Space Shuttle: Difference between revisions

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.

'''Full explanations about Entry shuttle guidance might be found there: [[Shuttle guidance - Entry guidance algorithm]]'''