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Yet Another Vacuum Cannon and materials selection

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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Yet Another Vacuum Cannon and materials selection

Unread postAuthor: nik282000 » Thu Feb 02, 2017 4:53 pm

Hi guys, I am on the quest to put a Ping Pong ball past mach 2 and this forum has a lot of great info.

To start, I am already getting speeds right around 500m/s which is satisfying/terrifying from little celluloid ball. The first GIF below was filmed at 1200fps with a 1/2000s shutter speed and only shows the ball for 2 damned frames!
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I was surprised to see "cartoon physics" as the back of the can exploded like a banana peel.
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I layout is nothing surprising, I have a 10'x1-1/2" ABS barrel with a 2'x4" vacuum accumulator on muzzle end. For the pressure chamber I erroneously chose ABS having read that PVC shatters and that DWV ABS is good up to 150psi, I've read through a number of threads and will be switching to PVC before I fire again, thanks guys. The front burst disk is made of space blanket and the rear disk is mylar strengthened with "tuck tape" and with a small wire between the layers that is heated for triggering.
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I have 2 questions of any one who has the time, is there an ideal ratio for barrel volume to air reservoir volume? And the same goes for the vacuum accumulator. I plan on turning my (old) pressure chamber into a new, larger, vacuum reservoir because I am still seeing the muzzle membrane inflate just before the ball exits.

Thanks and I look forward to a long career in the recreational abuse of plumbing.
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Re: Yet Another Vacuum Cannon and materials selection

Unread postAuthor: Ragnarok » Fri Feb 03, 2017 6:25 pm

nik282000 wrote:I have 2 questions of any one who has the time, is there an ideal ratio for barrel volume to air reservoir volume? And the same goes for the vacuum accumulator.

It depends how you define ideal, but the likely answer is no.

Two possible ideals are:
- A chamber volume where acceleration falls to zero at the muzzle, meaning all energy is extracted from the air.
- A chamber volume with the greatest possible velocity at the muzzle.

In the first case, such ratios exist (although depend on specific pressure and flow, meaning they're difficult to calculate), but are not usually particularly productive. Most of the barrel will have a very low acceleration, meaning that similar results could be achieved out of a barrel half the length.

In the second case, it's diminishing returns. Increasing chamber size will increase velocity (as the pressure will stay higher for longer), but with no limit. That said, in practice, the velocity usually plateaus fairly fast after a C:B ratio of about 2:1 (and even less for valves with poor flow).

a 2'x4" vacuum accumulator on muzzle end.

I was always disappointed that the Mythbusters missed this trick with their ping-pong cannon, but then their air cannons were never the most sophisticated.

(The other thing I wanted to see them do was use a lighter-than-air propellant, such as hydrogen or helium. With their kind of resources, their velocities of ~500 m/s were really quite lacklustre. With even half of their kind of budget, I'm sure some of our members could have pushed that past 1000 m/s, possibly even twice that).
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Does that thing kinda look like a big cat to you?
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