Modern spaceflight, let alone high altitude flight in aircraft, would not have been possible without the concurrent development of oxygen breathing systems for pilots and aircrew. This is a two-part history of that important aspect of modern aerospace technology. [The image accompanying this segment shows a typical German WW2 pilot helmet and Draeger oxygen mask, circa 1943]
A History of US Military
Aviation Oxygen Breathing Systems
(Part One of Two)
The need for special oxygen breathing systems for military aeronautical operations today is taken for granted as being one of the most critically important areas of aircrew life support concerns. When one considers that powered flight itself will shortly become 100 years old (2003), the fact that oxygen breathing systems in aircraft have only been in use for about 86 of those years takes on enhanced meaning. Despite the fact that oxygen delivery systems in military aircraft have been in use for so relatively brief a period, this should not obfuscate the fact that awareness of the need for human beings to breath supplemental oxygen as they rise higher in the atmosphere has been in existence and uniformly acknowledged by physiologists for several hundred years.
In the span of the short 86 years during which military aviation oxygen delivery systems have been in use, the sophistry of life support breathing systems in military aircraft has developed dramatically from the first simple use of heavy iron cylinders to store compressed oxygen (breathed through a short length of rubber hose and a ‘pipe-stem’ device) in 1917, through the latest molecular sieve concentration systems that extract oxygen from the ambient air and compress it for use by aircrew of high-performance jet aircraft (as found in the F-22 Raptor and Joint Strike Fighter prototype, the F-35).
The purpose of this brief paper is not to fully enumerate and chronicle the entire, complex history of military aviation oxygen breathing applications (nor is it to cover the several hundred years of pulmonary physiology research that have enabled development of today’s systems), but rather to present a broad overview of generalised developments as a starting point for further understanding of this subject. It is hoped that this will therefore serve to stimulate and encourage interest by a wide range of ‘non-experts’ in this important area of military flight physiology, the importance of which is today almost completely overlooked and taken as granted by the typical aviation minded individual or enthusiast.
Early History: World War One
Prior to the outbreak of war in Europe in 1914, there had been little if any concern with developing aviator oxygen systems. Although some research had been done on providing oxygen flasks for balloon pilots engaged in lighter-than-air altitude attempts (mostly in Europe, as part of a continuing tradition of medical interest in the effects of high altitude on balloonists), nothing in the way of an American aircraft oxygen system had been devised or even conceived in practical form.
The first flight of an aircraft using an purpose-provided oxygen system in fact seems to have occurred in 1913, when a French aviator (Georges Lagagneux) flew a Nieuport biplane to an altitude of 20,014 feet (this was 10 years after the internal combustion engine powered Wright Flyer took to the air in December of 1903).
American disregard for the possible importance of providing oxygen systems in aircraft disappeared almost overnight in August of 1914, when the new European war was declared. The rapid advances in aeronautical technology spurred onward by the new war resulted in successively faster and more powerful aircraft, able to fly higher and higher into the atmosphere. This, in turn quickly created awareness by the fledgling US Army medical service of the need to provide oxygen for pilots and aircrews.
This awareness came of practical experience with US aviators flying at higher altitudes (necessitated by the need to fly up beyond the range of ground fire) who needed oxygen to remain alert and be able to function reasonably well in their operation of aircraft. Reports of strange symptoms and seriously debilitating conditions (typical of hypoxia and including cyanosis, headache, weakened muscular tone, earache, vertigo, extreme lassitude) reached medical personnel in operational areas. Further strange, occasional, and unexplained losses of aircraft began to accumulate. It wasn’t long before the cause of most of these maladies was pinpointed as being attributable to the need for oxygen use for safe flight above 15,000 feet.
Germany was one of the earliest nations involved in the First World War to recognize and address the need by aviators of aircraft and dirigibles for supplemental oxygen. The great Zeppelin dirigibles, by virtue of their ability to fly at higher altitudes, were the first war craft outfitted with aircrew oxygen systems, which were at first of the conventional compressed gas type, contained in iron storage flasks. Soon, however, the heavy storage flasks were replaced by early liquid oxygen generating systems. These systems were devised and produced by the Draeger Company, a company long associated with respiratory and resuscitation equipment for mining use. Other systems were produced by the Ahrend and Heylandt Company. It wasn’t long before some higher flying German bombers and fighters were equipped with these small, lightweight liquid oxygen systems. Oxygen could be breathed from these small ‘personal’ liquid oxygen systems through use of a mouthpiece (frequently called a ‘pipe stem’) that could be held clenched in the mouth of an aviator. The tube providing the oxygen was attached (on the German systems) to a large rebreathing bag positioned nearer the unit than the ‘pipe stem’, so that although the oxygen flow rate was continuous, more of the gas could be saved and reused in the process that would have otherwise been wasted.
