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The operation of each component and subsystem is discussed in the following paragraphs. The operation of the system as a whole is discussed later in the chapter. Explosive charges are contained in an ejection gun initiator, JAU-56/A (figs. 5-3 and 5-4), and a secondary cartridge. Gas pressure from the seat firing system or the aircraft command sequencing system operates twin firing pins in the ejection gun initiator to fire the explosive charge. Gas enters the manifold valve through one or both of the inlet ports, depresses the check valves, and passes down through the vertical bore into the initiator. Gas pressure acts upon the twin firing pins in the initiator, shearing the shear pins and forcing the firing pins down to strike the percussion caps and ignite the explosive filling. The gas pressure generated within the catapult- 1. Passes to the ballistic latches to operate the pistons, which retain the multipurpose initiator static lanyards. 2. Propels the catapult piston upwards. The initial movement of the piston forces the spring-loaded top latch plunger out of the breech groove back into the barrel latch (fig. 5-4). The piston continues to rise, thrusting against the top crossbeam of the seat, the upward movement causing the shaped end of the top latch plunger to ride out of, and disengage from, the barrel latch. Further upward movement of the piston uncovers the secondary cartridge, which is fired by the pressure and heat of the initiator gas. After approximately 38 inches (965mm) of travel, the piston head strikes the guide bushing and shears the three dowel screws. After a further 4 inches (101mm) of travel, the piston separates from the barrel and moves away with the ejected seat. The main beams assembly supports the major components of the ejection seat. The operation of the components supported by the main beams assembly is discussed in the following paragraphs. SHOULDER HARNESS CONTROL SYS-TEM.- When the ejection control handle is pulled, gas from the RH seat initiation system is piped into the breech to operate the cartridge. The gas also passes to the operating piston in the governor housing, forcing it upwards to operate the trip lever and bring the locking pawl into contact with the ratchet wheel. The gas from the impulse cartridge in the breech impinges on the end of the piston forcing it along the cylinder. Horizontal movement of the piston is transmitted via the threaded drive screw to rotate the splined shaft, spool assemblies, and ratchet wheel, which winds in the webbing straps to pull back and restrain the occupant's shoulders. The engaged locking pawl locks the spools in the restrained position. Withdrawal of the webbing straps at excessive speed causes the two governor pawls to rotate outwards under centrifugal force and engage the teeth on the housing, preventing rotation of the splined shaft and spool assemblies and further withdrawal of the straps. This system prevents the occupant from being thrown forward on crash landing or sudden deceleration if the shoulder harness control lever is in the disengaged position. Easing of tension on the webbing straps allows the pawl springs to reassert themselves and disengage the pawls from the teeth, permitting free withdrawal of the straps again. PARACHUTE DEPLOYMENT ROCKET MOTOR.- Upon seat ejection, gas pressure from the primary and secondary cartridges passes to the rocket igniter cartridge to fire the rocket, shearing the flange of the rocket igniter cartridge. As the rocket moves upward, the stirrup slides down the rocket and aligns itself directly below the thrust axis line to extract and deploy the personnel parachute. In the event of sequencer failure, gas entering the unit through the gas inlet ports from the harness release unit cartridge or the emergency restraint release cartridge will initiate the secondary cartridge to face fire the primary. ELECTRONIC SEQUENCER.- When the ejection seat is fired, two onboard thermal batteries are immediately energized, supplying usable electrical power to the sequencer after just 100 milliseconds, with the seat having travelled about 5 inches up the ejection catapult. The sequencer's microprocessors then run through an initialization routine, and by 120 milliseconds the sequencer is ready and waiting to perform. As the seat rises from the cockpit, two steel cables (approximately 42 inches) are pulled from the multipurpose initiators, actuating two pyrotechnic cartridges. The gas generated by these two cartridges is piped around the seat to perform the following functions: Initiate the underseat rocket motor. Deploy the pitot tubes from the sides of the seat headbox. Close two electrical switches (sequencer "start" switches). The sequencer responds to the closure of either start switch by changing to the "ejection" mode. The switch starts an electronic clock, and all subsequent events are timed from this point. In the absence of a start switch signal, the sequencer will simply continue in the "wait" mode. This mode is a safety feature designed to ensure that the drogue and parachute can only be deployed after the seat has physically separated from the aircraft. The ignition of the underseat rocket motor is timed to occur just as the seat separates from the ejection catapult, at about 200 milliseconds, so as to maintain a uniform vertical acceleration profile on the seat and occupant. The motor has a burn time of 250 milliseconds. Once the sequencer is switched into the "ejection" mode, its first action is to electrically fire the drogue deployment canister, which occurs precisely 40 milliseconds after start switch (approximately 220 milliseconds from seat initiation), while the seat rocket motor is burning. This happens regardless of the speed and altitude conditions. The sequencer then enters its most crucial period, when it will sense the seat's airspeed and altitude and choose the appropriate timings from a set of five available sequences. This is done during a 60 millisecond "environmental sensing time window" that starts just after the drogue canister is fired, and is completed before the drogue is fully deployed and pulling on the back of the seat. The sequencer measures the speed and altitude from the information it receives from three types of sensor: pitot pressure, base pressure, and accelerometer. Several samples of each parameter are taken during the environmental window. These are used to determine the ejection conditions. The sequencer then selects the appropriate timings for the remaining events, known as "mode selection," and completes the sequence accordingly. Overview of Sequencer Electronics and Hardware.- The electronic sequencer and its thermal batteries are packaged in two separate units. The sequencer and associated electronics are contained in a cast aluminum enclosure, which is mounted between the main seat beams directly beneath the parachute container headbox. A total of nine shielded cables attached to the housing transmit electrical signals to and from the sequencer. The input and output actions are as follows : Input- thermal battery power supply lines (2) start switch circuits (2) output- drogue deployment canister squib-fire circuit drogue bridle release squib-fire circuit parachute extractor squib-fire circuit seat harness release squib-fire circuit seat harness release (backup) squib-fire circuit The sequencer also has air pressure couplings to connect it to the two pitot pressure tubes on the headbox and the two base pressure sensing points inside the main seat beams. A functional block diagram of the electronic circuitry is given in fig. 5-29.
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