Investigations on the effects of acceleration on the human body have become a traditional research topic in aerospace medicine with the introduction of high performance aircraft before and during World War II as a military necessity to win or at least survive air combat. Furthermore, with the technical ability to leave the atmosphere of the earth and reach stable orbits in space, a necessary pre-requisite for early astronauts was the ability to withstand the accelerations induced by powerful rocket engines for prolonged periods of time; a number of high-performance human centrifuges has been constructed for just this purpose. But of course, there was also the necessity to deal with the high-impact crash situation, quite similar to the automotive research community.
In aerospace research it has become useful to distinguish between short-term (< 1 s) and long-term (> 1 s) accelerations. While sustained acceleration in the order of several G - with primary impact on hydrostatic phenomena in the human body - is unique to aerospace physiology, due to high engine power in rocket-type spacecraft or high radial acceleration in air combat manouvres, the topic of short-term acceleration has common interfaces with crash research in the automotive area. However, in flight there are additional factors to be considered, especially associated to emergency egress from high speed aircraft, usually performed by ejection systems with parachute descent. As a minor topic from military aviation, also catapult starting and deck landing during carrier operations may be consi-dered with intermediate durations of 0.5 – 2 s.Due to the much higher economic impact and passenger transit numbers of ground-based traffic, crash research is a major item in automotive research compared to aerospace efforts, producing a large data base which principally leads to transfer of knowledge from automotive to aerospace research in this area. The aim of this paper is to submit some results from aircraft ejection technology, which may possibly be used to reconsider crash protection in road vehicles.
The main reason to use ejection systems for disabled aircraft escape is the high airspeed of modern jet-propulsed (military) aircraft. In conventional parachute egress, like in much slower piston-driven aircraft, the escapee has to get free from the aircraft by means of his own muscular force, in order not to be hit by any part of the vehicle. However, the high speed of jet aircraft makes a deadly hit by any element of the structure, especially the tail fin, a very likely situation, so that the initial acceleration of the seat – or, better, escape system – has to be high enough to instantly separate from the aircraft. The energy for this initial acceleration is usually provided by rocket or explosive devices "burning" for about 250 ms. Peak acceleration slopes are as high as 300 G/s, with an average acceleration in excess of 12 G; target structure for these forces in the human body is the spine, which has to support a considerable part of the accelerated body mass. The tolerance limit is approached with approximately 23 G.During the first part of the ejection process, the seat leaves the wind protection of the cockpit and is immediately braked down by air resistance - the windblast - before departing from the aircraft in a parabola trajectory. Especially at lower altitudes with high air density, the air impact alone is likely to induce injuries on the escapee, especially if arms or legs are not securely restrained. Furthermore, the windblast produces an arterial pulse which typically can lead to subconjunctival hemorrhage.Finally, when the system has sufficiently decelerated under the action of a drogue or stabilizing para-chute within 2 – 3 s, the main parachute produces an opening shock with peak accelerations in the order of 20 G which even increases with higher altitudes because of faster opening, opposite to the beneficial effect in case of the windblast. The descent velocity of the system after full opening is in the order of 10 m/s, finally producing another short-duration acceleration when hitting the ground, with some landing injuries to be expected.
The different phases of aircraft ejection with the associated strain on the human body structure described here may be useful for stress prediction in certain crash situations.