Turbopumps

As the name suggests, a turbopump comprisescool down and closed when the pump was cold.
basically two main components: a pump and aWith this fix, two additional runs were made in
driving turbine, both mounted on the same shaft.March 1949 and both were successful. Flow rate
A turbopump can refer to either of two types ofand pressure were found to be in approximate
pumps.agreement with theoretical predictions. The
Turbomolecular pumps are also called turbopumpsmaximum pressure was 26 atmospheres and the
and are used to obtain high vacuum.flow was 0.25 kilogram per second.
Another type of turbopump is a centrifugal orCentrifugal and Axial turbopumps
axial pump.Most turbopumps are centrifugal - the fluid enters
Early developmentthe pump near the axis and the rotor accelerates
Turbopumps were originally developed for firethe fluid circumferentially and compresses it
fighting for pumping water at high rates andagainst the rim, generating high pressures
pressures.(hundreds of bar is not uncommon), and if the
For rocket motors, the initial breakthrough foroutlet backpressure is not too high, high flow
turbopumps used in rocket motors occurredrates.
under Dr. Walter Thiel, during the development ofAxial turbopumps also exist - in this case the axle
the V2 in Germany. Prior to Dr. Thiel's work,has essentially propellers attached to the shaft
pressurized tanks had been used. The earlyand the fluid is forced by these parallel with the
rocket turbopumps were slightly modifiedmain axis of the pump. Generally, axial pumps
turbopumps originally intended for pumping water.tend to give much lower pressures than
Using turbopumps in rockets was a breakthrough;centrifugal pumps, a few bar is not uncommon.
the power of the rocket motors was increasedHowever they are still useful - axial pumps are
by an order of magnitude, making the lifting ofcommonly used as 'inducers' for centrifugal
heavy loads practical.pumps; these raise the inlet pressure enough to
Development from 1947 to 1949prevent excessive cavitation from occurring within
The principal engineer for turbopump developmentthe centrifugal portion of the pump.
at Aerojet was George Bosco. During the secondComplexities of centrifugal turbopumps
half of 1947, Bosco and his group learned aboutTurbopumps have a reputation for being
the pump work of others and made preliminaryextremely hard to design to get optimum
design studies. Aerojet representatives visitedperformance. Whereas a well engineered and
Ohio State University where Florant was workingdebugged pump can manage 70-90% efficiency,
on hydrogen pumps, and consulted Dietrichfigures less than half that are not uncommon.
Singelmann, a German pump expert at WrightLow efficiency may be acceptable in some
Field. Bosco subsequently used Singelmann's dataapplications, but in rocketry this is a severe
in designing Aerojet's first hydrogen pump.problem. Turbopumps in rockets are important
By mid-1948, Aerojet had selected centrifugaland problematic enough that launch vehicles using
pumps for both liquid hydrogen and liquid oxygen.one have been caustically described as a
They obtained some German radial-vane pumps'turbopump with a rocket attached'- up to 50% of
from the Navy and tested them during thethe total cost has been ascribed to this area.
second half of the year.Common issues include:
By the end of 1948, Aerojet had designed, built,excessive flow from the high pressure rim back
and tested a liquid hydrogen pump (15 cmto the low pressure inlet along the gap between
diameter). Initially, it used ball bearings that werethe casing of the pump and the rotor
run clean and dry, because the low temperatureexcessive recirculation of the fluid at inlet
made conventional lubrication impractical. Theexcessive vortexing of the fluid as it leaves the
pump was first operated at low speeds to allowcasing of the pump
its parts to cool down to operating temperature.In addition, the precise shape of the rotor itself is
When temperature gauges showed that liquidcritical.
hydrogen had reached the pump, an attempt wasDriving Turbopumps
made to accelerate from 5000 to 35 000Steam turbine powered turbopumps do exist and
revolutions per minute. The pump failed andare employed when there is a source of steam,
examination of the pieces pointed to a failure ofe.g. the boilers of steam ships. Nowadays gas
the bearing, as well as the impeller. After someturbines are usually used when electricity or
testing, super-precision bearings, lubricated by oilsteam is not available and place or weight
that was atomized and directed by a stream ofrestrictions permit the use of more efficient
gaseous nitrogen, were used. On the next run,sources of mechanical energy.
the bearings worked satisfactorily but theOne of such cases are rocket engines which need
stresses were too great for the brazed impellerto pump fuel and oxidizer into their combustion
and it flew apart. A new one was made by millingchamber. This is necessary for large liquid rockets,
from a solid block of aluminum. Time was runningsince forcing the fluids or gases to flow by simple
out, as the contract had less than six months topressurizing of the tanks is often not feasible: The
go. The next two runs with the new pump werehigh pressure needed for the required flow rates
a great disappointment; the instruments showedwould need strong and heavy tanks.
no significant flow or pressure rise. The problemRamjet motors are also usually fitted with
was traced to the exit diffuser of the pump,turbopumps, the turbine being driven either
which was too small and insufficiently cooleddirectly by external freestream ram air or
during the cool-down cycle so that it limited theinternally by airflow diverted from combustor
flow. This was corrected by adding vent holes inentry. In both cases the turbine exhaust stream
the pump housing; the vents were opened duringis dumped overboard.