WASHINGTON, July 27 — The return of a spacecraft from orbit is one of the hardest problems in engineering. For seven decades, every American capsule or shuttle that has come back to Earth has done so because of a thermal protection system designed to survive temperatures that would melt or burn most metals. The first U.S. reentry capsules — Mercury, Gemini, Apollo — relied on ablative heat shields.
These were made of materials such as phenolic resin that charred and eroded away during descent, carrying immense heat with them. Apollo’s design, developed by the Massachusetts Institute of Technology, successfully protected crews from temperatures above 2,500 °C.
It was a proven, if single-use, solution. In the 1970s, NASA introduced a radically different approach for the Space Shuttle: a reusable thermal protection system built from more than 24,000 silica-glass tiles bonded to an aluminum airframe. Each tile cost thousands of dollars to manufacture and install, and the system required intensive post-flight inspection and replacement.
The Shuttle fleet flew 135 missions between 1981 and 2011, but turnaround between flights averaged several months. The system worked, but it was labour-intensive and expensive.
When the Shuttle was retired, NASA’s Langley Research Center and Ames Research Center continued basic thermal protection research, but with sharply reduced budgets. The Hypersonic Inflatable Aerodynamic Decelerator (HIAD) project, for instance, received less than $10 million annually. Work went on, but at a slower pace.
A new direction with stainless steel
SpaceX’s Starship took a different path, departing from both ablatives and Shuttle-era tiles. The company chose stainless steel as the primary structure — it melts at about 1,400 °C — and attached hexagonal ceramic tiles designed to be cheaper and lighter than the Shuttle’s. Yet the fundamental challenge remains: any bonded tile system is susceptible to cracking, gap heating, and water intrusion.
And rapid reuse, which is central to Starship’s design, demands near-automated inspection and repair that has not yet been demonstrated. The history shows that no single thermal protection system has solved every problem.
Ablatives work but are expendable. Tiles are reusable but fragile and expensive. Stainless steel offers durability but still requires a coating that can fail.
The next step, whether it is automated inspection or a new material altogether, has yet to be proven at scale. For now, engineers at Langley, Ames, and private companies continue the work, knowing that the heat of reentry waits for no one.
Sources
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