Low-level sensors crucial to safe shuttle ascent

James Oberg NBC News space analyst

When a low-level sensor in Discovery’s propellant tank malfunctioned during Wednesday’s countdown, mission commander Eileen Collins must have had that “déjà vu” feeling. And she would have had no objection to calling off the launch — because the proper functioning of exactly such a sensor may have saved her spaceship, even her life, on her previous launch in 1999.

The sensors exist to warn the shuttle's guidance computers of an imminent exhaustion of either of the spaceship's two propellants, liquid oxygen and liquid hydrogen. Although normally the spaceship's main engines are shut down based on reaching a preset velocity, designers knew that unusual conditions might lead to running out of one or the other tank contents before that desired end condition is met.

In such a case, the high-speed engine turbopumps would be sucking gas instead of denser liquid and could overspeed and disintegrate within seconds. Or the mixture ratio of the two propellants would be thrown wildly out of kilter, resulting in an explosive situation that could destroy one or more of the engines.

In the worst case, such an engine fire or explosion could damage other hardware in the shuttle’s tail. Loss of the auxiliary maneuvering pods, or of the aerodynamic flaps in that area, would almost certainly doom the crew.

Exactly such an outcome was safely avoided in July 1999 when Collins was in command on shuttle Columbia. This was the STS-93 mission that was to deploy the Chandra X-Ray Telescope and its booster rocket into space.

The malfunctions that led to the shutdown were frighteningly familiar in light of other shuttle catastrophes. For Challenger in 1986 and Columbia in 2003, a chain of unlikely events cascaded without interruption toward disaster. But in July 1999, another potentially lethal failure cascade was stymied by the proper functioning of the low-level sensors.

Sensor saves the day
Just at engine ignition, a pin tore loose from inside one of Columbia’s engines, and as it flew out the back end, it dinged the engine nozzle. Liquid hydrogen is circulated through about a thousand small lines inside the nozzle to cool the metal wall against the combustion temperatures and to heat the hydrogen for more efficient burning.

Several of those lines were punctured by the falling pin, and some hydrogen began spewing out. It ignited, and in video taken during liftoff it was seen as a flame protruding from the inner wall of the nozzle. The rate of leakage was not too high, but the engine controller noticed a slight loss of power and tweaked up the flow of oxygen into the engine to compensate. This worked, but increased the consumption rate of the oxygen.

Nevertheless, the reserve supply of oxygen that had been loaded just in case of unusual situations was actually almost enough to make it all the way to the normal cutoff point.

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.

In postflight analysis, space engineers discovered that the actual oxygen load was also a bit lower than they had intended, but within what they had thought were adequately tight margins. Combined with the independent leak, these two separate occurrences came within one second of triggering an extremely hazardous emergency landing.

Such a “Trans-Atlantic Abort Landing” (or “TAL”) had never been performed before, and it would have put high stress on the shuttle's heat shield and on its crew. It probably would have worked — the crew was fully trained for attempting it — but would have been extraordinarily dangerous.

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