Riding the rocket

22/10/2012 — Leave a comment

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Give me warp speed Scotty! We’re blowing this disco!!

The picture below captures the moment of aircraft and seat separation as the seats rockets takeover from the catapult mortar. You can see the seats pitot tubes extended at 45 degrees to sample dynamic air pressure for the seats flight computer, as well as the high G induced body slump of the pilot.

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The ejection seat carrying the pilot to safety is the SJU-17 NACES Mk.14 seat, a third generation system that can eject a pilot from zero-zero to max altitude-600 KEAS+ with a high probability of survival.

In the next picture in the sequence the seats rockets have ejected the pilot clear of the cockpit, the asymmetric rocket thrust, as shown by the rocket plumes differing lengths, is intended to roll the pilot out of the way of the vertical tail fins. The seats stabilising drogue parachute has already been deployed and can be seen in the detail.

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Then the parachute is deployed as the pilot separates from the seat. Parachute deployment is programmed to occur in less than two second for a low speed, low altitude ejection scenario (called Mode 1).

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At this point the pilot is decelerating rapidly and separating from the aircraft trajectory, important because you don’t want to fall through the subsequent fireball!

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And of course the destruction of the aircraft on impact.

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Ejection systems are a very interesting example of balancing various competing safety risks.

On the one hand too high acceleration can lead to unacceptable pilot injury. On the other hand too slow separation from the aircraft and they are not effective.

Designers also have to contend with differing weights of individual aircrew, which affects seat acceleration and parachute decelleration, with potentially lethal consequences for lighter weight (e.g. female) crew in some ejection modes.

Finally the seat must perform its mission over a broad flight envelope which demands ‘mode’ sensitive functionality to deal with the fundamentally different demands of ejection at high altitude and high speed versus ejection in a low speed, low altitude environment.

Of course the idea of ‘trading off’ one safety risk against another does fundamentally contradict the concept of Zero Harm, unless you insist on ceasing the activity altogether as the solution.

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