The spectacular June 28 failure of a SpaceX Falcon 9 rocket apparently was triggered by a support strut inside the booster's second stage liquid oxygen tank that suddenly broke free, releasing a buoyant high-pressure helium bottle that caused the rocket to break apart from the inside out, company founder Elon Musk told reporters Monday.
Providing initial results of an internal failure investigation, Musk said the strut in question, one of several aboard the rocket that was purchased from an unidentified vendor, apparently failed well below its design specification, allowing the high-pressure helium bottle it was holding in place to twist away.
Helium, contained in several composite overwrap bottles at 5,500 pounds per square inch, is used to pressurize the first and second stage propellant tanks to maintain the rocket's structural integrity as liquid oxygen and kerosene fuel is consumed during the climb to space.
The bottles are immersed in the liquid oxygen tanks of the first and second stages to cool the gas before it is routed to the engines, warmed up and sent back to the tanks to maintain the proper pressure.
During the June 28 launch, the Falcon 9's first stage operated normally to boost the rocket out of the dense lower atmosphere. But near the end of its planned burn, about two minutes and 39 seconds into flight, the second stage appeared to rupture, spilling its load of propellant in a thick cloud of white vapor.
The first stage continued firing, but moments later the rocket broke up in a shower of debris. The Dragon cargo capsule it was carrying, loaded with more than 4,000 pounds of equipment and supplies bound for the International Space Station, apparently broke away from disintegrating rocket intact.
Musk said engineers were able to receive telemetry from the spacecraft until it disappeared over the horizon on the way to a catastrophic ocean impact. He said a software change will be implemented on future space station resupply flights to enable the Dragon's parachutes, normally used to lower the craft to a gentle ocean landing at the end of a mission, to deploy in the event of a launch failure.
"That's an unfortunate thing," Musk said. "We could have saved Dragon if we had the right software" on board.
It was the first failure in 19 flights of a SpaceX Falcon 9 and the second loss in a row for the space station program following the April 28 failure of a Russian Soyuz rocket carrying a Progress supply ship. The Russian cargo craft reached orbit, but it suffered severe damage moments before, or during, release from the booster's upper stage and crashed back to Earth on May 8.
Lost in the SpaceX mishap, along with supplies and equipment for the station crew, was the first of two docking mechanisms needed to enable commercial crew capsules being built by Boeing and SpaceX to dock with the lab complex starting in 2017.
Musk said the failure and subsequent launch delays likely will cost the company several hundred million dollars in lost revenue. But he said he's hopeful flights can resume as early as September, although it's not yet clear how the manifest will shake out or what customer will get the next launch slot.
He also said the debut launch of the company's powerful Falcon 9 Heavy, made up of three Falcon 9 first stages strapped together, will be deferred to next spring while the company focuses on recovering from the failure.
"This is the first time we've had a failure in seven years, and I think to some degree the company as a whole became maybe a little bit complacent," he said. "When you've only ever seen success you don't fear failure quite as much. ... Now everyone in the company appreciates how difficult it is to get rockets into orbit successfully, and we'll be stronger for it."
SpaceX holds a $1.6 billion contract with NASA for at least 12 missions to deliver more than 20 tons of equipment and supplies to the space station. The Dragon capsule launched on June 28 was the seventh operational flight under the NASA contract. A new contract is expected to be awarded later this year, and Musk said he did not expect the failure to cause any changes in the company's bid.
But solving the mystery of what went wrong has been a major challenge. From the first indication of a problem to the complete loss of telemetry took just 0.893 seconds.
Data from more than 3,000 telemetry channels were analyzed, along with tracking camera footage and video from on-board cameras. When all was said and done, a strut failure was the most likely explanation for the mishap. And that was determined by acoustic triangulation -- locating the exact position of the break by analyzing sound from different sensors -- not from any definitive data in the telemetry stream.
"At approximately 3.2 Gs, the strut holding down one of the helium bottles appears to have snapped and as a result, releasing a lot of helium into the upper stage oxygen tank and causing an overpressure event quite quickly," Musk said.
The steel struts measure about two feet long and an inch or so thick. They are certified to handle 10,000 pounds of stress, but the failure occurred at a calculated load of just 2,000 pounds.
Musk said the data were confusing because they initially showed a drop in helium pressure, which would be expected in a breach, "and then, somewhat strangely, a rise in the helium system back to approximately its starting pressure."
"This is quite confusing, but we think what may have happened is that as the helium bottle broke free and twisted around, it may have pinched off that line to the helium manifold and restored pressure in the helium system but released enough helium into the liquid oxygen tank to cause the liquid oxygen tank to fail," he said.
It was not clear whether the buoyant helium bottle might have shot up to the top of the oxygen tank, possibly damaging the structure, or whether it simply released enough helium to cause it to rupture. Either way, engineers believe, the failure of the oxygen tank triggered the rocket's destruction.
Musk said engineers examined close-out photos to make sure the struts used in the rocket that failed had been installed correctly. No problems were found. But during tests to measure the actual strength of struts in the SpaceX inventory, one broke at less than 2,000 pounds. Microscopic inspection revealed abnormalities.
Musk did not identify the vendor, but said SpaceX is considering switching to a different supplier and possibly a different material. In the meantime, he said, each individual strut delivered to SpaceX will be tested to verify its actual strength.
Musk repeatedly stressed that the investigation was ongoing and that the current theory -- a strut failure in the second stage -- was just that, a theory, and that additional analysis is planned. He also said engineers were on the lookout for any "close calls" that might have happened indicating a more subtle problem that could cause a failure down the road.
"The fundamental nature of rocketry is that is a case where a passing grade is 100 percent, every time," he said. "It's not possible to issue a recall or a patch or anything like that. From the moment of liftoff, it's 100 percent or nothing. So we'll be looking for any tiny thing that can improve things in the future.