Fun With: Rail/Coil Guns

[Hotfoot]

Ah, yes. My mistake. That does make a significant difference, obviously. Oh well, thank you for the correction good sir.

Edit: And THAT is why it's good to show your work, because if you fuck up something, someone can correct you.

[Patrick Degan]

Suffice to say, any device which can launch projectiles at relativistic velocities I would not waste on any application shy of long-range planetary bombardment. A pistol-sized rail/coil gun is simply nonsensical since it won't give you any more of an advantage than an ordinary rifle to justify its complexity in such a small package. And for ground artillery or tanks, a railgun impelling 50kg slugs at 3km/sec is sufficiently destructive. Likewise for space combat, that same type of ship-mounted gun impelling slugs at 20km/sec will do quite well for reducing an enemy starship to swiss cheese. For that, writing the gun as capable of the same ROF as a 40mm Oekerlion gun is a bit wank but not outrageously so. The only thing required for these is the assumption that most if not all of the present engineering problems in railgun development have been solved.

[Connor MacLeod]

A projectile with a significant fraction of c in the atmosphere simply won't work. At those speeds, any appreciable atmosphere might as well be an impenetrable barrier--in the amount of mass intercepted per unit time (initially), a 0.007c projectile at sea level is comparable to a slightly supersonic projectile hitting an iron wall, all else being equal. Obviously, the specifics of those two interactions will be different, but at that scale, it's still clear that we won't have a usable weapon.

I'm not sure where your infantry is going, but anywhere but hard vacuum is out, and that's ignoring all the other issues.

So could you fire some sort of DEW pulse before firing the projectile to make some sort of "path" like has been proposed for particle beams or lasers in an atmosphere (IIRC - its been awhile since I looked into any of the old DEW stuff like that.)

[Connor MacLeod]

I am also mildly suprrised this has gone on so far without someone mentioning John Ringo and the Posleen/Aldenata novles (brings back memories of the ACGS gravwank guns which were basically relatavistic projectile guns, even if Ringo has minimal concept of "math")

It also kinda touches on Andromeda, which was big on the whole "relatavistic missile attack" idea (including for orbital bombardment IIRC, since one ep they mention attacking a planet.)

[rhoenix]

I'm not sure where your infantry is going, but anywhere but hard vacuum is out, and that's ignoring all the other issues.

Ahh, Kuroneko - I do look forward to your input.

I've since revised the infantry design to "only" go about mach 18 (about 11,880mph), instead of the high relativistic speeds of before. If this is still excessive, I'm still reading responses to this thread avidly.

However, the ship-mounted ones I'm still keeping as relativistic until physics laughs in my direction.

[Kuroneko]

So could you fire some sort of DEW pulse before firing the projectile to make some sort of "path" like has been proposed for particle beams or lasers in an atmosphere (IIRC - its been awhile since I looked into any of the old DEW stuff like that.)

I'm not certain about the setup, but this trick would seem to be more useful to get rid of particulates and vapor in the air along the trajectory rather than the air itself. The sheer magnitude involved seems to prohibit it from being reasonable. Since frictional forces are roughly proportional to the density but also to v², we'd need to be aiming at decreasing the local atmospheric density roughly seven orders of magnitude in order to have similar heating characteristics to the normal Mach 1 projectile. If it turns into plasma (or even melts, for that matter), its range will be basically worthless, and it start to lose its aerodynamic properties well before that due to deformation. (And that's already about 15 times lower velocity than the OP wanted, and ignoring the heating from accelerating it in the first place.)

Essentially pointing a gigantic tracer beam onto your own location by turning the air along the trajectory into plasma is sort of a bright idea, but not exactly the kind of brightness one would want on a battlefield.

I've since revised the infantry design to "only" go about mach 18 (about 11,880mph), instead of the high relativistic speeds of before.

You might want to tone it down even more to keep the projectile from ionizing the air as it passes--turning every shot into a tracer round might not be advisable. I'm not certain at what speed this would happen, but it would be somewhat dependent on the geometry of the projectile. Something like Mach 10 or less in Earth-like atmosphere, perhaps with different ammunitions tailored toward different environments.

