Function relativistic_delta_v 

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

Relativistic rocket equation (constant-thrust, constant-exhaust-velocity).

For a relativistic rocket with exhaust velocity ve and mass ratio m0/mf, the final coordinate velocity is:

v_final = c × tanh(ve/c × ln(m0/mf))

This is the Ackeret relativistic rocket equation. When ve << c, tanh(x) ≈ x and this reduces to the classical Tsiolkovsky equation.

Note: This gives the velocity achievable from rest. For ve << c, v_final ≈ ve × ln(m0/mf) (classical limit).