The Armstrong limit, a boundary that marks the altitude above which the surrounding atmosphere can no longer keep warm water liquid, is a critical concept in aerospace physiology and flight protection. At 19 kilometers above Earth, the atmospheric pressure falls to approximately 6.3 kilopascals, which is also the vapor pressure of water at 37 degrees Celsius. This means that exposed water, such as saliva and tears, can begin turning to vapor, leading to a condition known as ebullism. The human body temperature is enough to cause this phase change, making the Armstrong limit a crucial threshold for aircrew safety.
The boiling point of water is not solely determined by temperature; it is also influenced by atmospheric pressure. At higher altitudes, where atmospheric pressure decreases, water boils at a lower temperature. This phenomenon is why aircrew above the Armstrong limit require sealed pressure suits rather than oxygen masks. An oxygen mask only increases the proportion of oxygen in each breath, but it does not provide the necessary pressure to keep the body and tissues from boiling.
The story of Jim LeBlanc, a NASA technician, illustrates the dangers of rapid high-altitude decompression. In 1966, LeBlanc was testing an Apollo suit in a high-altitude chamber when an oxygen coupling disconnected, causing the suit pressure to drop rapidly. He experienced symptoms such as steam blowing from his left side, fuzzy vision, and eventually, the water on his tongue began to boil. LeBlanc survived without lasting neurological injury, but this incident highlights the critical need for immediate emergency response and medical assessment in cases of rapid decompression.
The U-2 aircraft, which operates above the Armstrong limit, carries a cabin and a full-pressure suit. The suit is designed to provide a survivable atmosphere in case of cabin failure or other emergencies. The suit does not make the flight comfortable, and pilots must breathe pure oxygen before takeoff to reduce the risk of decompression sickness. The Cabin Altitude Reduction Effort has increased cockpit pressure, reducing the physiological load on pilots.
The Armstrong line is a critical boundary where the atmosphere stops doing invisible work. At sea level, atmospheric pressure feels like nothing, but above the Armstrong limit, aircrew require an artificial replacement for the missing environment. A sealed aircraft cabin or a full-pressure suit can provide this environment, while an oxygen mask can only contribute breathing gas. The survival of LeBlanc after seconds near vacuum demonstrates the importance of immediate emergency response and medical assessment in cases of rapid high-altitude decompression.
In conclusion, the Armstrong limit is a critical concept in aerospace physiology, and aircrew above this altitude require sealed pressure suits to ensure their safety. The boiling point of water is not solely determined by temperature, and the Armstrong limit marks the altitude where the surrounding atmosphere can no longer keep warm water liquid. The story of LeBlanc highlights the dangers of rapid high-altitude decompression and the critical need for immediate emergency response and medical assessment.
