A. NUCLEAR WEAPON EFFECTS ON PERSONNEL

     In addition to the natural radiation dangers which will confront the space traveler, we must also consider manmade perils which may exist during time of war. In particular, the use of nuclear weapons may pose a serious problem to manned military space operations. The singular emergence of man as the most vulnerable component of a space-weapon system becomes dramatically apparent when nuclear weapon effects in space are contrasted with the effects which occur within the Earth's atmosphere.

     When a nuclear weapon is detonated close to the Earth's surface the density of the air is sufficient to attenuate nuclear radiation (neutrons and gamma rays) to such a degree that the effects of these radiations are generally less important than the effects of blast and thermal radiation. The relative magnitudes of blast, thermal and nuclear radiation effects are shown in figure 1 for a nominal fission weapon (20 kilotons) at sea level.¹

     The solid portions of the three curves correspond to significant levels of blast, thermal, and nuclear radiation intensities. Blast overpressures of the order of 4 to 10 pounds per square inch will destroy most structures. Thermal intensities of the order of 4 to 10 calories per square centimeter will produce severe burns to exposed persons. Nuclear radiation dosages in the range 500 to 5,000 roentgens are required to produce death or quick incapacitation in humans.

1 The Effect of Nuclear Weapons, U. S. Department of Defense, published by the Atomic Energy Commission, June 1957.

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132 ASTRONAUTICS AND ITS APPLICATIONS

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     If a nuclear weapon is exploded in a vacuum-i. e., in space-the complexion of weapon effects changes drastically:

     First, in the absence of an atmosphere, blast disappears completely.

     Second, thermal radiation, as usually defined, also disappears. There is no longer any air for the blast wave to heat and much higher frequency radiation (x-rays and gamma rays) is emitted from the weapon itself.

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ASTRONAUTICS AND ITS APPLICATIONS 133

     Third, in the absence of the atmosphere, nuclear radiation will suffer no physical attenuation and the only degradation in intensity will arise from reduction with distance. As a result the range of significant dosages will be many times greater than is the case at sea level.

     Figure 2 shows the dosage-distance relationship for a 20-kiloton explosion when the burst takes place at sea level and when the burst takes place in space. We see that in the range 500 to 5,000 roentgens the space radii are of the order of 8 to 17 times as large as the sea-level radii. At lower dosages the difference between the two cases becomes even larger.

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134 ASTRONAUTICS AND ITS APPLICATIONS

     A yield of 20 kilotons has been used here as an example to show the dominance of nuclear radiation effects in space; however, it may well be that multimegaton warheads, rather than 20-kiloton warheads, will be far more representative of space defense applications. With such weapons the lethal radii (from nuclear radiation) in space may be of the order of hundreds of miles. The meaning of such huge lethal radii in possible future space warfare cannot now be assessed. It does seem clear, however, that manned space combat vehicles, unless heavy shielding is feasible, will be considerably more vulnerable to nuclear defense weapons than their unmanned counterparts.

B. POSSIBLE COMMUNICATION EFFECTS

     On August 1 and 12, 1958, nuclear warheads were detonated in missiles over Johnston Island in the Pacific.^2-3 These detonations were accompanied by impressive visual displays seen over wide areas, leading observers to the opinion that the detonations took place at very high altitudes.^4-7 These displays were even seen on Samoa, some 2,000 miles from Johnston Island.

     The visual displays were accompanied by disruptive effects on radio communications. Specifically, most commercial communication systems operating on the high-frequency (about 5 to 25 megacycles) bands in the Pacific noted substantial disturbances. Most links within a few hundred miles of Johnston Island experienced "outages" for as long as several hours, at various times over a period of about a day. In general, the effects on high-frequency communication links appear to have been quite similar to the effects produced by giant solar flares.

2 Note to Editors and Correspondents, U. S. Atomic Energy Commission, Department of Defense, Joint Office of Test Information, August 1, 1958
3 Note to Editors and Correspondents, U. S. Atomic Energy Commission, Department of Defense, Joint Office of Test Information, August 12, 1958.
4 Atomic-Like Flash Seen Here-Nuclear Rocket Test Indicated, The Honolulu Advertiser, August 1, 1958.
5 Samoa Bulletin, August 1, 1958.
6 Samoa Bulletin August 15. 1958.
7 Cullington, A Man-Made or Artificial Aurora, Nature, vol. 182, No. 4646, November 15, 1958, p. 1365.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The nuclear bomb is essentially a machine that assemble - smash together - a sub-critical mass of fissionable material into a supercritical mass, so the chain reaction would run in it. The trick is to do it fast: to smash the fissionable material together before the energy, released from nuclear fission, would vaporize it and throw it away.

If the assembly is not fast enough, then the chain reaction (with the massive release of energy) would start before the fissionable material mass is properly assembled. The result would be a pre-detonation: a weak thermal explosion, that destroy the bomb and threw the bulk of fissionable material around before it have a chance to get involved into chain reaction. Such situation during nuclear tests is often called “fizzle”.

Making nuclear bomb to fizzle is quite a good way to “disarm” it. Sure, we would still have an explosion – and quite dirty one, spreading a lot of radioactive dust around – but much weaker than the bomb was supposed to do. It would still have a power from sub-kiloton to low-kiloton level, thought. Still, dealing with consequences of low-level detonation is generally much more preferable than with full-power blast.

How exactly we could do it? Obviously, we could not slow down the assembly – at least, without physical access to the enemy bomb (which the enemy would not be eager to grant us, of course). But we could affect the other factor – the neutron breeding rate in fissionable material inside the bomb.

If we subject the enemy nuclear bomb to a powerful neutron flux – for example, by small nuclear detonation of our own nearby – then external neutrons, penetrating the fissionable materials of enemy bomb, would induce a fission reaction in it. Since the mass of fissionable materials is still subcritical, it would not cause the proper chain reaction. But it would create a lot of short-lived isotopes in bomb’s fissionable materials, pre-heating or “poisoning” it with a lot of neutrons.

And when such pre-heated/poisoned bomb would attempt to detonate? There would be a lot more free neutrons flying around inside its fissionable fuel than it was designed to dealt with. These additional neutrons would ignite the chain reaction long before the assembly is complete. The bomb would fizzle: it would destroy itself in low-level explosion.

The concept of pre-heating enemy bombs to make them fizzle was one of the main reasons behind putting nuclear warheads on surface-to-air missiles and air-to-air missiles (and at least one air-to-air unguided rocket) in 1950s and 1960s. They were supposed to not only knock down enemy planes and cruise missiles, but also “poison” their nuclear payload, so it would not detonate at full power. The same idea was behind early anti-ballistic missile defense systems as well.

So why nuclear warheads aren’t widely used on SAM’s and AAM’s nowadays? Well, the general reason is that nuclear technology marched on. “Poisoning” the fissionable fuel worked good enough against pure fission bombs, and fusion bombs with fission triggers – but not against boosted fission weapons. Which is the backbone of the modern nuclear stockpiles.

The boosted fission weapon is, essentially, a fission bomb, with a small amount of fusion fuel (deuterium-tritium mix) placed inside its hollow pit. When chain reaction starts in fission material, the heat and pressure cause the D-T mix inside to undergo fusion. The energy release from such fusion is negligible. But during fusion, a lot of high energy neutrons got released. They shoot into fissionable material around, causing a lot of additional fission, and boosting up the chain reaction. Essentially, the fusion worked as a source of additional neutrons to “afterburn” the fissionable material, ensuring that much more of it would undergo fission.

The boosted fission weapon is immune to neutron “poisoning”. The fusion of D-T mix starts long before the fission material assembly is complete. And high energy neutrons, released from such fusion, would “afterburn” the fissionable material of the bomb by themselves. So even if the boosted fission bomb is “poisoned”, it would still be able to ignite a fusion, and the fusion would boost it to full power.

So, do we have a “nuclear nullifier” here? Yes and no. The neutron poisoning could prevent enemy nuclear bombs from detonating at full power, but they would still explode, albeit at much reduced power. And the effect is not “blanketing”; you still need to go after each enemy bomb personally, with either a neutron bomb of your own, or a fictional neutron beam.

Most disappointing, though, is the fact, that this method is… obsolete. It did not work against boosted fission weapons. And pure fusion bombs (without fission trigger at all) would also be completely immune. So in modern, or futuristic warfare setting, this “nuclear nullifier” is nearly useless.

On the other hands, in retro warfare setting neutron poisoning could work just fine. From late 1940s to early 1970s it was a perfectly viable way of dealing with enemy nuclear weapons. So if your setting involves “Fat Man” type fission bombs dropped from piston-engine bombers (1940s), or megaton-scale thermonuclear bombs dropped from jet bombers (1950s), or “Orion” nuclear pulse spaceship clashing around the Moon (1960s), you actually could turn the enemy warheads into duds.

