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The launch rail: its direction, length and friction, and where a design’s rail buttons or launch lugs leave it.
The rocket starts with its aft end (the aft end of its body or its aft-most nozzle exit, whichever is further aft) at the foot of the rail, its axis along the rail. It is guided, with one degree of freedom along the rail, until the aft edge of its aft-most guide passes the top of the rail. A rocket with no rail buttons or lugs is guided until its aft end passes the top, as from a tower. Between the forward guide leaving and the aft guide leaving, a real rocket can pivot about the aft guide (“tip-off”); that rotation is not modeled, and the rocket leaves the rail with no angular velocity.
Friction is Coulomb friction, μ |N|, with N the rail’s reaction perpendicular to the rail:
the component of gravity, the aerodynamic force and the variable-mass terms across the rocket’s
axis. It opposes the motion along the rail, and on the pad it holds the rocket until the force
along the rail exceeds it.
Method: docs/physics/flight.md.