Newsgroups: sci.space.shuttle,sci.space.history,sci.space.policy
Subject: Re: Anniversary Marks Milestone of US Presence in Space
From: Henry Spencer <henry@zoo.toronto.edu>
Date: Tue, 8 Apr 1997 23:14:49 GMT

In article <m2k9meglwv.fsf@harvey.cyclic.com>,
Jim Kingdon <kingdon@harvey.cyclic.com> wrote:
>> And one unpleasant thing about solids is that their failures are
>> almost always catastrophic...
>
>That is particularly true if you define "failure" to mean "something
>catastrophic".  Just in the last year there have been a couple of
>in-flight problems with the shuttle solids (gases reaching o-rings,
>grooves) which were not catastrophic...

In my book, "failure" means "it stops working", as opposed to "some of
the parts show a bit more wear than usual". :-)  If you don't know you've
got a problem until you inspect the hardware afterward, it may still be
a headache but it wasn't an engine failure.

>Having said that, it is true
>that liquids have somewhat more possibilities for serious things to go
>wrong and have the vehicle survive...

Catastrophic failures of liquid engines have actually been amazingly rare.
All the more so when you eliminate cases where the control designer said
"well, the mission's lost in that case, so we don't care what happens" --
for example, not bothering to shut down the pumps when they're about to
start sucking air (okay, helium).  Provided that an attempt is actually
made to shut the thing down when there is clear evidence of trouble, the
engine itself may not be in great shape afterward, but things seldom get
violent enough that neighboring hardware would be endangered.  Pumps can
strip their gears, igniter lines can break and start spewing propellant,
engine-specific Pogo oscillation can reach astonishingly violent levels,
and provided you shut the ailing engine down, the rest just shrug and
carry on.

Whereas almost anything that causes a solid to stop working involves major
pyrotechnics, best observed from a good safe distance.  To some extent
this is simply because solids are so much simpler that those are their
only major failure modes; to some extent it's because preliminary signs of
such failures are hard to spot and difficult to do anything about if you
do spot them.
--
Committees do harm merely by existing.             |       Henry Spencer
                           -- Freeman Dyson        |   henry@zoo.toronto.edu