Are Heat Shields The Key to Unlocking a Future Space Economy?

When the Orion capsule experienced reentry during the Artemis I test flight in 2022, its thermal protection system (TPS), the heat shield, did not perform as expected, losing large chunks of material in a process called spallation. The cause was a build-up of gasses in the TPS that caused shredding and cracking. The heat shield is being redesigned.

When John Glenn, in 1962, during the first orbit of his Friendship 7 Mercury capsule, was alerted to the heat shield potentially becoming detached, a seven-orbit mission was reduced to three, and the retrorocket pack instead of being jettisoned before reentry was left attached to try and keep the thermal protection system (TPS) in place. Before the Friendship 7 flight, very few people were aware of TPS.

TPS came up again during the Apollo 13 crisis in 1970. Concern was raised about the heat shield of the command module being damaged when the explosion occurred inside the attached service module. This led to some tense moments during reentry.

TPS failure occurred in 2003 to cause the loss of the Columbia Space Shuttle and its crew when it was damaged by debris falling from its external fuel tank during launch.

TPS Advances Are Important For Commercial Space Operations

Every returning spacecraft has had to endure atmospheric reentry. It is a demanding challenge, and the solutions aren’t one-size-fits-all. Low-Earth orbit reentry temperatures peak at 1,600 Celsius (2,912 Fahrenheit). In comparison, reentry temperatures for the Apollo command modules returning from the Moon reached 2,760 Celsius (5,000 Fahrenheit). When the Galileo probe entered Jupiter’s atmosphere at the end of its mission, the temperatures recorded reached 16,000 Celsius (28,832 Fahrenheit).

When we think of TPS, we tend to describe the type of heat shields used in space capsules flown by human crews. But there is more to it than that, as SpaceX has found in developing its innovative Starship. Nevertheless, heat shields remain the prime component of any TPS as space capsules turn them to face the heat upon deorbiting to protect the integrity, equipment and travellers on board. While doing this, the shields need to be lightweight so that they don’t take away from the needs of the mission. That’s why the Space Shuttle designers chose TPS tiles that were each lighter than the lightest styrofoam block.

The Changing Nature of Space Travel

An IDTechEx report entitled “Heat Shields & Thermal Protection Systems for Spacecraft 2025-2035: Technologies and Market Outlook” describes the current state of TPS development and innovations, which include advanced ablators and novel inflatables. It describes how human activity in space is changing rapidly.

The 20th century witnessed the first satellites and humans sent into space. First, we were in low-Earth orbit and later, we sent probes across the Solar System to visit the Moon, Venus, Mars, Jupiter and Saturn. A lander named Huygens survived the heat of reentry to reach the surface of Titan, Saturn’s largest moon. Another spacecraft visited the dual dwarf planet systems of Pluto-Charon. We visited the two largest members of the Asteroid Belt. All of these missions were on the behest of government space agencies that included NASA, Roscosmos, JAXA, ESA and others.

In the 21st century, however, the government agencies are being supplanted by new players who have emerged seeking to profit from outer space. These for-profit space businesses are putting even greater demands on TPS technology because for them to be profitable they need reusability. We are talking about SpaceX, ULA (Boeing-Lockheed partnership), Blue Origin, RocketLab and many others.

ULA has provided space launch services to NASA for several decades. SpaceX is a later entry, but today is the prime commercial space launch company on the planet. Its constellation of Starlink telecommunications satellites is showing that space can be profitable. SpaceX has also perfected a space taxi business contracted by NASA to take supplies and crews to the International Space Station.

For the most part, payloads in the past launched by commercial space companies were there to stay in orbit. These were one-way missions. If the satellite was decommissioned, its orbit eventually degraded, and it burned up in the atmosphere. TPS for these missions was not a prime concern. The growing demand for launch and return services requires a TPS technology with reusability as the key driver.

