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

The goal of damage-mitigating control in reusable rocket engines is to achieve high performance without overstraining the mechanical structures; and the major benefit is an increase in structural durability with no significant loss of performance. This sequence of papers in two parts investigates the feasibility of damage mitigating control of a reusable rocket engine similar to the Space Shuttle Main Engine (SSME). The challenge here is to characterize the thermo-mechanical behavior of the structural materials for damage prediction in conjunction with dynamic performance analysis of the thermo-fluid process in the rocket engine, and then utilize this information in a mathematically and computationally tractable form for synthesizing decision and control algorithms. This paper is the first part and investigates the damage phenomena in the coolant channel ligament of the main thrust chamber of a rocket engine that 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 closedform 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 examined 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 rocket engine.