These German systems were carefully studied in the United States during the war, after specimens were recovered from downed German machines, and systems very similar in design to the original Draeger systems were soon devised and tested in American aircraft. It wasn’t long before several things became apparent. These were that the effects of cold at altitude frequently made it extremely difficult (and at best uncomfortable) to hold an oxygen ‘pipe stem’ in the mouth for a protracted period. This led to the design of a leather mask in which the small diameter rubber delivery hose could be inserted; the mask, it was felt, would succeed in both holding the tube in place near the mouth and (perhaps equally as important) provide substantially improved protection against the severe cold encountered in open cockpits at altitude.
Predictably, complaints were soon heard about how the use of a mask was ‘restrictive’ and would obstruct the wearer’s ability to move his head about, in search of a pursuing enemy aircraft. Others felt that the use of a mask would critically distract their attention during dogfighting. These objections were ultimately overcome, however, as the benefit of oxygen use became more widely acknowledged by fliers.
Further areas of study concerned regulation of the oxygen delivery, since originally the continuous flow oxygen systems had featured only a reducing valve that could be opened and closed. Some sort of ‘automatic’ pressure regulator would be a very desirable improvement in the system (if a successful design could be devised) and the need for a gauge to indicate oxygen supply remaining was also evident. During the war, the French came up with a system named after its creator, Dr. Paul Garsaux. This system used an aluminum ‘mask’ that could be shaped to conform somewhat to the wearer’s face and had inflatable face-sealing bladders to help insure a tight seal (it is interesting to note that the latest US Air Force oxygen masks incorporate a face-sealing bladder on the mask's face-seal, some 85 years after Garsaux's experiments with this feature!). The Garsaux system was tested in the USA and considered for adaptation to US biplanes, but was ultimately rejected owing to faults in the design that made it unreliable at certain times. An improved Garsaux system addressed these deficiencies, but although tested in the USA, none of the improved Garsaux systems were acquired and used in US flying machines.
In Britain, British RFC research soon proved beyond question the many benefits of oxygen to pilots flying combat missions, so the question of whether oxygen systems were necessary or not was thereupon mooted, and effort was dedicated towards devising lighter, more dependable, and more comfortable oxygen breathing systems for aviators. Siebe, Gorman, and Company, English pioneers in the design and production of respiratory equipment for miners and divers, produced the first practical military aviation oxygen breathing system for the RFC, although it was somewhat heavy and had certain disadvantages. It used a rubber mask without inlet or outlet valving, and featured single or doubled flasks of compressed oxygen (500 or 1000 liters capacity). This system could support one or two men, but after a year of flight experience with this system, the RFC abandoned it and returned to their original system of providing each aircrewman with his own individual oxygen supply.
Not long after this, the 'Dreyer Oxygen Equipment' system was developed by LC Geo. Dreyer of the RFC Medical Branch. Designed by Dreyer, after a cooperative RFC/French study of all the major military aviation oxygen delivery systems in use, the Dreyer oxygen system was adopted by the RFC and placed into production by the De Lestang Company in Paris (patient holder). The key to success in the Dreyer system was its use of an aneroid-controlled, automatic regulator design. This pressure-compensated delivery system was entirely automatic, being regulated entirely by the system and not by hand (as in the Siebe-Gorman system). Unfortunately, the system was so precise that each unit had to be manufactured entirely by hand, with the result that production output was limited and not rapid. American tests of the new Dreyer RFC system showed that it had some deficiencies, but that it was very, very rugged; a review of available systems by these same American researchers suggested that the United States should select and install the improved Dreyer system on American military aircraft.
Owing to the slow production of the Dreyer systems in France, in 1917 arrangements were made to manufacture and produce the improved Dreyer oxygen system. Upon consideration of this intent, it was clear that in order to rapidly mass produce the system in the USA, it would have to be completely redesigned. This proved no easy task in 1918, given the status quo of industry at that early time, however after a massive amount of study and preparation, the system was entirely reconfigured. The new American version of the Dreyer apparatus featured a leather and rubber mask in which a microphone was to be fitted, but it proved very unpopular due to various considerations that included the awkward size of the microphone, bulkiness, and placement of the components. Work done by the A.C. Clark Company on the improved Dreyer system resulted in a new reference to it as the ‘Clark-Dreyer System’ and many months after the studies began, complicated by extraordinarily perplexing developments, the system finally began to be produced and shipped to France for installation in American war planes.
When the Armistice was finally signed, fewer than 3000 sets of the ‘Clark-Dreyer System’ regulators and masks had been delivered to American AEF squadrons in France. By the time the war had ended for all combatants, oxygen had been permanently accepted as a necessary part of the life support equipment required by pilots to successfully fly and fight at altitude. Ultimately, however, historical research has seemingly demonstrated that few American combat aircraft flown in the war actually had been equipped with oxygen systems by war’s end.
As Glenn Sweeting has stated in his superb book, COMBAT FLYING EQUIPMENT, the problem of providing adequate oxygen breathing systems for military aviators in the First World War simply was too great for the amount of time available to devise a suitable system: "…it simply challenged the state of the art and came up short."
