A study in body ballistics: seat ejection

Abstract

Escape from high-speed aircraft has necessitated the development of ejection seats. This introduced the physiological problem of the maximum acceleration and rate of change of acceleration which can be sustained by the seated human body along the vertical axis of the spine. The ballistic behaviour of the human body has been studied by various experimental techniques. Natural frequencies and damping characteristics of the ejected system (man and seat) have been determined by subjecting the instrumented body to continuous vibrations over the frequency range from 1 to 20 c/s, and also by investigation of its response to spike- and step-force functions in sledge-hammer and seat-drop experiments. Results with different cushion and spring systems interposed between man and seat were compared and related to accelerometer records obtained from live subjects on ejection test rigs. The data derived from these experiments has been studied on an electronic servo simulator and the general physiological requirements for ejection-seat guns have been defined. Under these conditions a rate of change of acceleration of 300 g/s with a maximum peak acceleration of 25 g should be considered as limiting factors for accelerations tolerable by the body. This implies that the characteristics of the acceleration applied to the seat should be less than these figures by an amount depending on the elasticity of the cushion employed. It is suggested that the optimum duration of the thrust is approximately 0⋅23 s and a maximum overshoot in peak acceleration in the body would result with a rate of change of acceleration of 400 g/s applied to the seat.