A FIERY RETURN
Arguably the most audacious feature of an Apollo flight was to have the crew reenter Earth’s atmosphere in the manner that they did. In the final minutes of a mission, a lump of metal and plastic, three crewmen and a few dozen kilograms of moonrock, altogether weighing nearly six tonnes, came barrelling in from outer space at speeds approaching 11 kilometres per second as Earth’s gravity hauled them in. As it entered, the air in front of the blunt end of the command module was brutally compressed in a shock wave that generated temperatures approaching 3,000°C. All that stood in the way of the crew being incinerated by this extraordinary heat was a coating of resin and fibreglass that NASA’s engineers reckoned could withstand the punishment.
In truth, the heatshield that surrounded the Apollo command module was very conservatively engineered for two main reasons. When the spacecraft’s design was frozen, engineers still had a poor knowledge of how the superheated air of re-entry would flow around the upper walls of the spacecraft. Although this surface did not bear the brunt of the heat, they decided to cover almost all of the hull with the heatshield material. Additionally, the original specifications had required that the shield should tolerate a much longer passage through the atmosphere, 6,500 kilometres, than ever proved necessary. The command modules that returned from the Moon typically flew for only about 2,200 kilometres through the atmosphere, which nearly halved the overall amount of heat the shield had to endure. In practice, although the heatshield took a lot of punishment across its curved aft section, much of its conical surface was barely singed by re-entry. Even the reflective Kapton tape that had been glued to the spacecraft’s exterior for thermal control in space was usually found to be still adhering to much of the hull. On recovery, pieces of Kapton were occasionally peeled off by those in attendance and kept as souvenirs.
W. D. Woods, How Apollo Flew to the Moon, Springer Praxis Books,
DOI 10.1007/978-1-4419-7179-1 15. © Springer Science+Business Media. LLC 2011