SpaceX is aiming to return to flight on January 8th, as the company has officially determined what caused one of its Falcon 9 rockets to explode on a Florida launch pad this past September. In an online update this morning, SpaceX detailed the source of the rocket failure — a complex process that involved broken carbon fibers causing super cold oxygen to ignite. The company says it has identified all of the "credible causes" that may have been responsible for this combustible interplay of materials, and corrective actions have been taken to make sure the same accident doesn't happen again.
SpaceX was forced to ground all of its vehicles after the accident, which happened in Cape Canaveral, Florida on the morning of September 1st. The failure occurred as the Falcon 9 was being being fueled for an upcoming static fire test — a routine procedure SpaceX does before launch to see if the rocket's engines are working properly. During fueling, the Falcon 9 exploded in a huge fireball, leading to the destruction of the rocket and the communications satellite it was supposed to carry into space.
Video of the event revealed that the explosion began toward the top of the rocket, and SpaceX later narrowed down the source of the problem to the vehicle's upper oxygen tank. That's the tank that stores the super cold liquid oxygen propellant needed for the engine in the second stage, or the top portion of the vehicle. Along with liquid oxygen, the tank also houses three smaller vessels that store really cold helium. They're called COPVs, for composite overwrapped pressure vessels, and they're responsible for pressurizing the tank during flight; the helium from these vessels fills the empty space left behind by the oxygen when it leaves the tank. It seems that the materials used for these helium pressure vessels may have had a bad reaction with the oxygen in the tank.
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The vessels are made up of an aluminum liner surrounded by a carbon fiber composite material. SpaceX was able to recover some of the debris from the accident, and the company found something strange about the COPVs. Something had caused the vessels' aluminum liners to scrunch up, creating buckles in the material. The most likely scenario, according to SpaceX, is that some of the liquid oxygen in the tank pooled into these buckles and got trapped there. Then, some of the carbon fibers wrapping the helium vessel may have snapped, causing friction that ignited the trapped oxygen. Additionally, the helium in these vessels is incredibly cold too — so cold that it may have caused the pooled liquid oxygen to turn into a solid. And solid oxygen has an even greater chance of combusting due to friction.
The process more or less matches CEO Elon Musk's explanations for the failure last year. SpaceX says its accident investigation team — which collaborated with the Federal Aviation Administration, NASA, the National Transportation Safety Board, and the US Air Force — poured over 3,000 channels of data to figure out what happened. That may seem like a lot of information, but the timeline of the accident was incredibly brief. From the first sign of trouble to the loss of the vehicle, the entire event lasted just 93 milliseconds. A week after the accident, Musk said that the failure was shaping up to be the "most difficult and complex" one the company has ever had, noting that there was no apparent heat source for the explosion. In the end, SpaceX identified a number of possible causes for the failure, all of which involved the pooled oxygen scenario.
Now that the most likely cause for the failure has been identified, SpaceX says it is implementing a number of short- and long-term changes to avoid a similar situation in the future. For one, SpaceX is going to load warmer helium into the pressure vessels, apparently to avoid turning any of the liquid oxygen into a solid. Additionally, the company will also return to a previous way of loading helium that's been shown to be successful before. As for the long-term, SpaceX says it will be creating a new design that prevents the helium vessels from buckling, though the company did not specify what those design changes will be. It's still unclear exactly what caused the buckling in the first place.
With its accident investigation over, SpaceX's next planned launch is less than a week away. The flight will put 10 small probes into orbit for satellite operator Iridium. The launch will take place from SpaceX's site at Vandenberg Air Force Base in California, though an exact time for the flight has yet to be announced. If the flight does occur this weekend, it will be a remarkably short turnaround for the company. It took SpaceX six months to return to flight after one of its rockets exploded during launch in June 2015.