October 29, 2007
Updated Spacewalk Plans
As you may have heard, STS-120 spacewalkers Scott Parazynski and Dan Tani uncovered a problem in the starboard Solar Alpha Rotary Joint (SARJ) on Sunday. This joint is what allows the solar array wings to rotate around the y-axis of the station. (The x-axis runs along the length of the modules, and the y axis along the truss with the origin at Z1 atop the Unity node. Z is up and –z points toward Earth.) The joint has not been working properly, and Mission Control asked for an inspection.
Tani removed some covers and found “filings” jamming up the works. He used some tape to capture samples of these particles. Station Commander Peggy Whitson used paper and a magnet to find out that these particles are magnetic.
In Monday’s press briefing, ISS Program Manager Mike Suffredini said that the particles being magnetic rules out them coming from the aluminized Mylar thermal covers and indicates they are probably from the steel bearings or “race” inside the joint. I am not an engineer, and I’m sorry that I cannot tell you what a “race” is other than it is a steel part near the gear teeth. The teeth are the brakes that hold the SARJ in place after it has rotated to a new position. Some wear is expected on these teeth over time, and Mr. Suffredini said that life-time testing had shown some particulates—but nothing like the number seen by Tani.
Other parts of the SARJ that may be contributing to the problem include something called a TBA, trundle-bearing assembly, and a ?joggle that is a U-shaped piece. The ?joggle has a thousand-pound load on it. If the TBA turns out to be the source of the problem, then there is a procedure to replace it with a spare. The next shuttle flight would have to take it up. Mr. Suffredini called this replacement a “sophisticated effort,” meaning it is probably one heck of an EVA script, and one that the crew has not been trained to do.
The plan is to take some time from EVA 3 on Tuesday to inspect the port SARJ. This joint is working smoothly, and so will provide a comparison to the balky starboard joint. Ironically, the port joint is older than the starboard one. The port joint was installed last December, and the starboard just this past June. Mr. Suffredini also said it does not look like a wearing-down build-up kind of problem because the performance has changed in a stepwise fashion, not gradually gotten worse as it would if there were a gradual buildup of filings.
During EVA 3, Scott will take a tape sample of the port array to check for particles whether he sees any or not. A window of about 40 minutes has been carved out the EVA for this inspection. The rest of the EVA is devoted to the important job of attaching the P6 solar wing and getting it unfurled and working. If the starboard array is not able to rotate and track the sun (and thus produces much less power), then having this port array in full operation becomes even more critical. Therefore, if there is a problem with the array deployment, it will take priority over any SARJ work planned for the EVA 4 on Thursday.
The array deployment was originally scheduled to begin around 1:30 p.m. Eastern time. The port SARJ inspection is supposedly added somewhere in the middle of the EVA, so this time may be changed. (DONE EARLY! AROUND 11:30 Eastern!)
Assuming that the P6 attachment and deployment goes well Tuesday, then Thursday’s EVA will be devoted to a thorough inspection of the starboard joint. The plan is to remove all 22 thermal covers around the 10-foot diameter joint and look for filings and take samples. This will take longer than the previously planned spacewalk, and will thus require Friday's spacewalk by the station crew to be delayed a day to prepare the airlock.
Flight Extension/Flight Delay
To fit in the long EVA 4, the shuttle flight will be extended one day. The question was asked if the P6 deployment requires work on EVA 4, will another 6th EVA (a record) be added to deal with the starboard SARJ, and the answer was undecided., but probably not. To do so might jeopardize the launch of the next shuttle flight, planned for December 6. That launch will take the Columbus module up. Harmony must be moved to its permanent position between the PMA and Destiny before Columbus can be attached. EVA 5 by the station crew is therefore in the critical path of this action, and if delayed, would delay the launch of Columbus. Also, EVA 5 requires that the power-sharing system that allows the shuttle to use station power and stay longer, has to be disconnected. So a new EVA would have to be added in between EVA 4 and 5, and require 2 days addition to the shuttle flight, and that would impact the next shuttle launch. Just moving Friday’s EVA to Saturday may mean that the launch will slip from December 6 to 7. Constraints on the beta angle, the angle of the sun with the orbit plane of the station, require that launch to be no later than December 13.
Mr. Suffredini indicated that even if the starboard SARJ is locked in place and won’t allow the starboard array to rotate, with port side working, there is sufficient power for Columbus to be added. However, the Japanese module that is planned to go up next spring, will require the starboard arrays to rotate to produce the required power levels. So while the SARJ problem is serious, it will not cause any immediate change in operations on the station.
Space exploration is never easy, and challenges are to be expected. The good news is that the crew and flight control teams are fantastic problem solvers. I sure appreciate their willingness to let us “look over their shoulders” while they go through the process. I certainly wish them well!
Posted by m_dyson at October 29, 2007 01:05 PM