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1 September 2005

Simple sensors stall shuttle

The science behind the failed sensing devices that delayed the Discovery launch through July is similar to that of a resistance-temperature detector (RTD).

As temperature increases in one of these devices, the resistance (ohms – W) to the flow of electricity through that device also increases.

The problem surfaced after the shuttle's tank had filled up with super cold liquid hydrogen and liquid oxygen, the fuels that fire the orbiter's rocket engines during the trip to space.

When managers ran a test to make sure the sensors were operating properly, they found false readings.

Turn off its rocket engines

In the test, the computer signaled the hydrogen container was empty, even though it wasn't. Three of the sensors responded correctly by signaling they were dry. However, one sensor misfired, indicating it was still wet, or the container held fuel.

The four sensors at the base of the tank's hydrogen fuel container operate like a gasoline gauge in an automobile, telling Discovery's computer when the container is nearing empty.

That is to say, as the liquid propellant burns in the rockets, its level in the fuel tank goes down. When the cryogenic hydrogen no longer covers the sensor, the temperature rises considerably as does the resistance to electricity through the probe.

The shuttle theoretically could fly with fewer than four hydrogen fuel sensors working, however, NASA's flight rules require all the sensors to be functioning for liftoff.

The precaution is in place because of the crucial role played by the sensors in the shuttle's flight. The sensors tell the orbiter when to turn off its rocket engines. The engines must shut off before the tank runs dry.

The orbiter's main engines shut down when two of the sensors go from wet (low W) to dry (higher W) during launch. This is a safeguard to prevent engine pumps from running on empty. They pump cryogenic fluids at about half a ton per second, and when nothing is left, they operate at a higher speed and destroy themselves. However, this safeguard could actually be dangerous if two sensors failed and prematurely read dry during launch. Then the engines would shut down, and the shuttle would have to make an emergency landing (in Europe if it had not yet reached orbit, or back at the Kennedy Space Center if it had).

CBS News reported there are 24 propellant sensors in the shuttle's external tank, 12 each in the oxygen and hydrogen sections. Eight in each tank measure the amount of propellant present before launch. Four in each tank, known as engine cutoff (ECO) sensors, are part of a backup system intended to make sure the ship's engines don't shut down too early, resulting in an abort, or run too long, draining the tank dry with potentially catastrophic results.

NASA's original launch commit criteria required three operational ECO sensors for a countdown to proceed. However, in the wake of the 1986 Challenger disaster, the LCC amended to four-of-four because of concerns two sensors could go down with a single failure in an upstream electronic black box known as a multiplexer-demultiplexer. Engineering corrected the single-point failure during Discovery's last overhaul, but the four-of-four launch rule remains on the books.

As the foam falls

Incidentally, the falling foam that took the blame for the Columbia reentry crash and that continued its high profile in this most recent launch when foot long strips of fell away during blastoff is insulation to keep the fuel at cryogenic temperatures.

For hydrogen (-252°C) and oxygen (-182°C) to be in liquid phase the temperature has to be pretty nippy. The reason for using liquid phase is far more of the propellants fit in the tank in liquid form than in gaseous state.

Nicholas Sheble (nsheble@isa.org) edits the Sensors department.