![]() The first phase computes throttle and attitude commands based on three SSMEs and a constant thrust requirement until an acceleration of 3g is reached. The powered explicit guidance (PEG) scheme used by second stage guidance nominally operates in two phases. One limitation of second stage guidance is that it doesn't calculate if there is enough propellant to reach the desired MECO conditions. It solves this two point boundary value problem each cycle (every 1.92 seconds). Second stage guidance computes the control variables (essentially commanded attitude and attitude rates) and burn time to go (TGO) in such a way that the vehicle flies from the current state to the prescribed target conditions (altitude, velocity, flight path angle, and orbit plane) within trajectory constraints. Second stage guidance functions very differently from first stage guidance in that second stage guidance is closed loop. *A paper about enhancements made over the years to the original ascent guidance: /citations/20180002035 *Original formulation of the Unified Power Explicit Guidance with equations and algorithms: /citations/19740004402 *A deeper document with nice schematic drawings: Ascent Guidance Navigation and Control Shuttle Workbook (page 111) *A very detailled and complete topic about the guidance by Noiredd who implemented it in Matlab and KSP. The guidance is based on the real closed loop used in the Shuttle, known as Power Explicit Guidance. The purpose of this topic is to present and discuss about the new second stage ascent guidance (post SRB sep) for Nominal Orbital Insertion, and some Intact Aborts (TAL / AOA / ATO).
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