yee@ames.arc.nasa.gov (Peter E. Yee) (08/05/88)
KSC SPACE SHUTTLE PROCESSING REPORT FOR - Thurs., Aug. 4, 1988 PM
STS-26 - DISCOVERY (OV 103) - PAD 39-B
The Flight Readiness Firing for the Space Shuttle Discovery
was halted today at the T-5 second mark. The main engine
controller for Space Shuttle Main Engine No. 2 detected a a
sluggish fuel bleed valve in the engine's hydrogen system and
sent a major component failure command to the orbiter's onboard
computers which in turn halted the engine start sequence.
Engineers are troubleshooting the situation by cycling the
valve at ambient and at the cold temperatures the valve was
exposed to during the last seconds of the countdown. Engineers
expect this data, coupled with information on similar problems
experienced during acceptance testing of these valves, will
indicate whether the problem is temperature induced and can be
masked out with a software change or if there is a mechanical
problem with the valve which would require a changeout and some
retest. Documentation on this valve indicates it did pass
acceptance testing.
The criteria for the terminal portion of the countdown is
that the valve be closed prior to main engine ignition to keep
propellant out of certain areas. The valve, which is located
between the high pressure fuel turbo pump and the engine nozzle,
allows fuel to flow into the main combustion chamber and bleeds
off the helium purge so the engine will start properly with pure
hydrogen and not a mixture of helium.
Another possible cause for the cutoff today could be the
valve's position indicator which tells the engine controller
whether the valve is open or closed. A faulty indicator could be
giving the controller the wrong information.
In parallel with the troubleshooting, another valve is being
located for use on Discovery in case the valve needs replacing.
The valve can be replaced at the pad without removing the engine.
None of the three engines were started during today's FRF
attempt and therefore a cutoff was called by the shuttle's
redundant set launch sequencer (RSLS).
If hardware changes are not necessary before another FRF
attempt can be made, the earliest the countdown could pick up is
Saturday evening and the T-0 would occur no earlier than 7:30
a.m. Sunday, August 7. A specific time for FRF will be announced
once the problem is isolated and the remedy is factored into the
schedule.
Meanwhile, the external tank has been drained of its liquid
hydrogen and liquid oxygen propellants. The residual propellants
have to boil off before the pad is open for normal work - about 7
a.m. tomorrow. The Rotating Service Structure is scheduled to be
moved into the position around the orbiter at 10 a.m. tomorrow.