Propulsion Sizing Calculator

Every orbital manoeuvre — station-keeping, plane change, deorbit, transfer — costs a velocity increment, the delta-v (Δv). Sizing a propulsion system means turning that Δv budget into a propellant mass and, from there, into tank volume and burn time. The governing relation is the Tsiolkovsky rocket equation: Δv = ve · ln(m_wet / m_dry), where ve is the exhaust velocity. Solved for propellant mass from the dry mass it becomes m_p = m_dry · (e^(Δv/ve) − 1).

Exhaust velocity follows from specific impulse: ve = Isp · g0, with standard gravity g0 = 9.80665 m/s². Because propellant mass grows exponentially with Δv/ve, a higher-Isp propellant collapses the propellant requirement dramatically — the central trade in propulsion selection. Cold gas (Isp ≈ 50–90 s) is simple but heavy; hydrazine monopropellant (≈ 200–235 s) and MMH/NTO bipropellant (≈ 300–340 s) are denser and more efficient; electric propulsion (≈ 1000–3000 s) is the most mass-efficient but delivers only milli-newtons of thrust, so burns last weeks.

Tank volume is sized from the stored propellant density and an ullage fraction — the gas headspace left for pressurant and thermal expansion. The internal tank volume is V = m_p / ρ / (1 − ullage). Cold gas is the only family stored as a gas, so its low density (~56 kg/m³ at high pressure) makes its tanks far bulkier than the dense liquids. Burn time at constant thrust is t = m_p · ve / F; for low-thrust electric systems the tool also suggests a count of discrete burns, each capped at one day of thrusting.

What this tool does not capture: gravity and steering losses, finite-burn losses for non-impulsive manoeuvres, residual and trapped propellant, pressurant mass, feed-system and tankage structural mass, blow-down pressure decay, and throttling. The result is a first-order trade-study sizing — detailed mission design needs a full propulsion feed-system model and an integrated mass budget.