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Semi auto design

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Semi auto design

Unread postAuthor: daccel » Sat Jan 17, 2009 12:38 am

It is basically a coaxial piston hybrid. The spring behind the piston is compressed air with a divider having a check valve and a small hole to make it act like a shock and (relatively) slowly return the piston to allow all the combusted gas to vent.

The one part I see going wrong is if there are still hot enough gases in the chamber when it starts refilling it could prematurely ignite.

So, should it work?
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Unread postAuthor: starman » Sat Jan 17, 2009 2:13 am

You really aren't describing your idea very well here...the drawing only partially gets your idea across. It would be helpful if you put together a graphic movie showing proposed step by step operation...perhaps a more detailed description of operation.

Also, storing premixed high pressure fuel/air is dangerous and should be avoided...use seperate fuel and compressed air vessels and inject them in the chamber individually when ready to fire.
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Unread postAuthor: daccel » Sat Jan 17, 2009 3:03 am

Well things like the regulator and breech I didn't detail because they aren't new, but I suppose I could be more descriptive.

The basic operation is like the other piston hybrids that have been built, but this one is coaxial with the piston sealing on the right side of the chamber via a rod to minimize dead space.

Premixed fuel/air is regulated down for consistency, to prevent dropping power each shot. Assuming the conclusion of this topic is correct, the fuel/air ratio should be consistent every shot.

The chamber is pressurized through a check valve (to prevent ignition of storage tank) and then through the hole in the piston.

The space on the left of the piston is pressurized with air to seal the piston and act as a bumper spring. This space is divided in half with a small hole and a check valve (drew it as a flap) in the divider.

When it is fired the piston will move to the left and the air will easily flow through this check valve. When the chamber pressure drops below the air spring pressure the piston will start to return but the check valve will close and because the air can't flow very fast through the small hole the piston will return more slowly allowing all of the burnt gasses to leave the barrel.

When the chamber pressure drops below the storage tank pressure the filling check valve will open and start filling the chamber again. And repeat.

Obviously the storage tank would have to be capable of withstanding an accidental ignition at whatever mix it was pressurized to, so this would only work for lower mixes or extremely strong parts. If the tank is within the bounds of it's material strength, I don't see how it's more dangerous than than the chamber itself. I'm thinking about this in terms of a small scale cannon, so while I said tank, the storage would be pipe not a thin walled air tank or something.

I don't want to take on injecting individually because it's much more challenging, and I think this is probably difficult enough.
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Unread postAuthor: microman171 » Sat Jan 17, 2009 3:13 am

Well you could use a safety pop off? You could use a burst dick in the fuel chamber so if it did ignite, the pop off would go instead of the tank.

By what I gather:

<simple>
You are making a hybrid.
Said Hybrid will have a piston.
Said Piston will have a rear air spring to buffer the piston and to return the piston.
The combustibles will be stored in a separate tank (with safety).
The Hybrid will self reset and refuel.
</simple>

Have I got it right?
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Unread postAuthor: CS » Sat Jan 17, 2009 3:27 am

So to reinforce what you have said the air spring pressure would be higher then the fuel/oxygen mix, but less then the pressure created by the combustion.

From what you have said can I assume:
-The piston is spring loaded to aid its return, if not what would propel the piston to the right?

I don't want to take on injecting individually because it's much more challenging, and I think this is probably difficult enough.


No I don't think so. As I have said before it can simply be two 3-way valve meters. One for the fuel, and one for air/oxygen.

Great thread, I like the type of discussion you are provoking.

[EDIT:]
As suggested it would help tons if you made a bullet-proof animation. They make these conceptual types of things so much easier.
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Unread postAuthor: microman171 » Sat Jan 17, 2009 4:22 am

I think the air is going to move it back, the pressure on the left will be higher than the right after combustion.
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Unread postAuthor: spudamine » Sat Jan 17, 2009 6:25 am

On the subject of a combined fuel air tank, I use one on my (still to be completed) hybrid. To the probable dismay of many posters here I will own up to using a pair of 3L soda bottles for this. Instead of going for something which is unlikely to blow if ignited but which would severely maim me if it did I've opted for something that would definitely blow up but wouldn't do any damage provided Ear (and maybe eye) protection is in use. As microman171 suggested it's like using a burst disk on the tank, only using the whole tank as the burst disk.
Using this system I fill up the tank with around a 5x mix and then put it through a reg, solenoid and check valve into the chamber to give multiple shots at whatever x is required. It's not blown up yet :)
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Unread postAuthor: microman171 » Sat Jan 17, 2009 7:20 am

Living life on the edge huh? :wink:
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Unread postAuthor: sputnick » Sat Jan 17, 2009 8:58 am

Just to say, I very much doubt this will ever get built, between the air shock and the little hole that you manage to drill with a 90 degree turn in it, it's looking kind of sketchy... Also, if this were to be a reality, you would need a much higher oxygen content in your mix, because the chamber would be full of the gasses from the previous fire. You would need a check valve to clear out the previous gasses so you don't get a buildup of combusted gasses making it harder to fire every time. I have yet to see a combustion or hybrid that could fire multiple times while never being opened.