P.S. Also, it means that “Failsafe” movie is, sadly, not doing it right. If the “Vindicator” bombers were subjected to doses of radiation, fatal for their crews, then their 1960s bombs would be turned into nearly-harmless dud. So much for the drama…

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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Mass Schedule

System	Mass (kg)
Optics	9.1
Primary ACS	9.1
Secondary ACS	2.7
Communication	2.7
Warhead	90.7
TOTAL	114.3

The story is fictional, an alternate history novel. But the details about the Orion nuclear pulse drive and the casaba howitzer are meticulously researched and extrapolated where the details are classified.

Warhead has a length of 0.676 meters, infrared telescope has a length of 0.552 meters. Length when folded, about 1.23 meters. It is mostly a cylinder with a diameter of 0.387 meters, but there are four bumps near the top of the warhead that increase the diameter to 0.412 meters. I calculate the volume to be approximately 0.14 m³.

Nuclear device yield is 5 kilotons. Weapon jet velocity is 280,000 meters per second, containing a whopping 8,700 Ricks.

Blueprint legend:

The first generation of operational Casaba Howtizer units was first deployed in 1972 aboard the USSF Hornet. The units were composed of four primary assemblie… the modified small Orion pulse unit, a high-thrust, short-burn solid rocket booster, a 13-inch infrared telescope and a deployable communications module. All are stored and launched as a 15.25" (0.412 m) diameter cylinder. During the short boost phase, the freon fluid-injection TVC system directs the unit towards the target and roughly aims it using internally stored data obtained from the warship at the moment of launch. After booster separation the unit deploys the sensor and communication systems. A high-thrust monopropellant thruster system aims the weapon to within half a degree of the target. The infrared scope detects the target, using reflected laser light (projected from the warship); the cold gas thruster perform final aiming. Weapons initiation is commanded from the warship after confirmation of target lock.

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From author's afterword:

Discussion of Casaba-Howitzer

The Casaba-Howitzer was a real concept: a modified pulse unit that fired a jet of plasma. But instead of a jet of fairly dense plasma at a fairly wide angle, Casaba-Howitzer was to fire a lower density jet at a much tighter angle in order to serve as a weapon. Work continued well after the Orion program was terminated. And that, sadly, is about the sum total of the publicly available information on Casaba-Howitzer. Everything else about it is speculative. So, I speculated.

My first generation Casaba-Howitzer weapon is a modification of the pulse unit designed for the small 10-meter Orion. Exactly how a tight “beam” of nuclear death was to be generated, what sort of range could be expected… these are concepts about which I simply cannot speculate. But other areas sort of fall into place on their own. Was Casaba-Howitzer a weapon that would be fired from the ship, like a massive cannon? Given that the yield for a small pulse unit was a good fraction of a kiloton, trying to contain that energy in any sort of cannon-like object seems futile. So the pulse unit would be fired in free space. And likely you’d want to fire it at some distance from the ship. Therefore the pulse unit would need to be projected from the ship. This could be done via either gun or rocket; I’ve chosen rocket. In this case, a fast-burning, high-thrust booster similar to a Sprint motor, using Freon injection in the nozzle for thrust vectoring. The rocket would burn for only a second or so, tossing the projectile some considerable distance from the ship. After burnout, the projectile would unfold. I’ve given the projectile a sizable telescope with an IR scanner and a communications system. The presumption is that the weapon would be used to take out enemies at ranges of hundreds of kilometers, so it would need precise aim. If it was hundreds of yards from the ship, the only way to be sure of precise orientation with early 1970’s tech would be if the projectile could see what it was aiming at. The projectile would be aided by a laser on the warship; this would illuminate the target, making it stand out from the background, shining as a bright point in the distance. Computer aiming would be needed; even with a jet velocity of 2.8×10⁷ cm/sec (~174 miles/sec) — slightly less than twice that of the pulse unit — it will still take several seconds to hit a target. In that time the jet will have radiated away much of its heat as well as spreading out some distance, so the target will be hit with a shotgun blast of tiny particles. A thin cloud of dust moving at one tenth of one percent the speed of light.

The weapon has three attitude control systems. The first is the thrust vector control system on the booster; this is enough to get the unit within a few degrees of the target. The second system is a hydrazine monoprop thruster system which, once the system is properly deployed, quickly gets the weapon within a fraction of a degree of the target. The third is a simple cold gas (helium) system that has very low but precise thrust, used for getting the system precisely on target. Once the weapon has locked onto the target, the command to fire is issued by the warship. The weapon is initially launched with the telescope and radio communications system folded against the front of the system, but you wouldn’t want “stuff” immediately in front of the beam, as that would disrupt the blast.

The Casaba Howitzer yield is here given as 5 kilotons, about ten times the yield of a comparable pulse unit. The pulse units were at the low end of what was feasible for repeatable nukes; dialing one up to five kilotons would only be a matter of letting the base nuke be what it wants to be, rather than intentionally throttling it.

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(ed note: In 2018 Matter Beam discovered errors in the original calculation. The figures below have been updated)

The Casaba Howitzer is the result of research into reducing the spread of the particles produced by a nuclear pulse unit. Make the cone narrow enough and it becomes a destructive beam.

The original nuclear shaped charge design called for the use of a tungsten plate. The particles that resulted from the detonation of a pulse unit would fit inside a cone with a spread of 22.5°. The particles would be relatively slow (between 10 and 100km/s depending on thrust requirements) and rather cool (14000°C in transit, 67000°C after hitting the plate).

As noted before, using lighter elements, such as plastics or even hydrogen, in a thick and narrow instead of wide and flat shape, you can achieve a very narrow cone and very high particle velocities. A Science & Global Security report from 1990 used polystyrene as the propellant material to produce a particle beam with a spread of 5.7° and a velocity of 1000km/s.   

Particle velocity is derived from the Root Mean Square equation. It can be written as such:

• Particle velocity = (24939 * Temp / Mass) ^ 0.5

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

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

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

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

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

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Effectiveness

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

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

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

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

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

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

Let us calculate some examples:

Small Casaba Howitzer (50kg)
0.01 radian directivity (0.057 degrees)
5kt yield, 10% efficiency: 2.09TJ
Distance 1km: Irradiance = 673GJ/m^2
Distance 10km: Irradiance = 6.7GJ/m^2
Distance 100km: Irradiance = 67.2MJ/m^2
Distance 1000km: Irradiance = 672kJ/m^2

Large Casaba Howitzer (1000kg)
0.001 radian directivity (0.0057 degrees)
1Mt yield, 5% efficiency: 209TJ
Distance 1km: Irradiance = 6728TJ/m^2
Distance 10km: Irradiance = 67.3GJ/m^2
Distance 100km: Irradiance = 672MJ/m^2
Distance 1000km: Irradiance = 6.7MJ/m^2

Futuristic Megaton Nuclear lance
0.0001 radian directivity (0.00057 degrees)
1Mt yield, 20% efficiency:836TJ
Distance 1000km: Irradiance = 2691GJ/m^2
Distance 100000 km: Irradiance = 269MJ/m^2

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

We will describe the strike as a laser pulse of duration 1 microsecond, containing X energy and with Y spot radius. A 1.63 micrometer laser focused by a 2cm diameter mirror consistently produces the same spot sizes as a 0.01 radian beam. A 20cm mirror is used for 0.001 radian beams, and 200cm for 0.0001. We test penetration against Aluminium. 

Small Casaba Howitzer:
X = 2.09TJ
1km, Y = 0.994m: 734mm penetration
10km, Y = 9.94m: 0.73mm penetration

Large Casaba Howitzer:
X = 209TJ
50km, Y = 4.97m: 586mm penetration
500km, Y = 49.7m: 0.59mm penetration

Futuristic Megaton Nuclear lance:
X = 836TJ

1000km, Y = 9.94m: 293mm penetration
5000km, Y = 49.7m: 2.35mm penetration

The results reveal that the Casaba Howitzer is an extremely destructive weapon, with the larger models able to strike at distances usually reserved for lasers. Even a small Casaba Howitzer is effective at up to several kilometers, using technology tested in the 80s. Larger, more modern devices can strike at extreme distances. Futuristic devices will reach particle velocities of about 10000km/s, so time to target is negligible.

However, these distances are lower than those of powerful lasers, so the Casaba Howitzer will need a delivery system such as missile, or be used in defensive roles.

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Making use of the Casaba Howitzer

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

• Terminal warhead

• 

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

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

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

• 

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

• 

Using a Casaba Howitzer warhead solves this conundrum. 

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

• Point defense

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

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

• 

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

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

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

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

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

• Booster

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

• 

Instead of being used to generate high velocity particles in a narrow cone, a Casaba Howitzer can be used as a nuclear version of modern shaped charges. A metal cone is put in the way of a nuclear-heated beryllium filler. It is accelerated by the blast, like in an Explosively Formed Projectile. The only requirement is that the energy deposited into the metal lining is not sufficient to vaporize it.

Find out more here

  

• Particle beam weapon

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

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

• 

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

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

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Integrating the Nuclear Lance into your setting

The Casaba Howitzer is best used as an 'early technology' science fiction setting. When space exploration is still new, and opponents start out in the same orbit, the short-ranged but powerful nuclear shaped charges available are extremely effective.

