A question, and an observation: 320 seconds Isp sea-level, or vacuum?
(Vac, I'd guess.) Either way, very useful performance in a monoprop, of
course. If, that is, it can be persuaded to ignite only when in the
chamber and never ever before...
The observation is a generic one about high-energy monoprops: Running a
high-performance liquid-propellant rocket engine seems inevitably to
involve doing violent things to the propellant to get it into the
chamber at requisite pressure and flow. Low flow velocities and
plumbing carefully designed and operated to never cavitate or hammer or
phase-change may be (with considerable effort) doable on a test stand,
but at high-performance flight weight and volume constraints, not so much.
Never say never, but the history of such things seems to be full of
liquid monprops that could be babied into behaving right up until that
essential final step of firing it in an engine. I am inclined to
consider any hot new monoprop candidate coming along as a long shot
(possibly a worthwhile one, but nevertheless) until proven otherwise.
Henry
On 4/21/2018 8:49 PM, John Schilling wrote:
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".
If there's a serious effort to push forth a similar technology, I'll offer what advice I can. Starting with the word, "ethylene". But also with the recommendation that you don't touch anything like this without an eight-figure budget, because building custom remote-control semi-cryogenic propellant mixing facilities and engine test stands in bunkers in the desert is not cheap.
It would have been an honest 320-second monoprop, though. And it blew up REAL good.
John Schilling
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