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Registered Member #53
Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638
I've always been interested in vacuum cannons but only recently found out that with a high pressure boost it is possible to get objects up past the speed of sound. (The current record is over 800m/s)
So after a quick trip to the hardware store I built myself a pressure+vacuum cannon with vacuum accumulator. The I put a wire along the mylar foil on the high pressure side as a trigger, if it fails to fire from pressure I can just heat it up but as of yet it has always fired on its own. The vacuum accumulator at the business end is used to eliminate any back-pressure from air that blows by the ping pong ball as it accelerates.
After a few attempts I finally got a ball going at 360m/s! "Fast" doesn't cover how short the event is, the ball is evaporated before you get a chance to process what has happened.
To estimate the speed I (previously) measured the displacement between the target and the barrel and then was able to measure how far the ball moved in 1 frame (about 30cm). The shutter speed was 1/1250s but I'm not sure how much rolling shutter caused the streak to be lengthened so I am calculating conservatively.
The next step is to build a target enclosure to catch all the ball bits and target material that comes flying off.
Registered Member #230
Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284
I saw a vacuum cannon at HEAS a few years ago. That one went more than 1 Mach iforget how fast but the noise was stunning. The ping pong ball went through full sodapop cans (filledwith water). No need for accumulator but the unit did have a potato cannon to help it along
Registered Member #53
Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638
I my pressurized air boost is pretty much a potato cannon so it sounds like a similar setup. Looking at my high speed footage I can see the exit burst disk puff up just before the ball exits, indicating back pressure. I thought that the vacuum accumulator would take care of this but maybe I just haven't let it pump down far enough.
Registered Member #230
Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284
HEAS = high energy amateur scientists getotgether late Sept early oct once a year run by Richard Hull in Richmond Virginia. Two stage vac pump took around ten minutes to evacuate the tube
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
Interesting.. This could be a great tool to visualize to what extent air is in the way of moving objects. A couple of questions: How thick is the mylar you use as diafragm? How high pressure is in the pressure chamber? How do you sequence the breaking of the 2 seal, pressure vs vacuum?
Registered Member #53
Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638
It would be interesting to run it with varying levels of vacuum and see how much of a difference there is in the ball speed. At the moment I am getting about 40torr according to my amazon vacuum gauge but I think there are some leaks in the non-vacuum rated hose I am using.
The mylar I am using is sometimes known as "space blanket" and my crummy micrometer measures it to be ~15um.
At the moment I haven't had it above 80psi due to the length of airline between the cannon and my compressor, with the ABS I picked I can safely go up to 100psi with no fear of failure.
I have changed the setup a little and am now using only one membrane at the vacuum/pressure interface (having 2 was causing issues). My new membrane is made of poly propylene tape (tuck tape), a length of 30awg wire, mylar and then another strip of tape. This will easily hold the 80+psi and can be triggered by heating the wire with a little current, once the mylar and poly propylene have weakened they fail in a catastrophic way and release the pressure.
My last shot came out around 500m/s and put a measurable dent in a hard hat! (1200fps, 1/2000s shutter speed)
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Mach 1 over a weekend
