Self-Forging Rail-/Coilgun Projectiles

[Caiaphas]

So I recognize that it'd be practically useless anywhere outside of the atmosphere that it was tuned for and that you'd have major issues with the entire thing being destabilized as well as getting a consistently shaped projectile at the end of it, and you'd get more utility just dropping a shaped projectile in the first place, but I'm not entirely certain of my argument and I figured that it'd be an interesting thought exercise.

As it turns out, if you throw a flat disc of nickel iron molybdenum out of a coilgun at the ground at 24 km/s, you end up needing for it to pass through a medium with a density of around 2.2 kg/m^3 for it to reach its ultimate yield strength (around 700 MPa) from drag forces, assuming a Cd of 1.1, where air at sea level has half that density. So (leaving the fact that it's hitting the troposphere at around Mach 60+ aside) it would seem that what would happen is that the disc would deform rather than shatter, the edges bending backwards progressively as it punches into the atmosphere, until you end up with a much more aerodynamic, bullet-like shape when it impacts.

My gut is telling me that this isn't going to be the case and that a far more likely scenario is that it blows up before it hits the ground, but I haven't the background in fluid mechanics and aerodynamics to argue otherwise.

My math:

F = 0.5p(v^2)CdA from here
F/A = stress S = 0.5p(v^2)Cd
2S/(Cd*v^2) = p, where Cd = 1.1, from here, v = 24000 m/s, and S = 700 MPa
(2*700e6)/(1.1*24000^2) = 2.21 kg/m^3

[Imperial528]

I don't have a very high level of math or physics in terms of formal education, but my intuition tells me that any such projectile won't be evenly heated due to thermal inconsistencies in the medium. This will probably cause further abnormalities from the expected projectile shape and aerodynamics thus leading to the projectile going off-aim and slamming into the ground from the uneven drag forces.

If you really wanted to do this, say for stowage reasons (discs are easy to pack into a given space with high efficiency, ofc panels/cubes are better) it'd probably be much better to incorporate and forming and hardening mechanism into the weapon itself. At the very least you may want a half-way such that the disc is given a more aerodynamically stable shape before firing.

Even if you get that to work out, firing it at 24km/s means a significant amount of material will ablate off before it even hits the target if you're at any sort of range where the projectile has enough time to form. I'll see if I can run some math to see if that assumption holds as true as I think it should.

[Caiaphas]

Well, the assumption that I'm using is that you're shooting from at least geosynchronous orbit (around 42000 km) so that it needs to penetrate the full depth of the atmosphere before it hits, but not sure if that changes anything significantly about the math.

[Simon_Jester]

Why not just pre-forge the projectiles, then?

[Sea Skimmer]

To a point you are right. But the boundary layer heating would quickly vaporize the whole thing if you made it out of iron. So this isn't going to work out at all for reentry.

You don't need to make your EM gun projectiles out of iron though, realistically they'd just be pushed by armatures with some kind of actual winding in them. Ideally a super conductor, but short of that say, platinum would be amazing, if you could get enough of it ever. Then the projectile can be anything you want, the parasite weight being perhaps 20-25%, not worse then the latest state of the art sabots for powder guns presently.

Overall this idea makes absolutely no sense I can see, since building an EM gun to launch a disk would be harder then a rod shaped projectile because the bore diameter can be much smaller for the same overall muzzle energy. Meanwhile using the damn atmosphere to forge the projectile shape (which in physical terms does work as an idea) would mean a tremendous inefficiency in energy delivery to target. It would be much more efficient to fit a rod with a heat shield or make it out of a very high heat resistant materials from the get go.

The reverse makes more sense. Fire a rod into the atmosphere, and have it violently deeclerate by shape morphing (choose a method) say 100m above the target, resulting in sudden multiplication of the size of its shockwave its already making from moving that fast. As in it would more or less convert into a directional air burst hammering a large radius on the ground, letting your KE weapon have an explosive blast effect, and reducing the problem of most of the energy just going into a crater in the ground that holds back pure KE bombardment effectiveness.

If you had a 24/km firing velocity, and say 15km/s at time of morphing...that's going to result in a blast wave effect much more powerful then something like HMX explosive of the same mass could have produced. The morphing could be triggered by a nose unit which would also provide your guidance.

[Caiaphas]

Why not just pre-forge the projectiles, then?

