Rocket engine ignition structural shock

It is well known that an elastic compression wave develops during ignition of launch vehicles or rockets. This wave travels up the vehicle's structural stack, and creates significant shock loading on sensitive payloads at the top of the structure. For smaller vehicles, such as sounding rockets, having the capability to assess the effect of engine ignition structural shock in a simple and inexpensive manner is essential. This paper presents a mathematical and computational procedure to evaluate the amplification in steady state axial load and its associated bending moment due to transient loading from the rocket engine at ignition. Both liquid and solid rocket cases were simulated. The body of the rocket is discretized into a small number of elements which are connected by springs. The resulting system of coupled differential equations is solved using a 4th order Runge-Kutta algorithm, and the amplification factor due to the ignition shock is computed as a function of time after ignition. Maximum amplification factors, often in excess of 4, and bending moments for the various body elements are found to vary with location and time. An approximate result for the frequency to which secondary structure will be exposed is presented.