In the Beginning- man had just learned to fly with powered ...



In the Beginning- man had just learned to fly with powered aircraft at the beginning of the 20th Century. The early days were fraught with danger and many pilots experienced life-endangering experiences. Soon, the inventors of the day rushed to create safe methods of delivering aircrews from harm. The parachute, which had been used by showmen from balloons for some time now, was an obvious choice for returning the aircrew to the earth. Unfortunately, the designs of both the airplanes and parachutes were not up to the challenge of the time period. The parachutes were large and unwieldy and the aircraft were weak and difficult to exit, even on the ground.

Even so some experimenters worked diligently on the problem. One of the earliest was a J.S. Zerbe who in 1910 patented a gunpowder-operated ejection seat. A few years later in France, a Baron d’Odkolek demonstrated a device consisting of a mortar that fired a parachute on a platform into the air. This platform contained a set of three radially mounted guns that fired slugs attached to the skirt of the parachute. This rudimentary extraction system was used to pull a dummy off the wing of an airplane in flight. During WW-I most experimentation ground to a halt in England and the United States, although parachutes were used by observation balloon crews. The Germans utilized parachutes more often in aircraft though.

After the war, parachutes had improved significantly. Although they were still bulky, their use in cockpits was now possible. Some more experimentation on assisted escape was done in the United States, with a biplane fitted with a mechanism to separate the aircraft at the front of the cockpit to allow the aircrew a better chance at bailing out.

While the rest of the world proceeded slowly with development, the Germans rearmament continued in secret. As other countries had found, the aircraft were getting faster, and the Germans aircraft were no exception. Given their dramatic increases in airspeed, the aircrew were finding it far more difficult to leave the cockpit manually. As the Germans developed more revolutionary aircraft designs, they began to feel that the risks to the test pilots were rising at a rapid pace. To counter this, they began to experiment with methods of aircrew escape. Starting with a separable nose capsule, they began to try different methods of escape. The nose capsule was intended separate and a drogue parachute would slow its decent while the aircrew would open the hatch and bail out. Initial testing found that the drogue would get caught in the wake of the capsule and not deploy so a method was added to propel the drogue out into the airflow. .

The Germans later began to develop ejectable seats. The earliest of these was a seat for the Dornier Do-23. A co-pilot inadvertently used this seat in 1934 when the seat accidentally deployed. The seat raised the man to the tops of its rails and he was able to unhook his lap belt and bail out from there. It is believed this seat was powered by a spring or bungee system.

Later German seats used either compressed air or cartridges for their power source. German experimenters used ejection towers to test the acceleration forces of the seats. One example of the compressed air type seat is that fitted to the Do. 335 Pfiel. This seat featured armrests and foot rests to lessen the forces on the pilot during ejection. The seat was sometimes fitted with armor plate behind the headrest and back area. An example of the cartridge type is the Heinkel He-162 jet propelled aircraft used later in the war. This aircraft used nearly the same seat as fitted to the He-280 series of aircraft. It is particularly significant as examples of this seat were sent to England and the United States in 1945 for experimentation. The He-162 seat was fitted with foot rests and arm grips.

During the late 1930s th e Swedish firm Saab also began development of an ejection seat for the J-21 aircraft. This aircraft was a single seat fighter with a pusher propeller. The propeller being behind the cockpit made conventional over-the-side bailout hazardous and so the seat was decided to be a necessity. Saab developed a seat that consisted of a seat pan with a pair of catapult tubes, one along either side of the rear. This seat was a strictly manual seat in terms of seat separation. The seat was fired by a pair of cartridges, and the propeller was jettisoned as part of the pre-ejection sequence.

As WW-II progressed, Allied pilot’s began to report that the Germans were sometimes abandoning their aircraft in their seats. The Allies were already working on their own assisted egress devices though. The Americans had developed a pusher aircraft, the XP-54 in 1943. This aircraft, the “Swoose Goose” was equipped with an entry system that consisted of the pilot’s seat mounted on a pivoting boom. This boom was attached to the aft end of the aircraft and lowered the seat to the ground for the pilot to strap in and be raised up into the aircraft. In case of an emergency, the pilot would be pulled down and out of the aircraft by this boom. When the boom was fully extended downwards, the seat would pivot at its base and tip the airman off to allow him to deploy his parachute manually. This system was never used for an emergency.

