Last week's shuttle abort was caused by failure in the hydrogen level sense system in the big external fuel tank. Four engine cutoff sensors are supposed to signal when the hydrogen is all used up and cut off the engines. Otherwise the turbopumps will overspeed when the fuel line goes dry and they start spinning in air (or vacuum). Centrifugal forces from pump overspeed can cause the pumps to fly apart, flinging pump parts thru out the spacecraft at high velocity and triggering an explosion. Normal operating speed of the turbopumps is 39,000 RPM, which is faster than stink. For comparison, a car engine will blow up at a mere 5,ooo RPM, a turbojet can shuck turbine blades at 10,000 RPM. These pumps are running on the ragged edge of failure under normal conditions. Letting them spin faster when the hydrogen runs out naturally makes all hands pretty nervous, and rightly so.
The failure only shows up when the hydrogen tank is full of supercold liquid hydrogen. NASA was going to run a ground test yesterday by filling the tank with 385,000 gallons of liquid hydrogen. I don't know what hydrogen costs, but at gasoline prices, that's about $1,000,000 worth of fuel. A lot of it will boil off just sitting in the tank.
Apparently the problem has been there all along.
"It seems to me likely that we have been flying the entire history of the shuttle program with a false sense of security and that we never had reliable protection from a [catastrophic] liquid hydrogen low-level engine cutoff. That is a really sobering thought ." writes Wayne Hale, NASA program manager, in an email obtained by Aviation Week.
So far, the shuttle computers have been shutting down the engines when orbital velocity of 25,700 foot per second is reached. Up to now, there has been hydrogen left in the tank at engine cutoff. Any one of a number of malfunctions, starting with a leaky tank, could cause hydrogen to run out before orbital velocity is achieved, causing the shuttle to explode unless the hydrogen level sensors are working.
Managers had initial considered flying the 9 Dec mission with "relaxed ECO rules", NASA speak for flying with broken hydrogen sensors, but the Astronaut Office objected.