maintained.  However, everyone had better remain in p-suits; a blown fuse could otherwise result in the inconvenience of explosive decompression.  Note that a real world device called a Plasma Window can achieve much the same result. 
  • Landing Gear:  this example comes from the SF comedy Galaxina, and is employed by the spaceship Infinity to overcome the problems with landing on uneven ground.  Like the forcefield airlock this is well and good, until the power fails and several hundred or thousand tonnes of spaceship crashes to the ground.  While it would be somewhat foolhardy to equip a normal spaceship with these, they could serve for special landings where the ground is unhardened, say for military or rescue missions, with the forcefield acting like a futuristic giant snowshoe.
  • Structural Reinforcement: the ships of Star Trek's Federation are much maligned among engineers and those of scientific bent for their structurally stupid design.  The narrow struts connecting the hulls and warp nacelles are under incredible stress in any manoeuvre, and to cope with this the ship uses Structural Integrity Fields.  Again, good until the power goes out, which is probably when you don't want the ship snapping into three pieces. 
  • Emergency Containment:  although - yes, power (requirements) again - physical barriers are probably preferable for containing anything from prisoners to poisonous gasses, forcefields might prove indispensable in an emergency.  If they can power up in instants they could seal off an area far sooner than ponderous blast or containment doors, failing to seal off of dangerous situation in moments. 
  • Life Support:  the vacuum of space makes simple jobs a nightmare, pushing the costs or orbital construction sky high.  If a starship under construction in orbit could be enclosed with a forcefield just strong enough to contain an atmosphere, matters would be much simpler.  Even though for safety p-suits would still be worn, the 'air' could equally easily be there to enable plasma cutting as (well as) breathing.  A forcefield on a planet could be used as an emergency shelter from natural attacks such as volcanos, tsunamis, and hurricanes.
  • Power & Drive Reactors: the lower limit to the size and weight of a nuclear fission power supply is the critical mass needed for a sustained reaction.  However, if the forcefield is a perfect neutron reflector it is easy to see how it could cause even a few grams of uranium or plutonium to fission.  The resulting plasma could be confined by the forcefield and piped through a magnetohydrodynamic generator, the whole package limited in size only by the forcefield generators.  Fusion could be treated in the same way, opening up the way to abundant clean energy.  The most obtuse of the implications that result from this is the development of torch drive spacecraft, which I will discuss in a future post.
  • Energy Storage: antimatter is often proposed as the ultimate starship fuel; a misleading statement.  Like the use of hydrogen as car fuel, antimatter acts only as an energy storage device, a battery.  Starships need astronomical amounts of energy, so antimatter is used as giving the ultimate power-to-weight.  Create a hollow container from a forcefield, fill it with a vapour that absorbs a frequency of light allowed to pass through the field.  Energy is added to the 'battery' with lasers, and thanks to the phenomenon of electron shells the light released by the vapour is unable to pass the field, most of it at a different than original frequency.  A device such as this could have no limit to the energy contained, making 'nuclear hand-grenades' look like damp firecrackers.
  • Directed Energy Weapons: as anyone familiar with the Kzinti Lesson knows, the exhaust plume of a starship or torch drive spaceship is as deadly a weapon as can be found.  The Fission Fragment Rocket, possible to build in the real world, has an exhaust velocity of a few % of c, making it a deadly beam weapon at close range.  By piping the plasma from a fusion reaction out through a forcefield nozzle a devastating weapon could be created; it disadvantage a fairly short range due to the dispersion of the beam.
  • Airships: if a forcefield has no mass in and of itself, it makes the perfect airship hull.  Possibilities include vacuum airships, high temperature thermal, airships that change shape to attain supersonic speed, etc.  Nor would an airship built in this way suffer from the fragility of conventional designs, making it much more practical than any physical airship. 
  • Re-entry Shielding: for a large or fast spaceship a re-entry shield is much too heavy, despite the advantage in fuel reduction that the Leonov demonstrated.  A forcefield, especially if already fitted for military defence, is a perfect substitute, and if power fails, you're probably screwed anyway.  By allowing aerobraking the available delta Vee for a mission could be doubled, or the fuel load halved; a significant improvement.  Also, as it could be much larger than the actual ship, and of a more aerodynamic shape, g-load could be altered to be less taxing.
  • Cassions & Civil Engineering: a cassion is a temporary dry-dock of sorts, built to enable the construction of underwater structures.  A forcefield by its virtue of easy deployment - on/off switch - is ideal for rapid or emergency construction.  
  • Tools: this is seldom seen, the only example I have personally come across being in Asimov's extensive 'Verse, where forcefield tools are far more capable than mechanical counterparts.  The advantages of such theoretical tools are many, and are discussed on Atomic Rockets.
  • Ramscoop:  the greatest challenge of constructing a Bussard Ramjet, one of the most advanced and powerful starship designs, is the construction of a magnetic field capable of collecting the interstellar hydrogen needed by the design.  A forcefield could be just the solution, especially if it is massless and frictionless.
  • Solar Sail: the solar sail is an interesting design that cannot ever come to its full potential due to material constraints.  In essence the solar sail uses the momentum of reflected sunlight or solar wind, having theoretically infinite delta Vee.  However, the mass of the sail, combined with the inability of find a material that will withstand solar heat at the closer, and thus more effective, distances means it is not likely ever to be widely used.  A forcefield, however, might be massless and perfectly reflective, making it the ideal method of producing a solar sail, and thus providing a reliable method of interstellar transport.  In fact, this is such an effective interstellar design that I have decided to do a post on Interstellar Transport, featuring such a design and explaining its implications. 

