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hybrid cartridge protoype without burst disk

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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Unread postAuthor: jackssmirkingrevenge » Sun Sep 05, 2010 2:46 am

Ragnarok wrote:I imagine Jack is smart enough to be in possession of ear protection. Problem solved.


Jack has done some pretty stupid things in the past, let's not take his commitment to PPE for granted :roll:

What I'd like to do is fill my shock pump pipe with water to get an accurate figure for the volume, as so far it's been guestimated and probably comntributing to the lack of consistency.

edit: pump dead volume came to 1.75mL, I had calculated around 2 so that was pretty close. Using the water method cartridge volume is 14mL. Everything's drying out in the sun at the moment, we'll see if accurate metering gives better results.
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Unread postAuthor: Insomniac » Sun Sep 05, 2010 6:37 am

Judging by your complaints about ringing ears after your latest phail... I'll take that to mean that you are not smart enough to be in possession of ear protection... :D

I did the same thing testing a small amount of a certain magic dust with a hammer a while ago... I swear my right ear never did quite go back to normal :?
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Unread postAuthor: jackssmirkingrevenge » Sun Sep 12, 2010 2:44 am

Helped by the data gathered from playing with my mini-hybrid, new prototype in the works :)

One of the issues I was having with the cartridge is the seal not having enough grip to hold the projectile past 45 psi, which is only good enough for a 4x mix. Now that I proved that piezo ignition for much higher mixes is possible, I need better than that, so I plan on fitting the seal in a 6mm tube in a way that it will already be under compression before the projectile is inserted, meaning it should hold to higher pressures.

Prototype curing, as always watch this space ;)
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Unread postAuthor: ilovefire » Sun Sep 12, 2010 4:08 am

what is going to stop the spark from jumping from the schrader to the chamber on the out side? and i have tried something similar but when i pumped it up the projectile held for a bit but would slowly move forward until it would pop out, what have you got to stop that from happening?
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Unread postAuthor: jackssmirkingrevenge » Sun Sep 12, 2010 5:26 am

ilovefire wrote:what is going to stop the spark from jumping from the schrader to the chamber on the out side?


Simple, as you can see in the picture the spark gap is really tiny, so it is the path of least resistance for the spark to arc through, tested it and it works.

and i have tried something similar but when i pumped it up the projectile held for a bit but would slowly move forward until it would pop out, what have you got to stop that from happening?


This happens to me too at anything above 45 psi, so what I did was force the seal into a tighter tube in order to place it under compression and able to hold the projectile to higher pressures, we'll see how well it works.
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Unread postAuthor: POLAND_SPUD » Sun Sep 12, 2010 5:47 am

This happens to me too at anything above 45 psi, so what I did was force the seal into a tighter tube in order to place it under compression and able to hold the projectile to higher pressures, we'll see how well it works
try rubber washers... I have tested tubing too but it leaked, while washers did not
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Unread postAuthor: jackssmirkingrevenge » Sun Sep 12, 2010 6:24 am

POLAND_SPUD wrote:try rubber washers... I have tested tubing too but it leaked, while washers did not


Wouldn't that need threaded fittings though?
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Unread postAuthor: inonickname » Sun Sep 12, 2010 7:56 am

jackssmirkingrevenge wrote:
POLAND_SPUD wrote:try rubber washers... I have tested tubing too but it leaked, while washers did not


Wouldn't that need threaded fittings though?


The plan...she is falling into place :lol:

But in all honesty, not much harder than that little pvc piece you use as a union on your hybrids, and I assume you can get more.
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Unread postAuthor: POLAND_SPUD » Sun Sep 12, 2010 9:00 am

yeap that's the plan :D
ohh come on, 1/8" or 1/4" fittings are not that large... personally I think it would be a better idea to build an all metal contruction... probably it owuld be the best idea to combine both threaded and soldered fittings

At least it would be safe at higher mixes
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Unread postAuthor: lozz08 » Sun Sep 12, 2010 9:04 am

I have had problems where projectiles have been gradually worked through at a lower pressure than they should using a rubber washer and threaded fittings alone, it is a major PITA.
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Unread postAuthor: POLAND_SPUD » Sun Sep 12, 2010 10:02 am

I've never had problems with rubber washers.. maybe you're using different kind of washers.. or you didn't apply compression at all ?

I used them with 6mm ammo... the centre hole was probably close to 4-5mm and I didn't have to apply a lot of compression - just 1/4 - 2/5 turn for 300 - 400 psi

1/2 turn increased release pressure to 500 psi
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Unread postAuthor: spudamine » Sun Sep 12, 2010 11:35 am

personally I think it would be a better idea to build an all metal contruction...

only problem there is you need something insulating somewhere to get a spark gap.

Simple, as you can see in the picture the spark gap is really tiny, so it is the path of least resistance for the spark to arc through, tested it and it works.


still if you're firing at 10x your spark gap has to be 10 times smaller than the outside gap to have a lower resistance so you're probably getting close to the limit :(
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Unread postAuthor: jackssmirkingrevenge » Sun Sep 12, 2010 12:56 pm

POLAND_SPUD wrote:ohh come on, 1/8" or 1/4" fittings are not that large... personally I think it would be a better idea to build an all metal contruction... probably it owuld be the best idea to combine both threaded and soldered fittings


I feel quite safe with current construction, and the whole point of having "no burst disk" seals is to avoid threaded fittings.

if you're firing at 10x your spark gap has to be 10 times smaller than the outside gap to have a lower resistance so you're probably getting close to the limit


Fair point, we'll see how well it works in practice

edit: sadly, your prediction was correct - while it was ok at atmospheric pressure, when pumped to 75 psi for a 6x mix, the spark arced externally to the schrader :? so it seems my visions of a rifle sized cartridge are a little optimistic if schraders are to be used.

On the positive side, when set inside a 6mm brass tube instead of being cast directly into the epoxy, the seal I was using is happy to hold a 4.5mm at at least 100 psi, meaning that it should be good for an 8x mix.

Back to the drawing board...

edit 2: I took this cartridge, cut it down to 2" (now that I have relaible ignition and a good seal, I can afford to lose some chamber volume) and fitted a seal for a 0.177"BB, testing this evening and if all goes well I'll make a breech/barrel setup to evaluate performance.
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Unread postAuthor: jackssmirkingrevenge » Tue Sep 14, 2010 2:39 pm

[youtube]http://www.youtube.com/watch?v=bydZoR1LX0Q[/youtube]

letterbocks.

I'm beginning to think this whole "friction breech" idea is more trouble than it's worth, maybe I should just follow conventional wisdom and find myself some suitably sized threaded fittings...
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Unread postAuthor: Insomniac » Tue Sep 14, 2010 6:11 pm

Hey, that's a nice little rocket motor you've got there! :D
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