SpaceX Supply Flight Lost Due to Strut Failure

By Doug Mohney July 22, 2015

The June 28, 2015 loss of a SpaceX Falcon 9 and its Dragon supply mission to the International Space Station (ISS) was most likely due to the failure of a strut in the rocket's second stage liquid oxygen tank.  Estimates of SpaceX financial losses due to the accident are in the hundreds of millions.

SpaceX CEO and Lead Designer Elon Musk was the sole spokesperson discussing the company's preliminary findings in the loss of the CRS-7 supply flight in a press conference on July 20.

"A passing grade [for space flight] is 100 percent every time," said Musk. "From liftoff, its 100 percent or nothing."

It took barely a second—0.893 seconds by SpaceX reconstruction of the data—between any indication of a problem and loss of all data from the rocket, with the mishap taking place around 139 seconds into flight.   SpaceX gets around 3,000 channels of telemetry from the rocket and multiple video feeds, so it had plenty of data to sift through in order to determine the sequence of events.

A failed steel strut, around 2 feet long and an inch thick, held down a helium pressure bottle at the bottom of the second stage liquid oxygen tank. The helium tanks are put at the bottom of the tank in order to keep them super cold in order to pack in more gas. Since helium is lighter than oxygen, the tanks essentially want to "float" on top of the liquid oxygen if left unfettered.

When the strut failed, the helium bottle shot up to the top of the liquid oxygen tank and ruptured, releasing all of the helium at once, causing a sudden overpressure and rupture of the tank, resulting in the destruction of the second stage. The second stage continued to operate as designed for a few more seconds before being destroyed in the cloud of debris from the first stage, while the Dragon cargo ship continued to operate without problem until it hit the Atlantic Ocean.

Designed and certified by the manufacturer to withstand up to 10,000 pounds of force before breaking, the strut was purchased from an unspecified third-party. SpaceX engineers expected the part to take anywhere from 3000 to 3500 pounds of force during the course of flight, so they believed they had more than twice the structural margins necessary to operate safely.

Image via Shutterstock

An initial set of strut tests revealed a part that broke at 6,000 pounds, not good, but still within design tolerances.  A second round of tests using a much larger number of struts found at least one part out of the lot failed at around 2,000 pounds of force.

SpaceX plans to test all of its third-party parts moving forward, rather than relying on material certification from suppliers. A new supplier will be found for struts and the new strut could be made from Inconel rather than steel, slightly increasing cost on the rocket.

Musk said the investigation was not showing any other issues, but SpaceX  is going back and looking for any "near misses" where this or other problems might have occurred.  He said the company might have become "a little complacent" with over 20 launch successes since it had last lost a rocket seven years ago.

Between the ongoing safety review and engineering changes, Musk said the next Falcon 9 launch could take place in September. Discussions need to take place with its customers as to who will fly first and who will be willing to fly first, but the backlog is costing SpaceX hundreds of millions of dollars in revenue due to flight delays. No launches mean no payments.

SpaceX's other products have been affected in different ways. The cargo Dragon will get a software upgrade before its next flight to enable it to deploy parachutes in case of a launch failure, providing the possibility of saving the cargo. Falcon Heavy, SpaceX's largest rocket, has been "deprioritized" and is now slated to have its first launch in Spring 2016. 

Musk and NASA don't expect the failure to alter the workflow for its manned spaceflight plans, however. Development of the manned Dragon v2 capsule is proceeding as planned independently of the changes needed to the Falcon 9 rocket designed to carry it and up to 7 astronauts into orbit for space station crew transport. 




Edited by Dominick Sorrentino

Contributing Editor

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