  • (ed note: and as he points out in a later post, force fields allow Torch Drives)

From Force-Fields; Not Just Defence by Moran (2014)
Physical Nature of Shields

What do they do?

Before embarking on any speculation of mechanism, we should first determine what a shield does. Shields in sci-fi generally serve 2 purposes:

  1. Stop "energy weapons" (lasers, phasers, blasters, etc).

  2. Stop physical objects (bullets, knives, people, vehicles).

Fair enough, but what does it mean to "stop" energy weapons and physical objects? Physical objects tend to stop abruptly when they hit a shield, but not always: sometimes the "slow blade passes" concept from Dune seems to be in effect, and objects can slip through (see the Gungan dome shields in the Battle of Naboo). Sometimes incoming objects bounce, and sometimes they explode on impact. And what about "energy weapons"? Some ships (such as the Trade Federation battleship in the end of TPM) look as if they're taking every hit right on the hull, but their shields are said to remain up, as if they are coincident with the hull surface. Others project their shields out into space, and these shields light up in a multi-coloured display when hit.

Are shields forcefields?

The most popular candidate for shields is forcefields. In fact, shields and forcefields are often treated as interchangeable terms in the literature and dialogue. This is encouraging, because the term "forcefield" comes from real-world science, not science fiction. Unfortunately, the resemblance between real forcefields and sci-fi forcefields ends at the name.

The two types of forcefield you are most familiar with are electromagnetic and gravitational. Sure enough, those are the forces routinely mentioned in sci-fi. In the 1950's classic "War of the Worlds", the Martian spacecraft were said to be using an "electromagnetic blister", which easily warded off artillery shells and all other methods of attack. In Star Trek, the writers gleefully steal terminology from particle physicists and say that they're based on "gravitons" (the theoretical carrier particle for gravitational forces).

But electromagnetic and gravitational forcefields share an interesting characteristic: they are both long-ranged, and their effects weaken with the square of distance. So if you double your distance from the centre of the Earth, the force of gravity drops to one quarter of its original value. Simple enough, correct? Unfortunately, this creates a problem for our shields: you see, they typically have no effect whatsoever until you reach some invisible point. When a man runs into a forcefield on Star Trek, he feels nothing until he touches the invisible wall, which produces a sparkly effect.

Now, if this were a gravitational forcefield, he should have felt its effects from anywhere in the room, gradually increasing in strength as he approaches the window. A forcefield has a volume effect, hence the name "force field", not "force wall". But this is obviously not what we saw. Have you ever tried to force the positive poles of two magnets together? You can't do it, can you? And you will notice that the forcefield effect is gradual, not abrupt. It gradually increases as they approach, until it eventually becomes so large that you cannot force them any closer together.

And what about the fact that they wear down? We've all seen the displays: DEFLECTOR POWER 70%

In Star Trek, rather than simply being up or down, shields have a strength property. It wears down after multiple hits, and when it goes to zero, the shields are nonexistent. But why would a forcefield weaken after use? Does the magnetic field of an electromagnet get weaker each time you use it to pick something up? No, so why would shields get weaker? Is there a "fatigue" property? None of this is consistent with a forcefield.

Are shields made of energy?

Rather than imagining shields as forcefields, some people imagine them as a "wall of energy". That seems like an improvement (after all, there are no real walls of energy which we can use for comparison, so it's not as easy to say that it's wrong), but even if we disregard the question of how you would go about constructing this beast, some obvious questions leap to mind:

  1. What holds this energy in place? Pure energy is light, and moves at c. It does not sit in a particular spot, nor does it form walls of arbitrary shape. If you had some kind of mechanism which could control the energy and force it to move in a contour around the ship, why bother with the energy component? This mysterious energy-manipulating mechanism would obviously be capable of deflecting incoming energy weapons by itself if it can already manipulate a wall of energy to hold an arbitrary shape.

  2. Why aren't incoming objects destroyed by this energy? Not only can Picard touch one of these shields with his hand, but incoming objects such as Roga Danar's flimsy escape pod in "The Hunted" have been observed to bounce off a starship shield with no ill effects.

  3. Where does the energy go when they turn off the shields? If it's released, should it not be quite violent?

  4. Why would the energy necessarily interact with other energy?

The problems with the "wall of energy" idea are extremely difficult to resolve for many reasons, not least of which is the fact that the mechanism for manipulating the energy into a shield would perform the function of a shield all by itself.

What about frequency?

Star Trek shields have a "frequency" characteristic, which implies that they oscillate. It should be noted that this behaviour is unusual to Star Trek, and there is no reason to assume it is universal to all shield concepts. Many natural phenomena are frequency-based, but even a device based on a frequency-based principle need not be phase-coherent, so it would not exhibit an aggregate "frequency" characteristic. There are some general advantages and disadvantages of frequency coherence in shields:

Advantages

Disadvantages

Against a frequency-based attack, a phase-coherent shield could be theoretically optimized to give greater protection than a flat shield with the same average amplitude, by synchronizing with the attack.

Against a non-coherent attack, a phase-coherent shield would allow partial penetration even if it's working perfectly.

