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From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re:  New Shuttle Engines
Message-ID: <1990Nov16.211340.27611@zoo.toronto.edu>
Organization: U of Toronto Zoology
References: <11236@milton.u.washington.edu>
Date: Fri, 16 Nov 90 21:13:40 GMT

In article <11236@milton.u.washington.edu> brettvs@blake.u.washington.edu (Brett Vansteenwyk) writes:
>I am curious as to how much of a design difficulty it is to have an engine
>operate (well or otherwise) at both sea level and in the vacuum ...

It's not an enormously big deal, but it is a nuisance.  Ignoring some
details, one gets maximum thrust out of a given exhaust stream if it
exits the nozzle at roughly ambient pressure.  This means that a nozzle
optimized for high altitude will be a good bit longer than a sea-level
nozzle, to give the gas more expansion.  An underexpanded nozzle, with
the gas exiting at higher than ambient, results in some loss of thrust.
A badly overexpanded nozzle causes the gas flow to break away from the
nozzle wall before reaching the end, which causes all manner of nasty
turbulence and problems of various kinds.  The SSME nozzles are in fact
mildly overexpanded for use at sea level, for the sake of performance
at high altitude, but not enough to cause major trouble.

Multiple stages, with upper stages ignited only at high altitude, offer
somewhat of a way out of this.  The first stage is still a compromise,
because it finishes its burn in much thinner air than it started in,
but the upper stages can use fairly uncompromised high-altitude nozzles.
People have explored more devious schemes, like variable-length nozzles,
but no real use has been made of them yet.

>... If this is a heavy constraint, why light
>the SSMEs at sea level at all ...

One very important practical reason is that the #1 time for engine trouble
is at ignition.  Lighting the SSMEs on the pad means that the more benign
kinds of engine failures simply cause a pad abort.  There may also have been
some hope that gimballing the SSMEs would suffice for control, and the SRBs
wouldn't need gimballed nozzles, although in fact that didn't pan out.

>By the way, I remember that there is some consideration (or ongoing effort)
>to adapting the RL-10 to operate at sea level.  Is this a good example of
>this sort of design "difficulty"?

In principle, all you need is a shorter nozzle, although you have to think
about things like the effect on the cooling system.  There may be some
issues in ignition, too.
-- 
"I don't *want* to be normal!"         | Henry Spencer at U of Toronto Zoology
"Not to worry."                        |  henry@zoo.toronto.edu   utzoo!henry