SpaceX Uses Tiles And More For Thermal Conductivity

Insulating thermal tiles were first developed for NASA’s Space Shuttle. They were developed as a lightweight distributed TPS that was supposedly durable to allow for rapid mission turnover and reuse. Thousands of these tiles were applied to parts of the Shuttle vulnerable to heating upon reentry. The tile design wasn’t perfect, with hundreds to thousands damaged and having to be replaced after each shuttle flight. This turned rapid reusability for the Space Shuttle into a pipedream.

SpaceX, with its Dragon capsule, and Boeing, with Starliner, are the first commercial space companies to build crew-ready spacecraft for the post-Shuttle age. Both have borrowed heavily from NASA’s legacy in these first-generation commercial crew capsules.

Starship, SpaceX’s next-generation crew-ready spacecraft, however, is taking a different path. Instead of aluminum, Starships are being built using high heat-resistant, high-chromium stainless steel. Their TPS systems combine tiles and ablative backup materials. The tiles are attached to the forward edges. Instead of square, they are hexagonal to be more effective in blocking direct paths for hot gasses produced during reentry to reach Starship’s hull. The tiles on Starship can withstand temperatures up to 1,377 Celsius (2,510 Fahrenheit), less than the peak heat low-Earth orbit reentries produce. That’s why Starships have an ablative material undercoat to provide further heat protection while the stainless steel hull’s more heat-resistant hull proves its metal.

The Newer and Future TPS Ablative Materials

Starship’s tiles were not built to deal with reentry heat alone. SpaceX added ablative resin materials made of AVCOAT. AVCOAT is also being used by NASA for the Orion space capsule. It converts the resins into pyrolysis gasses to divert heat from the spacecraft.

What is AVCOAT? It is made from silica fibres embedded in an epoxy novolac resin within a honeycomb structure. First used in the Apollo Program for the Command Module, it was reformulated for Orion. AVCOAT surfaces char and erode during the heat of reentry to protect the underlying structure and can withstand temperatures up to 2,760 Celsius (5,000 Fahrenheit).

Another novel ablative material is called PICA. It is composed of a ceramic-carbon substrate with embedded phenolic resin. SpaceX’s Dragon capsule uses PICA for its TPS. PICA burns off layer by layer during reentry and endures similar temperatures to AVCOAT. Its biggest advantage is that it weighs less.

Other TPS materials include polymers and silicone ablators such as:

• SIRCA, a fibrous silica substrate containing silicone resin that can withstand 538 Celsius (1,000 Fahrenheit) temperatures. It has been used for the Mars Pathfinder mission in 1997 and the Mars Exploration Rovers, Spirit and Opportunity, in 2003.
• RICA, a carbon-phenolic composite with high-temperature TPS characteristics that has yet to be mission tested.
• Cyanate ester resin, polyimide resin, polybenzoxazine and polybenzimidazole, all under development to overcome the TPS limitations of phenolic resins.
• Composites containing different lightweight materials each with different thermal protection capacities. Composites are being used by China for its Shenzhou spacecraft, and India is using it for the Gaganyaan human-rated space capsule that has had a successful uncrewed test flight.

Inflatable Heat Shields Show Promise

Inflatable Thermal Protection Systems (ITPS), also called Inflatable Heat Shields, or Inflatable Atmospheric Decelerators, are lightweight. They can measure 10 metres (33 feet) or more in diameter. Their larger surface distributes the heat of reentry more evenly than the rigid heat shields used on spacecraft today. They can be used with large spacecraft and on planetary entry missions. ITPS are composed of ablative ceramic fibre composites and offer the ability to incorporate additional insulators such as aerogels.

ITPS is seen as highly reusable. That’s why commercial space companies looking to get into the suborbital and orbital cargo delivery business see the technology as a game-changer. One of these, a German commercial space company, ATMOS Space Cargo, plans to conduct a first test flight in 2025 of the PHOENIX 1 capsule that incorporates ITPS. Suborbital and orbital delivery services are seen as a high-growth business in this decade and beyond. Recently, the U.S. Space Force asked for bids on supply and operational tests. This space wing of the American military is seeking offshore spaceports for supporting its strategic movement of supplies and eventually even troops. We’ve come a long way since Sputnik first circled our planet in 1957.