Turbopump

This article is about the fuel pump. For turbine devices for producing a high vacuum, see Turbomolecular pump. For the vehicular air pump, see Turbocharger and Supercharger.

Part of an axial turbopump designed and built for the M-1 rocket engine

Cutaway of the turbopump used in the F1 rocket engine (Saturn V first stage)

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

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.

A common turbopump arrangement. Note there are many possible, and common, variations to this arrangement including mounting the turbine between the pumps, having a multistage turbine, using a stator upstream of the rotor wheel (the above without a stator is more common in impulse turbine designs), etc.

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.

History

The V-2 rocket used a circular turbopump to pressurize the propellant.

Early development

High-pressure pumps for larger missiles had been discussed by rocket pioneers such as Hermann Oberth. ^[1] In mid-1935 Wernher von Braun initiated a fuel pump project at the southwest German firm Klein, Schanzlin & Becker that was experienced in building large fire-fighting pumps. ^{[2] : 80} The V-2 rocket design used hydrogen peroxide decomposed through a Walter steam generator to power the uncontrolled turbopump ^{[2] : 81} produced at the Heinkel plant at Jenbach, ^[3] so V-2 turbopumps and combustion chamber were tested and matched to prevent the pump from overpressurizing the chamber. ^{[2] : 172} The first engine fired successfully in September, and on August 16, 1942, a trial rocket stopped in mid-air and crashed due to a failure in the turbopump. ^{[2] [4]} The first successful V-2 launch was on October 3, 1942. ^[5]

Starting from the 1938-1940, Robert H. Goddard's team also independently developed small turbopumps.

Development from 1947 to 1949

The principal engineer for turbopump development at Aerojet was George Bosco. During the second half of 1947, Bosco and his group learned about the pump work of others and made preliminary design studies. Aerojet representatives visited Ohio State University where Florant was working on hydrogen pumps, and consulted Dietrich Singelmann, a German pump expert at Wright Field. Bosco subsequently used Singelmann's data in designing Aerojet's first hydrogen pump. ^[6]

By mid-1948, Aerojet had selected centrifugal pumps for both liquid hydrogen and liquid oxygen. They obtained some German radial-vane pumps from the Navy and tested them during the second half of the year. ^[6]

By the end of 1948, Aerojet had designed, built, and tested a liquid hydrogen pump (15 cm diameter). Initially, it used ball bearings that were run clean and dry, because the low temperature made conventional lubrication impractical. The pump was first operated at low speeds to allow its parts to cool down to operating temperature. When temperature gauges showed that liquid hydrogen had reached the pump, an attempt was made to accelerate from 5000 to 35 000 revolutions per minute. The pump failed and examination of the pieces pointed to a failure of the bearing, as well as the impeller. After some testing, super-precision bearings, lubricated by oil that was atomized and directed by a stream of gaseous nitrogen, were used. On the next run, the bearings worked satisfactorily but the stresses were too great for the brazed impeller and it flew apart. A new one was made by milling from a solid block of aluminum. The next two runs with the new pump were a great disappointment; the instruments showed no significant flow or pressure rise. The problem was traced to the exit diffuser of the pump, which was too small and insufficiently cooled during the cool-down cycle so that it limited the flow. This was corrected by adding vent holes in the pump housing; the vents were opened during cool down and closed when the pump was cold. With this fix, two additional runs were made in March 1949 and both were successful. Flow rate and pressure were found to be in approximate agreement with theoretical predictions. The maximum pressure was 26 atmospheres (26 atm (2.6 MPa; 380 psi)) and the flow was 0.25 kilogram per second. ^[6]

After 1949

The Space Shuttle main engine's turbopumps spun at over 30,000 rpm, delivering 150 lb (68 kg) of liquid hydrogen and 896 lb (406 kg) of liquid oxygen to the engine per second. ^[7] While not technically a turbopump (in that it lacks a turbine), the Electron Rocket's Rutherford became the first engine to use an electrically-driven pump in flight in 2018. ^[8]

Impellers

A few criteria are used when sizing and designing impellers. The first is specific speed - this is a dimensionless parameter characterizing the impeller discharge, for which certain ranges of valves are empirically known to indicate different impeller designs would be optimal ^[9] .

${\displaystyle N_{s}={\frac {N_{RPM}\cdot Q_{gpm}^{0.50}}{H_{ft}^{0.75}}}\,\,\,\,\,\,\,\,n_{q}={\frac {N_{RPM}\cdot Q_{m^{3}/s}^{0.50}}{H_{m}^{0.75}}}\,\,\,\,\,\,\,\,\omega _{s}={\frac {N_{Hz}\cdot Q_{m^{3}/s}^{0.50}}{(g\cdot H_{m})^{0.75}}}}$

${\displaystyle N_{s}}$ is the imperial version, common in US literature. ${\displaystyle n_{q}}$ is common in European literature. ${\displaystyle \omega _{s}}$ is the dimensionless version, but is not yet commonly seen in pump literature. N is shaft speed, Q is the volumetric flow rate requirement, and H is the head rise requirement.

