Function propellant_consumed 

pub fn propellant_consumed(
    pre_burn_mass_kg: f64,
    delta_v_km_s: f64,
    isp_s: f64,
) -> f64

Compute propellant consumed during a burn using Tsiolkovsky equation.

Given the pre-burn total mass, ΔV, and exhaust velocity (from Isp), returns the propellant mass consumed (kg).

m_propellant = m_before × (1 - 1/mass_ratio) = m_before × (1 - exp(-ΔV/vₑ))