Engineering topics
Publications and source records attributed to Enright, Paul J..
A preliminary design of the Cassini Thrust Vector Control algorithm which controls the spacecraft attitude during main engine burns is described. The discussion includes software architecture, sensor/actuator characteristics, and vehicle dynamics, as well as controller design and margin analysis via clasical methods, and performance evaluation via simulation.
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Preliminary attitude and articulation controller designs for the CRAF/Cassini spacecraft have prompted a consideration of potential interactions with structural dynamics and propellant motion. The two algorithms significantly affected are attitude control during main engine burns, and articulation control of the high-precision scan platform (HPSP). For main engine burns, the primary concerns are the sloshing of the bipropellant, which constitutes 70 percent of initial spacecraft mass, and the vibration of the three large appendages, especially the HPSP, on which the gyros are mounted. For the HPSP pointing algorithm, the main concern is the fairly large offset of the HPSP center-of-mass from the gimbals, which is motivated by other system and subsystem considerations. This offset results in significant coupling between HPSP articulation and HPSP boom structural dynamics. This paper surveys the operational scenarios of these algorithms, and the impacts of nonrigid dynamics on controller design and performance.
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A new method is described for the determination of optimal spacecraft trajectories in an inverse-square field using finite, fixed thrust. The method employs a recently developed optimization technique which uses a piecewise polynomial representation for the state and controls, and collocation, thus converting the optimal control problem into a nonlinear programming problem, which is solved numerically. This technique has been modified to provide efficient handling of those portions of the trajectory which can be determined analytically, i.e., the coast arcs. Among the problems that have been solved using this method are optimal rendezvous and transfer (including multirevolution cases) and optimal multiburn orbit insertion from hyperbolic approach.
A recently developed method for solving optimal trajectory problems uses a piecewise-polynomial representation of the state and control variables, enforces the equations of motion via a collocation procedure, and thus approximates the original calculus-of-variations problem with a nonlinear-programming problem, which is solved numerically. This paper identifies this method as a direct transcription method and proceeds to investigate the relationship between the original optimal-control problem and the nonlinear-programming problem. The discretized adjoint equation of the collocation method is found to have deficient accuracy, and an alternate scheme which discretizes the equations of motion using an explicit Runge-Kutta parallel-shooting approach is developed. Both methods are applied to finite-thrust spacecraft trajectory problems, including a low-thrust escape spiral, a three-burn rendezvous, and a low-thrust transfer to the moon.