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A History of Military Aircraft Egress Systems (Part One of Three)
By Kalikiano Kalei
Last edited: Wednesday, February 27, 2008
Posted: Thursday, February 07, 2008



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This is part 1 of the fascinating story of how aircrew egress systems, know more commonly as 'ejection seats', came to be developed as an important safety device for saving the aircrew of disabled military aircraft. Known variously as 'bang-seats' in England and 'schleudersitz' in Germany, ejection seat systems work side by side with pressure suits and other pilot personal equipment to enable the pilot of today's modern military fighter aircraft to survive the inherent dangers that flight in fast jets pose. (The accompanying image shows a successful ejection at extreme low altitude by a US Air Force Thunderbirds pilot from his disabled F-16 at an airshow in 1996)




Achtung! Schleudersitzaparat!

A Brief History of the Development
Of Western Aircraft Egress Systems:
(Part One of Three)



INTRODUCTION:

The destructive power of a modern military aircraft is a thought-prevoking matter to reflect upon. When one considers the weaponry, performance and powerplant parameters of just about any contemporary military fighter aircraft in use today, it quickly becomes apparent that by comparison, the performance envelope of the body of the human being flying the machine is an order of magnitude lower than that of his aircraft. One only has to take note of the grim aircraft accident investigation findings regarding disposition of aircrew and/or passengers involved in a catastrophic air crash to be all too aware of just how fragile the human body is when subjected to G-force stresses beyond even a modest level.

The science of aviation technology has progressed far, far beyond the state of development which in December of 1903 saw what is generally recognized as the first (fully documented) successful powered, manned flight--the Wright Flyer’s brief but historic launch from the sands of Kitty, Hawk, North Carolina. However, even in that earliest phase of man’s pursuit of powered flight thoughts relating to human safety in this new and potentially dangerous environment were not unknown. Obscure references exist from that period (1910) regarding attempts to provide aircrew with escape from crippled flying machines (although of necessity, quite primitive).

The introduction of the new technology of air warfare into the hoary traditions of warfare etiquette resulted in no small amount of reactionary resistance to the phenomenon of aerial battle, let alone the startling concept of providing a safe operating environment for pilots of the early wood and fabric craft. Aviation was at that time an inherently hazardous undertaking, just as it still is today, but to a far greater extent due to the newness of the technology and operating environment. To a certain degree, resistance to early aviation safety concerns centered upon the ancient traditions of military honor and protocol. One did not wish to suffer the dishonor of retreat from battle, according to honorable custom, and therefore many felt that attempts to provide a safe exit from a crippled aircraft were somehow demeaning to the assumed bravery of aircrewmen.

Parachutes, the first practical aircrew safety device, were at first subjected to disdain by the higher echelons of English aviation command in World War One, who felt that their introduction would encourage a lack of firm resolve to 'stay the course' in aerial combat. This was apparently not the case in the Imperial German High Command, for the Germans soon recognized the value of the new device and started equipping their balloon crews with this escape device; according to reports, some German pilots also adopted the parachute shortly before the war ended.

Although the development of parachute technology developed marginally in the years immediately following the First World War, the same cannot be said for the concept of ejecting an aircrewman from a disabled aircraft. Perhaps this was due to the relatively low performance of the aircraft in use by the end of the war. To some degree, it must have been due to the general feeling that the successful resolution of the First World War had put the need for continued defense development on sustained hold. In any event, most defense technology activities experienced a significant slump immediately after the war, and it was not until Germany’s military resurgence of the 30s renewed international anxieties that defense concerns (including aircrew safety) again became reestablished.

In most cases, if a pilot found himself in trouble in the 20s, it was relatively easy to simply disengage the seat harness and jump over the side of the machine so that the parachute could be deployed for a safe descent. However, as the power and performance of aircraft continued to increase, it quickly became apparent that the risks involved in simply jumping over the side of an aircraft were significant. Especially so if the aircraft were in any manner aerodynamically disabled or damaged in a way that created unusual airframe gyrations or G forces for the aircrew who were trying to exit. Furthermore, as the designs of aircraft became more sophisticated, especially as found in aircraft designs with placement of the propeller driven engine in the rear of the aircraft (behind the crew compartment), the need for a device to successfully extract the aircrew safely beyond the tail assembly (and/or rear-positioned powerplant) became more fully focused in the minds of a few astute and enterprising individuals.

GERMAN DEVELOPMENTS THROUGH WORLD WAR TWO

As is the case with many innovations in technology, it was the Germans who were among the first to explore this area of aviation safety in a serious and innovative manner. Owing to the development of Germany’s covert military capability in the 1930s, much work was being done in aeronautical engineering research for military aviation applications. As Germany explored and researched a new generation of higher performance military aircraft as part of Hitler’s overall build-up of war materiel, the individuals concerned with aviation medicine in the new Luftwaffe recognized a coexistent, renewed need for aircrew safety. This was especially true , since much aeronautical flight testing of new designs was being undertaken, with all the implicit hazards pertaining to that area of technological investigation.

Many ideas occurred to German aeronautical engineers in the process of attempting to address this need for safe egress from crippled aircraft. One of the first was the "boom" concept, in which a pivoted fulcrum, attached to the pilot’s harness and powered by a compressed spring, pulls the pilot free of the cockpit in an emergency. Another of the early ideas was very simple--a compressed spring situated under the pilot’s seat which when released would eject the occupant (assuming that the canopy had been freed and cleared first, not always an easy thing to do using a manual technique and especially in a seriously disabled aircraft).

