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Mass flow and De Laval nozzles for your spuddy

A place to ask general spud cannon related questions.
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Unread postAuthor: jackssmirkingrevenge » Tue Dec 09, 2008 1:47 am

Ragnarok wrote:Actually, a better example is a Gas Ram rifle. It's my recollection that they use pressures of the order of a few hundred psi in the rams.


The starting pressure is around 800 psi if I recall correctly, but then it's much further compressed when the rifle is cocked. Can we at least agree that [bigger pushing piston = moar power!]? If nothing else, Hotwired's results prove that a bigger compressive force is needed for a noticable difference in power.
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Unread postAuthor: Ragnarok » Tue Dec 09, 2008 1:51 am

jackssmirkingrevenge wrote:Can we at least agree that [bigger pushing piston = moar power!]

More compression tube for the same barrel will give better results - but I think that the concept you proposed of having a larger driving piston than compressive one will however cause problems, mostly because of the piston being too heavy, amongst other things.
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Unread postAuthor: jackssmirkingrevenge » Tue Dec 09, 2008 2:40 am

Ragnarok wrote:mostly because of the piston being too heavy


Lolcat physics states that [Moar heavy=Moar momentum=MOAR POWER!], besides the increase in surface area should more than compensate for the increased weight.

Alternatively, you could keep Hotwired's design, however incorporate substantial vents midway down the chamber in order to allow the piston a period of non-compressing acceleration in order to gain momentum.
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Unread postAuthor: Ragnarok » Tue Dec 09, 2008 4:13 pm

jackssmirkingrevenge wrote:Lolcat physics states that [Moar heavy=Moar momentum=MOAR POWER!], besides the increase in surface area should more than compensate for the increased weight.

Not really. Firstly, you then have a heavier piston to stop.

Secondly, it's not so much about the momentum as the energy. I should really be talking about the inertia of the piston, not the momentum.
I'm being an idiot and using the public "understanding" that inertia = momentum, which is not the case.

Thirdly the speed of the compression also matters, because it's this rapid compression that heats the gases, so you can't sacrifice piston velocity for extreme mass, there needs to be a compromise.

Alternatively, you could keep Hotwired's design, however incorporate substantial vents midway down the chamber in order to allow the piston a period of non-compressing acceleration in order to gain momentum.

That would also ensure there was no accelerative force after the vent holes. The energy lost to compression for the first half of the travel is pretty insignificant anyway.

On an interesting note however, I once used almost that exact technique to try studying the effects of extreme acceleration on various projectiles.

Basically, I fired something from HEAL into an attachment (fixed straight on the end of the barrel), which incorporated vents to bleed off the firing pressure, and then after the vents, there was another length of 22mm copper about a foot long, capped off at the end.
In this 30 cm section, the projectile compressed and heated the air ahead of it to extreme levels, bringing the projectile from it's full velocity to a halt in just centimetres.

If I'd only modified the end cap so that there was a barrel coming from it, then this would have been almost exactly what your second method describes. However, I was interested in the effects of the acceleration on the projectile, not the pressure built up ahead of it.

I did it that way because although I can generate some pretty fancy accelerations, they were clearly not enough, and they also resulted in a projectile that was travelling at some speed and was therefore difficult to study. The rapid deceleration was a far more useful way to put such forces on the projectile.

The peak deceleration Gs on the projectile were vastly more than any I could have levelled on it during acceleration, into millions, I think my eventual estimate was - so many in fact that I'm highly surprised the projectile survived.

However, it proved that things could withstand accelerations of patently ridiculous levels for short spaces of time.
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Unread postAuthor: jackssmirkingrevenge » Tue Dec 09, 2008 4:37 pm

I was thinking two part piston, the coaxial rear ring would lock over the vent holes, it's complicated :roll: :D

I'm thinking it would be worth trying this out with a combustion propelling the piston, big bore combustions are easier to make than big bore pneumatics, plus through cooling the pressure behind it would be lost after firing.
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Unread postAuthor: Hotwired » Tue Dec 09, 2008 5:23 pm

Mmm, I had to unscrew the barrel afterwards very slowly so the trapped air could leak out.

Of course that's easily solved by having a venting valve built into the barrel.
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Unread postAuthor: Happy » Mon May 06, 2013 6:06 am

Sorry for posting on old threads but ive been kinda researching divergent convergent nozzles and came across this video.

http://www.youtube.com/watch?v=ohURkDRLxj4

You guys have probably seen this but I though this was a great video of the application of a de laval nozzle in a burst pneumatic cannon.
The cannon was fired at 90 psi and broke the sound barrier.

P.s the description in the video had some of the details of the cannon.
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Unread postAuthor: jackssmirkingrevenge » Mon May 06, 2013 6:13 am

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Unread postAuthor: Alanstone » Mon May 06, 2013 4:31 pm

Edit: sorry I didn't see this was an old thread. Here is a link to a 1960 patent of a compression ignition gun.

Patent US2947221 - COMPRESSION IGNITION GUN - http://www.google.com/url?q=http://www. ... dgEKX5iQ4g
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