Support strut probable cause of Falcon 9 failure

January 12, 2018 – 10:36 am

The Falcon 9 rocket disintegrated 2 minutes, 19 seconds after liftoff from Cape Canaveral on June 28. Credit: Walter Scriptunas II/Scriptunas ImagesUpdated at 9 p.m. EDT with SpaceX statement

A faulty support strut inside the Falcon 9 rocket’s second stage liquid oxygen tank likely broke free during a June 28 space station resupply launch, destroying the booster minutes after liftoff in a cloud of debris over the Atlantic Ocean, SpaceX chief Elon Musk said Monday.

While noting the investigation into last month’s failure is not complete, Musk told reporters Monday the evidence shows a weakened bracket holding a high-pressure helium vessel inside the Falcon 9’s second stage is the likely culprit.

Several helium tanks, each pressurized to about 5, 500 pounds per square inch, are mounted inside the rocket’s second stage liquid oxygen tank. The helium is routed through the second stage’s Merlin engine, where the helium warms up and injected into the rocket’s propellant tanks to pressurize the stage as the launcher burns fuel, keeping the tanks structurally sound.

Musk said SpaceX engineers analyzing extensive telemetry data from the rocket, along with physical testing on the ground, concluded a support strut holding one of the helium tanks likely fractured near a bolt attach point.

File photo of SpaceX founder and CEO Elon Musk. Credit: Gene Blevins/LA Daily NewsThe high-pressure helium tank, immersed inside the super-cold liquid oxygen, wanted to shoot to the top of the stage, similar to the way an inflated beach ball rises to the surface of a swimming pool, Musk said.

“It may seem sort of counterintuitive that, as the rocket’s accelerating, that something immersed in the tank would actually want to go up more, but that’s basically what happened, ” Musk said. “The buoyancy increases proprotionate to the G-loading. At approximately 3.2 Gs, this 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 over-pressure event quite quickly.”

Musk said less than nine-tenths of a second passed from the first indication of a problem until the data link between the rocket and mission control cut off.

Telemetry logged from the June 28 launch showed a momentary drop in helium pressure, then a rise back to the system’s starting pressure, Musk said, initially puzzling investigators probing the mishap.

One explanation for the data signature is that the tank broke free, introducing helium into the liquid oxygen tank until a kink in a feed line stopped the leak, allowing pressure in the helium system to rise again.

The failure occurred 2 minutes, 18 seconds after liftoff from Cape Canaveral, when the Falcon 9’s nine first stage Merlin engines were still firing, according to a SpaceX statement released Monday. Because the second stage was not ignited, the propellant tanks were full, leaving little room to add high-pressure gas before rupturing the stage.


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