"Shut-Off Piston" to save air for (semi)auto

[psycix]

I had the idea of simply pilotting a QEV gun with a slide check valve but the major problem is the air usage. Actually, in ALL full or semi auto guns, there just has to be something to stop the chamber from exhausting all of its air.

After some thinking, I came up with the shut off piston which shuts off the chamber at a specific time. Its all explained in the pic.
Sit back, read it, and tell me what you think.

[Hubb]

Now, time to put theory into practice. Unfortunately, I see this purging the entire chamber if the piston slides all the way to the rear. This should be fixed, with the piston stop you mentioned, however.

[daberno123]

Could this also improve performance by shock heating the air in front of the piston? Kinda like in a commercial springer, only with an air spring.

I also don't see how this saves air. You fill the chamber all the way, then fire and empty the air in front of the piston, then you still have to refill that volume again. The air that is put behind the piston doesn't help fill the volume in front of the piston.

If the area in front of the piston is about the volume of your average gun and them you also have a massive volume behind the piston, how does it save air to fill your normal reservoir again per shot plus a massive air spring?

Maybe I'm misinterpreting your design, but it just seems like its a normal gun with an massive air spring in the chamber. Please clarify if I'm missing something, or got something wrong.

[sputnick]

I think you could accieve the same result by having a smaller ait chamber, calculated for perfect air usage, and have a small hose bleed high pressures from your "bigass chamber extension"...

Seems like it would make more sense to me...

[Ragnarok]

This idea came up on UKSGC twice.
Fortunately, I noted the problem with it before anyone attempted to implement it.

Consider the chamber "shuttle". What kinds of speeds and energies do you think it's going to build up? Hint - they're going to be pretty large, and that means it will easily damage the chamber. If not the first time, perhaps the second, or third... but it will happen.

Sorry, it's just not safe or workable to do something like this.
If you want to pull back efficiency, there are ways to do it, but this is not one of them.

EDIT: I believe there's a type of watergun that does similar things (perhaps called a Precharger?) - however, water guns don't empty their chambers in mere milliseconds.

[sputnick]

It would be possible though, provided he put in a substantial bumper, don't you think? the design is not entirely unfeasible... just somewhat unnecessary.

[Ragnarok]

It would be possible though, provided he put in a substantial bumper, don't you think?

No, I don't think...

... that came out wrong. :tongue3:
I do think, but because of that, I know this is not a sensible idea.

To stop it tearing the chamber end off, it would slow the shuttle over a pretty long distance, and I still wouldn't want to put any faith in it.

I have taken projectiles from half the speed of sound to zero (or near it) in the space of a few inches in the past without damage to anything, but the same methods cannot be applied here.
I used adiabatic heating in one case, using it to generate high pressures ahead of the projectile - which requires an air cushion, which is clearly not present here.
The other case used magnets to form inductive currents in an conductive projectile, slowing it against the magnetic fields. This would require a ridiculous magnet to stop a chamber shuttle.

Neither is feasible here. The other thing was that I wasn't still fighting an accelerative force as well.

A much better plan would be a system that re-pressurizes the pilot volume at the right moment, closing the main valve - rather than a modification that is practically guaranteed to smash apart the cannon.

[psycix]

I think you could accieve the same result by having a smaller ait chamber, calculated for perfect air usage

Any idea what kind of lousy performance that gives?

I also don't see how this saves air. You fill the chamber all the way, then fire and empty the air in front of the piston, then you still have to refill that volume again.

You indeed have to fill up that volume again. But if the piston is halfway in the chamber when the chamber is loaded, then that means you only have to fill up 50% of the chamber.
You could also put the initial (loaded, pressurized) piston position more to the front to chop off the air earlier.
All automatic guns need to cut off the air before the projectile leaves the barrel by either a hammer valve or very rapidly closing QEV. Well this is another way to do that.

but it just seems like its a normal gun with an massive air spring in the chamber

Normally, the air which actually goes into the barrel is also pushed by a massive air spring: the air which is still in the chamber at the end of the shot.
Now with a shutoff piston, you make sure that this air doesnt get dumped.

What kinds of speeds and energies do you think it's going to build up?

Say: the projectile leaves the barrel with 200m/s and the projectile is 16mm dia. and the chamber is 40mm.
pi*8^2*200 = pi*20^2*X
pi * (8^2) * 200 = 40212
40.212/(pi*20^2) = 40 m/s
If the piston would go fast enough to keep up with the volume of the projectile, it should be travelling 40m/s at the end.
Note that I am thinking steel chamber and machined aluminium piston here.

[Ragnarok]

If the piston would go fast enough to keep up with the volume of the projectile, it should be travelling 40m/s at the end.

So you're happy to have something doing 90 miles an hour smashing into your chamber end repeatedly?

You've also ignored the weight of the piston in your calculations. You'll get a "lag". The piston's momentum will mean that it will take away power from the projectile, and because of that, also actually hit the chamber end somewhat faster than your simple maths predicts.

Note that I am thinking steel chamber and machined aluminium piston here.

I still wouldn't want to trust it myself.
Bear in mind, _Fnord cracked steel pipe with the piston on his piston hybrid, which was nowhere near as fast or energetic as what you're talking about here.

Ask yourself this, if you firmly hit a piece of pipe several dozen times with a hammer, would you then be as willing to trust it as a piece you'd never hit?

[Brian the brain]

Despite of all the others have said, I think it can be done.And it will work very well!
The chamber piston will allow you to have the smaller main chamber to act asif it were a large one, without the downside of having to pump it back up each time.

A modded QEV will work more efficiently, but this idea does seem worth the effort, without the risks of forking up the QEV, wich keeps it 100% function.

[ALIHISGREAT]

its an interesting concept... but from the diagrams I'm unsure of how it would work in practice???

would you have a toolies style piston in the chamber that would be pushed forward by the pressure to seal the chamber?

[Hotwired]

I wonder if you'd read my signature before coming up with the design

I've been working on exactly and I mean exactly the same thing for a while.

A piston is a PITA and an air bladder as used in accumulators has its own problems.

However I realised not too long ago that I've already got a working example of the system I wanted and it requires NO more moving parts than in a standard cannon.

I reserve the right to keep secrets until I get round to making a decent post about it though

[psycix]

@Ragnarok
True. An air cushion bumper should work. The same way pneumatic cylinders are air cushioned.
Yes, the lag of the piston takes of a bit of the performance. But you CAN enlarge the chamber without actually having to fill up more so that could make up for it.
Also, If the piston would cut off a bit earlier, then it doesnt have to reach that maximum speed and you conserve even more air.

@ALITHISISGREAT
Toolies are very long rods right? Its just a piston in the shape shown, no rod. It indeed gets pressed forward, but there are different ways in how you could seal. This one seals by the o-rings.

@Hotwired
Cant wait!

[spencetron]

Didn't BTB talk about a QEV mod which serves the same purpose in some other thread?

edit: BTB already mentioned it a few posts up... whoops

[Technician1002]

In all fairness, I have been toying with it with the QDV. If the pilot area is "sealed" and during the firing of the cannon, some pressure bleeds into the pilot past the trigger rod, the bounce on the trapped air could push the piston back closed. I have had this happen with no modifications on the marshmallow cannon. The hard slam into the bumper produces a recoil that re-closes the valve prior to the entire discharge. Often I have upwards of 20 PSI remaining. It could be optimized. With a heavy projectile, the dwell time would be naturally longer as it would wait for the barrel pressure to drop.