As a final note on First World War military oxygen breathing systems, it should be remarked that the systems devised in that extraordinarily compressed early "learning curve period" were not completely adequate, being given to failure and prone to faulty operation, which not infrequently resulted in a loss of both machines and men when the systems failed at higher than normal altitude. Had the war continued on into 1919 and beyond, there is little doubt that improvements would have resulted in far better systems reliability that existed at war’s end.
Post World War One Developments:
Although the less-than-satisfactory state of the art status in military aviator oxygen systems was well recognized after the Armistice, wartime demobilization was soon in full swing and all areas of military research and development suffered accordingly. Fortunately, a small but vital cadre of aviation medical researchers in the USA and the UK managed to continue the work begun during the war, albeit on a greatly reduced scale and with a radically lowered priority for funding. Much of this important work continued at Hazlehurst Field in New York (shortly to be renamed Roosevelt Field), a medical research laboratory, which had been part of the wartime US Army Signal Corps’ Air Service medical operations. In keeping with the reduction of the Signal Corps Air Service from about 200,000 personnel at peak strength in wartime, demobilization after the Armistice had resulted in a reduction in strength to about 20,000 men; of this almost half would be further released, per a plan established by Congress. Despite this discouraging situation, Dr. Louis Bauer, by now Chief of the Hazlehurst Field medical laboratory, was able to maintain the laboratory’s presence as the core of a fledgling Army Air Service flight surgeon program.
A relocation of the lab to Mitchel Field shortly before the beginning of 1920 (a location less than a mile from the original one), nearly coincided with the 1920 Army Reorganisation Act, a Congressional move that offered a considerable further autonomy to the US Army Signal Corps’ Air Service. In 1926 a further reorganisation of the military services resulted in the establishment of the US Army Air Corps (to replace the US Army Air Service). At about this same time, the US Army School of Aviation Medicine, formerly headquartered at Mitchel Field, had relocated to Brooks Field in Texas.
Meanwhile, the lack of a suitable system of military aviator oxygen supply for the Army’s aircraft provoked continuing research in this area, as new aircraft designs, benefiting from accelerated wartime technological developments, came into use. The old fashioned manually reduced, continuous flow oxygen system that the ‘Clark-Dreyer System’ had been intended to replace came back into interim use for most immediately post-war flight operations. By the mid-1920, when the ‘Type Designation System’ standard nomenclature came into US military use, the old Clark-Dreyer System regulator was designated the ‘Type A-1 Regulator’. This post war period saw much research (although on a limited scale) into the need for resolving the discrepancies of the old Clark-Dreyer System and a number of systems from both the USA and other nations were tested. A design eventually known as the ‘Prouty-Van Sicklen Apparatus’ was engineered by T.C. Prouty and manufactured by the Van Sicklen & Company corporation of Illinois. The automatic regulation of oxygen flow in this system was managed through use of a high-pressure/low-pressure system regulated by reducing valves in both chambers, and featuring a low pressure chamber controlled by an aneroid component assembly. An important difference between this system and the earlier Clark-Dreyer System was found in its use of a constant area orifice, driven by varying oxygen pressure (as opposed to using constant pressure and varying the control valve orifice size).
Extensive testing at McCook Field (the Army Engineering Division Laboratory) and Mineola (the Medical Research laboratory) showed that this new automatic control device had great promise as the core of a lightweight, compact, high pressure automatic regulation system. In 1919 a small number of these new devices had been acquired and tested, although not adopted by the Air Service.
In 1921 the US National Bureau of Standards carried out test on a wide range of oxygen breathing systems, in cooperation with the NACA (National Advisory Committee for Aeronautics) and the US Army and Navy Departments. This comprehensive study investigated both gaseous and liquid oxygen delivery systems, and included the older and newer Garsaux systems, as well as German wartime systems, the Siebe-Gorman system, and the Prouty Oxygen Regulator. Problems remained with liquid oxygen systems, although the US National Bureau of Standards project developed an American liquid oxygen system in conjunction with the study.
In 1922 a modified version of the Prouty Oxygen Regulator was finally developed and adopted by the US Army, installed as ‘standard’ on US Army aircraft, and eventually designated the Type A-2 Regulator. The improved Prouty regulator was entirely automatic throughout a range from 10,000 feet through 32,000 feet and only required being tuned on or off by the pilot. It was, as were all of the early systems, a ‘Continuous Flow’ system (in which there is no intermittent or cyclical delivery, as in a Demand System). The Type A-2 (Prouty) Continuous Flow oxygen regulator was designated ‘Limited Standard’ in 1927 and continued in use through 1930. Although the advantages of the lightweight A-2 regulator were recognized and appreciated, the problem posed by the weight and limited duration of gaseous oxygen storage system required to supply aircrew for prolonged periods of time remained somewhat thorny. Further, whereas today ‘Aviator Grade’ oxygen is produced ‘bone dry’ (devoid of water vapor), in the 20s the water vapor in oxygen supplies was mechanically filtered from the circuits through incorporation of a ‘purifier (a glass-wool and chemical scrubber tube); this was not optimal and freezing remained somewhat of a problem in all early oxygen circuits for this reason.
