Carl F. Lorenzo

Damage-Mitigating Control of Mechanical Systems: Part I—Conceptual Development and Model Formulation

A major goal in the control of complex mechanical systems such as advanced aircraft, spacecraft, and power plants is to achieve high performance with increased reliability, availability, component durability, and maintainability. The current state-of-the-art in control systems synthesis focuses on improving performance and diagnostic capabilities under constraints that often do not adequately represent the dynamic properties of the materials. The reason is that the traditional design is based upon the assumption of conventional materials with invariant characteristics. In view of high performance requirements and availability of improved materials, the lack of appropriate knowledge about the properties of these materials will lead to either less than achievable performance due to overly conservative design, or over-straining of the structure leading to unexpected failures and drastic reduction of the service life. The key idea of the research reported in this paper is that a significant improvement in service life could be achieved by a small reduction in the system dynamic performance. The concept of damage mitigation is introduced and a continuous-time model of fatigue damage dynamics is formulated in this paper which is the first part of a two-part paper. The second part which is a companion paper presents synthesis of an open loop control policy and the results of simulation experiments for transient operations of a reusable rocket engine.

Damage-Mitigating Control of Mechanical Systems: Part II—Formulation of an Optimal Policy and Simulation

The objective of damage-mitigating control introduced in the first part of this two-part paper is to achieve high performance without overstraining the mechanical structures. The major benefit is an increase in the functional life of critical plant components along with enhanced safety, operational reliability, and availability. Specifically, a methodology for modeling fatigue damage has been developed as an augmentation to control and diagnostics of complex dynamic processes such as advanced aircraft, spacecraft, and power plants. In this paper which is the second part, an optimal control policy is formulated via nonlinear programming under specified constraints of the damage rate and accumulated damage. The results of simulation experiments for upthrust transient operations of a reusable rocket engine are presented to demonstrate efficacy of the damage-mitigating control concept.

A reusable rocket engine intelligent control

An intelligent control system for reusable space propulsion systems for future launch vehicles is described. The system description includes a framework for the design. The framework consists of an execution level with high-speed control and diagnostics, and a coordination level which marries expert system concepts with traditional control. A comparison is made between air breathing and rocket engine control concepts to assess the relative levels of development and to determine the applicability of air breathing control concepts to future reusable rocket engine systems.

Damage-mitigating control of a reusable rocket engine

The goal of damage mitigating control in reusable rocket engines is to achieve high performance with increased durability of mechanical structures such that functional lives of the critical components are increased. The major benefit is an increase in structural durability with no significant loss of performance. This report investigates the feasibility of damage mitigating control of reusable rocket engines. Phenomenological models of creep and thermo-mechanical fatigue damage have been formulated in the state-variable setting such that these models can be combined with the plant model of a reusable rocket engine, such as the Space Shuttle Main Engine (SSME), for synthesizing an optimal control policy. Specifically, a creep damage model of the main thrust chamber wall is analytically derived based on the theories of sandwich beam and viscoplasticity. This model characterizes progressive bulging-out and incremental thinning of the coolant channel ligament leading to its eventual failure by tensile rupture. The objective is to generate a closed form solution of the wall thin-out phenomenon in real time where the ligament geometry is continuously updated to account for the resulting deformation. The results are in agreement with those obtained from the finite element analyses and experimental observation for both Oxygen Free High Conductivity (OFHC) copper and a copper-zerconium-silver alloy called NARloy-Z. Due to its computational efficiency, this damage model is suitable for on-line applications of life prediction and damage mitigating control, and also permits parametric studies for off-line synthesis of damage mitigating control systems. The results are presented to demonstrate the potential of life extension of reusable rocket engines via damage mitigating control. The control system has also been simulated on a testbed to observe how the damage at different critical points can be traded off without any significant loss of engine performance. The research work reported here is built upon concepts derived from the disciplines of Controls, Thermo-fluids, Structures, and Materials. The concept of damage mitigation, as presented in this report, is not restricted to control of rocket engines. It can be applied to any system where structural durability is an important issue.

An intelligent control system for rocket engines: need, vision, and issues

In this paper several components of intelligence are defined. Within these definitions an intelligent control system for rocket engines is described. The description includes a framework for development of an intelligent control system. The framework includes diagnostics, coordination, and direct control. Some current results and issues are presented.

Life Extending Control - A Concept Paper

The concept of Life Extending Control is defined. Life is defined In terms of mechanical fatigue life. A brief description is given of the current approach to life prediction using a local, cyclic, stress-strain approach for a critical system component. An alternative approach to life prediction based on a continuous functional relationship to component performance is proposed. Based on cyclic life prediction an approach to Life Extending Control, called the Life Management Approach is proposed. A second approach, also based on cyclic life prediction, called the Implicit Approach, is presented. Assuming the existence of the alternative functional life prediction approach, two additional concepts for Life Extending Control are presented. These are called the Measured Damage Approach, and the Estimated Damage Approach. A simple, hydraulic actuator driven, position control system example is used to illustrate the main ideas behind the Life Extending Control concept and is based on a cyclic fatigue damage model. Some results are given which demonstrate that system durability can be maximized by more effectively managing the resource of critical component life. Some additional research is proposed to develop a functional life prediction method and to demonstrate the concept of Life Extending Control.

