Clohessy-Wiltshire (Hill's) Equations

In low Earth orbit (~408 km), spacecraft relative motion does not behave like intuitive terrestrial driving. The Earth's gravitational gradient and orbital angular velocity create Coriolis and centrifugal accelerations. Firing forward (+X) raises your apoapsis, which causes you to gain altitude and actually drift backwards over time!

V-Bar vs R-Bar Approaches

V-Bar: The velocity vector corridor (approaching along the ISS path of travel). Starliner typically holds at Waypoint 1 (200m) and Waypoint 2 (120m) inside the Keep-Out Sphere (KOS) before continuing inside the 10° approach cone.

R-Bar: The radial vector pointing directly to or from Earth center, utilized for vertical transitions.

Capture Envelope & Safe Contact

International Docking System Standard (IDSS) capture mechanisms require relative closure speed below 0.10 m/s at contact, lateral misalignment under 0.10 meters, and angular misalignment under 2.5°. Excess speed damages the soft-capture ring; insufficient alignment prevents magnetic latching.