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SpaceX suspects bad steel strut caused Falcon 9 breakup

Irene Klotz, Spaceflight Correspondent
Jul 21, 2015, 6:30 UTC

Sen—SpaceX believes its Falcon 9 rocket exploded minutes after liftoff last month when a defective steel strut snapped, causing a bottle of helium to shoot to the top of the liquid oxygen tank inside the booster’s upper-stage, chief executive Elon Musk said Monday.

The accident claimed a Dragon cargo ship making a supply run to the International Space Station.

The cause of the accident remains under investigation, but SpaceX engineers tested thousands of the struts and found a few that failed far below the force they were certified to withstand.

“That’s what led us to think that there was one just far below its rated capability that happened to be at the wrong place at the wrong time,” Musk said.

The steel struts, supplied to SpaceX by a company Musk declined to identify, hold the composite-wrapped helium pressurization bottles inside the rocket’s first- and second-stage fuel tanks.

The struts are certified to withstand 10,000 pounds of force, but engineers believe one gave way at 2,000 pounds, triggering the rocket’s mid-air breakup.

“It failed five times below its nominal strength, which is pretty crazy,” Musk said.

SpaceX will buy new struts, most likely from a different vendor, and test each one, despite whatever materials certification the company provides.

It also plans to fly new software aboard its Dragon cargo ships so that in case of a launch accident the capsule’s parachutes will deploy. SpaceX already planned to include that feature on the passenger version of Dragon it is developing to fly crew to and from the space station.

The capsule survived the breakup of the rocket and was destroyed when it impacted the water. Dragon, which is the only station cargo ship that returns intact to Earth, already has parachutes aboard.

“If the software had initiated the parachute deployment then the Dragon spacecraft would have survived,” Musk said. “We’re now including contingency software that if something were to go wrong with the vehicle, Dragon will always attempt to save itself.”

The ongoing investigation, coupled with the strut testing and other changes, will keep the Falcon 9 fleet grounded until at least September, Musk said.

The company also will likely delay the debut flight of its 27-engine Falcon Heavy until next spring. The rocket had been slated to fly at the end of this year or early 2016.

Musk said it had not yet been decided which mission will be first in line when Falcon 9 returns to flight. The company had planned to fly an ocean-monitoring satellite in July, followed by a commercial communications satellite launch for SES in August, and another Dragon cargo mission to the space station for NASA in September.