The heat shield of Orion, returned by Artemis I, was battered and bruised over 100 times, and NASA intends to put four people on the next one. Artemis II is constructed on the basis of a mere promise with complex engineering behind it: a crewed orbit around the Moon and a safe reentry through the atmosphere of Earth. The spacecraft that highlighted that promise, Orion, contains a well-known weakness in the Avcoat heat shield, which will need to take the impacts of lunar reentry conditions that can reach 5,000degF (2,760degC) when the capsule bursts through the atmosphere resulting in lunar-return velocities.
It is not the heat shield “burning” that is the concern. It is designed to. What is feared is that, in Artemis I, Avcoat was not fading away; it broke and bled off, leaving pits throughout the shield. NASA even in its own watchdog recorded over 100 sites on the heat shield where material chipped away unexpectedly, an outcome that made engineers go a multi year ground test heavy hunt to find root cause.
That inquiry eventually narrowed down to one failure mode: trapped gas in Avcoat. In NASA technical explanation, gases created during the ablation process were unable to escape and dissipate on the desired manner since the material was not permeable and pressure arose until cracking took place and burnt material peeled off. The agency recreated the Artemis I scenario in arc jet facilities in the NASA Ames Research Center after which observations were compared with flight data. “Engineers determined as Orion was returning from its uncrewed mission around the Moon, gases generated inside the heat shield’s ablative outer material called Avcoat were not able to vent and dissipate as expected,” NASA’s statement explained. “This allowed pressure to build up and cracking to occur, causing some charred material to break off in several locations.”
In the case of Artemis II, the change of entry is the core mitigation of NASA, but not its structure. The reentry profile will retain the heat shield used on the return of the capsule, but the time spent in the narrow regime which caused the poorest cracking characteristics during the skip-style entry on Artemis I will be minimized. To put it another way, the agency is attempting to modify the environment in which the material experiences occurs as opposed to requesting the material to act otherwise.
The language is unusually open even to knowledgeable people that accept the flight rationale. It was termed as “a deviant heat shield” by former astronaut and materials scientist Dr. Danny Olivas and described as such, stating: “There’s no doubt about it: This is not the heat shield that NASA would want to give its astronauts.” However, he too claimed NASA “has its arms around the problem,” a division that runs through many of the external commentators: trust in the quality of engineering labor and uneasiness about placing a crew on equipment that has already astonished the designers.
A detail, which seems minor in the materials, sharpens that divide, namely, permeability. Future heat shields, NASA has stated, will be made more uniformly permeable, and even at Artemis I, some experts point that local permeable zones have not cracked. Meanwhile, the shield of Artemis II was constructed prior to the flight of Artemis I, which constrained the redesign choices without having to impact the schedule significantly. The outcome is an Artemis II risk posture that is no longer characterized by ideal hardware but by constrained circumstances an effort to ensure that the heat shield does not get into trouble by managing how long it spends in the heat.
It is also a process argument rather than a physics argument. Following the publicity sparked by images captured by NASA Inspector General, the agency was put under the microscope due to the duration of time sensitive information was kept secret. The current administrator at NASA, Jared Isaacman, has gone on record to say that he had “full confidence” in the plan, but went further to state to onlookers “That level of openness and transparency is exactly what should be expected of NASA.” Artemis II thus is two tests simultaneously: an engineering test of a re-architectured return path, and an institutional test of whether NASA can continue the hard lessons of anomaly response, to adapt the unexpected behavior as the basis of further knowledge, and not as normalizing behavior.
