Lunar descent using sequential engine shutdown

Abstract

The notion of sequential engine shutdown is introduced and its application to lunar descent is motivated. The concept calls for the utilization of multiple fixed thrust engines in place of a single continuously throttleable engine. Downrange position control is provided by properly timed engine shutdowns. The principle advantage offered is the potential cost savings that would result from the elimination of the development cost of a throttleable rocket engine. Past lunar landing efforts are reviewed and provide the foundation for a baseline vehicle definition. A descent from a lunar parking orbit is assumed. The powered descent is divided into two phases, and a sequential engine shutdown-based guidance scheme is developed for the earlier phase. The guidance scheme consists of a biased ignition point and an algorithm for calculating shutdown times combined with a linear tangent steering law to provide full terminal position control. The performance of the sequential engine shutdown guidance scheme is assessed against two alternative approaches.

(cont.) A statistical picture of the performance of each guidance scheme is obtained via Monte Carlo trials of a lunar descent simulation that captures, to first order, the interaction between the descent propulsion system, the navigation filter, and the guidance function, allowing a direct comparison to be made on the basis of accuracy and fuel consumption. The impact of variations in the number of engines available in the sequential engine shutdown case is analyzed. While the performance observed with sequential engine shutdown does not match that observed with a throttleable engine, the results suggest that it is a viable solution to the lunar descent guidance problem.