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→Shuttle guidance - Ascent guidance Powered Explicit Guidance (PEG)
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Operating the Shuttle AP properly is very different from operating airplane APs and requires a profound knowledge of OPS sequences and major mode transitions as well as strict adherence to the published procedures. | Operating the Shuttle AP properly is very different from operating airplane APs and requires a profound knowledge of OPS sequences and major mode transitions as well as strict adherence to the published procedures. | ||
=== [[Shuttle guidance - Ascent guidance Powered Explicit Guidance (PEG)]] == | === Ascent guidance Powered Explicit Guidance (PEG) === | ||
'''Full explanations about the Ascent guidance might be found there: [[Shuttle guidance - Ascent guidance Powered Explicit Guidance (PEG)]]''' | |||
The purpose of this section is to present and discuss about the second stage ascent guidance (post SRB sep) for Nominal Orbital Insertion, and some Intact Aborts (TAL / AOA / ATO). | |||
The guidance is based on the real closed loop used in the Shuttle, known as Power Explicit Guidance https://www.orbiterwiki.org/wiki/Powered_Explicit_Guidance. | |||
'''Documentations''' | |||
*A very detailled and complete topic about the guidance by Noiredd who implemented it in Matlab and KSP: https://github.com/Noiredd/PEGAS-MATLAB/blob/master/docs/upfg.md | |||
*A deeper document with nice schematic drawings: Ascent Guidance Navigation and Control Shuttle Workbook (page 111) https://www.google.com/search?client=firefox-b-d&q=ascent+guidance+workbook+shuttle | |||
*Original formulation of the Unified Power Explicit Guidance with equations and algorithms: ''ntrs.nasa.gov/citations/19740004402'' | |||
*A paper about enhancements made over the years to the original ascent guidance: ''ntrs.nasa.gov/citations/20180002035'' | |||
'''Overview''' | |||
Second stage guidance functions very differently from first stage guidance in that second stage guidance is closed loop. 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. It solves this two point boundary value problem each cycle (every 1.92 seconds). One limitation of second stage guidance is that it doesn't calculate if there is enough propellant to reach the desired MECO conditions. | |||
[[File:PEG Meco target.webp|400px|thumbnail|none]] | |||
The powered explicit guidance (PEG) scheme used by second stage guidance nominally operates in two phases. The first phase computes throttle and attitude commands based on three SSMEs and a constant thrust requirement until an acceleration of 3g is reached. At that time, the second phase, which uses variable throttle to maintain a constant acceleration, is entered. If an engine failure is detected, a third phase of PEG, which computes the necessary guidance commands using constant thrust to aim for the desired targets using two SSMEs, is entered (assuming no RTLS or TAL abort). | |||
During current shuttle operations, only two phases of PEG are used, constant thrust through 3g and then variable thrust through main engine cutoff (MECO). STS-1 and STS-26, in order to prevent or reduce abort gaps, flew higher than normal trajectories, called lofted or abort shaped. This method required the third PEG phase, which ran from SRB sep to T_FAIL (I-loaded MET) and achieved lofting by assuming that an engine would fail causing loss of performance at the time T_FAIL. When T_FAIL occurred, PEG stopped assuming that an engine would fail. A drawback with this method was discovered later, however. The lofted trajectories caused “black zones,” or regions where an unsurvivable entry/pullout condition would be created if two engines actually did fail (CA). For this reason and the fact that abort shaping costs thousands of pounds of nominal ascent performance (payload), the I-load, T_FAIL is now set to zero, and lofted trajectories are not currently planned. | |||
[[File:PEG step.webp|600px|thumbnail|none]] | |||
Second stage guidance performs yaw steering to achieve the desired orbit plane. The desired orbit plane is defined by the unitized negative angular momentum vector (I-loads), commonly referred to as the '''IY vector'''. The x and y components of the IY vector define the nodal crossing, while the z component defines the inclination. For missions which do not involve rendezvous with a vehicle already in orbit (referred to as the “target”), the IYs are defined during the flight design process approximately 6 months prior to launch. These missions employ “earth fixed” yaw steering since the trajectory relative to the earth remains the same regardless of launch time. In order to successfully launch into orbit and rendezvous with another vehicle already in space, the orbiter must end up in the same orbital plane and altitude as the other vehicle. | |||
[[File:PEG insertion.webp|600px|thumbnail|none]] | |||
Forty seconds prior to MECO, guidance no longer seeks to achieve the altitude and orbital plane position targets. Common terminology is, “at MECO minus 40 seconds, the position constraints are released.” Without this constraint release, when TGO becomes small, a small change in position error would produce large changes in the thrust turning rate vector and over controlling would result. Note also that the cutoff time (TGO) calculation includes the predicted velocity change from the time minimum throttle is commanded to burnout. This corresponds to the predicted tailoff impulse from each active SSME and is known as fine count. Fine count occurs 10 seconds prior to MECO for nominal ascent, ATO, and TAL and 6 seconds prior to powered pitchdown for RTLS. It is at fine count where second stage, closed loop guidance is terminated and the SSMEs are commanded to a lower power level, usually 67% for three engines running or 91% for one or two engines running (note that the SSMEs aren't throttled back until powered pitchdown during an RTLS). Thereafter, the flight path angle constraint is released, such that TGO is computed solely on the desired velocity change (VGO). When guidance sees the shuttle at the correct inertial velocity (VI), all SSMEs are commanded to shut down. | |||
=== Entry guidance algorithm === | === Entry guidance algorithm === | ||