First Hybrid Plans

[ALIHISGREAT]

Well i have made a few pneumatics and a ghettoo combustion, but i figured, if i'm going to make a combustion, don't just make a normal one, make a hybrid

i've done a bit of reading around, looking at stuff, thinking, and hopefully not rushing in with a silly design making me look incopentant and noobish

so i plan to use either a 3" or 2" pipe nipple for the chamber (ideally 3" but the fittings are damn expensive!)
28mm or 35mm copper for the barrel
would cam locks work for a burst disc holder?
3x mix maybe 4 or 5x
not sure about ignition, but probably a stungun circuit from BCarms (or will a piezo do the job?)
3 spark gaps
coathanger pieces epoxyed into a pipe nipple with endcap on for ignition system....
and the two ball valves are for the fuel meter and the other for pumping it up.

and tips on the ignition source, spark gap sizes etc. would be welcome

[Carlman]

peizo will do just fine for the sub 10x mixes mate, there is a few fueling methods avilable in the discussion section and there is a sticky called hybrid fueling 101

[ALIHISGREAT]

edited main post: 3" nipple for chamber is actually mild steel not malleable iron so i presume its stronger..

thanks Carlman, piezos work for multiple sparkgaps right?

[starman]

You won't need to worry about multiple gaps so much in a hybrid. Getting one to fire consistantly will be challenge enough. I would therefore recommend a stun gun/circuit if you can swing it. Your gap will have to be narrow.

Camlocks can work great for burst disking. Be sure to use the metal ones and keep your mixes lowish...your goal of 3 - 5X should be about the max.

Good lick with your hybrid! Keep us posted on progress.

[DYI]

Low mix hybrids really don't pose a whole lot more design problems than combustions - the density of the gases is still low enough that you don't have to worry very much about heat transfer, achieving ignition with the usual high-voltage methods, or busting the seals on ball valves with high temperature gases.

Now, a piezo might work, but maybe not consistently. Some people have had good luck with them on hybrids though. For a spark gap (you really only need one in most hybrids), take 1/2 the distance that your ignition source can create an arc across, then divide it by the mix you're using. Assuming that all your insulation holds up, you should be able to achieve a reliable arc inside the chamber.

Why do you need two ball valves and two tees? It means more fittings, and more to go wrong (ball valves always seem to break and start leaking at some point). Just attach the air inlet to the meter instead, then you can shut off the whole thing at once (at 5x, you really don't need to worry about back-pressure from the oxidiser bursting the propane tank ).

Finally, let me make my obligatory recommendation that you use a manometric rather than volumetric meter, as you'll thank yourself later that you did. If you do, you could use the same meter for this little launcher, and any 3" bore 30x monsters that you may want to build a few years down the road...

[ALIHISGREAT]

^^ two tees looks cool 8)

and thaks for the tips guys

[SpudFarm]

yeah the volumetric meters are outdated now. i am so happy i did not use it on my hybrid!

one spark gap should be fine on 7x with the piezo

[ALIHISGREAT]

i need to search for manometric meters then!

ok search doesn't bring up anything...

and what about camera flash igniton?

[SpudFarm]

i does use camera flash capacitor dumped into a ignition coil on 11x mixes in my hybrid. 0.25mm gap..

i think carlman can teach you a bit from all the stuff i sent him on msn yesterday. just add him to your list and ask for the pictures/document i made to get him understand the setup

[ALIHISGREAT]

i saw a tutorial in the wiki on camera flash ignition that i skimed through and might read properly later.

just added carlman

oh and is it worth getting some 1000psi rated stainless steel ball valves at £10 for 1/2" or is that kind of overkill for 3-5x mixes? (although you can never overkill safety may have just answered my own question)

[SpudFarm]

no need to use anything other than a ordinary valve.

you can start to think about a heavy protection valve on mixes that is getting up to 20x-30x

[psycix]

Looks like you should know some extra things on ignition systems:
-In the diagram, the spark gaps are wired in series. This will fail. Only one gap will jump that way. Wire them in series.

-When using multiple gaps, you can add up the distance from them to get the total distance. If your piezo can jump about 3/4" consistently, it should be able to power 3 spark gaps which are maximum 3/4" together so 1/4" each.

-On hybrids, ignition things get nasty. Each time the pre ignition pressure doubles, the maximum spark distance halves (more molecules in the way of the spark). Example: You got an ignitor which can jump 1" max. in atmospheric pressure. It can only jump 1/2" max in a 2x mix, 1 bar (14,7 PSI) environment! And it goes on: 1/3" in 3x mix (2 bar), 1/4" in 4x mix, 1/8" in 8xmix, 1/200 in 200x mix.

Due to this reason, people tend to use only one gap on their hybrids.
Also, with the use of a burst disk, the difference of multi or single spark is less.

[ALIHISGREAT]

so the multi spark gam won't work because it is in series, but i should wire them in seris? (don't quite understand)

thanks for the ignintion tips, but HGDT shows some pretty big performance differences when the "ign num" is changed, i assume this is number of spark gaps? oh and will a spark placed in the middle of the chamber work best?

[starman]

so the multi spark gam won't work because it is in series, but i should wire them in seris? (don't quite understand)

thanks for the ignintion tips, but HGDT shows some pretty big performance differences when the "ign num" is changed, i assume this is number of spark gaps? oh and will a spark placed in the middle of the chamber work best?

Yes spark gaps should be wired in series. Keep them as narrow as possible as psycix has very well spelled out.

And keep the gaps as much in the center of the chamber as possible so the ignition can operate in "spherical mode" as much as possible. Ideally for multiple gaps, space them so that they all will burn their own mini sphere within the chamber. This will ensure the fastest possible chamber burn.

[ALIHISGREAT]

still don't understand seris circuits (after reading the wikipiedia page on it ) i swear we did some stuff about it in physics last year, i'll look it up in my folder.