On 4/21/2018 11:04 AM, Henry Spencer wrote:
On Sat, 21 Apr 2018, Keith Henson wrote:Yes, 7:1 Nitrous/Acetylene. And for those of you who don't know, I was in charge of mission assurance for the propulsion system on ALASA, with a key emphasis on the part of the mission that involved not causing an F-15 to suffer what the USAF euphemistically calls a "Class A mishap". Between dilution of the acetylene, chilling and pressurization of the propellant, and what Henry describes as proprietary tricks, there were reasons to believe it could be made to work and to work safely enough for a manned system. And there were the resources to do it right, e.g. by remote control behind reinforced concrete in the middle of nowhere until safety had been more than adequately demonstrated. When instead we demonstrated excessive kaboomishness in spite of all precautions, we shut it down, but it was worth a try.
I believe John is referring to ALASA, not RASCAL et al. ALASA was using a
stock F-15 and a mixed-monoprop nitrous/acetylene rocket...
Given that acetylene can't be liquefied or compressed beyond 2 atmospheres without becoming shock sensitive, it amazes me that anyone would try to make such a devil's brew.
Bear in mind that it was a DARPA project, and DARPA is in the business of trying things that look speculative and iffy.
Given the very skewed mixture ratio -- 7:1 if I recall correctly -- the acetylene was pretty thinly dispersed in the other component, which is the way acetylene is routinely stored and handled in industry (although in acetone rather than nitrous). So the acetylene itself wasn't that big a concern. But nitrous with fuel in it, even a little bit of fuel, is notoriously much more sensitive than clean nitrous -- that was the more worrisome bit.
Set against that, (a) the mix was made and stored chilled well below room temperature, and (b) there was at least one proprietary trick also helping. So, as Mitch Burnside Clapp (then ALASA project manager) put it three years ago, "there are reasons why this is not entirely insane". And indeed, mixing, handling, storage, and cold-flow tests were free of loud surprises. (This was all being done very gingerly by remote control, of course.) But when it came to hot-fire tests, as John Schilling said, "it is still excessively enthusiastic about ignition".