Science Shuttle Atlantis problem not as clear cut as first thought | Shuttle Atlantis problem not as clear cut as first thought |
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| Written by William Atkins | |
| Saturday, 08 December 2007 | |
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However, NASA may launch without that backup if they can find a workaround plan. If launched with the two bad fuel tank cut-off sensors, NASA may have to manually monitor the amount of hydrogen fuel used to power the three Space Shuttle Main Engines (SSMEs). The three SSMEs use over 500,000 gallons (2.2 million liters) of liquid hydrogen and liquid oxygen as fuel, with is contained inside the external tank (ET), in a space of about 8.5 minutes to propel the space shuttle into space. Four sensors monitor the amount of liquid hydrogen left in the ET tank during the ascent. (This setup is similar to the sensor in your automobile that senses the amount of gasoline in your tank. When the amount is too low, a sensor light comes on saying "low on fuel.") These sensors also identify if there is a problem in one or more of the main engines. If a problem is noticed, that SSME would be shut down. In most cases, the shuttle can still reach orbit on only two SSMEs, however, it depends when the engine is shutdown during ascent into space. The sensors also tell the computers when to nominally shut down all three SSMEs; that is, when it is time to shut down the main engines, eject the ET, and push on into orbit. Redundancy in the system allows one sensor to fail, and still have an acceptable tolerance to fly the mission. Two bad sensors remove that built in safety redundancy in the system. Thus, if the four cut-off sensors fail to detect when the ET is empty, then the SSMEs could continue to operate without being fed any fuel. Eventually, under such circumstances, they could explode. According to the CBS article “NASA Considers Shuttle Launch ‘As Is’,” the problem seems to be a problem involving “an open circuit in the wiring between an electronic box in the shuttle's engine compartment and the sensors at the base of the hydrogen section of the external tank.” With only a short launch window (until December 13th), NASA engineers do not have time to inspect the engine compartment and still launch during this tight seven-day launch window. Such an inspection would cause the launch to be postponed until January 2008.
On Thursday, December 6, 2007, NASA official LeRoy Cain, chairman of the pre-flight mission management team, stated, “We'll try to put together an operational workaround plan that we can get comfortable with that will allow us to go fly on Saturday.” [CBS]
A Saturday launch is now scheduled to occur at 3:43:31 p.m. EST. The weather forecast for Saturday, December 8, 2007, is just slightly better than a 50% chance for acceptable weather to launch. However, Sunday’s forecast improves to about a 75% chance for good flying weather of space shuttle Atlantis and its STS-122 crew.
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