Avoiding a delay in STS-87

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.