In 1923, owing to investigations into the potential of liquid oxygen generating systems, a new ‘LOX’ system was devised and entered a period of protracted testing and evaluation by the Army. In addition to being lightweight, ‘LOX’ systems also were free of impurities and had no water vapor to worry about. Still to be overcome remained many smaller difficulties, such as the inoperability of an inverted LOX system and the fact that the inhalable oxygen produced thereby was super-cool and presented certain problems associated with monitoring of the remaining supplies of the gas.
The first flight with one of the new LOX systems installed occurred in 1923, when a DH-4B aircraft was flown at 13,000 feet for about two hours, as part of a test of a new improved LOX apparatus (designed by the US National Bureau of Standards) that would be designated as the Type B-4. Shortly thereafter a further test flight with this system was undertaken to 20,000 feet.
Several other automatic LOX system designs of the German design were also tested and designated as the B-1, B-2, and B-3 Types (none of these were adopted or procured for standard US Army use, however).
By 1926, with the establishment of the new US Army Air Corps, an even newer LOX apparatus was devised and designated the Type B-5; this system remained in experimental status, however, and was never standardised. At this time, owing to the complex problems presented by design and development of fully satisfactory LOX systems, the older gaseous oxygen systems remain in use. It is worth pointing out that at this time, very few routine operational flights took place above about 20,000 feet, thereby somewhat lessening the urgency of development of a completely satisfactory LOX system for standard use.
As of September 1927, US Army Air Corps TO 03-10 established the fact that there was only one standard oxygen regulator in US Army use: the Type A-2 (or Prouty) Continuous Flow Oxygen Regulator. This TO stipulated that supplemental oxygen was required for flights above 15,000 feet, but that the individual need for oxygen varied considerably, consequent to the level of exercise undertaken. A revision of that TO a year later (TO 03-10-1, dated June 1928) stated that the A-2 regulator was determined to be ‘not suitable’ for flights over 22,000 feet, owing to various operating parameters of the system. The TO went on to specify that a standard ‘manually operated’ regulator (such as was used in welding) was to be used in flights whose parameters lay outside the cited A-2 operating limits. A version of this type of commercial pressure-reducing regulator known as the ‘Rego’ was found to meet the requirement and that apparatus was approved as Limited Standard by August of 1927 (receiving the designation Type A-5); this manually operated A-5 ‘Rego’ regulator was not declared obsolete until as late as 1944! Coincidentally, TO 03-10-1 also contained illustrated, diagrammatic instructions for constructing a ‘pipe-stem’ type mouthpiece from wood, since a standardised oxygen facepiece (mask) was still not in standard use (nor would one be for a considerable period of time).
Prior to 1928, in view of the fact that no ‘standard’ oxygen facepiece (mask) design existed, instructions had been provided in US Army Air Service circulars of the period that if a mask were made (by individual flying units for use by their aviators), "…it should not be fabricated so that any metal parts came into contact with the face". It was further advised that a small tube, preferably made of hard rubber, could be inserted into a facemask made from leather, and retained by a strap. These pipe-stem mouthpieces were frequently made from wood and shaped with a rounded lip or ridge on the innermost end to help retain the tube in either mouth or mask orifice. Shortly after TO 03-10-1 was circulated in 1928, a rubber facepiece (mask) described as "a loose fitting oxygen mask" and designated as the Type A-1 mask was finally adopted. It featured a metal nipple in the mouth area to which a rubber oxygen tube of small diameter could be attached. The Type A-2 mask consisted of another design that combined the helmet and facemask as an integrated unit, but this design was consigned to experimental status and was never adopted. The Type A-3 mask was virtually identical to the earlier ‘winter’ (Type B-2) cold protection mask made of leather, to which a rubber oxygen tube fitted with a wooden ‘pipe-stem’ could be attached. This last Type A-3 mask was never taken out of ‘service test’ status (which it entered in 1929) and was finally declared ‘inactive’ in January of 1931.
In 1927 the Engineering Test Laboratory of the Army Air Corps moved from McCook Field to Wright Field. Shortly afterwards, an automatic oxygen regulator of a new design was fielded for advanced testing. This improved regulator, which would replace the A-2 ‘Prouty’ regulator, would be standardised in March of 1930 as the Type A-4 Continuous Flow Oxygen Regulator. Several advancements in the automatic regulation of oxygen flow were provided by the design, which used aneroid control to regulate one or two outlet tubes to provide two separate rates of flow. The regulator began self-actuation in the first models at 15,000 feet, but this was later lowered to 10,000 feet. Amount of flow was thereafter increased proportionate to altitude and would automatically decrease in reverse proportion. While this regulator marked certain advances in regulation of continuous flow oxygen, it suffered from similar faults as its predecessors at altitudes above 20,000 feet. By 1936, it had been declared obsolete. Amazingly, the US Army Air Corps once again reverted to use of the old fashioned manually adjusted regulator to control oxygen flow above 20,000 feet!
