SpaceX still hasn’t quite figured out what caused its Falcon 9 rocket to explode

Still horizontal.
Still horizontal.
Image: SpaceX
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When will SpaceX fly again?

That question has been asked with increasing urgency since September, when the company’s flagship rocket, the Falcon 9, was grounded after one exploded during a routine fueling operation. SpaceX and its client Iridium had hoped to fly ten communication satellites on Dec. 16, but were forced to acknowledge this week that the rocket will not be ready until early January.

That return would be quick, even for SpaceX. The company last recovered from a failure in six months, which was considered unusually fast in the industry.

To repeat the feat, Elon Musk’s space company needs a license from the Federal Aviation Administration. The FAA is waiting for SpaceX to submit a preliminary report explaining exactly what went wrong and how it will be fixed; then the regulator and the company can agree on a final report in advance of issuing a new launch license to fly Iridium’s satellites. Unfortunately, that investigation, which began minutes after the accident, still lacks a final conclusion. 

SpaceX has said its investigation focused on a specific system in the rocket, one that uses extremely cold helium to maintain pressure in tanks containing the liquid oxygen as it is combusted, with high-grade kerosene, to propel the rocket. The tanks suffered “a large breach” that became a massive fire as the rocket was being fueled for a test-firing days before launch.

Chilling the liquid oxygen to extremely low temperatures—below 200° C—allows SpaceX to cram more of it into the rocket, extending its range and power so that it can be reused. But the process also caused delays, as engineers worked to develop reliable fueling practices to get the cold liquid oxygen into the vehicle before it warmed.

This same system was involved with the company’s previous failure, a June 2015 mid-flight explosion of a Falcon 9 carrying cargo and experiments to the International Space Station. In its review of that incident, SpaceX determined that one of the struts used to secure the helium containers inside oxygen tanks snapped under the force of the launch, sending it careening through the tanks and leading to the explosion. Since isolating that failure, the company has launched nine Falcon 9 rockets successfully, and said in a statement that the struts were not to blame for the September fire.

Some outside the company have speculated that the strut was itself a scapegoat for other failures. A NASA investigation into the incident suggested that a number of problems in manufacturing and materials choices could have played a role in the issue, leading to a reprimand from NASA officials and an internal restructuring at SpaceX to improve quality-control procedures.

Many theories for the current problem revolve around how the helium tanks are constructed, and specifically a piece of technology called a composite over-wrapped pressure vessel, or COPV. Essentially a thin aluminum liner coated in carbon fiber, these containers are important to modern rockets because they are very light and very strong. The trade-off is that, submerged in liquid oxygen at ultra-low temperatures, it’s possible that the fiber shell can develop tiny cracks, become brittle, ignite under light pressure, or even combust spontaneously.

SpaceX has been torture-testing their helium COPVs to determine what went wrong. In October, the company wrote on its website that “through extensive testing in Texas, SpaceX has shown that it can re-create a COPV failure entirely through helium loading conditions,” suggesting that they had a solid lead on what to do to avoid repeating the incident. On a CNBC appearance in early November, Musk suggested the problem related to the liquid oxygen becoming so cold that it actually became solid.

Whatever their conclusions, SpaceX will still need to bring the FAA onboard before they can fly again. One company official estimated it took a few weeks to obtain a launch license following the submission of the 2015 failure report.

The delays are not helping the company meet its obligations to its customers. In addition to pushing its client’s Falcon 9 launches back, returning to flight is also stressing SpaceX’s ability to develop its Falcon Heavy rocket and bring it into service on schedule. This week, the British telecom company Inmarsat switched a planned 2017 launch from the Falcon Heavy to a rocket built by Arianespace, the European launch company. SpaceX still retains a contract to launch a future satellite with Inmarsat.

“We’re continuing to make progress with the investigation into our Sept. 1 anomaly and we are working to safely and reliably return to flight in early January,” SpaceX said in a statement. “Inmarsat is a long-time partner, and we wish them well with their upcoming mission.”