pump. Generally, axial pumps tend to give much lower pressures than centrifugal pumps, and a few bars is not uncommon. They are, however, still useful – axial pumps are commonly used as "inducers" for centrifugal pumps, which raise the inlet pressure of the centrifugal pump enough to prevent excessive cavitation from occurring therein. Complexities of centrifugal turbopumps Turbopumps have a reputation for being extremely hard to design to get optimal performance. Whereas a well engineered and debugged pump can manage 70–90% efficiency, figures less than half that are not uncommon. Low efficiency may be acceptable in some applications, but in rocketry this is a severe problem. Turbopumps in rockets are important and problematic enough that launch vehicles using one have been caustically described as a "turbopump with a rocket attached"–up to 55% of the total cost has been ascribed to this area. Common problems include: excessive flow from the high-pressure rim back to the low-pressure inlet along the gap between the casing of the pump and the rotor, excessive recirculation of the fluid at inlet, excessive vortexing of the fluid as it leaves the casing of the pump, damaging cavitation to impeller blade surfaces in low-pressure zones. In addition, the precise shape of the rotor itself is critical. Driving turbopumps Steam turbine-powered turbopumps are employed when there is a source of steam, e.g. the boilers of steam ships. Gas turbines are usually used when electricity or steam is not available and place or weight restrictions permit the use of more efficient sources of mechanical energy. One of such cases are rocket engines, which need to pump fuel and oxidizer into their combustion chamber. This is necessary for large liquid rockets, since forcing the fluids or gases to flow by simple pressurizing of the tanks is often not feasible; the high pressure needed for the required flow rates would need strong and thereby heavy tanks. Ramjet motors are also usually fitted with turbopumps, the turbine being driven either directly by external freestream ram air or internally by airflow diverted from combustor entry. In both cases the turbine exhaust stream is dumped overboard. See also Turboexpander Gas-generator cycle Staged combustion cycle Expander cycle References External links Book of Rocket Propulsion Turbines Pumps