The Challenge of Spacecraft Reentry

On Sunday morning, the Orion capsule will reenter the Earth’s atmosphere at great speed and need to arrest that speed very quickly to splash down safely. I wrote the below text to commemorate the 50th anniversary of the Apollo 11 return. It is as true for Orion, except Orion will be coming in a little faster and has twice the mass.

A spacecraft re-entering the Earth’s atmosphere is a wonderful demonstration of the integration of science and engineering. Scientists make observations of how the universe works and then engineers are inspired to make machines that take advantage of these observations.

A spacecraft, like the Apollo 11 Command Module, is traveling at a great speed when it enters the atmosphere. Apollo 11 entered the atmosphere at almost 24 thousand miles per hour (10.67 km/s). It had to shed that speed before the capsule landed in the water. Parachutes couldn’t be deployed until the speed was down to about 350 mph (150 m/s). Without engines to slow it down, the only mechanism of deceleration was to have its kinetic energy transferred to the air molecules that it slammed into. We are talking about a lot of kinetic energy – about 300 billion joules. That’s about the same energy as Hoover Dam produces in 2.5 minutes.

That transfer turns speed energy into heat energy. It gets very, very hot around the spacecraft – hotter than the surface of the Sun. This should incinerate the spacecraft, but this is where the wonderful integration of science and engineering comes into play.

When the spacecraft slams into molecules of air, those air molecules bounce off. The molecules that bounce off will then collide with other air molecules, below the spacecraft. The multitude of these collisions will result in a plasma shockwave in front of the spacecraft. The heat that is building will gather in two places – the boundary layer (essentially the surface of the spacecraft) and the shockwave. If more of that heat is gathered by the shockwave then less of the heat will affect the surface of the spacecraft. In the early 1950s, Julian Allen and Alfred Eggers discovered that the shape of the object affects how far away the shockwave forms from the object and that the relationship was counterintuitive – the shockwave was farthest from the object when the object has a blunt rather than aerodynamic shape.

In short, by making the bottom of the spacecraft a slightly rounded, but largely blunt shape, a lot of the heat could be prevented from ever touching the spacecraft.

But this discovery wasn’t enough to allow a spacecraft to survive the journey. A lot of heat was still imparted from those air molecules that did get to touch the surface. Another wonderful integration of science and engineering was needed.

If you’ve ever caramelized sugar, you’ve observed a phenomenon called pyrolysis. Pyrolysis occurs when an organic substance is heated above its decomposition temperature, breaking bonds and causing small charred pieces to break away. Engineers figured out how to use this process to reduce the amount of heat that travels from the surface of a spacecraft through to the interior.

When you look at pictures of the Apollo command module, it looks like shiny metal, but that shiny metal is merely a thin layer of aluminized kapton tape. Below that tape is a clay-like substance called AVCOAT 5026-39, an organic material – a type of resin. It formed the heat shield. As the spacecraft plummeted through the atmosphere, the outer molecules of the heat shield absorbed that heat and then as the molecules broke down, tiny pieces of the heat shield broke off and were carried away by the flow of air. With them, they carried much of the heat they had absorbed. The next layer would then repeat the process, heating, undergoing pyrolysis, breaking off little pieces that travel away from the spacecraft, carrying their heat with them.

The result of these two phenomena of nature being manipulated by engineers was that the cabin temperature of the Apollo 11 spacecraft, during reentry, was 55 degrees Fahrenheit (12.8 C), while mere inches away it was hotter than the Sun. Engineering is cool (pardon the pun).