It can be mounted on modern-technology missiles to allow them to be effective regardless of the impact velocity, alternatively, missiles will accelerate to low velocities then expend the majority of their dV in evasive maneuvers. It will more likely be used by the most technologically advanced nation to greater effectiveness, as the technology is far from well understood even 58 years after its conception.

• 

When the technology becomes widespread, such as following its development in nuclear pulse propulsion, it will still be the favorite of nations with greater access to fissile materials. While a fusion device allows greater yields, and would be better for propulsion, a Casaba Howitzer weapon does not benefit from the 1000km/s particle velocities. Easy to detonate fission charges are easier to handle and use.

They will however fall out of favor as lasers extend the range of combat beyond even their reach, into the tens of thousands of kilometers. More efficient missile propulsion, or the development of cold stealth technology, might change the battlefield even further.

However, as developments in propulsion continue, newer, simpler methods of detonating thermonuclear devices might become commonplace. Antimatter-catalyzed fusion or supercapacitors powering Z-pinch devices might allow Casaba Howitzers to return to the battlefield as cheap anti-missile defenses, free from the requirement of fissile materials.

Throughout history, however popular or effective they are, Casaba Howitzers will force states to carefully watch who and where to fissile materials are sold. Just 2 kilograms of uranium can be converted into a several kiloton-yield weapon, easily hidden in a civilian cargo-bay or remote satellite, and used to destroy an expensive warship in an instant.

• 

A follow-up to the popular Casaba Howitzer post, we now look more closely at the concept of nuclear shaped charges in both Explosively Formed Projectile and High-Explosive Anti-Tank (Monroe effect) forms.

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The concept

An explosive produces hot gasses that expand in all directions. A shaped charge focuses the energy of an explosive into a narrow cone. How effectively it does this is called 'directivity'.

The energy of a shaped charge can be used to accelerate a projectile. This projectile absorbs some of the gasses' kinetic energy and some of its thermal energy.

To maximize the amount of kinetic energy gained by the projectile and to reduce how much heat it absorbs, the projectiles are made as thin sheets of metal resting on a layer of explosive filler. The filler detonates and expands in only direction — into the projectile.

• 

The projectile henceforth will be referred to as the 'metal plate' or the 'liner'. You can find it called the 'flyer plate' or simply 'flyer' in literature.

The angle the metal plate forms with the explosive filler determines how much of the Monroe effect it uses. At shallow angles, we produce an Explosively Formed Projectile. At sharp angles, the Monroe effect is used to pinch together the walls of the cone and squirt out a very fast jet. The latter is used today in High Explosive Anti-Tank projectiles.

• 

Modern weapons use chemical energy. The explosive filler is also a chemical compound, so the maximum velocity of the metal plate is proportionate to the energy density and the amount of explosive filler being used.

In this post, we will consider inert fillers being heated by nuclear shaped charges. For more information on, read the Casaba Howitzers post.

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Existing performance

Chemical explosives today energy densities measured in megajoules per kg. TNT contains 4.184MJ/kg. HMX contains 6.27MJ/kg. Some chemical compounds such as some solid rocket propellants contain even higher energy densities, but do not produce the supersonic shockwave necessary in shaped charges.

These explosives create gasses reaching 3000 to 4000K. The rate of expansion of a gas, and therefore its kinetic energy, is strongly dependent on its temperature. Therefore, a hotter gas contains more energy and can accelerate a metal plate even faster.

Modern EFPs manage to propel their metal plates at velocities ranging from 2000 to 3000m/s. Attempting even higher velocities quickly requires huge amounts of explosive filler.

HEAT weapons manage to accelerate the tip of their jets to velocities ranging from 7 to 14km/s. The remainder of the metal lining reaches much slower 1-2km/s velocities, with the deepest segment not being accelerated at all. This velocity differential stretches out the jet until it fragments into ineffective pieces, which severely limits the effective range.

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Nuclear Explosive Formed Projectiles

The idea here is weaponize the nuclear pulse propulsion units designed for use in the Orion drive.

• 

From the original project, we know that 85% of the nuclear yield can be directed into a narrow cone of 22 degrees or less. Instead of allowing beryllium filler particles to fly out into space, we place a thick metal plate on top.

In a NEFP, the metal plate is at a very shallow angle.

• 

Research has already put into the concept, as published by Science and Global Security 1990.

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NEFP velocity

The main requirement of a NEFP is that the energy deposited into the metal lining is not sufficient to vaporize it.

Copper's melting point is about 1400K. Refractory materials such as tungsten can stay semi-solid at 3600K. Some materials can stay solid at even higher temperatures, but would not exhibit the plastic behaviour of metals. This limits the maximum metal plate temperatures to about 3500K.

We can use the contemporary performance of Explosively Formed Penetrators to estimate the maximum temperature of the filler in a nuclear design.

This study from Thermal Science 2016 tracked the temperatures and pressures in a copper plate being driven by Octol, a mix of TNT and HMX. Octol has a detonation velocity of 2000m/s and a specific energy of 6.3MJ/kg.

• 

We observe that the copper reaches temperatures around 622K if we average between the 545 and 698K in the last frame.The gasses driving it reach 4010K. In the experiment, the copper is 10mm thick, masses 12.5kg and is shaped as a hemisphere 150mm in radius, for an 'exposed' area of 0.14m^2.

Copper's heat capacity is 385kJ/kg/K and its heat conductivity is 385W/mK.

Tungsten's heat capacity is 133kJ/kg/K and its heat conductivity is 100W/mK.

If we substituted copper for tungsten, the metal plate would survive 3500K, a temperature 5.83 times higher, but requires only 2.04 times more energy due to the lower heat capacity.

Heat transfer by conduction is linear with the temperature difference. In the Thermal Science test, the copper started at 300K and ended up at 622K, averaging a 3548K temperature difference between the hot gasses and the metal plate.

A tungsten plate would heat up from 300K to 3500K, averaging 1900K. Its heat conductivity is 3.85 times lower than that of copper, so the temperature difference can be allowed to become 3.85 times higher for the same heating effect.

Considering all these factors, tungsten can survive a temperature difference 3.85 * 2.04 : 7.85 times higher.

This works out to a tungsten plate would average 1900K if it is accelerated by a gas of temperature just under 30000K.

This gas contains 7.42 times more energy than high explosive gas. It would accelerate a tungsten plate to a velocity 2.7 times faster.

We can safely say that Explosively Formed Projectiles can be propelled about three times faster using nuclear energy than using chemical explosives. This suggests velocities of about 6 to 9km/s.

Higher velocities can be achieved if we accept the fragmentation of the metal plate. These fragments have a theoretical velocity of 100km/s.

Even higher velocities, such as those cited in the Science & Global Security article, are the result of explosive fillers being heated to millions of Kelvins. They allow for velocities of up to 3% of the speed of light, as fast as the particles in a Casaba Howitzer. However, heating a metal liner and an explosive filler to those temperatures turn them into a plasma, and plasma-plasma interactions do not allow for much of the nuclear weapon's yield to be converted into kinetic energy.

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NEFP efficiency

• 

According to Friedwardt Winterberg, 50% of the nuclear blast is converted into the kinetic motion of the particles in the shaped charge's explosive filler. The rest goes into heating the filler.

• 

Since the nuclear blast also destroys everything aft of the explosive filler, the configuration is assumed to be an 'open-faced sandwich'. Roughly 50% of the filler's kinetic energy is used to accelerate the metal plate in the target's direction.

Using the 85% efficiency for the nuclear blast, 50% for the filler and 50% for the metal plate, about 21% of the nuclear yield ends up in the projectile.

This is better than the 5% efficiency listed in experimental studies.

In a NEFP, this means that a 1 kiloton yield warhead could propel more than 21.7 tons of metal at the target at 9km/s.

This literal boulder would be immune to most forms of anti-missile defenses, such as Whipple shields, lasers, missile interceptors or even wide-angle defensive Casaba Howitzers.

A 2m wide 21.7 ton tungsten projectile would be 352mm thick. Using the hydrodynamic penetration model, this projectile would penetrate 947mm of aluminium. Armor materials suited to resisting laser fire would be less dense and suffer greater penetration. This isn't an exceptional penetration depth for the mass invested in the weapon.

Instead, the metal has incredible momentum. Striking a 10000 ton target would knock the target back at 19.5m/s. In practice, this would break the target in half through sheer mechanical stress. The relatively size of the projectile makes the impact resemble a cannonball ploughing through a building.

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Spaced NEFP

In the Orion drive, the nuclear pulsed propulsion charges are detonated at a distance of 25 meters from the pusher plate. This spacing allows for the hot plasma (67000K) ejected by the nuclear charge to expand and cool down to 14000K. This greatly reduced the erosion and heating of the pusher plate.

• 

A similar concept can be used to allow nuclear EFPs to both use high-temperature gasses and the high kinetic efficiency of solid metal plates.