Sorry, but I just don't think it is a feasible build.
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Unread postAuthor: Fnord » Sat Jan 17, 2009 11:05 am

You need a flashback arrestor behind the check valve, and I wouldn't put more than 90 psi in the soda bottles. Wrapping them in tape will add more of a safety factor.

Also, (though sputnik said it already) we haven't even seen a semi combustion yet... I'm not saying don't shoot for the moon, but starting out simple will make more advanced projects go easier.
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Unread postAuthor: daccel » Sat Jan 17, 2009 12:13 pm

Have to run so I don't have time to discuss the points you guys brought up at the moment but here's a quick animation.

[youtube]http://www.youtube.com/watch?v=oWxPTUpiZsg[/youtube]
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Unread postAuthor: sputnick » Sat Jan 17, 2009 12:15 pm

Again, the gasses would still be there, and if your gas and air are pressurised, THAT would blow back your piston.

fail.
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Unread postAuthor: CS » Sat Jan 17, 2009 12:31 pm

Come on sputnick... BTW the mentioned 90 degree would be fairly easy to pull off.

Nice animation helped a lot. For some reason I totally missed one of the equalization holes.

you would need a much higher oxygen content in your mix, because the chamber would be full of the gasses from the previous fire.


Well we know air contains around 21 percent oxygen, so that would be a number to work off. Ultimately I think you would have to tinker with that amount, but should get you shooting in the right direction.
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Unread postAuthor: sputnick » Sat Jan 17, 2009 3:02 pm

Sorry pimpmann, but this is just far to optimistic, best case scenario, it gets mostly built, but then either he realises that with an airtight piton that DOES NOT equalise, the air being pumped in would blow it back and release into the barrel, so the barrel would be acting as the vent during fueling. Next, it would take MUCH tinkering to find a fuel mixture that is neutral enough to work for the first, second and so on shots, but since the contents are constantly changing, I don't know if that is even possible.

I'm sorry, but this is just alot of time and effort for nothing, if you wanted semi auto, why not just have a simple tank, with air and gas inputs, then a blow forward breach? and even that is not certain to work.
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Unread postAuthor: daccel » Sat Jan 17, 2009 4:28 pm

Yeah, first animation, I didn't realize they were so simple to make.. very useful.

Ok, I will try to reply to all:

microman - That's pretty much it, simple right :P. Good point on the safety release, which would allow use of a larger storage chamber.

pimpmann - Correct, the air spring would have a higher pressure than the fuel/air but lower than combustion, eg. 500/100/800psi respectively (off the top of my head).

It may be just as easy to do what you're suggesting with the 3 way valves, I'd have to look up those posts to better understand, but part of why I was looking at pre-mixed was in trying to make it as compact as possible. So by having the valving and gauges separate in a fueling station you are removing those components from the cannon.

spudamine - If that's how you like to get your thrills, so be it.. haha. But if you had a burst disk in a stronger storage tank, you could control the direction of failure which I'm sure your hands and face would appreciate.

sputnick - Thanks for the vote of confidence. I will assume your tone is well intentioned in trying to save me the frustration of not being able to make this work, rather than something else.

The components would take some care to build, but making a well sealing piston with o-rings is not beyond reach for spudders. Nor is drilling two holes and having them meet - though this actually isn't necessary. As you can see in the animation I moved the inlet to the chamber wall. Having it go through the piston was a remnant of an idea I had earlier.

The point of the shock/strut (not sure on correct term?) built into the air spring is that it slows the return of the piston to it's sealed position. This allows the pressure to return to 0 in the chamber. There would still be combusted gases left at atmospheric pressure, but these wouldn't have to be vented as in a combustion, because you are adding the correct ratio of oxygen/fuel when it refills. The combusted gases would simply act as a buffer gas like the nitrogen in the air does.

fnord - Wouldn't the check valve act as a flashback arrestor? To me, a hybrid seems simpler because you don't need the step of venting the chamber.

sputnick, again - I am well aware the piston does not equalize. The design is based upon this. The barrel would not vent during fueling because the pressure behind the piston is greater than the fueling pressure.
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