Apollo: The Race To The Moon

by Catherine Bly Cox

There was the problem of the heat shield. The specification the North American engineers were trying to meet required, logically enough, that the heat shield maintain its integrity under the extreme shifts from cold to heat that it would experience on its way to the moon and back, depending on whether it was in sun or shadow. But when they began testing the heat shield material under entry conditions, they found to their consternation that after exposure to severe cold it began to crack and craze and flake. They were going to have to create a new heat shield material, which would take untold quantities of money and time. Shea asked how long it took for the heat shield to cool down to the point where the problems began. The answer was about thirteen hours. So why did the spacecraft have to stay in the same attitude for that long? Why couldn’t it rotate, so the heat shield would remain nice and warm all the time? And that was the origin of what came to be known as the barbecue mode, or passive thermal control, in which the spacecraft rotated once an hour all the way out to the moon and back. Keep it simple.