SpaceX and NASA Still Determining Reasons for Falcon 9 Engine Failure, Other Anomalies
The overall success of SpaceX's first operational cargo mission to the International Space Station (ISS) last month overshadowed the fact that the mission also encountered several problems, including the failure of one of the nine Falcon 9 engines.
Speaking to the NASA Advisory Council's Human Exploration and Operations Committee today, ISS program manager Mike Suffredini said that Space X is still trying to determine what happened to the engine. NASA is participating in the investigation, he said, and a fault tree analysis is underway.
Several other problems also arose during the mission. While berthed to the ISS, one of the three computers on the Dragon spacecraft failed. Dragon can operate with only two computers, and SpaceX chose to proceed with the two functioning units rather than trying to fix the faulty unit while on orbit. According to Suffredini's charts, Flight Computer-B "de-synched" from the other two "due to a suspected radiation hit" and although it was rebooted successfully, it was "not resynched." Dragon experienced other anomalies because of radiation as well. One of three GPS units, the Propulsion and Trunk computers and Ethernet switch all experienced "suspected radiation hits," but all were recovered after a power cycle. Suffredini said that SpaceX is considering whether it needs to use radiation-hardened parts instead, but noted that "rad-hardened" computers, for example, not only are more expensive, but slower. He speculated that the company would ultimately decide to use rad-hardened components in the future unless it is cost-prohibitive.
Problems with Draco thruster sensors and a loss of all three coolant pumps after splashdown also marred the mission. The Glacier freezer onboard Dragon used to return scientific samples from the ISS was at -65 degrees Centigrade (C) instead of the required -95 degrees C when it was accessed three hours after splashdown. Suffredini said that some of the samples "exceeded limits" (presumably temperature limits), but that the limits were conservative. How much of a problem the warmer temperature could cause apparently is not yet clear.
in response to a question, Suffredini said that although NASA does not have go/no-go authority over SpaceX launches, it does have influence as a customer. The company's contract requires it to deliver a certain amount of cargo to the ISS over a fixed period of time. If NASA is not sufficiently confident that the system works, it will not put its cargo aboard and "they don't get paid if I don't fly."
Despite these outstanding issues, Suffredini presented a timeline chart showing the next SpaceX mission to ISS scheduled for March 2013. That chart also listed the first flight of Orbital Science's Cygnus cargo spacecraft to the ISS for April 2013, but Suffredini advised the committee not to bank on it because he expects that date to slip.
As the briefing continued it became clear that a delay might be needed in any case because of an unresolved problem that arose when Japan's HTV cargo spacecraft was released from the ISS. The HTV automatically aborted the release when it detected an "off-nominal trajectory" resulting from friction between the grapple fixture and Canadarm2 that caused Canadarm2 to pull HTV as it was trying to back away. Suffredini said they are still investigating what happened. SpaceX uses a different configuration, so it is not issue for the next Dragon flight, he said, but Cygnus uses the same configuration as HTV and "we need to sort it out before Cygnus flies."
Editor's note: This article has been clarified in the fourth paragraph to indicate that the problems with the Draco thrusters were with sensors, not the thrusters themselves. The NASA presentation identified problems with three sensors: injector resistance temperature detector on Draco thruster 4 in quad 3; pressure transducer in Draco thruster 3 in quad 3; and pressure transducer in Draco thruster 4 quad 2.
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