NASA studying options to fix solar array problems
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: December 18, 2007
Shuttle or space station astronauts likely will be asked to replace a faulty motor assembly on the right side of the lab's power truss early next year that is needed to pivot a solar blanket from side to side to improve power generation. A different problem in a massive rotary joint used to turn the right-side solar panels like a giant paddle wheel will take longer to resolve. But an exhaustive spacewalk inspection today gave engineers hope that near-term modifications may allow the joint to be operated in some fashion until a permanent fix can be implemented.
"Even though we're keeping all of our options open, our thrust today is to try to see if we can put ourselves in a posture to operate with this particular race for a while so we get more time to sort through root cause," said Mike Suffredini, space station program manager at the Johnson Space Center in Houston. "The obvious problem has not come forward to us yet."
The issue is confusing because engineers are faced with two problems involving separate systems on the right side of the space station's main power truss.
The starboard side of the truss is made up of a solar alpha rotary joint, or SARJ, and two solar panels making up the starboard 4 - S4 - power module. One wing of the array is known as S4-1A and the other, extending in the opposite direction, is known as S4-3A. The two panels stretch some 240 feet from tip to tip. A second set of arrays, known as S6, will be attached to the starboard power truss next fall.
To maximize power generation, the arrays must be constantly repositioned to keep them face on to the sun as the station orbits the Earth. The starboard SARJ, which features a 10-foot-wide motor-driven gear supported by 12 so-called trundle bearings, turns the outboard arrays like a giant paddle wheel, completing one 360-degree rotation per 90-minute orbit.
But the sun's position relative to the space station's orbital path changes from day to day and simply rotating the arrays end over end is not enough to maximize power production. To permit the panels to be aimed to either side of the station's orbital path while the SARJ rotates them as required, each array wing is equipped with a beta gimbal joint. The beta gimbal assembly pivots the blankets from side to side about their long axis in a motion similar to changing the pitch of an airplane propeller.
The port-side of the station's power truss is finished and now features four solar array wings. The BGAs on those four wings are working normally, as is the port-side SARJ.
But only one set of arrays is in place on the right side of the truss and one of them - panel S4-1A - suffered a BGA failure Dec. 8. Engineers initially believed a cable or some other component might have been hit by space debris or a micrometeoroid. But during an inspection today, no such damage was found and subsequent tests showed the problem involved a fault inside the BGA motor assembly itself.
The loss of a single BGA would not normally be a critical issue. But NASA is on the verge of launching European and Japanese research modules and the station needs all of the power it can generate.
What makes the BGA problem serious in the near term is that earlier this fall, engineers noticed high vibration levels in the starboard SARJ. Impromptu spacewalk inspections revealed unexpected damage to the surface of one bearing race surface and large amounts of metallic shavings, presumably the result of some sort of friction or grinding in the mechanism that eroded the outer layer of the bearing race in question. Today's inspection was ordered to examine the entire circumference of the race ring, the two drive motors and all 12 trundle bearings.
Data from sensors mounted around the joint indicated the highest vibrations were associated with trundle bearing No. 5, one of two bearing assemblies on either side of drive motor No. 2. Space station commander Peggy Whitson and Dan Tani reported slightly higher concentrations of debris around bearings 4 and 5, but no obvious problem that could explain the damage. Even so, bearing No. 5 was removed for eventual analysis on Earth.
The 10-foot-wide toothed gear at the heart of the SARJ is gripped by the trundle bearing assemblies, which each feature three rollers pressed against three race surfaces with 1,000 pounds of force. Going into today's spacewalk, engineers knew the surface layer of one race was damaged and one was not. Using a mirror to peer at the third race surface today, Tani reported it, too, was free of damage. The motor drive gears appeared properly engaged and there were no other signs of trouble.
"One of the important finds today was the crew used a mirror to look underneath the race, which is the third rolling surface, and told us that it looked clean," Suffredini told reporters. "And that's going to be important for us because one of the things we would like to do as a program is figure out how to live with this joint for a while, to stay on this particular race for a while, while we try to really isolate the cause of this problem."
