Idea for a rapid open valve

Why dont you use a solenoid valve? Pretty inexpensive and much more repeatable than aluminum foil.

Dear Chris W:

Roughly, a "rupture disk".

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I'd see if you can get chummy with someone at a company that makes rupture disks.

Your "screw back together" operation will provide the lion's share of your variability. The other source of variability will be the final thickness and temper of the metal film (after you are done "mangling" it to get it in place). High volume commercial films might be uniform, and might not.

Make sure the "tennis ball" is very close to the foil when it is ruptured. This will place most of the compressed air behind it...

You might consider forcing the rupture of a thicker film, so that it does not go off accidentally.

You might consider machining the bore of your gun (or at least an initial orifice place) so that the tennis ball provides the necessary back pressure (or at least some fill delay). That way a standard CO2 cartridge (or paintball gas cartridge) could do the job.

David A. Smith

Or a diaphragm in a shock tunnel. You could look in the literature for such.

I don't know about the repeatability issue you have raised. Such diaphragms are a big part of shock tube operation. However, if a solenoid valve with the appropriate flow area* Cv is available a known technique to get them open faster is to run them at much higher voltage, ie kV's on a 24V solenoid. Yes, they don't last as long, but what the hey. They are cheap.

Chris W wrote: It seems the most

There are a number of dump valve designs out there that will suffice. The most popular is a modified sprinkler solenoid. You can replace the electric solenoid with a mechanical trigger. You can also use two or more in parallel if you need more valve area. Burst disk valves are more usually employed on fuel vapor spud guns that are triggered by an electric ignition circuit, the burst disc there to aid in flame propagation and maximum energy. The valve is the easy part. Having some experience with tethered projectiles, I will say that I expect the most difficult part will not in fact be the dump valve, but instead be designing and building a mechanism that will allow your fishing line to play out at high velocity without snaring. It is not as simple as it initially seems. It is also more dangerous than you might think. If your two hundred yards worth of line snares at six feet with sufficient momentum in the tennis ball to drive it another 194 feet it can be far more exciting than you desire I expect.

Charles

Some one else mentioned a standard solenoid valve. A trick used in shock and expansion tube aero facilities is to use high voltage (multiple kV) on standard 24V solenoid valves to get them to open quicker where fluid injection into a model is desired.