By spacing the explosive filler from the metal plate, an initially very hot plasma can be accelerate a solid plate without vaporising the latter.

The advantage is that a very hot plasma allows for very fast EFPs and much lighter weapons. The disadvantage is that they will become much larger and there will be some efficiency losses from the metal plate not intercepting the entirety of the filler gasses.

Let us assume a 1 kt yield nuclear shaped charge with 85% directivity. We want the gasses arriving to accelerate a tungsten plate to be no hotter than 30000K, as calculated in our example above.

How hot can the initial filler get?

If we use the original 22.5 degree cone, and state that the filler starts out 1m wide (surface area 3.14m^2), then in 10 meters spacing it will have spread out to a disk 5m wide (19.47m^2). This linear expansion would cool the plasma by a factor 6.2. The initial plasma temperature can be 186000 K and allow velocities (186000/4010)^0.5 about 7 times higher than with chemical explosives.

If we increase the spacing to 20 meters, the plasma would cool by a factor 20. The initial plasma temperature can be 602400 K and velocities 12.25 higher.

We could instead reduce the radius of the filler down to 10cm and increase the propellant cone's angle to 45 degrees to achieve an expansion and cooling ratio within 10 meters of 7022, within 20 meters of 27755, allowing velocities 83 and 477 times faster!

Here is a simple equation to determine how the spacing and spread angle cools the plasma and allows for higher projectile velocities, based on the results from the experiment cited above.

Velocity factor = ((tanA * Spacing + Ri) / Ri ) ^ 2 * (Ts / Tc ))^0.5

Velocity factor is how much faster the NEFP projectile can be compared to a chemical EFP. Velocities for chemical EFPs at 2 to 3km/s.

A is half the spread angle. For the Orion drive, this is 11.25 degrees.
Spacing is the distance between the filler and the metal plate, in meters.
Ri is the initial radius of the filler, in meters.
Ts is the survivable temperature of the metal plate. For tungsten, it should be 30000 Kelvin.
Tc is the chemical gas temperature we are using as a reference. For our example, this is 4000 Kelvin.

Using this equation, we determine that a 1kt yield shaped charge with 85% directivity, spreading by 60 degrees (30 degree half-angle), Ri 15cm, and placed 10 meters away from a 16.7kg tungsten plate could reach velocities of up to 324km/s.

The same warhead with the same spread at 25 meters distance would be able to accelerate a 2.75kg plate to 798km/s.

A problem with very high spread angles is that some of the gas particle's kinetic energy is not perpendicular to the plate and therefore does not contribute to its acceleration. Great separation distances increases losses from gasses expanding laterally and not being intercepted by the plate. Overall efficiency would be lower in these cases.

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Nuclear HEAT or Nuclear Munroe Projectile

Using the Monroe effect on metal cones angled sharply inwards allows for jets with tip velocities 7 to 10 times greater than the velocity of the explosive gasses driving them.

• 

Modern HEAT weapons generate tip velocities of up to 14km/s using gasses that travel no faster than 2 or 3km/s.

A 'Nuclear Monroe Projectile' would therefore produce metal jets of 60 to 90km/s.

If the maximum particle velocity in a fusion shaped charge is 3% of the speed of light, then the Monroe effect can increase this velocity to 30%.

However, there are severe limitations that reduce the effectiveness of this type of weapon.

The first is the standoff distance.

• 

While the tip of the jet can reach astounding velocities, the main body of the projectile reaches much lower velocity, with the rearmost 'slug' remaining mostly stationary relative to the warhead.

The large velocity differential stretches out the jet to the point of fragmentation and uselessness. Tip velocities of several tens of kilometers per second would disrupt a jet in milliseconds, meaning that it has to be fired close enough to its target to penetrate with an intact jet.

The standoff distance would be measured in single meters.

The second is efficiency.

In a NEFP, 21% of the nuclear yield ends up as the kinetic energy of the projectile. In a NMP, the kinetic energy is shared between a small fast tip, a slow moving body and a mostly stationary slug concentrating most of the mass. This reduces the overall efficiency of the weapon to a few percent.

In a realistic space setting, getting an intact warhead close to the target before it detonates is a difficult task. In most cases, factors which make this easier (massed missile attacks, high velocity warheads) reduce the usefulness of nuclear warheads (high per-unit costs, waste of missile's kinetic energy).

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Performance compared to lasers and Casaba Howitzers

Lasers are generally taken to be low-efficiency, long-ranged weapons which require so many high-mass components that spaceships are built around them. Their extreme effective range can further be extended by relatively cheap methods (larger focusing mirror, laser webs) once the initial investment in radiators, reactors, cooling systems, electrical generators and so on, is made.

• 

Casaba Howitzers unlock the potential of nuclear energy at long distances. Conventional nuclear warheads waste their energy in spherical explosions that cannot harm spaceships beyond a few kilometers. A Casaba Howtizer focuses this nuclear energy into particle beams that can vaporize targets at close range and cover large swathes of space in burning plasma for only a few hundred kilograms per warhead.

At an average 1kW/kg from reactor to radiator through all the components required for a laser weapon, a gigawatt beam would require an investment of 1000 tons.

This 1000 ton weapon would maintain a 10mm/s penetration rate in Aluminium at about 25000km, using a 40m wide mirror and 400nm wavelength.

In comparison, a much less complex spaceship could arms itself with 285 Casaba Howitzers with 10 megaton yield and 0.001 radian directivity, with the 10000km effective range. The lack of huge radiators and power requirements means that some stealth tactics are possible, wherein the spaceship unloads its missiles and overwhelms its targets with multiple particle beams each.

However, if the Casaba Howitzer-equipped spaceship is detected and intercepted, it will lose to the laser. The laser can fire indefinitely and stay outside of the range of the particle beams.

Increase the yield of the nuclear warheads to reduce the range gap quickly reduces the mass advantage they have over a reusable laser. A 150 megaton yield warhead would be effective out to 25000km, but would mass more than 52 tons each.

The solution is the spaced NEFP. Its effective range is practically infinite. A 1 megaton warhead could propel a 2.7 ton projectile to 800km/s, while massing only about 3 tons. This projectile crosses the laser's effective range in about 30 seconds, gouges out a crater nearly a 100 meters deep and/or cracks the target in half with 2160 kN.m of momentum concentrated on a spot less than a meter wide.

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Consequences

The consequences of mature NEFP technology in a setting are similar to those of Casaba Howitzers.

Devastating effects, able to be projected at extreme ranges, requiring only small investments in terms of propulsion, energy and mass to be used. The smallest freighter can take down the largest warship in a surprise attack. Large specialized warships such as laser battleships would not be able to compete with swarms of NEFP-equipped fighters.

On the flip side, widespread use of shaped charges means that the Orion propulsion concept is viable. Spaceships would be able to sustain heavy-g burns for long periods, either for travel or for dodging projectiles.

Combat might evolve into a cross between a chess board and a pinball machine. Chess, when it comes to intercepting your target and setting up a cross-fire they cannot dodge, and pinball, for when the nuclear warheads detonate and you have 30 seconds to outsmart your opponent and out-manoeuvre their projectiles.

A secondary consequence is that widespread use of nuclear energy requires either inordinate amounts of fissile fuels (with proliferation and unconventional warfare effects) or a cheap way to ignite fusion fuels.

DIRECTED THERMONUCLEAR EXPLOSIVES

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

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

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

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

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

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

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

10-kiloton ASAT
Nuclear yield	10 kilotons
Number of beams	10
Mass per plate	32 kg
Mechanism	50 kilojoules per pellet impact kill
Assumptions	4 × 106 particles per beam uniformly spaced 1 per m2 at 2,000 kilometers
Range	2,000 kilometers

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

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

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

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

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

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

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

‡ The energy fluence per beam, E in J/m², is approximately ηY/(N_bR²θ²), where η is the fraction of overall yield transferred to the pellets, Y is the bomb yield (1 kiloton is equivalent to 4.2 × 10¹² joules), N_b is the number of individual beams being driven by one bomb, R is the distance to the target, and θ is the individual full-beam divergence angle. A maneuvering target could accelerate out of the path of the beam if a_mR/v_f² > θ, where a_m is the magnitude of the target's average acceleration, v_f is the particle velocity, and τ = R/v_f is the particle fly-out time. (For comparison, the average acceleration of ICBMs is about 40 m/s².) To deliver this energy requires a total mass per beam of M_b = 2E(Rθ)²/v_f².

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

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

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

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

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

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

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

RE: Casaba Howitzer

This apparently HAS been tested, although the results are a little different to a searing fiery death-beam. There isn't too much to go on, unsurprisingly, but it seems a test, codenamed "Chamita" was carried out in support of a "Project Prometheus" and was investigating using a orion-pulse-unit-style setup to project a "beam" of solid shrapnel at velocities in the 100km/s area in a cone 0.001 radians wide.

Whether this can really be extrapolated to megaton versions vaporising kilotons of metal at extreme ranges, is probably guesswork though. But what appears to have been empirically verified is still quite eye-opening.

