Spudchukker III - Piston Hybrid

[GalFisk]

This is wonderful, I'm glad to see that this style of hybrid valve is catching on at last. I'm sorry to say that my own proof of concept cannon using this type of valve has been sitting idle for >5 years now, with unresolved metering issues, in favor of other hobbies.

[jackssmirkingrevenge]

The list of old members popping back for a visit keeps growing

other hobbies

Like?

[GalFisk]

The list of old members popping back for a visit keeps growing

other hobbies

Like?

Skydiving, fireworks-making, and girlfriend+teenage stepdaughter are the things that have eaten my cannon time. I may dust them off and bring them to the skydiving club next year, as some of my friends there are interested in spudguns as well.
On topic: is the purpose of the cam lock only to enable quick breech loading? How do you like the igniter? It looks small and sleek, much prettier than my flyback circuits. Does it give a continuous spark, or a rapid series of sparks?

[jackssmirkingrevenge]

teenage stepdaughter

Sounds painful.

Oh well, the rest sounds like fun

[sudpuzzer]

Yes, you're right, the camlock is there only to facilitate breach loading at the rear end of the barrel. This section of the gun isn't pressurized pre-ignition, so it serves only for breach loading.

I love the igniter. It's of the gas-grill variety, so it releases a rapid repearing spark rather than a continuous arc. It is much smaller than the box it's contained in, I have two 9volt batteries and a bunch of wiring in there with it. It's small, powerful, and runs on 9v batteries.

[sudpuzzer]

Found some vids a friend sent me from his iPhone, made a gif:

[Juggernaut12121]

A little late but very nice!
A question if I may, it seems pretty consistent that the orafice for the pilot vent is somewhat small. Is there any harm in using a larger port? I was planning on using a 1 1/4" nipple (1.4" inner diameter), but I wasn't sure if it would be too large. The only problem foreseeable is that there's less of an air cushion when the piston retreats. Any feedback is a huge help, thanks!

[MrCrowley]

A little late but very nice!
A question if I may, it seems pretty consistent that the orafice for the pilot vent is somewhat small. Is there any harm in using a larger port? I was planning on using a 1 1/4" nipple (1.4" inner diameter), but I wasn't sure if it would be too large. The only problem foreseeable is that there's less of an air cushion when the piston retreats. Any feedback is a huge help, thanks!

The greater the diameter of the pilot vent, the more pressure you will need in the pilot area to hold the piston closed for a given mix. If you increase the pilot vent area, you increase the surface area of the force vector that is trying to push the piston back (e.g. open the valve). This means that you'd need to increase the pressure of the air in the pilot area to negate this problem. With a ~20mm diameter pilot vent, I need about 300PSI in the pilot area for a 10x mix. So you probably wouldn't want to increase the diameter too drastically although it'd be worth doing the calculations to figure it out.

Also, if you're using a bumper, you wouldn't want much of an air cushion as it causes piston bounce.

[Juggernaut12121]

Dear god I'm stupid, thanks!

[MrCrowley]

Haha we've all made that mistake before. It's worth drawing up a basic diagram of the valve with arrows indicating the direction of the forces and numbers indicating the cross-sectional area of each surface. It really helps when it comes down to calculating the pressure you'll need in the pilot area for each mix.

[sudpuzzer]

Err, yeah. What he said.

[Brian the brain]

Skydiving, fireworks-making, and girlfriend+teenage stepdaughter are the things that have eaten my cannon time. I may dust them off and bring them to the skydiving club next year

So the guys at the skydivingclub don't like dusty girlfiends and stepdaughters?

[Juggernaut12121]

Another dumb question, does your piston slide back far enough to where the front end goes back into the housing? The way I have mine out on paper it does slightly and I'm worried that it could catch the wall of the housing (being somehow off center) and potentially kill itself. And (MrCrowley, if you're reading this) how well does a solid piston work out/is there too much stress?

[sudpuzzer]

I don't have the exact measurements in front of me, but I constructed the piston in such a way that at the most open point, the o-ring section of the piston is at the back of the nipple against the rubber bumper (which isn't shown), and the sealing face is still inside the 'Tee' a little bit. In the full closed position, I don't think the o-ring section of the piston is right at the 'Tee' end of the nipple, like my renders show. The piece of 1/2" threaded rod I used at the front end of the piston is longer in real life than it is in the renders. I did that for illustration purposes and ease of modelling, although I suppose I should have made them accurate. The lengths of rod and dimensions of the piston need to be worked out so that the stroke of the piston is 1/4 of the inner diameter of the barrel, so it doesn't need to move far. Although mine moves more like 1/2 of the inner diameter of the barrel. It doesn't hurt for it to open more rather than less, but not so far that the piston starts to retreat into the nipple.

See http://www.spudfiles.com/spud_wiki/inde ... mon_traits

[Juggernaut12121]

Nice to know, thanks!