Darned ECO sensors acting up again...

So if you are following the STS-122 launch attempt you'll note that the launch is being held up by these fuel tank ECO sensors- again. So I thought it would be interesting to examine- just what are these sensors? Why are they so important?

The Shuttle external tank has eight sensors to detect empty fuel tanks; four in the LOX tank and four in the LH2 tank. If these sensors detect an empty tank they shut down the engines; otherwise you'd run an excessively fuel-rich (with depleted LOX) or oxidizer-rich (depleted LH2) mixture, both of which will damage your engines.

MECO (Main Engine Cutoff) ordinarily occurs based on the velocity. But if any two of the four fuel tank sensors in either tank sense an empty tank, the engines are shut down early to avoid the engines running themselves to pieces on no propellant.

The hydrogen ECO sensors on mounted on an object called a shock mount that spans the entrance to the valve that carries LH2 to the combustion chamber. If two LH2 ECO sensors detect an empty tank, the engines are shut down. If the ECO sensors do not function, the engine could explode (if it ran too long) or leave the crew in a dangerously low orbit or suborbital condition (if it falsely registered a dry tank).


NASA has a rule that three of these four ECO sensors must be working before launch. These sensors have caused headaches STS-114, STS-121, and STS-115, which all had ECO problems. And now they're happening again. It is unclear exactly what the problem is with these irritating sensors. Now NASA wants all four ECO sensors to function correctly for launch to proceed tomorrow.


As the Space.com article quotes Wayne Hale as saying:
"I would like to quit talking about ECO sensors..We're going to have to pursue it at an even more vigorous level."

I have to agree. Obviously there's some underlying problem with the sensor system itself.

What analysis can we derive from this for the future of the space program? Not much; this is a fairly specific problem. But it's a good indication of the complexity and coordination required for a Shuttle launch; these four little sensors are holding up the entire half-billion dollar mission. Shuttle hardware is indeed aging.

Some background on the ECO sensors