combustion coxial

[jimmy101]

Willard:

I agree completely, different builders have different goals. Clever design is always of interest.

I just try to correct misconceptions that many people post on this site.

For example, you said "Any additional heat loss is to the interior fo the gun, thus minimizing the effect." Sorry, but that is not true. Where the heat is lost to is irrelevant. Lost is lost. The high heat capacity and high mass of the pvc will suck a tremendous amount of energy out of the low heat capacity, low mass, but high temperature, combustion gases.

Or, "Use a spark strip," spark strips are OK but not very effective. It takes two sparks on a strip to equal one spark on the central axis of the chamber. And, the sparks must be spaced roughly the chamber diameter apart for the multiple sparks to make much difference.

So, back to my original post, a coaxial design is interesting but it basicly sucks.

[lukemc]

could you make a coaxial hybrid? a regular piston valve but instead of droping the pressure on the pilot side increase the pressure on the chamber side (combustion) or have a simultaneous ignition pilot valve release

You can, you just need to use a compression spring instead of trying to use a pneumatically pressurized piston. The problem comes in trying to release the pilot at the same moment moment of combustion... get it wrong and you just made a bomb.

hence the burst disk on the diagram i showed

[spudthug]

ok so back to the being ok for combustion part...so u guys dont think this will blow up and kill me??


edit: i plan on using propane...

edit 2...ok since im bored ill go try it..if i die someone who lives in pa search for me and take my cannons...take care of them for me mmkay

[SpudBlaster15]

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[FeLeX]

Are you dead? You posted an hour ago.

[spudthug]

actually i posted like 15 mins ago...and it works but since i am not using metere propane how many seconds of propane should i use??? ill go try again with more propane and post back



omg this thing sucks...i used right guard i used propane and the potato comes out like "phwoop" and goes like 20 feet....what is wrong??

[FeLeX]

Wrong mix. Happend with my cannon once too, my friends almost peeeeed themselves. On my cannon I use 30-40 in warm weather and 50-60 when its cold. But it really depends on the size of your gun. Id say start at 15-20 seconds and then go up by 5 seconds. Find the sweet spot and rember it, at least tahts how I did it. Good luck!

[boilingleadbath]

...I wonder if putting lengthwise beads on the outside of the barrel would increase performance enough to be noticed. (think schelkin spiral)

Regardless, I think we have two contradicting factors with one's standard coaxial launcher:
1) Increased chamber volume
2) Increased heat loss and increased time of combustion - discounting going overboard on the ignition points.

As such, I'm not certain it's as strait forwards as "coaxials won't preform as well".

[FeLeX]

You mean decreased volume since the barrel is in the chamber.

[SpudBlaster15]

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[BC Pneumatics]

A step back? That is like saying a bolt action is a 'step back' form a semi auto. Both have there places. Performance isn't always everything.

The best place of ignition is as far from the barrel as possible, so the flamefront can accelerate, as far as anyone has ever been able to show. the whole 'put it in the middle some more is burning' has never been proven as effective. (Most people seem to think the opposite, it hurts performance.

[boilingleadbath]

I was assuming that the standard coaxial has the chamber extend for most of the length of the barrel, in which case you'd normally have a pretty high ratio.

Hmmm....
Perhaps one could use the small barrel-chamber gap to our advantage in the coaxial design, by ribbing the barrel? (again, think schelkin spiral)

This idea should be really f'ing scary to anyone who knew what a schelkin spiral is designed to do, and what 4' of schelkin spiral is capable of, but perhaps one could make a only 'slightly coaxial' chamber with, say, 6" of 'submerged' ribbed barrel?

With the obvious caveat that it is never, <i>ever</i>, to be used with enriched or sensitive fuel-air mixtures.

[frankrede]

The barrel, if used in used in a hybrid mixture would collapse on itself from the pressure. SCH-80 pvc actually is weaker when pressure is coming from the exterior.

[SpudBlaster15]

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[LucyInTheSky]

For the hybrid idea, you could just make the piolet abit larger and only fill it to about 5psi higher than the pressure of the mix. This way the piston will still be able to move back as it will just compress the air behind it until it is equal with the combustion at the front so it wont be a bomb, plus it is like a self setting bust disc.