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semi-auto combustion #3

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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semi-auto combustion #3

Unread postAuthor: auxiliary » Sun Nov 02, 2008 7:44 pm

(look at the diagram first)

So here's how it works... The spark in the combustion chamber ignites the propane. The BB shoots out of the barrel and the piston is pushed back. The piston maintains enough momentum to go past vent holes in the side of the copper pipe. The piston returns with enough force to push back a spool valve that lets in propane. Then the spool valve pushes back against the weaker spring loaded piston to load more propane, yet keeping it from spewing into the chamber. The gun is charged...

The trigger is pulled and the barrel moves back, to chamber a round (pushed by the brass tube). Once the round is pushed into the barrel, the trigger continues back until it pushes the button of the igniter and makes the spark. The cycle repeats...

My main worries about this design are that the aluminum heat shield behind the epoxy won't protect the epoxy from the heat build-up and that the piston won't go past the vent holes very far in order to vent properly.

To solve the heat build-up problem, a tank filled with water and with rubber couplers can be host clamped down onto the copper pipe.

Comments, questions, and criticisms are welcome:
(100% zoom makes for the clearest picture)
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Unread postAuthor: Hubb » Sun Nov 02, 2008 11:42 pm

Maybe one step closer? But, right away, I see a few problems besides the ones already mentioned.

First off, assuming the tension on the spring piston is just right, there will be an equal amount of force pushing on said piston as there is pushing on the projectile. If the piston reaches the vent holes before the projectile leaves the barrel (which is likely if the C:B ratio is not exact), then the projectile will exit the barrel from it's own momentum (meaning a crappy shot).
Proposed solution: I have no idea.

The next problem I see is in the metering system. If this design does work as intended, then the metering valve will be open very briefly. This may not allow for the correct amount of fuel to enter the chamber.
Proposed solution: Make the meter as small as possible.

Lastly is venting. As is, it would exhaust. But what about regaining fresh air? On all of the combustion launchers I've seen and made, it takes a little more than "very briefly" to vent a chamber and reintroduce fresh air.
Proposed solution: Introduce a secondary air supply, such as compressed air.

Oh, and when the launcher does exhaust, where will the hot gasses go? Shoulder mounted = head and shoulder burns, hip mounted = well, that's just an all-around bad place for hot gases to be.

Have you seen how the Tippmann C3 works? This idea reminds me of that.
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 03, 2008 2:56 am

Save your images as jpegs or gifs and upload them directly, they'll be more accesibly and you're more likely to get feedback ;)
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Unread postAuthor: psycix » Mon Nov 03, 2008 6:17 pm

And there's topic no3.... :lol:

Well, atleast you arent venting off fresh propane anymore!

Clever fueling system i have to say.
Only flaw: I think that the piston will return back to its original position within half a second or shorter. It does not have enough time to vent and refresh.
Its not only about getting the burned gases out, but its mostly about getting oxygen rich air IN.

As hubb said: you could go with compressed air. But if you want to do it good: grab pure oxygen.

@hubb:
If the piston is heavier then the projectile, then the projectile will already have left the barrel as soon as the piston reaches the vent holes.
This ofcourse only works if the piston travel is long enough, the barrel short enough, the piston heavy enough and the projectile light enough.


@ aux
(btw, you MAY post design no 4 in this same thread to prevent a whole row of your topics, you can also rename the title by editing your starting post and make an update)
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Unread postAuthor: auxiliary » Mon Nov 03, 2008 6:25 pm

hubb017: Thanks for the feedback, I keep on spitting out these weird designs, but I think that inevitably I am going to have to use a system that uses some sort of compressed air to allow for proper venting (at the very least a fan). What makes me think that I could pull it off using a fan is this diagram of a combustion nail gun that JSR showed a short while back: http://static.howstuffworks.com/flash/nail-gun-combustion.swf

Also I need to improve on this blow back concept or just lose it. My goal is to harness as much power from the combustion as possible in order to make the trigger pull actuate as little things as possible.

Do you guys think that a blow forward loading design like clide's will work? I don't think so because the momentary burst of power provided by combustion will not allow the hot gases to push a mechanism forward and maintain enough pressure to rush air down a barrel to propel a BB.

JSR: Thanks, but how do I save a bmp file as jpg? Do I just retype it with jpg instead?

psycix: Ya, I think I will do what you suggested just as long as the time span isn't to long when I posted the last design.
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Unread postAuthor: jackssmirkingrevenge » Tue Nov 04, 2008 12:21 am

auxiliary wrote:JSR: Thanks, but how do I save a bmp file as jpg? Do I just retype it with jpg instead?


MS paint should give you the option in the "Save as type" field.
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Unread postAuthor: ALIHISGREAT » Tue Nov 04, 2008 2:44 am

its a good concept for the propane metering, and maybe you should have the same system but with two inlets, one for propane and one for compressed air... but then there is the problem of piston movement which i can't envisage being very much at all.
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Unread postAuthor: psycix » Tue Nov 04, 2008 2:08 pm

I would just drop the whole piston thing.
Another bad thing of it is that when the piston moves back, the chamber expands and thus you lose pressure.
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Unread postAuthor: Ragnarok » Tue Nov 04, 2008 2:52 pm

psycix wrote:Another bad thing of it is that when the piston moves back, the chamber expands and thus you lose pressure.

This can be compensated for with a heavy piston. This will slow the piston recoil and cycling, which might make the idea more feasible.
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Unread postAuthor: auxiliary » Tue Nov 04, 2008 8:31 pm

I believe that the only way that I get the venting/air input thing to work is to use a motorized piston vent or a compressed air/ pure oxygen injection system, both of which I do not intend on using. The very least I could get away with is a fan vent/drawing in air, but the rate of fire would be hindered so greatly that a pump action will be actually faster. And that brings be to my new design. I feel like taking the relatively easy route and making a pump action very similar to the Tippmann C3, but just adapting the mechanisms for ease of construction. Some ideas I have come up with such as an impact spool valve injection system will be implemented.

There is one thing that still gives me hope though, and that is the Tippmann stake pounder: [youtube]http://www.youtube.com/watch?v=j2Hiojx_XPA[/youtube]

I do not know if it uses combustion or if it just uses the pressure of the propane... It is probably combustion because it can drive hundreds of posts with one 14oz. propane canister and has 700lbs of impact force.

I sure you have all seen this diagram of the C3, but here it is if you haven't:Image

EDIT: I found some info on the "propane hammer" http://propanehammer.com/marketing_flyer.pdf

Here's an instructional video of it:
[youtube]http://www.youtube.com/watch?v=MhJxnHdh650[/youtube]
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Unread postAuthor: clide » Sun Nov 09, 2008 5:05 pm

Based on that instructional video and some transparent models on their site it looks like that propane hammer has nearly the same mechanism as the C3. The only difference is it uses the inertia of that ring to perform the pumping action rather than manually doing it.
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Unread postAuthor: auxiliary » Mon Nov 10, 2008 7:39 pm

That is exactly how it works, I couldn't have figured that out without the help of this brochure I got from emailing Tippmann...
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