Interestingly, both these early ideas were also conceived and evaluated by counterparts in England, at the end of the Second World War and without knowledge of the original pre-war German research. In fact, back in 1930 an enterprising RAF Flight Officer, concerned with the increased performance of a new aircraft his squadron was transitioning into, formulated a design for an "escape seat." Consisting of a pilot’s seat mounted on two telescoping tubes, each containing a highly compressed spring, the seat would be pushed with its occupant up into the slipstream of the aircraft when the seat’s catch was released. While the seat was not actually ejected from the aircraft, the idea was that in this position it would be easier for the pilot to roll out of his seat and free himself of the stricken plane. The idea was submitted to the English Air Ministry, along with blueprints and a working model, but no further development was ever undertaken, and thus English innovations in aircrew escape would languish until immediately after the Second World War, when the captured German research and development would stimulate new interest in Allied aircrew escape technology.

In Germany, developments in aeronautical technology were accelerating with the introduction of the jet engine, while by 1939, the Luftwaffe’s Aviation Medicine branch was actively experimenting with ejection systems, using physiological testing devices that included instruments for measuring the forces of gravity and acceleration on the human body. Their tests has determined rough physiological parameters of human ability to withstand G force onset of about +20G for a duration of about 0.1 second. The German preference was at this time for a compressed gas system of ejecting the aircrew seat, although explosive cartridge propelled seats were also under development. The need for adequate aircrew escape from dive-bombing aircraft such as the Ju-87 Stuka, with its sustained high positive G loading during pull-out, significantly motivated investigations into use of high-pressure systems to eject aircrew. The German manufacturer Heinkel maintained chief engineering responsibility for development of all aircraft escape systems, throughout the war, and by late 1942 all German experimental aircraft being flight tested were equipped with some form of Heinkel ejection seat.

With aircraft development feverishly continuing in wartime Germany, Heinkel developed ejection seats finally started being installed in production aircraft, as radical new designs came into use. Although the singular Messerschmitt 262, twin-engined production jet fighter-bomber (Schwalb) did not feature such a schleudersitzaparat (the German term for 'ejection seat', which translates roughly to "seat catapult device") in its earliest permutations, reports suggest that at least a few late versions (Sturmvogel) had what has been described as a catapult-seat (although it is not clear whether the seat was driven by an explosive charge or by a spring mechanism). Other aircraft, such as the Heinkel He-162 Volksjäger, were provided with a compressed air propelled ejection seat. Further aircraft types to feature similar systems included the Dornier Do-335 Pfeil, the Arado Ar-234B Nachtigal, the Heinkel He-177, the Heinkel He-219 Uhu, the Dornier DFS-228, and the rocket-powered Messerschmitt Me-163 Komet (this last system was spring powered). Additionally, earlier research begun in the late 30s by Heinkel had resulted in the first recorded example of a completely ejectable crew compartment being developed. The rocket-powered Heinkel He-176 (the world’s first rocket propelled aircraft) featured a nose section which could be jettisoned in the event of an emergency. Development problems involving successful deployment of the main parachute designed to slow descent of the ejected crew compartment resulted in several innovative engineering designs, and subsequent testing demonstrated that in the event of the crew being disabled, the He-176’s crew compartment would enable its occupant to survive a landing within the escape pod with only minor injuries.

The Heinkel explosively ejected seat consisted of a seat bucket assembly mounted on 4 rollers which moved in two parallel channels 42 inches long. The charge used in the two tube catapult consisted of 30 grams of powder, firing much like a conventional projectile cartridge, with the two part catapult tube fixed to the upper end of the seat and at the lower end to the aircraft frame. Ejection velocity achieved was 35 fps, with a stroke of 28 inches and an acceleration of about 12 Gs. Although an experimental compressed gas system achieved a ejection velocity of 57 fps and an acceleration of 27 Gs, the system was massively heavy and presented considerable field maintenance problems with over 1700 psig pressures required for successful operation.

On 13 January 1942, the first known emergency use of an ejection seat occurred during a test flight of the Heinkel He-280 jet fighter. The pilot, a man named Schenck, ejected from his iced-up aircraft after jettisoning his canopy, successfully achieving safe egress from the machine. Although Schenck’s was the first known use of an ejection seat for emergency egress, the Heinkel compressed air driven seat used by Schenck was purportedly tested in an experimental ejection from a test aircraft by a Heinkel employee named Busch, prior to Schenck’s 1942 ejection. (These facts was unknown outside of Germany until after the end of the Second World War, and for a while it was thought that the first successful emergency ejection was made by a Swedish pilot from his crippled SAAB J21-A1 aircraft--with "pusher" type propeller powerplant--on 29 July 1946).

By the end of the Second World War, more than 60 emergency ejections had been made by Luftwaffe personnel, and German Aviation Medicine branch research in aircrew egress technology had progressed substantially into areas of high-performance aircraft escape systems at proportionately high physiological limits of human endurance. Among the important facts that had became clear to German researchers, however, was that compressed gas driven systems were prohibitively heavy, requiring installation of heavy system components to contain the gas under high enough pressure to perform as required; they were furthermore quite difficult to maintain and keep operational under actual battlefield conditions. Ballistic (explosive) systems were lighter, but the requisite technology, while promising, was still in its infancy. These facts were not lost on American or British scientists going over the German research at war's end.

