Demons always threatening fast-moving humans are the G-forces (one G equals normal gravity) that strike during changes of direction or speed. They are felt when an airplane makes a turn or pulls out of a dive, when a rocket takes off, when any vehicle is braked or hits an obstacle. As man moves faster and faster, the Gs become more and more dangerous.
The effects of moderate Gs developed by a turning airplane have been studied in whirling "human centrifuges," but the brief, intense forces of sudden starting or stopping cannot be simulated in this way. The Air Force uses, instead, a...