WASHINGTON — An investigation into a failed Astra launch last June concluded that a complex series of events caused a fuel leak that kept the rocket’s upper stage from reaching orbit.
In a March 1 statement, Astra said it received a letter from the Federal Aviation Administration formally concluding the investigation into the June 2022 launch of its Rocket 3.3 vehicle from Cape Canaveral, Florida. The rocket’s upper stage shut down prematurely, keeping its payload of two NASA TROPICS cubesats from reaching orbit.
The company said that the upper stage ran out of kerosene fuel early, shutting down the engine while only at 80% of orbital velocity. Engineers concluded there was a combustion wall burn-through in the upper stage engine 18 seconds after ignition. That burn-through allowed fuel, used in the engine’s regenerative cooling system, to leak.
The investigation blamed the burn-through on a partial blockage in the engine’s fuel injector, which slowed the rate fuel passed through cooling channels and made it hotter, to the point where some fuel boiled and reduced its ability to cool the chamber.
Extensive testing ruled out both solid debris and helium gas as the source of the blockage. That left gaseous fuel as the likely source. That had not been seen in ground tests of the engine, but Astra concluded several factors, such as limitations of ground tests of the upper-stage engine, meant that it was possible for the fuel to get warmer than expected. That left the engine with “thin margins” for keeping the fuel from boiling.
“Given this thin margin, small factors — like the warm sunny weather in Cape Canaveral on the day of launch, which meant the fuel was slightly warmer than in prior flights – helped to tip the fuel over into a boiling regime on the TROPICS-1 mission,” the company wrote in a statement by Andrew Griggs, head of mission assurance at Astra, and Adam London, chief technical officer.
A contributing factor, Astra said, was an eroded thermal barrier coating in the combustion chamber. That erosion was noticed before the launch but engineers concluded at the time that it was acceptable given its location in the chamber. Instead, the investigation found the need for that coating was greater than originally believed based on limits of testing.
The TROPICS mission was the last flight of the Rocket 3.3 vehicle. The company announced in August it was retiring the vehicle to focus company resources on the Rocket 4, a larger vehicle whose first test launch could be before the end of the year.
“Rocket 4 incorporates key architectural choices (most notably, a different upper stage engine design and a different fuel) that completely eliminate the causes of this mishap,” Griggs and London wrote. The company has made other changes to the engine intended to avoid other potential failure modes, like solid debris or helium gas blockage of the injector.
They said the company also looked at ways for “improving our processes, systems, and culture to increase the reliability” of Rocket 4. Examples of those improvements are an updated design review process and a “test-like-you-fly” qualification process. They concluded they are “confident we now have the right team and systems in place to make Rocket 4 a success.”