SWEDEN: CO-PIONEER IN EVOLVING EGRESS TECHNOLOGY

While America’s interest in emergency aircrew egress languished for the most part until after the end of the war, Germany was not alone in recognizing the need for increased air crew safety during the 1940s. In Sweden, SAAB Aircraft was developing a pusher-type aircraft to be designated the SAAB J21, and with the attendant work came understandable concerns about how to effectively provide J21 aircrew with an effective emergency escape system. 1939 saw initiation of studies in ejection systems, and by 1941 primary design studies had been finished, with actual flight testing of the explosively deployed seat being carried out in 1942. Bryan Philpott, in his interesting book Eject! Eject!, remarks that Sweden’s egress program developed in the early 1940s, "...when war, so often the mother of invention, did not provide the spur to the Swedes that it did to the combatant nations..." In fact, Sweden was very much spurred on by the immense threat of being drawn into war against its will, and had long maintained a tradition, its political neutrality notwithstanding, of providing itself with innovative and advanced defense technology--a tradition it shares with Switzerland, and for similar reasons. Regardless of their motivations, with its excellent reputation for scientific and industrial excellence, Sweden enthusiastically went ahead with its initial research into aircrew egress systems. By the end of the war, Sweden’s technical and scientific database on emergency escape systems for aircraft was well advanced, although ostensibly entirely independent of any research undertaken by Germany in the same war-years period (this has not been verified, as Sweden, although technically neutral, had strong sympathies with Germany and maintained continuing diplomatic relations throughout the war--thus the possibility of exchange of technical or research information may not be entirely ruled out).

The Type 1/Type 21 seat ultimately evolved into their later Type 2 (Type 29) ejection seat. The Type 2 seat was intended for use in the SAAB J29 (known informally as the ‘Tunna’, but also as The Flying Barrel), a post-war, swept-wing, barrel shaped aircraft of single engine design. Although substantially similar to the earlier Type 1 (or Type 21) seat, anticipating the highly touted modern McDonnell-Douglas ACES II seat concept as used three decades later in the F16 Viper, SAAB canted the seat back at a 30 degree angle to both reduce frontal area and improve pilot G-tolerance. At the onset of Swedish investigations into ejection seat design, a decision was made, as remarked earlier, to utilise an explosive cartridge type ejection gun. In the seat used on the SAAB J29, this took the form of 60 gm of explosive fired in a three-stage charge. A further preference of the SAAB designers was for a pilot worn back-type parachute configuration (not attached to the seat as on the Martin-Baker system), employing a seat-contained survival kit. Although seat ejection was originally to have been initiated through use of two strap-devices located at shoulder level, the design was changed to employ a face-curtain actuator similar to that used by the Martin-Baker system (English). A back-up, or secondary, seat actuating device was situated between the pilot’s knees, where it was conveniently at hand in the event unfavorable G forces made grasping the face blind actuator difficult (it is interesting to note that this innovation is now used almost uniformly by all current ejection seat systems). Further enhancements to the Type 29 seat were spring footrests intended to protect the pilot’s lower extremities and reduce flail injuries. On the whole, the philosophical approach of the SAAB seat was to eject the seat and occupant, thereafter immediately separating them. Successful man-seat separation was further provided for by the introduction of an apron device which was actuated by a separate power gun, actuated after the central harness unlocked after ejection. When man and seat had cleared each other, the idea was then for the pilot to either manually deploy his back-type parachute or rely on the automatic barometric chute deployment device.

EARLY AMERICAN EFFORTS: THE US AIR FORCE and US NAVY DIVERGE

By the end of the war, captured German research came into the possession of the Allies. Although emergency egress concerns were not significant in the United States until later in the war, the US had made a few insubstantial efforts in this direction in the late 30s and 40s. Among them in 1937 was the establishment by a Dr. J.W. Heim of an impact facility in a hanger at Wright Field (later to become Wright-Patterson Air Force Base / Air Development Center) to investigate the forces of abrupt accelerations on human beings. A simple swing-device suspended on cables, the system could achieve parameters of about 16 Gs with a pulse of 0.15 seconds. There are further unsubstantiated reports of a late 1930s Army Air Corps experiment using a spring-driven, pivoted fulcrum arm to snatch the pilot out of a rear-engined aircraft under development to fling him to safety beyond the propeller arc. Beyond these examples, there is little to indicate a strong interest in emergency egress technology by American aviation medicine proponents until the later stages of the war, when German research and developments in this area came to light. With the acquisition of both German databases in egress research and actual examples of the German Heinkel explosive cartridge ejection seat by the US, immediately the war had ended, the US began to vigorously attempt to gain greater knowledge in this overlooked area of aviation technology.

