Damage mitigating control of a reusable rocket engine: part I - life prediction of the main thrust chamber wall

This paper is the first of the two parts and investigates the damage phenomena in the coolant channel ligament of the main thrust chamber of the Space Shuttle Main Engine (SSME), which are characterized by progressive bulging-out and incremental thinning leading to eventual failure by tensile rupture. A creep damage model is analytically derived based on the theories of sandwich beam and viscoplasticity. The objective of this model is to generate a closed form solution of the wall thin-out in real time where the ligament geometry is continuously updated to account for the resulting deformation. The creep damage model has been verified for both single-cycle and multi-cycle stress-strain behavior, and the results are in agreement with those obtained from the finite element analyses and experimental observation. Due to its computational efficiency, this damage/life prediction model is suitable for on-line applications of decision and control, and also permits parametric studies for off-line synthesis of damage mitigating control systems. The second part, which is a companion paper, develops an optimal policy for damage mitigating control of the SSME.