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Transonic and supersonic pneumatic gun directory

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: Ragnarok » Thu Jul 30, 2009 6:50 am

inonickname wrote:With hydrogen it exceeds the U.K. legal limit for air rifles..with a Q-tip..

Unlicensed air rifles to be specific. Appropriately licensed, there's no energy limit beyond what rifles you can actually buy in this country.
Also, to be honest, the sensible man's limit for unlicensed air rifles is 11 ft-lbf/15 joules. The merest fraction above 12 ft-lbf (in police tests, which aren't precisely defined) can cause one to be detained at her majesty's pleasure, so the wise leave a margin to avoid any possible problems.

As a closing point, I would say that my air rifle gets ~11 ft-lbf in an 9.5" barrel. You're getting ~14 ft-lbf in a barrel over 3 times the length and using a larger bore...
Impressive muzzle energy in its own right, but you're hardly comparing like with like.
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Unread postAuthor: inonickname » Thu Jul 30, 2009 7:01 am

I could go to a smaller bore, though there's no need. (ammunition can fit slightly smaller, as well as slightly larger) As for the barrel length, 80cm really isn't that much. Dropping it down to 9.5", I can still almost get 10 ft/lbs.

Exceedingly short airgun barrels has always annoyed me, though of course lengthening the barrel on a springer has it's limitations.

Hell, even with a 2cm long barrel i still exceed the performance of a lot of similar bore launchers here.
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Unread postAuthor: btrettel » Fri Jul 31, 2009 8:00 pm

Ragnarok, thanks for clearing up what approach you're taking. It definitely verifies that I'm on the right track. I'm thinking about eventually trying the axisymmetric approach as well as it seemed to be the next simplest approach, though, I'm not certain it would be necessary (then again, I don't know as I haven't tried it).
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All spud gun related projects are currently on hold.

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Unread postAuthor: Technician1002 » Fri Jul 31, 2009 8:14 pm

To add to the supersonic club, the team I mentored in making t shirt launchers are the ones that got me starting in trying for supersonic marshmallows. They built a marshmallow launcher and using the principals listed above have gotten miniature marshmallows supersonic.

They did it by building a launcher using a 1 inch sprinkler valve with a reducing section to 1/2 inch. This 4X decrease in cross sectional area and resulting shock heating launches light mini marshmallows well into supersonic. They haven't told me what pressure they are using, but the sprinkler valve is handling it.

:shock: They beat me supersonic. and I'm mentoring them?

Image
961 MPH.. 1409 FPS..
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Unread postAuthor: inonickname » Fri Jul 31, 2009 8:18 pm

That is an interesting way of doing it tech, so you believe having a very over kill valve porting to a lower cross sectioned area will produce shock heating?

If so, I'll increase the sealing/porting on the next co-ax by 3 or 4 times the barrel, and have a small amount of dead space for heating.. That, with 400+ psi, a super light super efficient piston, minimal pilot volume and fast pilot porting should get me supersonic.. Right?
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Unread postAuthor: Technician1002 » Fri Jul 31, 2009 8:49 pm

inonickname wrote:That is an interesting way of doing it tech, so you believe having a very over kill valve porting to a lower cross sectioned area will produce shock heating?

If so, I'll increase the sealing/porting on the next co-ax by 3 or 4 times the barrel, and have a small amount of dead space for heating.. That, with 400+ psi, a super light super efficient piston, minimal pilot volume and fast pilot porting should get me supersonic.. Right?


A fast valve regardless of a fast pilot or snap action should do fine with reasonable coef and proper reduction in area with dead space for shock heating after the valve.

The expanding gas from the chamber is cold.. The existing air in the dead space is heated by compression. Yes, shock heating.

The theory is based loosely on oceanside blow holes. As the mass enters a smaller area and slows down, the release of energy accelerates the front of the mass so the speed increases well above the average mass which is slowing down.

[youtube]http://www.youtube.com/watch?v=RE1ur61FcoA[/youtube]

To do a 4X reduction in area only requires 1/2 the diameter. For example 2 inch to 1 inch and 1 inch to 1/2 inch.

I don't have a chronograph. My magnets are too heavy, so I don't know if I am supersonic on marshmallows yet. I should be with the 1/2 inch barrel on the marshmallow cannon and the smooth venturi transition to 1/2 inch.

At 110 PSI on the marshmallow cannon with the 1/2 inch barrel, I should be near or over supersonic with mini marshmallows.

I'll find out later.
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Unread postAuthor: Ragnarok » Sat Aug 01, 2009 3:50 am

Technician1002 wrote:961 MPH.. 1409 FPS..

Based on the distance between the muzzle and chrony, I'd expect error in there. Can't tell you how much, but that's far too short a distance for an air cannon.

The existing air in the dead space is heated by compression. Yes, shock heating.

Can I get this straight before people get (deeper) into bad habits?

Shock heating ≠ Adiabatic heating.

Shock heating is heating across a shock front (or shock wave if you prefer).
Adiabatic heating is heating due to compression (or equally cooling due to decompression).

Both can occur in spudguns (and indeed, do - often simultaneously), but they are not the same thing.

And yes, before anyone gets... well, anything, I know I've occasionally made the same error in the past when not thinking straight.
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