Starship Second Flight: Now We Know What Happened

The Federal Aviation Administration (FAA) has wrapped up its inquiry into the mishap involving SpaceX’s Starship Super Heavy Orbital Test Flight 2 (OTF-2), which took place on November 18, 2023.

In a statement released on Monday, the FAA disclosed that SpaceX has identified the root causes of the incident, with both parties agreeing on 17 corrective measures outlined in SpaceX’s mishap report.

*Starship clears the launch pad during its second orbital test flight. Credits: SpaceX*

However, the closure of the mishap investigation does not signify an immediate green light for the next Starship launch. SpaceX must first implement all corrective actions and obtain a license modification from the FAA, a standard process that we also saw after the first test flight.

“The FAA is evaluating SpaceX’s license modification request and expects SpaceX to submit additional required information before a final determination can be made,” said the agency in the statement.

What happened to the booster

In a statement provided by SpaceX, the company highlighted the milestones achieved during the OTF-2 mission, but also shared insight about the failure.

At liftoff, all 33 Raptor engines on the Super Heavy Booster ignited successfully, completing a full-duration burn during ascent. Additionally, the mission saw the successful execution of a hot-stage separation, a technique previously untested on a vehicle of this magnitude, which was the riskiest part of the flight.

*Tracking camera views of Starship hot-staging separation. Credits: SpaceX*

However, the mission encountered challenges during the booster boostback burn phase: 13 engines were commanded to restart, but “several engines began shutting down before one engine failed energetically, quickly cascading to a rapid unscheduled disassembly (RUD) of the booster”.

The vehicle breakup occurred at an altitude of ~90 km over the Gulf of Mexico. SpaceX attributed the root cause (most likely) to “filter blockage where liquid oxygen is supplied to the engines”, resulting in a loss of vehicle. To address this issue, SpaceX has implemented hardware modifications and operational refinements aimed at enhancing propellant filtration and reliability.

What about the Starship upper stage?

On the other hand, the Starship upper stage successfully lit all six Raptor engines and flew a normal ascent until approximately seven minutes into the flight. At that point begun a planned vent of excess liquid oxygen propellant, loaded on the spacecraft in order to simulate future payload deploy mission.

A leak in the aft section of the spacecraft started, resulting “in a combustion event and subsequent fires that led to a loss of communication between the spacecraft’s flight computers”. This commanded the shut down of all six engines prior to completion of the ascent burn, followed by the Autonomous Flight Safety System (AFSS) triggering, leading to vehicle breakup.

*Starship and Super Heavy ascending during the second test flight. Credits: SpaceX*

SpaceX has implemented hardware upgrades and operational enhancements to mitigate potential leaks and improve fire protection measures. Also, they’re switching from hydraulic steering system to electric system, which reduces fires probability.

Going forward

The corrective actions mentioned by the FAA in the report, categorized into seven for the Super Heavy Booster and ten for the Starship vehicle, include a spectrum of measures ranging from hardware redesigns to operational adjustments.

For the Super Heavy Booster, SpaceX planned hardware redesigns, updated control system modeling, and reevaluation of engine analyses based on OTF-2 flight data, among others. Similarly, corrective actions for the Starship vehicle involve hardware redesigns, operational changes, and updates to flammability analyses and guidance systems.

Moving forward, SpaceX plans to incorporate lessons learned from the OTF-2 mission into future Starship and Super Heavy launches. Upgrades derived from the flight test, “along with planned performance enhancements” aim to bolster the reliability and efficiency of the world’s most powerful launch system.

SpaceX noted in the end that is also “improving the speed of propellant loading operations prior to launch”, and that the water-cooled flame deflector and other pad upgrades made after the first flight test performed as expected.

Iterative design and improvements remain paramount for SpaceX, additional Starships are ready for flight: “we’re not dead yet”.