By 1928, further American advances in the design of LOX supply (vaporiser) systems had been made. A series of improved models were developed and tested, beginning with the B-6 LOX system (standardised in 1929), the B-7 and B-8 systems (similar to the B-6 but with varying capacities), the B-9 (this was an experimental system that was actually a redesigned B-4 type), the Type B-10 (standardised in 1930, which had a supply sufficient for a two hour flight to 30,000 feet), and eventually, the B-11 system (which was adopted in 1932 and declared obsolete in 1944), the Type B-12 LOX system (an improved system with a 5 liter capacity, that was also standardised in 1932 and became obsolescent in 1944), and finally, a B-13 system, which was similar to the earlier systems, except that it had a 10 liter capacity. Of interest is the fact that the last main LOX supply systems (B-10, B-11, and B-12) were declared ‘Limited Standard’ in 1936, when the US Army Air Corps returned to the original gaseous oxygen system as its primary and standard system (gaseous oxygen stored under pressure in steel flasks).
By 1930, it is reasonably clear that technical problems specific to the imperfect operation of all extant US constant flow automatic oxygen regulators above 20,000 feet had severely handicapped further advancements in life support equipment development suitable to match the increasing performance of aircraft. Consequent to this awareness, it was suggested to NACA that LOX systems remain the only suitable recourse for flights above 20,000 feet by key personnel in the Experimental Engineering Section (at Wright Field). Compounding the use of gaseous oxygen systems was the fact that it was still virtually impossible to generate commercial quantities of ‘bone-dry’ aviator oxygen and however small the quantity of water vapor that remained in aviator breathing oxygen of the time, it was too much to preclude recurrent problems with oxygen line freezing and similar difficulties.
In 1930 a US Navy aircraft reached an altitude of 43,166 feet, with Naval Aviator Lt./ Apollo Soucek wearing "…an oxygen mask that covered his nose and mouth, but still suffering some impairment at maximum altitude". The ‘impairment’ is understandable, given that at this altitude some sort of pressure-demand supported augmentative oxygenation is almost mandatory. Of special note is the fact that this flight was carried out in an open cockpit aircraft (practically the last altitude record attempt thereupon undertaken). Later developments in the development of pressurised crew cabins would contribute immeasurably in raising the human functional operational ceiling and eliminating any physiological limits further imposed by the inherent limitations of attempting high-altitude flight in an open-cockpit aircraft equipped with only a constant flow oxygen system.
The Immediate Pre-World War II Period:
Fortunately, in 1933, at the Army’s Aeromedical Lab at Wright Field (headed by co-founders Major Malcolm Grow and Captain Harry Armstrong--the latter to become generally known as the ‘Father of Aerospace Medicine’), awareness of the need for rapidly advanced, systematized, and scientific research in life support technology was taking hold. After assaying the available knowledge database in aviation physiology, Dr. Armstrong suddenly became acutely aware that there was simply not enough basic physiological knowledge of the effects of altitude upon which to carry out development programs for new pressurised crew cabins and advanced oxygen equipment. A recommendation was made to establish a modern outgrowth of the old Hazlehurst Medical Research Facility, equipped with new high-pressure hypobaric chambers, at the Wright Field installation. This was successful and the Wright Aeromedical Laboratory work coordinated with Technical Engineering Branch studies to achieve the stated objectives.
Among the chief concerns facing Armstrong and his confreres was the problem of devising a suitable oxygen facemask for delivery of oxygen at higher altitudes. Beyond that, new pressure regulators would be required to complement the design of pressure cabins for aircrew in newer aircraft.
In 1931, a newer oxygen facepiece (mask) had been devised designated the Type A-4 mask. It was based heavily upon experience with the original A-1 leather mask (with its pipe-stem tube), and work with the experimental Types A-2 and A-3 masks. This A-4 mask was a combined ‘winter’ and oxygen delivery mask that comprised a chamois leather lined outer leather shell, fitted to the face through use of an elasticized strap and buckle attachment. The exterior of the mask was wind-resistant, while openings in the facepiece for the eyes allowed standard glass goggles to be used on it. Rubber inlet tubes allowed a small-bore oxygen delivery hose to be attached, while exhalation channels in the leather allowed exhaled air to be routed from the nose area to outlets above the cheek areas.
Padded protrusions (filled with Kapok) were situated at the lower margins of the eye holes, so as to shield the goggles from fogging effects of moisture laden exhaled air. The A-4 mask was standardised in 1931 and was carried in limited standard use until 1933. It was not obsolesced until stocks were depleted in 1942. The A-5 mask, a similar full-face combined wind-protection and oxygen delivery mask made from chamois-lined horsehide, was standardised in 1935 and dropped as obsolete in 1943. The last of these leather, combined wind and oxygen mask designs was the Type A-6 mask. The A-6 comprised a multiple-layer mask, with chamois lining, wind-proof exterior surface, and an intermediate layer. The A-6 was not a full-face mask, however, and covered only the lower portion of the face; it therefore came closer to the eventual distinctive shape that subsequent rubber facepiece oxygen masks would feature. It allowed a metal oxygen distributor attached to a small-bore rubber hose to be fitted to the mouth section’s exterior, for use with a LOX type delivery system. The mask could be worn without the oxygen distribution tube as a wind protection mask alone, such as might be required in an open-cockpit machine flying at lower altitudes. This last of the old style leather oxygen delivery masks was standardised in 1933, made Limited Standard in 1939, and obsolesced with the Type A-5 mask in 1943. Evidence suggests that it was continued as a ‘non-standard and expendable’ facemask in USAAC inventories somewhat after that date, however.
