[AR] Re: thinking big once more
- From: Henry Vanderbilt <hvanderbilt@xxxxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Fri, 30 Sep 2016 09:44:49 -0700
On 9/30/2016 9:12 AM, Henry Spencer wrote:
On Fri, 30 Sep 2016, John Dom wrote:
Correct me if I?m wrong, but I do not think there ever was a rocket of
which an engine (cluster or not) exploded which did not become toast.
I said exploded, not malfunctioned / flamed out.
Falcon 9 #4: engine explosion (not a "malfunction", not a "flameout" --
an explosion) followed by successful insertion into orbit.
The SpaceX press releases, and later statements about investigation
results, were cleverly worded to make you think the engine was still
running and shut down on command, but if you read them very carefully,
they do not actually say that. Which is itself quite revealing.
This was in fact a bit surprising. Engine explosions *usually* don't
end well. However, liquid-engine failures are usually not explosions,
given a sensible control system which shuts down the engine when
disaster is clearly imminent. Usually the engine either doesn't start
at all, or just suddenly shuts down.
A bit of photometry I did at the time showed that roughly one Merlin
chamber's worth of Merlin-chamber-pressure hot gas was released
essentially instantaneously. IE, the chamber unzipped along a seam. (I
believe SpaceX at some point in the fine print did say some such thing
had happened, at the injector end IIRC.)
Given that I was quibbling quite recently over whether the F9 that went
south during fueling recently exploded, or only fractionally exploded
but mostly burned, I'm not going to take sides on whether this chamber
unzipping was an explosion.
But it'll do till the real thing comes along. It was a pretty violent
event, ~10 cu ft (IIRC) of ~1000 psi several-thousands-F gas releasing
instantaneously. It's about the most energetic event you're going to
get from a liquid biprop short of mixing a significant quantity of unlit
fuel and oxidizer in the chamber then lighting it, IE a major hard start
detonation.
I believe SpaceX was on record beforehand that they'd engineered the F9
engine installation to isolate engine failures and let the flight
continue, but yes, it was a bit surprising how well all that actually
worked. The two keys were avoiding damage to the other engines and
quickly shutting down propellant feed to the failed engine, and they
accomplished both.
Getting back to the matter at hand, they key question seems to be
whether it's cost&weight-effective to armor sufficiently against a
similar event between engines running at so much higher pressures.
Hmm. It just occurs to me that, to first approximation, larger,
lower-pressure engines would require larger areas of thinner armor, and
thus the anti-fratricide armor mass for a given number of engines at a
given thrust may not vary all that much with engine pressure.
Any actual engineers care to comment?
Henry (V)
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