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 sub- sequent 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 se- quence 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 5-27