Recoilless Railguns

[Neko_Oni]

So I keep coming across the brainbug that railguns for some reason have no recoil. Now usually when I call people out on it and explain it to them they understand that, yes, railguns do in fact have recoil.

However recently someone I encountered someone who argued that railguns would have very low (negligible) recoil due to the lack of exhaust gasses. Saying that most of the recoil in a firearms comes from the expulsion of exhaust gasses. After some digging I've been totally unable to find any information on the ratio between bullet-recoil and exhaust gas-recoil for firearms. Would anyone be able to help me out?

[Stark]

The recoil is the reaction force of whatever propels the projectile. In a regular gun this is the explosion of propellant - it's the force of that explosion on the gun, not the muzzle flash itself. A railgun will have a reaction applied to it as the projectile is accelerated in the same way.

[MKSheppard]

Now usually when I call people out on it and explain it to them they understand that, yes, railguns do in fact have recoil.

They have very low recoil forces because....they can spread out the acceleration-time of the projectile.

In a conventional firearm; all the acceleration happens within an absurdly tiny amount of time and space.

With a railgun, you can have the projectile accelerate much slower, but over a longer length (like the length of the railgun barrel); so the perceived/felt recoil is much lower.

[Neko_Oni]

I understand that they will have recoil, but doesn't the ejection of the exhaust gasses also contribute to recoil? Hence why muzzle brakes reduces recoil. So a railgun of similar power to a gun should have slightly less recoil than a conventional firearm of similar power.

[Lord of the Abyss]

I understand that they will have recoil, but doesn't the ejection of the exhaust gasses also contribute to recoil? Hence why muzzle brakes reduces recoil. So a railgun of similar power to a gun should have slightly less recoil than a conventional firearm of similar power.

I don't think so. In a normal gun, those gasses come from the portion of the ammunition that was propellant; in a railgun there's no propellant so the whole thing comes out as a piece.

At any rate, in order to get the same amount of mass moving the same speed, the same amount of force needs to be applied regardless of the source.

[Korvan]

The exhaust gases had to be accelerated to leave the barrel and the reaction to that acceleration will produce recoil. This recoil will depend on the total mass of the gases (for modern propellants, I guess this will be equal to the initial mass of the propellant plus the mass of the air inside the barrel). It will also depend on the acceleration of the gases, and for that I think is were things get pretty complicated.

If I had to simplify things, I'd just add the mass of the gases to the mass of the bullet and knowing the muzzle velocity, work out the acceleration and thus the total recoil force, then by using the mass of the mass of the gases / total mass, work out the component of the recoil due to the exhaust gases.

[Korvan]

Just took a look at some numbers, since we just want the relative recoil for a given load, we don't need to worry about muzzle velocities and barrel lengths. As F=ma and we are assuming acceleration of the combined bullet+exhaust gases, the ratio of the gas mass / total mass will give us the relative recoil of the exhaust gases.

A further assumption is the mass of the exhaust gases equals the mass of the powder (we will ignore the mass of air in the barrel and any combustion products).

This ratio will be different depending on the ammunition looked at, for this test case I looked up a particular .357 magnum load. Mass of bullet = 125 gr, mass of powder = 15.3 gr.

So recoil attributed to exhaust gases is 15.3 gr / (125 gr +15.3 gr) = 0.109, or just under 11%.

[Sea Skimmer]

You can’t count the mass of the projectile and the mass of the propellant charge as being equal, powder charge guns do not work so neatly. The propellant wants to expand faster then the muzzle velocity of the projectile, and the moment the projectile leaves the barrel the propellant is free to accelerate to its maximum speed while still exerting pressure to the rear down the barrel. The only time this wouldn't be true would be on some kind of super magnum gun with a very long barrel, like Paris gun kind of super barrel. That means normally powder exit velocity is usually going to be substantially higher then the projectile muzzle velocity, and thus the recoil momentum of the propellent is much higher in turn. Because of this muzzle breaks can manage as much as 50% recoil reductions.