It should be possible for a ship to fire outgoing weapons out through its own phase-coherent shield by matching frequencies but being 180 degrees out of phase. You would have to open a small hole in a flat shield in order to fire through it, which requires fine control over shield geometry.

The knife cuts both ways. An attacker could penetrate a phase-coherent shield by matching frequencies and being 180 degrees out of phase.

Note that it's possible to oscillate with respect to amplitude or vector, although we would expect a vector oscillation would cause significant scattering effects with outgoing beams (turning a tight beam into more of a spray), and we generally don't see that with Trek weapon/shield interactions, or those of any other sci-fi series for that matter. A square-wave (as opposed to sinusoidal wave) would allow perfect penetration with a synchronized weapon, thus eliminating the scattering effect of a vector oscillation, but it would also allow 50% penetration from an incoherent weapon.

There are interesting theoretical possibilities for a shield which oscillates, but the disadvantages outweigh the advantages, depending on what kinds of weapons the enemy is using, what kind of control you have over shield geometry, etc. Worse yet, if the enemy has sensors which can detect the activity of your shields, it should be trivially easy for him to match frequencies, synchronize phase, and shoot through your defenses. Ultimately, the idea of a phase-coherent frequency-based shield seems more attractive for its script-writing flexibility than its tactical attributes.


Miscellaneous

Sci-fi is diverse, and not all shield systems do the same thing. But there are a few questions you can ask to narrow down what basic phenomenon a shield represents (note that this is different from trying to invent some technobabble explanation for how it works).

  1. Do energy bolts or beams "bounce off" the shields, still intact? If so, you are looking at reflection (for examples, see the trash compactor scene in ANH, or the battle droid shot which ricocheted off Anakin's Naboo starfighter in TPM).

  2. Do energy bolts or beams splinter, or break apart into a shower of smaller bolts? If so, you are looking at scattering (for an example, see the ISD turbolaser bolt that struck the blockade runner's shields in the opening scene of ANH).

  3. Do energy bolts or beams make a large area of the shield glow? If so, you are looking at absorption, conduction, and subsequent retransmission (for examples, see most TNG-era Trek shield incidents, as well as the incident in TESB where the Falcon was knocked about its longitudinal axis by a turbolaser hit).

  4. Does shield geometry affect the shield/energy interaction? If so, it may be a vector effect, deflecting the bolt in specific directions based on geometry (phrases like "angle the deflector shield" in SW or "continuously vary the shield geometry" in ST hint at this possibility).

  5. Does the shield completely block incoming energy, or does it allow a portion through even if it is still functional?

Ultimately, there are far more questions than answers when it comes to shielding, and one must be careful not to leap to facile conclusions.


Conclusions

Shielding is more complex than it may appear on first glance. There are more issues to consider, and more difficulties involved in evaluating strength than one may initially realize. However, this hardly means that the exercise is futile. On the contrary, a thorough and systematic examination of observed events can be used to determine realistic limits, given certain caveats:

  1. The fact that they often call it a "forcefield&quoquot; does not mean it actually conforms to the description of one.

  2. There is no intrinsic need for a great volume of energy in a shield, so it is wrong to assume that shields must consume large amounts of energy. Do not become attached to a particular model of shields simply because everyone else seems to accept it. A solid case can even be made for the idea that shields have mass.

  3. Always remember to consider the weakest link in the chain, not the strongest link. This is an important lesson from real-life engineering which is often lost on sci-fi debaters, who tend to conceptualize sci-fi in a purely abstract theoretical sense in which they pick one particular phenomenon and concentrate on that phenomenon to the exclusion of all others (ie- focus on the non-physical shields and ignore physical constraints).

  4. Do not assume that all energy shields employ the same mechanisms. We can view the behaviour of energy shields in action and see that there is significant diversity in their operation, and this must be considered when attempting to synthesize a consistent model of their operation.

  5. At no point do any of these theories require that the shield must draw as much energy from the ship's systems as the incoming weapon carries with it. Yet I have often noted that virtually everyone assumes this energy equivalency to be the case, without making the slightest attempt to explain the logic. Why should a shield require energy equivalent to the weapon? Does a piece of armour on a tank consume energy when a shell bounces off its surface? Did you ever wonder why an air conditioner's rate of cooling can exceed its electrical power draw in watts?

Before making any leap in logic about how much power a shield must need or how it must work, just ask yourself whether your assumptions are coming from observation and logic, or from common practice. Because ultimately, common practice is a poor justification for anything.

From Physical Nature of Shields at StarDestroyer.net by Michael Wong (2002)
Shields: Physical Impact Issues

Introduction

It is widely assumed that if a sci-fi shield can withstand X joules of energy from a laser, it must be able to withstand X joules of energy from a physical impactor. However, this is not necessarily the case. As attractive as the simplistic numbers game is, if we apply a little bit of physics knowledge to the situation, we can see that if anyone were to build such a beast, the situation would be more complex than that.

So what would make physical impactors more dangerous? The answer to that question comes down to damage mechanisms. To put it simply, a physical impactor inflicts damage upon its target in a variety of ways. While an energy weapon will generally attempt to heat the target, thus permitting specialized one-dimensional defensive strategies, a large, fast-moving physical impactor presents a more complex threat:

Threat type

Damage mechanisms

Energy weapon (laser, phaser, turbolaser bolt, etc.)

Heats the target surface.

