# Man-portable 5-20 MJ railgun?

Not to say it can be fired without a tripod and/or a mount to disperse the recoil, but could railgun artillery or anti-tank rifles work in a near future setting with miniaturized fusion reactors? I'd imagine there would be a battery with enough juice for one shot built into the actual weapon and the soldier could also carry a larger battery pack to power additional shots.

Would this be plausible?

Recoiless rifles like the Carl Gustav balance the recoil force of the projectile by allowing 4/5 of the propellant gas to exit the venturi in the rear. For man portable weapons like the Carl Gustav or various Soviet era 80-90mm recoiless rifles, this produces a pretty impressive backblast, which limits where the soldiers can fire from (allowing the sudden rush of gas into an enclosed room or reflecting the backblast off a nearby wall has some pretty obvious issues for the soldier).

Carl Gustav firing

However, this suggests a way to get a man portable railgun without the extra mass and expense of a fusion reactor. The round is contained in a recoiless casing like a Carl Gustav, but the 4/5 of the propellant isn't only countering the recoil force of the round moving down the barrel, but also powering a MHD generator built into the venturi.

MHD generator as it might appear in a venturi

The round is loaded and the venturi/generator is locked in place. The firing of the round accelerates the projectile into the rails while the backblast powers the MHD generator, which energizes the rails and takes the moving round from a few hundred meters/second to thousands of meters/second in a fraction of a second.

• If your powering the device with a chemical propellant, why not just use it to launch the projectile, without bothering with the rail-gun part? – Drgabble Oct 12 '16 at 20:59
• The OP is looking for a feasible way to have a man portable railgun. As well, conversion of chemical energy to electrical energy allows the projectile to be accelerated well beyond the level possible in a straight chemically powered hand held weapon. – Thucydides Oct 12 '16 at 22:30
• Is that so? Is it not efficient? – Drgabble Oct 12 '16 at 23:37
• @Thucydides I don't think this works. The muzzle energy of the shot comes from the linear motor, which in turn transfers the reaction force to the rail itself. There is no propellant to eject, and I don't see how the MHD generator can absorb or otherwise negate the reaction force. I don't think there is any way to buffer down that force to something a human can handle. – kingledion Oct 13 '16 at 0:04
• @kingledion -- the querent said that a tripod or other sort of mount to deal with the recoil problem was OK. I could see a redesigned "over the shoulder" stock working well for a prone shooter -- sure, the recoil's huge, but transmitting most of it through the stock and into the ground next to the shooter could very well be a practical solution – Shalvenay Oct 13 '16 at 3:20

# No, because of recoil

When they tried putting a 120mm smooth bore on a CV 90, they found that the recoil bent and twisted the chassis out of shape. The forces were such that a tracked armored fighting vehicle could not stand up to it. A 120mm sabot shot has a muzzle energy of about 5 to 7 MJ.

You want to have something that...

• Is man portable that is to say: much smaller and flimsier than a vehicle
• Fires a projectile at up to 4 times the muzzle energy, which results in the equivalent increase in recoil forces
• Has a much shorter barrel, which means it needs to accelerate the projectile to muzzle velocity much quicker, which means even higher recoil forces

So in short: no, this is not plausible. A "tripod, or mount" cannot handle those forces. The kind of "tripod, or mount" that you need to hold up to these forces need to look something like this:

## Plausible?

Back of envelope maths, assuming your lower-bound on energy and a $1kg$ mass: $$E_k = 1/2 m v^2$$ $$5\times10^6 = 1/2\times 1\times v^2$$ Moving all the known numbers over to one side, and doing the maths. $$v^2 = 10,000,000$$ $$\therefore v = 3,162 m/s$$

Which is like nine times the speed of sound.

Some more back of envelope maths, based on the numbers Wikipedia gives for the Carl Gustav's projectile weight and velocity:

$$E_k = 1/2 \times m \times v^2 = 1/2 \times 3.1kg \times (255m/s)^2$$ $$E_k = 0.2 MJ$$

That's a $25\times$ increase in kinetic energy. Which is a lot, but makes sense because the hurty-part of the Carl Gustav is the explosion. A railgun's hurty-part comes from the impact.

You can trade increasing projectile weight for decreasing velocity at the cost increasing recoil, but heavier projectiles are less likely to glance off if they impact at an angle.

• Is the kinetic energy of the Carl Gustav relevant? It uses HE to cause damage, surely you should compare the kinetic energy of the rail-gun to the chemical energy of the HE warhead? – Drgabble Oct 12 '16 at 21:01

Certainly!

Currently the limiting factor on a man portable rail-gun is several things, first and foremost providing enough energy to fire one. Conventional ordnance stores the energy in chemical form, which is energy dense (4.6MJ/Kg for TNT) whereas the batteries required for an electrical rail-gun are far less effective (at most 0.46-0.72MJ/Kg). Assuming a ten fold increase in battery capacity, rail-guns would be very competitive.

Having a portable energy source such as miniature fusion reactor would be much harder to develop, but this would allow much higher energy densities (576TJ/Kg for Deuterium-Tritium, ignoring the weight of the reactor).

Rail-guns also suffer from rapid wear caused by the severe friction during firing. This would either require more resistant conductive materials, or more frequent barrel changes than conventional ordnance (but not a huge challenge).

As for effectiveness, we can compare (as another question did) to the Carl Gustaf. If we assume all of it's ~4kg projectile is TNT or equivalent, it would have an explosive force of ~18.4MJ (and we can assume kinetic energy is minimal). This compares very favourably to 20MJ in your specification for the rail-gun.

In short, with improvements in battery or other portable electrical supplies, a rail-gun is a very feasible option.

• Fusion power solves the energy problem, but there is still the recoil problem. A 5 MJ blast with a 1kg projectile imparts 3162 kg*m/s to a man holding it. More explicitly, that means a 100 kg man will be going 31.62 m/s backwards after firing it. – kingledion Oct 13 '16 at 0:09
• @kingledion Good point.. tripod definitely required there then. Or it shoots half the energy both ways... – Drgabble Oct 13 '16 at 8:34
• @Drgabble You mean like the Carl Gustav mentioned by Thucydides? – a CVn Oct 13 '16 at 11:21