SCIENCE -- May 2, 2011 at 5:00 PM ET
Just Ask: What's Behind Space Shuttle Endeavour's Electrical Glitch?
Photo courtesy of NASA.
Updated May 2, 6:00 p.m. ET | NASA now says that repairs to replace and retest a faulty power distribution box has pushed the launch date to May 10 at the earliest.
At the root of Friday's scrubbed space shuttle launch is a simple fact about fuel. Hydrazine fuel must be kept warm or the fuel lines will freeze. The shuttle's built-in heaters keep this from happening.
Hydrazine is both toxic and flammable. Frozen hydrazine could expand and crack open the fuel line, exposing the fuel, which could cause a fire upon reentry.
Endeavour's launch was postponed on Friday when heaters in one of the shuttle's power units failed.
Endeavour is powered by three fuel cells. From each of its three auxiliary power units, electricity gets distributed by something called a load control assembly, which is essentially a 48-pound box of electrical switches. Power is routed through this switch box to the two thermostats and two heaters on each of the shuttle's fuel lines. Shuttle managers think the problem is associated with wiring from the one of these switch boxes -- the aft load control assembly-2 -- to one of the heaters.
"We were not able to get [the heater] to come to life no matter what we did," said NASA's shuttle launch director Mike Leinbach on Friday. "We tried to get the line to cool down by normal means to see if the thermostat on the heater would kick in. That did not happen...so we know we have a hard failure in that heater for that one auxiliary power unit."
Engineers were still troubleshooting on Monday to identify the precise source of the electrical glitch, according to NASA spokesman Allard Beutel. The plan calls for removing the switch box Monday and installing its replacement Tuesday.
"They've been methodically testing for the past couple of days," Beutel said. "They know it wasn't the thermostat, and it wasn't the circuit breaker associated with that. They're ruling out system after system to see what it is that's preventing power from going to the heater." The heaters work fine when they get power, he added.
Each fuel line has a backup heater in case the main heater fails. But the system controls all hydraulics for the shuttle, so launching without a backup is a bad idea.
The team is equipped with a new controller box ready to swap in. But installation of the new box must be followed by another exhaustive round of testing. The box is located in the back section of the shuttle and provides power distribution not only to the heater, but to nine different systems, which together control about 90 different functions.
"Once you replace the box, you're breaking connection to all systems that were working just fine on Friday," Beutel said. "So you have to go back and do extensive testing of all these systems."
Draining and refueling the shuttle's external tank with liquid oxygen and liquid hydrogen propellants for the rescheduled launch will cost about $500,000. Propellant from Friday's planned liftoff had to be burned off and drained after the delay.
Meanwhile, astronauts are now back in pre-flight medical quarantine at Johnson Space Center in Houston. NASA astronaut Gregory Johnson sent out this tweet early Monday morning:
Enjoyed an 18 hour reprieve in Houston spending an evening at home. Now we're back in quarantine ... reminds me of the movie "Groundhog Day"
During its 14-day mission, the crew will deliver to the International Space Station a $2 billion particle physics detector, know as the Alpha Magnetic Spectrometer, or AMS. AMS is a massive, supercooled magnet designed to soak up cosmic rays and search for dark matter and antimatter. The crew will also deliver spare parts, including two communication antennas and a high-pressure gas tank.
Beutel expects a new launch date to be set within the next few days. "There's that ambiguity of an exact new target date, only because we've got to give the teams time to do the testing they need to do," he said. "We'll get there. We're just not there yet."
We're still taking questions for our special LIVE interview with the Endeavour crew, which is expected to take place three days after launch. It's part of a collaboration with Google and YouTube.
Email your Just Ask! questions to firstname.lastname@example.org with "Science Question" in the subject line.