Impact of uncommanded effector motions on Shuttle consumables

Abstract

Uncommanded thrust vectoring and aerosurface motions have concerned Space Shuttle designers, engineers, and flight planners since these motions were first discovered during early testing and flight simulation. These motions use hydraulic power and consume additional fuel. Clearly, uncommanded motion of these controls during Shuttle ascent, entry, and descent represents a potential impact to consumables margins and to payload weight. This article defines the two basic sources of uncommanded activity as hardware and flight controlrelated sources. The impact of these sources of uncommanded activity on consumables was successfully modeled by the authors. The model was also employed to allocate consumables reserves. The intent of the authors is to share with the reader the observed incidences of uncommanded activity, the formulation of the model, its application, and the reconciliation of model results with actual flight data. IntroductionU NCOMMANDED effector motion is important to the control system designer, the power hydraulics subsystem designer, the consumables analyst, and the mission planner. Space Shuttle effectors under consideration include: inboard and outboard elevohs, rudder, speed brake, body flap, and the main engine thrust vector control. These units control the motion of the vehicle during aerodynamic flight so that only the ascent and entry phases of a mission are germane to this discussion. Idealized use of energy comes from providing actuator motion to control the vehicle (aerodynamic force encountered in flight). There are also nonideal uses of energy that require the effectors to consume energy beyond their idealized thermodynamic requirements. The first group of nonidealities, familiar to the consumables analyst, consists of mission and subsystem performance uncertainties which deviate from ideal performance. These include pump characteristic uncertainties, specific fuel consumption uncertainties, turbulence, use of automatic vs control stick control steering, and timeline deviations. The second type of nonideality consists of noncommanded effector vibrations such as flight control limit cycling and power drive unit dither. These vibrations consume energy and, therefore, propellant. The motions can be very complex. They have been analyzed only recently for Shuttle systems and are the major topic of this paper.Uncommanded effector motion could cause excess propellant consumption and violate planned mission margins. It is important, therefore, that these motions be understood and accounted for in mission planning and postflight data analysis.To understand these motions, one must understand Shuttle subsystems and missions/ The important related subsystems involved are the auxiliary power unit (APU) (Fig. 1) and the hydraulic subsystem (Fig. 2).