The high-pressure fuel turbopump of the Space Shuttle main engine was about the size of a car's engine block and produced more than 70 000 horsepower. Where does an engine get that much power, and what does it cost to take it?
How much power does a pump need?
A pump raises the pressure of a liquid. The power it takes is the volume flow times the pressure rise, divided by the pump's efficiency:
The density in the denominator is the whole story of hydrogen pumps. Liquid hydrogen is 16 times less dense than liquid oxygen, so pumping the same mass to the same pressure takes 16 times the power. In a hydrogen engine the fuel pump is the monster; in a kerosene engine the oxygen pump is. And because the discharge pressure must exceed chamber pressure by the injector and cooling-jacket losses — often by 30–60%, and far more in staged-combustion cycles — pump power grows faster than chamber pressure.
A turbopump is a pump (or two) on a shaft with a turbine. Modern ones spin at 20 000–40 000 rpm, and their speed is limited by one phenomenon above all.
Cavitation
Liquid speeding up onto a pump blade drops in pressure, just as air does over a wing. If the local pressure falls below the propellant's vapour pressure, the liquid boils right there on the blade. Those bubbles collapse violently as they move into higher-pressure regions, pitting the metal, and the pump's pressure rise collapses with them. This is CavitationVapour bubbles forming where local pressure drops below vapour pressure; they collapse violently and destroy pump head..
The margin against it is the NPSHNet positive suction head: how far the pump-inlet pressure sits above the propellant’s vapour pressure. Too little and the pump cavitates.: how far the pressure at the pump inlet sits above the vapour pressure, measured in metres of liquid. The pump needs a certain NPSH, which rises steeply with speed; the vehicle supplies an available NPSH from tank pressure, the liquid column above the pump (multiplied by the vehicle's acceleration) and the propellant's temperature.
Designers attack cavitation from both ends. Inducers — small, gently loaded axial screws ahead of the main impeller — raise the inlet pressure a little while tolerating some cavitation themselves, cutting the required NPSH several-fold. Tank pressure helps directly but costs tank mass (Lesson 5). Subcooled, densified propellants, like SpaceX's LOX chilled to 66 K, have a much lower vapour pressure and are denser too.
Engine cycles: who pays for the pumps?
The turbine needs a hot, high-pressure gas. The engine cycle is the answer to where it comes from, and Sutton's family tree has five main branches:
- Gas generator (open cycle). A small side combustor burns a little propellant fuel-rich, at a temperature the turbine blades can survive, and the turbine exhaust is dumped overboard. Simple and decoupled, but the dumped gas makes little thrust: a few percent of the propellant is wasted. Merlin, F-1, Vulcain, RS-68.
- Expander. Fuel warmed in the cooling jacket drives the turbine and then goes to the injector. Nothing is burned outside the chamber and nothing is dumped — an elegant, gentle cycle. But the available heat scales with the chamber's surface, while the pump power scales with its volume flow, so it works only for small, hydrogen- or methane-fuelled engines. RL10, Vinci.
- Staged combustion (closed cycle). A preburner burns all of one propellant with a little of the other, drives the turbine with a huge mass flow, and sends the entire turbine exhaust into the main chamber to finish burning. Nothing is dumped, so chamber pressure can go very high — but the pumps must now deliver far above chamber pressure to push gas through the turbine and still get in. Fuel-rich (RS-25) or oxygen-rich (RD-180, NK-33, BE-4).
- Full-flow staged combustion. Two preburners, one fuel-rich and one oxygen-rich, each driving its own turbopump. Every molecule of propellant passes through a turbine, so each turbine needs only a small pressure ratio and runs relatively cool. The highest chamber pressures of any cycle. RD-270 (tested), Raptor.
- Electric pump. Batteries and electric motors drive the pumps. Wonderfully simple — and heavy, because batteries store far less energy than propellant. Viable only for small engines. Rutherford.
The "ladder" beneath the builder summarizes the chapter in one chart: each cycle has a ceiling set by its power balance. It is the reason high-pressure engines are all staged-combustion designs, and the reason the Isp gap between a gas-generator and a staged engine using the same propellants is a few percent — for a launch vehicle, often worth years of development.
Going deeper
Starting, throttling and controlling an engine
Sutton devotes a chapter to engine systems and controls, because an engine that cannot start smoothly or shut down safely is useless. A turbopump engine has a chicken-and-egg start: the turbine needs gas, which needs propellant, which needs the pump. Gas-generator engines are spun up by a start cartridge or by tank-head pressure flowing through the system; staged-combustion engines use carefully timed valve sequences, sometimes with spin-start helium. Ignition uses pyrotechnic igniters, spark torches, or a slug of hypergolic TEA–TEB (as on the Merlin and F-1).
In operation, a controller trims valves to hold thrust and mixture ratio against drifts in propellant temperature and pressure. Throttling is limited mostly by the injector: at low flow, the pressure drop across it falls with the square of the flow, and the injector loses stiffness (hello, chugging — Lesson 7). Deep-throttling engines use variable-area injectors such as pintles.
- Pump power : low-density hydrogen needs enormous pump power.
- Cavitation limits pump speed; the margin is NPSH. Inducers, tank pressure and subcooled propellants buy it back.
- Gas generator: simple, dumps a few percent of propellant; practical up to ~150 bar.
- Expander: heat from the cooling jacket; clean but limited to small engines.
- Staged combustion: preburner gas goes to the main chamber; ~200–260 bar. Full-flow: both propellants through turbines; up to ~300 bar.
- Electric pumps: simplest, heaviest; small engines only.
Next: a rocket with no pumps, no valves and no turning back — the solid rocket motor.