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.

${\displaystyle N_{ss}={\frac {N\cdot Q^{0.5}}{(NPSH_{R})^{0.75}}}}$

NPSH is net positive suction head; the amount of head required (subscript R) to be generated in the fluid before it reaches the impeller inlet in order to not excessively cavitate 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%.

Centrifugal (Radial) Impellers

Main article: Centrifugal pump

Most turbopumps have centrifugal impellers - the fluid enters the pump along its rotational axis and the impeller accelerates the fluid to high speed. The fluid then passes through a volute (which spirals outwards to the outlet) or a diffuser, which is a ring with multiple diverging channels. This causes a large increase in dynamic pressure as fluid velocity is lost. The volute or diffuser turns the high kinetic energy into high pressures (hundreds of bar is not uncommon), and if the outlet backpressure is not too high, high flow rates can be achieved.

In centrifugal turbopumps a rotating disk throws the fluid to the rim.

Axial Impellers

Main article: Axial-flow pump

Axial impellers also exist. In this case the shaft essentially has (sometimes multiple) rotors and stators along the shaft, and the pump the fluid in a direction parallel with the main axis of the pump. Compared to centrifugal impellers, axial impellers trade lower head generation for higher volumetric flowrates of propellants. For this reason they are common for pumping liquid hydrogen, because of its significantly lower density than essentially all other propellants which use centrifugal pump designs.

Inducers

Main article: Inducer

It is very common for turbopumps to feature inducers as well, upstream of the impellers. The inducer is an axial, spiral design that raises the fluid pressure enough to prevent cavitation when it reaches the entrance to the impeller. The head pressure that the fluid rises over the length of the inducer is termed the NPSH_A (NPSH available). This must be above the NPSH_R of the impeller: NPSH_margin = NPSH_A / NPSH_R. Turbopumps also require a certain NPSH before it even reaches the inducer, again termed the NPSH_R for the inducer (so the inducer and impeller both have their own individual NPSH_R). This is achieved by pressurizing the propellant tanks to some extent; a few bar is typical. Inducers for cryogenic propellants usually cannot be designed to have zero NPSH_R because a rocket usually fills cryogenic propellants at their saturated state, meaning NPSH_A in the tank is zero. This gives no margin and thus cavitation at the inducer blades becomes likely. This can possibly be overcome with subcooled / densified propellants (e.g. Falcon 9). Regardless, some tank pressure is often desirable for structural stability of the rocket itself and so increases the NPSH_A, reducing the NPSH_R of the inducer (and so probably it's axial length) as a side benefit.

Turbines

Main article: Gas turbine

Turbopumps, by definition, are driven by gas turbines. Turbines are typically either of an impulse design (common in gas generator and other open cycles) or of a reaction design (common in staged combustion and other closed cycles). They can consist of one or more stages, where each stage has both a stator, which can be bladed or nozzles, and a wheel (sometimes referred to as a rotor in older papers and aero focused papers).

Open cycles aim to increase efficiency by minimizing mass flow through the turbine, making up for it by maximizing pressure drop. This is because the mass flow is dumped overboard, a performance hit. Comparatively, maximizing pressure drop is easy to do because it dumps to ambient pressure, which will be significantly lower than the gas generator chamber pressure. This is true even if GG pressure were the same value as the main chamber pressure, which the pumps have to work hard enough to discharge to anyways. These considerations drive the designer towards impulse designs on the turbine, with gas flow expanded via converging-diverging blades or nozzles to supersonic velocities that then impinge on the turbine wheel.

Closed cycles aim to increase efficiency by minimizing pressure drop across the turbine, making up for it by maximizing mass flow. This is because the downstream pressure must be higher than the main chamber pressure. It is often significantly higher because of injector and regen jacket pressure losses. Consequently, the only method for increasing pressure drop is to increase chamber pressure in the preburner much higher than the main chamber. This puts significantly more load on the pumps which must have a high pressure discharge for the preburner. Comparatively, high mass flow is easy to accomplish because none is being dumped overboard - so it is common to route the entire mass flow of one propellant through the preburner and turbine. Full flow staged combustion cycles take this a step further by routing the entire mass flow (hence 'full flow') of both propellants through preburners and turbines, taking advantage of essentially 100% of possible mass flow through the engine to generate shaft power for the turbopumps. These considerations drive the designer towards reaction type designs on the turbine where gas flow is subsonically expelled from, and reacting against, the wheel blades.

Complexities of centrifugal turbopumps

Turbopumps have a reputation for being difficult to design for 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. ^[10]

Common problems include:

1. 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,
2. excessive recirculation of the fluid at inlet,
3. excessive vortexing of the fluid as it leaves the casing of the pump,
4. damaging cavitation to impeller blade surfaces in low-pressure zones.

