WASHINGTON -- A busted nut, not human error, is to blame for
the fuel leak that doomed the Falcon 1 rocket on is 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, 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] 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 too 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 factor. 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. "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."
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 steel 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 is also 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.
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