This document ( http://extremal-mechanics.org/wp-content/uploads/2012/11/Fenstermacher.pdf) makes reference to a "The one known NKEW test (having yield under 20 kilotons) occurred on 17 August 1985 and was named "Chamita."

This document lists "chamita" as a 20kt test burst in a shaft, listed as "weapons development"

This document states in a reference:

"[Ref#]68. Christopher E Paine, unclassified presentation at the Washington Test Ban Workshop, 20 March 1990. It has been reported that the 17 August 1985 “Chamita” test, in support of a nuclear-powered kinetic energy weapon, accelerated a 1-kilogram tungsten-molybdenum plate to 70 kilometres per second and that five known x-ray laser tests occurred between 14 November 1980 and 28 December 1985, all but the first of which having yields in the range 20-150 kilotons."

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

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

Note that 5% figure — not the 60-80% figure that is often reported alongside Project Orion materials.

Also note that this is a "beam" of solid particles, not a beam of x-rays.

Also note the predicted degredation with larger yields.

Further:

"There is also a fundamental problem with both the Casaba and Prometheus concepts that becomes relevant at higher yields. Despite the alleged success in directing 5 percent of the energy of a small nuclear explosion into flying debris, a good portion of the remaining energy inevitably becomes blackbody radiation, which would quickly overtake the pellets. Even at 1 kiloton with optimistic assumptions, this poses the risk that most of the particles will be vaporized or even ionized, rendering them ineffective: The NKEW concept is thus one that may require subkiloton explosives to be feasible. If its feasibility also depends on employing shaped thermonuclear explosives to help direct the pellets or dust more efficiently, then the concept is further burdened by the difficulty of designing thermonuclear devices with yields less than 1 kiloton. Whatever the case may be, it is clear that demonstrating a rush of hypervelocity pellets from a nuclear blast, while perhaps impressive, in no way guarantees that a useful weapon will ever be derived from this concept." [emphasis mine]

Luckily, Diaz was facing the other way when the missile exploded. It was too far off to blind him permanently, but the retinal burns would have taken a week or more to heal. He saw the glare reflected in his view lenses.

As a ground soldier he would have hit the rock and tried to claw himself a hole. But there was no ground here, no up or down, concealment or shelter, on a fragment of spaceship orbiting through the darkness beyond Mars. Diaz went loose in his armor. Countdown: brow, jaw, neck, shoulders, back, chest, belly…

No blast came, to slam him against the end of his lifeline and break any bones whose muscles were not relaxed. So it had not been a shaped charge shell, firing a cone of atomic-powered concussion through space. Or if it was, he had not been caught in the danger zone.

As for radiation, he needn’t worry much about that. Whatever particles and gamma photons he got at this distance should not be too big a dose for the anti-X in his body to handle the effects.

Against blackness and a million wintry stars, a gas cloud expanded. It glowed in many soft hues, the center still bright, edges fading into vacuum. Shaped explosions did not behave like that, thought the calculator part of Diaz; this had been a standard fireball type. But the cloud was nonspherical. Hence a ship had been hit, a big ship, but whose?

From the bizarre experimental weapons files:

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

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

Gentlesophs, I give you Project Casaba-Derringer.

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

• 

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

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

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

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

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

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

K_e = 0.5 * M * V²

therefore

M = K_e / (0.5 * V²)

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

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

Sneaky the cat watches with bright interest as a space-suited Floyd carries the cat's litterbox into the airlock, emptying all the sand and the lumps into the path of the raider…

• 

      When spacecraft and satellites travel through space they encounter tiny, fast moving particles of space dust and debris. If the particle travels fast enough, its impact appears to create electromagnetic radiation (in the form of radio waves) that can damage or even disable the craft's electronic systems.

     A new study published this week in the journal Physics of Plasmas, uses computer simulations to show that the cloud of plasma generated from the particle's impact is responsible for creating the damaging electromagnetic pulse. They show that as the plasma expands into the surrounding vacuum, the ions and electrons travel at different speeds and separate in a way that creates radio frequency emissions.
     "For the last few decades researchers have studied these hypervelocity impacts and we've noticed that there's radiation from the impacts when the particles are going sufficiently fast," said lead author Alex Fletcher, now a postdoctoral researcher at the Boston University Center for Space Physics. "No one has really been able to explain why it's there, where it comes from or the physical mechanism behind it."
     The study is a step towards verifying the theory of senior author Sigrid Close, associate professor of aeronautics and astronautics at Stanford University. In 2010, Close and colleagues published the initial hypothesis that hypervelocity impact plasmas are responsible for a few satellite failures.

     To simulate the results from a hypervelocity impact plasma, researchers used a method called particle-in-cell simulation that allows them to model the plasma and the electromagnetic fields simultaneously. They fed the simulation details from a previously developed hydrocode—a computational tool they used to model the fluid and solid dynamics of the impact. The researchers let the simulation evolve and calculated the radiation produced by the plasma.
     When a particle hits a hard surface at high speeds, it vaporizes and ionizes the target, releasing a cloud of dust, gas and plasma. As the plasma expands into the surrounding vacuum (of space), its density drops and it enters a collisionless state where its particles no longer interact directly with one another.

     In the current study, the researchers make the assumption that the electrons in this collisionless plasma then travel faster than the larger ions. Their simulation predicts that this large-scale charge separation generates the radiation. The model's results are consistent with Close's initial theory, but predict a higher frequency for the emission than researchers have detected experimentally.
     The authors point out that the assumption that the electrons move en masse as they separate from the ions deserves more careful attention. The group is building new simulations to test whether the shift to a collisionless state is sufficient to create the separation.

     Fletcher also notes that they have neglected to account for the dust.
     "The impact creates dust particles that interact with the plasma," Fletcher said. The dynamics of these "dusty plasmas" are an area for future research.
     The next step in the work is to use the simulation to quantify the radiation generated so they can assess the threat to satellites, and devise ways to protect satellites and spacecraft from meteoroids and orbital debris.
     "More than half of electrical failures are unexplained because it's very hard to do diagnostics on a satellite that fails in orbit," Fletcher said. "We believe we can attribute some of these failures to this mechanism."

     More information: "Particle-in-cell simulations of an RF emission mechanism associated with hypervelocity impact plasmas," Physics of Plasmas, May 2, 2017. DOI: 10.1063/1.4980833

Presently the flare guttered out. The pyre cloud faded to nothing. The raft deck was between Diaz and the shrunken sun. But the stars that crowded on every side gave ample soft light. He allowed his gullet, which felt like sandpaper, a suck from his one water flask. Otherwise he had several air bottles, an oxygen reclaim unit, and a ridiculously large box of Q rations. His raft was a section of inner plating, torn off when the Argonne encountered the ball storm. She was only a pursuit cruiser, unarmored against such weapons. At thirty miles per second, relative (260 Ricks! Each 1kg ball does 50% the damage of a Tomahawk cruise missile), the little steel spheres tossed in her path by some Unasian gun had not left much but junk and corpses. Diaz had found no other survivors. He’d lashed what he could salvage onto this raft, including a shaped torp charge that rocketed him clear of the ruins. This far spaceward he didn’t need screen fields against solar particle radiation. So he had had a small hope of rescue. Maybe bigger than small, now.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

• 

Hey all, we have a guest post from writer and scientist M.T. Reiten about the technology behind the railguns in The Expanse universe. He talked through this with me years ago, long before we were producing the show, but it’s taken this long to get his guest post written and up because he’s an actual scientist who works on government projects and we had to be sure he wasn’t violating any non-disclosure kind of stuff. But we finally have permission, so here it is!

 

So I was at a party with Ty Franck and talking about science fiction. Specifically ship-to-ship combat and I shared an idea that I had been playing with. Because that’s what you do at parties.

A few months previously, I had gone in to talk with my old postdoc mentor and he asked me what I thought about railguns. I thought they were cool and admitted to wanting to build a miniature-scale railgun using semiconductor industry techniques in grad school. (Not because it was useful, but because it would be fun to have a one-shot millimeter-sized launcher that would require an ultrafast laser to trigger.) Then we talked about putting a railgun as a micro-satellite launch system. This would require putting it on an airplane. We thought we had a research proposal in the making. However, nothing came of it and we moved on to other things, but the idea had stuck in my writer’s brain.

A real railgun, as you can read about on Wikipedia or see in numerous Youtube videos, uses electromagnetic forces (Lorenz Force to be precise) to accelerate a projectile. It’s somewhat related to the Jacob’s Ladders that are sometimes seen in old movies featuring a mad scientist of some flavor. Except that bit of electrical discharge passes through a conductor which moves and can be used to fling a projectile. The nice thing is that it doesn’t use chemical explosives to accelerate a kinetic projectile to very respectable velocities. And we can do it right now. (Laser weapon technology still has limitations, but that’s a whole different discussion.)

The longer the accelerating force can be applied to a projectile, the higher the muzzle velocity will be when it exits the launcher. This is why longer barreled rifles typically have a much greater range than short barreled handguns using the exact same cartridge. But space on aircraft is at a premium. So how would one extend the barrel without adding more weight?