The Martin-Baker Aircraft Company (Martin-Baker) began to develop escape devices at around the same time. Their first device was a single point release for the Spitfire canopy system. This system simplified the egress from the Spitfire. They then turned to designing more active devices. The first concept explored was a spring-loaded arm mounted along the top of the fuselage on an existing aircraft. The arm would be connected to the pilot’s parachute harness and would lift the pilot up into the air and toss him over the tail of the aircraft. A model of the device was developed but the device was never fielded.

Martin-Baker then turned to the concept of jettisoning the pilot’s seat along with the pilot. They explored springs, bungees, and compressed gas before selecting a cartridge-fired catapult. To test the seat concept an inclined rail was developed. This allowed a seat to be fired upwards with either an instrumented load, or a person on board to determine the effects. Testing went well, however an injury occurred to a test subject at what had been an accepted acceleration. This led to further research on the limits of human endurance. The result of this experimentation was to develop a catapult that used a primary cartridge to begin the seat movement, and one or more secondary cartridges to incrementally increase the acceleration. This was successfully implemented. Another development of the research was in terms of body positioning of the ejectee. The determination was made that to properly set the airman’s back, overhead handles on a face curtain would be used to initiate the seat. The action of reaching up and pulling the handles would straighten the spine and place the ejectee in good position for the forces involved. The face curtain would also protect the airman’s face and eyes against the windblast effects during high-speed ejections.

The early seats (referred to as Pre-Mk. 1 seats) were then tested in many ways leading up to test firings of dummies during 1946. Later the same year, on July 24, Bernard Lynch became the first British ejectee from a modified Gloster Meteor Mk. III. .

The United States Army Air Force (USAAF) had been working at the same time using a captured He-162 seat as an example to develop their own ejection seat. They also examined a Saab seat but decided to use the He-162 seat as the basis for their research. The main problem had been the lack of a suitable cartridge to replicate the German originals, so a new catapult was developed for the seat. This was done, and along with a modified seat it was live tested on 17-August-1946 by 1st Sgt. Lawrence Lambert. The test was done from the gunner’s compartment cockpit of a P-61 aircraft.

The United States Navy (USN) had also been investigating ejection seats. They purchased equipment from Martin-Baker for evaluation including an ejection test tower, and a Pre-Mk. 1 seat. Lt. Furtek live tested this seat from a JD-1 aircraft on 1-November-1946.

With this testing complete, both the USAAF and USN began to acquire aircraft factory-equipped with ejection seats. The P-84 was one of the first USAAF aircraft to be so equipped. Earlier a German seat had been test fitted to a P-80. Other aircraft from the inventory were tested with ejection seats for fit and clearance testing.

This time period of the late 1940s was marked by the entry into the egress field by many of the aircraft designers, and a few companies that specialized in certain types of seats. Designers preferred to design their own seat for the aircraft. This led to many different designs being used, most with significantly varied equipment and activation requirements.

The U.S. Navy requirements were for face curtain activation, and for a drogue chute to stabilize and decelerate the seat. These requirements were generally followed although with much variety. The U.S. Army Air Force requirements were for foot rests and arm rests. These were also generally followed, although again with many variations. These design requirements stemmed from different points of view of the methods of safeguarding the aircrew using the seats. The U.S. Navy requirements were based on the experiences of testing the Martin-Baker pre-Mk. 1 seat and the technical information gained from Martin-Baker. The U.S. Army Air Corp requirements were based on the German research and seat development.

Martin-Baker continued their development adding such innovations as leg restraint lines and automatic releases for the seat occupant. The drogue mentioned before was an integral part of the seat systems and was extracted from the top of the seat by means of a gun-propelled slug. Early trials of this system showed that the drogue could be damaged at higher speeds so a smaller drogue was included to withdraw the larger drogue. In addition to the slowing effect of the duplex drogues was the added stability of the seat. The drogues stopped the seat and man from tumbling. This allowed for a more controlled recovery parachute deployment.

The early Martin-Baker seats were soon equipped with automatic harness release mechanisms to allow for seat separation. The aircrew would still have to manually actuate his rip-cord to deploy the recovery parachute. Feedback from the tests and early actual emergency ejections led to a more completely automated seat system. This system consisted of the aforementioned drogue parachutes which were connected to the main recovery parachute withdrawal line via a set of devices called the drogue shackle and scissors shackle. The scissor shackle held the drogue shackle until the seat mounted release mechanism would open it. This mechanism performed the same function as the earlier automatic harness release, but with the addition of the scissor shackle, it allowed the drogues to withdraw the main parachute from its pack and thus recover the airman with no further action necessary on his part.

