Rail gun question.

I was reading some cool stuff about rail guns, and came across an interesting article, that has one statement that left me puzzled,


What’s the big deal about a lump of metal in space? That seems to me to be the easy part of the design. Does Tungsten freak out in a vacuum or something? I’m pretty sure I could easily walk outside and find a billion projectiles on the ground that will “handle four minutes in outer space” with 0% failure rate.

My guess is that it’s not space itself that’s the problem, it’s the re-entry. It’s sort of like the old adage about how to survive a 10,000 foot fall: Fall from 10,010 feet.

The materials not only have to survive reentry, but the rail gun munitions are expected to have some sort of terminal guidance, like GPS, so that the shots going extreme distances are actually landing where they are supposed to. Figuring out how to make a survivable material that isn’t simply a slug of metal, but has components and the ability to aim itself, isn’t easy.

Just some WAG on my part…

The first thing that comes to mind is the hard radiation that the guidance system will be exposed to while outside the Earth’s atmsphere. Satellites electronics are hardened and shielded against the constant exposure, but that level of protection might be too heavy or fragile to withstand the 40,000 G acceleration during launch.

Another consideration might be the extreme cold of space -v- the heat generated during the firing and ascent phase. Many materials don’t take extreme transitions gracefully. Even steel shatters somewhere around -130C IIRC.

Love this excerpt from the article:

Bolded part sounds like something my dad, a former Marine, would say.

The radiation risk depends on how long you’re exposed, and what chance of failure is considered acceptable. For all the longer these things are going to be above the belts, most of them will survive without any hardnening, and if a few don’t, it’s no big deal.

And the temperature transition would be no issue at all. For practical purposes, it’s much more accurate to say that “space has no temperature”, not “space is cold”: You get very little heat transfer through vacuum. And even to the extent that space does have a temperature (from radiative heat transfer), the space near Earth has about the same temperature as Earth does. If you’re hot when you go up, you’re pretty much going to stay hot while you’re up there.

The speculations provided so far are all on point; however, there is an additional issue: during hypersonic reentry any real world material, be it tungsten or carbon-carbon composite, will ablate, and the speed is so great that even very small variations in surface geometry and roughness can result in dramatic changes in flight characterization. (The control surfaces on MaRV-type vehicles are usually on close order of 6 sq inches.) Because they’re purely kinetic, contact weapons these projectiles require a high degree of precision and controllability on reentry; if they hit even 100m away from their target they probably won’t do much damage. Making a material for which the ablation can be accurately characterized and modeled for a wide range of reentry modes and conditions is non-trivial, especially if you want a circular probability of error measured in single digits of meters.

The article is the typical journalistic muddle of facts, factoids, and misconceptions; while they get the general ideas right, there are a number of details that are sufficiently unclear or inaccurate that make it clear that the author is merely repeating someone else’s simplified explanation.


The vacuum and sudden reentry might be handled by the metal just fine, but remember, these things are loaded with explosives. (Kinetic weapons are effective, but also limited.) A thin metal shell that explodes miles up during reentry probably isn’t going to win too many wars. If it’s too solid, it might be unreliable for proper detonation when it arrives. As most explosives are purposely volatile, the casing would need to protect the contents, but not be a hindrance once the package arrives. Add in 40,000 G’s, and the other effects (lightning at the destination is one, IIRC), and you basically need a lunar capsule.

Of course, those aren’t cheap, which is what this really boils down to. They need something that can survive, is reliable, and cheap enough to actually justify it’s use. It’s nice to be able to sit at home and hit any location on the globe as needed, but if it costs too much, it’s pointless. Also, my understanding is that part of this program relates to the possibility of using these things for emergency relief, so no doubt that plays a (subtle) part in the statement, as well.

And let’s not forget, the main job of a military contractor isn’t necessarily success, it’s to keep the bucks rolling in. “Good enough” might work during wartime, but when you’re eyeing that Caribbean Island for retirement, it’s important to work towards “perfection”. :slight_smile:

I’m not sure of all the things I’ve read today(the link wont work for some reason), but one of the advantages of rain guns is that is is a kinetic weapn, and has no explosive, so is cheaper. Mach 7 kenetic penetration is what the thing does.