Damage-mitigating control of space propulsion systems for high performance and extended life

A major goal in the control of complex mechanical system such as spacecraft rocket engines advanced aircraft, and power plants is to achieve high performance with increased reliability, component durability, and maintainability. The current practice of decision and control systems synthesis focuses on improving performance and diagnostic capabilities under constraints that often do not adequately represent the materials degradation. In view of the high performance requirements of the system and availability of improved materials, the lack of appropriate knowledge about the properties of these materials will lead to either less than achievable performance due to overly conservative design, or over-straining of the structure leading to unexpected failures and drastic reduction of the service life. The key idea in this report is that a significant improvement in service life could be achieved by a small reduction in the system dynamic performance. The major task is to characterize the damage generation process, and then utilize this information in a mathematical form to synthesize a control law that would meet the system requirements and simultaneously satisfy the constraints that are imposed by the material and structural properties of the critical components. The concept of damage mitigation is introduced for control of mechanical systems to achieve high performance with a prolonged life span. A model of fatigue damage dynamics is formulated in the continuous-time setting, instead of a cycle-based representation, for direct application to control systems synthesis. An optimal control policy is then formulated via nonlinear programming under specified constraints of the damage rate and accumulated damage. The results of simulation experiments for the transient upthrust of a bipropellant rocket engine are presented to demonstrate efficacy of the damage-mitigating control concept.

Screening studies of advanced control concepts for airbreathing engines

The application of advanced control concepts to airbreathing engines may yield significant improvements in aircraft/engine performance and operability. Accordingly, the NASA Lewis Research Center has conducted screening studies of advanced control concepts for airbreathing engines to determine their potential impact on turbine engine performance and operability. The purpose of the studies was to identify concepts which offered high potential yet may incur high research and development risk. A target suite of proposed concepts was formulated by NASA and industry. These concepts were evaluated in a two phase study to quantify each concepts impact on desired engine characteristics. To aid in the evaluation, three target aircraft/engine combinations were considered: a military high performance fighter mission, a high speed civil transport mission, and a civil tiltrotor mission. Each of the advanced control concepts considered in the study were defined and described. The concepts potential impact on engine performance was determined. Relevant figures of merit on which to evaluate the concepts were also determined. Finally, the concepts were ranked with respect to the target aircraft/engine missions.

Nonlinear Life-Extending Control of a Rocket Engine

ROCKET engine has a number of critical components that operate close to mechanical design limits. These components often typify behavior of the remaining components and hence are indicators of the effective service life of a reusable rocket engine. Fatigue damage in the turbine blades is one of the most serious causes for engine failure. This Note focuses on the conceptual development of a nonlinear life-extending control system for rocket engines via damage mitigation in both the fuel (H2) and oxidizer (O2) turbine blades. The fundamental concept of life-extending control (LEC) was introduced by Lorenzo and Merrill. 1 Subsequently, a growing body of literature has emerged for feedforward 2,3 and feedback 4 control of rocket engines for life extension. Whereas the LEC technology was developed initially for rocket engines, it has broad applications for other systems such as fossil-fueled power plants 5 and mechanical structures, 6 where both dynamic performance and structural durability are critical issues. ThedesignapproachpresentedinthisNoteisdifferentfrompreviousworkinthesensethatthisapproachallowsadaptationoftheLEC featuretoaugmentaconventionalperformancecontrollerofarocket engine. Unlike the previously reported design approaches, 2,3,5 the proposed technique does not require an optimal feedforward control sequence, which is sensitive to plant modeling uncertainties and variations in the initial conditions. Furthermore, for other control applications such as military aircraft, the life extension feature ofthecontrolsystemcanbeactivatedordeactivatedattheoperator’ s discretion.

Life extending controller design for reusable rocket engines

The goal of life extending control (LEC) is to enhance structural durability of complex mechanical systems, such as aircraft, spacecraft, and energy conversion devices, without incurring any significant loss of performance. This paper presents a concept of robust life-extending controller design for reusable rocket engines, similar to the Space Shuttle Main Engine (SSME), via damage mitigation in both fuel and oxidiser turbines while achieving the required performance for transient responses of the main combustion chamber pressure and the oxidant/fuel mixture ratio. The design procedure makes use of a combination of linear robust control synthesis and nonlinear optimisation techniques. Results of simulation experiments on the model of a reusable rocket engine are presented to this effect.