Flight 2 Mishap Investigation closed as SpaceX preps for another WDR

by Ryan Weber

After the Federal Aviation Administration (FAA) officially closed the mishap investigation into the second flight of Starship, which occurred on Nov 18, 2023. SpaceX is preparing to restack Booster 10 and Ship 28 onto the OLM before another Wet Dress Reheral WDR attempt that would set the stage for a March launch attempt.

Next Up, Another WDR Attempt

Ship 28 has now been removed from Suborbital Pad B, where it completed a spin prime test to verify the connections on the engines that were swapped or removed for better access. That same day Booster 10 was rolled out to the OLM and then lifted onto the pad.

SpaceX could be ready for another Wet Dress Rehearsal (WDR) attempt as early as next week. In the update on SpaceX’s website, it is confirmed that teams will try for a faster propellant loading time as seen in the two aborted attempts.

Once the WDR is completed, SpaceX may have all the data the FAA needs to sign off on a license modification. Assuming all goes well during the WDR, only one final stack may be needed for FTS installation and arming before Flight 3. 

The FAA Statement

In the Statement by the FAA, the agency confirms that the mishap investigation into Starship Flight 3 has been completed and closed.

The investigation details 17 separate corrective actions for SpaceX to take that involve the result of the Booster 9 and Ship 25 flight.

In the letter received by NSF, the mishap report for each vehicle was completed separately, with Booster 10 being finished on Feb. 5 and Ship 25 on Feb. 16. The investigation was evaluated and closed on Feb. 26, just ten days later, which is around the same amount of time as the Flight 1 investigation.

Some of the 17 corrective actions include hardware redesigns, reevaluation of engine analyses based on OFT-2 data, robustness updates to ship, and several more items. 

Ship 25

For Ship 25, SpaceX stated that during a planned Liquid Oxygen (LOX) dump near the end of the burn, a leak developed, which led to an ignition and fires onboard in the engine section. This LOX dump was part of the mission plan as SpaceX had loaded Ship 25 with extra propellant to gather data on future payload missions, and this extra propellant needed to be dumped before reentry.

Looking back at Booster 7 from Flight 1, the fires in the engine section caused damage to the wiring, which led to the engines losing connection to the flight computers. This led to the subsequent loss of control and the activation of the Flight Termination System (FTS).

A similar problem occurred on Ship 25, where the fire severed the wiring to the flight computers, which violated a mission rule as SpaceX no longer had control over the vehicle. This then led to the Automated Flight Safety System, otherwise known as the FTS, being activated, which resulted in the loss of Ship 25

Some of the changes that SpaceX needs to make to Ship, according to the letter sent by the FAA, are upgrades to robustness and reduction in complexity. These are already prevalent on Ship 28 compared to Ship 25, with additional stringers in the LOX tank for more robustness and reduced vents for less complexity.

Ship 28 set to fly next, via Sean Doherty for NSF

Hardware changes to reduce leaking could include better seals, valves, and tubing overall to prevent leaks. Installation of additional fire protection would consist of adding fire-resistant insulation to the wiring and rerouting the wiring to safer locations.

Some of the other actions are the performance of transient load analysis and modeling updates, which is the process of finding out the dynamic response of a structure. This is how SpaceX can better understand how the Ship’s structure behaves while it flies and if any changes need to be made.

SpaceX also plans on eliminating the LOX dump before engine cutoff, which could help prevent another failure of a ship.

Booster 9

Booster 9 failure mode was more interesting. According to SpaceX, when the booster tried to relight the middle ring of 10 engines, some engines began to shut down one after another until a single engine exploded. This explosion caused a chain reaction, which led to the loss of the booster.

In the update, SpaceX explained the likely reason for this failure was a filter blockage on the inlet of the LOX turbopump for this Raptor engine. A blockage would cut off LOX to the engine and the inlet line, leading to the loss of inlet pressure and the failure of the turbo pump.

The reason this causes a catastrophic failure is that when operating normally, a turbopump is spinning at thousands of rotations per minute (RPM) while pumping liquids into the engines and keeping a stable inlet pressure. When the inlet pressure suddenly drops, cavitation can occur. This is where air bubbles start to form on the pump blades, which start damaging the blades, and when moving at thousands of RPM, this can quickly destroy a pump and lead to an explosion.

However, it is unknown what caused the other engines to shut down before the explosion. Only SpaceX and the FAA may know what else happened on the booster.

To fix this primary issue, SpaceX is redesigning the tank filtration. The letter also stated that SpaceX intends to improve propellant slosh, which could’ve been an issue during the flip and boostback burn. Other actions for SpaceX to take are updated thrust vector control (TVC) modeling or gimbaling, which would better help steer the booster and would make sense since Booster 9 was the first vehicle to fly with electric TVC. 

Booster 9 in Flight all Engines Running (Credit: Max for NSF)

Along with TVC modeling, SpaceX is using the data during Flight 2 to reevaluate engine analyses and update engine control algorithms. This will allow Raptor to be more reliable over time.

What Went Right

Overall, this was a very successful flight by Starship, with the stack getting to stage separation and performing hot staging, which is a first for a vehicle of this size. Ship 25 reached an altitude of 150 kilometers with a velocity of 24,000 kilometers per hour, just 3,000 kilometers per hour short of orbital velocity.

During ascent, all 33 engines on Booster 9 worked flawlessly up to engine cutoff (MECO). Then, Ship 25’s six engines burned all the way to the leak and subsequent FTS activation, which was eight minutes and five seconds into flight. Ship 25 became the first Starship to reach space.

As stated by the FAA, the completion and closing of this mishap investigation does not mean that SpaceX can fly Flight 3. These corrective actions will need to be implemented and reviewed by the FAA before a modification to the launch license is approved.

Booster 9 and Ship 25 in flight Credit: Mary (@bocachicagal) For NSF

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