The new American developmental research spurred on by acquisition of German wartime data branched off into two distinctly different approaches towards the same end, one taken by the US Air Force and one by the US Navy. With the political rivalry between these two services to attain ascendant aviation technology acting as a further goad, two schools of thought on military egress systems formed. The US Air Force researchers greatly favored many of the German findings, among which was the belief that the best manner of obtaining a favorable pilot posture that would minimise spinal injuries was having firing actuation controls built into armrests on the seat. The US Navy, on the other hand, tended to credit the British approach toward establishing correct spinal alignment to best absorb the punishing catapult forces; this school of thought advocated the use of a face-blind pull-handle firing actuator, the theory being that the act of reaching up to pull a blind down over the face automatically positioned the pilot for the most favorable spinal alignment. Neither of the two techniques were ultimately found to be completely perfect, but this resulted in initial US Air Force reliance upon arm-rest triggers and early US Navy adoption of the English Martin-Baker face-blind actuator system. This variance in developmental approach would remain a primary distinction between early US Air Force and US Navy ejection seat designs well into the 70s.

In May and June of 1945, three members of the Army Air Force’s Wright Aero Medical Research Laboratory (Colonel Randy Lovelace, Dr. Edward Baldes, and Lt. Verner Wulff) flew to Europe and gathered technical information and research data on egress systems in England, Sweden, and capitulated Nazi Germany. In August of that year a technical report was published (AAF Memorandum Report TSEAL-3-696-74C) summarising their findings. The Wright Aero Lab shortly initiated an egress systems research program on the basis of the two month data gathering tour immediately afterwards, beginning in July, when a vertical seat testing tower was constructed. Specimens of the German Heinkel He-162 had been obtained during the summer investigations and an American version of that seat was the first seat tested on the new ejection tower. To power the ballistic charge catapult of that Americanised Heinkel seat, a pyrotechnic unit was manufactured by the Frankford Arsenal (designated T-2). By November, the first two human ejection tower tests had been carried out with accelerations noted as being about 11 Gs.

As regards the catapult itself, suffice it to say that, as in nearly every instance in which captured German research and technological innovations had been acquired, the cartridge-actuated Heinkel seat figured prominently in early US Army Air Force investigations into emergency egress systems; clearly, compressed gas systems (although functionally adequate) required far too great a compromise in terms of added airframe weight and loading.

With the first American jet fighter already in the air (a squadron of the Lockheed P-80 Shooting Star was actually on the ground in Italy, awaiting orders to operate when VE Day occurred ), some sort of ejection system was needed that would extract crew from these fast new machines under emergency conditions. Although the term 'bail-out' remained in use after the first ejection systems were installed in American fighters, the ballistically fired ejection seat provided far more positive results than a simple 'bail' over the side of the aircraft.

Army Air Force emphasis on rapid development of ejection systems for US aircraft resulted in a formal policy of sharing the extant egress database on ejection systems technology with many different (US) aircraft manufacturers, once knowledge of the captured German developments in the field had been analysised and initial conclusions arrived at by the Wright Aero Lab. The opinion has been offered and formally stated in Bryan Philpott’s excellent book on the subject that this ultimately had the effect of creating more problems in the long term than it solved for the US Air Force. Whereas in Sweden, Germany and England, responsibility for ejection seat development had been given over to a single industrial concern, the proliferation of US corporate approaches to the problem ironically acted against the easy solution of common design obstacles. Consequently, At least 6 or 7 aircraft manufacturers in the US designed and developed their own ejection seat systems, installing proprietory designs in their own aircraft; thus design uniformity was largely absent, and attendant system and logistical complexities resulted which would later present considerable technical and materiel challenges for field maintenance crews. The obvious advantage obtained, on the other hand, would be a proliferation of widely different researches that would together provide a broader database upon which to draw in carrying out future egress R&D work.

In marked contrast to this Army Air Force approach and as early as 1946, the US Navy had determined that England's Martin-Baker Company had satisfactorily pioneered the basic functional principles of pilot egress technology well enough that duplicate or redundant studies by Navy researchers would be both expensive and needless. Thus, the US Navy arranged with Martin-Baker to provide a seat somewhat tailored to US Navy requirements as well as technical support from that company to develop it. Photographs of this first M-B provided US Navy seat show it to be quite similar to that company's 'pre-Mk.I' seat. Along with the actual seat, a 105 foot tall test firing tower was also acquired and a converted Martin B-26 Invader (US Navy JD-1) was specially fitted for planned aerial test firings.

With at least 4 different companies working on US Navy jet aircraft designs in 1946, it was decided to issue an ejection seat specification and allow the individual Navy contractors to develop their own systems, as long as each stayed within specific general parameters. These parameters were based largely on the Martin-Baker seat and the standard features that characterised the English egress philosophy founded by Martin-Baker (face-curtain actuator, drogue gun, etc.) and mandated use of standard US Navy parachutes, survival kits, life-rafts, and other equipment. It is worth noting here that both the first US Navy and US Army Air Force seats were designed to use seat-type personal parachutes (this is evident in the use of characteristic rounded seat pans in all early US Navy seats) that followed the German example (Heinkel, et al).

Initial US Air Force research showed that the Heinkel ballistic system was not powerful enough to use on anticipated Air Force jet aircraft, as the catapult velocity was insufficient for safe ejection at the new Lockheed P-80's maximum operating speed. However, a fairly simple redesign of the basic Heinkel seat by Wright Field resulted in a prototype US seat, which was tested by a live volunteer from a specially modified P-61B Black Widow on 17 August of 1946  (the test subject was AAF Sgt. Larry Lambert and the ejection occurred at 280 KIAS, at 7800 feet).

