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Two-part cartridge style hybrid??

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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Two-part cartridge style hybrid??

Unread postAuthor: dongfang » Thu May 24, 2007 10:53 am

Hi,

Just an idea. Has anyone tried it? Any thoughts?

It is a concept for a union, disk and valve free hybrid gun. The gas mixture (O_2 and butane/propane at 2-3-4 bar) is contained in a disposable PET soda pop bottle. The bottle is loaded into a chamber that fits rather tightly around it (needs not be exact, just not too much dead space around). The bottom of the bottle points towards the projectile, and the cap, with a Schrader valve through it, points back.

When ignited (in some way), the gas in the bottle explodes, blows the bottom out of the bottle and the well-known acceleration of potatoes takes place. The bottle could be charged before or after loading, depending on skill/stupidity of the gunner.

The breech (in red on drawing) is specially turned out, or maybe bolted together from several plugs. Part of the threads in the breech and the chamber could be cut away (since the breech is so long), allowing the breech to be plugged right into the chamber and locked with 1/8 turn. There might or might not be a hole through it for the Schrader valve.

The potato could be loaded using a loader tool - basically a cylinder with outer diameter close to the inner diameter of the chamber, a cavity in the front holding the spud, and a stick through it to push the spud into the barrel.

Cool things abt it:
- After ironing out reliability etc. problems, particularaly in ignition, one could prepare a number of cartridges (bottles precharged with gas mixture) and some spuds. It should be possible to fire this in very rapid succession - breech out, (clean out remainders of previous bottle - maybe some kind of ejector tool could help), load spud, load cartridge, secure breech, fire. No venting needed.
- Very little apparatus on the gun. Charges are loaded into the gun, not prepared there (in an initial stage, one could load the bottles empty and fill them up when in the chamber. And even use a simpler breech w/o threads)
- Generally a cool gadget ;)

On the down side:
- Chamber and breech hard to machine. Threads particularly difficult.
- Charge size limited by bottles available (1.5 litres or so - not much. But a 4 bar pure fuel/oxy charge much bigger than than would be too scary anyway.)
- Ignition problem needs resolution: Maybe one of you with one of those 15 gigavolt stun guns could check whether it can strike throgh a PET bottle? Or drop a paper clip into the bottle, and microwave it for a few millisecs (bye bye portability).

Has it already been done?
Soren
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Unread postAuthor: ShowNoMercy » Thu May 24, 2007 11:08 am

That is a nice idea, however you may run into problems with the ATF, it is a preloaded cartidge sort of and they may think its a gun. Anyway, how do you plan on igniting the cartidge? I was thinking perhaps a pair of nails, your ignitors, could be slammed into the cartidge and then ignite the mix. Other than that nice idea. Itd be nice to see in working sense. Also it doesnt need to be a hybrid, it could work with propane, or even a puenmatic.
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Unread postAuthor: jimmy101 » Thu May 24, 2007 12:08 pm

Interesting idea.

If you restrict the design to 1x mixes then you wouldn't need the schrader valve in the bottle cap. (You can fuel the round by water displacement, no need for any valve at all.) Saves some money and complexity. Plus, the schraders might not last very long inside the combustion chamber. And, without the scrader sticking out of the bottle cap the breech design would be a lot simpler.

A very simple way to do it would be to just fill a cleanout plug with bondo then push a bottle+lid (or bottle+lid+schrader) into it to set the shape.

Didn't some one post on a ~1/4 turn breech plug design made from standard cleanouts? (Latke at spudtech?)

Getting a spark into the bottle might be tricky. I doubt that even a high voltage stun gun will spark through the bottle itself.

Perhaps use roofing nails (the ones with a plastic washer) that are heated then forced through the side of the bottle. A big glob of suitable expoxy (or liquid nails or ...) on the bottom of the washer to seal the hole and keep the nail from coming back out. On the inside of the breech you'll need two more of the nails, this time perhaps pointing out of the chamber. Same deal, big glob of glue under the washers to hold and seal. The nails in the chamber need to align pretty well with the nails in the round. Use a stun gun to get a spark to jump from a chamber nail to an ammo nail, to the other ammo nail (inside the bottle) then back to the other chamber nail. (Might have to bend one of the ammo nails so it is close enough to the other.)

Or, if you omit the scrhader, put the nails in the bottle cap. Might even be able to make the electrical contacts using a pair of wires with aligator clips that live inside the chamber. (Kind of like the wiring of an Estes rocket.)

Could you set the chamber dimensions such that a spud could be taped (glued, nailed?) to the top of the bottle so that the spud is loaded when the bottle is loaded?
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Unread postAuthor: homedepotpro » Thu May 24, 2007 2:27 pm

stick the ignition in the cap with the shrader, then all you have to replace is the bottle each time you shoot
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Unread postAuthor: Fnord » Thu May 24, 2007 2:52 pm

Personally, I would put the schader valve at the other end. If you have the cap facing the barrel, it will act like a burst disk when the mixture ignites. Just take out the bottle, change the cap and refill.

