When a person talks about Ejector seats, they are most commonly found in aircraft. 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. There have been exceptions; like the famous ejection seat in James Bond
's Aston Martin
in the film Goldfinger
, which was used to get rid of unwanted passengers. 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 which descends safely to earth.
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 WW2, 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. Today's ejector seats can still operate if the plane is safely stopped on the ground.
Since the 1950's the mechanism would automatically open up the parachute for the crewman. Despite what some movies show, the seat separates from the pilot, who is attached to the parachute. The sea.t detaches in a number of ways, either due to the parachute yanking the pilot from the seat, or a strap pulling them apart, or a rocket called an "ear-burner" activating and blasting the seat away from the pilot (guess why it's called an ear-burner.)
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.
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, and it was unusable during takeoff and landing. Later on, a more powerful upward-firing ejector seat became availible. 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 embedded within the perspex of the canopy, which shatters it simultaneously with the firing of the seat.
B-47, RB-47, XB-52, RB-66, XF-104, F-104 and USAF Development all had downward-firing ejection seats. YB-47, FJ-2, F-106A, F4H, P6M, and USAF Development all had upward-firing ejection seats.
The B-58 and some aircraft prototypes featured Ejection Capsules, where the pilot's seat was enclosed with a plastic cocoon that sealed into an egg-shape before ejecting in one piece. They had built-in parachutes and floatation devices, and were said to be roomy, unless you had size 12 shoes or disliked being forced into a fetal position when you ejected.
Some helicopters had ejector seats designed for use, such as the Russian Kaman Ka-50 Black Shark and the American AH-1 Cobra, whereupon activating the ejector system also triggers explosive bolts in the helicopter blades, making them fly off, followed by jettisoning the canopy, and using a tractor rocket system to eject the seat and person, with parachutes automatically deploying seconds later. I do not know what happens if there are other people in the helicopter, presumably they should probably eject as well.
Boeing, Lockheed, Northrup, and Stanley Aviation all manufactured ejector seats at one point. 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.
The purpose of an ejection seat is survival, not pilot comfort. Many pilots have suffered career-ending injuries while using ejector seats, including crushed vertebra. The pilot typically experiences a G force of about 12 to 14 times the Earth's gravity as he is hurled out of the airplane. Supposedly Air Force scientists had to cut several holes in the current model helmets because the wind shear could take an ejecting person's head off otherwise.
NASA's Gemini spacecraft had ejection seats, though earlier and later designs like Apollo and Mercury instead used an escape capsule. The Columbia space shuttle was equipped with ejection seats for NASA's first four shuttle missions, when only a commander and pilot were aboard. The system, however, was removed when NASA began flying larger crews. The Challenger accident rekindled the debate on how to create a safer ejection system for the crew. The original ejector seats weren't feasible, because it would mean only the pilot and shuttle commander could eject, and flying with only a 2-person crew would mean NASA couldn't do most missions. Many escape module designs were proposed, but most were dismissed as far too costly or unfeasible. Instead, all astronauts now wear pressure suits and parachutes during launch and reentry, known as the Inflight Crew Escape System.
Facts taken from Wikipedia, ejectionsite.com (a treasure trove of info), http://science.howstuffworks.com/ejection-seat1.htm and personal memory