7.17 How could the fragile LM withstand temperature extremes so well?

IN A NUTSHELL: There’s a good reason why the lunar module could stand on the Moon with one side exposed to the sun and the opposite side in shadow, without overheating or freezing: it was insulated by a highly efficient multilayer thermal blanket. This gave it its characteristic “tin foil” appearance, which made it seem fragile while it was actually better protected against temperature variations than the rest of the Apollo spacecraft.

THE DETAILS: During the voyage to and from the Moon, the great thermal differences between the side of the spacecraft that was in full sunlight and the side in shadow required the astronauts to slowly roll the Apollo vehicle about its longitudinal axis to prevent it from overheating on one side and freezing on the other. This was known formally as Passive Thermal Control and less formally as barbecue mode.

However, the apparently fragile lunar module, when it landed on the Moon, could no longer roll. It kept the same side exposed to the incessant heat of the sun and the opposite side exposed to the cold darkness of shadow for up to three days, without overheating or freezing.

This apparent technical contradiction actually has a very practical explanation: the service module and the command module were more sensitive to thermal variations than the LM. In the service modules, the fuel tanks for its sixteen thrusters were close to the outer skin and had to remain within very strict temperature and pressure ranges. The command module also had a heat shield that would crack and flake if left to cool off in shadow in space for more than thirteen hours, becoming unusable and leading to crew loss upon atmospheric reentry. The slow roll was introduced to provide a more uniform and less extreme heating and cooling of these components of the Apollo spacecraft.

The LM instead didn’t have these limitations: differently from the other modules, it didn’t have to cope with the aerodynamic stresses of the liftoff from Earth (during which it was protected by a streamlined fairing), it didn’t have a delicate heat shield to protect and it had no fuel tanks in direct contact with the outside skin. Accordingly, it could be equipped with a more effective thermal control system, which included a thermal blanket made of multiple layers of Mylar or Kapton. Spacers formed an insulating gap between the blanket and the pressurized crew compartment. The LM also had a sublimator similar to the one used for the spacesuits.

The apparently fragile, tin foil-like appearance of the LM was produced by this thermal blanket, which concealed the normal underlying metal structure shown in Figure 7.22-1.

Figure 7.22-1. A prototype of the Lunar Module, preserved at the Smithsonian National Air and Space Museum, reveals the metallic structure inside the thermal protection covering. Credit: NASM.