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CliftonM

Would it be feasible to actually use guns as a propulsion source?

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We've all seen it. Using BDArmory to make a gun propelled rocket. But would it actually be feasible, or would the rocket equation make people scream for mercy?

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A rocket's efficiency (ISP) increases with exhaust velocity. Just expelling hot gas without further ado will give you a much higher exhaust velocity than if you tamp down on it with a metal bullet. That's if you use chemistry as a driving force. If you try electrics (e.g. mass driver), you can reach quite insane muzzle velocities... typically these go up as the size of your projectile goes down. Taken to the extreme, you end up with a particle accelerator emitting a stream of ions. Hmm, I've heard that somewhere...

Thrust depends on mass expelled per time. It's much easier to expel $mass of metal than the same mass of gas, so in first approximation a gun should have pretty high thrust -- but only for an instant, then zero thrust while you load the next projectile. Your typical machine gun spends most of it's time reloading rather than firing, and the truly fast-firing weapons buy their cadence at a lot of extra weight. So average thrust probably isn't too impressive, thrust-per-weight will be pretty bad... but depending on purpose it may suffice -- one XKCD What If dealt with a AK47-propelled rocket IIRC.

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We've all seen it. Using BDArmory to make a gun propelled rocket. But would it actually be feasible, or would the rocket equation make people scream for mercy?

CliftonM,

The force applied to your vehicle is 1/2*m*v^2. Since the force is proportional to the mass you're ejecting but the square of the velocity, the velocity is clearly the most important part. Moreover, the mass of the bullets is effectively your "fuel" so you're not going to get a very good wet/ dry ratio using bullets.

Best,

-Slashy

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The GAU-8 avenger can generate 45Kn of recoil, weight 281 Kg when empty and have a muzzle velocity of 1000 m/S (can it be considered exhaust velocity?) I don't think that's enough for a rocket (a plane may be) and that gun is one of the most powerful I know. So no, it's not feasible a normal bullet firing gun.

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45 kN force at 281kg dry weight is a TWR of over 16, so that's very easily enough for a rocket! Even with 3 tons of ammunition, it would still have a TWR of 1.4. The Isp is of course abysmal, but it can definitely lift itself and its whole ammo compartment no problem.

Accidentally slowing the plane below stall speed is in fact a legitimate concern for A-10 pilots firing the gun :P

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45 kN force at 281kg dry weight is a TWR of over 16, so that's very easily enough for a rocket! Even with 3 tons of ammunition, it would still have a TWR of 1.4. The Isp is of course abysmal, but it can definitely lift itself and its whole ammo compartment no problem.

Accidentally slowing the plane below stall speed is in fact a legitimate concern for A-10 pilots firing the gun :P

Man you are wright! I mix up my number:confused:, got something like 0.016 of TWR. Well yeah, it is feasible IRL with this gun a least.

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CliftonM,

The force applied to your vehicle is 1/2*m*v^2. Since the force is proportional to the mass you're ejecting but the square of the velocity, the velocity is clearly the most important part. Moreover, the mass of the bullets is effectively your "fuel" so you're not going to get a very good wet/ dry ratio using bullets.

Best,

-Slashy

That's energy, not force. I.E: and ion drive will have stupendous power (V is high), but negligible thrust (m is tiny).

Rune. The force is actually proportional to m*v, without the exponent (impulse).

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Part of the problem with using a gun as a propulsion device is that a significant fraction of your propellant mass (roughly a third to a half, depending on what cartridge you're using) is wasted: the case. This just gets ejected to one side, you really can't do anything else with it. You could try using a caseless round (such as the ill-fated G11), but these bring their own problems to the table: cookoff, propellant fracture, etc. A mass driver would work better, all of your propellant mass gets ejected, but you still have to carry a separate power source. This is why rocket engines work so well at our level of technology: the rocket fuel provides its own energy and it is all used as propellant mass with no waste.

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This tread reminds me of my old Fallout 3 idea for vertices. They was talking about motorcycles, atv or at most something like an armored motorhome/ base.

I was thinking of an combination of an pongo stick and an orion nuclear pulse engine running on mininukes.

Much more fun :)

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The GAU-8 avenger can generate 45Kn of recoil, weight 281 Kg when empty and have a muzzle velocity of 1000 m/S (can it be considered exhaust velocity?) I don't think that's enough for a rocket (a plane may be) and that gun is one of the most powerful I know. So no, it's not feasible a normal bullet firing gun.

All Hail The Almighty BBBRRRRRTTTT!!!

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All Hail The Almighty BBBRRRRRTTTT!!!

One interesting thing is that staffing runs with airplane guns has become more common for hitting stuff like machine gun nests or parked trucks.

Smart software combines the autopilot and the gunsight making the autopilot direct the plane so the bullet trajectory will overlap target so you can fire even with an fighter jet.

Not sure if the A-10 have this but will surprise me if not.

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A sci Fi writer wrote a short story about these people trapped on a destroyed space ship, PART of the ship that is, I forget the name, author or many of the details.... (so sue me, I suffer from M.S. and with it M.S. induced dementia) ... they had a view port and could see space, they saw this airless moon and then had a brainwave. they took a ray run and super heated a water container which was part of the ship they were left with... when the steam came on line, they opened a hose and .... not sure, I'm pretty sure the story ended before they actually landed but he assumption was they were all going to survive.

Steam power in the space age..... :)

Pretty sure they also made a movie out of this as well...

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I have thought for a while that it'd be a really cool sci-fi engine if you essentially took a railgun and, instead of feeding it bullets, fed it a continuous stream of conductive fluid or plasma that it would shoot out in a similar fashion to a VASIMR.

Such a design could maintain high thrust (if given sufficient electric power), but retain Isp savings akin to those of an ion drive by virtue of expelling exhaust linearly as opposed to just blasting it out via pressure in whichever direction it wants to go.

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A rocket's efficiency (ISP) increases with exhaust velocity. Just expelling hot gas without further ado will give you a much higher exhaust velocity than if you tamp down on it with a metal bullet. That's if you use chemistry as a driving force. If you try electrics (e.g. mass driver), you can reach quite insane muzzle velocities... typically these go up as the size of your projectile goes down. Taken to the extreme, you end up with a particle accelerator emitting a stream of ions. Hmm, I've heard that somewhere...

Thrust depends on mass expelled per time. It's much easier to expel $mass of metal than the same mass of gas, so in first approximation a gun should have pretty high thrust -- but only for an instant, then zero thrust while you load the next projectile. Your typical machine gun spends most of it's time reloading rather than firing, and the truly fast-firing weapons buy their cadence at a lot of extra weight. So average thrust probably isn't too impressive, thrust-per-weight will be pretty bad... but depending on purpose it may suffice -- one XKCD What If dealt with a AK47-propelled rocket IIRC.

I think the vibrations would be worse than Ares 1.

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