A visual side-by-side comparison of the original Heinkel seat and the modified American seat reveals distinct and clearly identifiable design concept sharing. Progress was made quickly in developing the concept, however, and the straight-winged Republic P-84 Thunderjet became the first production American jet fighter to be equipped with an ejection seat in 1948.

As mentioned earlier, the USAF approach to ejection seat design called for armrest-actuating triggers (following the German example) which were first raised and then squeezed. Although a conventional seat-type chute was at first used in the original production American seat (used in the P-84B), a back-type parachute was later incorporated; these were not, of course, attached to the seat (similar to the Swedish approach, but contrary to the English design), but were worn by the pilot. Further, all early American seats were not automatic in that the aircraft canopy first had to be opened or ejected as an initial and discrete action. Actuating the seat’s right hand armrest grip would then eject the seat from the aircraft. Once free of the aircraft, the pilot freed himself from the ejected seat first by unfastening his seat and shoulder restraints, then pushing himself away from the seat; finally, he would manually deploy his personal seat or back-type chute by pulling its ripcord.

Several things quickly became apparent. Chief among these was the fact that once ejected, the seats would tumble and rotate wildly, subjecting the occupant to severe G effects that made coordinated efforts to release and separate from the seat extremely difficult to carry out. Another concern emerged in the form of powerful wind-blast effects that could inflict severe injury to the head and limbs of the occupant of the seat. Only after further study of these effects did refinements come to be incorporated into the system, which included leg restraint provisions, correct separation sequencing, fully automatic seat-separations, and automatic personnel parachute deployment--all or most of these having been addressed or at least recognized as desirable early on by England’s Martin-Baker Company and quite soon thereafter by the US investigators.

Regardless of the rapid pace of American development in design and production of ejection seats for military aircraft, after the war, there was a significant amount of resistance to the use of ejection seats on the part of American aircrew, who were somewhat reluctant to fly aircraft equipped with them. More than a few pilots likened the prospect of being seated directly upon a seat's live explosive charge akin to sitting on a powder keg with a short fuse.

Fortunately, these anxieties were substantially ameliorated in 1949 by a series of demonstration ejections carried out by Air Force Captain Vince Mazza from the aft cockpit of a specially modified TF-80C Shooting Star (later standardized as the T-33 jet trainer). The first US Navy emergency use of the new seat occurred later in the same year when the pilot of a McDonnell F2H-1 Banshee was forced to eject at 597 mph over South Carolina. After these events took place, acceptance of the new device was more readily forthcoming among US air crews. In August of 1949 the pilot of an Air Force F-86 Sabre also made a successful emergency escape using the new type seat (North American model T-4E-1 catapult seat), again demonstrating functional performance under adverse conditions.


Again, partly due to the broad approach taken towards development of ejection seat systems among US manufacturers, a number of unusual designs were produced. One in particular, developed for use on the pusher-engined XP-54 Swoose Goose featured a downward-accessed pilot seat, which would lower the aircrewman below the belly of the aircraft so as to clear the arc of the prop. While not strictly an ejection seat, the XP-54 design anticipated several future developments of a downward firing type seat on the Boeing B-47 Stratojet, and the Lockheed F104A Starfighter, as well as the downward firing cockpit system for the experimental XF108 supersonic interceptor (development discontinued after mockups were completed) and the Douglas X-3 Stiletto twin jet research aircraft.

At the time, the official emphasis on development of a successful US system was on functional adequacy at high altitude and sustained high speed, as this was the envisioned performance area within which safe aircrew egress would be most critical; the obvious need for safe ejection at lower, slower speeds took a markedly subordinate priority in this overall Air Force conceptual view. Given the notable rate of engine failures and marginal reliability that characterised the early turbojet engines, this misplaced priority would subsequently have substantial adverse consequences, as well demonstrated in US Air Force aircrew survival statistics of the late 40s and early 50s period.

Principal among the influences governing the US Air Force decision to continue putting emphasis on use of the downward ejecting system was the fact that early ballistic catapult inertial acceleration velocities were not completely adequate to ensure clearance of jet aircraft vertical stabiliser assemblies as airspeed capabilities continued to rise. This, in combination with emergent anticipation of the future importance of high altitude interception mission requirements, resulted in a failure to adequately address safety concerns related to 'low & slow' modes of flight performance in downward ejection seat aircraft. Unfortunately, the 'downward egress' concept was somewhat less than favorable for a number of reasons, not the least of them being its unsuitability for the critical low-altitude or zero/zero mode ejection. This fact was later sadly and graphically demonstrated when emergency use of a downward firing Stanley model C-1 seat on a test flight of the Lockheed F104A Starfighter resulted in the death of Captain Iven Kincheloe in 1958 (it is extremely ironic that Kincheloe was an early and outspoken critic of downward ejection seats in high-speed fighter aircraft).

