Turbopump

A turbopump is a fluid pump with two main components: a liquid pump driven by a gas turbine, usually both mounted on the same shaft, or sometimes geared together. They were initially developed in Germany in the early 1940s. The most common purpose of a turbopump is to produce a high-pressure fluid for feeding a combustion chamber. While other use cases exist, they are most commonly found in liquid rocket engines. Turbopump fed systems scale much more favorably in large rockets than pressure fed systems, which require increasingly thick and heavy tanks to supply high chamber pressures in the engines.

There are two common types of pumps used in turbopumps: a centrifugal pump, where the pumping is done by throwing fluid outward at high speed, or an axial flow pump, where alternating rotating and static blades progressively raise the pressure of a fluid. Axial flow pumps have small diameters but give relatively modest pressure increases. Although multiple compression stages are needed, axial flow pumps work well with low density fluids. Centrifugal pumps are far more powerful for high-density fluids but require large diameters for low density fluids.

The pump side of turbopumps consist of impellers that spin at very high speeds (thousands of RPM) in order to pump liquid propellants. Impellers are mounted on a central shaft, which also has a turbine mounted to it (or in some cases geared off on a different shaft). The turbine supplies shaft power, which is then consumed by the impellers in order to impart energy to the liquid propellants. Impellers mostly impart this energy by accelerating the liquid to a high velocity. However the ultimate goal is not a fast liquid, but a high pressure one; so surrounding the impeller is either a volute or a diffuser - these are specially shaped housings to decelerate the flow which then consequently dramatically increases its pressure (via Bernoulli's principle). The liquid is then discharged to the rest of the rocket engine, or in some cases to a second impeller and volute/diffuser stage which increases the pressure even further.

Turbopumps on liquid rocket engines virtually always have inducers as well, upstream of the impellers. Inducers are spiral shaped pumping elements that serve to gently raise the pressure of the incoming fluid enough to prevent it cavitating when it reaches the impeller. In many cases the impeller and inducer are manufactured as a single component, with a gradual transition between the axial spiral and the radial blades.

The turbine side of turbopumps consist of one or more stages, where each stage has a stator and a rotor. Individual rotor discs in a turbine are more commonly referred to as wheels in the modern day. These turbines are virtually always of the axial type, because of the very high gas flow (volumetrically) needed to supply enough shaft power for a liquid rocket engine. Contrast this with turbochargers, which usually feature radial turbine designs because of their much lower gas flow.

Upstream of the turbine is the turbine manifold, which collects gas from whatever source that rocket engine's cycle has upstream of it, and then disperses it circumferentially along the rim of the turbine. It then flows from the manifold axially downwards to the stages of the turbine. Stators are typically bladed, though it is also quite common (where pressure drop is particularly high, as in gas generator cycles) to forgo blades and drill angled nozzles directly off of the manifold itself to then impinge on the turbine wheel.

Downstream of the turbine varies based on cycle - in closed cycles it leads to the main injector of the engine, where (depending on whether the turbine is fuel-rich or ox-rich), one of the propellants can be injected into the main combustion chamber as a gas which can be very advantageous for promoting propellant atomization and mixing. In open cycles it is dumped to atmosphere. This can either mean dumped overboard directly off the side of the engine, or it can also lead to a manifold on the rocket engine nozzle which then injects it in the main flowpath, far downstream of the throat where ambient pressure is much lower than the chamber. The purpose here is to provide extra film cooling to the nozzle, since the hot gas leaving the turbine is nevertheless much cooler than the gas in the main combustion chamber. the latter option is common in vacuum optimized open cycle engines because they have much larger nozzles (with correspondingly large areas that need cooling, often without a regen jacket at its furthest extremes). It is important to note that the dumped gas from the turbine can still provide a non-negligible portion of the engine thrust. For this reason even if it is dumped overboard directly, there will usually still be a housing and a mild converging-diverging nozzle downstream of the turbine to take full advantage of the extra thrust opportunity. There is also an opportunity to extract waste heat from the flow at this point via heat exchangers; useful for heating up repressurizing gas for the tanks, for example.

Turbomachinery / engine cycle design looks very different in liquid rocket engines compared to air-breathing engines (turbojets) for essentially one main reason: