Gc

Two main pumps and two booster pumps, each with its own gas turbine

With gear case, turbine can run faster. The two pumps have different speeds.

Two main pumps and two booster pumps, each with its own gas turbine

FIGURE 10-4. Simplified diagrams of different design arrangements of turbopumps. F is fuel pump, O is oxidizer pump, T is turbine, G is hot gas, and GC is gear case.

turbopumps for the fuel and the oxidizer can give the lowest energy and overall mass (J-2, SSME, LE-7, Vulcain 60).

Usually, the preliminary analysis for the pump is done first. Avoiding excessive cavitation sets a key pump parameter, namely the maximum shaft speed. This is the highest possible shaft speed, which in turn allows the lightest turbopump mass, without excessive cavitation in the pump. If excessive cavitation occurs at the leading edge of the first impeller (inducer or main impeller), then the flow will become unsteady and variable, leading to lower thrust and possible combustion instability. The amount of pressure in the vehicle (gas pressure in propellant tank plus the static elevation pressure) that can be made available to the engine (at the pump inlet) for suppressing cavitation has to be larger than the impeller vanes' own pressure limit to cavitate. This allows us then to determine the shaft speed, which in turn can establish the approximate pump efficiencies, impeller tip speed (usually limited by the material strength of the impeller), number of pump stages, key dimensions of the impeller, and the pump power requirements. All this will be discussed further (including key equations) in the pump section of this chapter.

The key turbine parameter can be estimated, because the power output of the turbine essentially has to equal the power demand of the pump. If the pump is driven directly, that is without a gear case, then the pump speed and the turbine speed are equal. From the properties of the turbine drive gas (temperature, specific heat, etc.), the strength limits of the turbine materials, and the likely pressure drop, it is possible to determine the basic dimensions of the blades (pitch line velocity, turbine nozzle outlet velocity, number of rows (stages) of blades, turbine type, or turbine efficiency). The particular arrangement or geometry of the major turbopump components is related to their selection process. Most propellant pumps have a single-stage main impeller. For liquid hydrogen with its low density, a two- or three-stage pump is normally needed. Usually some design limit is reached which requires one or more iterations, each with a new changed approach or parameter. The arrangement of the major turbopump components (Fig. 10^) is also influenced by the position of the bearings on the shaft. For example, we do not want to place a bearing in front of an impeller inlet because it will cause turbulence, distort the flow distribution, raise the suction pressure requirement, and make cavitation more likely to occur. Also, bearings positioned close to a turbine will experience high temperatures, which influences the lubrication by propellant and may demand more cooling of the bearings.

The use of booster pumps allows lower tank pressure, and thus lower inert vehicle mass, and provides adequate suction pressures to the main pump inlet. Booster pumps are used in the Space Shuttle main engine and the Russian RD-170, as seen in Figs. 6-12 and 10-11. Some booster pumps have been driven by a liquid booster turbine using a small flow of high-pressure liquid propellant that has been tapped off the discharge side of the main pump. The discharged turbine liquid then mixes with the main propellant flow at the discharge of the booster pump.

Later in this section a few of the equations that apply to the steady-state (full thrust) operating condition will be described. However, no detailed discussion will be given of the transient starting conditions, such as the filling of pipes, pumps, or manifolds with liquid propellants, or the filling of turbines and their manifolds with high-pressure gas. These dynamic conditions can be complex, are related to the combustion reactions, and are sometimes difficult to analyze, yet they are very significant in the proper and safe operation of the engine. Each major rocket engine manufacturer has developed some methodology, usually analysis and hydraulic models, for these system dynamics that are often peculiar to specific engines and hardware (see Refs. 10-3 and 10^1).

Mass is at a premium in all flying installations, and the feed system is selected to have a minimum combined mass of tubines, pumps, gas generator, valves, tanks, and gas generator propellants. Some of the considerations in the design of turbopumps are the thermal stresses, warpage due to thermal expansion or contraction, axial loads, adequate clearances to prevent rubbing yet minimize leakage, alignment of bearings, provisions for dynamic balancing of rotating parts, mounting on an elastic vehicle frame without inducing external forces, and avoiding undue pressure loads in the liquid and gas pipes.

