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This topic is offered for NewMars members to collect knowledge about coordinate systems in use for spacecraft.
The subject has become necessary because GW Johnson is moving steadily from theoretical spacecraft design to the real thing.
Post #3 will contain a brief Gemini summary of the coordinate systems that are in play on Earth and in space in 2026.
It appears that the coordinate systems are literally all over the place.
However, a team building a space craft needs an agreed-upon coordinate system.
At present, GW's design follows the spin stabilized mode described in Post #3, but the location of Positive or Negative Z is not yet established.
The difference matters.
On Earth, in the assembly room, the heat shield is down (negative Z) and the docking collar is up (positive Z)
In flight, the heat shield is in front (positive Z) and the docking collar is in back (negative Z)
The (small) project team is waiting for GW Johnson to decided which is which for this particular spacecraft.
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Post #3:: Summary of coordinated systems used for spacecraft prepared by Gemini
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This post is intended to contain a short summary of coordinate systems used for space craft in development and in service, in 2026.
Spacecraft Z-Axis Conventions Summary
In standard spacecraft and aerospace engineering, the assignment of the Z-axis depends on whether you are using a body-fixed coordinate system (relative to the vehicle itself) or an orbital reference frame (relative to its position in space).
1. Spacecraft Body-Fixed Coordinate Systems
For attitude control, structural analysis, and moments of inertia, spacecraft generally adhere to right-handed Cartesian coordinate systems (X, Y, Z):
NASA / AIAA Standard Body Frame:
X-axis: Typically aligned with the longitudinal axis pointing forward (toward the nose or primary direction of thrust).
Y-axis: Points to the starboard (right) side of the vehicle.
Z-axis: Points downward (ventral side) perpendicular to the X-Y plane, completing the right-handed triad.
Alternative Launcher / Structural Convention:
In some launch vehicle architectures, the longitudinal axis along the height of the rocket is designated as the Z-axis, with X and Y forming the transverse cross-section plane.
[h][color=#0055AA]2. Orbital Reference Frames (L V L H / Nadir-Pointing)[/color][/h]When a spacecraft is operating in orbit and maintaining a local vertical orientation, orbital reference frames dictate sensor and antenna pointing:
Local-Vertical Local-Horizontal (LVLH):Z-axis: Points directly toward the center of the primary body (the nadir vector, downward toward Earth).
X-axis: Points in the direction of orbital velocity along the orbital plane.
Y-axis: Points along the orbit normal to complete the right-handed system.
3. Principal Axes of Inertia
For dynamics and control (I_xx, I_yy, I_zz), the Z-axis frequently represents the axis of symmetry for cylindrical or axisymmetric spacecraft, where spin is established around the principal Z-axis to maximize gyroscopic stability.(th)
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