[net.columbia] Discovery's 1st stage performance low?

pyle@ut-ngp.UUCP (Keith Pyle) (08/31/84)

Just after SRB separation on Discovery's launch, Capcom informed the
shuttle crew: "First stage - nominal."  Shortly thereafter, Capcom came
back with a revised evaluation that first stage performance was "low."
Interestingly, the CNN people didn't pick up on this and nothing else
was exchanged between the crew and ground (at least nothing that CNN
aired) on the subject.

What is considered in this evaluation?  Was the SSME and/or SRB thrust
too low?  For that matter, how much is "low?"

Keith Pyle

UUCP: . . .{ihnp4,seismo,ctvax,kpno,gatech}!ut-sally!ut-ngp!pyle
ARPA: pyle@ut-ngp

jcp@brl-tgr.ARPA (Joe Pistritto <jcp>) (09/07/84)

The 'SRB Performance Low' call you heard has occurred on almost every
shuttle mission, and represents the Solids not achieving nominal
thrust parameters.  Due to the way solid rocket engines work, their
performance is much more variable than equivalent liquids.  The Discovery
automatically compensates for low SRB performance by boosting thrust
on the Space Shuttle Main Engines, or moving the throttle-back point
slightly, (the region of maximum dynamic pressure).  Typical 'low' values
are 3 to 7 percent below nominal, I believe.  Sufficient reserve fuel
is always carried to compensate for low solid performance, as the
shuttle must reach a fairly precise 'window in space' at the time
of Main Engine shutdown in order to the OMS system, (which is much smaller
than the SSMEs) to execute the orbit circularization burn successfully.
None of the shuttle flights have had their orbital parameters significantly
affected by low 1st stage performance.

Most of this I learned from articles on the subject in Aviation Week

							-JCP-