As the Martin-Baker seat was becoming automated the parachute was also changed from a seat pan located pack to a back pack style parachute. This had two effects. First, it allowed for a larger amount of survival equipment to be stored in the seat pan for use after an ejection, and second it allowed for a more direct deployment of the parachute, with less stress on the crewman. Leg movement during ejection was becoming a more pressing concern and the decision was made to incorporate a set of restraint lines for the aircrew’s legs. These restraint lines were routed from the cockpit floor up through the seat via a snubber mechanism. They were then connected to the airman’s legs via a set of garters that were worn on the lower legs. The lines then continued back to the seat pan where they were secured. On ejection, the lines would be pulled taut as the seat rose up the rails. As the airman’s legs dropped back against the seat bucket, the lines would hold them there by the action of the snubbers. The lines would separate from the cockpit floor by means of shear pins in the ends of the lines. The action of the release mechanism would release the clamps holding the end of the lines and allow the airman to separate cleanly.

The updated Martin-Baker seats rapidly became the standard seats in the Royal Air Force and Royal Navy.

In the mean time, the U.S. Navy aircraft were being equipped with a variety of seats from the various aircraft manufacturers. For example, Grumman had developed an ejection seat that was used in the F9F2 and F9F4 aircraft. Using the NAMC-type catapult gun, this seat was primarily made out of sheet aluminum. As was required by the U.S. Navy, the seat used a face curtain firing mechanism and had a drogue gun. The canopy needed to be jettisoned before the seat could be fired. Seat separation was completely manual. The airman would have to undo his lap belt and push free of the seat before pulling the rip-corp on the seat pan parachute. Vought, Douglas and other companies also produced seats for Navy aircraft at the time. They had many similarities to the Grumman seat although they varied considerably in method of manufacture and mechanisms.

The U.S. Army Air Force Wright Air Development Center had developed a seat known as the WADC seat, which they intended for all manufacturers to incorporate in their cockpit designs. This was not to be, however as each company began to design their own.

The North American Aircraft F-86 for example had a seat that consisted of a seat bucket of sheet metal that could be adjusted on the back structure to keep the airman’s head in the proper position. Typically of the time, the ejection procedure was somewhat complicated. A firing handle on the right side of the seat bucket was raised to jettison the canopy. Due to the minimal clearance over the airman’s head he was cautioned to lower his head before actuating this lever. Afterwards, he could return upright, raise the left handle to lock the shoulder harness, place his feet on the footrests and then he was finally free to squeeze the trigger under the right handgrip to fire the seat.

Seat separation was manual with the airman releasing the lap belt, pushing away from the seat and then pulling his rip-cord. The parachute was a backpack type. A survival kit was fitted to the seat pan.

The next major phase of American ejection seat evolution occurred in the 1949/1950 time frame with two major inventions becoming available. The first was the barometric parachute release. This device allowed a parachute to be armed on seat separation and would allow the airman to fall to the pre-set barometric pressure altitude at which time the parachute would be deployed. This altitude could be set, and was often set to 10,000ft. The release incorporated a timer mechanism to delay for a few seconds after the altitude was reached as well. If a airman ejected below the preset altitude, this delay would allow for the seat to drift further away before parachute deployment. The second major device developed was the automatic lap belt. These belts were designed to be actuated by a delay cartridge. On actuation, the airman would likely fall free from the seat. By combining this with a lanyard to activate the barometric parachute opener, the seats were basically converted to automatic operation.

At this time other concerns were beginning to come to light. Bomber aircraft used more than one flight deck for the crews so it was necessary to come up with a method of allowing the crews on lower decks to escape as well. Stanley Aviation was soon tapped to develop downward ejection seat systems. The U.S. Air Force also contracted with Republic Aviation to develop upward ejection seats.

Ejection seats were of necessity required to evolve as well. The sound barrier had been broken by more than one aircraft and record speeds were being attained with amazing regularity. It was clear that just getting an airman out of the aircraft by an ejection seat was not adequate. The higher speeds were making it difficult to produce a catapult which could force the seat and airman up past the tail of the aircraft without exceeding the physiological limits of the airman. It became apparent that an additional source of power would be required, and rockets were examined for this. There were three basic methods that were examined. Martin-Baker developed an underseat rocket system that worked along with their already existing seat technology rather well. The underseat rocket was bolted on to the seat bucket and would provide additional thrust over a period of time. The U.S. Navy developed a system known as the RAPEC system. The RAPEC was a rocket-catapult or ROCAT. This is a device that fits into the same space on an ejection seat occupied by the typical catapult tube and consists of a catapult cartridge and a sustainer rocket. The rocket is ignited near the end of the catapult stroke and continues the acceleration of the seat after the seat has cleared the cockpit rails. The third system developed by Rocket Power Inc. was the Seat Back Rocket (SBR). The SBR system usually is made up of two rockets mounted on either side of the seat bucket and works along with the seat mounted catapult charge.

