It is almost 51 years since, on 24 July, 1946, the first live test-ejection took place using a Martin-Baker ejection seat, and 49 years since Jo Lancaster made the first emergency Martin-Baker ejection from the prototype Armstrong Whitworth AW.52 flying wing.
Those ejections used pre-production versions of Martin-Baker's seats - each little more than a seat and parachute which could be fired from an aircraft. Today's Mk16 seat is more akin to a small, high-performance, fly-by-wire, rocket-powered aircraft, complete with its own environmental-control and life-support systems.
So far, the three applications for the Mk16 seat family are: the Dassault Rafale; the Eurofighter EF2000 and both candidates for the US Joint Strike Fighter (JSF) programme, the Lockheed Martin X-35 and the Boeing X-32.
The whole structural concept of the Mk16 seat is different from those of its forebears. They were effectively seats to which various propulsion and survival mechanisms had been attached; in the Mk16, the propulsion-system components form the structure of the seat itself.
For durability and lightness, most of the seat is made of light alloy, with Kevlar/carbon composites being used for the personal survival-kit container, the backrest and fairings.
The main structure is based around the U-shaped ejection gun. The two light-alloy telescopic barrels (or tubes) of this gun, 57mm in internal diameter, run up each side of the seat, closed ends uppermost. The ejection guns each have a stroke of 1,000mm. They are joined at their bases by a breech containing the gun propellant cartridge and two initiator cartridges. These gun-tubes run in guide-rails which are bolted firmly to the aircraft structure and which in the aircraft when the seat is ejected.
Previous ejection guns were literally that: a small gunpowder charge lit off a greater quantity of nitro-cellulose, which ejected the seat - resulting in very-high-peak g loads being imposed on the pilot. The new gun uses what is effectively a stick of rocket fuel as a charge, and this burns (still extremely rapidly) in a more progressive manner.
As Brian Miller, Martin-Baker's head of marketing, explains, this rocket "gun" also provides an elegant solution to the problem of adjusting ejection-seat performance to the weight of the occupant. "The use of a choked cartridge that produces a controlled energy output means that the gun automatically compensates for a higher ejected mass," he says.
This automatic, integral, compensation allows for a much greater spread of aircrew size for the seat. The specification of the simplified lightweight Mk16Lseat for the Beech T-6 MkII calls for a spread of pilot nude weights of 52-111kg, and Martin-Baker has even been asked to make a seat to accommodate a pilot of just 47kg weight.
Forward of the gun-breech at the base of the seat is mounted the under-seat rocket motor, which is the principal propulsion unit for the seat, responsible for sending it well clear of the aircraft once the ejection gun has ejected it from the cockpit. This rocket motor is a steel tube some 445mm long and 86mm in outside diameter, with end-caps each containing two angled nozzles .The nozzles on one side of the motor are larger than those on the other, to give an asymmetric thrust to roll the seat slightly as it leaves the aircraft. This ensures that the parachute will be clear of the seat when it deploys, and able to correctly re-align it as the canopy inflates. For two-seat aircraft, the front and rear seats have rocket motors with opposing asymmetric thrusts, so that they will diverge to opposite sides of the aircraft, increasing the separation between them.
The solid-rocket propellant is cast in the form of thick-walled concentric cylinders, separated by air-gaps to ensure that the maximum possible surface area of the propellant is exposed to the flame from the ignition cartridge.
Each of the rocket nozzles is hermetically sealed by a stainless-steel disc. When the fuel is fully ignited, these caps rupture, releasing the rocket gases to the atmosphere. The rocket motor has a peak output of 20kN(4,500lb) thrust, but burns for only 0.25s.
Parachutes
The small drogue chute, which stabilises the ejected seat before the main parachute opens, is stored at the upper back centre of the seat, with its bridle attached at three points to the seat structure (one at shoulder level, and two at the base) and is stowed in channels running down the back of the seat behind the gun tubes. This drogue is deployed by a small explosive cartridge when required.
The drogue itself is to a new design. Earlier drogues had only single-point attachment to the seat. Where the old design merely aligned the seat correctly, the new one also stabilises it. The drogue (which opens to form a 1.07m-diameter canopy) deploys progressively as the container moves away from the seat. The three-point attachment means that the drogue stabilises the seat in pitch and yaw, but allows it freedom in roll. When the drogue is released, the lower attachments are released first, thus ensuring that the seat is aligned before the main parachute is deployed.
The square aluminium-alloy box which contains the main GQ5000 aeroconical parachute made by GQ Parachutes is mounted at the top of the seat structure. This parachute has a flying diameter of 6.5m, and consists of 20 gores, or segments, each gore being made up of seven panels of ripstop nylon cloth and one of polyester netting, with Kevlar-tape reinforcing on all radial and circumferential seams. The gores are of differing colours (ranging from bright orange through white to dull green and sand) to allow a downed pilot to choose between high visibility (for rescue) and low visibility (for concealment from enemy forces). The nylon panels are arranged in decreasing porosity and increasing strength towards the top of the canopy. This ensures that the canopy fills progressively and that the seat occupant is not subjected to excessive g loadings as it deploys.
The canopy has two 1.35m-long leMoigne slots (slits surrounded by excess fabric, normally held tightly in place to keep the slit narrow, but which can be released to allow the fabric to balloon and the slit to widen), in rear-facing panels, which are opened by hand-lines for steering and controlling the forward glide-speed. At the base of every second panel is a water-pocket, which promotes rapid deflation of the canopy following a descent into water.