Digging back to my original interest in railguns, I thought of ultrashort high power laser pulses in the atmosphere. The cool thing about short laser pulses is that they compress a lot of energy into a very tight package. So you end up having these photon pancakes whizzing about at (nearly) the speed of light. All very good, but what does this have to do with railguns? This many photons corresponds to a very intense electric field. This intense electric field tears apart the gas molecules in the atmosphere creating an ionized plasma. Since the laser pulse is traveling in a straight line, the plasma stretches behind the pulse resulting in a plasma channel. This plasma channel can conduct electricity. If connected to the active elements of the railgun, the plasma can become a virtual barrel, imparting extra kick to a payload. A longer barrel for an aircraft-based micro-satellite launching railgun. Problem solved, except for all the hard work that will keep a dozen engineers employed for a decade.

But I write science fiction and how would this work on a spaceship? No readily available atmosphere in space! Simple. High velocity shock of gas spurts out the railgun port. The gas expands rapidly. Pump the volume with a short pulse laser to create the virtual barrel. Carefully shape the electromagnetic pulse to keep the plasma contained. FIRE!

So the awesome visual effects, with swirling ionized gases, is based in plausibility.

M. T. Reiten (www.mtreiten.com)

• 

A coilgun, also known as a Gauss rifle is a type of projectile accelerator consisting of one or more coils used as electromagnets in the configuration of a linear motor that accelerate a ferromagnetic or conducting projectile to high velocity. In almost all coilgun configurations, the coils and the gun barrel are arranged on a common axis. It is not a rifle as the barrel is not rifled. The name "Gauss" is in reference to Carl Friedrich Gauss, who formulated mathematical descriptions of the magnetic effect used by magnetic accelerator cannons.

Coilguns generally consist of one or more coils arranged along a barrel, so the path of the accelerating projectile lies along the central axis of the coils. The coils are switched on and off in a precisely timed sequence, causing the projectile to be accelerated quickly along the barrel via magnetic forces. Coilguns are distinct from railguns, as the direction of acceleration in a railgun is at right angles to the central axis of the current loop formed by the conducting rails. In addition, railguns usually require the use of sliding contacts to pass a large current through the projectile or sabot but coilguns do not necessarily require sliding contacts. While some simple coilgun concepts can use ferromagnetic projectiles or even permanent magnet projectiles, most designs for high velocities actually incorporate a coupled coil as part of the projectile. Another form of Gauss rifle is one which consists of a strong magnet on a rail. There are two metal balls on one end of the magnet. Another ball is placed next to the magnet, but not attracted to it. When the ball is pushed toward the magnet, it accelerates until it hits the magnet with some force and velocity. The momentum is transferred through the magnet to the last ball, which flies off the end with nearly as much force as the first ball started with.

The History

The oldest electromagnetic gun came in the form of the coilgun, the first of which was invented by Norwegian scientist Kristian Birkeland at the University of Kristiania (today Oslo). The invention was officially patented in 1904, although its development reportedly started as early as 1845. According to his accounts, Birkeland accelerated a 500 g projectile to 50 m/s (110 mph; 180 km/h; 160 ft/s).

In 1933, Texan inventor Virgil Rigsby developed a stationary coilgun that was designed to be used like a machine gun. It was powered by a large electrical motor and generator. It appeared in many contemporary science publications, but never piqued the interest of any armed forces.

Construction

There are two main types or setups of a coilgun: single-stage and multistage. A single-stage coilgun uses one electromagnet to propel a projectile. A multistage coilgun uses several electromagnets in succession to progressively increase the speed of the projectile.

Ferromagnetic projectiles

For ferromagnetic projectiles, a single-stage coilgun can be formed by a coil of wire, an electromagnet, with a ferromagnetic projectile placed at one of its ends. This type of coilgun is formed like the solenoid used in an electromechanical relay, i.e. a current-carrying coil which will draw a ferromagnetic object through its center. A large current is pulsed through the coil of wire and a strong magnetic field forms, pulling the projectile to the center of the coil. When the projectile nears this point the electromagnet must be switched off, to prevent the projectile from becoming arrested at the center of the electromagnet.

In a multistage design, further electromagnets are then used to repeat this process, progressively accelerating the projectile. In common coilgun designs, the "barrel" of the gun is made up of a track that the projectile rides on, with the driver into the magnetic coils around the track. Power is supplied to the electromagnet from some sort of fast discharge storage device, typically a battery, or high-capacity high voltage capacitors (one per electromagnet), designed for fast energy discharge. A diode is used to protect polarity sensitive components (such as semiconductors or electrolytic capacitors) from damage due to inverse polarity of the voltage after turning off the coil.

Many hobbyists use low-cost rudimentary designs to experiment with coilguns, for example using photoflash capacitors from a disposable camera, or a capacitor from a standard cathode-ray tube television as the energy source, and a low inductance coil to propel the projectile forward.

Non-ferromagnetic projectiles

Some designs have non-ferromagnetic projectiles, of materials such as aluminium or copper, with the armature of the projectile acting as an electromagnet with internal current induced by pulses of the acceleration coils. A superconducting coilgun called a quench gun could be created by successively quenching a line of adjacent coaxial superconducting coils forming a gun barrel, generating a wave of magnetic field gradient traveling at any desired speed. A traveling superconducting coil might be made to ride this wave like a surfboard. The device would be a mass driver or linear synchronous motor with the propulsion energy stored directly in the drive coils. Another method would have non-superconducting acceleration coils and propulsion energy stored outside them but a projectile with superconducting magnets.

Though the cost of power switching and other factors can limit projectile energy, a notable benefit of some coilgun designs over simpler railguns is avoiding an intrinsic velocity limit from hypervelocity physical contact and erosion. By having the projectile pulled towards or levitated within the center of the coils as it is accelerated, no physical friction with the walls of the bore occurs. If the bore is a total vacuum (such as a tube with a plasma window), there is no friction at all, which helps prolong the period of reusability.

Switching

One main obstacle in coilgun design is switching the power through the coils. There are several common solutions—the simplest (and probably least effective) is the spark gap, which releases the stored energy through the coil when the voltage reaches a certain threshold. A better option is to use solid-state switches; these include IGBTs or power MOSFETs (which can be switched off mid-pulse) and SCRs (which release all stored energy before turning off).

A quick-and-dirty method for switching, especially for those using a flash camera for the main components, is to use the flash tube itself as a switch. By wiring it in series with the coil, it can silently and non-destructively (assuming that the energy in the capacitor is kept below the tube's safe operating limits) allow a large amount of current to pass through to the coil. Like any flash tube, ionizing the gas in the tube with a high voltage triggers it. However, a large amount of the energy will be dissipated as heat and light, and, because of the tube being a spark gap, the tube will stop conducting once the voltage across it drops sufficiently, leaving some charge remaining on the capacitor.

Resistance

The electrical resistance of the coils and the equivalent series resistance (ESR) of the current source dissipate considerable power.

At low speeds the heating of the coils dominates the percentage efficiency of the coilgun, giving exceptionally low efficiency. However, as speeds climb, mechanical power grows proportional to the speed, but, correctly switched, the resistive losses are largely unaffected, and thus these resistive losses become much smaller in percentage terms.

Magnetic circuit

Ideally, 100% of the magnetic flux generated by the coil would be delivered to and act on the projectile; in reality this is impossible due to energy losses always present in a real system, which cannot be entirely eliminated.

With a simple air-cored solenoid, the majority of the magnetic flux is not coupled into the projectile because of the magnetic circuit's high reluctance. The uncoupled flux generates a magnetic field that stores energy in the surrounding air. The energy that is stored in this field does not simply disappear from the magnetic circuit once the capacitor finishes discharging, instead returning to the coilgun's electric circuit. Because the coilgun's electric circuit is inherently analogous to an LC oscillator, the unused energy returns in the reverse direction ('ringing'), which can seriously damage polarized capacitors such as electrolytic capacitors.

Reverse charging can be prevented by a diode connected in reverse-parallel across the capacitor terminals; as a result, the current keeps flowing until the diode and the coil's resistance dissipate the field energy as heat. While this is a simple and frequently utilized solution, it requires an additional expensive high-power diode and a well-designed coil with enough thermal mass and heat dissipation capability in order to prevent component failure.

Some designs attempt to recover the energy stored in the magnetic field by using a pair of diodes. These diodes, instead of being forced to dissipate the remaining energy, recharge the capacitors with the right polarity for the next discharge cycle. This will also avoid the need to fully recharge the capacitors, thus significantly reducing charge times. However, the practicality of this solution is limited by the resulting high recharge current through the equivalent series resistance (ESR) of the capacitors; the ESR will dissipate some of the recharge current, generating heat within the capacitors and potentially shortening their lifetime.

