Agility Analysis of the James Webb Space Telescope

Abstract

The agility of a rigid-body spacecraft can be expressed in terms of a geometric, three-dimensional, solid called the agilitoid. Originally developed as a means for explaining the concept of “hidden agility” made visible through the use of optimal control techniques, a modified agilitoid called an agility envelope is presented here that is compatible with conventional eigenaxis maneuvers. This paper demonstrates how the agility envelope can be applied to size an attitude control system (ACS) and/or assess the capability of an existing design. Analysis of the James Webb Space Telescope (JWST) ACS shows that the agility envelope accurately predicts the true capability of the ACS: a 90 deg maneuver can actually be completed 15% faster than the conventional back-of-the-envelope slew-sizing equations suggest. The utility of the agility envelope is further illustrated by showing how an alternative control allocation scheme can reduce the JWST torque and momentum requirements by 40%. The otherwise hidden agility can be recovered to enhance the slew performance of the JWST or allow the reaction wheel array to be reduced from six to five wheels, while meeting existing maneuver requirements. The agility envelope allows such design trades to be studied without the need to perform detailed simulations of the attitude control system.