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Sunday review confirms Discovery's readiness to fly BY WILLIAM HARWOOD STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION Posted: August 23, 2009 
NASA managers reviewed a few final technical issues Sunday and cleared Discovery for launch early Tuesday on a 13-day three-spacewalk mission to deliver 7.5 tons of equipment and supplies to the International Space Station. The only issue going into the final hours of the countdown is the weather, with a 40 percent chance of afternoon thunderstorms Monday that could delay fueling. Assuming the weather cooperates and fueling gets under way within a few hours of the 4:11 p.m. EDT start time, the forecast calls for an 80 percent chance of good weather for launch Tuesday at 1:36:05 a.m. "Overall the weather is looking good for launch, we've just got to get there," said shuttle weather officer Kathy Winters. "And so the tanking weather is what we'll be watching very closely. But by the time we get to launch, there is just a 20 percent chance of KSC weather prohibiting launch." The forecast for a 24-hour delay calls for a 20 percent chance of afternoon thunderstorms that would delay fueling Tuesday and a 30 percent chance of bad weather at launch time early Wednesday. The forecast is the same for a launch Thursday, but in that case forecasters only expect a 10 percent chance of bad weather for tanking. Because of a conflict with the Air Force Eastern Range, which provides telemetry and tracking support for all rockets launched from Florida, Discovery must get off by Aug. 30 or the flight likely will slip to mid-October, after upcoming Japanese and Russian space station missions. NASA's Mission Management Team met Sunday for a traditional launch-minus-two-day readiness review to close out open paperwork and discuss any unresolved technical issues.
The shuttle is equipped with power switching assemblies tied into each of the ship's three electrical buses to route electrical power to forward rocket thrusters, payload bay hardware and the shuttle's aft reaction control system thrusters. Forward power control assembly No. 3 failed aboard Discovery during launch pad checkout, forcing engineers to replace the unit with a spare. That failure, plus two similar malfunctions in the Shuttle Avionics and Integration Laboratory at Johnson, prompted an in-depth analysis. Disassembling the units, engineers discovered presumed metal fatigue problems with a solenoid relay system. It is not yet known whether the observed fatigue was caused by poor workmanship, the stress of normal operations since the boxes were built or both. The boxes cannot be opened and inspected without voiding their flight certification and triggering a lengthy delay. As a result, mission managers do not know the condition of the suspect components installed aboard Discovery, all currently powered on and operating normally. An assessment of the potential impacts of an in-orbit failure dominated Sunday's engineering review. Mike Moses, chairman of NASA's pre-launch Mission Management Team, said a failure does not change the state of the device. If the suspect controller component failed when it was powered on, the downstream circuits would remain powered. If it was off and then failed when commands were sent to turn it on, the circuit would remain unpowered. Moses said a thorough review of shuttle systems identified just one major area of concern: how a failure might impact the crew's ability to operate the shuttle's forward and aft maneuvering jets. Power to the "reaction jet drivers," or RJDs, normally is turned off after the shuttle reaches orbit and then turned on only when needed to conserve propellant and to prevent jarring thruster firings during crew sleep or when the shuttle is docked to the space station. To protect against a power control assembly failure during Discovery's flight, the RJDs will remain on through the shuttle's docking with the International Space Station and then turned off. If a failure occurs at that point, the jets would remain powered and flight controllers would disable them as required using the shuttle's flight software. If the device worked normally to turn off the RJDs after docking and then failed during power-up just before undocking, Moses said the crew would be able to proceed because the aft thrusters have an alternate source of power and the shuttle's forward thrusters are not needed for undocking and re-entry. It would take multiple failures to cause a real problem and the odds of that were considered acceptably low. "Everybody was fine with our flight rationale," Moses said of the decision to proceed. One other technical issue was discussed Sunday involving analysis and inspections of a flow control valve used aboard the shuttle Endeavour last month. The flow control valves are used to maintain the proper pressure in the shuttle's external tank during the climb to space. Recent problems with cracks prompted NASA to implement strict new inspection guidelines to minimize the chances for development of cracks that might release debris in the system. As it turns out, the guidelines were not strict enough. Engineers discovered a valve flown aboard Endeavour last month developed a crack even though it passed its pre-launch "eddy current" inspection. New acceptance guidelines have been implemented and the threshold for rejecting a valve has been tightened. Moses said all three of the flow control valves aboard Discovery met the new guidelines.
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The official crew patch for Discovery's flight to deliver and research gear to the space station
