Turbopump

 ( Turbines ) 

A turbopump is a fluid pump with two main components: a liquid rotodynamic 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.

Turbopumps on liquid rocket engines virtually always have Pump inducer 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 Cavitation 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.

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 exchanger; useful for heating up repressurizing gas for the tanks, for example.

Turbine materials cannot survive combustion chamber temperatures.
     

Rocket engine cycles are all various workarounds to this fundamental problem.
     

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 (whose gas is ultimately dumped overboard) or the preburner (whose "pre-burnt" gas eventually reaches the main combustion chamber after passing through the turbine). These smaller chambers run very far from stoichiometric, either with way too much fuel or way too much oxidizer. Hence you can have "fuel rich" and "ox rich" gas generator and staged combustion cycles. You could also have two preburners, one fuel rich and one ox rich, which is termed "full flow staged combustion".

Beyond these, there are also , where liquid propellant is heated (usually fuel) 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 to rapidly cool the re-routed gas before it reaches the turbine.

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 (for the other propellant), or a turbine wheel which will also have preloaded bolt(s) 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(s). 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 and/or Shear pin, which are common for components attached to the circular face of the shaft (i.e. turbine wheels).

The dynamic seals in turbopumps have quite specialized requirements compared to seals in most systems. They must support very high shaft speeds on shaft of significant diameter, meaning rubbing velocities are very high. They usually need to be cryogenically compatible as well, and oxygen-compatible on the seals exposed to the oxidizer side. This eliminates the possibility of elastomer based seals, which will embrittle (and cannot hold up at these speeds anyways). Spring loaded and other compression type seals are also not practical at these speeds.

In practice, turbopumps primarily use three seals: Labyrinth seal, face seals, and carbon ring seals

In practice all three of these seal types will leak to some extent. A large part of seal design is providing safe flow paths for this leakage. Most imperative is that the interpropellant seal (IPS), which the vast majority of engines have at least one of, does not leak fuel and oxidizer together. This is often accomplished by having a central cavity that is continuously purged with inert gas (e.g. helium, nitrogen) at a higher pressure than the propellants on either side, so that the IPS will leak that inert buffer gas outwards from the cavity instead of propellants inwards to the cavity. The only engines that are able to forgo an IPS entirely are full flow staged combustion cycles, because they have one entirely fuel rich turbopump and one entirely ox rich turbopump that don't interact with each other.

$N\_s\; =\; \backslash frac\{N\_\{RPM\}\; \backslash cdot\; Q\_\{gpm\}^\{0.50\}\}\{H\_\{ft\}^\{0.75\}\}\; \backslash ,\backslash ,\backslash ,\backslash ,\backslash ,\backslash ,\backslash ,\backslash ,\; n\_q\; =\; \backslash frac\{N\_\{RPM\}\; \backslash cdot\; Q\_\{m^3/s\}^\{0.50\}\}\{H\_m^\{0.75\}\}\; \backslash ,\backslash ,\backslash ,\backslash ,\backslash ,\backslash ,\backslash ,\backslash ,\; \backslash omega\_s\; =\; \backslash frac\{N\_\{Hz\}\; \backslash cdot\; Q\_\{m^3/s\}^\{0.50\}\}\{(g\; \backslash cdot\; H\_m)^\{0.75\}\}$

$\backslash omega\_s\; =\; \backslash frac\{N\_s\}\{2730\}\; =\; \backslash frac\{n\_q\}\{52.9\}$

• $H$ = Hydraulic head
• $Q$ = volumetric flowrate
• $N$ = shaft speed

$N\_s$ is the imperial version, common in US literature. $n\_q$ is common in European literature. $\backslash omega\_s$ is the dimensionless version, but is not yet commonly seen in pump literature. The second parameter is similar: the suction specific speed. This characterizes the impeller's inlet (suction) conditions, and is used to quantify the required inducer and tank pressures upstream of the impeller.

$N\_\{ss\}\; =\; \backslash frac\{N\; \backslash cdot\; Q^\{0.5\}\}\{(NPSH\_R)^\{0.75\}\}$

NPSH is net positive suction head; NPSH_R is the amount of head required to be generated in the fluid before it reaches the impeller inlet in order to not excessively Cavitation in the impeller. "Excessive" is often defined as the level of cavitation that would degrade the pump's discharge head by 3% – hence it is common to see NPSH_R defined as NPSH_3%.

Another key parameter is the impeller's head coefficient $\backslash psi$. This characterizes how effective a given tip speed $u\_\{out\}$ is at generating head. Head coefficient is typically selected (for a given specific speed) from empirical curves generated by previous industry experience.

$\backslash psi=\backslash frac\{2gH\}\{u\_\{out\}^2\}$ in some sources; $\backslash psi=\backslash frac\{gH\}\{u\_\{out\}^2\}$ in others

$H=\; \backslash eta\; (u\_\{out\}\; v\_\{out\}\; -\; u\_\{in\}\; v\_\{in\})\; =\; \backslash eta\; \backslash omega\; (r\_\{out\}\; v\_\{out\}\; -\; r\_\{in\}\; v\_\{in\})$