No. The projectiles in the rail gun system described in the link in the o.p are purely kinetic weapons whose destructive power comes from sheer momentum and kinetic energy. The amount of acceleration in a rail gun is such that it would be hard to design anything that was not a solid, nearly rigid continuum that would survive; certainly, at a sustained acceleration of tens of thousands of Gs, even the most stabilized plastique explosive would flow or crack, making it unreliable at best.

It’s actually not that hard to make modern explosives that are pretty much insensitive to high thermal impulse, high temperature gas jets, mechanical shock, or electrical shock. Insensitive high explosives (IHE) like cyclotetramethylene tetranitramine (HMX or trade name ‘Octol’ when combined with TNT) and triaminotrinitrobenzene (TATB). HMX is used in military tactical munitions and solid rocket motor propellant compositions; TATB is used as the implosive shell in modern nuclear fission weapons and Primaries to prevent accidental detonation. (Some older ‘physics packages’ like the W-62 originally deployed on Minuteman II/III ICBMs and the W-76 deployed on the Trident C4 and D5 SLBMs are now considered unsafe to the point that they can only be air-transported with high level approval.) See Cooper’s Explosives Engineering for more details.


That depends on what your target is. If you’re shooting for that particular tank right there, then yes, you need to hit the bulls-eye. If you’re shooting for that big column of tanks over there, then any tank is as good a target as any other, and it might not matter if the tank you hit is 100 m away from the tank you expected to hit.

What’s all this about guidance systems? I thought a rail gun was just an artillery piece which didn’t require explosive propellant?

The article says the projectile will be guided by GPS, so there must be some sort of electronics along with something like movable tail fins. I imagine the railgun aiming will get it within a mile or so and the guidance corrects it to within the 100M goal.

True; however, it is pretty widely acknowledged that the era of of unguided indirect fire and saturation bombing is nearing its end. The buzzwords today are “surgical strike”, “precision guided munitions”, and “minimal collateral impact [euph: damage]”, and the capability to make this more than press release chatter continues to improve.

And for the most part, tankers have learned not to advance in columns, or otherwise in symmetric or obvious formation. Doing so made sense back when communications consisted of follow the leader and flashing signals down the line; today, with modern cellular communications it is possible for a combat director to coordinate attacks for a platoon or even company of tanks and armored fighting vehicles, making suppressive fire less effective and saturation bombing wasteful and even counterproductive. (The Iraqis tried saturating attacks on American Abrams M1A1 tanks during both Gulf Wars; the net result was one unnerved but uninjured tank crew calling in for air support, followed by a quick, directed attack by precision artillery, air cavalry, or ground attack air support, and a lot of dead Iraqi solders and shattered equipment.) Modern tanker tactics is to zip forward to front lines, find a hidey-hole or berm to hunker down behind, wait for the enemy to appear, and pummel them with a few quick shots, then back up and redeploy.

As mentioned previously, at SRBM distances (which is what the velocity imparted by the rail gun would result in) end accuracy depends on a number of factors. ICBM systems rely upon the post-boost guidance system to correct for trajectory before releasing the non-maneuvering reentry vehicle (RV). The RV ablation has been characterized for the limited range of angle-of-attack it may see, and so accuracy can be estimated to a pretty high degree (within 100m CEP for modern ICBMs). The rail gun projectile probably doesn’t have any midcourse correction capability so it’ll need to have some kind of guidance control surfaces on the body to help adjust course on reentry after it hits atmo. These control surfaces won’t be fins; between the resultant stresses from imparted acceleration, and aeroheating from reentry speed in the upper atmosphere, even fixed fins wouldn’t survive much less something capable of articulation. The control surfaces are probably small nubs or tiny flaps somewhere around or a bit aft of the center of pressure.