Human beings function best at mean sea level, where the atmospheric pressure is approximately 14.7 pounds per square inch. As aerospace technology developed vehicles to travel to regions where the pressure is zero (space), parallel life support research produced the means by which human beings could survive in regions inherently hostile to biological physiology as we know it.
SURVIVAL IN THE
Man is a surface-dwelling life form, evolved over countless millions of years to survive in a more or less standard environment. This atmosphere surrounding us comprises a gaseous environment consisting of a mix of breathable gases exerting a combined average pressure of approximately 14.7 pounds per square inch on the human body. The ocean of gases found on the surface of the Earth contains a ratio of 20% oxygen, 5% carbon dioxide, and 80% Nitrogen (there are also ‘trace’ gases present, but they remain very slight and negligible for all practical purposes). This may be regarded generally as ‘Standard Atmospheric Conditions’ and the ratio of gases to each other remains the same, regardless of altitude, however air density decreases as one rises higher into the atmosphere. Temperature also decreases proportionately. This progressively diminishing air density and lowered air temperature that varies inversely with increases in altitude poses special concerns, as shall be seen below.
While certain weather disturbances (temperature, high & low pressure areas, varying humidity) may cause slight variations in this set of conditions required for normal human survival, for the most part they may be regarded as constant and unvarying to any significant degree, as long as man remains on the surface of the Earth (i.e. at sea level). If man does not venture higher in the Earth’s atmosphere, there is generally only the need to protect individuals from variations in temperature that normally occur at sea level. However, due to the fact that air pressure lessens as distance from sea level increases (vertically from the surface) as man rises to higher altitudes (whether in climbing a mountain, or flying in an aircraft), and the fact that temperature decreases with altitude, these changes in sea level ‘standard conditions’ must be compensated for if human beings are to be able to remain alive. A principal consideration is that although the percentage of gases in the atmosphere at higher altitudes maintains exactly the same ratio it has at sea level, ever diminishing pressure as a pilot flies higher poses severe challenges to aircrew survival. Chief among the needs required to offset those hazards is increased supplemental oxygen to assure normal human function. However, at a set altitude (about 35,000 feet), even supplemental oxygen alone is not sufficient and artificial pressure must be used to deliver oxygen to a pilot’s lungs. At still higher altitudes (between 45,000 and 50,000 feet), “pressure-breathing” of oxygen is insufficient to sustain life and some sort of pressurised cabin (with or in addition to pressure suit) must be used to operate an aircraft safely. At an altitude of 55,000 feet, without a pressure suit and/or pressure cabin, water vapor in the human body boils away; at only slightly higher altitudes (63,000 feet) blood in the human body will boil away. All of these serious threats to human safety at altitude are the direct result of lessened air density and temperature changes experienced as one rises higher into the air.
With specific regard to aerospace flight, a number of progressive advancements have been made (since man first rose into the atmosphere in a balloon) in medical physiology and protection technology to allow man to fly ever higher and faster in the Earth’s atmosphere. Without these steady advancements in knowledge, and without consequent improvements in science and technology, aviation and space flight as we know them today would be impossible.
Although man has flown off the surface of the Earth in balloons for nearly 3 centuries (variously estimated to be from about the mid-1700s), and despite the fact that men have been climbing mountains almost since the dawn of recorded history, it has only been since the development of manned flight in airplanes at the end of the 19th century that the need to devise truly effective mans of protecting human beings from the lethal effects of high altitude has occurred.
Early balloonists and mountain climbers were aware of the need for protection from effects of cold and oxygen deficit. Despite this awareness, the science of high altitude medicine took many centuries to advance to more than a rude level of basic knowledge, and it was not until the pioneering experiments of medical clinicians in the 18th & 19th centuries were undertaken that what has become known in today’s world as high ‘altitude physiology’ began to develop. As knowledge of the harmful effects of high altitude, lowered pressure, and the requirements of the human body’s metabolism at altitude became better known, it had become clear that if man was to develop powered aircraft capable of rising higher and faster above the earth, scientific exploration of ways and means of protecting their pilots from these hostile conditions would have to develop concurrently. The progression of these human ‘physiological support sciences’ has today resulted in a number of related but discrete areas of technological activity dedicated to enabling aircrew of air and space vehicles to operate safely in the hostile upper atmospheric and space environment.