As technical advances continued in seat design, and as catapult power increased, it was quite clear that the problem of providing adequate clearance of fast moving aircraft tail structures presented collateral concerns in terms of increased rates of spinal compression injuries related to catapult inertial acceleration forces. Problems associated with having the catapult thrust force located behind the seat center of gravity included high multidirectional G loading due to aerodynamic tumbling forces, wind-flail injuries, wind-blast effects, man-seat separation problems, and parachute entanglements due to aerodynamic instability of the seats after ejection. The problems attending higher inertial acceleration rates to clear the aircraft were obviously not going to be easily solved with continued use of simple explosive pyrotechnic devices. Despite the statistical evidence of only marginal success in achieving safe ejections in the outermost corners of the flight performance envelope, explosively fired catapults of necessity remained in service until roughly 1958, at which time the first rocket catapults were introduced to American ejection seat design (the Convair F102 Delta Dart was the first aircraft fitted with a rocket catapult fired seat, designed by Weber Aircraft Company). [Of interest is the fact that the proof of concept design for the F102--the Convair XF-92A research aircraft--was fitted with an early explosively fired catapult ejection seat originally designed for the Convair XP-81. When the two examples of this combined prop and jet propelled prototype were retired from testing in 1947, this seat was fitted to the XF-92A.

Perhaps fortunately, it was never put to 'test' use throughout the duration of that aircraft's flight test program.] Other innovations that were prompted by high rates of spinal compression injury associated with G-onset forces during ejection included the use of variable-density compressible foam in seat cushions, to help reduce or offset accelerative effects on the spinal column. (This same principle is employed today in crash helmet design, for the same purpose.)

Dr. Robert E. van Patten, former Chief of the Acceleration Effects Branch, Biodynamics and Bioengineering Division of Armstrong Aerospace Medical Research Laboratory (Dayton, Ohio), cites the rapid progress made by the United States in adopting emergency egress systems for its Air Force after the slow initial start at war’s end in reference to the fact that in 1955 the first successful supersonic ejection was made by a pilot from his stricken North American F100A Super Sabre after the aircraft went into an uncontrollable dive. The ejection occurred at Mach 1.05 during a test flight, and while he was injured in the process, pilot George Smith survived the accident and fully recovered. This incident took place less than a decade after the first real investigations into egress systems had begun in America at war’s end. Although the technology was improving rapidly, most US ejection systems were engineered to perform best under 'ideal' emergency situations. However, success in achieving a substantial safety record (and vindication of the new rocket powered catapult systems) is demonstrated by the fact that during the Vietnam conflict (1963 through 1975), more than 25% of the US Navy’s RA-5 (A3J) Vigilante combat ejections took place at speeds greater than Mach 1.0 (the system in use in that aircraft was the North American Aviation produced HS-1 rocket powered seat, with zero/zero to 700 KIAS capability).

Certainly the Korean War provided much valuable information to the US Air Force and American aerospace manufacturers regarding egress design assessment. Philpott in his book cites the fact that in almost 2000 combat ejections experienced by the US Air Forces during the Korean conflict, 60% of the aircrew ejecting experienced no problems during egress. The other 31% experienced difficulties ranging from seat actuation, canopy release, maintenance failures, incorrect ejection posture, slipstream, through-canopy-ejection, premature seat actuation, and so forth. Similar difficulties, as well as ones unique to carrier operations, were reported by US Navy pilots. Although most of the problems were addressed, Philpott suggests that again the vast number of dissimilar ejection seat designs in use in various aircraft compounded quick resolution. A look at the official US Air Force statistics themselves shows a slightly different picture, with an overall (USAF) aircrew survival rate of 77% during the first 5 years of ejection seat operational experience (1949-1953) . In a recorded 4626 emergency ejections incurred under non-combat conditions, from 1949 through 1980, fatal injuries occurred in 838 (or 18%) of those ejections. With the refinements of automatic release restraint systems, automatic man-seat separators,  variably-staged parachute deployment systems, and aerodynamic deployment stabilisation devices such as the DART system, survival rates went up in the 1954-1958 period to 81%. Throughout the period of the mid 50s through mid 60s, most USAF aircraft egress systems received continual updating as operational experience provided new engineering understanding of optimal design features. The overall survival rate thereafter remained roughly at a plateau of about 80% until the 1975-1980 period, in which these values fell significantly for USAF crews (introduction of the ACES system, with enhanced ejection safety features that evolved from the McDonnell-Douglas ESCAPAC seat system).

ENGLAND’S MARTIN-BAKER: DEDICATED COMMITMENT

With revelation of the early and substantial foundations in emergency aircraft egress technology developed by Germany and Sweden at the war’s end, awareness was acute in England that the logarithmic increase in aeronautical performance parameters required a corresponding capability in aircrew protection and escape. The English Air Ministry sought assistance from Sir James Martin, founder of the Martin-Baker Aircraft Company. Baker was a brilliant self-made engineer and designer who had originally set up his company (in 1929) to manufacture machines and specialised vehicles. Development of his firm into the aircraft business in co-partnership with Capt. Valentine Baker (ex-RAF) had met with marginal success, although it was generally agreed that his abilities were quite substantial and his designs for three successive prototype aircraft had been innovative and progressive. In 1942, the third aircraft (designated the MB3) was lost on a test flight which also killed Capt. Baker. Profoundly affected by the loss of his flight testing partner, it is said that James Martin developed a keen interest in aircraft safety and aircrew survival from this experience.

During the war the company manufactured a variety of products ranging from ammunition feeder belt mechanisms to wing-mounted cable cutters and canopy quick-jettison equipment. During the Battle of Britain Martin’s canopy emergency release device quickly gained him recognition, and in 1944 the RAF Air Staff approached the Martin-Baker Company to develop a system whereby the pilot of a disabled aircraft could be safely ejected for a normal parachute recovery. An incident involving a pilot having been killed by colliding with the tail assembly of his aircraft (a jet powered Meteor prototype) on bailing out was the instigating cause.