To reduce component size, weight, durability requirements, and most importantly, cost, the magnetic circuit must be optimized to deliver more energy to the projectile for a given energy input. This has been addressed to some extent by the use of back iron and end iron, which are pieces of magnetic material that enclose the coil and create paths of lower reluctance in order to improve the amount of magnetic flux coupled into the projectile. Results can vary widely, depending on the materials used; hobbyist designs may use, for example, materials ranging anywhere from magnetic steel (more effective, lower reluctance) to video tape (little improvement in reluctance). Moreover, the additional pieces of magnetic material in the magnetic circuit can potentially exacerbate the possibility of flux saturation and other magnetic losses.

Ferromagnetic projectile saturation

Another significant limitation of the coilgun is the occurrence of magnetic saturation in the ferromagnetic projectile. When the flux in the projectile lies in the linear portion of its material's B(H) curve, the force applied to the core is proportional to the square of coil current (I)—the field (H) is linearly dependent on I, B is linearly dependent on H and force is linearly dependent on the product BI. This relationship continues until the core is saturated; once this happens B will only increase marginally with H (and thus with I), so force gain is linear. Since losses are proportional to I², increasing current beyond this point eventually decreases efficiency although it may increase the force. This puts an absolute limit on how much a given projectile can be accelerated with a single stage at acceptable efficiency.

Projectile magnetization and reaction time

Apart from saturation, the B(H) dependency often contains a hysteresis loop and the reaction time of the projectile material may be significant. The hysteresis means that the projectile becomes permanently magnetized and some energy will be lost as a permanent magnetic field of the projectile. The projectile reaction time, on the other hand, makes the projectile reluctant to respond to abrupt B changes; the flux will not rise as fast as desired while current is applied and a B tail will occur after the coil field has disappeared. This delay decreases the force, which would be maximized if the H and B were in phase.

Induction coilguns

Most of the work to develop coilguns as hyper-velocity launchers has used "air-cored" systems to get around the limitations associated with ferromagnetic projectiles. In these systems, the projectile is accelerated by a moving coil "armature". If the armature is configured as one or more "shorted turns" then induced currents will result as a consequence of the time variation of the current in the static launcher coil (or coils).

In principle, coilguns can also be constructed in which the moving coils are fed with current via sliding contacts. However, the practical construction of such arrangements requires the provision of reliable high speed sliding contacts. Although feeding current to a multi-turn coil armature might not require currents as large as those required in a railgun, the elimination of the need for high speed sliding contacts is an obvious potential advantage of the induction coilgun relative to the railgun.

Air cored systems also introduce the penalty that much higher currents may be needed than in an "iron cored" system. Ultimately though, subject to the provision of appropriately rated power supplies, air cored systems can operate with much greater magnetic field strengths than "iron cored" systems, so that, ultimately, much higher accelerations and forces should be possible.

Potential uses

Small coilguns are recreationally made by hobbyists, typically up to several joules to tens of joules projectile energy (the latter comparable to a typical air gun and an order of magnitude less than a firearm) while ranging from under one percent to several percent efficiency.

In 2018, a Los Angeles-based company Arcflash Labs offered the first coilgun for sale to the general public. It fired 6-gram steel slugs at 45 m/s with a muzzle energy of approximately 5 joules.

Much higher efficiency and energy can be obtained with designs of greater expense and sophistication. In 1978, Bondaletov in the USSR achieved record acceleration with a single stage by sending a 2-gram ring to 5000 m/s in 1 cm of length, but the most efficient modern designs tend to involve many stages. Above 90% efficiency is estimated for some vastly larger superconducting concepts for space launch. An experimental 45-stage DARPA coilgun mortar design is 22% efficient, with 1.6 megajoules KE delivered to a round.

Though facing the challenge of competitiveness versus conventional guns (and sometimes railgun alternatives), coilguns are being researched for weaponry.

The DARPA Electromagnetic Mortar program is an example of potential benefits, if practical challenges like sufficiently low weight can be managed. The coilgun would be relatively silent with no smoke giving away its position, though a coilgun projectile would still create a sonic boom if supersonic. Adjustable yet smooth acceleration of the projectile throughout the barrel can allow somewhat higher velocity, with a predicted range increase of 30% for a 120mm EM mortar over the conventional version of similar length. With no separate propellant charges to load, the researchers envision the firing rate to approximately double.

In 2006, a 120mm prototype was under construction for evaluation, though time before reaching field deployment, if such occurs, was estimated then as 5 to 10+ years by Sandia National Laboratories. In 2011, development was proposed of an 81mm coilgun mortar to operate with a hybrid-electric version of the future Joint Light Tactical Vehicle.

Electromagnetic aircraft catapults are planned, including on board future U.S. Gerald R. Ford class aircraft carriers. An experimental induction coilgun version of an Electromagnetic Missile Launcher (EMML) has been tested for launching Tomahawk missiles. A coilgun-based active defense system for tanks is under development at HIT in China.

Coilgun potential has been perceived as extending beyond military applications. Challenging and corresponding to a magnitude of capital investment that few entities could readily fund, gigantic coilguns with projectile mass and velocity on the scale of gigajoules of kinetic energy (as opposed to megajoules or less) have not been developed so far, but such have been proposed as launchers from the Moon or from Earth:

• An ambitious lunar-base proposal considered within a 1975 NASA study would have involved a 4000-ton coilgun sending 10 million tons of lunar material to L5 in support of massive space colonization (cumulatively over years, utilizing a large 9900-ton power plant).
• A 1992 NASA study calculated that a 330-ton lunar superconducting quenchgun could launch annually 4400 projectiles, each 1.5 tons and mostly liquid oxygen payload, using a relatively small amount of power, 350 kW average.
• After NASA Ames estimated how to meet aerothermal requirements for heat shields with terrestrial surface launch, Sandia National Laboratories investigated electromagnetic launchers to orbit, in addition to researching other EML applications, both railguns and coilguns. In 1990, a kilometer-long coilgun was proposed for launch of small satellites.
• Later investigations at Sandia included a 2005 study of the StarTram concept for an extremely long coilgun, one version conceived as launching passengers to orbit with survivable acceleration.
• A mass driver is essentially a coilgun that magnetically accelerates a package consisting of a magnetizable holder containing a payload. Once the payload has been accelerated, the two separate, and the holder is slowed and recycled for another payload.

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

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

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

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

Also find the projectile cross-sectional area

area = radius² * π

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

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

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

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

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

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

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

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

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

K = 400 kJ/m³/T²

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

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

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

length = volume / area

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

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

(ed note: the question was: Is the projectile accelerated by a coil gun (not a railgun) heated by the acceleration process? To cut to the chase, skip down to equation 9)

2. General Relations

     Ideally, an electromagnetic launcher should be designed to achieve a given muzzle velocity using as short a barrel as possible. This means that the acceleration and the thrust should be as high as possible, consistent with the strength of the materials. As will be shown, these requirements translate into accelerations approaching half a million times that of gravity. For comparison, the antitank copperhead shell is subjected to accelerations of only 9000 g's.

     The armature of the projectile is subjected to mechanical, electromagnetic, and thermal stresses which are impulsive in character (the projectile proper is wrapped in a conductive armature like a sabot. The coilgun accelerates the armature, and the armature is firmly attached to the projectile). Therefore, in order to separate their effects, it is useful to determine the order of magnitude of the speed with which each stress propagates. Mechanical stresses propagate with the velocity of sound which, in solids, is in the order of (10³) m/s. Since the materials of interest are good conductors, the propagation of electromagnetic and thermal stresses is governed by diffusion equations. Introducing a characteristic length L which is typically in the order of 1 cm one can write for the diffusion velocity ν_d

where α is the diffusivity. Denoting by γ the electrical conductivity and by μ the magnetic permeability, one obtains for the diffusivity of the electromagnetic stress α_e,m

This corresponds to a velocity of 1 m/s. Denoting by λ(Wm^-1K^-1) the heat conductivity and by c the specific heat per unit volume, one obtains for the thermal diffusivity α_t

corresponding to a velocity of 10^-2 m/s.

     The large diiferences in the propagation velocities of the mechanical, electromagnetic, and thermal stresses thus suggest that one can assume that the mechanical stresses are established instantaneously, that the electrical stresses are established next, and last, that all the heat is dissipated in one skin depth and is absorbed locally; that is, the thermal process is adiabatic.

     These considerations permit some general relationships to be derived by considering a unit volume of the projectile armature. Let J denote the current density, B the magnetic flux density, ξ the mass density of the armature conductor, ν the ratio of the overall mass of the projectile to the mass of the armature conductor, and θ the temperature rise over the ambient. If one neglects friction losses, the increment of kinetic energy Δw_kin from the breech velocity v_b to the muzzle velocity v_m equals the work done by the electromagnetic force J × B over the length l of the barrel or

The energy dissipated in the conductor w_diss is

In the ideal case of uniform current distribution and perpendicular orientation of the vectors J, B, and dl one can eliminate J and obtain:

where B should be assigned an average value.

