They identify the cause of the problem that the Orion spacecraft suffered during the Artemis 1 mission

After extensive analysis and testing, the US space agency (NASA) has identified the technical cause of the unexpected loss of material in the heat shield of the Orion spacecraft during the Artemis 1 mission, in November 2022.

Engineers determined that as Orion returned from its uncrewed mission around the Moon, gases generated within the heat shield’s outer ablative material, called Avcoat, could not be vented and dissipated as planned. This allowed pressure to build up and cracks to occur, causing some of the material to break off in several places.

To find out, the researchers methodically examined many things: they performed detailed sampling of the heat shield, reviewed recorded images, and analyzed data from the spacecraft’s sensors. In addition, they carried out exhaustive tests and analyzes on the ground.

During the Artemis 1 mission, engineers used the double atmospheric reentry guidance technique for Orion’s return to Earth. This technique offers more flexibility by extending Orion’s flight range after the re-entry point to take it to a landing site in the Pacific Ocean. With this maneuver, Orion plunged into the upper part of Earth’s atmosphere and used atmospheric drag to reduce its speed. Orion then used the aerodynamic lift of the capsule to bounce back out of the atmosphere, then re-enter the final parachute descent for splashdown.

Using data from the response of the Avcoat material during the Artemis 1 mission, the research team was able to simulate the environment of the spacecraft’s entry trajectory (a key part of understanding the cause of the problem) within a Research Center facility. NASA Ames in California, United States. They observed that, during the period between atmospheric dives, heating rates slowed and thermal energy accumulated within the Avcoat material of the heat shield. This led to the buildup of gases that are part of the intended ablation (wearing) process. Because Avcoat had no “permeability,” internal pressure built up and caused cracking and uneven peeling of the outer layer.

Technicians conducted extensive ground testing to simulate the reentry bounce phenomenon before the Artemis 1 mission. However, they tested at much higher heating rates than the spacecraft experienced during its flight. The high heating rates tested on land allowed the permeable material to form and wear away as planned, releasing gas pressure. The less severe heating observed during the actual reentry of the spacecraft slowed the material formation process, while continuing to create gases in this layer of material. The gas pressure built up to the point of cracking the Avcoat and releasing parts of the affected layer.

While the craft on the Artemis 1 mission was unmanned, flight data showed that if the crew had been on board, they would have been safe. Temperature data from the crew module systems inside the cabin were also within limits and remained stable, with temperatures around 24 degrees Celsius. Heat shield performance exceeded expectations.

Engineers understand both the material phenomenon and the environment with which materials interact during atmospheric entry. By changing the material or environment, they can predict how the spacecraft will respond. NASA technicians unanimously agreed that the agency can develop an acceptable flight analysis that will keep the crew safe using the current heat shield on the Artemis 2 mission with operational changes for entry into the atmosphere.

The Moon and part of the Orion spacecraft, photographed by a camera installed on one of the spacecraft’s solar panels on the ninth day of the Artemis 1 mission. (Photo: NASA)

NASA’s research process

Shortly after NASA engineers discovered the conditions of the heat shield, the agency began an extensive investigation process, which included a multidisciplinary team of experts in thermal protection systems, aerothermodynamics, thermal testing and analysis, stress analysis (materials fatigue), materials testing and analysis, and many other related technical fields. NASA’s Engineering and Safety Center also participated to provide technical expertise, including nondestructive evaluation, thermal and structural analysis, fault tree analysis, and other methods to support testing.

The heat shield on the Artemis 1 mission was heavily instrumented for this flight, including pressure sensors, extensometers, and thermocouples at different depths of the ablative material. Data from these instruments augmented analysis of physical samples, allowing the team to validate computer models, create environmental reconstructions, provide internal temperature profiles, and provide information on the timing of material loss.

About 200 samples of Avcoat were removed from the heat shield at NASA’s Marshall Space Flight Center in Alabama for analysis and inspection. The team conducted a non-destructive evaluation to “see” inside the heat shield.

One of the most important findings in the examination of these samples was that some surfaces in the permeable Avcoat zone, which had been identified before the flight, did not suffer cracking or material loss. Since these surfaces were permeable at the beginning of atmospheric entry, the gases produced by the ablation could be adequately vented, eliminating pressure buildup, cracking, and loss of char.

Engineers conducted eight separate post-flight thermal test campaigns to support the analysis of the origin of these conditions, completing 121 individual tests. These tests were carried out in facilities in different places in the United States that have unique capabilities, among them.

In spring 2024, NASA created an independent review team that conducted a comprehensive review of the agency’s research process, findings, and results. The independent review was led by Paul Hill, a former NASA official who served as chief space shuttle flight director for the Return to Flight program after the Columbia accident, who also headed the Space Shuttle Operations Directorate. NASA Missions and is a current member of the agency’s Aerospace Safety Advisory Panel. The review was conducted over a three-month period to evaluate post-flight heat shield conditions, atmospheric entry environment data, thermal response of the ablative material, and progress of POT. The review team agreed with NASA’s findings on the technical cause of the heat shield’s physical behavior.

Advances in heat shield

Knowing that Avcoat permeability is a key parameter to avoid or minimize material loss, NASA has the right information to ensure crew safety and improve the performance of future heat shields in the Artemis program. Throughout its history, NASA has learned from each of its flights and incorporated improvements in hardware and operations. Data collected throughout the test flight on the Artemis 1 mission has provided engineers with invaluable information to guide future designs and refinements. Performance data from the lunar return flight and a robust ground test qualification program, enhanced after the experience of the Artemis 1 mission, are supporting improvements in Orion’s heat shield production. Future heat shields for Orion’s return Artemis lunar landing missions are in production to achieve consistent uniformity and permeability. The qualification program is currently being completed, along with production of more permeable Avcoat blocks. (Source: NASA)