In addition, the precise shape of the rotor itself is critical.

Turbopump Examples

Engine	Cycle	Fuel	Oxidizer	Pump Type	Shafts	Shaft Speed, RPM	Outlet Pressure, barA	Turbine Stages	Geared
F-1	Gas Generator	RP-1	LOX	Radial	Single	5488 [11]	110 / 128	2	No
RS-25 / SSME	Fuel Rich Staged [i]	Hydrogen	LOX	Axial/Radial	Quad	36000 HPFTP / 16185 LPFTP / 28120 HPOTP / 5150 LPOTP [12]	357 / 585	4 / 6	No
RS-68	Gas Generator	Hydrogen	LOX	Radial	Dual	21000 / 8700 [13]		2 / 2	No
J-2	Gas Generator	Hydrogen	LOX	Axial/Radial	Dual	27130 / 8753 [11]	77 / 85	2 / 2	No
RL10	Expander (Closed)	Hydrogen	LOX	Radial	Dual	30250 / 12100 [11]	41 / 68	2	Yes
RD-107	Catalyst Gas Generator	RP-1	LOX	Radial	Single [ii]			1	Yes [ii]
RD-180	Ox Rich Staged	RP-1	LOX	Radial	Triple [14]			1	No
RD-275	Ox Rich Staged	N2O4	UDMH	Radial	Single				No
YF-100	Ox Rich Staged	RP-1	LOX	Radial	Triple [15]				No
Merlin	Gas Generator	RP-1	LOX	Radial	Single			1	No
Raptor	Full Flow Staged	Methane	LOX	Radial	Dual				No
Archimedes	Ox Rich Staged	Methane	LOX	Radial	Single				No
Rutherford [iii]	Electric [iii]	RP-1	LOX	Radial	Dual				No
Reaver	Tap-Off	RP-1	LOX	Radial	Single				No
Lightning	Tap-Off	RP-1	LOX	Radial	Single			1	No
E-2	Ox Rich Staged	RP-1	LOX	Radial	Single	30000			No
Aeon-R	Gas Generator	Methane	LOX	Radial	Dual			1 / 1	No
Hadley	Ox Rich Staged	RP-1	LOX	Radial	Single				No
Zenith	Full Flow Staged	Methane	LOX	Radial	Dual				No

Where two values are given, fuel side listed first and oxidizer side listed second.

Notes

1. The low pressure fuel turbopump of the RS-25 is technically driven via expander cycle
2. The RD-107 technically has two more secondary shafts to pump hydrogen peroxide and nitrogen
3. Technically Rutherford is not a turbopump because it does not have a turbine. Shown for comparison

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
• Components of jet engines

References

1. Rakete zu den Planetenräumen; 1923
2. Neufeld, Michael J. (1995). The Rocket and the Reich. The Smithsonian Institution. pp. 80–1, 156, 172. ISBN   0-674-77650-X.
3. Ordway, Frederick I III; Sharpe, Mitchell R (1979). The Rocket Team. Apogee Books Space Series 36. New York: Thomas Y. Crowell. p. 140. ISBN   1-894959-00-0. Archived from the original on 2012-03-04.
4. Neufeld, Michael (2017-04-12). Von Braun: Dreamer of Space, Engineer of War. Knopf Doubleday Publishing Group. ISBN   978-0-525-43591-4.
5. Dornberger, Walter (1954) [1952]. Der Schuss ins Weltall / V-2 . US translation from German. Esslingan; New York: Bechtle Verlag (German); Viking Press (English). p.  17.
6. "Liquid Hydrogen as a Propulsion Fuel, 1945-1959". NASA. Archived from the original on 2017-12-25. Retrieved 2017-07-12.
7. Hill, P & Peterson, C.(1992) Mechanics and Thermodynamics of Propulsion. New York: Addison-Wesley ISBN   0-201-14659-2
8. Brügge, Norbert. "Electron Propulsion". B14643.de. Archived from the original on 26 January 2018. Retrieved 20 September 2016.
9. Gülich, Johann Friedrich (2020). "Centrifugal Pumps". SpringerLink. doi:10.1007/978-3-030-14788-4.
10. Wu, Yulin, et al. Vibration of hydraulic machinery. Berlin: Springer, 2013.
11. "Turbopump systems for liquid rocket engines". NASA. 1974.
12. "SPACE TRANSPORTATION SYSTEM" (PDF). HAER No. TX-116.
13. "Brush Seal Arrangement for the RS-68 Turbopump Set". NASA. 2006.
14. "Atlas V". www.ulalaunch.com. Retrieved 2025-10-08.
15. "YF-100". www.astronautix.com. Retrieved 2025-10-08.

External links

• Book of Rocket Propulsion
• M. L. "Joe" Stangeland (Summer 1988). "Turbopumps for Liquid Rocket Engines". Threshold – Engineering Journal of Power Technology. Rocketdyne. Archived from the original on 2009-09-24.