What is a turbopump in a rocket engine?
A turbopump is the pump that forces fuel and oxygen into the combustion chamber, driven by a small turbine spun by the rocket’s own burning propellant. Without it the engine could not push propellant in faster than the fire pushes back out.
A rocket chamber is not a calm place. A Merlin runs at 9.7 MPa — roughly a hundred times the air pressure around you. To get propellant in at all, it has to arrive at a higher pressure than that, and no tank could simply be squeezed hard enough without walls so thick the rocket would never fly.
So the engine pumps. A turbine spins a pair of impellers, one for kerosene and one for liquid oxygen, and they shove propellant into the chamber against everything coming the other way.
The clever part is what drives the turbine. A small amount of the propellant is burned separately, in a gas generator, purely to make hot gas to spin it. Merlin uses that arrangement, which is why its exhaust carries a dark line of soot: the gas-generator exhaust is deliberately fuel-rich and is dumped overboard rather than burned properly.
The numbers are the striking bit. A Merlin weighs 470 kg dry and produces 845 kN of thrust at sea level — a thrust-to-weight ratio of about 184. Almost none of that would be possible without a pump doing the work in the middle.
See it happen
Meet a RocketEvery part, namedSource: Merlin rocket engine family