Posted by Davin Flateau on 14 Jul 2005 at 7:33 pm.
Filed under Astronomy.
James Oberg on MSBNC.com writes a fascinating piece detailing the importance of the intermittent fuel sensors that caused the Shuttle scrub on Wednesday. The low level sensors inside the liquid hydrogen fuel tank saved the shuttle from disaster in 1999 during Columbia’s STS-93 mission to deploy the Chandra X-Ray Observatory. The mission commander? You guessed it - Eileen Collins, the same mission commander for the current mission.
Back in 1999, just as the main engines fired up at launch, a pin came loose and punctured hydrogen lines, causing some fuel to leak out of the engine and - ignite! The new flames weren’t really the problem, but the hydrogen loss casued the engine “bell” to heat up and lose power, so controllers on the ground had to punch up the liquid oxygen flow to compensate. As Columbia accelerated skyward, it readied to cut off its main engines at just the right point to get it into earth orbit. Oberg picks it up from here:
But only seconds before reaching that point, sensors in Columbia’s oxygen tank went “dry,” indicating exhaustion of the supply. First one, and then another of the four sensors signaled “dry” — and the rule is to require two votes to persuade the computer the signal is genuine. At that point, emergency engine shutdown was ordered, and all three engines stopped firing.
It was just 0.15 seconds prior to the nominal shutdown. Since the shuttle was accelerating at 3 G’s, three times the force of gravity (32.2 feet/second/second), it was still picking up speed at a rate of about 100 feet per second every second. In terms of the orbit it was aiming for, it was raising the far end of its circular earth-girdling path by about 60 miles every second.
So if the shutdown had been ordered only a second earlier in the 520-second climb into space — if the hydrogen leak had been a fraction of a percent faster — Columbia would have fallen short of a stable, safe orbit. It would have had to immediately head back to Earth for an emergency landing in West Africa.
Wow. To make things even more harrowing, this wasn’t the mission’s only close call.
The underspeed was hardly noticed, because another far more serious failure on the launch had brought the crew much closer to loss of vehicle and crew. Two separate short circuits in wires connecting main engine control computers threatened to shut off one — or more — of the engines early in the flight, throwing the mission into an abort mode without any airfield in range. The crew would have had to bail out in the final moments of their descent toward the ocean.
Spaceflight is a dangerous business, and a vehicle as old and complicated as the Shuttle makes it even more so. The Shuttle is due to be retired by 2010. Its main mission between now and then will be to complete construction of the International Space Station. Let’s all cross our fingers that the aerospace industry can successfully build a shuttle replacement before then, and avoid the mistakes of the costly and time consuming X-33 project. The last “shuttle replacement”, the problem-plagued X-33 project was scrapped in 2001.
