Function delta_v_correction_fraction 

pub fn delta_v_correction_fraction(
    exhaust_velocity: KmPerSec,
    mass_ratio: f64,
) -> f64

Fractional correction between relativistic and classical ΔV.

Returns (classical - relativistic) / classical.

A positive value means the relativistic ΔV is less than the classical (as expected — tanh(x) < x for x > 0). This represents how much the classical equation overestimates the achievable velocity.