Function propellant_fraction 

pub fn propellant_fraction(delta_v: KmPerSec, ve: KmPerSec) -> f64

Propellant mass fraction: 1 - 1/mass_ratio = 1 - exp(-ΔV/vₑ).

The fraction of initial mass that must be propellant. Returns a value in [0, 1). For ΔV >> vₑ this approaches 1.