Failed Strut Likely Cause of Falcon 9 Failure, But Investigation Continues

SpaceX’s Elon Musk told reporters during a media teleconference that preliminary conclusions point to an upper stage strut that “broke free” as the likely cause of the Falcon 9 failure on June 28.  He did not state when the rocket would return to flight, only that it would not be before September.

Musk said that initial assessments point to the failure of a metal strut inside the rocket’s upper stage as the likely cause of the explosion that destroyed a Dragon spacecraft carrying cargo bound for the International Space Station (ISS).  It was the company’s seventh operational cargo resupply mission for NASA under the Commercial Resupply Services (CRS) contract – SpaceX CRS-7 or SpX-7.  (Musk and other SpaceX officials use “second stage” and “upper stage” synonymously when referring to the segment that failed.)

Musk explained that the steel struts are designed to hold high-pressure helium bottles inside the upper stage’s liquid oxygen tank, but that one of them snapped while the stage was accelerating. When the strut broke, the helium bottle “shot to the top of the tank at high speed,” overpressurizing the tank and likely causing the explosion.

The strut, which is provided by a supplier that Musk did not want to name to avoid unnecessary “recrimination,” failed at 2,000 lbs of thrust – five times below what it is designed to withstand. SpaceX has been able to replicate the failure, conducting tests on thousands of these struts and finding that a few others snapped at a point far below their rated force level. As a result, Musk said SpaceX will move to individual testing of each strut independent of outside certification. This, he said, will result in a cost increase, but not “of a significant amount” so that the price of the vehicle should remain unaffected.

Musk said that the failed strut was the “most probable, but not definitive outcome” of the ongoing investigation, noting that there is still work to do.  Investigators are still puzzling over telemetry data that shows a drop in helium pressure, and then a rise back to starting pressure, something he described as “quite confusing.”  Analysis is ongoing.

The investigation also revealed that if the Dragon had deployed its parachutes before falling into the ocean, the spacecraft would have survived. The software in this cargo version of Dragon (Dragon 1), Musk explained, is inert on ascent and was not programmed to release the parachute in the event of a failure.  Software in the version of Dragon under development for taking people into space (Dragon 2 or Crew Dragon) is programmed to do just that.  Musk said they would be working on software fixes to ensure that the Dragon 1 cargo spacecraft can do what it needs to survive. “We could have saved Dragon if we had the right software there,” he said.

Musk said SpaceX customers, including NASA and the U.S. Air Force, had been briefed and were very supportive, indicating “no diminished faith in SpaceX.”

He indicated that return to flight would happen no sooner than September and that who the next customer will be is not clear. While addressing the strut issue is “fairly straightforward” Musk said he wants to ensure the issue is diagnosed correctly and that flights do not resume without everyone being “on board” with the changes.  In a press release issued after the media teleconference, the company said it expects to “return to flight this fall and fly all the customers we intended to fly in 2015 by the end of the year.”

This was SpaceX’s first launch failure in seven years, and the only one for the majority of its 4,000 employees who joined the company during that time.   Musk noted that to some degree the company became “a little bit complacent,” and that this failure was an “important lesson” moving forward.

SpaceX said in its press release that the failure was “regrettable,” but the review process ultimately will “yield a safer and more reliable launch vehicle.”