     It appears that in the limit v_b → 0, the length l of the barrel increases as the cube of the muzzle velocity. One can now arrive at an estimate of the numerical values by relating the maximum allowable mechanical stress σ_m (N/m²) to the force acting on a unit surface of the armature conductor. Letting

where K_p is the surface current density (A/m) of the armature conductor and ε is a non-dimensional factor, which depends on the geometry, and is always less than 1/2, one obtains

     Contemplating the use of hard drawn, oxygen-free copper, as armature material, allowing a temperature rise θ = 800 K, i.e. about 80% of the melting temperature and letting

(ed note:

• θ Temperature rise over the ambient = 800 K
• σ_m Maximum allowable mechanical stress = 2.5 × 10⁸ Pa
• ξ Mass density of the armature conductor = 8.93 × 10³ kg/m³ (density of copper)
• γ Electrical conductivity = 10⁷ S/m (conductivity of copper, actually more like 5.96 × 10⁷ S/m)
• c Specific heat per unit volume = 3.47 × 10⁶ JK^-1 m^-3
• ε Non-dimensional factor (which depends on the geometry and is always less than 1/2) = 0.35
• ν ratio of the overall mass of the projectile to the mass of the armature conductor = 1 (I guess this means the entire projectile is the armature)
• v_m muzzle velocity = 10 km/s = 10,000 m/s
• l length of the barrel = 12.93 m

)

Equation (8) yields:

so that for v_m = 10 km/s and ν = 1 the length of the barrel l = is 12.93 m. This corresponds to an average acceleration of 3.86 × 10⁶ m/s² or 3.94 × 10⁵ g's.

     Considering that the ideal condition of uniform current distribution is difficult to attain, because of the skin effect [8], it appears that with a solid armature, it will be difficult to achieve velocities far in excess of 10 km/s. This, however, is quite adequate for endo-atmospheric applications, because, due to the resistance of the air, the nose of the projectile would melt for velocities in excess of about 8 km/s, unless it was protected by ablative cones (there is zero air resistance in the exo-atmospheric vacuum of space, where spaceships fly).

(ed note: Anders Sandberg said that "The temperature seems to increase with the cube of the speed if everything else is constant. Superconductors avoid this, but have critical field and current.")

(ed note: if I have done my arithmetic correctly:

l = ((v_m / 4.26×10³ )³ ) * v²

)

(ed note: the question was: Is the projectile accelerated by a coil gun (not a railgun) heated by the acceleration process?. This is very much a first approximation calculation)

If you want to play with parameters you can explore a lot using this tool: LS-DYNA Examples.

And, of course, I assume you’re interested in the non-atmospheric case, cause friction heating is a whole ‘nuther thing.

But here’s what I would do for a space game “hand wavium” argument:

Assume a 1 Kg steel projectile is shot out of a coil gun (rail gun or EM launcher) at a speed of, let’s say, 8 Km/s. That means it acquires a kinetic energy of 0.5 * 1 * 8000^2 = 32E6 J. If the launcher is 90% efficient, then 32E5 J winds up as heat somewhere. Let’s assume half of it goes into the slug. The the slug gets 16E5 J hotter.

The specific heat of steel is 420 J/Kg/C, so 160,000 / 420 = 381 C, so it would go from room temp to oven on the cleaning cycle or thereabouts.

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

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

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

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

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

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

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

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

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

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

• 

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

It all depends on speed of impact versus the speed of sound in the target (what is called the Mach number, where Mach 1 means the speed of sound, Mach 2 is twice the speed of sound, etc), and the speed of the ravioli versus the speed of light in the target (which I'll call the Cerenkov number, where Cerenkov 1 is the speed of light in anything; Cerenkov 1.3 is the speed of high-energy protons in a water-cooled reactor (that's why you get that nifty blue glow), and you can get up to Cerenkov 2.4 using diamonds and nuclear accellerators. In the late 40's people used to talk about Cerenkov numbers, but they don't anymore. Pity.). Lastly, there's the ravioli velocity expressed as a fraction of the speed of light in a vacuum (that is, as a fraction of "c"). "C" velocities are always between 0.0 and 1.0

At low speeds (REAL low) the ravioli will simply flow over the surface, yielding a space-cruiser with a distinctly Italian paint job.

Faster (still well below speed-of-sound in the target) the metal of the space-cruiser's skin will distort downward, making what we Boston drivers call a "small dent".

Faster still, you may have a "big dent" or maybe even a "big dent with a hole in the middle", caused by the ravioli having enough energy to push the dent through, stretching and thinning the hull metal till the metal finally tears in the middle of the dent.

• 

Getting up past Mach 1 (say, 5000 feet/sec for steel), you start to get punch-a-hole-shaped-like-the-object effects, because the metal is being asked to move faster than the binding forces in the object can propagate the "HEY! MOVE!" information. (After all, sound is just the binding forces between atoms in a material moving the adjacent atoms — and the speed of sound is how fast the message to "move" can propagate.) From this, we see that WileE Coyote often reached far-supersonic speeds because he often punched silhouette-type holes in rocks, cliffs, trucks, etc.

Around Mach 4 or so, another phenomenon starts — compressive heating. This is where the leading edge of the ravioli actually starts being heated by compression (remember PV=nRT, the ideal gas law?) Well, ravioli isn't a gas, but under enough pressure, ravioli behaves as a gas. It is compressed at the instant of impact and gets hot — very hot. Likewise, the impact point on the hull is compressed and gets hot. Both turn to gasses — real gasses, glowing-white-hot gasses. The gasses expand spherically, causing crater-like effects, including a raised rim and a basically parabolic shape. In the center of the crater, some material is vaporized, then there's a melt zone, then a larger "bent" zone, and the raised rim is caused because the gas expansion bubble center point (the bending force) is actually inside the hull plate. If the hull plate isn't thick enough, then the gas-expansion bubble pushes through to the other side, and you get a structural breach event (technically speaking, a "big hole") in the side of the space-cruiser.

Compressive heating really hits the stride up around 20,000 feet/sec (Mach 4 in steel, Mach 15 in air) and continues as a major factor all the way up to the high fractional Cerenkov speeds, where nuclear forces begin to take effect.

Aside: the "re-entry friction heating" that spacecraft endure when the reenter the atmosphere is NOT friction. It's really compressive heating of the air in the path. As long as the spacecraft is faster than Mach 1, the air can't know to get out of the way, so it bunches up in front of the spacecraft. When you squeeze any gas, it gets hot. So, the glowing "reentry gas" is really just squeezed air, which heats the spacecraft heat shield by conduction and infrared. The hypersonic ravioli can be expected to behave similarly.

As we increase speed from the high Mach numbers (about 10 miles/sec) all the way up to about 150,000 miles/sec, not much different happens except that the amount of kinetic energy (which turns into compressive heat) increases. This is a huge range of velocity, but it's uninteresting velocity.

At high fractional Cerenkov speeds, the ravioli is now beginning to travel at relativistic velocities. Among other things, this means that the ravioli is aging more slowly than usual, and the ravioli can looks compressed in the direction of travel. But that's really not important right now.

As we pass Cerenkov 1.0 in the target, we get a new phenomenon — Cerenkov radiation. This is that distinctive blue glow seen around water-cooled reactors. It's just (relatively) harmless light (harmless compared to the other blast effects, that is). I mention it only because it's so nifty...

At around .9 c (Cerenkov 1.1) , the ravioli starts to perceptibly weigh more. It's just a relativistic mass increase — all the additional weight is actually energy, available to do compressive heating upon impact. The extra weight is converted to heat energy according to the equation E=mc²; it looks like compressive heating but it's not.

[Here's where I'm a little hazy on the numbers; I'm at work and don't have time to rederive the Lorentz transformations.]

At around .985 c (Cerenkov 1.2 or so), the ravioli now weighs twice what it used to weigh. For a one pound can, that's two pounds... or about sixty megatons of excess energy. All of it turns to heat on impact. Probably very little is left of the space-cruiser.

At around .998 c, the impacting ravioli begins to behave less like ravioli and more like an extremely intense radiation beam. Protons in the water of the ravioli begin to successfully penetrate the nuclei of the hull metal. Thermonuclear interactions, such as hydrogen fusion, may take place in the tomato sauce.

At around .9998 c, the ravioli radiation beam is still wimpy as far as nuclear accellerator energy is concerned, but because there is so much of it, we can expect a truly powerful blast of mixed radiation coming out of the impact site. Radiation, not mechanical blast, may become the largest hazard to any surviving crew members.

At around .9999999 c, the ravioli radiation may begin to produce "interesting" nuclear particles and events (heavy, short-lived particles).

At around .999999999999 c, the ravioli impact site may begin to resemble conditions in the original "big bang"; equilibrium between matter and energy; free pair production; antimatter and matter coexisting in equilibrium with a very intense gamma-ray flux, etc.[1]

Past that, who knows? It may be possible to generate quantum black holes given a sufficiently high velocity can of ravioli.

—xxx

[1] According to physicist W. Murray, we may also expect raining frogs, plagues of locusts, cats and dogs living together, real Old Testament destruction. You get the idea...