Module shifts 

Mass that moves along the airframe in flight: a ballast weight or a payload slid fore or aft on a trigger, with the rocket’s mass properties and its equations of motion following it.

A MassShift moves one part carried inside the airframe, with everything inside it, by a set distance along the axis over a set time, starting on a trigger as a recovery device does. It follows a cycloid (the cam designer’s “cycloidal motion”): its speed and acceleration are zero at both ends, so the center of mass moves smoothly. The rocket’s mass is unchanged; its center of mass and inertia follow the part (hpr_design::MassProperties::with_part_moved). The equations of motion take the moving center of mass’s terms as they take a burning motor’s, and add the part’s angular momentum relative to the airframe, which is not zero when it moves off the axis (the decision record on moving mass, ADR-087).

Method: the documentation site’s Moving mass page.

Structs

MassShift

A part carried inside the airframe moving along its axis: travel_m aft (forward when negative) over duration_s, starting on trigger.

Constants

MIN_SHIFT_DURATION_S

The shortest time a MassShift may take, s.

SHIFT_STOPS

How many equal intervals a shift’s time is cut into by stop times, so that an integrator takes at least this many steps across it: a fixed step as long as the move would take it in one, and the part’s Coriolis-like term would be weighed at a single midpoint. A numerical detail, which may change.