Ejector seat
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In (mostly military) aircraft, the ejector seat is a system designed to rescue the pilot or other crew in the event of the aircraft becoming unflyable. In most designs, the seat is propelled out of the aircraft by a rocket motor, carrying the pilot with it. The concept of an ejectable escape capsule has also been tried. Once clear of the aircraft, the ejector seat deploys a parachute, and descends safely to earth.
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History
While a bungee-assisted escape from an aircraft took place in 1910, the ejector seat as we recognise it today was invented in Germany during World War II. Prior to this, the only means of escape from an incapacitated aircraft was to jump clear, and in many cases this was difficult due to injury, the difficulty of egress from a confined space, the airflow past the aircraft and other factors.
The first ejector seats were developed during the war by Heinkel. Early models were powered by compressed air and the first aircraft to be fitted with such a system was the Heinkel He 280 prototype jet fighter in 1941. One of the He 280 test pilots, Helmut Schenk, became the first person to escape from a stricken aircraft with an ejector seat on January 13 1942 after his control surfaces iced up and became inoperable. This aircraft never reached production status, and the first operational type to provide ejector seats for the crew was the Heinkel He 219 night fighter in 1942.
In late 1944, the Heinkel He 162 featured a new type of ejector seat, this time fired by an explosive cartridge. In this system the seat rode on wheels set between two pipes running up the back of the cockpit. When lowered into position, caps at the top of the seat fitted over the pipes to close them. Cartridges, basically identical to shotgun shells, were placed in the bottom of the pipes, facing upward. When fired the gases would fill the pipes, "popping" the caps off the end and thereby forcing the seat to ride up the pipes on its wheels, and out of the aircraft.
After World War 2, the need for such systems became pressing, as aircraft speeds were getting ever higher, and it was not long before the sound barrier was broken. Manual escape at such speeds would be impossible. The United States Army Air Corps experimented with downward-ejecting systems operated by a spring, but it was the work of the British company Martin-Baker that was to prove crucial.
The first live flight test of the M-B system took place on July 24th, 1946, when Bernard Lynch ejected from a Gloster Meteor Mk III. Shortly afterwards, on August 17th, 1946, 1st Sgt. Larry Lambert was the first live US ejectee. M-B ejector seats were fitted to prototype and production aircraft from the late 1940s, and the first emergency use of a Martin-Baker seat occurred in 1949 while testing the Armstrong-Whitworth AW.52 Flying Wing.
Early seats used a solid propellant charge to drive the seat out, by exploding the charge inside a telescoping tube attached to the seat. Effectively the seat was fired from the aircraft like a bullet from a gun. As jet speeds increased still further, this method proved inadequate to get the pilot sufficiently clear of the airframe, so experiments with rocket propulsion began. The F-102 Delta Dagger was the first aircraft to be fitted with a rocket propelled seat, in 1958. MB developed a similar design, using multiple rocket units feeding a single nozzle. This had the advantage of being able to eject the pilot to a safe height even if the aircraft itself was on or very near the ground.
In the early 1960s, deployment began of rocket-powered ejection seats designed for bailout at supersonic speeds, in such planes as the F-106 Delta Dart. Six pilots have ejected at speeds exceeding 700 knots (805mph) and the highest altitude a M-B seat was deployed at was 57,000ft (from a Canberra in 1958). It has been rumoured but not confirmed that a SR-71 pilot ejected at Mach 3 at an altitude of 80,000ft. Despite these records, most ejections occur at fairly low speeds and at fairly low altitudes.
Pilot safety
The purpose of an ejection seat is pilot survival, not pilot comfort. Many pilots have suffered career-ending injuries while using ejector seats, including crushed vertebrae. The pilot typically experiences an acceleration of about 12 to 14 g (120 to 140 m/s²). Western seats usually impose lighter loads on the pilots, ex-soviet technology often goes up to 20-22 Gs. Career-ending is quite common, partly because eastern military pilots usually continue to fly into their late-40s or early-50s, while most western jet jockeys quit in their late-30's to pursue civilian life.
By December 2003, Martin-Baker ejector seats had saved 7028 lives. The total figure for all types of seat is unknown but must be considerably higher.
Soviet K-36 series
The utmost in contemporary ejection seat technology is incorporated in the K-36 series designed by the Russian Zvezda bureau. The K-36D has repeatedly stunned the world with seemingly impossible crew rescue events in a series of high-profile accidents affecting ex-soviet fighter planes which participated at western airshows.
The USA conducted tests of the upgraded, digitally-controlled "K-36 3.5" system, but refused to standardize it despite exceptional results. Some of the reasons were:
- The K-36 system is more than twice as heavy as the western ACES II (up to 225 kilograms vs. 90 kilograms)
- The bulbous headrest / parachute container of the K-36 seat greatly reduces the pilot's backwards field of vision
- The US mentality favours better armed and better cloaked planes, so that the pilot never gets shot down. Thus, the performance of crew ejection systems is less important. This, however, fails to address the issue of peacetime accidents, which account for the majority of crashes.
Non-standard ejection systems
The F-104 Starfighter was equipped, uniquely, with a downward firing ejection seat as the T-tail was judged likely to cut the pilot in half. In order to make this work, the pilot was equipped with "spurs" which were attached to cables that would pull the legs inwards so the pilot could be ejected. Note that such a system is of no use on or near the ground. Aircraft designed for low-level usage sometimes will have ejector seats which fire through the plastic of the canopy, as waiting for the canopy to be ejected is too slow. Many aircraft types (e.g. BAe Hawk) have an explosive cord (MDC - Miniature Detonation Cord) embedded within the perspex of the canopy, which shatters it simultaneously with the firing of the seat. Soviet Yak-38 VTOL naval fighter planes were equipped with automatically activated ejection seats, mandated by the notorious unreliability of their vertical lifting powerplants.
Some aircraft designs, such as the General Dynamics F-111, do not have individual ejector seats, but instead, the entire section of the airframe containing the crew can be ejected as a single capsule. In this system, very powerful rockets are used, and multiple large parachutes are used to bring the capsule down, in a manner very similar to the Launch Escape System of the Apollo Spacecraft. On landing, an airbag system is used to cushion the landing, and this also acts as a flotation device if the capsule lands in water.
Ejector seats in other aircraft
The Kamov Ka-50 was the first helicopter to be fitted with an ejector seat. The system is very similar to that of a conventional fixed-wing aircraft; the main rotor is equipped with explosive bolts and is designed to disintegrate moments before the seat rocket is fired.
Controversially, the US space shuttle is not fitted with ejector seats nor an escape capsule system of any kind. The soviet's shuttle copycat "Buran" was planned to be fitted with K-36DM seats, but the first, unmanned, flight in 1988, on which the crew cabin was not yet installed, would be the only one.
Passenger planes are unlikely candidates to receive ejection technology any time soon. The weight, complexity and economic requirements prohibit fitting jetliners with seat or capsule based ejectors and the benefits are not obvious, because most accidents happen during take-off or landing. However, some ultralight and Cessna 172 category general aviation aircraft have been refitted with ballistically deployed parachutes recently. These canisterized packages save the entire airframe upon deployment and thus could be considered an extreme capsule-based ejection system.
External links
- Ejectionsite.com - "... details on one of the most interesting fields of engineering." (http://www.ejectionsite.com/)de:Schleudersitz