Function brachistochrone_dv 

pub fn brachistochrone_dv(distance: Km, time: Seconds) -> KmPerSec

ΔV for a brachistochrone transfer (constant thrust, flip at midpoint).

ΔV = a_required * t = 4 * d / t

Returns ΔV in km/s.

Assumption: straight-line path, constant thrust, no gravity, rest-to-rest.