FIELD JOURNAL
Avoiding a delay in STS-87
by Jim Draus
Interviewer Brandt Secosh
July 29, l998
I wanted to take moment to share with all of you
a recent problem that we addressed concerning the Rotating Service Structure
(RSS) at Pad B. The RSS is a large steel tower that is rotated about a
hinge column through a 120 degree arc to provide access to the Space Shuttle
as it sits on the Launch Pad. The RSS is driven by two trucks that support
1,250,000 pounds each of the RSS weight and ride on rails at a top speed
of 18 feet per minute. The trucks are powered by four DC motors each along
with a series of spur gears.
The problem we encountered was a binding within the
spur gears. I have included a copy of my weekly notes to recap the events
that took place. Please keep in mind that there was a lot of teamwork
that took place to resolve this issue! These pictures should orient you
to the layout of the RSS and the "trucks" that are referred to in my notes.
A larger version may be seen by clicking on each image.
PAD B ROTATING SERVICE STRUCTURE (RSS) DRIVE PROBLEM
(DRAUS)
On October 21, 1997 an attempt was made by United
Space Alliance (USA) Structures and USA Special Power to rotate the Pad
B RSS into the mate position as part of a routine RSS drive validation
procedure. Several pinched wires at the brake thrusters, several burned
out heater overloads, a blown 10 Amp fuse, and a brake thruster motor
with low insulation resistance were problems discovered and addressed
prior to attempting first motion. At noon first motion was initiated toward
the mate position. The RSS moved approximately six to seven inches before
an over-speed indication on truck #2 shut the drive down. During this
brief move a loud bang was heard at truck #2 and a two-inch scrape was
observed at the rail behind the wheel driven by truck #2 motor #3. Several
attempts were then made to operate the drive which again resulted in over-speed
indications.
An uncoupled test of both drives was then performed
which resulted in all eight motors operating nominally. The motors were
then re-coupled, and one more attempt was made to run the truck #2 drive
only. Armature currents were observed to reach three times their nominal
values prior to a manual command to shut the drive down. At this point
it appeared the problem was not the motors or the electrical system, but
was a binding problem, possibly In the mechanical drive system or at the
hinge column bearings.
A thorough investigation of the eight gear trains,
four on each truck, was then performed. Both trucks were pressure cleaned
on October 22. On October 23 a fiber optic visual inspection was performed
at the spur gears and at the wheel/rail interface. Both trucks were jacked
up and their gear trains were rotated, cleaned, and inspected on October
24. No evidence of any interference or binding was found. Once all jacking
operations were complete, an uncoupled test of both truck drives was then
performed on October 25 with both drives operating nominally. The RSS
was then moved back to the full park position, approximately six to seven
inches at 10:30 a.m., with all motors and gear trains operating nominally.
The RSS was then cycled through a short move, about
eight inches, towards the mate position and then back to park. This was
performed two more times to verify nominal operation of the drive system.
At 1:00 p.m., a full rotation of the RSS to the nominal mate position
was initiated. Loud banging was immediately heard from the hinge column
bearings which resulted in the RSS returning to the park position to facilitate
greasing of the upper and lower bearings.
At 2:00 the RSS began moving toward the mate position
again with some popping still being heard at the hinge column bearings.
This popping/banging disappeared immediately upon switching to full speed
at about 12 feet out from park. Some very loud banging was then encountered
at the truck wheels as the trucks moved over the flame trench bridge.
This however has been heard on previous moves and is due to the rust scale
buildup on the rails. The RSS was in the mate position at 3:10 p.m. At
3:25 p.m. motion back to the park position was initiated with the move
complete at 3:50 p.m. No sounds were heard at the hinge column at all.
The cause of the binding problem was never positively
identified since nothing was actually found binding any of the gears,
such as a bolt, nut, tool or some other debris. It was determined that
upon jacking the trucks and removing the RSS load from the wheels, the
binding between the gears was relieved and the problem disappeared. All
agreed that the binding may have been a combination of several factors
such as an interference at the gears, wheels and possibly the hinge column
bearings.
As a result of the drive problem being corrected
in just four days, the processing of STS-87 at Pad B was not affected
and the RSS was able to provide nominal support of the Launch Countdown
through T-0 on November 19. Due to the potential impact of this problem
on the processing of STS-87, all levels of NASA and USA management up
to the Center Director were being given frequent status reports. Had this
problem not received the immediate and undivided attention of NASA and
USA, this problem could have resulted in a transfer of STS-87 over to
Pad A and possibly a costly slip in the launch schedule.
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