# Thrust & acceleration produced by a rocket engine

For given propellants, with their mass flow decided by the rpm of the turbo pump, what decides the thrust and acceleration produced by a rocket engine? Can we control them independently?

## 1 Answer

The mass flow rate multiplied by the average exhaust velocity determines the thrust.

The thrust divided by the remaining mass of the rocket determines the acceleration. The thrust and acceleration can’t be controlled independently, except of course by discarding (or, I suppose, collecting) mass.

The design of the combustion chamber and nozzle and the chemistry of the combustion determines the exhaust velocity. Varying the mass flow rate changes the exhaust velocity slightly given a fixed engine design.

• Borogove: Important information - "The thrust & the acceleration cannot be controlled independently". It means we cannot really "economize" on the propellant, for a given payload to make it orbit? I was thinking that we can, with a given quantity of propellants, & CD nozzle designed to give thrust enough to launch a payload above atmosphere, control the acceleration to the minimum value (to overcome "g") so as to use balance fuel to impart orbital velocity to payload. Not possible? What would be the bare minimum acceleration that would be required by the payload, to cross Karman line? Jan 12, 2023 at 6:21
• I think you may be misunderstanding what thrust actually is. Jan 13, 2023 at 6:08
• Borogove: My understanding of thrust is "the force exerted by the engine in downward direction (in case of a vertical rocket) so as to push a certain mass upwards". The higher the thrust, the more is the mass which can be pushed upwards. Acceleration is the rate of change of the velocity with which that mass moves upwards. Shall be grateful if you correct my perception if it is wrong. Jan 14, 2023 at 8:32
• Your definitions are correct, but I get the impression you're thinking of thrust as something to be conserved. Limiting thrust to the minimum value to overcome g actually wastes propellant. Consider the limiting case where thrust is exactly equal to weight; you are burning propellant and producing thrust and getting nowhere. In the absence of atmospheric drag, the most efficient ascent is maximum thrust, because gravity costs you 9.8m/s of velocity for every second you spend in vertical ascent. Jan 14, 2023 at 22:05