The problem, considered from its medical and physiological standpoints, is a thorny one, and although the German studies of the 30s and 40s had produced a baseline database on the practical limits of human G tolerance, the critical principles of "jolt" and pulse pressure--in other words the rate at which accelerative force is applied--were not fully or completely fathomed. It was not until 1944 in studies carried out in Germany by one Dr. Weisehofer that this last factor became more adequately understood. In England, the concept was grasped more quickly and efforts began to determine how to devise a system which would achieve the required pilot extraction without exceeding the ability of the human body to withstand severe rates of accelerative force. One very early idea was the previously mentioned pivoted fulcrum principle, in which a beam attached to the pilot’s harness is actuated by a strongly compressed spring mechanism to pull him free of the cockpit and toss him away from the aircraft’s tail assembly. The idea had a certain appeal in that it could conceivably be retrofitted to existing aircraft fuselages with a minimum of difficulty.

For various reasons, this idea was discarded and exploration into an ejectable seat system began in earnest. To develop a keen understanding of the limits of G force sustainable without injury by the human spine (which was the critical structure susceptible to damage) in vertical acceleration, it was necessary to develop a vertically inclined tower, up which a test seat could be shot at various rates of G. Both Sweden and Germany had constructed similar test equipment in their ejection seat programs. In the course of tests, it soon became apparent that it was not the peak gravity (or G) sustained which was the limiting factor in human tolerance, but the rate at which the acceleration occurred. Furthermore, onset of G ought not to exceed 300 G/sec, and proper body alignment during the application of force was critical to avoid spinal injury.

These insights led the English to the development of a two-cartridge gun, which met the physiological limitations while still providing peak G needed, and to various refinements in a seat which would assure proper posture (these included a face-blind actuation device and foot rests, both of which helped the ejectee to assume the desired anatomical alignment for a safe ejection). Still another consideration was revealed, after further study of the physiological function of the human spine while subjected to vertical compression forces. This was what is termed 'acceleration overshoot,' and is the amount of acceleration experienced by the seated person in excess to that of the seat, due to the construction of the body’s structure. It was found that spongy tissues of the body’s nether side tended to increase acceleration overshoot forces, just as did excessively soft seat cushions.

The earliest prototype Martin-Baker seats were configured with the parachute pack in the seat pan and the water-survival pack & inflatable raft fitted behind the back. This was also the configuration for the earliest production model, the Martin Baker Mk.I seat. This design allowed for increased possibility of acceleration overshoot injuries, as was discovered, and in subsequent Marks, the parachute was moved to a position behind the back, with the water-survival kit stowed in the seat-pan. In subsequent Mark developments, the parachute pack remained in the back stowed position, although with the present generation M-B seats (Mark IX and later), the chute has been moved to the seat’s head-support area.

In September of 1945, development of the prototype Martin-Baker ejection seat had proceeded far enough that a contract was placed for two experimental units which would be flight tested in a high-speed aircraft. The latter turned out to be a Meteor F3, and after a number of dummy ejections and corresponding modifications of such components as the drogue chute and its deployment device (a gun was incorporated in place of a spring deployment mechanism), the first English live ejection test was made by test subject Bernard Lynch on 24 July 1946.

It is interesting to note that only 5 days after Bernard Lynch conducted the first successful live test of the Martin-Baker prototype ejection seat in England, a Swedish pilot used the SAAB Type 1 (Type 21) ejection seat in his SAAB J21-A1 pusher-engine aircraft in an actual emergency. Furthermore, over 60 German Luftwaffe aircrew had previously used their Heinkel ejection seats for successful emergency wartime (WWII) ejections. Also at this time, the United States Army Air Force was still weighing the merits of ejection seat systems, investigating German wartime developments, and had not even a prototype of its own designed for possible American military aircraft use.

EGRESS MATURES: EJECTION SYSTEMS DEVELOP FURTHER

In June of 1947 the decision was made in England to fit the new Martin-Baker ejection seat (Mark I) to all British tactical military aircraft. A number of versions of the original Mk.I seat were produced for a variety of jet fighters and bombers. Various small changes in the Mark I application for each aircraft type reflected the unique requirements or constraints of that particular plane. The Meteor fighter was initially fitted with the M-B Mk.I, and later with the Mk.IE seat. The Attacker was fitted with the Mk.IA (it was later upgraded to the M-B Mk.IIA model). The Wyvern was fitted with the Mk.IB model (later upgraded to Mk.IIB). The Canberra bomber received the M-B Mk.IC seat, and the Seahawk aircraft was fitted with the M-B Mk.ID seat (later upgraded to M-B Mk.IID seat). The M-B Mk.IF seat went to the new Venom fighter (later upgraded to the M-B Mk.IIF seat).