So, at around v = 3.2km/s with a m = 25g projectile and .50m effective barrel length [1], the average force on the projectile will be F = (mv²)/(2d) = 2.6e5N. If the projectile base cross-section is A = 1.25e-4m² [2], this gives F/A = 2.1GPa. It doesn't seem to be completely outlandish, although considering that this should be at a high temperature and that the material should also have a very good electrical conductivity, high melting point, and a decent specific heat, perhaps it is pushing it. Hopefully an engineer would be willing to comment on that.

You can play around with different parameters to see how far you're willing to go.

[1] Comparison: the M61 Vulcan has 100g bullets at 1.05km/s, and a .45m barrel. Thus, this is more than than 230% the kinetic energy per projectile, but 3/4th the momentum compared to the Vulcan. (Even with power armor, it's best not to make the ammunition too heavy.)
[2] I simply scaled down the Vulcan ammunition (20mm) by a factor of 4^(-1/3), since the mass is 1/4, effectively making the caliber 12.6mm. This is somewhat arbitrary, since the geometry of the projectile is unlikely to stay the same, and it will most certainly be made of different materials.

[Edited because I can't think this late.]

[Jaepheth]

If you can manufacture them cheaply enough and energy storage/density is hand-waved out, then you could just launch one-time-use rail guns as primary projectiles or missiles and then when it approaches its target the rail gun fires a super-high velocity or relativistic velocity secondary projectile at close range.

Is that a feasible solution?

[rhoenix]

If you can manufacture them cheaply enough and energy storage/density is hand-waved out, then you could just launch one-time-use rail guns as primary projectiles or missiles and then when it approaches its target the rail gun fires a super-high velocity or relativistic velocity secondary projectile at close range.

Is that a feasible solution?

Now that's not a bad idea - it would allow for more surgical aim with railgun strikes as well, which is by no means a bad thing.

I'll keep this idea on the back burner for now, as it is a good one.

[kinnison]

The infantry version scaled down to mach 18 and a 1g projectile might be reasonable from the point of view of recoil, as I believe a reasonable figure for the mass of a rifle bullet is 10g and mach 1.8 isn't unreasonable either - which comes to the same momentum exchange and therefore recoil as a heavy-calibre (7.62?) rifle bullet. However, the kinetic energy of the projectile is ten times as high, so you somehow have to impart the energy equivalent of maybe 300 grams of cordite to the slug. A lot, but not ridiculous.

However, you still have the problem of the projectile melting or vapourising in air - I would think that the slug, or at least its outer shell, has to be made out of something pretty refractory - tungsten, tantalum or some sort of super-ceramic perhaps. (In the latter case, the core would have to be metal, but this may make it more effective - because the shell shatters and releases the molten lead (for example) core. The precise details probably depend on the intended target.

Incidentally, experiments on ultravelocity impacts have been done, as part of a study by NASA on cratering on airless worlds. The results seem to be that the projectile doesn't penetrate; instead it blows up along with a lot of the target. The study was done on impacts into loose dust, and penetration was still measurable in single-digit numbers of inches.

Armour/naval and space applications? I would think that a 100km/s projectile massing maybe 100g would be usable for tanks - comparable to modern penetrator rounds in terms of momentum. The energy requirements are still considerable - 100 times the power of a 120mm tank round, which is maybe a ton of TNT energy equivalent.

For space-to-space applications, relativistic speeds perhaps can be used, simply because there is no air to get in the way, but the projectile would have to be small - perhaps a microgram at 0.1c.

Personally, I believe that relativistic kinetic-kills are more reasonable as missiles, simply because the acceleration is much more reasonable. For this application you'd probably need a fusion motor and a long run-up; I haven't done the maths for this. And this becomes a weapon suitable only for planetary bombardment, and that only for a planet you don't want; 100kg (low for a working fusion rocket) at 0.1c makes a hell of a mess. I can't find the conversion factor between tons of TNT and joules, or I'd do the maths.

[Starglider]

I can't find the conversion factor between tons of TNT and joules, or I'd do the maths.

Here you go.

[kinnison]

I can't find the conversion factor between tons of TNT and joules, or I'd do the maths.

Here you go.

Thank you kindly, sir. OK. 1 ton TNT is approx. 4E9 J. 100 tonnes at 0.1c has a KE of 1E5 x (3E7^2) J. Which is 9E19 approx. Therefore the impact energy here is approx. 2E10 tons of TNT - 20 gigatons. Actually, this isn't as much as I thought - but you are scarcely likely to be using only one.