So taking away the powder is a really big gain in recoil reduction, which should allow for a more lightly stressed mounting. However on the other hand any remotely realistic coilgun or railgun barrel will be very heavy from the magnets and loaded with cooling and power supply gear beyond that so total weigh of installations savings might not occur. You’ve also got competing economies between conventional ammo storage and the pulse alternators or capacitors required to power the railgun.

The biggest reason to want the powder gone is not recoil, but simply to eliminate the fire/explosive hazard the powder represents. Powder also has a limited shelf life and it gets expensive replacing it over the years if you don’t fire it in action or training. Powder ignites more easily then high explosives, but it can then set off high explosives stored nearby in turn.

[Darth Yoshi]

Huh. I hadn't realized that convention firearms wasted so much energy like that. Now that I think about it though, it makes sense, and explains why black powder cannons have such huge smoke plumes.

[ShadowDragon8685]

Try explaining to the people who give you this brainbug the difference between recoil and perceived recoil. Use a medieval siege engine for your example - a catapult, or if they're fancy and know their stuff, a trebuchet or a ballasta.

Remind them that for every action, there must be an equal and opposite reaction. The only question is where that reaction goes. When you fire a firearm, the reaction goes suddenly into the body of the shooter - that's perceived recoil. When the guy with the axe cuts the string that fires the siege engine, he doesn't feel any tremendous kick to himself proportionate to the energy being used to unleash hell on the enemy, because the machine and it's mechanics are absorbing that recoil.

That should illustrate the difference between recoil and perceived recoil. Get them on the same page with you as far as that, then go from there. Any given weapon might have less or more perceived recoil; it's entirely possible for a vastly more powerful weapon to put much less recoil into the shooter.

[Korvan]

Heh, so much for my shortcut calculation then! I take some solace in getting within one order of magnitude at least.

[Sea Skimmer]

Heh, so much for my shortcut calculation then! I take some solace in getting within one order of magnitude at least.

You're thinking was correct, its just more variables are involved then you thought. But you could create a gun in which all propellant velocity was used up, how fast this is depends on the type of powder, its just that usually its more weight economical to have a big chamber and lots of powder, to keep down barrel length. You waste energy, but that’s life.

A couple sites can be found online which have recoil calculators, including this one which lets you choose several different powder velocities to see the difference it makes.
http://kwk.us/recoil.html
I can’t vouch for the specific accuracy of any of them as I am far too lazy to do math to verify it, but I would be surprised if they are inaccurate.

Huh. I hadn't realized that convention firearms wasted so much energy like that. Now that I think about it though, it makes sense, and explains why black powder cannons have such huge smoke plumes.

Well its wasted energy; and black powder is also just sucky as a propellant. It produces so much smoke in large part because it simply does not fully combust and is not hot enough to finish combusting after being blown out of the barrel. Lots of its mass was just straight carbon. Smokeless powder may not full combust in the barrel, but anything that blows out unburnt will almost instantly burn up in the air which is why modern artillery and tank guns can make one hell of a flash when firing, but not much smoke.

Brown Powder was actually even worse at making smoke then black powder, but it produced a smoother pressure curve and so enjoyed a short but important life prior to the invention of smokeless powder in the early 1880s. You needed some insanely large brown powder charges, as much as 960lbs for the British 16.25in gun. This compared to about 150lb of smokeless powder for the 16in guns on the Iowa class, which also fire much heavier shells faster in spite of the slightly smaller caliber.

[CaptainChewbacca]

The only way you can have a truely recoil-less weapon is when the projectile is self-propelled, like a gyrojet weapon.

Of course, the more commonly used name for something like that is 'rocket'.

[Korto]

The early recoiless rifles, IIRC, used a double charge, and propelled an equal projectile out the back (before they found that was unnecessary, and a gas exhaust would do). Presumably, you could use the same for a rail-gun, a double barrel, one firing backwards, to cancel out the recoil.
Of course, that would double the power requirements, restrict weapon placement, and overall be totally impractical and stupid, but I imagine it's do-able.

