Problems with Orion’s heat shield – Kosmonautix.cz

Manned spaceflight is inherently daring, and the Artemis campaign is no exception. The safe manned flight of Artemis II to the Moon and back presents a number of unique challenges and risks that NASA not faced since the era of Apollo. The Artemis I unmanned test flight in November and December 2022 was a significant success, providing important data and lessons learned from hardware, software and process testing that are used in preparation for the next mission. Artemis I was designed as an unmanned test designed to identify problems that might occur that could not be predicted by modeling or ground testing. NASA expected to solve the problems before the Artemis II mission. This process of finding and solving technical problems is a natural part of the design, test and subsequent error correction process in order to maximize the probability of success of the next flight.

Indeed, the Artemis I test flight revealed technical problems that must be resolved before the Artemis II mission, so that the astronauts are not exposed to any undue risk. These are particularly anomalies with Orion’s heat shield and separation screws, which pose a significant risk to crew safety. NASA has identified more than a hundred locations where the Avcoat ablative thermal protection material from the heat shield has worn off differently than expected during entry into the Earth’s atmosphere.

Ablation, or the removal of the charred layers of Avcoat due to friction with the atmosphere, was expected to take place by melting them. However, during the first phase of the skip entry maneuver, which bisected the descent, the Avcoat began to crack and tear apart.

The heat shield test was a key objective of the Artemis I flight. The biggest challenge now is to identify and mitigate the root cause of heat shield erosion and separation screw melting during re-entry into Earth’s atmosphere. Engineers are therefore exploring options to mitigate material loss by altering Orion’s reentry trajectory or modifying the heat shield.

Orion’s heat shield for the EFT-1 test in December 2014 was constructed in the same way as Apollo’s – it was monolithic. Even before EFT-1 launched, however, the large monolithic Avcoat heat shield showed a tendency to crack. It turned out that the monolithic shield was suitable for Apollo, but not for the larger Orion. Cracks in the heat shield for EFT-1 were repaired and Orion’s flight test, entering the atmosphere at 8.9 km/s, was successful. The ablation pattern was very similar to the Apollo including sharply defined edges where the charred layer of Avcoat was peeling off. The cracking problem worried NASA to the point that a heat shield with glued blocks of Avcoat was decided for the next missions.

Orion’s heat shield contains 186 Avcoat blocks. Four separation screws run through the shield. More details in our article.
Source: https://encrypted-tbn0.gstatic.com

The large segmental heat shield from Avcoat first flew on the Artemis I mission, during which Orion entered the atmosphere at nearly 11.2 km/s. Due to its return from the Moon, Orion’s velocity was about 40% faster than that encountered by spacecraft returning from the International Space Station. During the descent, the heat shield was exposed to temperatures of approximately 2,750°C.

The first information became public in a briefing on March 7, 2023. NASA’s Orion program manager Howard Hu said at the time that the biggest problem during the flight was the crew cabin’s heat shield, which was releasing material differently than engineers expected based on ground-based tests and computer models. Hu also said that this did not pose a safety risk to the Orion because there was a significant reserve of intact Avcoat left on the heat shield, which successfully protected the cabin and its systems. At that time, engineers began a detailed analysis of the heat shield to determine why it behaved differently.

Although Orion landed safely, Avcoat’s unexpected behavior is a potential risk for future missions where the heat shield may not adequately protect the crew and cabin systems from extreme heat. NASA therefore set out to understand the root cause of Avcoat breaking off.

Apollo 16 heat shield
Source: https://pbs.twimg.com

Last year and this year, extensive testing of Avcoat block panels was carried out. In ground tests, the loss of charred pieces showed the same features seen on the shield from the Artemis I mission. Currently, the team is completing the translation of the test results into a theory of the root cause of erosion. In May 2024, formal presentations should take place at technical forums as part of the preparation of recommendations to the Orion Program Review Board.

The recommendation is to describe the root cause of the erosion, the operational capabilities of the heat shield for the Artemis II mission, based on the latest thermal analysis work, and the necessary corrective actions. Currently, it is assumed that among the measures to mitigate the loss of Avcoat will be a change in the return trajectory, or a modification of the heat shield structure. The recommendation will then be reviewed by an independent review team. The estimated date of completion of the analysis and the plan for the next course of action is by June 30, 2024.

Field inspections also revealed that the three separation screws had developed cracks due to erosion of the heat shield, resulting in increased heating and melting of the inside of the screws. The crew module has a total of four separation screws. These screws firmly connect the crew module to the service module. Only before entering the atmosphere does the separation of the service module take place. It then burns up in the atmosphere.

Orion EFT-1 heat shield
Source: https://media.al.com/

Melting of the separation screws could lead to hot gases entering beyond the Avcoat layer and exceeding Orion’s design limits. Although the separation screws are surrounded by Avcoat thermal protection blocks and sealed with a heat shield, to alleviate the problem for Artemis II, minor modifications were made to the design of the separation screws and additional thermal protection material was added around them. The propellers were installed on Orion for Artemis II in September 2023. Possible changes to the return trajectory of the Artemis II mission, which NASA is evaluating to avoid Avcoat losses, are also being considered to avoid heating the propellers. For later missions, the agency plans to redesign the propellers.

Further testing and analysis of Orion’s separation screws is currently underway using updated models based on the findings of the heat shield investigation team. But NASA cannot complete a final evaluation of the screws until they complete the Avcoat loss assessment and update the thermal model to account for the full range of design and operational changes. However, the work is on track for a planned completion date of June 30, 2024.

At a meeting of NASA’s Manned Exploration and Operations Advisory Board on April 26, 2024, Amit Kshatriya, deputy associate administrator for the Moon to Mars program, said that understanding the behavior of the heat shield is a top priority for reducing the risks of the Artemis II mission.

Heat shield for the Artemis I mission during preparation
Source: https://www.nasa.gov/

The emphasis, he says, is on understanding the basic physical properties of the heat shield material and the release process of Avcoat during reentry. “We are nearing a final answer regarding this cause”, he said. He also emphasized that potential changes in the return trajectory are being analyzed to change the thermal load on the cabin. “When we put it all together, we either have a rationale for flight or we don’t”he concluded.

The rationale for the flight would mean continuing preparations with a launch date of September 2025. However, if the heat shield was deemed too risky for the flight, the launch date would be postponed. Shield modifications would be manufactured and certified. The service module would be disconnected, the heat shield including the separation screws uninstalled, modified, reinstalled and the Orion then retested.

Orion’s heat shield for the Artemis II mission
Source: https://upload.wikimedia.org/

NASA plans the Artemis II mission as a 10-day lunar orbiter. After performing a 23.5-hour check of the spacecraft’s systems in high Earth orbit, Orion is set to embark on a trajectory that will fly past the far side of the Moon before returning to Earth. With the crew on board, NASA plans to evaluate critical life support systems. The crew is also to test manual control of Orion’s flight and attitude while approaching another object represented by the ICPS upper stage.

There is currently no talk of crew withdrawal from the Artemis II mission. Personally, I think such a move would be rather surprising. Orion spacecraft and launch vehicles SLS doesn’t have NASA to spare. The time-consuming modification of the heat shield for Artemis II and the transfer of the crew to the third mission in the face of recent reductions in the number of Boeing employees working on the SLS would likely push back the moon landing date for the fourth mission until around 2030. And getting their astronauts to The moon by 2030 is also China’s goal.

Source of information:
https://oig.nasa.gov/

Image Sources:
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https://upload.wikimedia.org/wikipedia/commons/e/e2/Artemis_II_Heat_Shield.jpg