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Compressed gases or physical striking?

A place to ask general spud cannon related questions.
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Compressed gases or physical striking?

Unread postAuthor: SNDM » Tue Jun 03, 2008 4:02 pm

I was just wondering wether 300 psi will shift a projectile better than a piston pushed by 300psi?

I understand that some energy will be lost getting the piston moving, but I just wondered wether there was some sort of factor I was not aware of.

Thanks
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Unread postAuthor: MrCrowley » Tue Jun 03, 2008 4:20 pm

Well it depends on the valve.

I think you are confused a bit, do you think in a piston valve the piston physically strikes the projectile? Because it doesn't. The piston is simply a valve to let the compressed gas get to the projectile when you want it to.
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Unread postAuthor: psycix » Tue Jun 03, 2008 4:24 pm

If you want some sort of piston to push a projectile, you should look into sabot rounds.
A sabot is basically a lightweight thing around or behind the projectile wich fits tight in the barrel. A simple yet effective sabot could be made of foam.
Some sabots fall off as soon the projectile + sabot leaves the barrel.

With this you can shoot stuff that doesnt fit tight in the barrel, for instance, you can shoot a handfull of paintballs or bb's or fire off a round with a smaller diameter then the barrel.

The last thing is pretty awesome. Because of the big area of the sabot where the pressure acts out force on, the relatively small projectile gets alot of force on it and accelerates to very high velocities. When the sabot falls of, your small but deadly round slams holes through alot of stuff.

Hint:
Google for APDSFS :D
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Unread postAuthor: jackssmirkingrevenge » Wed Jun 04, 2008 3:59 am

I had pondered this point back in the spudtech days in this thread, with the following analogy:

The analogy I thought of was a golf ball being launched by a club. The sabot can be compared to taping the ball to the club and releasing it through inertia, whilst a piston striking the projectile can be compared to the club striking the golf ball in a conventional manner. Surely in the latter case the ball will go further?
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Unread postAuthor: SNDM » Wed Jun 04, 2008 3:46 pm

Dont worry, I understand that pistons do not hit the projectile, in fact they usually move in the opposite direction.

I was just wondering kind of what JSR was saying...

Ima gonna check that link out.

And google APDSFS, whatever the hell it is...
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Unread postAuthor: jackssmirkingrevenge » Wed Jun 04, 2008 5:03 pm

Armour Piercing Fin Stabilised Discarding Sabot, basically the best anti-tank round available to modern military forces.

As to firearms that actually use pistons, look at the Russian SP-4 silent round:

Image

It's basically a sabot that doesn't leave the cartridge, thus avoiding impact - though this is possibly to avoid impact noise.
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Unread postAuthor: Hotwired » Wed Jun 04, 2008 6:31 pm

jackssmirkingrevenge wrote:I had pondered this point back in the spudtech days in this thread, with the following analogy:

The analogy I thought of was a golf ball being launched by a club. The sabot can be compared to taping the ball to the club and releasing it through inertia, whilst a piston striking the projectile can be compared to the club striking the golf ball in a conventional manner. Surely in the latter case the ball will go further?


Analogy could do with some work

If you accelerate a projectile down a tube into the air with compressed gas, all available energy has been transferred to the projectile.

If you accelerate a piston down the same tube then have it connect with the projectile to transfer energy... more energy cannot possibly be transferred to the projectile because of the piston adding extra mass and resistance.

A trapped piston to launch a projectile would make an interesting silenced cannon, much in the manner of those sealed russian cartridges... but much larger.
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Unread postAuthor: jackssmirkingrevenge » Thu Jun 05, 2008 12:41 am

Hotwired wrote:If you accelerate a projectile down a tube into the air with compressed gas, all available energy has been transferred to the projectile.

If you accelerate a piston down the same tube then have it connect with the projectile to transfer energy... more energy cannot possibly be transferred to the projectile because of the piston adding extra mass and resistance.


The way you have to look at it however is that the piston can be of a much larger diameter than the projectile, meaning a lot more energy is available to it. This is essentially the point behind pneumatic nailers, the piston diameter is massive compared to the actual nails.

The debate here is what is the best, accelerating the projectile with the piston, or allowing the piston to accelerate then strike the projectile at the end of its travel?

A trapped piston to launch a projectile would make an interesting silenced cannon, much in the manner of those sealed russian cartridges... but much larger.


It needs to be of very strong construction however, lest your barrel is prompted to part company with the rest of your launcher and sail off beyond the horizon with your projectile *cough* :roll: :D

Another variation of the idea was the AAI Telecartridge, a shotgun shell made for "tunnel rats" in Vietnam that used a metal diaphragm instead of a piston like the soviet designs:

unfired:

Image

fired:

Image

That was in 12 gauge, but they also had an interesting round made for modified 44 magnum revolvers for the same purpose.
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