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With Starship’s tenth suborbital test flight launching from Boca Chica/Starbase as early as 6:30 p.m. Sunday, SpaceX has revealed what it thinks went wrong with Flight 9 on May 27, when Starship S35 was lost before it could land in the Indian Ocean near Australia as planned.

The company has also provided details on the loss of the Super Heavy booster rocket on May 27 as well as the destruction of Starship S36 in a massive fireball on the test stand on June 18.

According to results of the SpaceX-led mishap investigation into Flight 9 overseen by the Federal Aviation Administration, Starship S35 began to run into trouble after all six engines were lit and the vehicle had separated from the booster.

“Approximately three minutes into the burn, sensors in the nosecone detected a steady increase in methane levels,” according to the company. “This continued until approximately five minutes into the burn when pressure began to rapidly decrease in the main fuel tank while pressure simultaneously increased in the nosecone.”

Attitude errors soon began, but were temporarily brought under control (long enough for the ship to complete its ascent burn and reach planned velocity), though ultimately the flight was doomed as further complications arose. Starship, having automatically vented all its propellant into space, reentered Earth’s atmosphere “in an off-nominal attitude and communication was lost during entry,” SpaceX said.

“Final telemetry from Starship was received approximately 46 minutes into the flight test, while the vehicle was approximately (36.6 miles) in altitude and inside the designated entry area over the Indian Ocean,” the company said.

The most likely root cause was a failure of the main fuel tank pressurization system “diffuser” that optimizes the transfer of pressurized propellant into the main tank, SpaceX said.

“Cameras inside the vehicle showed a visible failure on the fuel diffuser canister, which is located inside the nosecone volume on the forward dome of the main fuel tank,” the company said. “While pre-flight analysis did not show a predicted failure, SpaceX engineers were able to recreate the failure using flight conditions when testing at our facility in McGregor, Texas.”

SpaceX said the diffuser has been redesigned in hopes of avoiding a repeat.

“The new design underwent a more rigorous qualification campaign, subjecting it to flight-like stresses and running for more than ten times the expected service life with no damage,” the company said.

Starship’s flight termination system was not initiated, and no violations of FAA rules governing autonomous flight safety systems occurred during Flight 9, according to the company. The FAA announced on Aug. 15 that the Flight 9 mishap investigation was closed and Flight 10 cleared to proceed.

The explosion of Starship S36 on June 18 happened while the ship was being loaded with cryogenic (very cold) propellant for a six-engine static-fire test in preparation for Flight 10. SpaceX said a sudden “energetic event” caused the total destruction of the Starship and damage to the immediate area around the test stand, within the safety zone, though no injuries or safety violations were reported.

According to that investigation, the mostly likely root cause of the explosion was “undetectable or under-screened damage to a composite overwrapped pressure vessel” resulting in structural failure that led to propellant mixing and igniting, the company said.

To prevent it from happening again, going forward the pressure vessel will operate at lower pressure; additional inspections and proof tests will be conducted before loading propellant; and new external covers have been added to the pressure vessels, affording another “layer of protection and visual indication of potential damage,” SpaceX said.

As for the loss of the Super Heavy B14-2 booster before it could make a planned landing in the Gulf of Mexico on May 27, the most probable cause was structural failure resulting in methane and liquid oxygen mixing and igniting, according to the company. The structural failure was likely caused by greater-than-predicted physical forces on the booster structure during Super Heavy’s descent, which was intentionally set at a high “angle of attack” in order to test the vehicle’s physical limits, SpaceX said.

Angle of attack is the angle of a wing — the booster’s centerline in this case — relative to the oncoming air. SpaceX said it will use a lower angle of attack for Super Heavy’s descent during future flights.

“Every lesson learned, through both flight and ground testing, continues to feed directly into designs for the next generation of Starship and Super Heavy,” the company said. “Two flights remain with the current generation, each with test objectives designed to expand the envelope on vehicle capabilities as we iterate towards fully and rapidly reusable, reliable rockets.”

Cameron County has announced that Boca Chica Beach and S.H. 4 from FM 1419 (Oklahoma Avenue) will be closed for SpaceX flight testing from 11 a.m. to 10 p.m. on Aug. 24, with alternate dates of Aug. 25-26 during the same 11-hour window.

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