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Converting a pneumatic into hybrid using shock heating

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Unread postAuthor: psycix » Wed May 13, 2009 7:29 am

Because I do not want to burn it for more pressure, but I want to burn it for a higher speed of sound.
The chamber will probably not stand up to a 30X mix, yet still I'd like a 450 psi pre-ignition.

My goal is not to use a pneumatic to build a hybrid, but to have a pneumatic with a hot gas, in order to achieve the same thing as with a lighter gas like helium. At 200m/s and higher, the sound barrier is the limiting factor.
I wanted to use a pneumatic to build a hybrid, I would simply slap on a spark system.
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Unread postAuthor: jeepkahn » Wed May 13, 2009 10:52 am

If you're looking at increasing the velocities, trying vacuuming the barrel... There's been lots of debate about the effectiveness, but I have learned that it is quite effective in taking a subsonic gun into the supersonic realm, but it offers only limited(but still noticeable) improvement if you are already past mach 1.5 or so...

If there's anyone out there who debates this, feel free to suck a vacuum to the barrel and fire it past a chrony....
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Unread postAuthor: psycix » Thu May 14, 2009 7:07 am

I once had that in mind. Point is that I don't feel like using burst disks.
I may try it one day though.
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Unread postAuthor: jeepkahn » Thu May 14, 2009 8:10 am

psycix wrote:I once had that in mind. Point is that I don't feel like using burst disks.
I may try it one day though.


You really don't have to use burst disc's per se, what I do is about .75" from the end of my barrels I drill a small (about .125") hole where I can attach my vacuum pump, then on the muzzle I use the bottom of a plastic disposable cup, smear a little grease around the edge of the muzzle, and the vacuum will hold the plactic in place and will usually hold about 28.5" of vacuum... No burst disc couplers needed...
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Unread postAuthor: daccel » Thu May 14, 2009 10:31 pm

Well what you're suggesting is essentially a 30x hybrid with a burst pressure of equal to the pre-ignition pressure.

Modeling the same dimension of cannon and light projectile in HGDT with a 4x @ 350 psi burst, vs a 30x @ 450 psi burst, the 4x has higher velocity, much higher barrel temperature, and they both peak at 450 psi.

So it seems that converting a pneumatic to a hybrid with a peak pressure equal to the pneumatic pressure would be a better solution to the speed of sound problem.

That said, shock igniting with air would be undeniably awesome, and should be attempted :D.

What about making your projectiles with the secondary mixture contained to make it more practical to load? Take something like an airgun pellet, put a piece of tape over the hollow tail, inject fuel with (extremely small) syringe, apply second layer of tape to seal hole, and presto!

For that matter, would the pressure be enough to set off a cap, that would be much easier than a secondary fuel/air.
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Unread postAuthor: Technician1002 » Fri May 15, 2009 11:27 am

daccel wrote:Well what you're suggesting is essentially a 30x hybrid with a burst pressure of equal to the pre-ignition pressure.

Modeling the same dimension of cannon and light projectile in HGDT with a 4x @ 350 psi burst, vs a 30x @ 450 psi burst, the 4x has higher velocity, much higher barrel temperature, and they both peak at 450 psi.

So it seems that converting a pneumatic to a hybrid with a peak pressure equal to the pneumatic pressure would be a better solution to the speed of sound problem.

That said, shock igniting with air would be undeniably awesome, and should be attempted :D.

What about making your projectiles with the secondary mixture contained to make it more practical to load? Take something like an airgun pellet, put a piece of tape over the hollow tail, inject fuel with (extremely small) syringe, apply second layer of tape to seal hole, and presto!

For that matter, would the pressure be enough to set off a cap, that would be much easier than a secondary fuel/air.


I'm playing with the idea that this science experiment is based on.

A cannon launches a piston, the piston hits the projectile. Do you think it will work to go supersonic?

http://www.metacafe.com/watch/834618/high_bounce/
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Unread postAuthor: KineticAmbitions » Fri May 15, 2009 9:51 pm

If they are sealed, the piston will not hit the projectile, it will compress and heat the air between the two, much like a spring piston airgun. Done properly, this concept is the same as the velocity doubler which is briefly described here, and is easily capable of Mach 1+ in an air powered pneumatic, assuming that the piston is heavier than the projectile, and near the speed of sound itself (a burst disc which does not restrict the movement of the piston would be an advantage here, allowing more pressure buildup and heating before the projectile moves).

Even without a burst disk, it would be an interesting thing to test, if one had the equipment to do so (due to the piston following the projectile out the barrel, traditional Chronying may not be the best method).
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Unread postAuthor: psycix » Sun May 17, 2009 6:16 am

You are now talking about light gas guns. Have the piston have a larger diameter then the projectile.

A cannon launches a piston, the piston hits the projectile. Do you think it will work to go supersonic?
Then your piston would also have to be SS, unless you are using the bounce thing, which would be very hard and unpredictable.
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Unread postAuthor: KineticAmbitions » Sun May 17, 2009 3:14 pm

psycix wrote:You are now talking about light gas guns. Have the piston have a larger diameter then the projectile.


The piston need not be a larger diameter than the projectile, just significantly heavier. In fact, unless you're trying to launch something at several kilometres per second, having the two the same diameter makes things far easier, as the piston doesn't extrude itself into the barrel after each shot, requiring it to be machined out.

And, once again, unless there's a lot of leakage going on, the piston will not HIT the projectile. Sealing things properly is in your favour here, as the shock of the piston hitting the projectile could do all sorts of damage (the slug expanding in the barrel and ripping the whole thing in half comes to mind). UHMWPE should be a sufficient piston material.
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