Design of Life Extending Controls Using Nonlinear Parameter Optimization

Summary This report presents the conceptual development of a life extending control system where the objective is to achieve highperformance and structural durability of the plant. A life extending controller is designed for a reusable rocket engine via damagemitigation in both the fuel and oxidizer turbines while achieving high performance for transient responses of the combustion chamberpressure and the O2/H 2 mixture ratio. This design approach makes use of a combination of linear and nonlinear controller synthesistechniques and also allows adaptation of the life extending controller module to augment a conventional performance controller ofa rocket engine. The nonlinear aspect of the design is achieved using nonlinear parameter optimization of a prescribed controlstructure. 1. Introduction Systems with high performance requirements and high power densities such as the Space Shuttle Main Engine (SSME),hypersonic propulsion engines, and gas turbine engines often have a small number of critical components that operate close tomechanical design limits. The critical components are also indicators of the effective lifetime of the entire system. These componentsoften experience maximum stress conditions during transients, and it is during such transients that large decrements in the componentlife are experienced. Possible damage modes include spalling, creep, corrosion, and fatigue. Simply minimizing stress levels is notalways a solution to these problems because it will typically result in an excessive loss of dynamic performance.The effect of thermal transient loading on the Space Shuttle Main Engine (SSME) turbine blades during startup and shutdownis a typical example of the above scenario. It was against the backdrop of the durability problems of the SSME that the concept oflife extending control (LEC) has evolved. The fundamental concept of life extending control is to control the rates of change andoperating domains of some performance variables to minimize damage (or damage rates) of the critical components whilesimultaneously maximizing the dynamic performance of the plant. While the life extending control technology was developedinitially for the SSME, it has broad application to many situations where both rapid response through transients and long life arerequired.The fundamental concept of life extending control has been forwarded by Lorenzo and Merrill (1991a and 1991b). Thefollowing basic approaches have been considered: (1) implicit life extending controls, which use current cycle based damage laws,and (2) direct life extending controls which assume the development of a continuous form of damage law. The availability of acontinuum damage model allows a more straightforward development of the life extending control concept and, hence, a simplerimplementation. Inthe life extending control implementations that have been considered, in addition to the plant and the performancecontroller, a su'uctural estimator must be addended which provides the stress, strain, and temperature states of the criticalcomponents. These are used by an appropriate continuum damage model which in turn provides estimates of the current damagerates for the damage controller. A continuum fatigue damage model based on the local stress method has been developed by Lorenzo(1994).Ray et al. (1994a and 1994b) have shown that, in an open-loop setting, it is possible to reduce the fatigue damage rate andaccumulation in the turbine blades of a reusable rocket engine (e.g., the space shuttle main engine) with little sacrifice in plantperformance. Their damage reduction procedure, however, is based on an extensive off-line optimization and does not takeadvantage of on-line damage predictions or measurements. Also, the resulting feedforward signal is optimized for a particular setof initial conditions and a maneuver which must be specified apriori. This method may not be applicable to maneuvers and/or initialconditions not used in the optimization procedure. Dai and Ray (1996) applied the same procedure to creep damage in the main thrustchamber wall of the same rocket engine.NASA TP-3700 1