Meanwhile, by 1935, the USAAC had acquired considerable experience with both gaseous and LOX type systems in aircraft and felt that it had enough research and practical experience to decide which system was best suited to support present and future aircraft development. A comprehensive report was prepared comparing the advantages and disadvantages of both gaseous and LOX type systems by a committee of the Army Air Corps’ Equipment Branch (the chairman of this committee was Dr. Harry Armstrong). Without going into specific details, the report concluded that although there were certain advantages to be derived from LOX systems, the ‘state-of-the-art’ was still such that these systems posed practical obstacles in terms of storage, field servicing, maintenance, and supply that would make them unsuited to the sort of minimal support conditions applicable to any conceivable wartime scenario in the immediate future. Thus, after due consideration of these findings by USAAF HQ, Air Material Division, and subsequent review by Wright’s Aeromedical Lab and Technical Engineering Branches, the decision was made to gradually phase out all LOX type systems and replace them with standardised gaseous oxygen supply systems from 1936 onwards. Once again, the ‘state-of-the-art’ had been challenged on a technical front and found lacking at that juncture in time. However, during the following WWII period, renewed interest in the advantages of LOX systems, combined with technical improvements in equipment and methodologies, resulted in a renewed LOX project that finally saw fruition with installation of newer, more satisfactory, and easier to service LOX supply systems in the new jet-powered aircraft of the 50s.
As a consequence, from 1936 until somewhat after the end of WWII, both US Army and Navy aviation relied almost exclusively upon gaseous oxygen delivery systems. With the decision to standardise on gaseous oxygen equipment, renewed importance and priority was placed upon perfecting and improving gaseous oxygen delivery & storage systems; this impetus was spearheaded by the Equipment Branch and Aeromedical Laboratories at Wright Field and the result was development of improved equipment suited to the emerging new requirements.
The Type A-6 Continuous Flow Oxygen Regulator was a manually-adjusted device that emerged from this program, was standardised in 1936 and carried as Limited Standard in 1940, until declared obsolete in 1944. It featured pre-set delivery rates for specific altitudes in increments of 10,000 feet, and was of simple, rugged, and easy to operate design. Initially designed to supply large quantities of oxygen required by the relatively inefficient ‘pipe-stem’ delivery system, the settings were later changed to suit the lowered demands of the soon-to-follow newer generation oxygen facemasks. The standard US Navy regulator of this period was designated the Type A-7 and used in service-test status for two further years (from 1936 through 1938). Along with these delivery improvements, newer, stronger and lighter compressed oxygen storage cylinders were introduced that allowed a volume equal to a pressure of about 1800 PSIG to be contained within. High pressure extensions carried the gaseous oxygen from central storage cylinders to the various crew station regulators, whereupon a small-bore rubber tube supplied then oxygen under reduced pressure to the aircrew facemask.
With these improvements having been settled upon, and with work on establishing them as standards in new aircraft being manufactured, the more pressing requirement then became development of suitably redesigned and enhanced automatic oxygen regulators for use at higher altitudes, as well as for more efficient modern oxygen delivery masks. Research was immediately undertaken by Dr. Harry Armstrong, Dr. John Helm of the Physiological Research Laboratory, and colleagues in the Wright Equipment Branch to produce a new generation oxygen facemask. Interestingly, at about this time (mid-30s), the success of new fully pressurised aircrew cabins led some investigators to believe that personal oxygen delivery equipment would soon be obviated by the introduction of such refinements (as the pressure-cabin) in the crew compartment of military aircraft. Despite this sentiment maintained by some, research continued at some speed to develop a new aircrew mask. In conjunction with the Army Air Corps agencies charged with this research, commercial and civilian participation in this research was encouraged. Thus medical research organisations such as the Mayo Clinic Foundation in Minnesota participated actively in the investigation of new oxygen breathing equipment. This decision to invite non-military participation in mask design and development soon would soon prove to be extremely fortunate and foresighted action.
The First 'Modern' American Aviator Oxygen Mask: The B.L.B.
Faced with the daunting task of heading up a project charged with developing a suitable military aviator’s oxygen breathing mask, Dr. Harry Armstrong and his colleagues at Wright Field soon sought additional ideas and input from the civilian medical sector. The task of designing and producing a functional, yet comfortable and efficient oxygen mask was not by any means a simple one, and similar challenges had been faced with varying degrees of success by others interested in protecting the human respiratory system from chemical gases from about 1900 onwards. Some of the problems associated with use of a facemask include discomfort from having a mask in contact with the skin, inefficient function due to imperfect face sealing, difficulty communicating, and (prior to about 1900, at least) the relative unsuitability of available materials from which to fabricate a satisfactory airtight oronasal mask. Clearly, old fashioned pipe-stem delivery devices were not well suited for use at altitude—especially in open cockpit aircraft—where the effects of cold simply compounded the difficulty of holding a delivery tube mouthpiece in place between the teeth.