Although there are a number of related areas of technological and medical activity dedicated to supporting high altitude flight in the 21st Century, they may be conveniently broken down into the following major divisions: 1) Aerospace Medicine, or hypobaric medical physiology dedicated to the maintenance of normal human function at altitude; 2) Life Support Technology, or the technological hardware systems by which the physiological needs of aircrew are met that will allow them to function normally in the high altitude and space environment; 3) Egress Technology, or the technological systems developed to permit aircrew to escape a crippled air or space craft safely; and 4) Crash-Rescue Sciences, which consists of the techniques used and hardware systems employed to rescue aircrew from air or space craft that have crashed. Taken together, these areas of aviation and space flight activity may be referred to henceforward simply as aerospace 'life support sciences'.
A BRIEF HISTORY OF MODERN AEROSPACE LIFE SUPPORT SCIENCES
The earliest need of pioneer aviators (and 'aeronauts', as balloonists were sometimes known, in the mid 1800s) was for protection from effects of severe cold, since the temperature of air decreases about 3 degrees for every 1000 feet of elevation gain. Although the first powered (and controlled—an important distinction) heavier-than-air aircraft (for example, the Wright Brother’s “flier” that was capable at the onset of an altitude of a scant hundred feet and speeds of less than 50 mph) permitted their pilots to wear ordinary street clothes, rapid progress in the development of internal combustion engine powerplants for airplanes soon enabled substantial increases in both speed and altitude. Until the latter part of the First World War, however, maximum altitudes reached were still less than 15,000 feet and speeds had not surpassed 150 mph. Thus the only protection pilots required was warm clothing that protected them from the wind-chill typically experienced in the era’s open-cockpit biplane. This typically consisted of leather coats, helmets, goggles, boots, pants, and gauntlets, frequently insulated with fleece.
As aviation knowledge increased, it was not long before further rapid advances permitted aircraft to routinely fly higher than 15,000 feet, at which point it was recognized that some sort of oxygen equipment would be necessary if pilots were to keep from lapsing into unconsciousness during flight. Thus the first aircraft oxygen systems were developed. These systems consisted at first of compressed cylinders of oxygen with a simple constant-flow valve that allowed aviators to receive oxygen via a mouth-held ‘pipe-stem’ device, delivered by a rubber hose at a rate of several liters per minute.
At about the same time, the fact that early airplanes were still notoriously unsafe and unreliable resulted in the introduction of the personal parachute—an invention first demonstrated as practical in the late 1800s. With a parachute, a pilot could jump out of a crippled aircraft while still in flight (although no lower than a thousand feet or so, safely) and float gently to earth. [Interestingly enough Germany was the first nation to recognize and advocate the use of parachutes by its aviators in the latter part of the First World War, whereas Great Britain felt that to allow its pilots to carry parachutes was somehow unmanly and did not permit their pilots similar safety provisions!]
As has been the case with all wars since, the First World War (1914 to 1919) resulted in substantial leaps forward in aviation knowledge and aircraft development. These advances, while they permitted pilots to fly higher and faster than ever, also brought about further demands for protection for aircrews. One of the most significant developments of the mid-to-late 1930s was the supercharger for aircraft reciprocating engines. Since aircraft carburetors also needed more oxygen in order to combust efficiently at higher altitudes (conventionally carbureted reciprocating engine power output drops to zero at between 55,000 and 60,000 feet), perfection of the supercharger for engines also conveniently provided a new aircrew safety development: the pressurised cabin.
Use of a supercharger to both enhance engine performance in the lowered air density found at higher altitudes and to allow generation of sufficient pressure inside the pilot’s cabin (which by this time were typically no longer open, but closed) permitted extended safe flight to higher altitudes for aviators than ever before. By the time the Second World War began, many commercial aircraft were being designed with pressurised cabins. Most of the combat taking place in that war occurred at altitudes of no more than about 20,000 feet, although in later war years high altitude (considered in that context to be above 25,000 feet) bomber formations were forced to fly their missions in the lower Stratosphere to avoid enemy anti-aircraft fire. Despite this fact, pressure cabins in military combat aircraft were not common until the post-war period.
