"One concern is a potential problem with one of four hydrogen propellant engine cutoff sensors in the new tank, says Mike Leinbach, Kennedy shuttle launch director. The same system in two different tanks delayed Discovery's initial return to flight last summer.
"The wayward sensor is showing off nominal current readings. If the unusual readings prove to have a predictable value that can be accounted for during countdowns, the sensor would be viable for use. But if several tests show divergent readings, that could mean a time-consuming sensor changeout, Leinbach said. All four of the tank's hydrogen ECO sensors must remain valid in the final phases of a launch countdown."(link)
I'm forced to shake my head at this one. Back during the last launch I was struck by how no one was paying attention to this issue after NASA "said" it was fixed.
To provide a brief recap to people, the "engine cut off" (ECO) sensors showed signs of failure months before last years launch. The sensors detect if the fuel in the External Tank runs out. This is critical, because the shuttle main engines run from fuel provided by the main tank. If the main tank runs out of fuel, and the main engines continue to burn - this is a kinda "kaboom" moment here folks.
Engineers requested that another fueling test be done to confirm that the fix had worked. It was ignored. They tried to launch, and the sensors went all whacky again and it was delayed. Then they tried swapping grounding cables for the sensors, hoping that the problem was in the grounding, and hence the problem should follow the grounding. Another fueling test was requested, only to be denied. NASA needed to launch, and further delay was obviously unacceptable.
I've blogged extensively on this topic before.
I provided this commentary back then, when the shuttle finally flew:
"...regardless they should at least understand what the crap is going on before they flip the switch. NASA wouldn't have had to spend the extra money now, if it had just listened to NASA engineers back in April, when they asked for another fueling test to verify that the fix they made back then on the ECO sensors worked.
What now? Well I hope they don't forget about this, and figure it out for the next flight.
Now a new tank, and still the same problems. I think it's should be obvious to anyone: the fix didn't work. They still don't know what is going on.
Maybe they should figure it out? That's just some Canucks opinion eh...