Turbopump
A turbopump is an assembly consisting of a liquid pump driven by a gas turbine, connected via a shaft. They were initially developed in the US and Germany in the 1930s and 1940s. While other use cases can exist, the primary purpose of turbopumps is to dramatically raise the pressure of liquid propellants and feed them to the combustion chamber of a rocket engine. While they have considerably higher design complexity, 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.
Two types of pumps have been used in turbopumps: most common are centrifugal pumps, where the pumping is done by throwing fluid outward at high speed, while much rarer are axial-flow pumps, where alternating rotating and static blades progressively raise the pressure of a fluid. Centrifugal designs give high head but modest flow rates, while axial flow rates give high flow rates but modest head, and so usually need many stages in series.
Design principles
Hydrodynamic design
The pump side of turbopumps consist of impellers that spin at very high speeds in order to pump liquid propellants. Impellers are mounted on a central shaft, which also has a turbine mounted to it. 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. 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.
Aerodynamic design
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 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 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, 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. 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.
Cycle design
Turbomachinery / engine cycle design looks very different in liquid rocket engines compared to air-breathing engines for essentially one main reason: turbine materials cannot survive combustion chamber temperatures. Rocket engine cycles are all various workarounds to this fundamental problem.The first ever turbopump designs did partially put the turbine into the main combustion chamber flow with regenerative cooling. By the time of actual builds in the later 1930s, he had moved on to a rudimentary expander cycle, and then ultimately a gas generator cycle.
The turbine of a turbopump is always driven by high pressure gas. The exact source of this gas is the primary differentiator between the various rocket engine cycles. Air-breathing engines mount their turbine downstream of the burner and take direct advantage of the full flow and pressure of the engine. Rocket engines have never been able to do this because their mixture ratios are much closer to stoichiometric and thus the flame temperature in the combustion chamber is dramatically higher. They are so high that nearly all possible materials would melt, and even the few that do have very little structural strength left at these temperatures.
For this reason, rocket engine cycles are all various schemes to circumvent this and supply hot gas to the turbine that is nevertheless much cooler than the main combustion chamber gas. Gas generator and staged combustion cycles do this by mounting an entirely separate and smaller combustion chamber to the engine, termed the gas generator or the preburner. These smaller chambers run very far from stoichiometric, either with way too much fuel or way too much oxidizer. Hence, one can have "fuel rich" and "ox rich" gas generator and staged combustion cycles. One could also have two preburners, one fuel rich and one ox rich, which is termed "full flow staged combustion".
Beyond these, there are also expander cycles, where liquid propellant is heated in the regenerative cooling loop of the main combustion chamber, to the point of boiling, and then fed as gas to the turbine. The last major cycle is the tap off cycle, where a portion of the main combustion gas is "tapped off" and routed to the turbine. Because of the aforementioned temperature problem, tap off cycles require large dedicated heat exchangers to rapidly cool the re-routed gas before it reaches the turbine.
Mechanical design
The collection of all rotating components in a turbopump are collectively known as the "rotor". The rotor is spinning at extreme angular velocities: shaft speeds in the tens of thousands of RPM are common. Nominally the only mechanical connection between the rotor and the rest of the turbopump is via the bearings. Most common by far are ball bearings, with some modern exceptions pivoting to hydrodynamic bearings. The goal of bearing selection is to minimize friction - both because high friction can wear out the bearing, and also because any frictional energy losses are dissipated as heat that must be carried away rapidly to not destroy the bearing. The extra challenge in turbopump design is that the local environment in the pumps is very often at cryogenic temperatures, which virtually all greases and oils normally used to lubricate bearings are not compatible with. Therefore, turbopump bearings do not use lubricants at all in the traditional sense. Rather, they are installed as bare metal, and some amount of cold propellant is intentionally routed through them to dissipate the heat generated by their friction. This bearing cooling circuit is a secondary flow that the hydrodynamic designer must also design in addition to the primary flow of the propellant through the inducer/impeller/volute.Turbopumps can be very sensitive to the exact placement of components and the loads/stresses developed in them. Hydrodynamic considerations typically demand very tight clearances between the impellers / inducers and the pump housings, as well as aerodynamic considerations demanding tight clearances between turbine wheels and stators / manifolds. Furthermore, rotordynamics demands a high stiffness coupling of the rotating components with the shaft, especially when it comes to the turbine wheel.
These considerations and more demand high precision and high stiffness mechanical design. Bolted joints are generally the default method by which to join parts; some turbopumps have welded joints as well but require more careful consideration and analysis because of their generally lower stiffness, potential for thermally induced warpage of the parts during the welding process, as well as increased risk of fatigue over the life of the turbopump. In order for the rotor to act structurally as one rigid object, all of the components are stacked into one long stackup that envelops the entire shaft and then is preloaded onto it from both ends. This moderately loads the ball bearings, which are usually of the angular contact type, which increases their stiffness. Typically the preload is supplied one end by a bolt clamping onto the nose of the inducer and threaded into the end of the shaft below it. Depending on the exact configuration of the turbopump, the other end could be another inducer, or a turbine wheel which will also have preloaded bolt onto the end of the shaft.
Design of the shaft itself is driven by the need to carry high torque; the more torque it can carry the more power can be transferred from the turbine to the pump. Shaft power is the product of shaft speed and shaft torque. This high torque requirement drives the designer to maximizing the polar moment of inertia of the shaft. It is not uncommon for shafts to be hollow, as this maximizes this polar moment of inertia for a given weight of material. Shaft also need to transfer torque to the components of the rotor stackup. This can be accomplished via keyways, which carry less torque but are easier to manufacture, splines which generally carry higher torque but more difficult to manufacture, or shear pins, which are common for components attached to the circular face of the shaft.