A busted nut, not human error, is to blame for the fuel leak that doomed the Falcon 1 rocket on its maiden flight, according to the findings of a government review board chartered to investigate the March 24 launch failure.

The rocket’s manufacturer, Space Exploration Technologies (SpaceX) of El Segundo, Calif., initially suspected that the Falcon 1’s fuel leak was caused by a tiny pipe fitting loosened and accidentally left untightened by technicians doing work on the rocket the day before launch.

The U.S. Defense Advanced Research Projects Agency (DARPA), the sponsor of the Falcon 1’s inaugural flight, announced July 18 that the review board concluded a small aluminum nut designed to hold the fuel pipe fitting in place failed due to subsurface corrosion not visible to the naked eye. The resulting kerosene leak caused the main engine to catch fire shortly after the rocket cleared the launch pad, bringing the flight to a premature end.

“The board determined that the only plausible cause of the fire was the failure of an aluminum B-nut on the fuel pump inlet pressure transducer due to inter-granular corrosion cracking,” the DARPA release states. “This caused [Refined Petroleum-1, or RP-1] fuel to leak onto the engine and down the outside of the thrust chamber. Once the engine ignited, the leaking fuel caught fire. The fire, over time, resulted in a loss of pneumatic pressure, causing the RP-1 and liquid oxygen pre-valves to close, terminating engine thrust 34 seconds after ignition.”

After the engine shut down, the Falcon 1 fell back to Earth, crashing into a reef less than a kilometer from SpaceX’s private Pacific Ocean island launch facility in the Kwajalein Atoll. Upon impact, the rocket’s payload, the experimental FalconSat-2 spacecraft built by cadets at the U.S. Air Force Academy, was thrown skyward and came crashing down through the roof of an unoccupied storage shed on the island and came to rest next to the shipping container in which it had arrived.

Within days of the launch failure, SpaceX’s chairman and chief executive officer Elon Musk said that human error probably was to blame.

While he has not ruled out that mistakes were made in the hours leading up to launch and is taking steps to reduce the likelihood that human error will botch another Falcon 1 launch, Musk told Space News that he accepts the findings of the board, which included SpaceX personnel.

“It appears at this point there was not a pad processing error but, in a stroke of unbelievably bad luck, the nut actually broke sometime in the 18 hours prior to launch,” Musk said.

Musk said an inspection conducted the day before the launch found that the nut was still intact and holding its pipe fitting securely in place. Rocket parts recovered after the accident show that nut’s lock wire was still attached, an indication that technicians working on the rocket the day before had properly tightened the pipe fitting before closing up.

Why the nut cracked is still a bit of a mystery. Musk said the “prevailing thought” among investigators, which included a mix of government and SpaceX personnel, is that the heat and humidity on Omelek Island was a contributing factor. “We had a series of countdowns [during which] the rocket was exposed for quite a bit of time,” Musk said. “And the vehicle hangar for about three months was not climate controlled.”

Though the nut was anodized to guard against corrosion, Musk said, it may have been scratched at some point, compromising its protection against the elements.

Another possible contributing factor, Musk said, is that there was an adverse reaction — so-called galvanic corrosion — between the aluminum nut and the pipe fitting itself, which was made of stainless steel.

Musk said SpaceX will replace the $5-a-piece aluminum nuts with less-expensive stainless steal nuts to avoid that problem in the future.

“The irony is we are replacing them with a cheaper component to increase reliability,” he said.

SpaceX’s planned Falcon 1 changes do not stop there.

In addition to what Musk characterized as small design changes, such as switching to all stainless steel nuts, fire-proofing the main engine components, and wrapping the vehicle in a protective shroud to protect it from the elements while it awaits launch, SpaceX also is implementing bigger changes. These include more rigorous pre launch quality control processes, improved vehicle health monitoring and a more highly automated countdown intended to reduce the risk of human-induced error, Musk said.

Musk said that while these changes will increase the development cost of the $6.9-million-per-launch Falcon 1 somewhat, he does not intend to pass the costs on to his customers. “The only investor I’m accountable to is me,” he said.

Musk, who made a fortune before his 30th birthday creating the electronic payment service PayPal and selling it to eBay, has invested about $100 million of his own money to date in the development of the Falcon 1, its larger follow-on the Falcon 9, and a proposed crewed capsule dubbed Dragon he hopes to sell to NASA for international space station missions. Revenues from 10 booked launches to date, Musk said, account for most of an additional $40 million to $50 million SpaceX has spent so far.

The next Falcon 1 launch is slated for November from Kwajalein. Musk said the DARPA-funded launch will not carry a satellite but will carry some DARPA-provided instruments, which he declined to describe. He said the launch is primarily a test flight meant to demonstrate the design and procedural changes SpaceX is making.

Assuming the flight goes well, SpaceX is due to launch TacSat-1 for the Pentagon early next year.

Comments: bberger@space.com