Turbopumps are used with rocket engines to provide pressure for the fuel and oxidizer to be injected into the combustion chamber. A turbopump is used when it is infeasable to use a pressure-fed engine, which requires much stronger tanks to hold the fuel and oxidizer.
Turbomolecular pumps are also called turbo pumps and are used to obtain high vacuum, for instance in the manipulation of matter or chemicals in which high purity of products or contamination is at issue, or in which high vacuum is necessary for its insulative properties. In these cases, some other form of mechanical "backing" pump is used to lower the pressure to a point at which the turbopump can operate effectively and sustainably. For high vacuum applications, turbopumps have in some instances replaced diffusion pumps, many of which used toxic mercury as their working fluid.