Propulsion Sizing Calculator

Tsiolkovsky rocket equation. propellant mass from dry mass, exhaust velocity from Isp, first-order tank + burn-time sizing. cold-gas / monoprop / bipropellant / electric. engineering trade-study accuracy.

This calculator solves the Tsiolkovsky rocket equation - Δv = ve · ln(m_wet / m_dry) - turning a delta-v (Δv) budget into propellant mass, wet mass, mass ratio, tank volume, and burn time, with exhaust velocity from specific impulse (Isp): ve = Isp · g0. The propellant families span cold gas, hydrazine monopropellant, MMH/NTO bipropellant, and electric, with Isp and density values in the ranges of SMAD ch. 17 (Wertz et al., 2011) and Sutton's Rocket Propulsion Elements - which makes it equally at home as a CubeSat propulsion calculator or a smallsat deorbit-burn sizer.

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.