In a press conference today, SpaceX CEO Elon Musk said a preliminary investigation has identified the failure of a strut in the second stage liquid oxygen (LOX) tank as the failure of a Falcon 9 rocket last month.
Musk said engineers believe a strut holding a helium bottom snapped inside the tank, cause the bottle to shoot up to the top of the tank and releasing helium. The helium caused an overpressure event that ruptured the second stage, causing a Dragon supply ship headed for the International Space Station (ISS) to fall into the sea.
Musk stressed that the conclusion was preliminary at this point, not definitive. Engineers are continuing to investigate the accident and looking at other failure modes.
He said the company would be switching to a new strut design, probably manufactured by a different outside supplier. SpaceX would also individually test each strut and not rely on certifications from the supplier.
The SpaceX founder was non-committal on when flights of the Falcon 9 would resume. He thought they could begin again no earlier than September, but possibly at a later time. The inaugural flight of the larger Falcon Heavy, which had been planned for later this year, has been pushed to spring 2016.
Musk said there were no problems with the first stage, which continued to fire normally even as the second stage was breaking up above it. The Dragon capsule survived the accident and continued to send telemetry to controllers until it disappeared over the horizon.
The Dragon could have been saved if the software aboard it had been programmed to activate the parachute system during an abort, Musk said. The company had planned to upgrade the software on the cargo version of the Dragon, but engineers had not yet done so prior to the accident. This change will be made in the future.
The strut was rated to survive pressures up to 10,000 pounds per square inch (psi), but it failed at about 2,000 psi, Musk said. After extensive testing of struts after the accident, engineers found one that failed at below that level.
Musk said SpaceX had previously flown thousands of struts of the exact design in both the first and second stages of the Falcon 9 without incident.
He added that acoustic triangulation in tank pointed to a failure of the strut, not a bursting of the helium tank. Engineers would have seen a more steady stream of helium being released had the tank broken.