The in-flight failure of a critical support strut inside the second stage liquid oxygen tank holding a high pressure helium tank in the Falcon 9 rocket, is the likely cause of the failed SpaceX launch three weeks ago on June 28, revealed SpaceX CEO and chief designer Elon Musk during a briefing for reporters held today, July 20, to explain why the critical cargo delivery run for NASA to the space station suddenly turned into a total disaster after a promising start.
The commercial booster and its cargo Dragon payload were unexpectedly destroyed by an overpressure event 139 seconds after a picture perfect blastoff from Space Launch Complex 40 at Cape Canaveral Air Force Station in Florida on June 28 at 10:21 a.m. EDT.
The root cause appears to be that the second stage strut holding the high pressure helium tank inside the 2nd stage broke at a bolt – far below its design specification and thereby allowing the tank to break free and swing away.
“The strut that we believe failed was designed and certified to handle 10, 000 lbs of force, but failed at 2, 000 lbs, a five-fold difference, ” Musk explained.
“During acceleration of the rocket to 3.2 G’s, the strut holding down the helium tank failed. Helium was released, causing the over pressurization event.”
To date no other issues have been identified as possible failure modes, Musk elaborated.
The helium tanks are pressurized to 5500 psi and were breached during the over pressurization. The purpose of the helium tanks is to pressurize the first and second stage propellant tanks.
“We tested several hundred struts. On the outside they looked normal. But inside there was a problem, ” Musk explained
“Detailed close-out photos of stage construction show no visible flaws or damage of any kind, ” according to a SpaceX statement.
The struts are produced by an outside vendor that Musk would not identify. He added that in the future, SpaceX will likely choose a different vendor to manufacture the struts.
He said the struts were made from a type of stainless steel and would also likely be redesigned.
“The material of construction will be changed to Inconel, ” Musk told me in response to a question.
Hundreds of the original type struts have been used to date on the first and second stages of the Falcon 9 with no issues. In the future, they will also be independently certified for use, by an outside contractor instead of the vendor.
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