Control of the heights has long been important in war:
Two centuries ago the Montgolfier hot air balloon took this to a new dimension — high in the air. When Benjamin Franklin saw one of the first flights, he envisioned an armada of thousands of balloons carrying airborne troops to battle. France formed the first air force — the “Compagnie d’Aerostiers” — and on 2 June 1794 two French soldiers were taken high above gunshot to observe and report the disposition of the Austrian army ranged against them. For many decades such balloons did not ascend more than a few thousand feet. However, in 1875, a huge balloon carried three men to over 25,000 ft; two were dead from hypoxia when they landed. The danger of great altitude was thus dramatically proven: if men were to fight in the air, the hazards of high altitude must be overcome.
By 1920, powered aircraft could reach altitudes that incapacitated pilots if they did not breathe extra oxygen. Twenty-five years later, aircraft could go even higher, beyond the level where pure oxygen, delivered under ambient pressure, sufficed. Pressurized cabins were thought impracticable for battle; other approaches were sought.
By then it was clear that exposure to gradually increasing altitude enabled men to survive, whereas the unacclimatized soon lost consciousness. Some air forces kept pilots for weeks at moderate elevations in the mountains to enable them to fly higher than their adversaries. This was somewhat effective but impractical, and it suggested that air crews might become acclimatized by repeated exposure to simulated altitude in decompression chambers.
To examine this concept, in 1946 the US Navy authorized a study of acclimatization called Operation Everest, which was conducted at the Naval Air Station, Pensacola, Florida. Four men were taken to a simulated altitude of 8,800 m (29,000 ft) during 35 days of slow ascent. Samples of arterial and venous blood, expired air, and urine, taken at rest and after exercise, were analyzed, and subjects were examined repeatedly during the ascent. Electrocardiograms and occasional chest roentgenograms monitored cardiac function.
Finally two of the four reached a simulated altitude of 8,800 m, where they were able to do light work for 20 minutes. Next day, breathing 100% oxygen, they were taken slowly from 8,800 to 15,200 m (50,500 ft), where they were able to exercise, although only lightly. This portion of the study showed that when breathing pure oxygen, these partially acclimatized men could go much higher than could the unacclimatized; at the time, this proved that acclimatization could give a small edge to the fighting pilot at extreme altitude. Soon, however, the use of pressurized cabins made moot the issue of acclimatization. Today, for military purposes, the altitude ceiling of the aircraft, rather than the pilot’s tolerance, is the limiting factor.