Tight packing
The main parachute container is airtight and, to ensure that the parachute occupies the minimum volume possible, it is packed into this container by a special machine. This packing process takes three days, under a pressure of 690bar (10,000lb/in2), resulting in an almost-solid block of nylon: "If you melted the nylon in, you'd have about the same volume," says Miller.
The main parachute is deployed by firing the container off the seat, with the lines unfurling before the canopy as they leave the container. In this way, the canopy is extracted from the container, and the lines are taut before the canopy starts to open. There is a small auxiliary parachute packed on top of the main canopy and attached to the parachute box. On deployment, this ensures that the discarded box is carried away from the main parachute.
On each side of the seat pan and on top of the main parachute box are aerodynamic panels, which are deployed to keep the seat in the correct attitude as it is ejected from the aircraft and before it is stabilised by the drogue. These aerodynamic surfaces are deployed by small explosive cartridges. On either side of the top of the seat are two hard steel points, designed to break the aircraft canopy as the seat hits it, if the canopy has failed to jettison.
The need for the deployable aerodynamic surfaces is a unique requirement stemming from the close proximity to the cockpit of the tall centre-line fin of the EF2000. This means that the drogue has to be deployed when the seat is further from the aircraft than in the case of, for instance, a twin-tailed design such as the McDonnell Douglas F-18 or any of the proposals for the US Joint Strike Fighter. Without the stability provided by the aerodynamic surfaces, the sudden deployment of the drogue could impose very high g-loadings on the crew.
Also deployed as the seat leaves the aircraft are two pitot tubes, which provide aerodynamic-pressure readings to the electronic sequencer which controls the whole operation of the seat. In the latest version of the seat (but not shown on our drawing) there are cooling-air ducts built into the back of the pilot's headrest.
The headrest sits well behind the back of the pilot's helmet in normal flying, to allow him/her maximum head-movement freedom, essential in a fighter. Because of this gap, the front of the headrest is made of a progressively crushable aluminium honeycomb, so that the impact of the pilot's helmet on to the headrest during ejection is cushioned.)
The modern ejection seat provides much more than just a means of rapid egress from a failing aircraft. It also acts as the interface with the aircraft while it is flying, and provides the pilot with an environmental-control system after ejection. Thus, for example, the aircraft's onboard oxygen-generating system (OBOGS) is plumbed through the seat, which also contains an auxiliary oxygen bottle. If the aircraft's OBOGS fails, the seat bottle takes over. If the OBOGS comes back on line, the auxiliary bottle shuts off.
Supplies maintained
Once the seat has been ejected from the aircraft and the main oxygen lines are disconnected and automatically sealed, the auxiliary bottle (mounted in the seatback) automatically becomes the primary source of oxygen for up to 30min of flying time. The seat also contains all the controls for electric adjustments for height and reach, and connections for telecommunications and anti-g suit supplies.
The main pressure lines for the pilot's high-altitude pressure-suit are also plumbed in and, when they are disconnected by ejection, the seat's own pressure reservoirs take over to ensure that the suit remains pressurised until the pilot has descended to a safe altitude.
The main seat-safety handle by the pilot's right thigh is interconnected with the aircraft's main warning panel, which shows "safe" if the seat is not armed. For the pilot to make an emergency exit from the aircraft on the ground, only two simple release actions are required. The pilot or rescuers can press the integral handle-latch and turn the safety lever beyond the normal "safe" position, to release all of his/her connections to the seat except for the harness, and the harness quick-release button can then be operated.
The pilot's shoulder-harness straps are connected to a piston-powered reel driven by a choked cartridge (having been triggered by gas from the seat-firing cartridge), which tighten and pull the pilot back into the seat as soon as the ejection sequence begins.
In a major change from previous seat designs, the pilot's leg restraints are passive: there are no separate leg-garters to do up, and there is no restraint on the legs until it is needed. In the EF2000 installation, the leg lines are clipped around the foot tunnels, so, simply by putting his/her feet on the pedals, the pilot has put them in the right place. The pilot's arm-restraints are built into the clothing, but connected through the quick-release fitting of the harness, so that all straps can be released with one movement. This allows a quick exit on the ground.
The restraints are attached to the seat through one-way snubbers and shear-rivets. Upward movement of the seat on ejection automatically tightens these restraints. The shear-rivets break as the correct tension is reached, and the snubbers keep the lines tight. The legs are held quite tightly, but the purpose of the arm restraints is merely to limit movement, so that they are not swept round behind the pilot's body, with the risk of being broken.
Unlike earlier seats, the Mk16A is installed, handled and removed from the aircraft fully armed, which means that there is no need to take explosive charges separately into the cockpit. The seat-handling system has been designed in such a way that it is impossible to attach a lifting sling to the seat if the safe/armed lever is not in "safe". There is, says Miller,"-no way of firing this seat other than by pulling the handle".
The attention paid to weight-saving means that, although this seat carries a great deal more equipment, it is lighter than the equivalent Navy Aircrew Common Ejection Seat used by the US Navy. Complete with the largest pilot for which it is designed, plus all his equipment and the other non-seat-specific items which are mounted on it, a Mk16A seat weighs some 225kg, compared with 89kg for the complete, but bare, seat.
Shorter time
All this means, says the company, that "-the crew is on a safe descent in a shorter time than with any other seat in the world". So far, the Mk16A has not been used in any of the more than 6,500+ successful ejections using Martin-Baker seats, but with it now entering full production, and the statistical likelihood that one in ten seats will be used in anger, it will not be long before its sophistication is put to the real test.
Source: Flight International
