Trajectory optimization is the process of designing a
trajectorythat minimizes or maximizes some measure of performance. While not exactly the same, the goal of solving a trajectory optimization problem is essentially the same as solving an optimal controlproblem. The selection of flight profiles that yield the greatest performance plays a substantial role in the preliminary design of flight vehicles, since the use of ad-hoc profile or control policies to evaluate competing configurations may inappropriately penalize the performance of one configuration over another. Thus, to guarantee the selection of the best vehicle design, it is important to optimize the profile and control policy for each configuration early in the design process.
Consider this example. For
tactical missiles, the flight profiles are determined by the thrust and load factor(lift) histories. These histories can be controlled by a number of means including such techniques as using an angle of attackcommand history or an altitude/downrange schedule that the missile must follow. Each combination of missile design factors, desired missile performance, and system constraints results in a new set of optimal control parameters. [Phillips, C.A, "Energy Management for a Multiple Pulse Missile", AIAA Paper 88-0334, Jan., 1988]
Trajectory optimization began in earnest in the 1950s as digital computers became available for the computation of trajectories. The first efforts were based on
optimal controlapproaches which grew out of the calculus of variationsdeveloped at the University of Chicago in the first half of the 20th century most notably by Gilbert Ames Bliss. Pontryagin[L.S. Pontyragin, The Mathematical Theory of Optimal Processes, New York, Intersciences, 1962] in Russia and Bryson [Bryson, Ho,Applied Optimal Control, Blaisdell Publishing Company, 1969, p 246.)] in America were prominent researchers in the development of optimal control. Early application of trajectory optimization had to do with the optimization of rocket thrust profiles in a vacuum and in an atmosphere. From the early work, much of the givens about rocket propulsion optimization were discovered. Another successful application was the climb to altitude trajectories for the early jet aircraft. Because of the high drag associated with the transonic drag region and the low thrust of early jet aircraft, trajectory optimization was the key to maximizing climb to altitude performance. Optimal control based trajectories were responsible for some of the world records. In these situations, the pilot followed a Mach versus altitude schedule based on optimal control solutions.
In the early phase of trajectory optimization; many of the solutions were plagued by the issue of singular subarcs. For such problems, the term multiplying the control variable goes to zero and it becomes impossible to directly solve for the optimal control. Instead one is left with a family of feasible solutions. At that point, the investigators had to numerically evaluate each member of the family to determine the optimal solution. A breakthrough occurred with a condition sometimes referred to as the Kelley condition. While not a sufficient condition, this provided an additional necessary condition that allowed downselection to a trajectory that is usually the optimal. [ H.J. Kelley, R.E. Kopp, and H.G. Moyer, "Singular Extremals", Topics in Optimization, G. Leitmann (ed.) Vol. II Chapter 2 New York, Academic Press, 1966]
The techniques available to solve optimization problems fall into two broad categories: the optimal control methodology that allows solution by either analytical or numerical procedures and an approximation to the optimal-control problem through the use of nonlinear programming that allows solution by numerical procedures. The optimal control problem is an infinite dimensional problem while the nonlinear programming approach approximates the problem by a finite dimensional problem. Trajectory optimization shares the same optimization algorithms as other optimization problems. The numerical optimal control methodology can produce the best answers but converging to a solution is difficult. Convergence is rapid when the initial guess is good, otherwise the search may fail. The ascent trajectories for the US space program (Gemini and Apollo) were designed using numerical optimal control. The very tight tolerances associated with space launchers allowed optimal control to be a useful tool. For systems with less controlled environments such as missiles, numerical optimal control would not prove as useful.
The analytic solution of the optimal control often involves extensive approximations but can still produce useful algorithms. An example is given in Ohlmeyer & Phillips [ Ohlmeyer, E.J., Phillips, C.A., Generalized Vector Explicit Guidance Journal of Guidance, Control, and Dynamics 2006; 0731-5090 vol.29 no.2 (261-268)] .
Many numerical procedures exist to solve parameter optimization problems. The simplest procedures are the gradient or steepest descent techniques. Second-order methods are also available to improve the rate of convergence, for example, the Newton–Raphson iteration, which requires the evaluation of the Hessian matrix. Quasi-Newton or variable-metric methods avoid the evaluation of the Hessian matrix by using iterative evaluation of first-order information to approximate the Hessian matrix. The nonlinear programming methods such as
BFGSand SQPmay be used to solve the finite dimensional problem. The nonlinear programming approach is generally more robust in terms of finding a solution than numerical optimal control, but many of the gradient or Newton-Raphson methods require "smoothness" in the function algorithms to be successful. Smoothness is continuity in the first derivative. The smoothness requirement imposes a burden on flight trajectory analysts in that most highly detailed trajectory simulations do not exhibit smoothness. This restriction was a problem in the early days of trajectory optimization when computer computation speed was an issue. Often, special approximate trajectory models had to be used to work with non-linear programming models. As computation time has become cheap compared to manpower, direct sample methods have evolved as the optimization algorithms of choice. These algorithms may require orders of magnitude increases in the number of functional samples but exhibit robustness to non-smoothness in the trajectory code. Examples include: genetic algorithms, stochastic sampling methods, and hill climbingalgorithms. An excellent overview of the state of the art in numerical methods is given in Betts. [Survey of Numerical Methods for Trajectory Optimization;John T. Betts Journal of Guidance, Control, and Dynamics 1998;0731-5090 vol.21 no.2 (193-207)]
When dealing with complex payoff functions that are pertinent to realistic engineering problems, an alternative method is one of the multi-level techniques. These approaches allow models to be used in the optimization in a tiered approach by the passing of constraints to the lower levels and selecting the optimal value of the constraint value in the upper levels. An early paper in this area presents this idea for the optimization of the performance of a missile. [Trajectory Optimization for a Missile Using a Multitier Approach; C.A. Phillips, J.C. Drake, JOURNAL OF SPACECRAFT AND ROCKETS ; Vol. 37, No. 5, September–October 2000]
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