Physical impactor (asteroid, high-velocity ramming attack, hyper-velocity railgun, etc.)

Subjects the target to severe structural stresses, usually resulting in penetration. If it fails to penetrate, it pulverizes and/or vapourizes at the point of contact due to internal stresses and work-heating, thus producing a large cloud of high-temperature material at the target surface. This cloud heats the target surface through convection and radiation.

That is why an effective defense strategy would use guns to destroy large physical impactors, forcefields to deflect small physical impactors, and shields to reflect, absorb and retransmit, or scatter energy weapons rather than the "one size fits all" approach that seems to be popular among fanboys.

Collision Physics

When a physical object strikes a shielded vessel, it must be decelerated by the vessel's defensive systems. Most people tend to assume that if the shield holds, the ship is undamaged. However, this is not necessarily the case. Consider the following image (and please, keep in mind that I am not a professional graphic artist):

Let's assume that the rectangular assembly at left is a shielded starship (yes, I know, it looks cheesy, but please bear with me). The big brown rock at right is hitting the ship's shields, and it is being decelerated (hence the rightward force F being applied to the rock by the forcefield). For every action, there is an equal and opposite reaction, so there must be a counter-balancing force for that forcefield. A forcefield must be coupled to something, and in this case, it would obviously be the shield generator. Therefore, there is a leftward force F being applied to the shield generator (the blue square) in the middle of the ship. But the shield generator cannot move relative to the ship or it would be torn loose from its moorings, so its mounting brackets (the four red blocks) must each apply a rightward force 0.25F in order to hold the shield generator in place. These four reaction forces, in turn, push the entire ship to the left with force F, so the net result is to stop the impactor while accelerating the ship.

Are we clear on that? Now here's where it gets interesting: what if the shield generator's projected forcefield is easily strong enough to decelerate the asteroid to zero before the moment of impact, but the four little red blocks aren't strong enough to hold the generator in place? Guess what: the shield generator will be torn from its moorings, and the rock will slam into the ship. This is where momentum can rule over energy; a low-momentum, high-energy weapon such as a laser might not be as dangerous to a shielded vessel as a high-momentum, low-energy physical impactor. In this scenario, the potential points of failure are the shield generator itself, the points where it is mounted to the vessel, and the structure of the vessel itself. In other words, the mounting brackets, bolts, welds, shield generator internal mechanisms, shield generator forcefield strength, and all other connecting bits are parts of a chain through which reaction forces must go in order to make the end-to-end connection between the ship and the impactor. It can be thought of as a chain, and as in any chain, it is the weakest link that will cause your downfall.

As you can see, even if it was possible to build a deflector shield generator of virtually infinite strength, the overall effectiveness of the system would still be limited by good old-fashioned structural limits. Ultimately, the survivability of a shielded spacecraft against physical impacts could (and would, given sufficient shield strength) conceivably come down to a set of bolts holding a shield generator onto the ship's spaceframe. This example highlights the severe problem with most attempts to rationalize sci-fi technologies, which is that people tend to look for the strongest link in the chain, not the weakest link in the chain.

Shield Collision Physics Summary

Physical impacts and energy weapons should not be treated as functionally identical, particularly in terms of the relationship of energy to structural stress in the target. Collision physics are still ruled by Newton, and all of the deflector shields and fancy tricks in sci-fi will not prevent reaction forces from acting upon the physical structure of a target spacecraft.

Ramming tactics are widely used in sci-fi (click here for an example analysis of a collision event). In Star Trek, Worf called for "ramming speed" in STFC, Jem'Hadar vessels rammed the USS Odyssey and destroyed it in DS9, and Commander Riker prepared to ram the Borg Cube in the TNG two-part episode "Best of Both Worlds". In Babylon 5, we saw a Starfury crash into and through a Minbari war cruiser's dorsal fin in the Battle of the Line as shown in the movie "In the Beginning", and Jeffrey Sinclair tried to ram another war cruiser later in that same battle. We also saw an Earth-force cruiser ramming a Minbari war cruiser in a brief flashback to the events leading up to that battle. Other examples include Battlestar Galactica, where Cylon raiders routinely crashed into the Galactica's flight decks (thus making the viewer wonder why there were no weapon emplacements near these flight deck entrances), Transformers (where the Autobots' stolen Quintesson corkscrew-ship rammed through Unicron's eye), and of course, ROTJ, where an A-wing crashed through the bridge windows of the Executor after its bridge shields were knocked out (which would imply that the Executor's unshielded bridge windows are similar in strength to the dorsal fin of a Minbari war cruiser).

The effectiveness of these popular ramming tactics has often been used as an excuse to downgrade shield estimates against energy weapons. But this implies an equivalency which does not exist. The "real-world" explanation for the effectiveness of ramming in sci-fi is that ramming is a very dramatic event, filled with imagery of martyrs and heroes. But the physics of collisions and reaction forces provide us with an "in-universe" explanation that works just as well.

From Shields: Physical Impact Issues at StarDestroyer.net by Michael Wong (2002)

Langston Field

In space combat it pretty much looks like the first to get a hit wins. This isn't really surprising; it's true of most combat these days (air combat, submarine combat, etc.). The weapons will be devastating enough that one hit will put a ship out of combat, if not vaporize it outright (i.e., they will have a very high Single Shot Kill Probability).

