Artemis II Reentry Change Validates Orion Heat Shield Strategy
Artemis II’s most important finding may not be the flight around the moon. It may be that NASA has solved a critical thermal-protection issue without having to modify the design of Orion’s heat shield. That’s crucial because Artemis II was essentially a mission to test the systems, including the flight profile, during the roughly 10-day mission. Orion delivered four astronauts to a free return around the moon 700,237 miles in all, with a closest approach of 4,067 miles. The spacecraft’s systems, navigation, and return trajectory to Earth were all part of the mission parameters. However, from an engineering standpoint, the crucial test was the entry.
The skip-entry profile used for Artemis I resulted in unexpected loss of avcoat the ablative material used to protect the spacecraft and cracked and eroded areas after the first Artemis mission’s return from lunar orbit. According to the engineers, the root cause of the issue was the improper venting of gases generated within the Avcoat during the skip-entry profile. On the flight of Artemis I, the gases coming off the material built up pressure as the shuttle skimmed the atmosphere.
However, since NASA does not appear to have abandoned the fundamental notion that Avcoat is a reliable material for protecting the spacecraft, engineers were able to adapt to the new conditions by changing the re-entry profile for Artemis II. Instead of changing the material for the spacecraft’s heat shield, engineers designed the mission so that Orion would adopt a steeper entry angle to reduce the time spent in the problematic gas-building regime.
In other words, it is a standard aerospace engineering practice to alter the flight environment if the operating parameters are understood well enough. In this case, it appears that changing the angle was a far more straight forward solution than re-engineering the heat shield.
The inspection of the spacecraft after landing appears to confirm that the solution worked. The spacecraft’s heat shield experienced only localized loss of material with no apparent damage to the underlying structure and the bonding layer in good condition. For the flight around the moon, those are precisely the results that the program managers and engineers wanted to see.
Not that the heat shield is something small, as the spacecraft features the largest ablative heat shield ever built. The 16.5-foot-diameter Orion heat shield was built from 186 machined Avcoat blocks attached to a titanium skeleton using composite skins and forming compression pads at critical attachment points. It has to endure the extreme conditions of the spacecraft’s re-entry, which means reaching temperatures nearing 5,000 degrees Fahrenheit at 24,000 mph.
An ablative heat shield like the one used on the Orion spacecraft means that the material is not designed to keep the heat out. Instead, the heat shield is designed to eat away, carrying the heat with it, in order to protect the spacecraft’s internal systems and structure. Therefore, the material loss is normal, and the primary concern for the engineers is whether the loss follows the expected rates within the acceptable limits. It seems that so far, for Artemis II, things are looking good.
However, it is also worth noting that the confidence in the solution comes from NASA’s ability to identify the root cause of the issue. According to the engineers, the problem was caused by the inability of the Avcoat to vent the gases properly during the skipped entry.
The investigation into the root cause included the examination of the material samples, sensor data, nondestructive evaluation, arc-jet testing, wind tunnel tests, and other relevant research. The data allowed the engineers to conclude that the problem was caused by inadequate venting of the internal gases in the Avcoat during the skip entry. Therefore, NASA had two options: to change the material properties on the Avcoat or adjust the entry profile to ensure that those properties were not a factor in the mission. It seems that so far, the solution that they have chosen is working well.
As for the flight itself, Artemis II was a momentous occasion for the crews, as the five-day mission marked the first lunar mission beyond low Earth orbit in more than 50 years. It also marked the first trip for Victor Glover, who is set to become the first Black astronaut to fly near our natural satellite. It was the first mission for Christina Koch as well, who is set to make history as the first woman to travel near the moon. Meanwhile, Jeremy Hansen will become the first Canadian to visit the moon. In many ways, it is too early to say whether Artemis II has been a success.
However, the engineers can breathe easy knowing that the solution that they developed for the spacecraft appears to be working well. In the bigger context, the flight of Artemis II served to demonstrate that deep-space missions beyond low Earth orbit are logistically viable. After all, the long-range spacecraft and their propulsion systems have to be tested, as do the orbital dynamics that will allow them to return safely to Earth. In essence, Artemis II is a validation of NASA’s ability to solve the problems that crop up during missions by studying them meticulously, identifying root causes, and developing solutions. In practice, that means that the issues with the heat shield in Artemis I were recognized and resolved for Artemis II.
Ultimately, those kinds of tests are crucial for future missions beyond low Earth orbit. After all, NASA is going to need to send these kinds of spacecraft to the moon frequently if it wishes to meet its goals. By getting ahead of the problems and addressing them when they emerge, NASA can move quickly to ensure that each spacecraft is ready for subsequent missions.
By David Whitaker, associate editor for AMI’s aerospace and drone systems desk, covering stories related to flight systems, commercial aircraft, spaceflight, and UAVs.
