Momentum based gun

[Korvan]

I was doing some thinking about a gun whose basic operating principle is simple momentum and collision. Basically, you have have a flywheel (electrically driven or otherwise) connected to a piston in a barrel. The barrel and piston are designed in such a way that air is not pushed ahead or compressed by the piston. This is not a compressed air gun, the projectile is launched by the piston physically colliding with it.

A steel piston colliding with a steel bearing is pretty close to being a perfectly elastic collision, so we will assume the collision is perfectly elastic. In reality, actual performance will likely be about 90%. Since the mass of the piston and the moment of inertia of the flywheel are much great than the mass of the bearing, we can greatly simplify our work and just assume the speed of the bearing after the collision will be twice the speed of the piston.

So, if we want our bearing to have a muzzle velocity of 1000 m/s, the piston needs a maximum velocity of 500 m/s (which means the collision has to occur when the piston is at its maximum velocity). A quick google search turned up a number of piston speed calculators and I worked out a piston with a stroke length of one meter driven at 15000 rpm, will attain a maximum velocity of 500 m/s.

We do have a couple problems. First, as I mentioned before, the collision isn't perfectly elastic, it's about 90-93% elastic. That means the energy lost in the collision goes into fucking up the gun. The other problem is while 15000 rpm isn't a big deal and is easily obtainable by an electric motor, 500 m/s is fucking huge for the speed of a piston. For instance, a dragster's piston only reach about 25 m/s. This leads me to think the stresses involved in accelerating and decelerating the piston, not to mention friction, would be too large for even modern materials to handle.

A collision based gun does have some advantages if you could get one working. It is energy efficient, especially if electrically driven. Within a certain range, the velocity of projectiles of varying weight is constant. So the gun could fire a variety of shells that would follow the same trajectory. The speed of the shell can be varied by varying the speed of the piston.

I'd be interested in knowing just how unlikely a piston speed of 500 m/s is, so if there's any mechanical engineers in the house, please illuminate me.

[Sarevok]

Can your piston gun fire instantly at the pull of a trigger ? Or does its flywheel mechanism need to be spun up first ?

[Ilya Muromets]

I'm no engineer, but to my untrained ear the design sounds more complicated than a standard exploding propellant gun with little real advantage over said standard gun. The more complicated a gun is, the more expensive it'll be and the more reliability issues you'll have to face. And at the speeds needed for a piston to hurl projectiles as fast as a propellant gun? That's just a part failure waiting to happen, which is more likely since said system has more parts than a propellant gun. Also, all those parts and the powersource needed to run said parts take up more space than the mechanism of an equivalent propellant gun.

[Ilya Muromets]

You know, now that I think about it, this gun also has a physical impactor ramming something at 500 m/s repeatedly and a rotating section spinning at 15,000 rpm. That's gonna put an enormous amount of stress on the gun. The design doesn't just sound like a part failure waiting to happen -- it sounds like a catastrophic disaster waiting to happen.

[haard]

Either way its hard to see what would make this superior to a conventional firearm with muzzle break (or silencer); you can load or even buy factory loaded ammunition that gives similar ballistics for different bullet.
Sure, you'd not have to manufacture and store explosive ammunition, but you'd have to have some awesome machine tools and spare parts aplenty. And access to electricity at all times.

[Stuart]

Yikes. How much is this thing going to weigh? What powers it? What powers the thing that powers it? Will materials technology be up to the collision between piston and bullet? How is the steel bullet going to engage the rifling in the barrel. If it isn't, how are you going to stabilize the bullet and ensure accuracy?

Above all, what advantage does this contraption offer?

Sorry but this seems like another idea that is being cute for the sake of being cute.

[Shroom Man 777]

Could just be a thought experiment, there's nothing wrong with a novelty ingenious device that shoots bullets at things for experimental scientific exercise purposes instead of mass-produced weapons purposes. It would actually be nicer than seeing a new kind of weapon that can kill more people.

[Ilya Muromets]

But if this thing fails in the middle of superspeed piston pumping and insane rpm flywheel rotating, then the resulting catastrophe could kill more people than the failure of an equivalent propellant gun.