Meanwhile, advancements in aviation oxygen breathing systems had resulted in development of the first constant-flow oxygen masks in the mid 1930s (typically, this consisted of an oronasal mask that received oxygen at a constant flow rate); an example of this type of mask would be the A-8 mask; these masks were useful up to about15,000 feet. The next development was the Demand Oxygen Breathing System that came into use in the early 40s. This system used a pressure-sensitive flow valve to provide oxygen only on intake (“demand”), with the flow being shut off on exhalation by a system of exhaust valves in the mask and oxygen delivery regulator. The demand system permitted flights to about 35,000 feet and the standard US aviator demand mask used from 1944 up until about 1965 was known as the A-14 mask. Still further refinements in aircraft capability resulted in even higher operating altitudes being achieved, which in turn resulted in an improved form of the demand system known as the Pressure-Demand System (introduced in about 1944-45, and good to an altitude of about 45,000 feet). With this system, variable oxygen concentrations were delivered to the aviator via a facemask under pressure, that automatically adjusted both the oxygen concentration and the pressure supplied proportionate to the altitude at which it was used. The Pressure-Demand Oxygen Breathing System remains the standard military oxygen system still in common use today (in combination with the pressure cabin found on all modern, high performance jet powered aircraft) and with present improvements is adequate for operating an aircraft up to about 50,000 feet, concurrent with use of a pressurised cabin.
Today’s modern military aircraft , while still using the basic principle of the Pressure Demand System, features refinements never dreamed of in previous decades, such as the OBOGS (an 'On-Board Oxygen Generation System' that employs a state-of-the-art molecular sieve to concentrate atmospheric oxygen into a breathable form. This differs widely from the early applications of oxygen in aircraft that used bottles of compressed gas, and even from more recent systems that used liquid oxygen reservoirs for aviator breathing oxygen.
The Second World War again produced a greatly accelerated body of knowledge in aeronautical design and engineering, a substantial amount of it coming out of secret German aviation research projects that were only completely revealed after that nation surrendered to the Allies in 1945. During the war, Germany tried hard to counterbalance the numerical superiority of its enemies by giving greater emphasis to advanced aviation research programs that it hoped would ultimately provide scientific superiority in weapons technology.
Fortunately for the Allies, the war ended before Germany could produce most of these weapons, but after the war ended the captured knowledge gathered by the Allies was distributed among the Western nations, many of which applied the advanced German research to their own aviation research and development programs. Among the innovative products of German wartime aeronautical research was the 'ejection seat', a concept that consisted of an ejectable aircrew seat that allowed aviators to rapidly (and safely) bail-out of a fast-moving, disabled warplane. The need for such a system was becoming evident as warplanes traveled even faster, rendering the old system of sliding back the canopy and simply jumping over the side unfeasible. Early German ejection seats were explosively ejected from crippled aircraft using a number of methods that included highly compressed springs, compressed gases, and explosive charges. German investigations into the effectiveness and suitability of various ejection (or 'egress', as the process is known in the military) methods appeared to point to use of a ballistic charged catapult tube attached to the pilot’s seat as being most favorable, and several of these seats were actually installed and used operationally in late-war German aircraft (1944-45) by the time the war ended.
Another startling advance quickly recognised as having merit for wartime applications by the German aviation researchers was the jet turbine engine. Although research into the jet turbine engine went on concurrently in several nations during the 30s and 40s, it was Germany that succeeded first in developing practical, mass-produced jet turbine powered military combat aircraft. By war’s end, Germany had several jet propelled fighter and bomber aircraft operating at squadron strength in combat against the Allies.
These two developments, the ejectable aircrew seat and the jet turbine engine, again extended airspeeds and altitude limits dramatically. The result for the West (as for the Soviets), in the 1950s, was a remarkable series of technological developments that effectively applied many of Germany’s advanced theoretical studies in aeronautical design to push the threshold of manned flight into the regions of near-space.
This acceleration in aircraft operating capability, much of it based extensively upon captured enemy wartime research, imposed an entirely new and even more demanding requirement upon human life support sciences to devise ways of allowing aviators to fly safely at these very high altitudes and supersonic speeds (the term ‘supersonic flight’ means flight in excess of the speed of sound, which varies with altitude, air density, and temperature, from about 761 mph at sea level to about 660 mph at 45,000 feet).
