On June 28, SpaceX suffered a major setback when its unmanned CRS-7 mission carrying supplies to the International Space Station (ISS) exploded shortly after liftoff. In a press conference today, SpaceX founder and CEO Elon Musk laid out the preliminary findings of the post-flight investigation which indicate the cause may have been a faulty strut.
According to Musk, the investigation of the incident required the combined efforts of SpaceX, the FAA, NASA, and the US Air Force. Multiple lines of enquiry were followed aimed at seeking not only the cause of the midair explosion, but also other problems that could have led to other failures. He emphasised that the investigation is ongoing and that today’s findings were only tentative.
One difficulty in determining what happened when CRS-7 was destroyed about two minutes and 18 seconds after liftoff is that the accident occurred so quickly. Musk says that the time from the first indication of trouble to loss of all data was 0.893 seconds, which means that the investigation team was parsing telemetry records by the millisecond.
What is known so far is that the Falcon 9 rocket's first stage operated normally and that the fault was in the second stage. Specifically, Musk says that the failure appeared to be due to a faulty strut used to secure a helium tank, which broke loose under the g forces of liftoff.
The helium tank is used to maintain pressure in the oxygen tank as the oxygen is consumed by the second stage rocket engine, which had not yet fired because the stages hadn't yet separated. The helium tank is located inside the oxygen tank so that the liquid oxygen can help keep it cold.
According to Musk, the buoyancy of the helium tank floating in the oxygen increases proportionate to the g loading. The stresses produced by this increase snapped the strut and broke the helium tank free. Helium then flooded into the oxygen tank, over-pressurising it and causing the explosion.
He went on to say that even if it had been a Crew Dragon carrying astronauts, the passengers would have survived, so the CRS-7 accident will not delay SpaceX’s plans to ferry crews to the ISS or alter future launch dates. In addition, all future Dragon missions will be programmed for emergency parachute deployment.
In the meantime, the struts in all Falcon 9 launch vehicles will be replaced and all struts will be individually tested. Though Musk was not apportioning blame, he said that the struts came from an outside vendor and tests on samples indicated that they were not performing to specifications.
Musk says that the investigation is continuing and that a remote submersible will be used to recover parts from the Falcon 9 and the Dragon capsule. He also says that the first failure in seven years has dispelled an air of complacency that had descended on the company.