Written By: Zac Aubert
Published: Wed, Mar 1, 2023 10:05 PM
Latest Update: Wed, Mar 1, 2023 10:28 PM
Astra has concluded its investigation into the TROPICS-1 mishap that occurred on June 12, 2022, after six months of rigorous testing and analysis.
The investigation revealed that a combustion chamber wall burn-through, caused by a fuel injector blockage, led to the anomalous fuel consumption during the upper stage flight. The failure prevented the upper stage from delivering the payloads to orbit and resulted in the mission's termination. Astra has taken steps to prevent similar issues in future missions and is committed to ensuring safety and success.
The receipt of the closure letter from the FAA concludes the TROPICS-1 mishap investigation, allowing Astra to move forward with their spaceflight plans. The company has stated its commitment to learning from the experience and ensuring the safety and success of future missions.
TROPICS-1 launched using the Rocket 3.3, serial number LV0010. The rocket had a nominal first stage flight, stage separation, and upper stage ignition. However, shortly after the ignition of the upper stage engine, the upper stage's fuel consumption rate increased and remained anomalously high for the remainder of the flight.
Further analysis revealed that the anomalous fuel consumption was due to a combustion chamber wall burn-through that occurred 18 seconds into the upper stage flight. Flight data showed that the burn-through was caused by a substantial blockage of the fuel injector, which had a complex relationship with the mechanics of combustion and regenerative cooling.
As a result of the burn-through, the upper stage was only able to obtain about 80% of the required orbital velocity, and the payloads were not delivered to orbit. The stage subsequently re-entered the atmosphere, ending the mission.
The investigation required extensive testing and analysis to recreate the failure mode and understand the root cause of the injector blockage and burn-through.
Astra's investigation into the TROPICS-1 launch failure has concluded that the primary factor leading to the combustion chamber burn-through was a partial blockage of the injector.
During the upper stage flight, the rate of fuel passing through the cooling channels decreased due to the blockage, reducing the amount of heat the fuel could absorb and making the combustion chamber wall hotter. If the wall temperature exceeds the local boiling point of the fuel, some of the fuel boils along the wall inside the channels, causing the wall to fail and "burn through," dumping a portion of the fuel flow directly into the combustion chamber and essentially wasting it.
While partial blockage of the injector was determined to be the primary factor for the burn-through, Astra has also identified a secondary factor, which was thermal barrier coating erosion.
Some portions of the upper stage engine's combustion chamber have a thermal barrier coating on the inside to insulate the chamber wall and reduce the heat that the fuel is required to absorb as a coolant. If there is missing thermal barrier coating, even a small amount, that portion of the wall can get much hotter and increase the likelihood of a local burn-through.
During the investigation, Astra found that there was a small amount of missing thermal barrier coating on the LV0010 upper stage engine, which was considered acceptable by engineers at the time. However, further analysis showed that the need for coating in that region was underestimated under flight conditions. Astra's investigation also revealed that the mechanics of combustion and regenerative cooling are complex, and failures like the one during the TROPICS-1 launch do not have an immediately apparent root cause.
Astra's investigation report says it was initially "straightforward" to determine that a blockage of the injector had occurred during flight, However, "it took much longer to conclusively determine what had caused the blockage". Astra used a combination of analysis and testing to systematically investigate each potential blockage source, including foreign object debris (“FOD”), gaseous helium, and gaseous fuel.
Following their review of the flight results and testing to recreate the failure, they have concluded that the injector blockage was caused by a gas, ruling out solid foreign object debris such as metal particles. Next, their team examined helium as a possible cause, since it is used for the upper stage’s pneumatic and pressurization systems and theoretically could have leaked into the fuel lines. However, after performing a barrage of tests on the pneumatic systems, Astra was unable to substantiate any meaningful leaks or find any data from the LV0010 upper stage indicating leaks before or during flight.
The only other credible source of helium in the upper stage is in the pressurization system, found at the start of upper stage flight in a small “ullage” bubble at the top of the fuel tank. Analysis showed that it is highly unlikely that this helium could have migrated to the bottom of the tank and been ingested into the engine at the beginning of the burn. Even if it had migrated, it is even more unlikely that this helium could have remained in the bottom of the tank and sustained the injector blockage for the amount of time seen in flight, as helium tends to migrate toward the top of the tank as soon as the engine lights and the stage begins accelerating.
Astra was left with gaseous fuel as the main suspect, during ground acceptance testing, the fuel in the upper stage engine gets warm, but boiling or near-boiling within the cooling channels had never been observed. However, the exhaust jet of the upper stage engine on the ground is “separated” from the inside of the engine nozzle by the pressure of the atmosphere around it, and therefore transfers less heat into the fuel. In flight, the engine is surrounded by vacuum and the exhaust jet expands to become “fully attached” to the inside of the nozzle. Therefore, the fuel passing through the engine during flight is heated to a higher temperature than during ground testing.
To better understand this phenomenon, Astra conducted numerous experimental engine tests with fuel pre-heated to various temperatures to simulate the effect of full attachment. This allowed them to create a more sophisticated thermal model to predict the temperature of the fuel inside the engine with better accuracy than before. This analysis showed that, in flight, the fuel at the injector would have thin margins with respect to its boiling point. The most significant contributors to this thin margin were unique to the Rocket 3 architecture: a pressure-fed upper stage engine that operates at relatively low pressures, as well as the selection of a kerosene-like fuel with a higher vapor pressure than traditional rocket-grade kerosene (e.g. RP-1) to simplify testing and operations.
Given this thin margin, small factors such as 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.
Astra's analysis concluded that the boiling fuel caused the partial injector blockage in flight and, together with the eroded thermal barrier coating mentioned previously, caused a thermal “run-away” event that worsened the injector blockage and eventually led to the burn-through.
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