[Count Chocula]

One simplistic rebuttal would be to posit that both a powder gun and a rail gun would accelerate the same weight projectile to the same velocity. Powder guns accelerate the bullet to speed very quickly (expansion of gases from one explosion), while railguns accelerate their projectiles over a longer period of time. A powder gun would have a higher recoil impulse, forcing the shooter to handle the recoil in a short interval. A railgun would spread the same magnitude recoil impulse over a longer period of time, which is easier to handle. It's kinda like a car suspension handling a speed bump better than a pothole.

[starslayer]

No, they would impart the same impulse, but not the same force. Impulse is the actual change in momentum, while force is the rate of change of momentum with respect to time (i.e., dp/dt). The power weapon imparts a greater force than the rail gun because it generates the same impulse over a much shorter time. Of course, this discounts the gases themselves, which as already described, do add significantly to the recoil.

[Sarevok]

Don't you get enormous Lorentz force on the rails ? IIRC a weapons version of a railgun will require very sturdy construction to prevent the rails from breaking themselves as they push each other apart.

[Admiral Valdemar]

Don't you get enormous Lorentz force on the rails ? IIRC a weapons version of a railgun will require very sturdy construction to prevent the rails from breaking themselves as they push each other apart.

Yes, but the railguns will typically be far sturdier due to the required magnets and cooling and given the cost of the barrel due to this, whereas a traditional barrel is far simpler and cheaper. You could get over the heating effect causing ablation after repeated shots (muzzle flare down to plasma) by having a Gatling arrangement, which is doable, albeit, even more costly due to the barrel composition. You won't have just a twin fork barrel, the two rails will be within a barrel itself with the armature (projectile) being snugly fitted within.

[ShadowDragon8685]

Don't you get enormous Lorentz force on the rails ? IIRC a weapons version of a railgun will require very sturdy construction to prevent the rails from breaking themselves as they push each other apart.

Yes, but the railguns will typically be far sturdier due to the required magnets and cooling and given the cost of the barrel due to this, whereas a traditional barrel is far simpler and cheaper. You could get over the heating effect causing ablation after repeated shots (muzzle flare down to plasma) by having a Gatling arrangement, which is doable, albeit, even more costly due to the barrel composition. You won't have just a twin fork barrel, the two rails will be within a barrel itself with the armature (projectile) being snugly fitted within.

What exactly is the point of a Gatling railgun? Assuming you're not just saying "we'll solve the problem of the barrels wearing out by using eight barrels so the whole thing lasts eight times as long at sixteen times the cost," the other advantage of a rail-gun is accuracy. But aren't Gatling weapons kind of hard (like, near-impossible hard) to make uber-accurate in the way a railgun needs to be?

And what in the world do you need that kind of rate of fire for in a mass driver? Maybe space-combat applications, but terrestrially I thought railguns would be more useful as hyper-glorified, hype-worthy artillery pieces.

[Sea Skimmer]

What exactly is the point of a Gatling railgun? Assuming you're not just saying "we'll solve the problem of the barrels wearing out by using eight barrels so the whole thing lasts eight times as long at sixteen times the cost," the other advantage of a rail-gun is accuracy. But aren't Gatling weapons kind of hard (like, near-impossible hard) to make uber-accurate in the way a railgun needs to be?

Galting gun sights compensate for the centrifugal force, the loss in accuracy is minimal (you would also loose a slight bit of range) and present primarily when the gun is spinning up to operating speed. That can be solved by spinning up the barrels before you open fire, this is how the Phalanx CIWS works. How much accuracy a railgun needs depends on what it is supposed to do, just like any other weapon. If it for example primarily fired guided shells to very long ranges then accuracy would be unimportant. Pretty much all kinds of artillery from mortars to tank guns are moving towards guided ammo as we speak.

