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120,000 Ft. Cosmic rays may be hazardous, with heavy nuclei in the raw. One can bore through a man, killing a "column" of tissue of 1,500 to 3,000 cells. An unlucky hit on the macula, the point of clearest vision at the center of the retina, could cause partial blindness. Even the rarefied air at this level will create enough friction, at speeds already foreseeable, to raise cockpit temperatures to 400° or 500° F. Air conditioning is a must, but to cool the air to a comfortable 80° would require forbiddingly heavy gear, so some designers have hit upon 120° as an attainable mark (if man can be equipped to withstand this sweltering heat).
The Dangers of Gravity
Next to the dangers caused by low atmospheric pressure at high altitude, the biggest perils on the vertical frontier are gravity forces. Every time a human body is subjected to acceleration (a word that scientists use broadly to include slowdowns and changes in direction as well as speed-ups), it feels the pull or push of gravity, or "G" forces. Common example: the passenger in the hot-rod who is thrown against the seat-back when the driver makes a jackrabbit start. In an airplane the crew is subject to sharp acceleration forces in any quick burst of speed, e.g., a jet-assisted takeoff, or in an abrupt change in direction of flight. At high speeds even gentle turns can set up heavy acceleration forces. Thus when a fighter pilot makes a turn, his blood and guts are pulled by acceleration to the outside of the turn: blood flows from head to feet, and organs in the abdomen are pushed down to the pelvis. If the turn is sharp enough or fast enough to develop forces of two Gs (double the ordinary force of gravity) or more, the drainage of blood from the head and heart may make him black out.
One G is a force equal to that of gravity, which makes a body in free fall (without air drag) pick up speed at the rate of 32 ft. per second for every second of fall. The human body's ability to withstand G forces without injury varies enormously, from about two Gs to 15 or more, depending on the position and protection afforded. Damage is smallest if the shock is taken through the body's smallest dimension, from nose to nape and from the navel to the small of the back. Damage is greater if the shock is taken so that blood rushes from head to feet (positive Gs), and worst of all from feet to head (negative Gs). Dr. Armstrong calculates that when a man jumps from a table 30 in. high and lands flat-footed on a hard floor, he subjects himself to the frightening force of 16 Gs, but is not harmed because the shock is taken head to feet. The daring experiments of Lieut. Colonel John Paul Stapp on a rocket sled show that the human frame can withstand great stresses (up to 45 Gs) if it is properly supported and can take them in the right direction.