Investigators find no root cause for Taurus mishap
BY STEPHEN CLARK
Posted: July 17, 2009
Investigators could not find a definitive cause of the February launch failure that doomed a $273 million environmental satellite, but officials recommended a series of improvements for the Taurus rocket in a report released Friday.
Rick Obenschain, chairman of the board, said each potential cause has both supporting and refuting data and all investigators know for sure is the fairing was not jettisoned.
"That's as far as we can go absolutely for sure," Obenschain said. "All we can say is any of these four (findings), if they had happened, would have resulted in the fairing not separating."
The 63-inch fairing, made of two halves designed to break apart, stayed attached to the rocket throughout the launch, weighing down the booster and preventing the NASA-owned spacecraft from reaching orbit.
The Orbiting Carbon Observatory and the Taurus upper stage fell back to Earth near Antarctica after launching from the California coast on Feb. 24.
"The board is very confident that if these four potential causes and the associated recommendations are followed, we will not have a repetition of a fairing not separating on a Taurus vehicle. It won't happen again," Obenschain said in an interview Friday.
During a normal Taurus launch, the fairing halves separate along frangible joints holding the fairing together and attaching the shroud to the rocket. The frangible joints contain small explosive charges to sever the connections, allowing the fairing to fall away.
Small pneumatic thrusters, pressurized by a hot gas generator, push the fairing parts outward in a hinge-like motion.
The potential causes for February's mishap include an incomplete fracture of the frangible joint base ring between the fairing and the launch vehicle. Investigators could not determine if the joint operated as designed from available data.
Obenschain said the board was also not sure whether the fairing's pyrotechnics received enough current from an electrical subsystem. The ordnance requires about 24 amps to fire correctly, but live telemetry from the rocket only measures a range up to 13.9 amps.
The committee recommended changing the telemetry to cover the entire range of expected electrical currents.
Another focus of the board was on the pneumatic system designed to force apart the fairing halves after the explosive joints fire.
A hot gas generator provides pressure for the system, but analysts were not able to determine if the pneumatic thrusters had enough power to push the fairing outward as designed.
The report listed five corrections for the hot gas generator, including design changes, more stringent acceptance criteria, and better testing of pressure cartridges used to ignite the generator.
Obenschain said Orbital should replace the hot gas system with an alternative if engineers are not able to make the recommended changes.
Hot gas generators are also used by Orbital's air-launched Pegasus program. A larger 92-inch fairing employed by other Taurus vehicles uses a cold gas system that does not employ the troublesome pressure cartridges.
A fourth potential cause could be a snagged detonating cord that prevented the fairing from separating, but further analysis may show this possibility is remote.
"These are our four potential causes, but we aren't ranking them as likely or unlikely," Obenschain said.
Obenschain, also deputy director of NASA's Goddard Space Flight Center, said engineers were not able to confirm whether the potential causes actually occurred during February's launch.
But any of the potential causes could have led to the fairing's failure to separate, Obenschain said.
"While we didn't find the classic root cause, we do have a way to prevent a repetition in the future, which is a good thing," Obenschain said.
Investigators impounded and reviewed more than 2,000 pieces of data and conducted 78 personnel interviews, according to NASA.
Orbital Sciences Corp., builder of the Taurus rocket, conducted a concurrent independent investigation into the accident to help inform designers of necessary changes to the launch vehicle.
The company also reviewed the NASA report, which was not released publicly due to concerns about proprietary information and international arms regulations.
"The company intends to address the board's findings prior to the next mission of the Taurus rocket. In fact, Orbital has already begun the process of implementing certain corrective actions and fully expects that all necessary steps will have been taken to assure mission success by early 2010 when the company is scheduled to launch NASA's Glory satellite aboard a Taurus rocket," Orbital said in a written statement.
Glory's launch is currently targeted for no earlier than next April.
"Whatever path we decide to take, we have plenty of time to accommodate it," Obenschain said.