Larry Niven and Jerry Pournelle knew this, but wanted to write about dramatic extended space combat anyway. They contracted physicist Dr. Dan Alderson to design a self-consistent science-fictional gadget to allow this. He created the Langston Field.

In the SF trade, the Langston field is a "capacitor" or "tank" field. The field drinks up energy. It will absorb a laser beam, a nuclear blast or the kinetic energy in a coilgun shot. It then tries to radiate the energy away. However, the field cannot radiate away the energy as fast as the enemy can load the field with weapons fire. The field can only hold so much, and when the limit is reached, the field explodes, vaporizing the ship.

Also, the more destructive energy currently being held in the field, the more of the ship's own power that will be required to keep the field from exploding. If the field gets too full, the ship will not have energy to spare for movement or its own weapons.

A torpedo had penetrated her defensive fire to explode somewhere near the hull. The Langston Field, opaque to radiant energy, was able to absorb and redistribute the energy evenly throughout the field; but at cost. There had been been an overload at the place nearest the bomb: energy flaring inward...

All through Defiant nonessential systems died. It took power to maintain the Langston Field, and the more energy the Field had to contain the more internal power was needed to keep the Field from radiating inward. Local overloads produced burnthroughs, partial collapses sending bursts of energetic photons to punch holes in the hull. The Field moved toward full collapse, and when that happened, the energies it contained would vaporize Defiant. Total defeat in space is a clean death.

From "Reflex" by Larry Niven and Jerry Pournelle (the deleted first chapter of The Mote in God's Eye, collected in There Will Be War I)

Temporary "portals" or "holes" can be opened in the field to allow the ship's laser fire to hit enemy starships. Otherwise the laser beams will hit the underside of their own field. Of course the more energy being held in the field, the more difficult it is to open a hole.

Sensors are on booms so they can be extended outside of the field, otherwise the ship is blind. As the exposed sensors are blown away, the booms are retracted and fresh sensors are mounted. If the attack is ferocious enough, a ship can become blinded (i.e., all exposed sensors destroyed before any new ones can be deployed), and the enemy will quickly move out of the path of the ship's weapon fire while still pouring death and destruction into the blind ship's field. Then the blind ship frantically tries to deploy enough sensors so that at least one will last long enough to plot the position of the attacker.

Unfortunately, if the field becomes too full of energy, sensors or any other item being extended through the field will be fried or vaporized by the contained energy.

A hot field will also fry any object attempting to pass through the field en route to the ship inside (such as a shuttle containing a boarding party). Any object would also become embedded in the field, since the field also absorbs kinetic energy, unless is was moving really fast.

In a nod to E.E."Doc" Smith, when radiating, the field starts glowing red, then moves its way up the spectrum. The only thing a blinded ship can see is the color of the inside of its field.

Note the implication. When a ship's field is ten seconds from detonation, the ship is near death. But nothing has been physically damaged. If the ship is left alone long enough the field will cool off and the ship is as good as new. This made surrender a tricky proposition. If you gave too much mercy to the surrendering ship, it would recover and you'd be right back where you started.

The solution was interesting. If a ship with hot fields surrendered to you, the captain asks for a volunteer from the midshipmen. If nobody volunteers, the captain shrugs and signals to destroy the enemy ship anyway. But if there is a volunteer, they get to strap on their chest a tactical nuclear weapon with a hand detonator (dead-man switch or other fail-deadly type). Under pain of destruction, the surrendering ship has to allow the midshipmen to board, and let the midshipmen go to the control room or other vulnerable spot. You can now allow the surrendering ship's field to cool off. If it doesn't do exactly what you say, the midshipmen will detonate the bomb (you hope).

A ship in Defiant's situation, her screens overloaded, bombarded by torpedoes and fired on by an enemy she cannot locate, is utterly helpless; but she has been damaged hardly at all. Given time she can radiate the screen energies to space. She can erect antennas to find her enemy. When the screens cool, she can move and she can shoot. Even when she has been damaged by partial collapses, her enemy cannot know that.

Thus, surrender is difficult and requires a precise ritual...

...Weapons in the hand of a defeated enemy are still dangerous. Indeed, the Scottish skean dhu is said to be carried in the stocking so that it may be reached as its owner kneels in supplication...

Defiant erected a simple antenna suitable only for radio signals. Any other form of sensor would have been a hostile act and would earn instant destruction. The Imperial captain observed and sent instructions.

Meanwhile, torpedoes were being maneuvered alongside Defiant. (Captain) Colvin couldn't see them. He knew they must be in place when the next signal came through. The Imperial ship was sending an officer to take command.

Colvin felt some of the tension go out of him. If no one had volunteered for the job, Defiant would have been destroyed.

Something massive thumped against the hull. A port had already been opened for the Imperial. He entered carrying a bulky object: a bomb.

"Midshipman Horst Staley, Imperial Battlecruiser MacArthur," the officer announced as he was conducted to the bridge. ... "I am to take command of this ship, sir."

Captain Colvin nodded. "I give her to you. You'll want this," he added, handing the boy the microphone. "Thank you for coming."

..."Midshipman Staley reporting, sir. I am on the bridge and the enemy has surrendered." He listened for a few seconds, then turned to Colvin. "I am to ask you to leave me alone on the bridge except for yourself, sir. And to tell you that if anyone else comes on the bridge before our Marines have secured the ship, I will detonate the bomb I carry. Will you comply?"

