A steel strut which failed under much less load than it was rated to carry was the cause of the recent mid-air explosion of a SpaceX Falcon 9 rocket bound for the International Space Station, according to SpaceX chief Elon Musk.
In a teleconference with journalists, Musk said that the 2ft (60.96cm) long and one inch (2.54cm) wide steel strut was holding down a helium pressure vessel situated in the liquid oxygen fuel tank on the Falcon 9's second stage. When the strut failed, it caused a momentary overpressure in the tank, which destroyed the second stage.
"It's fundamental to the nature of rocketry that the passing grade is 100 per cent every time," Musk said. "You can't recall or patch things. From the moment of takeoff it's 100 per cent or nothing."
The rocket's liquid oxygen tanks need to be pressurized as they empty, which is where the helium bottles come in. The bottles, which store the helium at 5,500psi, become more buoyant as the g-force levels increase, and although the strut was verified to withstand 10,000lb of pressure, it failed at 2,000lb.
The SpaceX engineers have spent the last three weeks examining data from the accident, in particular the 0.863 seconds between when the rocket went from all systems green to failure. Sensors on board were triangulated to identify the source of the failure, and a "huge" amount of testing on the remaining struts has found that a few units were not as strong as they should be.
Musk said it was probable that after the strut broke, the helium bottle shifted and released a burst of helium before twisting and closing the gas release valve, but that single overpressuring of the oxygen tanks caused the breakup.
The rest of the rocket performed admirably, he said, with the first stage continuing to fire through the wreckage and on upwards for several seconds after the second stage was destroyed. The Dragon cargo capsule was unharmed and remained in contact with ground control until it hit the ocean and broke up.
"The saddest thing about this is that with just a different piece of software, Dragon would have made it," Musk said. "If the software had initiated the parachute deployment then the Dragon would have survived. Future flights will have contingency software installed on Dragon so it will attempt to save itself, and the same system will be included in Dragon 2."
Sadly, the remains of the capsule haven't yet been found. SpaceX is sending a remote-operated submersible to the Dragon's landing spot to try and find any wreckage, but there's a lot of ocean floor to search.
SpaceX has now ordered a redesigned strut from a new supplier and will be testing each one individually before installing them in future rockets. Additional testing and analysis are also ongoing and the strut issue was described as the provisional cause, not a definitive one by Musk.
That single strut failure will, unfortunately, cost SpaceX several hundreds of millions of dollars, Musk said. The Falcon 9 rocket won't fly until at least September, and the inaugural flight of the Falcon Heavy rocket now won't happen until April next year at the earliest, which will hurt the firm's cash flow.
Musk thanks his customers, NASA, and the US Air Force for its support on the matter and their trust in still supporting SpaceX. The incident had shocked SpaceX staff, many of whom had never experienced the firm having a failed launch.
"Before every flight I always email the entire company saying that if anyone can think of any reason to hold off then to call or email me immediately, whether their manager agrees or not," Musk said.
"After the 20th time it seems like 'Elon being paranoid again,' but now people appreciate how difficult this is, and we'll be stronger for it." ®