It’s a sign of just how cautious NASA has become that they decided to stand down today. The questionable engine cut-off sensor was one of four that are used to shutdown the engine if the hydrogen fuel runs low. The shuttle needs just two of its four sensors working, and the whole cut-off system itself is a backup: the navigational computers typically turn things off automatically at the right altitude and speed. Even if they don’t, Atlantis carries an extra 300 kilos of hydrogen to prevent the engines running dry.
The risk was so minimal that only two members of the roughly twenty-man mission management team voted “no-go.” One of them was team chairman LeRoy Cain, so that sealed the deal.
Nevertheless, shuttle programme manager Wayne Hale said, if the shuttle did run out of hydrogen, it would be a “very bad day.” In static tests, shuttle engines running on oxygen without the presence of hydrogen underwent what engine designers euphemistically call “uncontained failure.”
Mission managers say that they will try again tomorrow, and unless they see something really weird in other sensors, they’ll go ahead regardless of the bad engine cut-off sensor’s status. If tomorrow doesn’t happen, they’ll wait until after an 18 September Soyuz mission.
“Tomorrow is a deadline because I have to go find a Laundromat,” says Hale.
Amen to that.