I recognize that pre-forging would be boatloads more efficient in terms of energy delivery, since you're not wasting energy in deformation, but I'm unsure about the rest of the physics involved.

Overall this idea makes absolutely no sense I can see, since building an EM gun to launch a disk would be harder then a rod shaped projectile because the bore diameter can be much smaller for the same overall muzzle energy. Meanwhile using the damn atmosphere to forge the projectile shape (which in physical terms does work as an idea) would mean a tremendous inefficiency in energy delivery to target. It would be much more efficient to fit a rod with a heat shield or make it out of a very high heat resistant materials from the get go.

Yeah, I figured, but it's nice to have more concrete reasoning as to why, thank you.

The reverse makes more sense. Fire a rod into the atmosphere, and have it violently deeclerate by shape morphing (choose a method) say 100m above the target, resulting in sudden multiplication of the size of its shockwave its already making from moving that fast. As in it would more or less convert into a directional air burst hammering a large radius on the ground, letting your KE weapon have an explosive blast effect, and reducing the problem of most of the energy just going into a crater in the ground that holds back pure KE bombardment effectiveness.

If you had a 24/km firing velocity, and say 15km/s at time of morphing...that's going to result in a blast wave effect much more powerful then something like HMX explosive of the same mass could have produced. The morphing could be triggered by a nose unit which would also provide your guidance.

Mm. So something like an EFP? Slap an explosive sufficient to vaporize the entire hypervelocity rod o'death and a basic proximity fuse on it (although as I write this I have my doubts that it'd survive reentry) and let basic physics do the rest?

[Sea Skimmer]

I recognize that pre-forging would be boatloads more efficient in terms of energy delivery, since you're not wasting energy in deformation, but I'm unsure about the rest of the physics involved.

The problem is you aren't applying the energy quickly enough, and it becomes the doom of it. Any applied force before you reach that hydrodynamic forging threshold is basically useless and wasted. It will also cause velocity to drop, further delaying the point at which forging can occur, yet further wasting energy. Meanwhile your heating the projectile up like crazy while you wait and all that happens, and your trying to reenter at three times the speed of anything actually built. That's going to ensure its vaporized in this application. It certainly cannot be unshielded in the lower atmosphere at anything like that speed; below 100,000ft going over about 3,000m/s becomes a real problem with any known heat shielding, even ablative. ICBM warheads have typically already slowed to this kind of speed at that point, and then slow down to as low as subsonic-1km/s by the time of surface impact.

Shaped charges don't melt the projectile because it's only exposed to the heat of the explosion for a very short period of time, and launched away in fact before the explosive is actually done combusting. Also high heat explosives are themselves (like ones doped with aluminum powder often seen for aircraft bombs) poor choices for making HEAT/EFP warheads, you want ones with maximum shattering effect like HMX.

Also in any remotely realistic sense this idea just won't work because real atmospheres have winds, ones going upwards of several hundred miles an hour at very high altitudes any reentry must pass through. Those winds, plus slight imperfections in the material itself will make the heating and shaping process uneven, which at this kind of pressure will lead to violent instability.

The heat shields on spacecraft and missile warheads don't just keep the inside of the vehicle from cooking, they also act to even out this heating problem. They also use shape to maintain stability, and your idea of dropping a disk flat side down is unstable in the first place, I'm certain some kind of tail could be devised to make it work out, but that tail has to fire out of your gun too.

Yeah, I figured, but it's nice to have more concrete reasoning as to why, thank you.

Were you trying to work something out for a specific purpose? Maaany things are physically possible with EM guns, and EM driven EFP warheads, if you've got a reason to build them. The later tech is now claims as entirely proven by the US government, just classifmified on the exact details.

[quote
Mm. So something like an EFP?[/quote]

Sort of, in that it would go off near the target and then project something at it. Realistically it would just need to squish a rod of metal into a wider disk or a finned slug, by using explosives at each end. At that point you could also have multiple air burst modes, and a just about ground burst mode radial penetrator mode too. That's where relying on high explosives is a damn lot more appealing, among other reasons, its not hard at this point to engineer in multiple functions. Meanwhile anything you make your EFP do, will maintain the existing KE of the projectile... just in a lateral spread fashion. And you'd have the option to not detonate the EFP at all, and still just strike at a single unitary projectile if you want maximum cratering.