[Eulogy]

But if this thing fails in the middle of superspeed piston pumping and insane rpm flywheel rotating, then the resulting catastrophe could kill more people than the failure of an equivalent propellant gun.

Momentum-based mines?

[Ilya Muromets]

Again, more overcomplicated than simple and reliable explosive mines.

[Eddie Van Helsing]

We already have momentum-based guns. The momentum is supplied via chemical reaction. Your proposed design might be handy in space, where there's no oxygen to react with gunpowder to facilitate the chemical reaction, but even that situation seems unlikely. You could always put the weapon inside an old pressure suit. Firing would blow a hole in the suit, but that's fine if all you need is to fire a shot or a short burst.

[Stuart]

Your proposed design might be handy in space, where there's no oxygen to react with gunpowder to facilitate the chemical reaction, but even that situation seems unlikely. You could always put the weapon inside an old pressure suit. Firing would blow a hole in the suit, but that's fine if all you need is to fire a shot or a short burst.

One doesn't need ambient oxygen to fire a gun. They work in vacuum or under water OK as long as the barrel isn't blocked.

[Eddie Van Helsing]

One doesn't need ambient oxygen to fire a gun. They work in vacuum or under water OK as long as the barrel isn't blocked.

I stand corrected. Is it because oxygen gets packed in with the powder during the process of manufacturing the ammunition?

[Stuart]

I stand corrected. Is it because oxygen gets packed in with the powder during the process of manufacturing the ammunition?

Some of the chemical compunds contained within the propellent are oxygen-rich and thus supply the oxygen necessary for the burn. The big problem with firing guns underwater is the barrel filling and thus being effectively blocked. In vacuo, the primary problem is the lubricant boils off and leaves the gun without any.

The Glock manual actually advises against firing the gun underwater which suggests it's been tried. . . .. .

[Admiral Valdemar]

I was under the impression the GLOCK 17 and other models had a basic capacity to fire underwater, or after being submerged, just that it's not ideal.

In any case, for propellantless guns, stick with railguns or coilguns. This idea is a nice mental exercise, but it's far too bulky and dangerous otherwise.

[Eddie Van Helsing]

In vacuo, the primary problem is the lubricant boils off and leaves the gun without any.

I didn't even think of that.

[Simon_Jester]

I stand corrected. Is it because oxygen gets packed in with the powder during the process of manufacturing the ammunition?

Some of the chemical compunds contained within the propellent are oxygen-rich and thus supply the oxygen necessary for the burn. The big problem with firing guns underwater is the barrel filling and thus being effectively blocked. In vacuo, the primary problem is the lubricant boils off and leaves the gun without any.

There might be workarounds for this, but I don't know enough about firearm design to say. The first thing that pops into my mind is the use of materials such as teflon, which have low friction and are vacuum-safe.*

*So long as you don't care about maintaining the quality of the vacuum, which in outer space you do not.

[Korvan]

I was under the impression the GLOCK 17 and other models had a basic capacity to fire underwater, or after being submerged, just that it's not ideal.

In any case, for propellantless guns, stick with railguns or coilguns. This idea is a nice mental exercise, but it's far too bulky and dangerous otherwise.

Yeah, I'm pretty much getting that! This idea came to me while reading the railgun thread, just after having gotten caught up on the Girl Genius web comic, so impractical and user-maiming was pretty much the theme I was going for. I was just wondering if if was even possible to construct such a gun that would even fire only once without exploding.

What I did find interesting was that piston driven guns actually exist, they just use the pistons to compress air and then use the compressed air to fire. I really should've known this, as I did own a couple of air guns as a kid. Given the performance of electrically driven airsoft guns, it wouldn't take much scaling to get them match the performance of firearms. Still, you end up with the problem with increased complexity for no real gain.

[Seggybop]

I was under the impression the GLOCK 17 and other models had a basic capacity to fire underwater, or after being submerged, just that it's not ideal.

In any case, for propellantless guns, stick with railguns or coilguns. This idea is a nice mental exercise, but it's far too bulky and dangerous otherwise.