Entirely new life support programs were established in the 50s at aviation research facilities, such as the well known Edwards Air Force Flight Test Center in California, to test new aircraft ejection seat systems that would allow pilots to safely escape crippled jet aircraft flying as high as 60,000 feet and speeds approaching Mach 2 (twice the speed of sound). Since conventional ejection seats could not fully protect aircrew from severe wind-blast effects encountered at high Mach numbers, new ideas emerged involving ejectable, full encapsulated escape pods and ejectable cockpit sections of aircraft. Among the latter category of egress system were the aircrew escape pods of the delta winged Mach II Convair B-58 Hustler and the Mach III North American XB-70 Valkyrie. Increasingly sophisticated biomedical research programs aimed at devising ways of protecting future astronauts from the effects of space on orbital flights resulted in the refinement of conventional aviation medicine into the complex new area of biomedical science known as Aerospace Medicine, as NASA prepared to launch the first Earth orbiting space satellites. Life support sciences in turn further evolved into correspondingly complicated areas of applied technology concerned with aircrew protection and safety—all mandated by the significantly higher and faster operating capabilities of the new generation of modern jet and rocket powered aircraft.
As the Korean and Vietnam Wars of the 50s and 60s began using large numbers of combat aircraft to a greater degree than ever before, another area of closely related support (prompted by the need to ensure safety of aircrew shot down in combat zones) was that of aircrew survival equipment, an important and discrete discipline unto itself. The activity focused on development and maintenance of individual survival equipment items that the aircrewman could carry on his person (and in ejection seat survival kits), and formulated methodologies that would allow an ejected airman to survive on remote ground or in unfriendly territory while awaiting rescue.
Concurrent with this period of rapid advances in aviation and space technology, investigations into pressure suit systems for aircrew rose to new levels of sophistication. Although exploration of the concept of a pressurised suit to allow safe operating of aircraft at high altitude had begun several decades earlier, the 19509s and 60s focal period brought with it an entirely new impetus driven by the promise of rocket engine technology to boost human occupants into near-space Earth obits. In the United States, the simple anti-G suit used as early as the Second World War to protect aviators from G-induced blackout during high speed maneuvering, quickly evolved into what became known as the mechanical partial pressure suit, or ‘capstan suit’. This system employed a tight-fitting suit equipped with inflatable tubes (capstans) along the torso, arms and legs of aircrewmen to constrict the material around the body, providing artificial pressure that would permit operation of aircraft for limited periods of time above 50,000 feet. The suits were typically used in combination with pressurised cabins to extend the ability to function at high altitude somewhat further, since the suits by themselves were by design very uncomfortable to wear when fully pressurised. Moreover, in the event of a pressurised cabin failure, the partial pressure suit alone could also be used as a stand-alone emergency ‘get-me-down’ safety system to allow the pilot to quickly descend to a lower and safer altitude (i.e. below 30,000 feet). These early partial pressure suits quickly proved their value when in 1952 a test pilot flying the Bell X-1A rocket research aircraft high over the Mojave Desert used his suit to survive an explosive decompression, when his canopy cracked at 60,000 feet.
A related development of even more promise, beginning in the early 50s, was that of the full pressure suit assembly. This concept in high altitude aircrew protection enclosed the pilot entirely within a hermetically sealed suit and helmet combination, effectively surrounding him with a wearable mobile pressurised environment maintained at about 3.5 PSIG (roughly 40% of sea-level pressure), permitting more extensive periods of flight time at extreme altitude. Biomedical studies demonstrated the fact that the wearer of such a suit could operate safely in near-space for a substantially extended period. The application quickly led directly to the first Earth orbit 'space suits' of the Project Mercury Program. Although crude by comparison to later space protective garments (for example, the Apollo moon landing suits), the first American NASA astronauts flying in the Mercury orbital program of the 60s wore early high altitude military full pressure suits that had been specially modified to meet the requirements of the Mercury capsule cabin. In that program the suits were worn solely for emergency use in the event of a cabin pressure failure, and were not worn inflated throughout the standard Mercury orbital mission.