Slap an explosive sufficient to vaporize the entire hypervelocity rod o'death and a basic proximity fuse on it (although as I write this I have my doubts that it'd survive reentry) and let basic physics do the rest?

Yeah that's kind of the problem. 24km/s on firing is one thing, 24km/s at sea level is very much another, and I'm pretty damn sure that's not actually possible, though half that probably is. Hypersonic flight is extremely violent from the get go, its not just that you get hot, but chemically the atmosphere attacks everything because it's become so hot and reactive, and that generates endless tiny explosions across the surface of the vehicle which produces lots of turbulence. We have serious problems with this just at mach 5! Its a reason why reentry is imprecise, and its a reason why most metals are doomed if exposed at all, they'd be massive oxidized and stripped away.

I have a chart in front of me from 1994 of interceptor velocity vs aero heating rate vs altitude for ABM stuff. The 100MW per m2 curve is hit at 10km altitude at only 1.6km/s. At 55km altitude and 10km/s you get the same heating rate as the top end of that curve. 100MW m2 isn't a limit for one use vehicles, but at 24km/s your going to hit much much higher rates when you plunge below 40km at which point you'll just explode. My idea is to do something to control the burst height of that effect, so it can be militarily useful without having a tremendously high yield.

[Caiaphas]

Were you trying to work something out for a specific purpose? Maaany things are physically possible with EM guns, and EM driven EFP warheads, if you've got a reason to build them. The later tech is now claims as entirely proven by the US government, just classifmified on the exact details.

Mostly I was just trying to figure out what would happen with a (bugged) coilgun from CoaDE if someone fired it at the ground, and then it sorta evolved into "how can I make a hypervelocity space-based railgun whose rounds waste as little energy as possible on boring into the ground?"

Something else that occurred to me as I was writing this, more of a note to myself to look it up when I have the time, but how many gees and how many Teslas could military-grade electronics take before something fizzles?

[LaCroix]

Thing is, a disk fired flat side first would be the worst possible shape for a railgun projectile.
Using a round object, you have either minimized the contact area to transfer current or you have uneven current flow through the disc (as the contact areas fore each half of the barrel will be curved, so the current will prefer flowing trough the edges closest to each other)
Both will result in massive overheating and erosing of the projectile during firing.
Even correcting for that and using a square plate, if it is flat enough, you might face instant melting or vaporization.
That's why using a pre-forged slug is the better option.

[Caiaphas]

Thing is, a disk fired flat side first would be the worst possible shape for a railgun projectile.
Using a round object, you have either minimized the contact area to transfer current or you have uneven current flow through the disc (as the contact areas fore each half of the barrel will be curved, so the current will prefer flowing trough the edges closest to each other)
Both will result in massive overheating and erosing of the projectile during firing.
Even correcting for that and using a square plate, if it is flat enough, you might face instant melting or vaporization.
That's why using a pre-forged slug is the better option.

Focusing on the bolded bit, wouldn't this also have a not-insignificant chance of causing the projectile to accelerate unevenly through the barrel of the railgun?

[LaCroix]

uneven current flow through the disc (as the contact areas fore each half of the barrel will be curved, so the current will prefer flowing trough the edges closest to each other)

Focusing on the bolded bit, wouldn't this also have a not-insignificant chance of causing the projectile to accelerate unevenly through the barrel of the railgun?

Not really, for it will be symmetrical. it just means that no current will flow thought the middle of the disc, but through the sides wher the contact points are closest.
If the disc is thick enough to withstand these forces, it might just mean reduced magnetic field and less efficiency, but most likely, it will mean that these parts of the disc will just melt off and ruin the gun.

[Sea Skimmer]

Mostly I was just trying to figure out what would happen with a (bugged) coilgun from CoaDE if someone fired it at the ground, and then it sorta evolved into "how can I make a hypervelocity space-based railgun whose rounds waste as little energy as possible on boring into the ground?"

If you want maximum DOP into a planet you'd probably want a segmented heavy metal penetrator then, with all the projectiles stacked inside a common aeroshell for heat shielding. Assuming that's possible. The stacked effect means you gain a small amount of successive bonus time delays in which the overall round can penetrate before hydrodynamic erosion totally destroys it end to end and all the energy rebounds into blowing debris out of the crater.