Aircrews ejecting at higher speeds were found to be much more likely to be injured than those at lower speeds. Various designs began to be examined to solve this problem.

The Douglas D-558-2 Skyrocket was one of the earliest aircraft to have a fielded solution to this problem. The Skyrocket was designed with an ejectable capsule that included the entire cockpit. After ejection, the pilot would have to manually bail out of the capsule. Other designers began to examine the concept of encapsulating only the airman’s seat.

One avenue of doing this was the Stanley Canopy Capsule. This included a pilot’s seat hinged at the rear that on actuation would retract the pilot’s legs, and then the seat would rotate on the hinges up into the canopy frame. The entire unit would then be fired off the aircraft by means of a rocket system mounted on the seat back. Although the system was not implemented in the U.S., the Russians developed a similar system known as the SK-1 for the MiG-21.

The Convair Rotational Ejection Seat was one seat that did get fielded in the F-106A/B model aircraft. The B-seat as it is often called was a very advanced open ejection seat. The seat consisted of a seat bucket with a special backpack parachute and survival kit. Its more unique features were in its mechanisms though. After the canopy was jettisoned and the seat activated, the aircrew’s feet would be retracted by means of cables attached to spurs on his boots. A set of foot ramps would tip up to protect the feet from the airblast and then the seat would rise to the top of the cockpit and pitch up to place the aircrew’s spine parallel to the aircraft fuselage. A pair of telescoping booms would be extended upward (relative to the seat itself) from the back of the seat. The seat would then be severed from the cockpit by explosive bolts and the rocket in the center of the back of the seat would thrust to separate the seat from the aircraft. The booms would serve to stabilize the seat and the curved bottom of the seat would protect the aircrew until seat separation.

During the same time period, Stanley Aviation developed for the B-58 Hustler an encapsulated seat that consisted of a relatively standard upward ejection seat enclosed in a clamshell-like enclosure. The clamshell was normally stowed up over the aircrew’s head. The aircrew would raise one of the handles that would actuate the leg retraction mechanism. The clamshell would then close and the capsule would automatically pressurize. The pilot’s capsule was designed to encompass the control stick for the airplane, and this stick had several special controls on it. Along with the window in the front of the clamshell, this allowed the pilot to control the aircraft during a decent from altitude if the problem was only a pressure loss. If the need arose to eject, pulling either trigger would actuate the canopy jettison and fire the rocket-catapult on the capsule. A drogue parachute and pair of fins would deploy to stabilize the capsule in the airflow. After a preset interval, the main recovery parachute was deployed, along with an impact attenuation bag under the capsule. The capsule was designed with flotation bags as well, and could be used as a survival shelter on both land and sea.

Low speeds and altitudes were a concern in the 1950s as well. The U.S. Air Force developed a system using a lanyard attached to the parachute rip-cord for extreme low altitude use. This lanyard would bypass the 1-2 second delay from the barometric parachute release and allow for faster parachute deployment. Some altitude was still necessary for this to be effective however.

Martin-Baker had continued development of their seats during this time and the improvements included better control of the timer release mechanism sequences to allowed for ejection at ground level. During 1957 this ability was demonstrated at the Patuxent River Naval Air Station by Flt. LT. Hughes of the RAF for the benefit of the U.S. Navy. He ejected from the aft seat of a Grumman F9-F8T Cougar at take-off speed. The successful test led to the decision to use Martin-Baker seats in most U.S. Navy aircraft at the time. By 1961 the underseat rocket improved this capability to be useful at zero-airspeed/zero-altitude (0-0).

During the late 1950s Weber Aircraft was working with the U.S. Air Force to replace the Convair B-seat in the F-106 aircraft with a better low-altitude performance seat. The seat they developed included a rocket-catapult and was also capable of 0-0 performance. This seat was installed in the F-106 beginning in the late 1950s, and was live tested by Maj. James Hall in 1965. The key to the system was a gun-deployed backpack parachute. This parachute pack included a gun that would fire a slug to open the pack and withdraw the pilot parachute. Another feature that was becoming commonplace at this time was a powered inertia reel system. This helped position the airman for ejection by retracting straps connected to the airman’s shoulder harness. This would help the airman by placing him in a more upright posture in the seat.

