Newsgroups: sci.space.shuttle
From: henry@spsystems.net (Henry Spencer)
Subject: Re: ET stretch.
Date: Sun, 18 Jun 2000 17:29:12 GMT

In article <394932B3.1FD9D856@ist.ucf.edu>,
T L Clarke  <tclarke@ist.ucf.edu> wrote:
>> >...run the SSME even more hydrogen rich ...
>> Well, the general effect of such a change is higher Isp but lower thrust.
>> Whether this turns into an increase in *vehicle* performance...
>
>I recall reading that the Saturn was run with a variable mixture ratio
>optimized for the flight trajectory.  Do they do this with the shuttle?

No, the SSMEs run at a constant ratio all the way up, partly because they
are complicated and cranky engines and nobody wants to qualify them for
the wider range of conditions that would be needed.  The benefits of doing
it would be small unless you changed the mixture ratio quite a bit.

>Further, the book I read said that this was discovered accidentally
>on the early flights.  Some flights gave better vehicle performance than
>others.  Someone noticed a correlatiion with the way the mixture ratio
>varied during the flight.  Then they had the idea to vary the mixture
>ration *on purpose* so as to optimize performance.

Almost right -- it was the *simulations* which varied slightly.  The
original second-stage design used a slightly variable mixture ratio to
make sure both propellants ran out at the same time.  (Any leftover
propellant counts fairly directly against payload, and at the rate rocket
engines use the stuff, it doesn't take much of a mismatch in when the two
propellants run out to leave you with quite a bit of dead weight.)
However, this did produce slightly unpredictable performance, and while
everybody was scratching their heads trying to figure out how to reduce
this effect, one bright lad (Bud Brux) suggested trying to maximize it
instead.

It worked.  Careful preflight calibration and precise propellant loading
were good enough to give reasonably predictable propellant exhaustion, and
a preprogrammed shift of mixture ratio, from maximum lean to maximum rich,
about 2/3 of the way through the second-stage burn added a ton or so of
payload.  One or both of the unmanned test flights flew with the original
control scheme, but on Apollo 8 the automatic sensors were disconnected
and a preprogrammed shift replaced them, and later on the hardware was
simplified by changing the original continuously-variable valves to a
two-position design optimized for the preprogrammed shift.
--
Microsoft shouldn't be broken up.       |  Henry Spencer   henry@spsystems.net
It should be shut down.  -- Phil Agre   |      (aka henry@zoo.toronto.edu)