by A “runaway incident” caused the catastrophic loss of Astra’s last Rocket 3 launch last June, the company announced on Wednesday (March 1).
Astra’s Launch Vehicle 0010 lost two NASA hurricane-tracking cube sets on June 12, 2022 after a failure in the second stage of the booster, called Rocket 3.3. The overall Rocket 3 line, which faced a reported five failures in seven launches, was canceled in August; Astra is working on making improvements for a more powerful Rocket 4 version.
“This was easily the most complex investigation that Astra has ever conducted,” a joint statement on the accident said (opens in a new tab) from Astra co-founder Adam London, together with head of assignment insurance Andrew Griggs.
NASA had tapped Astra to launch six Time-Resolved Observations of Precipitation Structure and Storm Intensity with a Constellation of Smallsats (TROPICS) cube satellite at a cost of $7.95 million (opens in a new tab) agreement for the company. After the loss of the rocket, NASA said in October that it would seek alternative suppliers to get all six satellites into space by the end of 2023.
Video: See Astra’s LV0010 rocket launch failure with NASA satellites
Initial analysis showed that the defective rocket stage burned through its fuel supply faster than expected. With reduced fuel available, the upper stage could only reach 80% of the speed (velocity) required to achieve orbit, meaning NASA’s satellites could not reach their target. But learning how this problem occurred took months of investigation, Astra outlined in the lengthy statement.
The upper stage burned through the fuel too quickly due to a fault in the combustion chamber wall that created a break or “burn through,” investigators determined. The principle failure was in the rocket’s regenerative cooling system, which keeps the chamber wall cool during the extreme heating experienced during rocket launches.
“Most liquid rocket engines require cooling to prevent the very hot combustion gases from melting the chamber wall and causing the engine to fail,” Astra officials wrote.
“In a regeneratively cooled rocket engine, cooling is achieved by passing the fuel through many cooling channels, built into the combustion chamber wall. This allows heat from the wall to be absorbed into the flowing fuel, keeping the wall at a low enough temperature to prevent failure.”
Related: How Rockets Work: A Complete Guide
In the case of the TROPICS-1 launch failure, the primary cause of this reduced engine chamber cooling was a partial blockage of the fuel injector. The blockage reduced the speed at which the fuel moved through the cooling chambers. With less fuel available in the cooling chambers, not as much heat could be absorbed, which caused the combustion chamber wall to heat up.
Eventually the wall temperature became hotter than the fuel’s boiling point. As the fuel heated up, the ability to cool was “significantly and adversely affected” and the wall temperature rose so high that part of it failed. Some of the fuel then flowed out of the cooling system and into the combustion chamber, “essentially wasting it,” the Astra officials said.
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But there were several problems as well. A “small amount” of thermal barrier coating was missing from part of the engine’s upper stage combustion chamber. Not the entire chamber was coated, as engineers had initially thought it was not necessary. “We had underestimated the need for cover in this region under flight conditions,” Astra officials said.
Finding the source of the fuel injector blockage also took some time, but the ultimate cause was problems with gaseous fuel. Ground testing showed that the fuel never boiled or even came close to boiling in the cooling ducts, but again, differences emerged during the errant flight of Rocket 3.
The margin of error between boiling and non-boiling fuel temperatures was much narrower than expected, partly due to the type of high vapor pressure kerosene fuel used and partly due to the engine design.
For example: Engineers found that the exhaust jet inside the rocket nozzle unexpectedly expanded and stuck to the inside of the nozzle due to vacuum conditions high in the atmosphere. Because this phenomenon was not observed during ground testing, the fuel was heated to a higher temperature than the engine was designed for, contributing to the failure.
Rocket 4, Astra officials added, uses a different upper stage engine design and a different fuel “which completely eliminates the causes of this mishap.”
Rocket 4’s design will also include additional safeguards against future failures, such as “upgrading our helium diffuser design to prevent foaming in the propellant tanks and ingestion of helium into the engine.” This was not a problem on the Rocket 3.3, but the investigation determined that the rocket’s design was susceptible to the problem.
The company is also working to improve “processes, systems and culture to increase the reliability of our fourth-generation rocket.”
Some of the company changes regarding personnel include “an overhauled design review process, a more robust test-as-you-fly qualification process and a renewed set of Astra core values,” the statement added.
Elizabeth Howell is the co-author of “Why am I taller? (opens in a new tab)?” (ECW Press, 2022; with Canadian astronaut Dave Williams), a book about space medicine. Follow her on Twitter @howellspace (opens in a new tab). Follow us on Twitter @Spacedotcom (opens in a new tab) or Facebook (opens in a new tab).
