SpaceX has been grounded ever since a launchpad explosion on September 1st resulted in the loss of a Falcon 9 rocket. The failure destroyed a Facebook internet satellite and called into question the design of SpaceX’s fuel tanks. This was the second tank failure leading to the loss of a Falcon 9, though this situation has proved to be much more complex. After narrowing its investigation to the helium tanks, SpaceX has now released its findings and hopes to launch again as soon as January 8th.
The anomaly (a nice way to say giant fireball) occurred while helium was being loaded into the rocket on the launchpad. The engines were not firing, and there was no active heat source. The team had precious little data to go on with just 93 milliseconds between the first sign of a problem and complete loss of telemetry. Still, engineers zeroed in on the design of the helium composite overwrapped pressure vessels (COPVs) early on. The incident was all the more concerning because this is the same design that will be used to launch the manned Dragon crew capsule in the next several years.
The Falcon 9 has three COPVs inside the upper-stage liquid oxygen fuel tank. Because liquid-fueled rockets can turn on and off, they need to maintain pressure inside the tank as fuel is used. Helium is a natural choice for this as it’s inert. SpaceX now says it has traced the cause of the explosion to one of the three COPVs at a point where the tank “buckles” and accumulates oxygen. It was this accumulated oxygen that ignited from friction and caused the vehicle to explode.
SpaceX previously noted that it believed variations in helium temperature had caused the incident. But now we have more detail on what that means. The super-chilled liquid helium being loaded into the rocket that day may actually have been cold enough to freeze the liquid oxygen accumulated between the COPV liner and overwrap. Solid oxygen can’t flow, making a “friction ignition” much more likely.
Having worked out the likely cause of the incident, SpaceX has identified several corrective actions. On the immediate agenda are small changes to the COPV design to stop oxygen from pooling around it. SpaceX will also lower the temperature of the liquid hydrogen it loads. Over the long term, the company plans to redesign the COPV completely.
SpaceX is awaiting FAA approval to move forward with its launch on January 8th. However, it has already prepped a payload of ten IridiumNEXT communications satellites in the nose cone of a Falcon 9 in anticipation of approval. SpaceX plans to launch a total of 70 IridiumNEXT satellites over the coming months, assuming the upcoming launch goes as planned.