Yeah, I'm pretty much getting that! This idea came to me while reading the railgun thread, just after having gotten caught up on the Girl Genius web comic, so impractical and user-maiming was pretty much the theme I was going for. I was just wondering if if was even possible to construct such a gun that would even fire only once without exploding.

What I did find interesting was that piston driven guns actually exist, they just use the pistons to compress air and then use the compressed air to fire. I really should've known this, as I did own a couple of air guns as a kid. Given the performance of electrically driven airsoft guns, it wouldn't take much scaling to get them match the performance of firearms. Still, you end up with the problem with increased complexity for no real gain.

Have you read anything about light gas guns? They use a conventional explosive charge to propel a piston which then squeezes a chamber filled with hydrogen, which is then released at extremely high pressure to accelerate the projectile. The maximum speed of the expanding hydrogen is way higher than the reaction gases of gunpowder or other normal propellants, so the projectile can theoretically have a much greater muzzle velocity. The cannons proposed for launching stuff into space, like the HAARP project or Saddam's giant gun would have been of this type.

[starslayer]

I was under the impression the GLOCK 17 and other models had a basic capacity to fire underwater, or after being submerged, just that it's not ideal.

That is correct; the Mythbusters actually tested this. The problem for semi-automatic or automatic pistols and rifles is that the case ejection system is designed to work in air, not water, so the mechanism can only partially eject the shell. The resulting jam can usually be cleared pretty easily, though. Revolvers and bolt-action rifles actually work fine.

In any case, the barrel filling with water instead of air does produce a ton of extra stress on the gun when it is fired, so it's really not advisable to try this experiment yourself, because it could potentially cause the gun to explode.

Have you read anything about light gas guns? They use a conventional explosive charge to propel a piston which then squeezes a chamber filled with hydrogen, which is then released at extremely high pressure to accelerate the projectile. The maximum speed of the expanding hydrogen is way higher than the reaction gases of gunpowder or other normal propellants, so the projectile can theoretically have a much greater muzzle velocity. The cannons proposed for launching stuff into space, like the HAARP project or Saddam's giant gun would have been of this type.

Not just theoretically; NASA and other research institutions use these or their helium based cousins to model asteroid impacts, or space junk impacts on spacecraft and such.

[Rossum]

Lets look at some existing non-gunpowder projectile weapons:

1). Bows or crossbows- Been around for ages and rely on elasticity for the momentum. The first bows relied on flexible wood and string to provide the push while composite bows have tough frames and strings and use springs to supply the energy for the shot.

2). BB guns or airguns- I'm not 100% sure about the mechanics as I've never used them but they use either compressed air or springs to fire small BB pellets. Kids used to be able to use them as toys or whatever but it seems they were still pretty darn dangerous and could take somebodies eye out (and in some cases even break glass or punch through a persons skull). Not exactly a high grade military weapon but can still be dangerous in some conditions and maybe a person could increase the power with the right materials.

3). Steam cannon- On an episode of mythbusters they were able to build a cannon that ran on steam. Use a boiler to create the steam and when the pressure is enough then open a valve to send it into a cannon to launch the cannonball. They were able to launch a volleyball pretty far with it.

4). Tennis Ball Launchers - I don't know what the exact name for those things are, but they are those machines people use to practice tennis with. They have two wheels that spin and can launch a tennis ball that goes between them. I doubt one could make a particularly accurate kinetic weapon with such a setup, but it can at least launch things.



Here's the first thing I came across when searching for Tennis Ball Launchers on youtube... and this is a homemade pedal powered tennis ball launcher. It looks pretty neat and can launch stuff but I doubt it would have any gunlike applications. At most, a mechanised version of it could be used to launch grenades or something.

[Eddie Van Helsing]

At most, a mechanised version of it could be used to launch grenades or something.

The M-79 does a perfectly good job of that, if you don't mind the fact that you have to reload after every shot.

[Surlethe]

The other problem with your idea is that you won't be able to turn your gun after you spin up the flywheel --- it will have tremendous angular momentum.

Anyway, looks like the idea has been sorted through. Thread locked.