SpaceX details Starship's feisty mid-flight explosion after FAA closed mishap investigation

The second integrated test flight of the biggest space rocket that mankind ever built, SpaceX’s Starship, was a huge success. The future moon landing system lifted off from Texas’ Boca Chica in mid-November, successfully separated its two stages, and reached space for the first time in its short history.

And even though both stages independently experienced a rapid unscheduled disassembly – a fancy space term for blowing up – the mission named Orbital Test Flight 2 (OTF-2) was still undoubtedly a success.

However, despite all the spaceflight enthusiasts being extremely curious, SpaceX and its CEO Elon Musk have said very little about the reasons why Starship and its first stage called Super Heavy rained from the sky.

That changed late Monday after both SpaceX and the Federal Aviation Administration (FAA) released statements regarding the mishap investigation of OTF-2.

SpaceX said in an update that following the lift-off, all 33 Raptor engines on the Super Heave first-stage booster successfully completed a full-duration burn, up until the point of hot-stagging.

Hot-stagging was attempted by SpaceX for the first time. Usually, the first-stage engines are turned off shortly before the separation. In hot-stagging, however, some of the Super Heavy Raptors still burn after the second-stage engines ignite.

Both stages of the rocket survived the risky process and Super Heavy initiated the boostback burn, where 13 engines ignite again to send the booster toward its landing location in the Gulf of Mexico.

The planned soft landing on the waters of the Gulf of Mexico didn’t happen, though. The rocket burst into plumes in seconds, sparking speculations about the flight termination system (FTS) being initialized. However, as SpaceX detailed in the latest statement, several engines began shutting down before one engine “failed energetically”, quickly cascading to a rocket’s loss:

“The most likely root cause for the booster RUD was determined to be filter blockage where liquid oxygen is supplied to the engines, leading to a loss of inlet pressure in engine oxidizer turbopumps that eventually resulted in one engine failing in a way that resulted in loss of the vehicle. SpaceX has since implemented hardware changes inside future booster oxidizer tanks to improve propellant filtration capabilities and refined operations to increase reliability.”

The Starship upper-stage continued to space powered by its six Raptor engines. Approximately seven minutes into the flight, the vehicle conducted a planned vent of excess liquid oxygen propellant that served as ballast to simulate the weight of the payload.

SpaceX wanted to dispose of this additional weight before reentering the atmosphere to get more representative data. The company explains:

“A leak in the aft section of the spacecraft that developed when the liquid oxygen vent was initiated resulted in a combustion event and subsequent fires that led to a loss of communication between the spacecraft’s flight computers. This resulted in a commanded shut down of all six engines prior to completion of the ascent burn, followed by the Autonomous Flight Safety System detecting a mission rule violation and activating the flight termination system, leading to vehicle breakup.”

Musk previously stated that if the rocket had carried an actual payload, the explosion would be avoided.

FAA said in a statement cited by Reuters that it officially closed the SpaceX-led investigation into the mishap. The investigation identified the root causes and 17 corrective actions that the FAA accepted:

“Seven corrective actions were identified for the Super Heavy booster including vehicle hardware redesigns, updated control system modelling, reevaluation of engine analyses based on OTF-2 flight data, and updated engine control algorithms.

“Ten corrective actions were identified for the Starship vehicle including vehicle hardware redesigns, operational changes, flammability analysis updates, installation of additional fire protection, and guidance and modelling updates.”

The closure of the mishap investigation is important as it opens the doors for the upcoming third flight. That might happen early to mid-March, as previously reported by Ars Technica.

However, FAA’s announcement is just part of the process to grant SpaceX a license for the next Starship flight. Musk’s space company first has to implement all those corrective actions and send additional paperwork to get FAA’s thumbs up for its third installment to its “Making Life Multiplanetary” series.