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SpaceX facility at Vandenberg
SpaceX’s return to flight is due to take place Jan. 8 at its launch facility at Vandenberg Air Force Base in California. (SpaceX Photo)

SpaceX says an investigation has concluded that the Sept. 1 explosion of its Falcon 9 rocket occurred due to the failure of a helium pressure vessel, and it’s taking steps to avoid the problem for its return to flight, set for Jan. 8.

That launch will send 10 Iridium Next communication satellites into orbit from California’s Vandenberg Air Force Base. Meanwhile, repairs are continuing at SpaceX’s launch pad at Cape Canaveral Air Force Station in Florida, where September’s blow-up occurred during a pre-launch fueling test.

The California-based launch company’s founder, Elon Musk, had said previously that the supercooled helium tanks played a role in the accident, which led to the fiery loss of the rocket and its commercial Amos-6 satellite payload. Today’s update adds lots more detail to that diagnosis, and explains what SpaceX is doing to address the issue.

The investigation – led by SpaceX but also involving experts from the Federal Aviation Administration, U.S. Air Force, NASA, the National Transportation Safety Board and elsewhere – checked readings from more than 3,000 channels of data, focusing on the 93 milliseconds between the first sign of an anomaly and the loss of the Falcon 9’s second stage.

The analysis led investigators to focus on the composite-wrapped vessels that sit inside the second-stage liquid oxygen tank. Those vessels hold superchilled helium that helps maintain pressure in the oxygen tank.

After reviewing the data and video of the blow-up, and checking the debris, the investigators concluded that liquid oxygen pooled up in a void or a buckle between a helium vessel’s aluminum inner liner and the carbon composite layer wrapped over the aluminum.

“When pressurized, oxygen pooled in this buckle can become trapped; in turn, breaking fibers or friction can ignite the oxygen in the overwrap,” SpaceX explained in today’s update.

Investigators concluded that this is what caused the helium tank to fail, setting off a chain of events that resulted in the second-stage explosion. The helium was so cold that some of the liquid oxygen could have turned solid, increasing the likelihood of failure. SpaceX said it ran a variety of tests at its facilities in California and Texas to validate the investigation’s findings.

SpaceX came up with corrective actions that it said would address the credible causes in the short term as well as in the long term.

In the short term, the company will go back to a tried-and-true, albeit slower, procedure for loading helium into the tanks at a warmer temperature. In the long term, SpaceX will change the design of the pressure tanks so that there are no buckles in which the liquid oxygen can pool. That should allow for faster helium loading procedures to be used again, SpaceX said.

“SpaceX greatly appreciates the support of our customers and partners throughout this process, and we look forward to fulfilling our manifest in 2017 and beyond,” the company said. SpaceX’s next customer, Iridium, voiced its approval as well.

There was no official word on whether the FAA had signed off on the investigation’s findings or the Jan. 8 launch date, but the fact that SpaceX provided a detailed update today suggests that it’s anticipating the FAA’s approval.

The months-long suspension of Falcon 9 launches had caused uncertainty for SpaceX’s customers, and put a crimp in the privately held company’s finances. SpaceX’s next resupply mission to the International Space Station and its first reflight of a previously flown Falcon 9 booster are among the key launches that have been held up.

The end of the investigation is also likely to clear the way for the first flight of SpaceX’s Falcon Heavy, a heavy-lift rocket that’s due to make its debut this year.

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