Falcon 9 rockets use liquid oxygen as propellant, and the fuel tanks are backfilled with helium, which maintains pressure in the tank as the fuel is burned. The two-foot-long struts holding down helium canisters are designed to withstand 10,000 pounds of force, but on June 28, one failed at just 2,000 pounds of force, Musk said. As the rocket careened skyward, the buoyancy forces of liftoff would have shot the bottle to the top of the tank in the rocket’s upper stage. This caused helium to spew into the oxygen tank and overpressurize it. All this happened within about nine-tenths of a second.