Prior to full-scale production of the Martin-Baker Mk.I series, the Saunders-Roe Aviation Company placed an order for an ejection seat for use on its Saro A-1 jet flying boat/fighter. This seat lacked some of the more sophisticated features and detailing of the standard Mk.I production model, and was officially designated the Pre-Mark.I Seat. Altogether the Mk.I production seat weighed some 172 pounds (143 pounds of which was ejectable). A list of its innovations includes: 1) a two cartridge ejection gun; 2) gas-pressure release mechanism unlocking the ejection piston from the firing cylinder immediately upon firing of the first cartridge; 3) an adjustable seat pan; 4) spring-loaded footrests to facilitate foot placement for ejection; 5) a face-curtain seat actuation system, which assured proper ejection posture and which protected the pilot’s face from blast injury; 6) an explosive drogue gun method of deploying the seat’s stabilising drogue chute; 7) retractable seat-pan raising handle; 8) the Martin-Baker back-type water survival/dingy pack, and seat-cushion type personal parachute; 9) integral thigh restraining seat-pan extrusions. By 1950, the Mark I series seat was standard on all first line RAF and RN fighters.

It ought to be noted that there was considerable difference in the physical appearance of the various Mk.I models, and that not all of the dash variations featured a seat type chute. For example, the M-B Mk.IA variant (for Attacker aircraft) used a Mk.3A back-type parachute rather than a seat chute, and used a seat sea survival kit/inflatable raft--very much as later seats would. The Mk.IB, ID, IE, and IF seats all featured Type 3A M-B back-style parachute configurations. Only the Pre-Mk.I, Mk.I, and Mk.IC featured seat-mounted chute assemblies. In some cases, the seat style chute was used due to the constraints of the aircraft’s cockpit, despite established awareness that acceleration overshoot problems were more of a concern with the seat-style chute design. All subsequent Mk.II series seats featured the back-style chute design.

As experienced in the US, there were initially some reservations about use of the new ejection systems on the part of both aircrew and aircraft manufacturers, who felt that the structural modifications required to fit the seats were costly and technically complex. Again, time and effort was needed to address the reluctance of aircrew to feel comfortable with the new safety devices, but it was not long before several actual in-flight emergency ejections proved the value of the Mk.I type system and these examples of its safe operational value put further hesitance to rest.

Within the first few years the results of about 70 successful ejections (50 of them successful) provided valuable information on much needed modifications to the original Martin-Baker design. The Mk.I system was, after all, a manual seat which did nothing more than successfully get the aircrewman out of his disabled aircraft safely. It was clear that further progress was needed in development, specifically in automating the man-seat separation process and in deploying the personal parachute. In instances where disorientation, confusion and/or disablement were factors in emergency ejections (as they frequently are, especially in aerial combat situations), automated systems would make the difference between saving a pilot or not.

The Mk.II seat addressed these problems. Among the modifications featured on the Mk.II series seats were 1) modification to the drogue shackle, and addition of a Time Release Unit (TRU) for deploying the drogue; 2) a mechanism for releasing the seat harness; 3) addition of an "apron" which would link the drogue chute to the pilot’s personal parachute and withdraw it after man-seat separation occurred; 4) a barostatic unit, which was fitted to the TRU so as to prevent deployment above 10,000 feet (oxygen deficit and hypothermia hazards); and 5) further modification of the drogue gun. By 1953 a retroactive modification program was carried out on all RAF and RN aircraft fitted with the original Mk.I series seats to bring them up to Mk.II standards. The effectiveness of the new design resulted in a reduction of ejection fatalities from the earlier, higher figure applicable to the Mk.I seats to about 10% of the Mk.II series ejections in 1954 14

Attention soon focused, however, on the parameters of emergency egress wherein the Mk.II series seats still fell short. Two of these were in high-speed and low altitude situations. Changes in the ejection gun ensued which increased ejection trajectory without significantly increasing peak acceleration. Additionally, the adoption of a two-stage drogue chute design resulted in faster, quicker and safer deployment of the pilot’s personal parachute at higher speeds. Finally, in order to prevent lower extremity flail injury, which frequently were a consequence of high-speed ejections, a system was devised using nylon strap restraints to secure the legs to the seat on actuation where they were secured until the man-seat separation occurred. These modifications resulted in a system which allowed successful ejection from altitudes of 50 feet altitude and 130 knots IAS. The new system was designated the Mk.III series seat.

Further modifications were carried out on the Mk.III series seat over the next few years to achieve a further refinement in the parachute deployment process (primarily a reduction in the drogue/mainchute deployment sequence timing), which resulted in successful ejections from zero altitude and 90 knots IAS. The seat was also tested in ejections above 40,000 feet which proved that the seat was usable in both extreme polar limits of the possible performance parameters.

In the same time period (1954 through 1955), a system was devised whereby automatic explosive canopy jettison was incorporated as part of the seat actuation protocol, so that no time was wasted in clearing the canopy before egress was initiated. Owing to possible complications, however, the Martin-Baker seats were capable of ejecting through the canopy of aircraft if necessary, and in some RAF aircraft the egress drill was standardised to blast through the canopy routinely (an example in point was the Canberra B Mk.II aircraft).

By the time the mid-1950s had arrived, the continuing evolution of the standard British Martin-Baker emergency egress system had enabled the Martin-Baker seats to prove their sound and carefully considered design handsomely, and this fact was not overlooked by many of the other nations around the world who were growing increasingly interested in providing adequate emergency escape capabilities for their air force personnel flying high-performance jet aircraft.

 

(Next: Part Two)
 
 
 
 
 
 
 

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