From "Reflex" by Larry Niven and Jerry Pournelle (the deleted first chapter of The Mote in God's Eye, collected in There Will Be War I)

For dramatic purposes, Dr. Alderson decreed that the Langston field was subject to "local burn-throughs". That is, a given weapon strike might be too intense to be absorbed all at once, so a fraction of the damage pokes through the field into the ship. This gives enough damage to the ship to be cinematically interesting, but not enough to vaporize the ship outright or something boring like that.

This had the intended side-effect of ensuring that the ship with the best damage control crew would win the battle.

The Langston field may be science fiction, but at least it is internally self-consistent. Niven and Pournelle used it in their novel "The Mote in God's Eye", which Heinlein said was "possibly the finest science fiction novel I have ever read." High praise indeed.

In principle Defiant was a better ship than she'd been when she left New Chicago. The engineers had automated all routine spacekeeping tasks, and no United Republic spacer needed to do a job that a robot could perform. Like all of New Chicago's ships, and like few of the Imperial Navy's, Defiant was as automated as a merchantman.

Colvin wondered. Merchantmen do not fight battles. A merchant captain need not worry about random holes punched through his hull. He can ignore the risk that any given piece of equipment will be smashed at any instant. He will never have only minutes to keep his ship fighting or see her destroyed in an instant of blinding heat.

No robot could cope with the complexity of decisions damage control could generate, and if there were such a robot it might easily be the first item destroyed in battle. Colvin had been a merchant captain and had seen no reason to object to the Republic's naval policies, but now that he had experience in warship command, he understood why the Imperials automated as little as possible and kept the crew in working routine tasks: washing down corridors and changing air filters, scrubbing pots and inspecting the hull. Imperial crews might grumble about the work, but they were never idle. After six months, Defiant was a better ship, but...

From "Reflex" by Larry Niven and Jerry Pournelle (the deleted first chapter of The Mote in God's Eye, collected in There Will Be War I)

Erickson's Model

And now for something totally different. Leonard Erickson came up with an interesting model for force fields: use the equation for gas pressure.

Best fit between real formulas and the desired behavior/model was one of the gas equations. The one that has Energy equaling pressure times volume times a constant. This works ok for a closed surface type field. And leads to some interesting performance issues.

P * V = k * E

where:

  • P = pressure
  • V = volume
  • E energy
  • k = constant

Assuming k=1, you get something like 42 joules for a 1 meter radius sphere with 1 atmosphere of pressure inside. If you make k smaller, the energy requirements go up.

It "makes sense" that the bigger the enclosed volume, the more energy it'll take. And likewise for higher pressure (i.e. "stronger") fields taking more energy. One unexpected, but nice detail is that the field doesn't "use" energy. It takes energy to set it up, but that energy is "stored" in the field. So, aside from losses, you don't need to keep pumping energy in.

On the other hand, when you start considering the strength or "resistance to penetration" of a force field in terms of "pressure" (i.e. force per unit area), you suddenly realize that while holding in air is cheap, stopping bullets is gonna cost.

Another nice thing is that it would seem likely that "puncturing" the field doesn't hurt it. On the other hand, this is little comfort to the user when he finds that it wasn't turned up high enough to stop that bullet.

Leonard Erickson

Potential Barrier

Yet another possibility is the system described in Poul Anderson's novel Shield.

"So what is your invisible screen? A potential barrier?"

Surprised, he nodded. "How did you guess?"

"Seemed reasonable. A two-way potential barrier, I suppose, analogous to a mountain ridge between the user and the rest of the world. But I've determined myself, today, that it builds from zero to maximum within the space of a few centimeters. Nothing gets through that hasn't the needful energy, sort of like the escape velocity needed to get off a planet. So a bullet which hits the screen can't get through, and falls to the ground. But what happens to the kinetic energy?"

"The field absorbs it," he said, "and stores it in the power pack from which the field is generated in the first place. If a bullet did travel fast enough to penetrate, it'd get back its speed as it passed through the inner half of the barrier. The field would push it, so to speak, drawing energy from the pack to do so. But penetration velocity for the unit I've got, at its present adjustment, is about fifteen miles per second."

She whistled. "Is that the limit?"

"No. You can push the potential barrier as high as you like, until you even exclude electromagnetic radiation. That would take a much larger energy storage capacity, of course. For a given capacity, such as my unit has, you can expand the surface of the barrier at the price of lowering its height. For instance, you could enclose an entire house in a sphere centered on my unit, but penetration velocity would be correspondingly less-maybe only one mile a second, though I'd have to calculate it out to be certain."

From Shield, by Poul Anderson's (1963)

Point Defense

Point Defense is a fancy name for all the short ranged weapons and anti-missile missiles used to shoot at incoming enemy missiles. They are analogous to anti-aircraft guns.

A low powered weapon would do for defense against nuclear warheads. John Schilling says that nuclear weapons are rather complex and fragile devices, and it doesn't take much to put them out of action. And they do not undergo sympathetic detonation, i.e., they don't go boom just because you hit them real hard. So if your point-defense system can score a solid hit, the nuke is effectively useless.

Kinetic PD

Eric Rozier has an on-line calculator here that does calculation of Kinetic point defense hit probabilities (i.e., a point defence using bullets).