If you had a Galting rail gun purely for longer barrel life; and not for higher rate of fire then you could just fire with the barrels completely stationary, then unlock, rotate to the next one, halt, lock and fire again. It would be easy to have a 60-100rpm ROF doing that which is more then ample for destroying enemy armor or serving as artillery, and probably faster then the barrels could cool down and pulse alternator charge back up. You should on paper loose no accuracy at all if the barrel locked before firing. Gatling guns are of course not the only way to mount multiple barrels either, a battery gun is an option too in which you simply had a rigid block of barrels. Not a common arrangement in the modern day, only the Spanish 20mm Meroka uses this configuration, but it was somewhat common in the second half of the 19th century.

The trouble is, barring some major technology and materials science breakthroughs, we are still stuck with just a few shots per railgun barrel, and even 20 barrels would not be enough to make the weapon viable.

[Einhander Sn0m4n]

Actually it appears hypervelocity gouging in railgun barrels might be solved. We might get our first railguns by 2020ish.

[Sea Skimmer]

That is rather old and from a period when the USN was heavily pimping railguns in connection with its DDG-21 and then DD(X) programs. Not a time when I would trust predictions from. It then goes on to say they haven’t solved anything full scale yet, which is the key bit. Currently even today the largest lab test weapon the Navy has is still only half scale of what they want.

Gouging is only one barrel destruction problem, and one which basically would instantly ruin the barrel. The destruction stemming from shear heat involved is the real issue. At the velocities the military want out of rail guns no gun barrel will last very long, let alone one that has to be wrapped in magnets and wiring and which is being additionally stressed by active cooling. If they don’t get higher velocities, then its hard to abandon the simplicity and reliability of a conventional cannon artillery piece.

The most recent test videos of the half scale rail gun, admittedly I think the newest are close to two years old now, they have still shows it blasting out a colossal cloud of vaporized material. This is from a single shot, no previous heating as a service weapon will be subject too. Even if it’s the armature providing most of the vaporized material, its impossible that this will not also erode the barrel. Until they can change that barrel life will suck. Conventional artillery actually puts wax in the powder to help lubricate the barrel and insulate the rifling, I am sure ideas like that are being explored heavily, but once more you run into the problem of very high velocities to allow small projectiles to be effective. Really heavy projectiles require much larger barrels, which becomes its own technological challenge. The 2020s may be plausible, but of course I very much do expect us to invent new materials by then too so it doesn’t change what I’m saying. The power supply side of the technology isn't without its troubles either, though that's not likely to be what holds back R&D. It will be what holds back deployment of a service weapon.

[adam_grif]

I suppose they could try some sort of automated lubrication system between every shot or every couple of shots. Or they could do a Gatling style setup with 2 or 3 barrels, and when they reach the non-firing positions they're freshly lubricated. By magic. Don't ask how the lubrication happens.

[defanatic]

Why would a railgun necessarily be longer? With guns, you'd just make the barrel long enough so that the bullet is always accelerating while inside the barrel. Making it any longer would be kind of pointless.

Railguns still have recoil, though. Recoilless railguns would need to somehow compensate for the impulse of the project (like another projectile shot out the back).

[Sapper007]

in a convential weapon, recoil comes to the wielder in a few parts...
1. Combustion of propellent...(the initial release of energy)
2. Action of the weapon... (AK's piston driven blowback system)
3. Dispersion of Gases from step 1...

But not all of this is the reason recoil happens... recoil from conventional firearms is from the directed forward force put on the projectile, obeying newtons 3rd law, and the returning force...

Many manufacturers have made different things to absorb this residual energy from the bullet being fired and many have harnessed it to perform various actions within the weapon system (semi-auto, full auto etc)

But railguns work on completely different principles...
Will there be recoil... yes...
but considering that the projectile is being propelled in a completely different manner the recoil should be almost nil to the user,(assuming a shoulder fired rail gun is the discussion here) and absorbed almost entirely by the weapon system itself. almost lending to they concept of a forward recoil... more akin to a slingshot or bow and arrow than a rifle...