This last fact had been discovered by US Army researchers in the late first decade of the 20th Century, while attempting to devise a functional gas protection mask to defense against German war gases. They found that even on the ground, the muscles of the mouth have difficulty holding a pipe-stem device or similar mouthpiece tightly between the teeth for even limited amounts of time. The severe effects of cold brought about by flight in the upper atmosphere added considerably to this difficulty. Further, the ‘state-of-the-art’ of molded rubber was (prior to about 1915) simply not yet sufficient progressed to allow the complex design and molding of a facemask that could be mass produced in a range of sizes suited to fit almost every variation in the broad range of human facial contours. Haldane, along with Messers Draeger and Siebe-Gorman, had all explored this area of work to some extent, facilitated by years of research and experimentation in the development of mining rescue breathing apparatus. Yet none of the existing designs for a reliable, functional, and yet still comfortable face mask were suited for use in upper atmosphere flight.
In the medical field, parallel concerns had somewhat fortuitously developed among medical practitioners, who were interested in devising a method of administering oxygen to critically ill patients by means other than the somewhat dangerous and old style ‘oxygen tent’.
At the Mayo Clinic & Foundation in Minnesota, Drs. Walter Boothby, Randolph Lovelace II, and Arthur Bulbulian had been exploring the issue, concerned with the administration of anaesthesia for surgical patients and the administration of oxygen to their respiratory patients. Holding reserve commissions in the US Army, they had also established a special aviation medicine department at the Mayo facility. In 1938, they designed and developed a new type of molded latex rubber mask, which became known as the ‘BLB Mask’ (named after its inventors). This mask, which covered only the nose, had twin oxygen inlet tubes that skirted the frontal mouth area to once again join together on the chin, just below the mouth. At that juncture was situated a metal valve that attached to a thin rubber bag and the main small-bore rubber delivery tube. Available in two basic sizes and secured to the wearer’s head via straps, the benefits of this mask were immediately obvious for medical applications; when the new mask was subsequently demonstrated to military and civilian aviation medical experts, the applications of this new mask for high altitude flight became apparent to Dr. Armstrong and his team, as well.
A series of tests of the new mask in simulated high altitude situations proved successful, although some problems were encountered with the small valve freezing in cold conditions due to water accumulation. The latex bag attached to the valve had a small plug in the distal end so that any accumulation of water could be drained away, but the bag itself permitted far more efficient use of oxygen flow. Since exhaled human breath contains about 16% oxygen (as opposed to about 21% in ambient air), the exhaled breath could be breathed back into the ‘rebreathing bag’ where it could effectively be recycled instead of being breathed out into ambient air (and thereby wasted). This permitted lower flow rates of inlet oxygen, among other things—a decided benefit that allowed available oxygen supplies to be extended by a significant margin. Otherwise, the BLB Mask allowed the wearer to talk, drink, and even eat without interrupting the supply of oxygen.
The only obvious drawback of the new mask for flight applications was found in the metal exhalation valve under the chin, which had a tendency to freeze in severe cold conditions (i.e. at altitude). Since most aircraft in the late 30s were starting to be equipped with enclosed cockpit areas, the relatively fragile nature of the thin latex rebreather bag with its susceptibility to turbulent wind conditions could be overlooked. Thus, after evaluation by the Wright team headed by Dr. Armstrong, the BLB Mask was officially adopted by the Army Air Corps and designated Standard Type A-7 in July of 1939. The Type A-7 mask could be used with the manually regulated A-6 or A-8 regulators, a combination which proved highly economical in comparison to earlier ‘pip-stem’ type delivery systems.
In order to address the problem of the tendency of the A-7’s mask's metal valve to freeze, an improved version designated the A-7A was developed in short order. This design did away with the old metal valve and incorporated twin sponge exhalation valves on either side of the hose structure, in place of the metal valve body; the improved A-7A mask was standardized in June of 1943. A further subsequent modification of the A-7A led to the A-7B mask, which appeared quite similar to the A-7A (this mask was standardised in June of 1945).
Meanwhile, problems using the A-7 mask in certain combat situations emerged. Among these were the fact that combat conditions frequently made aircrew forget to breath through their noses; furthermore, nasal passages were prone to congestion and blockage, whereas the larger oral air passage was rarely occluded. These insights led to the development of the A-8 oronasal mask, which covered both nose and mouth. The A-8 mask was secured by straps to a flier’s head or helmet in a manner identical to the A-7. It featured a single large frontal exhalation valve, covered by a sponge disc which could be cleared of ice with a simple squeeze. Unforeseen was the fact that this feature served to partly obscure the forward, downward field of vision, however (important for an aircrew function such as that of the bombardier), so a further refinement was produced that featured bilateral sponge covered exhalation ports on each side of the facepiece. This left room in front (where the exhalation valve was formerly located) for a small carbon-element microphone to be installed. The improved A-8 modification of the BLB Mask concept was designated the A-8A mask and standardised in February of 1941. A subsequent further modification of the basic oronasal mask was designated the A-8B mask in November of 1941, and it is worth noting that so successful was the A-8B mask design that it continued to be produced and has been used by military forces until well into the 1980s! Accordingly, it is not unusual to today find specimens with mid-1980s production dates stamped upon them!
