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Sci Tech
Atlantis’ sensors: when redundancy matters
K.S. RAJGOPAL
— Photo: AFP
New year launch: The launch of the Space shuttle Atlantis on December 9 has been rescheduled for January 10.
NASA (National Aeronautics and Space Administration (U.S.) will celebrate its 50th anniversary of founding next year. It will add yet another feather to its cap if the space shuttle Atlantis mission to the International Space Station, scheduled for January 10, is successful. A highlight of the mission will be the installation on the ISS of the Columbus Laboratory developed and built in Europe. The cutting-edge research module will be used by institutions based in Europe to
study space and the effects of weightlessness.
‘Wet’ indication
Atlantis was originally scheduled to be launched on December 6. But on that day just before the launch a problem with the liquid hydrogen sensors was detected and the shuttle launch postponed by an hour.
The problem was that three of the liquid hydrogen sensors indicated ‘wet’ when in fact the liquid hydrogen tank was dry. The problem could not be resolved within an hour and the launch was postponed to December 9. On December 9, one of the four sensors malfunctioned and the launch has been rescheduled for early January.
Backup system
Why is it important that the sensors indicate correctly when the propellants in the main engine run out? According to NASA spokesperson Michael Curie, the normal main engine cut-off command comes from the shuttle’s computers. The liquid hydrogen cut-off sensors serve as a backup system.
The main engines operate on a mixture of liquid hydrogen and liquid oxygen, and it is important for that mixture to be maintained.
If, for some reason, the main engines use more liquid hydrogen than predicted, leading to a low level of liquid hydrogen in the tank the sensor system detects such a condition.
The main engines then shut down to prevent liquid oxygen alone from flowing through the engines. This is essential as in the absence of a proper mixture, the main engines’ turbo pumps would spin at an ever-increasing rate, which would lead to the destruction of the engines and the shuttle.
On December 9, the launch window was reduced from 5 minutes to one minute as a precaution. What is the launch window? Why did NASA shrink its launch window?
Typically, the launch window for shuttle missions to the International Space Station is about five minutes. The exact launch time that is picked provides the most direct path for the shuttle to catch up to the space station.
That direct route uses the least amount of fuel. However, the shuttle can still catch up to the space station for another five minutes by using more liquid hydrogen and liquid oxygen.
The one-minute launch window was selected to use the least amount of liquid hydrogen possible, thereby leaving more hydrogen in the external tank and minimizing the chance that the sensors would be required to shut down the engines.
Since the levels of liquid oxygen and liquid hydrogen are important, the shuttle is equipped with four sensors. But what determines the number of gauges that should be functioning properly for a launch ‘go-ahead?’
Being sure
“There are four gauges to provide redundancy, to be sure multiple sensors agree that the tank is either empty, or not.
The Space Shuttle Program management decides the number that need to be functioning properly for launch,” explains Curie in an email communication to this correspondent.
Space exploration, a venture into the still largely unknown, is fraught with perils and an organisation of the stature of NASA does not take chances.
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