The shrader valve should also be used to carry the current to the spark gap. That way you will only need to make one other hole in the bottle, Unless you can use high-frequency, high voltage(tesla coil), in which case I believe you will only need a single lead.

You will probably run into heat build-up problems, as plastic bottles dont take much to melt.
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Unread postAuthor: dongfang » Thu May 24, 2007 4:06 pm

Hi,

Thanks for all the comments.

I just got an idea for the ignition: I think Heinrich Hertz's first wireless experiment was a demonstration that an arc could be made to jump across the ends of an open loop of wire, several meters away from the transmitter. It should be possible to repeat that experiment, with a receiver loop inside the bottle.
Otherwise, one needs another electrode into the bottle apart from the Schrader. Maybe just a thin wire twisted around the cap's thread. The breech could then be made to make contact w the wire and the Schrader. OK it does not sound all that reliable...

Nice idea also to turn the bottle around. Putting a valve through the bottom of each bottle sounds boring, though ;)

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Soren
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Unread postAuthor: jackssmirkingrevenge » Thu May 24, 2007 5:12 pm

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Unread postAuthor: jimmy101 » Fri May 25, 2007 12:25 pm

I just got an idea for the ignition: I think Heinrich Hertz's first wireless experiment was a demonstration that an arc could be made to jump across the ends of an open loop of wire, several meters away from the transmitter.


Uh, I don't think you are going to be able to reproduce that experiment with stuff you have lying out in the garage or can reasonably expect to get at the local hardware store. Do you have any idea how much energy the transmitter would have to put out to get a spark across the antenna? If you walk up to your local neighborhood radio transmitter with a loop of wire, the exactly right length and shape, you are not going to get a spark. That tranmsitter is putting out what, hundreds of kilowatts of power?

=+=+=+=+=

I've been thinking about the 1x mixtures.... I don't think a 1x mixture will work, the pressure won't be high enough to cause the bottle to fail (too bad since it would be a good place to start and avoids the need for the schrader). Or, the bottle will fail sometimes but not other times. You might have to use plastic bottles from non-carbonated beverages, they are not necisarily pressure rated like soda bottles are.
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Unread postAuthor: jackssmirkingrevenge » Fri May 25, 2007 12:54 pm

jimmy101 wrote:You might have to use plastic bottles from non-carbonated beverages, they are not necisarily pressure rated like soda bottles are.


I doubt that would be practical - I used to test PET bottles made for mineral water at my old job, 2 litre bottles would typically fail at 80 psi, a little more for the 0.5 litre bottles.

Maybe you could cut a hole in the cap and put a burst disk between the cap and the thread.
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Unread postAuthor: jimmy101 » Fri May 25, 2007 5:02 pm

Jackssmirk..: Cool, someone with experience with PET bottles. But, why would a failure pressure of 80 PSI be a problem? The OP is for up to ~60 PSI (4x) charge pressure. I think the OP wants the bottle to fail when it is ignited.

I was more worried about using a pressure rated bottle that won't fail, hence the gun won't work, with a 1x mixture, and perhaps not even with a 2x mixture. Pressure rated PET bottles will typically take what, at least 200 PSI before failing? The occasional bottle, especially if it is contained in a chamber, might be hable to handle 300 PSI without failing.
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Unread postAuthor: jackssmirkingrevenge » Sat May 26, 2007 12:44 am

I meant they wouldn't fail with a 1x mixture, it certainly wouldn't take a hybrid mix.

My experience is with mineral water bottles and bottles for iced tea. The latter was a non carbonated beverage but the headspace between the liquid level and the cap was filled with nitrogen, both as a preservative and to give the bottle rigidity for stacking reasons, meaning it has to take an internal pressure of at least 30 psi - typically these bottles fail at around 300 psi.

Using one of these would be the highlight of my day, seeing the bottles swell like a balloon then fail dramatically :D Sometime's we'd draw smileys on the bottle and watch them expand and get progressively more worried before exploding :) happy days hehe
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Unread postAuthor: dongfang » Mon May 28, 2007 5:22 pm

Hi

Jimmy, you could be right about that wireless spark. Hmm I just junked some nice PA stages for 70 centimeter waves recently. Didnt think about trying out the spark experiment. Anyway the field just beneath a big transmitter isn't all that strong -- the antenna is designed to send very little power in that direction.

About bottles (not) failing: I think I have seen empirical data (from someone using them as pneumatic chambers) saying such bottles are reliable at 8 bar. The peak pressure with a 1 bar propane oxy charge is (gaseq) 18 bar, and I am rather sure they will fail there.

Anyway I noticed that the cap can be screwed on so tightly that is jumps the threads, and that the threads seem to get weakened by that. Maybe doing that, reorienting the bottle to cap forward, and letting it pop off the cap is an idea.

Worst problem is still how to ignite reliably... ok 2 tiny pinholes in the bottom of the bottle would do the trick. But placing and connecting needles is neither very fast nor reliable...

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Unread postAuthor: jackssmirkingrevenge » Mon May 28, 2007 6:55 pm

There were some interesting suggestions here
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