SpaceX says harmonic response caused Starship failure
As SpaceX prepares for a Friday launch of its next Starship flight test, Elon Musk’s biz has explained that the failure of the previous test was due to a harmonic response.
The return of the Super Heavy booster to its launch site and subsequent capture by the launch tower’s arms was a near total success, even though the boostback burn didn’t go as planned due to one of the 13 Raptor engines used in the burn aborting on startup because of a low power condition in the igniter system.
All 13 middle ring and center Raptor engines – including the engine that had aborted during startup – were successfully lit for the landing burn, and the Super Heavy was captured.
Things did not go so well for Starship. The vehicle separated from the Super Heavy booster and successfully lit its six second-stage engines. All was good until around two minutes into the burn, when a flash was seen near one of the Raptor engines in the aft section of the vehicle.
Referred to by SpaceX as “the attic,” the unpressurized section is located between the bottom of the liquid oxygen tank and the aft heatshield. After the flash, sensors detected a pressure rise in the attic that suggested a leak.
After approximately another two minutes, a second flash was seen, followed by “sustained fires” in the attic. All but one of the engines eventually shut down and, while communication ceased, SpaceX reckons that the safety system triggered autonomously as expected. Debris from the termination was showered over the Turks and Caicos Islands.
According to SpaceX, the most probable root cause for the test’s premature end was a “harmonic response” several times stronger in flight than had been seen in previous testing. The result was increased stress in the propulsion system, which sprung a leak. The leaks overwhelmed the venting capability of the attic area, and fires ensued.
Harmonic response is the vibration of a structure in response to regular oscillations. Computer makers might worry about fans, but SpaceX has an altogether more explosive issue.
The solution? An extended static fire of the Starship to be used in Flight Eight to recreate the harmonic response experienced during Flight Seven. SpaceX said: “Findings from the static fire informed hardware changes to the fuel feedlines to vacuum engines, adjustments to propellant temperatures, and a new operating thrust target that will be used on the upcoming flight test.”
The longer-term plan is to move to the Raptor 3 engine and reduce the number of joints prone to leaking, but for the current generation of Starships, SpaceX will add more vents and a new purge system utilizing gaseous nitrogen.
According to SpaceX boss Elon Musk, Starship Flight Eight is set for this Friday. ®