If you had a proximity fuse for the penetrators then they could also break apart 300ft above the ground for a pesudo airburst effect from self combustion, and say 30ft up they could breakup so that they impact the ground solid, to blow as wide a crater as possible, but not to a great depth, by creating a cluster of smaller overlapping craters, total ejection of all collective material if done just right. This is predicated of course on a relatively heavy impact projectile, say 100kg. Fire it inside one heat shielded aeroshell, the flight control wouldn't be fins, it'd be some kind of reaction jet out the butt. Elaborate as you want armature on the rear, and possibly a second one pulling forward.

Of course this would be expensive if we can ever survive reentry in the first place, but it's just a brain bug to think anything need or would be cheap or simple about EM gun ammo. All the more so in space where the ammunition mass is highly relevant to your ship design. No cost incentive will exist to low ball it.

You may also need some kind of further protective shroud over your heat shielded orbit-surface projectile to protect the heat shield while in the bore, I'm not sure how that would pan out, it'd burn off on reentry, but some heat shielding materials in real life would be electrically conducting, and notional future composite materials may be that way too.

Something else that occurred to me as I was writing this, more of a note to myself to look it up when I have the time, but how many gees and how many Teslas could military-grade electronics take before something fizzles?

30,000g is the reliable state of the art for artillery guidance stuff. 80-100,000g has been long experimented with and is where that US Navy HVP projectile aims to finally demonstrate a serviceable round.

If I did this right 100,000g to reach 24km/s needs about a 300m long barrel. Not really out of the question in space, but it'd be very heavy too since the whole length needs to be equally rigid, it wont get lighter towards one end the way normal artillery pieces do.


Not sure on actual tesla limits, I believe the present railgun demonstrator is operating at about 2T. Generally the acceleration problem is going to be what limits you, because guidance electronics in a projectile don't really need to function while its still in the barrel. Maybe a simply circuit does to remember some kind of preset command, but otherwise in principle everything can be off or in some kind of hibernate mode, which will render it far less vulnerable to EM fields then when its operating. Soon as it clears the muzzle an acceleration based trigger can flip it on. Meanwhile everything just being crushed....that's a lot harder to engineer around. Its also a place I figure 3D printing will eventually pay off massively and let us make things like guidance units as one solid integrated piece.

[Caiaphas]

On a related note, could anyone please point me in the direction of some mechanical models of what happens to solids when struck by a hypervelocity impactor? Just reading through a few articles it seems as through the consensus is "the thing you hit will behave as a fluid", but I can't quite reconcile that with Newton's impact depth appoximation and with what my background says should happen when you hit a surface with a projectile going faster than the speed of sound in that substance (from my understanding it's going to be moving so quickly that the bonds holding the substance together are literally torn apart before they have the opportunity to communicate the information "we are being hit" to surrounding molecules, so on and so forth until it slows below the speed of sound).

[Sea Skimmer]

On a related note, could anyone please point me in the direction of some mechanical models of what happens to solids when struck by a hypervelocity impactor?

Searching DTIC is a good place to start, though I suggest using google instead of the places own search function which sucks. And only search for PDFs because they have lots of useless abstracts.

Just reading through a few articles it seems as through the consensus is "the thing you hit will behave as a fluid", but I can't quite reconcile that with Newton's impact depth appoximation and with what my background says should happen when you hit a surface with a projectile going faster than the speed of sound in that substance (from my understanding it's going to be moving so quickly that the bonds holding the substance together are literally torn apart before they have the opportunity to communicate the information "we are being hit" to surrounding molecules, so on and so forth until it slows below the speed of sound).

At sufficient pressure all penetration is hydrodynamic. This will cause solids to flow like fluids, but they do not lose their bonded grain structures in the process outside of a violent boundry layer, they may be transformed, the material may even be folded over, which is what happens in shaped charges, and in solid rock in the face of large asteroid impacts.

At the boundary layer both materials will progressively eroded in the most violent manner possible, and the penetration event will end when the penetrator is fully eroded. Then you have a tremendous rebound effect, which at high enough velocity can include an opposed hydrodynamic flow. Hydrodynamic flow in solids can naturally be turbulant flow, and the material may be broken apart into chunks or otherwise fragment, but it will not turn into actual liquid.

Newton's impact depth approximation is purely based on momentum and won't work for erosive pentration. Its only going to be useful for something like a concrete piercing bomb in a situation where the concrete has no chance of deflecting or erroding the bomb.