PTC (Passive Thermal Control)

PTC (Passive Thermal Control)
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Strong, unfiltered sunlight, with its large infrared component, heats one side of the spacecraft. Meanwhile, the great heatsink of deep space chills the other side as energy is radiated away at infrared wavelengths. The story is told in the excellent book, Apollo: The Race to the Moon, by Murray and Cox, of the difficulties the spacecraft designers were having with the heatshield material around the Command Module, trying to make it withstand extreme cold as well as heat. Joe Shea, a brilliant and intuitive engineer who was the NASA chief overseeing the CSM's difficult birth, asked how long it took for the heatshield to cool to the point where it began to crack and flake. The answer of thirteen hours prompted him to suggest that they simply keep changing the spacecraft's attitude by rotating it slowly in the sunlight. The maneuver which resulted was PTC or Passive Thermal Control, dubbed by many the 'barbecue' mode.

The integrity of the heatshield is not the only reason for PTC. The RCS quads, SPS propellant tanks and the structure, propellant and battery systems of the LM also needed to be evenly heated or cooled.(ap15fj)

    This article is based on material extracted, with permission, from the [Apollo Flight Journal] by David Woods and Frank O'Brien. The NASA Apollo Flight Journal files are copyright © 1998 - 2004. W. David Woods and Frank O'Brien.(more...)
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