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Wait, what? I do understand and I honestly don't know what all this is, but nasa's technical documents all seem to state this.
Here are some more:
NASA Technical Note; Calculation of Wind Compensation for Launching of Unguided Rockets (Page 8 Trajectory Simulation, 2nd Paragraph) ..."this simulation assumes ... the missile position in space is computed relative to a flat nonrotating Earth"
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040008097.pdf
NASA Technical Paper 2768; User's Manual for LINEAR, a FORTRAN Program to Derive Linear Aircraft Models (Page 12, Program Overview) ... “Within the program, the nonlinear equations of motion include 12 states representing a rigid aircraft flying in a stationary atmosphere over a flat nonrotating Earth”
https://www.nasa.gov/centers/dryden/pdf/88072main_H-1259.pdf
NASA Technical Paper 2835; "User's Manual for LINEAR, a FORTRAN Program to Derive Linear Aircraft Models" (Page 1, Summary) AND (Page 126 , Report Documentation Page, Section 16) "The nonlinear equations of motion used are six-degree-of-freedom equations with stationary atmosphere and flat, nonrotating earth assumptions."
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19890007066.pdf
NASA Technical Memorandum; Determination of Angles of Attack and Sideslip from Radar Data and a Roll Stabilized Platform (Page 2, Section 16.) “The method is limited, however, to application where a flat, nonrotating earth may be assumed.”
NASA Contractor Report 186019; An Aircraft Model for the AIAA Controls Design Challenge (Page 11, Equation of Motion and Atmospheric Model) ... “The nonlinear equations of motion used in this model are general six-degree-of-freedom equations representing the flight dynamics of a rigid aircraft flying in a stationary atmosphere over a flat nonrotating Earth.”
https://www.nasa.gov/centers/dryden/pdf/88248main_H-1777.pdf
NASA Contractor Report 3073; Investigation of Aircraft Landing in Variable Wind Fields (Page 6, Chapter II - Aircraft Landing Model) ... "The Aircraft trajectory model employed in this study was derived based on the following assumptions: a) The Earth is flat and non-rotating. "
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19790005472.pdf
NASA Technical Memorandum 81238; A Mathematical Model of the CH-53 Helicopter (Page 17, Equations of Motion) .. "The helicopter equations of motion are given in body axes with respect to a flat, nonrotating Earth."
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19810003557.pdf
Engineering Experiment Station, Georgia Institute of Technology, Prepared for NASA; Atmospheric Oscillations (Page 10) ... "A model frequently used is that of a flat, nonrotating earth." ... (next paragraph) .. "The most one can profitably simplify the problem is to consider an isothermal atmosphere, plane level surface, and a nonrotating Earth."
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19650015408.pdf
NASA Tecnical Paper 2002-210718; Stability and Control Estimation Flight Test Results for the SR-71 Aircraft With Externally Mounted Experiments (Pages 10-11 Equations of Motion) ... "These equations assume a rigid vehicle and a flat, nonrotating Earth."
https://www.nasa.gov/centers/dryden/pdf/88733main_H-2465.pdf
NASA Technical Memorandum 100996; Flight Testing a VSTOL Aircraft to Identify a Full-Envelope Aerodynamic Model (Pages 4-5, State Estimation) ... “For aircraft problems, the state and measurement models together represent the kinematics of a rigid body for describing motion over a flat, nonrotating Earth…”
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19880014378.pdf
NASA Ames Research Center; Singular Arc Time-Optimal Climb Trajectory of Aircraft in a Two-Dimensional Wind Field (Page 2, Section II. Singular Arc Optimal Control) ... “In our minimum time-to-climb problem, the aircraft is modeled as a point mass and the flight trajectory is strictly confined in a vertical plane on a non-rotating, flat Earth."
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20060053337.pdf
C'mon, these are their own TECHNICAL documents.
It just looks like studies where they take out that variable to look at a particular effect. The more variables you add to the study, the less effective you get at looking at whatever you're looking at.
That's like college level science 101 dude. If you have to add into rotations of multiple bodies and the effect on the atmosphere, it becomes a much more complicated problem to look at.