Along with the previously mentioned encapsulated seats, Stanley Aviation worked with Lockheed Aircraft to develop ejection seats for the F-104 Starfighter. Since the Starfighter was designed to fly at high speeds, in the Mach 2 range, the fear was that the catapults of the day would not allow for clearing the rather high tail of the aircraft. The decision was made to work with a downward ejection seat, which Stanley had much experience with from the B-45, and B-47 programs. The seat that resulted initially, included several features found on the earlier downward seats including thigh guards which were rotated into place prior to ejection. The feet were held in manacles (clamps actuated by bringing the feet back into the footrests) and the seat was fired by a center pull handle. The hatch below the cockpit was jettisoned by cartridge and could be forced out by seat movement if necessary. Another safety feature of the seat was the ability to use gravity to allow the seat to exit if the catapult failed. This was done by disconnecting the catapult attachment.

Later versions of the seat automated the foot retraction by use of a cable retract/boot spur system, and an arm net was added to restrain the arms of the aircrew. The nets were automatically deployed by the thigh guards that were now extended by thruster from under the seat. Later on several low altitude accidents led to the seat being redesigned for upward use. This was accomplished with the addition of telescoping canopy jettison thrusters and a newly designed ROCAT with adequate thrust to clear the tail. Seat separation was accomplished by a delay cartridge that would sever the cables on the spurs, fire the automatic lap belt release, and then activate a gas-operated rotary actuator in the seat back. This rotary actuator would retract a seat separation strap that was stretched under the backpack parachute and below the survival kit. The effect was to force the airman off the seat. The barostatic parachute deployment device was activated by a lanyard on the lap belt. A separate lanyard could be attached to the rip-cord for extreme low altitude flight to provide more rapid parachute deployment.

Douglas Aircraft was also developing seats and had developed a lightweight ejection seat that was significantly lighter than their previous seat. The new seat was developed for the A-4 Skyhawk and was initially known as the RAPEC seat after the rocket-catapult installed. Later the designation ESCAPAC 1A became the standard. The Escape Package series of seats became one of the widest used family of seats in the United States. The letter designation helped identify the seat based on the features installed as the seat structure varied relatively little between variants. The early seats were equipped with seat separation bladders. These were inflatable bags behind the backpack parachute and below the survival kit. After ejection a delay cartridge would fire a thruster to unlock the lap belt, inertia reel straps, and open a valve on a seat mounted nitrogen bottle to inflate the bladders.

Later ESCAPAC seats were improved in stability by the addition of the Stencel Aero Engineering-developed DART system. The Directional Automatic Realignment of Trajectory system is a bolt-on subsystem that was used on earlier seats including the F-100D ejection seat. The DART is a lanyard connected to the cockpit floor and connects to the seat via a frame to position it and a set of braking reels. As the seat leaves the cockpit the DART automatically deploys and resists seat tumbling by the controlled pay-out of the line. The line spools off the reel at the end of travel and the seat continues its flight.

A Stencel Aero Engineering-developed parachute spreader gun was also implemented in the ESCAPAC series of seats for more rapid parachute deployment. This spreader gun is attached to the skirt of the parachute and as the parachute nears line stretch is fired to radially deploy a number of slugs. These slugs open the skirt of the parachute to fill it more rapidly.

Returning to the issue of high-speed ejection, another concept developed during the 1950s-1960s was the concept of a detachable nose capsule. As previously mentioned, the Germans had experimented with this concept in the 1930s, but the difference in speed made this a more dangerous issue. The Bell X-2 was indeed flown with a nose capsule. Later designs were developed for the F-8 Crusader and the F-104, although neither was fielded. Several other nose capsules were examined but none were fielded.

A similar concept was taken for the F-111 aircraft. The entire cockpit of the F-111 was developed by General Dynamics and McDonnell to be a self-contained ejection module. This module was developed as an integral portion of the airframe and included not only the cockpit itself, but also a section of the front edge of the wings known as the glove vane. This area helped provide aerodynamic stability during high-speed ejections. The crew escape module is separated from the aircraft by linear explosive strips that sever the fuselage, guillotines which sever the control lines and a very large rocket to force the crew escape module away from the aircraft. The module is additionally stabilized by a drogue parachute and flaps that are deployed on aircraft separation. The module is then recovered by means of a 70ft diameter parachute and a series of airbags under the module that work in combination to cushion the decent of the more than three thousand pounds of module and crew. The module is equipped with complete survival kits and is designed to float with several flotation and self-righting bladders. The pilot’s control stick can be connected to a pump to help control seepage into the cockpit area when afloat.

