Thrust

Intermediate

Force on the rocket from exhaust.

Key relationF = v_e dm/dt

The force from ejecting mass is F = vₑ(dm/dt) (plus pressure terms). It is the rate of change of momentum of the exhaust.

Worked example

1000 kg/s at 3000 m/s gives 3 MN.

Common mistake

Thrust is independent of air.

In more detail

Thrust is the force a rocket or jet exerts from expelling mass, equal to the exhaust speed times the rate of mass loss, F = v_e dm/dt.

Rocket engines are rated by their thrust in kilonewtons.
Jet engines produce thrust by accelerating air backward.
Engineers calculate the thrust of a jet engine from the mass of air it expels each second and the exhaust speed.

How can thrust be increased?

By increasing the exhaust speed or the mass flow rate.

Is pressure also involved?

In real rockets, pressure at the nozzle exit adds to the thrust.

Why is thrust larger with higher exhaust speed?

Thrust equals the mass flow rate times the exhaust speed, so a faster exhaust gives a larger push for the same fuel flow.