Musk: SpaceX rocket explosion: steel strut failed, causing helium bottle to brake free inside and cause high pressure, explosion
A steel strut holding a helium bottle in place inside the doomed Falcon 9 rocket broke, causing a chain reaction less than a second long that ended with the rocket exploding over Cape Canaveral June 26.
That's the preliminary assessment offered by SpaceX founder Elon Musk Monday afternoon in a teleconference explaining how the rocket explosion likely occurred, and what the company intends to do to fix it.
Musk said the fix should be easy -- a new kind of strut. He said that his company expects to be launching rockets from Cape Canaveral Air Force Station again within a few months, perhaps as soon as September.
The reason for the strut failure remains somewhat of a mystery because it was designed to handle five times as much force as it was experiencing at the time it failed, about two minutes after launch. The key pieces of rocket debris were not recovered.
Musk speculated that the alloy structure of the steel was not up to specifications, and said SpaceX's subsequent tests of other struts in inventory found others that appeared inadequate. He would not name the supplier that made the struts, but indicated a new supplier would be found.
Musk emphasized the theory is a preliminary finding and more investigation will be done.
He said SpaceX has informed its customers and the government of the theory and shared data, and they have agreed that the explanation makes sense. The explosion destroyed the $70 million rocket, its Dragon I capsule and 4,000 pounds of supplies that was headed to the International Space Station. None of it was insured.
SpaceX already has postponed its planned launches in August, but Musk said he was hopeful the delays would not be long.
Initially, SpaceX did not suspect strut failure, because they were rated to handle 10,000 pounds of load, but were experiencing just 2,000 pounds when something went wrong. All future struts going into the rockets will be individually tested, he said.
"At 3.2 Gs [of pressure] the strut holding down one of the helium bottles appears to have snapped. As a result, releasing a lot of helium into the upper-stage oxygen tank and causing a pressure event quite quickly," Musk said.
The whole event, from the instant that on-board monitors detected the first unusual noise to the point where the rocket had exploded enough that SpaceX lost all contact, was just .893 second, he said.
Musk said the capsule and its contents could have been saved had SpaceX loaded it with the same emergency software that it has designed for its astronaut-rated Dragon II capsules. He said there was no reason the software couldn't be on the cargo capsules, and it will be included on all future launches.
The Dragon capsule actually survived the explosion but was lost when it fell into the Atlantic Ocean, he said.
"If the software had initiated the parachute deployment, then the Dragon spacecraft would have survived," he said. "That was the frustrating thing. We could have saved Dragon if we had the software in there."
Helium bottles play crucial roles in SpaceX Falcon 9 rocket boosters. Several -- the number varies per launch -- are installed inside the liquid oxygen tanks of each stage, held in place by struts. As the rocket burns its oxygen and rocket fuel, the helium bottles discharge and replace those materials with helium, keeping the tanks pressurized.
In SpaceX's preliminary theory, when the strut in the upper stage broke, the helium bottle twisted loose. Because of the buoyancy of helium within a liquid oxygen core, the bottle shot to the top of the tank and discharged its contents. That over-pressurized the oxygen tank, causing it to rupture, and then the booster to explode.
Most of what was left of the second-stage booster fell into the ocean and sank to the bottom, Musk said. It has not been recovered, though he said SpaceX was planning to send a submersible down to see what could be salvaged. That debris on the bottom of the ocean could verify the preliminary findings.
For now, SpaceX is relying on telemetry it received from about 3,000 different sensors in the rockets. Among them were sound sensors that were able to precisely triangulate the point where the first unusual sound came from, and that appeared to be a breaking strut on one of the helium tanks.
"The best of what we know so far, this is an initial assessment, and further investigation may or may not reveal more over time," Musk said.
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A failed strut caused the SpaceX rocket explosion
(NASA TV)
The SpaceX explosion on June 28th was caused by a failed strut
in the rocket's upper stage liquid oxygen tank, SpaceX chief executive
officer Elon Musk said today. The strut was one of several hundred used
to hold together the helium pressure vessels in the tank, which help to
pressurize and maintain the buoyancy of the rocket. According to Musk,
the strut was designed to handle 10,000 pounds of force, but failed at
just 2,000 pounds of force.
"This is the best of what we know thus far."
"This is the best of what we know thus
far," said Musk during a press conference. "We emphasize this is an
initial assessment, and further investigation may reveal more over
time."
The strut was a steel rod that’s about
two feet long and one inch thick. Struts like the one that failed have
flown on several previous Falcon 9 flights before. Musk said SpaceX
still doesn’t know why the steel rod snapped, but it’s possible that its
material was faulty. Musk didn’t name the strut's supplier, but did say
it may be that one strut of thousands wasn’t up to code.
When the strut snapped, the helium tanks released a lot of
helium into the upper-stage liquid oxygen tank, Musk said. That put too
much pressure on the tank and caused it to explode.
The Dragon capsule, filled with supplies
for the International Space Station, survived the explosion and
continued to communicate data to ground control for a while afterward.
If the Dragon had been equipped with different software, the capsule
would have been able to deploy its parachutes and save its contents.
Future versions of the Dragon cargo capsule will have this capability,
Musk said.
"It's the first time we've had a failure
in seven years so, to some degree, the company as a whole got a little
complacent," said Musk. "Especially with all the successes in a row, I
think this is an important lesson and something we'll take with us into
the future."
"This is an important lesson and something we'll take with us into the future."
At the 2015 ISS Research and Development Conference, Musk described the failure as a "huge blow for SpaceX." Until the June incident, SpaceX had an impeccable launch record. This recent rocket loss was the first major failure out of 19 total Falcon 9 launches. As a result of the accident, SpaceX has postponed its launches of the Falcon 9 for the next couple of months. Their next launch, which was originally scheduled for August 9th, will carry the National Oceanic and Atmospheric Administration's Jason-3 Earth observation satellite to orbit. Musk said the launch postponement will result in hundreds of millions of dollars in lost revenue.
Update, 4:30PM, July 20th: SpaceX
will switch to different struts with even better safety records,
according to Musk. The company will also change their procedures for
testing the struts before lift off — SpaceX employees will
conduct individual unit tests on every single steel beam. That shouldn’t
raise the costs a "significant amount," he said.
Musk said the Falcon 9 failure shouldn’t
affect the company’s contract with NASA for its Commercial Crew Program,
which tasks SpaceX with ferrying astronauts to and from the ISS.
However, the first test launch of SpaceX’s Falcon Heavy rocket, which
will carry these astronauts into space, has been postponed. Originally
scheduled for later this year, the first Falcon Heavy launch is now
slated for spring of 2016.