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Design help with metal hybrid pweease...

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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Design help with metal hybrid pweease...

Unread postAuthor: Icesoda » Thu Aug 30, 2007 10:32 am

I'm slowly(Yet steadily) constructing a smallish handheld hybrid and I need some help and and a few pointers to keep it going...
The chamber is a hollow steel axle from a big boat cart, looks like it's rolled cause I can see something that might be a seam on the inside, but it's a little cruddy and hard to see. Anyway, the wall thickness is 5 mm so I don't expect this to be a limiting factor.

Chamber:
Length 18,5 cm
Inner radius 1,9 cm
Volume 209 cc

I'm keeping square 8mm thick plexiglass endcaps in place along with rubber gaskets by four 23 cm M6 screws running along the entire length of the chamber. The same screws are supposed to secure a third plexiglass piece -with barrel- and between these two pieces there's supposed to be a flange out of washers. Not all is completely planned yet... I'll have a cad drawing up as soon as that program is.. uhh, delivered to my computer. :roll:
I think I'm gonna need some help and hints to ever get it done, it's been in the lazy planning and thinking stage for like a year or something... :oops:

1: I understand the plexiglass endcaps will severely limit the gun compared to what the chamber should be capable of, but about how much? I really have no idea what to expect of the plexi, will it withstand 4x(Or even 6-7x?) mixes or should i just stick with low mixes? I plan to someday make these endcaps out of steel instead, I just couldn't find any proper sheets, so I thought this might do for now.

2: I'm pretty sure threading into the chambers wall for ignition and fueling is better than threading the plexi, will this significantly weaken it? I doubt it but I'm rather safe than sorry.

3: Can I thread a schrader through the chamber wall(For air)? They are obviously not rated to pressures up in the hundreds of psi, but safety margins and the short exposure time might mean there's no problem at all. I really don't know.

4: If the ignition is hooked up so that the chamber is ground, I won't get shocked... Right? :P

I will most likely come up with more questions, and I hope anyone is willing to answer.
Thanks :)
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Last edited by Icesoda on Thu Aug 30, 2007 10:50 am, edited 1 time in total.

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Unread postAuthor: Hubb » Thu Aug 30, 2007 10:49 am

I would recommend against using the plexiglass in a hybrid, especially one that you plan on holding.
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Unread postAuthor: Fnord » Thu Aug 30, 2007 2:43 pm

1.Yeah, plexi is generally a bad idea in hybrids. It's more brittle than pvc, and doesn't really like having machine screws in it either.

2.You should be fine drilling into the chamber walls. 5mm doesn't sound like a lot but pipe that small can take quite a bit of punishment.

3.As long as the valve is closed when the gas is ignited, it will survive. If it is opened at the time by say, an air chuck, the gaskets inside the schrader will be blown out of place and it won't seal anymore.
Even if it does get damaged, you can always unscrew the center part of the valve with tweezers and replace it.

4.I generally like to run the spark circuit apart from the chamber, especially with stuff like a stungun/ignition coil. If you do it right there won't be much of a chance of getting shocked, even if you take it swimming (Don't try).
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Unread postAuthor: dongfang » Thu Aug 30, 2007 4:19 pm

Hi,

Plexiglass endcaps? COOL! But not for a handheld. If you have the time, by all means complete it, and videofilm the flash seen through it. At a safe distance, that is.

I have been looking a little for transparent PVC for a combustion chamber for a demonstrator of the Jet Gun (an 1 year old invention of mine - a gun that injects and burns fuel downstream of the source of compressed air (the valve), like a jet engine. It´s supposed to be a detonation-safe, burst disk free hybrid). It´s rather expensive.

Regards
Soren
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Unread postAuthor: frankrede » Thu Aug 30, 2007 6:14 pm

dongfang wrote:Hi,

Plexiglass endcaps? COOL! But not for a handheld. If you have the time, by all means complete it, and videofilm the flash seen through it. At a safe distance, that is.

I have been looking a little for transparent PVC for a combustion chamber for a demonstrator of the Jet Gun (an 1 year old invention of mine - a gun that injects and burns fuel downstream of the source of compressed air (the valve), like a jet engine. It´s supposed to be a detonation-safe, burst disk free hybrid). It´s rather expensive.

Regards
Soren


Thats amazing!
Wow, what size is it?
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Unread postAuthor: dongfang » Mon Sep 03, 2007 3:53 am

Hi,

I think I could get all of 40, 50, 63, 75 mm and maybe even larger.

It costs about 3 - 5 times the price of grey pipe, though.. The rating is either PN10 or PN16, I forgot to check.

Regards
Soren
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Unread postAuthor: mark.f » Mon Sep 03, 2007 8:10 am

How old is this axle? If it's got some impressive mileage on it, I'd maybe think a little harder on using it for a chamber. If you've ever driven behind a loaded trailer in a low-riding vehicle such as a sedan, you've seen the stresses on trailer axles at interstate speeds. Also, being that it's not rated for pressure, I'd think twice about it anyway.

You won't be shocked by holding the ground electrode, (the cannon), as long as you don't touch the positive electrode or wiring. You might want to look at an automotive spark plug and an old wireset from a place like U-Pullit. It would suffice, (current-wise), for single cannon ignition, and it would insulate the spark plugs and current from your ignition source better than anything else.

You should be good threaded holes into the chamber. Just think about where you thread them. Not along the seam and not in the middle of the chamber. If you're worried about the schrader valve, you can plumb in a small ball-valve between it and the chamber.
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