A rocket pushes itself forward by throwing mass backwards at high speed. Most thrust comes from momentum (ṁv_e), with a correction for any difference between exhaust and ambient pressure at the nozzle exit – why the same engine gives more thrust in vacuum.
How to use the Rocket Thrust Calculator
- Exhaust velocity v_e
- Propellant mass burned
- Burn time
- Nozzle exit area A_e
- Exit pressure p_e
- Ambient pressure p_a
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the exhaust velocity, the propellant burned and the burn time (or the mass flow), and the pressures and nozzle exit area.
Tips
- Orbital formulas use the mass of the central body, not the satellite.
- Altitude is measured from the surface; orbital radius from the centre.
- Astronomical units, light-years and parsecs are all in the unit menus.
- Real missions also need margins for drag, gravity losses and steering.
Common mistakes to avoid
- Using altitude where the orbital radius is needed.
- Mixing sidereal and synodic periods.
- Forgetting that delta-v depends on the mass ratio, not the fuel mass alone.
Inputs explained
| Field | What to enter |
|---|---|
| Exhaust velocity v_e | Units: m/s, km/h, mph, ft/s, knots, km/s |
| Propellant mass burned | Units: kg, g, mg, t, lb, oz |
| Burn time | Units: s, ms, min, h, days |
| Nozzle exit area A_e | Units: m², cm², mm², km², in², ft² |
| Exit pressure p_e | Units: kPa, Pa, MPa, bar, atm, psi |
| Ambient pressure p_a | Units: kPa, Pa, MPa, bar, atm, psi |
What each result means
- Mass flow rate ṁ = [Propellant mass burned]÷[Burn time]
- Thrust F = [Propellant mass burned]÷[Burn time]×[Exhaust velocity v_e] + ([Exit pressure p_e] – [Ambient pressure p_a])×[Nozzle exit area A_e]
- Momentum thrust = [Propellant mass burned]÷[Burn time]×[Exhaust velocity v_e]
- Effective specific impulse = [Thrust F]÷([Mass flow rate ṁ]×9.80665)
Example values
How the main result changes when Exhaust velocity v_e changes and everything else stays at the example values above:
| Exhaust velocity v_e (m/s) | Thrust F (kN) |
|---|---|
| 1,500 | 380.3454 |
| 2,250 | 583.0481 |
| 3,000 | 785.7508 |
| 3,750 | 988.4535 |
| 4,500 | 1,191.1562 |
| 6,000 | 1,596.5616 |
Formula
F = ṁ·v_e + (p_e − p_a)·A_e, ṁ = m_prop / tWorked example
Inputs — Exhaust velocity v_e: 3,000 m/s; Propellant mass burned: 1,000 kg; Burn time: 3.7 s; Nozzle exit area A_e: 0.8 m²; Exit pressure p_e: 70 kPa; Ambient pressure p_a: 101.325 kPa.
Results — Mass flow rate ṁ = 270.27027 kg/s; Thrust F = 785.75081 kN; Momentum thrust = 810.81081 kN; Effective specific impulse = 296.4599 s.
Key terms
- Mass flow rate
- Propellant mass expelled per second.
Frequently asked questions
Why do rockets perform better in vacuum?
Ambient pressure no longer pushes back on the nozzle exit.
What is an optimally expanded nozzle?
One where the exit pressure equals ambient pressure.
How much thrust does a Merlin engine make?
About 845 kN at sea level.
Can a rocket work in space without air to push against?
Yes – it pushes against its own exhaust (Newton's third law).
What formula does the Rocket Thrust Calculator use?
F = ṁ·v_e + (p_e − p_a)·A_e, ṁ = m_prop / t.