As with the most modern seats, the S-III-S is equipped with torso restraint inertia reels that are gas fired on ejection to retract the pilot to the upright position. It is fitted with a limb restraint system to hold the pilot's legs firmly against the seat during the first few seconds after ejection. The limb restraint garters are attatched by lanyards through a set or ratchet assemblies and on to the catapult manifold, which remains in the aircraft during ejection. The result of this routing is that as the seat rises up the catapult tubes, the pilot's legs are drawn back by the lanyards to be held against the seat by the ratchet assemblies. The lanyards have a rip stich point that tears free once the legs are firmly in place.
The S-III-S is normally equipped to eject through the canopy for more rapid response. This is accomplished in two ways. First on the Harrier, the canopy is shattered by a line of Mild Detonating Cord (MDC) which is applied to the canopy, and visible in most photos of the Harrier. This is fired as part of the seat ignition and shatters the canopy enough for the pilot to clear easily. In the RAF, there were some incidents that indicated that the pilots may have been being injured by small fragments of the plexiglass/acrylic canopy cutting their exposed faces, and they made it manditory that the pilots fly with thier helmet visors down at all times to protect their faces. Should the canopy destruct system fail, the secondary ability of the seat is its integral canopy breakers. The four breakers are mounted on the headrest such that the right aft breaker strikes the canopy first, which cracks it. The left rear breaker strikes next, propagating the breaks, and then the front pair strike to push aside enough of the transparency for the pilot to get through uninjured.
Seat stability is a special concern, and is controlled in two ways by the S-III-S. One of these is the DART® system which consists of a lanyard that is connected to the cockpit floor and plays out of a mounting system under the seat bucket at a set deployment force. This keeps the seat from pitching or rolling significantly while the Wind Oriented Rocket Deployment (WORD) system begins to withdraw the drogue chute. The WORD rocket can be seen clearly in the center of this photo. Once the seat has reached the full length of the DART bridle it disconnects and the seat continues with the drogue stabilizing it. The DART implementation on the S-III-S includes a fold-down panel that has aerodynamic stabilizing qualities as well.
Parachute deployment is accomplished by a series of events that include the WORD rocket firing, and launching the drogue, the headrest opening at the preset time, and the WORD and Drogue pulling the chute lines to full extension, whereupon a spreading charge fires the parachute skirt full open for rapid deployment. (This only occurs in Mode 1, low and slow ejection.) Seat separation follows with the parachute force aligning the seat/man mass with the parachute drag vector. Once the force builds to a set point, the seat/man separation lanyard actuates the separation system.
The separation lanyard rotates the seat panel release shaft. This bellcrank activated shaft allows the seat pan to release from the seat bucket assembly, and releases the leg restraint ratchet assemblies. It also activates a gas-generator to fire a guillotine to cut the inertia reel straps. These actions leave the crewman free of the seat structure, with the survival kit, seat pan, and emergency oxygen system.
While I have been discussing mainly the Harrier seat, versions of the seat have been used in such aircraft as the Alpha Jet, F-16 FSED prototypes, the A-7 Corsair, the Argentine 1A-63, the YF-17 prototype, the Northrup F-20, the Beech Starship, and the Scaled composite ARES.
|Three different varients of the S-III-S, Left is the SJU-4, Middle an early prototype, and right is the Escapac Replacement (ER) SJU-8 used in the Chance/Voight A-7 Corsair II|
These systems often have different appearance and nomenclature although they use the same basic systems. UPCo is looking toward the future, and they are working on developing new and improved versions for possible use. The Stencel S-4-S system is a prototype seat based on the S-III-S with additional folding stabilizers and other improvements for high speed ejection.
Pictures compliments of
Universal Propulsion Co, Inc, (UPCO)
Thanks to Larry Hadley, Mid-Western Regional Manager, UPCO
Additional photos of Stencel seats can be found on these pages:
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