The mid-to-late 1960s saw minor improvements and changes to seats, including the introduction of a seat separation rocket to the ESCAPAC seats. These rockets were mounted on the shoulder of the seat and would fire after the survival kit and inertia reel were unlocked. The thrust of the rocket was vectored up and forward to allow the seat to rotate aft off the airman and his personal equipment.

Another seat to see much modification was the C-2 series of seats. For high altitude flight the seat had been modified by the addition of a stabilizing/retardation drogue parachute. This version was designated as the S/R-2 and was used in variants of the F-104. The A-12, YF-12A, and SR-71 series of aircraft used a version of the C-2 which had a headrest containing a different style of drogue, and incorporated a backpack parachute equipped with its own drogue parachute to decelerate and stabilize the occupant after seat separation. This version did away with the arm nets as the pressure suits used with the seat were not fully compatible with the nets and the suit provided some protection for the arms. The series of seats was modified further by a simplification of the seat bucket shape, deletion of the thigh guards and addition of an improved drogue on the seat. This version was used in the YF-12A and SR-71 aircraft and is known as the SR-1 seat. The main functional difference with this version is that seat separation is delayed until the seat is below the barostatic altitude set on the dual barostats on the headrest. The parachute is also different, deleting the drogue and incorporating a different gun for deployment.

Martin-Baker began to develop improved versions of their seats during the 1960s as well. The Mk. 8A for the T.S.R.-2 was optimized for high altitude and speed while the Mk. 9A for the Harrier was more developed for the low speed, high sink rate area of the envelope. One major feature shared by these seats were the mounting of the seat bucket to the main beam assembly via a pair of tubes bolted to the front of the beam. The Mk. 9A had a more conventional parachute location in the backrest, while the Mk. 8A used a headrest mounted main recovery parachute, which allowed for a slightly faster deployment with less risk of entanglement with the seat. This location of the main parachute is now used in many seats.

During the late 1960s and early 1970s the USAF and McDonnell-Douglas worked together to develop a new Advanced Concept Ejection Seat. Using lessons learned over the past few decades, which included both the Korean and Vietnam conflicts, they sought to improve aircrew safety. Among the features tested in the ACES program were an electronic sequencer to make a decision as to the timing of the recovery based on the altitude and airspeed at the time of ejection. This system utilized a set of pop-up pitots and a static air pressure transducer to determine the airspeed and altitude as the seat left the aircraft. (Some earlier seats, most notably the Stencel S-III-S and some North American Aviation seats, used a system which was actuated by the aircraft systems.) This system would determine which mode of activation the seat should use. For example, at high-speed the seat would determine it should deploy the drogue parachute. At low speed the drogue would not be deployed to save time. Instead, the main recovery parachute would be mortared off the seat for rapid deployment. The seat also featured arm paddles similar to the F-106 Weber seat’s. The backrest of the seat would separate along with the airman to protect his back during landing. The survival kit was packed in a soft pack and stowed under the seat pan lid. This lid was hinged at the front and would rotate at seat separation to allow the kit to slip off the rear edge.

Complexity, cost and weight issues led to the program being reworked with a new seat developed. This seat, the ACES Ii is much lighter and simpler to produce. The seat chassis is designed ergonomically to assist in keeping the aircrew comfortable and in good posture for ejection. The Stabilization Package (STAPAC) rocket was installed on the bottom of the seat to prevent excessive pitch of the seat. Using a gyroscopic weight spun up by a gas charge, the STAPAC would rotate a rocket under the seat counter to the direction of pitch.

Later seats from Martin-Baker such as the Mk. 14 NACES and Mk. 16 series featured digital sequencers for more varied response to the ejection conditions. The Mk. 16 series utilizes a new structure arrangement of twin catapult tubes forming the beam assemblies. This echoed the designs of the Stencel S-III-S, and earlier Swedish designs from WW-II and before. The Mk. 16 also uses a redesigned underseat rocket.

The 4th Generation Ejection Seat prototype was developed as an off-shoot of the USAF Crew Escape Technologies (CREST) seat program. In concert with the USN and Aerojet Propulsion, the 4th Gen seat used an ACES II seat chassis with a special designed four nozzle rocket system. This system used an inertial guidance system and was optimized for ground avoidance, stability, and parachute deployment. The seat was capable of damping pitch, roll and yaw deviations by varying the thrust of the four pintle-controlled nozzles.

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