SpudFiles

Most of my previous [Nerf] projects to date have been powered by HPA or CO2. That's all well and good, but expensive and difficult to use at times. I decided to develop a Nerf gun with the "always ready" convenience of CO2 without the cost. I quickly realized propane was the solution and got working on my first combustion gun in some 5 years.

Here's what I've got so far:






That cap on the left in the top picture consists of the metering and ignition system. Propane is supplied to a 3-way N.C. valve specially selected for the fact that it exhausts air around its button. The gun is pump action; on the backward pump stroke, the piston (to be explained later) impacts the button causing propane to fill into a small, premeasured reservoir. On the forward stroke, air is released from the reservoir around the button and into the gun's combustion chamber. This system is essentially the same as the double-ball-valve volumetric metering system except that the whole process is performed with the push and release of a single button.

Attached to the cap is a small check valve which allows fresh air to enter the combustion chamber when vacuum is produced by the piston's forward stroke. Also attached is a spark screw; the NC valve is used as the ground instead of a second screw.

The piston contains a check valve to release gas both during the backward pump stroke and during firing, yet remains closed to prevent diffusion of propane while the gun isn't being used. This check valve is actually a safety pop-off valve with an extremely light spring. It's mounted inline with the barrel and offers very high flow during combustion.

The pieces will be mounted on a simple wood receiver such that the combustion chamber and barrel will be in fixed positions while the piston will be free to move. In its forward position, the piston will plug into the barrel. In its backward position, the barrel will be exposed for breech loading. In a sense it could be considered "bolt action".

As usual my explanation probably sucked so take a look at a diagram of the Tippmann C3 for a better understanding of my gun (they're quite similar):




Special thanks to JSR for the advice he provided me.

It lives! If you make this work combustion repeaters will be within reach of the masses, looking forward to seeing it in action

Oh I was thinking of a similar design just the other day. It was for a semi auto version though (it uses the pressure of the butane to cycle, of course other pressured gases would work, maybe even better).


This isn't really done to any scale and the parts could be laid out in a much better way.


Sadly I won't be able to test this until I move back home, and I'm not sure when that will be.

It seems you're up to a good start there.

I look forward to seeing how this goes, pump combustions interest me, but of the few I've seen, most aren't explained well.

Don't post this of NerfHaven though, you''l probably get banned for posting a combustion.

(I haven't had a bad experience there, and I'm in favour of very harsh moderation. But combustions are looked at as black majiks that will blind everyone on the field)

hi here a diagram of a working micro repeater. add magazine and push button fuel so the pump-action is easy to archive.

easy to build
by me it works with 8mm barrel and syringe fueling power is bad by the small barrel diameter but in 1inch size it will work fine.

i hope you get the diagram
green is epoxy and grey are pvc with two endcaps

I temporarily pieced together the combustion chamber and did some tests. Even with the smallest propane reservoir I can possibly make, the meter injected too much propane for proper combustion. It took one pump plus a partial pump to push out some propane and draw in clean air. I'm not concerned though since I was running unregged propane. If I regulate it I'll be able to find the right pressure so just one pump does the trick.

The gun is a bit weak; it only gets about 30 feet with Nerf darts. The pop-off valve will need further modification to give me the results I want, but I must say I'm still pleased. The test shots were an underrepresentation of what the gun can do since I was using a short, loose barrel. I'm confident that with some fine tuning, there is a lot of potential here. More updates to come...


jean- very nice layout. I like it a lot. I may do a similar variant sometime.


Edit 6/8: After giving some thought to jean's design, I realized how much could be gained from doing away with the pop-off valve. I may just take it out and load the projectile into the piston rather than the barrel. This way I can get full flow-through, prevent diffusion, and the check valve still serves a purpose.

Firing Video: Click

When regulated to ~50psi, one pump is all that's needed to fire the dart just like I wanted. The piston is now completely flow-through. You just push it back to clear the waste gasses, load the dart, push back slightly further to inject the propane, then pull it forward. It's stupid powerful compared to before. I just have to tidy up the components and mount them on a simple wood frame.


The gun is noticeably weaker if you let it sit there for a few minutes after loading and filling. I was sure I sealed it up well. Any thoughts?

mh i think it is to much fuel
if the air comes in- the mix near the spark is right and if it sit down the fuel mix over time with the air and the combustion is weak.
try it with less fuel and let ist sit down for a minute or so.
first is what i mean with the fuel
second pic for a fuelmeter to mixing fine
jean

Interesting thoughts, I was wondering if something like that was taking place. The problem seems to have gone away after I lubricated the combustion chamber so the o-ring makes a better seal. There might have been a leak after all.

My DCV metering system is also very consistent; I'll keep that the way it is.

Here is the new piston: