SpaceX the Alan Notes

White vapor (probably liquid oxygen) blasts away from the Falcon 9 rocket after a strut snapped in the second stage, leading to a series of events that led to the loss of the vehicle.

Photo by NASA/SpaceX, from the video

On June 28, 2015, disaster hit an uncrewed SpaceX Falcon 9 rocket 2 minutes 20 seconds after launch. Video made it clear something went wrong in the second stage, and at a press conference on Monday, CEO Elon Musk announced the most probable cause: a strut failure that led to the loss of the vehicle.

As fuel is used up during launch, high-pressure helium is fed into the liquid oxygen tank to maintain the pressure in that tank. The helium is kept at 5, 500 PSI inside a bottle.

During launch, a strut holding the helium bottle down snapped. Counterintuitively, during the high acceleration of launch the helium tank feels a large force upward, because the helium in the tank is highly buoyant. The larger the acceleration is, the larger the buoyant force gets, and the more force the helium tank feels upwards.

The strut that snapped was designed to hold the bottle down against this force. When it snapped, the helium tank shot upward, slamming into and rupturing the liquid oxygen tank. The super-cold oxygen then boiled, expanded rapidly, and breached the second stage walls.

In the video, you can see that what happened was not technically an explosion; there’s no fireball, but you can see the expanding vapor from the oxygen tank flying away.

These struts are certified before launch, but Musk has promised that they will do a second test individually on each strut from here on out; this will mean a slightly higher cost for each launch but not a significant one.

This strut failure is hard to test for on the ground. As Musk pointed out, boosters are tested strapped down on the ground, so they only ever feel 1 g of acceleration (due to gravity). This failure happened when the rocket was undergoing 3.2 g, which cannot be realistically tested except in launch. This is a “really odd failure mode” and “quite a puzzle, ” Musk said. The root cause of the failure still isn’t known; the strut failed at well under the rated stress level (it’s rated for 10, 000 pounds of force, and failed at 2, 000).

They are continuing to look into the telemetry; this series of events is the preliminary conclusion of the failure, not the definitive one.