Henrik Schultz wrote:
As long as you have a reasonably studly igniter running in the main
chamber (for oneshot use an Estes D6-0 motor works quite well), treating them like any other non-hypergolic propellants should work. Your mileage may vary, don't try this at home, warranty void if seal broken...
Thanks Doug, this supports my main direction of thinking. When you say "studly", is that to be interpreted as "sturdy", "powerful" and/or "robust", or are you saying that it should be running all the time?
Robust, really- "studly" in american vernacular is another form of manly or tough. Just some silly wordplay. Anyway, I used those Estes motors as igniters for a heatsink engine running on N2O-Propane years ago. The solid motor was in a can built up from pipe components, threaded into the side of the main chamber with the exhaust aimed across the face of the injector about an inch down. The igniter leads for the solid were fed in through through the side port and out the main nozzle, then the can was threaded into the chamber. Doing this without dislodging the igniter squib from the solid is a bit tricky- fastening the leads to the outside of the solid during assembly of its can helps.
The firing sequence started the solid, then opened the main propellant valves. The solid ran for about 2-3 seconds, but as soon as the main chamber ignites the igniter is surplus. The real key is that the igniter device should have a sonic choked throat so that the initial pressure fluctuations in the main chamber can't affect the igniter operation (this is really what I meant by "studly"- sorry for the ambiguity). I've seen hard starts on engines using flare-type igniters, where the main propellant flow suppressed the igniter long enough for a detonable mixture to accumulate.
Although I got away without it it a few times, I recommend using a burn-through wire or pressure switch to detect the solid operation before opening the mains- if the solid lights off late with the main propellants already flowing, the engine most likely *will* explode.
[Insert standard at-your-own-risk disclaimers here, etc]
Doug
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