Modeling kinetic point defense is no easy task, the simulation I've created is a discrete event simulator which simulates individual bullets fired from a Phalanx style weapons system. The initial parameters for the CIWS are equivalent to a Phalanx with a perfect targeting computer. You can increase the number of CIWS firing at an incoming missle by increasing the number of linked CIWS, it is not as simple as multiplying the probability.

Target parameters are set to those of an AIM-9 Sidewinder missle with "infinite fuel", i.e. it will accelerate continuously during the entire simulation, regardless of the distance.

The simulation begins firing at the given range to the target in meters, simulating each shot to the target, and calculating the percentage of a hit based on the apparent velocity of the missle (muzzle velocity + target velocity), and the acceleration capabilities of the target (much as in the laser calculations on your page, but with slower than light bullets).

During each time step (of length indicated by the intershot time), all of the linked CIWS simulate a firing and calculate a hit probability, the missle then accelerates to a new velocity, the distance is shortened and provided the missle hasn't closed to minimum targeting distance, the CIWS take another shot and the joint probability is recomputed.

Eric Rozier

Missile PD

The indefatigable Eric Rozier has an on-line calculator here that does calculation of Missile point defense hit probabilities (i.e., a point defence using anti-missile missiles).

Ok, I think I've got a pretty well justified CIWS missile system. It models anti-missile missile point defense, similar to the RAM system in development by the military.

Cm = (Rb + Rt)2 * π

  • Rb is the blast radius of the kill zone for the nuclear CIWS missile.
  • Rt is the radius of the missile we're attempting to kill.
  • Cm is the cross-sectional area we must hit to kill the missile.

Hp = Cm / (π * d2)

  • d is the displacement which can be achieved by the target missile
  • Hp is the hit probability

d = 0.5 * (9.8 * (At - (Ac * e)) * t2

  • At is the acceleration (in Gs) of the target missile
  • Ac is the acceleration (in Gs) of the CIWS missile
  • e is the effectiveness of the tracking system on the CIWS missile
  • t is the time to intercept

t is calculated in my model by approximating an integral which takes into account the increasing velocity due to acceleration of both the CIWS missile and the target missile.

The end model basically models the system by calculating when the two missiles will hit, and then calculating the possible displacement the missiles can achieve. Normally with a purely kinetic kill vehicle this is calculated by the acceleration potential of the target missile during the time it takes us to intercept. In this case since we can supply active thrust, we can cancel out some of this acceleration potential. Our ability to do so is modeled as our acceleration potential multiplied by an effectiveness of our tracking system. If we have a perfect tracking system, we match them move per move to the extent our acceleration allows (i.e. if At = Ac, we hit, if At > Ac we usually miss). If it is imperfect we only get a fraction of our acceleration, as a portion of the time we are correcting mistakes, (i.e. in general if At < Ac by a ratio proportional to effectiveness we hit, otherwise we usually miss).

Eric Rozier

Laser vs. Missile

When it comes to laser point defense vs incoming missiles, there is some controversy. This is the subject of a long-running "Purple/Green" debate on SFConSim-L.

Purple/Green?

The term "Purple/Green" comes from an episode of Babylon-5 called "The Geometry of Shadows". The episode involving the ritual Drazi civil war, where the sides are chosen by randomly choosing colored sashes from a barrel. It is a science-fictional version of Miller Lite partisans shouting "Tastes Great!" and "Less Filling!".

More specifcally, as Christopher Weuve explains:

"It's the SFConsim-L brevity phrase meaning 'an argument in which no actual agreement can be reached, usually (but not always) because it is dependent on going-in assumptions.'"

Anyway the argument is about what happens in the last hundred kilometers to the target ship.

For an in-depth look go to the Rocketpunk Manifesto and read Battle of Spherical War Cows: Purple vs Green and Further Battles of Spherical War Cows. For a brief summary, see below:

The laser gang asserts that they can zap a missile before it ever gets to kill range, even for a nuclear warhead. And do it every time, at least so much of the time that missiles aren't worth firing. Even if the missile fragments into 10,000 pieces of shrapnel (each with substantial killing power), tracking gear can determine the fragments that will hit, and zap them before they reach target.

The laser gang's theory is that lasers never miss. If you can paint the target with photons to see it, you can hit it with a laser. In addition: missiles, by definition, need to close on the target, which means there are some trigonometry tricks that will allow you to lock them up hard with lasers - they can't laterally juke in space without missiing the target, for example.

The missile gang contends that laser point defense can always be saturated. Fire a big enough missile, or a salvo of missiles, coming in fast enough, and there will just be more mosquitoes than the bug zappers can zap in the short time till impact.

The missile gang's theory is that you can derive the number of missiles needed to overwhelm a given number of lasers by inputting some variables, like amount of energy per square cm needed to guarentee a kill on a missile, the wattage of output of the lasers, and the cycle/recharge time of the lasers. Lasers do require some time to recharge, and need some time to cool off.

The laser gang reply that lasers have the advantage in that they are reusable, unlike missiles. If lasers are dominant, it's also an offensive weapon to zap enemy ships, not a purely defensive one.

The missile gang retorts that the missile can be fired outside of laser range, and if it does penetrate point defense and smoke your ship, your laser is no longer reusable, now is it?

There is the cost effectiveness argument. Can you afford to carry point-defense lasers that can stop my missiles? Can I afford to carry missiles that can penetrate your point defense? Which is cheaper?

Can there be any tactics in a long-range duel between two missile armed ships? It comes down to whether you can afford to fire a missile on anything but a certain intercept, this is also ultimately a matter of cost.

