Re: Test Failure of SpaceX Merlin VTS1-221Engine


"Pat Flannery" <flanner@xxxxxxxxxx> wrote in message
news:11iechab29m934d@xxxxxxxxxxxxxxxxxxxxx
>
>
> Jeff Findley wrote:
>
> >You're missing the possibility that this could have been a purely
structural
> >failure. Sometimes structures fail with little to no warning when
they're
> >placed under load. If this is the case, then there wouldn't be much of
> >anything to detect before the failure occurred.
>
> And that's what concerns me; one of the main arguments for clustering
> large numbers of liquid engines is that you can afford to shut one or
> more down in their performance goes out of spec, and still reach the
> final cutoff velocity for the stage by extending the burn time of the
> remaining motors. This shutdown ability is used as an argument in favor
> of liquid over solid motors.
>
> Fine and dandy as long as you get an operating anomaly where you have
> time to shut down the engine before it catastrophically fails.
> In this case that didn't happen- if the engine had been part of the
> nine-engine cluster on the base of a Falcon 9, its catastrophic failure
> would probable damage the nearby engines and lead to the loss of the
> vehicle.

True.

When solids go "boom" it's usually structural failure, which seems to give
you nearly the same result as a liquid engine going "boom" due to structural
failure. The only difference may be how the "boom" of each impacts the
payload (i.e. people) riding on top. That would surely take a detailed
analysis that's beyond the free time use posting to these groups. ;-)

> It would be interesting to dig up a detailed history of failed
> liquid-fueled engines on space launch vehicles and find out how many
> times the failed in a benign manner and how often catastrophically.
> Three launches come immediately to mind - the double J-2 shutdown on
> Apollo 6's second stage, the single J-2 shutdown on Apollo 13's second
> stage, and the "abort to orbit" that the Shuttle performed after it had
> a SSME shut down during ascent. In the case of the Shuttle, the shutdown
> was completely benign. IIRC, in the case of the Apollo 13 failure
> shutdown occurred just shortly before the second stage would have
> disintegrated due to "pogo" oscillation in that engine.
> I'm not sure of the details of the Apollo 6 dual motor failure.
> In some cases even a benign shutdown in a cluster of engines will doom
> the vehicle- the Soviets had a Proton (maybe more than one) that
> suffered a single first stage engine failure at launch- even though the
> engine didn't explode, the asymmetric thrust was beyond the ability of
> the other five engine's gimbaling to compensate for and the vehicle went
> out of control and had to be destroyed.

Some of this has to do with how you cluster engines. On Proton, each engine
is away from the center of the vehicle and each has its own fuel tank, but
they all share the center oxidizer tank. In this sort of a configuration,
you can't tolerate engine failures very well since the fuel for the failed
engine can't make its way to any other engine. So even if the control
system could have compensated for the thrust imbalance (by engine
gimballing), it wouldn't have made orbit anyway.

Saturn V handled engine shutdowns a bit better, since all engines shared the
same tanks. This lets you burn the remaining engines until all of the fuel
runs out, hopefully leaving you in an orbit that's acceptable. Although I'm
sure there were times when losing an engine meant losing the vehicle. From
memory, I don't think a Saturn V could handle the loss of an F-1 engine
right after liftoff and still make orbit.

If you're going to have multiple liquid engines, you'd better hope that the
catastrophic failures are removed during the testing phase. Even better is
if you "derate" your engines so that they're running at reduced thrust.
This would hopefully improve reliability and reduce the chances of
catastrophic failure. The added bonus here is that in an emergency, you
have the option to increase thrust on the remaining engines. For example,
if your nominal launch has you running five engines at 80% thrust, then you
can shut down an engine and throttle the remaining four up to 100% to
maintain the same trajectory.

Jeff
--
Remove icky phrase from email address to get a valid address.


.

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