SPACE SHUTTLE GN&C EQUATION DOCUMENT No.14 (Revision 1) Deorbit Targeting

摘要

The deorbit targeting program is divided into two sections. The first sec¬tion establishes an initial guess of the landing time based on updating the vehicle in its orbit until its position coincides with the landing site position projected into the plane of the orbit. If the crossrange at this point is not acceptable, the vehicle is updated through successive orbital periods until an acceptable crossrange distance is at-tained. The second section utilizes the Unified Powered Flight Guidance (UPFG) routine - which is based -on velocity-to-be-gained and Linear Tangent Guidance concepts — in order to approximate the effects of the finite deorbit maneuver. This is necessary as the anticipated thrust/weight ratio for the Shuttle during deorbit using a single OHMS engine is 0.025, resulting in long burn arcs. The UPFG rou¬tine is called in an iterative loop which searches for the minimum fuel deorbit trajec-tory.