Vibrations of turbopumps have caused problems during development. The analyses of the various vibrations (shaft, turbine blades, liquid oscillations, gas flow oscillations, or bearing vibrations) are not given here. At the critical speed the natural structural resonance frequency of the rotating assembly (shaft, impellers, turbine disk, etc.) coincides with the rotation operating speed. A slight unbalance can be amplified to cause significant shaft deflections (in bending), bearing failure, and other damage. The operating speed therefore is usually lower and sometimes higher than the critical speed. A large diameter stiff shaft, rigid bearings, and stiff bearing supports will increase this critical speed, and damping (such as the liquid lubricant film in the bearing) will reduce the vibration amplitude. Also, this critical shaft frequency or the operating speed should not coincide with and excite other natural vibration frequencies, such as those of various parts (piping, bellows, manifolds, or injector dome). The solving of various internal vibrations problems, such as whirl in bearings and blade vibrations, is reported in Ref. 10-5.

Bearings in most existing turbopumps are high precision, special alloy ball or roller bearings. Some ball bearings can take both radial and axial loads. Ball and roller bearings are limited in the loads and speeds at which they can operate reliably. In some turbopump designs this maximum bearing speed determines the minimum size of turbopump, rather than the cavitation limit of the pump. More recently, we use hydrostatic bearings where the shaft rides on a high-pressure fluid film; they have good radial load capacity, can provide some damping of oscillations and a stiff support. Axial loads (due to pressure unbalance on impellers and turbine blades) can be taken by special hydrostatic bearings. Since there is no direct contact between rolling and stationary assemblies, there is little or no wear and the life expectancy of these hydrostatic bearings is long. However, there is rubbing contact and wear at low speeds, namely during start or shutdown (see Ref. 10-6).

Cooling and lubricating the bearings and seals is essential for preventing bearing problems. A small flow of one of the propellants is used. Hydrocarbon fuels are usually good lubricants and hydrogen is a good coolant, but a marginal lubricant. If an oxidizer is used as the coolant and lubricant, then the materials used for bearings and seals have to be resistant to oxidation when heated during operation.

If the turbopump is part of a reusable rocket engine, it becomes more complex. For example, it can include provision to allow for inspection and automatic condition evaluation after each mission or flight. This can include an inspection of bearings through access holes for boroscope instruments, checking for cracks in highly stressed parts (turbine blade roots or hot-gas high-pressure manifolds), or the measurement of shaft torques (to detect possible binding or warpage).

Pumps

Classification and Description. The centrifugal pump is generally considered the most suitable for pumping propellant in large rocket units. For the large flows and high pressures involved, they are efficient as well as economical in terms of mass and space requirement.

Figure 10-5 is a schematic drawing of a centrifugal pump. Fluid entering the impeller, which is essentially a wheel with spiral curved vanes rotating within a casing, is accelerated within the impeller channels and leaves the impeller periphery with a high velocity to enter the volute, or collector, and thereafter the diffuser, where conversion from kinetic energy (velocity) to potential energy (pressure) takes place. In some pumps the curved diffuser vanes are upstream of the collector. The three-dimensional hydraulic design of impeller vanes, diffuser vanes, and volute passages can be accomplished by computer programs to give high efficiency and adequate strength. Internal leakage, or circulation between the high-pressure (discharge) side and the low-pressure (suction) side of an impeller, is held to a minimum by maintaining close clearances between the rotating and stationary parts at the seals or wear ring surfaces. External leakage along the shaft is minimized or prevented by the use of a shaft seal. Single-stage pumps (one impeller only) are stress-limited in the pressure rise they can impart to the liquid, and multiple-stage pumps are therefore needed for high pump head,* such as with liquid hydrogen. References 105 to 10-7 give information on different pumps. There is a free passage of flow through the pump at all times, and no positive means for shutoff are provided. The pump characteristics, that is, the pressure rise, flow, and efficiency, are functions of the pump speed, the impeller, the vane shape, and the casing configuration. Figure 10-6 shows a typical set of curves for centrifugal

FIGURE 10-5. Simplified schematic half cross section of a typical centrifugal pump. *See footnote on page 367.

FIGURE 10-5. Simplified schematic half cross section of a typical centrifugal pump. *See footnote on page 367.

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