The A-8B mask was secured to the flying helmet, rather than to the head itself, using straps that hooked to the helmet. The A-9 fabric flying helmet was the first flight helmet to be produced specifically with these hooks to attach the A-8B type mask. Early examples of the A-8B mask mask also featured leather straps, while the later specimens used elasticated fabric; the much later versions used nylon straps. Surprisingly, the A-8B mask could also be converted into a demand type oxygen mask through use of components in a special adaptation kit that included one-way valves and a quick-attach corrugated, large-bore rubber tubing used in in place of the rebreather bag.
The Type A-7 and A-8 family of masks quickly replaced the old pipe-stem delivery system and a new manual regulator designated the Type A-8 was developed for use with these masks. The A-8 regulator was standardised in 1940 and featured a 3000 PSIG capacity, rendering it capable of being used only with the new masks (and not the earlier 'pipe-stem' devices). A modification of the A-8 was designated the A-8A regulator in 1941 and it remained in service as the new standard regulator, until both (A-7 & A-8) regulators were changed to limited standard when the new ‘low-pressure’ oxygen delivery systems were developed and introduced in about 1941.
Low Pressure Oxygen Systems Replace the Old High-pressure Systems, as War in Europe Looms Anew:
Advances in aeronautical physiology continued apace with improvements in the oxygen equipment technology itself, as might well be understood given the awareness of the need for a more up-to-date knowledge database in USAAC circles. By 1939, many were convinced that a new international conflict was taking shape as Germany continued to build up its military might and Japan continued to challenge the United States for economic hegemony in the Pacific Rim region.
Among the cooperative results of the Wright Aeromedical Lab group’s work at Wright Field and the similar studies conducted by the USAAC’s School of Aviation Medicine was the publication of USAAC TO 01-1H-1B (dated April 1938). This Technical Order stated that all personnel would use supplemental oxygen at all times in flights above 15,000 feet and that oxygen would be used at all times during any flying done at night (in order to help preserve good vision and motor skills coordination). Further breakthroughs in aeromedical physiological understanding were accomplished, consequent to determination of recommended ‘standard’ altitude/oxygen levels that postulated (among other things) that flight above 40,000 feet (even with supplemental oxygen) should not be allowed. This was well prior to the development of pressure-demand oxygen breathing systems that would soon appear in about 1944, and sophisticated pressurised crew cabins that would work with the pressure-demand systems to allow exceptionally high operational ceilings to become commonplace.
Concurrent with these enhancements in flight physiology knowledge, experimentation was also initiated in other areas of work, such as on the development of pressure suits and the introduction of low-pressure Demand Type oxygen breathing systems. German aeromedical research in particular had been most vigorous and by the time war broke out in Europe (September 1939), the German Luftwaffe had already introduced a fully functioning, standard low-pressure Demand Type oxygen breathing system into all of its latest aircraft designs. These systems, built by the Draeger Company in Lubeck and also by the Auer Company, pioneered the new concept of pressure-reduced oxygen delivered only on demand by the aviator wearing a mask. Italy, England, and Germany furthermore had all experimented with early full pressure suit prototype assemblies in attempts to permit flight to higher altitudes. Supercharger technology improvements (and later turbocharger systems) also permitted development of moderately sophisticated experimental pressurised cabins for aircraft, particularly in Germany, where aeromedical research had enjoyed intensive support from Hitler’s Luftwaffe.
Thus, as Europe again headed into an all encompassing major conflict, the United States was still attempting to catch up in what was soon to be the vitally important area of aeromedical physiology and life support technology (despite the intensive and remarkable research work undertaken by Armstrong and others in America from about 1935 onwards). Most American combat aircraft were at that time still equipped with the constant flow oxygen breathing system that used compressed oxygen stored in high pressure flasks on board the aircraft and delivered to individual crew stations, where it would be dispersed by the old manually manipulated A-6 type regulators and small-bore latex rubber hoses to the ‘new’ A-7 and A-8 type rebreather masks (that had only recently started to replace old style ‘pipe-stem’ oxygen delivery devices). A great number of American aircraft (most particularly trainers) at this time were also of the open-cockpit type, wherein effects of wind and severe cold were still considered formidable threats to aircrew efficiency.
Time and circumstance, however, proved favorable to the United States, in that while all of Europe was immediately plunged into war by the events of September 1939, America remained safely out of the conflict. This provided not just an early alert to the inadequacies of the old style oxygen systems, but a ‘buffer’ of about two years in which to accelerate the response needed to update and replace these old systems sufficiently to meet the challenge of ‘modern’ war.
(Next: Part 2.)