In a worst-case scenario, the 12 bearing assemblies and two drive motors could be moved to a redundant inboard gear, work that would require four or more spacewalks. But engineers do not want to consider such a drastic step until they figure out what is causing the problem with the active gear and race ring.
"We've talked a lot about being able to swap over to the inboard race and one of the concerns for doing that, until we understand root cause we're likely to cause similar damage to that race as well," Suffredini said. "And of course, that is our only backup race left. So ultimately the best thing for us would be to figure out how to operate for a while on this race."
NASA plans to launch a spare drive lock assembly, or motor, aboard the shuttle Atlantis next month and a full set of 12 trundle bearings on the flight after that in mid February. Engineers may be able to reduce the bearing pre-load to permit SARJ rotation while minimizing additional race ring damage. That would give the team more time to come up with a permanent fix.
But if and when the new bearings might be installed, who would do it and how the work would play into the tightly scripted space station assembly sequence is not yet clear. The next space station crew, scheduled for launch in April, features two Russian cosmonauts unfamiliar with the U.S. power system and possible repair techniques. While a shuttle crew could, in theory, carry out extensive repairs, any such work would disrupt the station assembly sequence.
"We proved to ourselves that the DLA gear teeth are in good shape, that the DLAs ... didn't appear to be loose," Suffredini said. "We got a look at almost all of the trundle bearings, they all looked relatively fine, some a little bit dirtier than the others. So we didn't find anything that stood out, but that in and of itself is an enormous amount of data for us. It would be really nice if something stood out and said 'hey, I'm the cause of your problem.' We didn't get that, but we do now know more about the condition of the ring.
"We're rolling on three surfaces. What this data told us today is two of the three surfaces are smooth. A, that's information, because the two canted surfaces are loaded the same and they're treated the same so it'll be an interesting decisions about ... why the bottom surface is not similarly damaged. ... We want to make sure that we're not eating up the ring such that it can't carry the load. Again, rememb, is hanging off these race rings. They have to hold.
"The fact that only one side is pitted means we feel better about the structural integrity of this ring over time and in fact, maybe we can roll on it," Suffredini said. "As we roll on it, we'll wear a little more of the surface away but since we know the other two surfaces are clean, that'll give us more confidence in our ability to do that for a while before we start to eat into the structural life of the ring."
With the starboard SARJ locked in place pending additional analysis, NASA needs to fix the S4-1A beta gimbal joint as soon as possible to boost the station's power production. A spare BGA motor is on board the station, but it's not yet clear whether Whitson and Tani will be asked to stage another spacewalk before Atlantis takes off next month or whether the work might be added to the shuttle mission.
"We believe the failure mode is inside the beta gimbal motor itself," Suffredini said. "And so we need to recover that in order to get our power balance up to the level we need for the subsequent flights. We can get through 1E (Atlantis' mission), but 1-J/A (the February flight) looks very challenging. It's hard enough when you don't have the joint that tracks the sun in the alpha angle as you go around the Earth. But one of the things we got even when that joint wasn't working was at least we were able to point to the sun from a beta perspective. When you take both of those away, then it gets very difficult to get the power we need out of the starboard side. So that's become our priority as a program. We've asked the ops team to go find a spot to do this change out."
Whether a BGA swap out might be added to Atlantis' mission will depend in large part on when the shuttle can be cleared for flight. During launch attempts Dec. 6 and 9, trouble with intermittent engine-cutoff - ECO - sensors in the ship's external tank forced NASA managers to ground spaceplane and its cargo, the European Space Agency's Columbus research module.
Launch now is targeted for no earlier than Jan. 10 but that assumes engineers figure out a solution to the ECO sensor problem. During a fueling test at the Kennedy Space Center earlier today, sensors 1 and 3 "failed wet." Special test instrumentation spliced into the ECO sensor circuitry appears to show the trouble involves a so-called feed-through connector at the base of the tank. But how the problem might be resolved is not yet known.