Jul 30 1985
From The Space Library
NASA announced that, based on a review of engine data following the July 29 Space Shuttle mission 51-F launch, Marshall Space Flight Center and Kennedy Space Center engineers believed that failed sensors-not an engine failure-were the probable cause of the main engine No. 1 shut down [see Space Transportation System/Missions, July 291 The data suggested the high-pressure fuel turbopump on that engine performed normally, because there were no indications the pump was running hot. If the pump were running hot, Johnson Space Center flight engineers, who monitored engine performance during ascent, would have expected to see changes in the position of an oxidizer valve indicating an increased flow to supply the pump with more power. Data showed the valve position did not change. A second data indication of normal pump performance was present in the engine discharge pressures, which were monitored by ground controllers and the main engine controller.
Based on these indications, NASA engineers reviewing the cause of the engine shutdown tentatively concluded that the main engine controller was receiving faulty data from failed sensors. Resistance measurements taken by the engine controller provided an indicator of pump temperatures, because the resistance increased as the temperature increased. The controller logic was able to distinguish, within certain limits, a valid reading from one that indicated the sensor wire, a very thin wire, was broken or had otherwise failed.
The controller recognized the bad reading from the Channel B sensor; the reading from Channel A, however, drifted, indicating a temperature rise above the redline limit. Because of the failure mode in the second sensor, which gave readings above the redline limit but below those that would cause the controller to disregard them as unreasonable and thus indicative of a sensor failure, the engine controller believed there was a possible pump problem and shut down the engine.
A temperature sensor on main engine No. 3 also failed. Because there was a history of these temperature sensors failing on previous Space Shuttle flights, NASA decided to develop a new sensor for use on future Space Shuttle flights beginning with mission 51-1.
Until such time as NASA confirmed this explanation by an examination of the engine and its various systems, NASA would not launch another mission. However, the agency did not anticipate that this would result in a delay in the next mission.
The call "abort to orbit" used to describe the action ordered for the July 29 Space Shuttle flight was the preferred type of Space Shuttle launch "abort" in that it gave the crew, the orbiter, and the ground controllers time to review options and to ascertain that the crew and orbiter were in good condition. In nearly all cases, the abort-to-orbit contingency would allow completion of most of a particular mission's flight requirements.
Once NASA determined that an orbiter was in a safe orbit (minimum would be about 105 nautical miles circular), then the crew and ground flight controllers could begin to assess the impact on planned mission activities. In the case of mission 51-F, the orbit achieved was 106 by 146 nautical miles, 40 miles short of the planned designations; the orbiter was at a velocity of 25,760 feet-per-second, which was 110 feet-per-second less than planned.
NASA determined all abort modes based on the weight of the orbiter, the inclination of the desired orbit, and other characteristics. Each mode had a specific applicable time and duration during the ascent phase, and crew and ground controllers practiced these contingency situations regularly. (NASA Release 85-114)
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