Can there be any tactics in a long-range duel between two laser armed ships? It can be argued that it is the equivalent of two crack marksmen at opposite ends of a football field, shooting at each other with scope-equipped, tripod-mounted sniper rifles.

Given equal quality lasers, if I can zap you, you can zap me. Given laser ranges of at least a few hundred km, maybe a few thousand how can ships maneuver? If they are slow, it will take minutes to change position, meanwhile zapping away with multimegajoule lasers. If they are fast, they'll hurtle past each other in a drive-by, then take hours to swing around for another pass, unless they have science-fictional levels of acceleration. Possible solutions include long recharge and/or cooling-off times between laser volleys, and restricted firing arcs on the laser turrets.

The argument rages on, which probably means you can just pick which side appeals to you and be able to justify it. By carefully selecting, say, the proper minimum laser recycle time one can decide whether missiles are a viable weapon or not.

The Attack Vector: Tactical wargame adds an additional wrinkle. The laser recycle time is set such that missiles are viable. However, laser cannons have a limited number of "flash cooler" loads which can drastically cut the recycle time. But once you've used up your flash cooler loads, the laser is stuck at the standard recycle time.

Waste Heat

Radiators

The "Achilles Heel" of combat spacecraft are the heat radiators. Drives, power plants, and most weapons generate incredible amounts of waste heat. For unlimited operations, the heat has to be disposed of with radiators. However, since by their nature radiators are difficult or impossible to armor, radiators will probably be the first thing shot off by hostile weapons fire. Then you have about thirty seconds to scram the ship's reactor before the engineering section turns into a sea of molten metal. This is because shooting a hole in a spacecraft's radiator will have the same effect as shooting a hole in your automobile's radiator, except at a much higher temperature.

Droplet style heat radiators cannot be armored, but they are relatively immune to hostile weapons fire, since they are basically liquid sprays of coolant instead of physical panels. There are some notes on weapon radiators here. And before somebody mentions the "refrigerator laser" from David Brin's novel SUNDIVER, there appears to be certain theoretical reasons why it would not work. For one it probably violates the second law of thermodynamics.

And no, you cannot solve the problem by using a thermocouple to convert the heat into electricity.

Having said all that, Isaac Kuo is having second thoughts about the impossibility of armoring radiators.

The general wisdom, promulgated by the Atomic Rocket website, is that it's hard to armor a radiator and still have it be an efficient radiator.

Eric Henry

I know that's the general wisdom, and I used to believe in it...but I'm now skeptical of it. All you really need is for your armor material to be transparent to your desired operating frequency range. If diamond-like carbon is too rich for your tech, then try IR grade quartz. Either way, the basic idea is the same--your coolant fluid simply flows through tunnels in the transparent armor material. This actually makes for a more efficient radiator than the traditional design.

The traditional radiator involves coolant which conducts heat to the surrounding tubes, which then conduct heat to the radiator surface, which then radiates away heat. This armored radiator skips the conduction steps and simply radiates heat directly from the coolant.

Isaac Kuo

I tend to assume radiator wings will be lightly armored because they're big, and therefore heavy to armor. However, it doesn't seem that difficult to make radiators that are damage-tolerant enough to not be very tempting targets (the sails on age of sail ships weren't armored, but it wasn't terribly useful to shoot at them).

Anthony Jackson

Jens Bartmann disagrees with Anthony Jackson:

I beg to differ.

A sail, or more specific, the whole rigging was a very desirable target. Up to the point, special ammunition was created for bringing it down.

This is understandable, given that the rigging was the "motor" of a sailing vessel and bringing it down renders the opposing ship utterly helpless. While your opponent drifts along, you can attack at an angle where he cannot shoot back, or you can sail away if you so wish with no fear of persecution. (at least for the moment as damage control will try to set something up, Its all wood ropes and canvas after all and any self respecting ship had plenty of spares of that.)

For anti rigging weaponry have a look here around page 56. This should be somewhat similar to a spaceship with its radiators shot of, as it would be equally or even worse hampered in its abilities.

Jens Bartmann

Heat Sinks

In AV:T, ships going into battle retract their radiators into armored cubbies. They then rely upon internal heat sinks to dispose of waste heat. The good thing is that the heat sinks are armored. The bad news is that they can only store a few minutes worth of heat. This puts a severe time limit on the length of combat. Naturally a battleship will have a larger heat sink than a destroyer, but it will also have a higher waste heat level to dissipate.

If one's heat sink fills up too soon, the only option is to "strike the colors" and signal surrender to the enemy by extending the heat sinks (sort of like a dog in a dogfight surrendering by lying on its back and baring its throat). The alternative is being roasted alive as your ship melts.

Equations for heat sinks can be found here.

One concept for an orbital laser fort was to use a largish asteroid nudged into an appropriate orbit. The idea was to use the immense mass of the asteroid as a heat sink. During those long uneventful months the fort would use its radiators to cool down the asteroid as much as possible. When an attack occurs, all the fort's radiators are of course immediately retracted or are shot off by hostile fire. Both the fort and the hostiles will then commence lobbing laser beams at each other, and filling up their heat sinks with waste heat from laser cannon. However, when it comes to heat sinks, the internal sink of a warship is miniscule compared to the millions of tons of cold rock in an asteroid. It will require a fleet or two in order to even those odds.

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