It is worthwhile to consider that the immediate post-war period of the late 40s and early 50s was focused substantially on providing a safe operating environment for aircrews flying at very high altitudes and supersonic speeds, due to the fact that military strategists of that early Cold War period theorised that aerial combat would be most likely taking place high up in the atmosphere at very fast speeds (in the form of air space penetration by enemy bombers). Although the need for high altitude combat operations was soon nullified by the widespread proliferation of intercontinental ballistic missiles, this vast body of research ultimately resulted in, among other things, sophisticated new high altitude physiological techniques and more modern full pressure suit assemblies that went on to support the Lockheed SR-71 and U-2 high altitude strategic reconnaissance flights (they also figured significantly in the design of protective space suits for the US post STS-51L space shuttle crews).
From these early full pressure suit programs, the development of advanced space suit life support systems to protect lunar landing astronauts was a logical follow-on application of that early 50s and 60s high altitude research. Consisting of multiple layers of specialized material and containing their own oxygen generation and waste products recycling systems, the lunar suits used by the Apollo astronauts were truly a ‘portable, mobile, and wearable individual personal protective environment’ that allowed human beings to survive and function normally in that most hostile region, true ‘outer space’.
Still another area of advanced life support research that occurred in the 1980s was NBC (nuclear, biological, and chemical) defense for aircrews. The challenge of incorporating protection against chemical and biological war agents into flight protective gear and clothing would take years of study and result in a number of proposals, including integrating pressure suit and NBC defense garments into a combined, multipurpose aircrew assembly that met all needs. Due to the unique requirements posed by each of the concerns (high altitude protection, chemical defense, and heating/cooling systems) complete success in these efforts was never fully achieved and related study continues at present.
Today, the emphasis in military flight is not as focused as it formerly was on extreme high altitude flight and multi-Mach speeds, but on the lower, but high-maneuverability flight ‘envelope’ permitted by new materials technology and the use of computers to control flight of atmospheric vehicle. State of the art modern military aircraft engineering today allows the design and production of airplanes that have far more maneuvering capability than their human aircrew occupants can physiologically withstand, since the human body can only function normally up to specific limits of so many positive or minus Gs (the term 'G force' is commonly expressed as a multiple of the force of gravity). Beyond these physical limits unconsciousness occurs, so contemporary life support research has more recently focused (and to a great extent) on developing aircrew life support systems that will ameliorate or more adequately modulate the deleterious physical effects on human occupants produced by today’s radically advanced and ultra-maneuverable jet (and rocket) aircraft. Finally, as man extends his operational environment into true ‘outer’ space for protracted periods (i.e. beyond actual Earth orbit, as in missions to the Moon or Mars), entirely new areas of concern dealing with psychological, social, and behavioral aspects of extended human survival in the hostile space environment will take on increasingly important relevance.
It may be fairly stated, in light of the above brief historical overview, that the story of progress in aircraft design and performance capability is inextricably dependent upon collateral developments in aerospace medicine, aircrew life support, egress, and (to a slightly lesser degree) crash-rescue support technologies. These have continually advanced apace, as the many new challenges of high altitude atmospheric and space flight operations have emerged. It almost goes without saying that without the thousands of skilled doctors, technologists, and specialists employed in these many support activities and applied technology research programs, safe modern military flight and indeed all space flight would be impossible to contemplate.
Although there are inevitable failures to be expected as the search for new means of protecting aircrews in the aerospace environment continues (two sad examples are the loss of the Space Shuttles Challenger and Columbia, in which adequate escape and survival systems were deficit), no area of research and development has perfect foresight. The level of success achieved in the aircrew life support sciences is and continues to be admirable, given the often perplexing challenge of problems that have been encountered and successfully dealt with, over the past 60 years of high altitude and space flight.
In today’s world, in which science and technology are so much taken for granted, so common have these support activities become that the average individual rarely considers more than the airplane (or the spacecraft) and its pilot when he thinks at all of the world of flight. The critical role that aviation and aerospace supportive technologies play (and have historically played) in enabling man to fly farther, faster, and higher in the hostile environments of high altitude atmospheric flight and space are therefore frequently overlooked and misunderstood, becoming submerged and lost within the complexities of our manned atmospheric and space flight programs.
To illustrate this point using a common, but meaningful analogue, the aircraft and its pilot of today’s aerospace world are only the tip of a huge, figurative iceberg, the vast bulk of which lies hidden from sight beneath the surface of the water. Accordingly, it is our purpose here to highlight and accentuate this awareness of the important of these underemphasised aerospace support technologies for the benefit of the aviation-minded public.