Experimental study on the internal temperature rise of a high-speed canned motor pump for liquid rocket engines

High-speed electric pumps are now gradually applied in liquid rocket engines. Compared with the pumps with dynamic seals, the canned motor pumps are more reliable, and thus suitable for delivering risky propellants. However, the partial flow diverted to the air-gap for cooling and lubrication faces the risk of overheating caused by its friction against the rotor. This paper performed an experiment on this issue with a high-speed canned motor pump for a notional rocket engine. Impellers with and without balance holes, as well as different configurations of jet holes, were adopted and their effects were analysed. Results showed that the internal temperature rise decreased by up to 78% when the balance holes were removed, and decreased by up to 40% when the jet hole configuration was modified. Both factors influenced the internal temperature rise by varying the flow rate of the cooling recirculation. The study suggested that the high-speed canned motor pump should be designed with an appropriate flow rate of the cooling recirculation, so as to control the temperature rise of the propellant and ensure safe operation of the rocket engine.


Introduction
In pump-fed systems for liquid rocket engines, the propellants are pressurized and delivered to the thrust chamber by the pumps (usually centrifugal pumps).To develop high pressure with light structural weight, the propellant pumps in rocket engines feature high speed that typically ranges from 20000 to 60000 r/min.In traditional pump-fed systems, the pumps are driven by turbines, so related gas supply components are also required, resulting in a complicated system composition and a limited regulating range.With the development of advanced motors and batteries, electric pumps are gradually applied in low-thrust liquid rocket engines.Compared with the turbopump-fed system, the electric pump-fed system removes high-temperature components such as the turbine and the gas generator, featuring a simple system composition and high reliability.As the pump speed is directly controlled by the motor controller-rather than determined by the complex coupling relationship existing in the turbopump-fed system, a wider range of flow regulation can be easily achieved, and multiple restarts are unlimitedly supported.In addition, the electric pump-fed engine takes shorter time and lower cost for research & development.Therefore, its broad application can be expected, especially in the context of current development of low-cost, reusable launch vehicles.
Attempts to apply electric pumps in rocket engines can be traced back to 1984.Bell Aerospace Textron developed a 4.5 kN thrust engine operating with dinitrogen tetroxide/monomethyl hydrazine (NTO/MMH), and equipped it with an electric pump with a speed of 24000 r/min and power of 6.8 kW [1].The pump-fed engine was tested successfully and the feasibility of electric pump-fed rocket engines was demonstrated.Over 1985Over -1988, the European Space Agency (ESA) developed an Electric Propellant Pump System (EPPS) for MMH/NTO [2].To fulfil the requirements for an apogee engine of 3 kN thrust, small centrifugal pumps with low specific speeds were designed and tested by Volvo Flygmotor.The MMH and NTO pump was individually driven by its brushless DC motor, of which the speed and power reached 30000 r/min and 3 kW.Each motor rotor compartment was sealed off from the propellants, and the rotor was supported by two grease lubricated ball bearings.In the frame of the European FP7, the In-Space Propulsion (ISP-1) project was carried out during 2009-2012 and an electric pump for cryogenic propellants (LH2 and CH4) was developed by Snecma [3,4].No dynamic seals existed as a canned motor was adopted, and the bearings and the motor were internally cooled by a propellant flow picked at the impeller outlet and re-injected at the inlet.Electron rocket built by Rocket Lab is the first launch vehicle powered by electric pump-fed rocket engines.It was first launched in 2017 and 38 orbital launches have been accomplished as of July 2023.Its engine, named Rutherford, generates 24 kN thrust with liquid oxygen (LOX) and kerosene RP-1 as the propellants [5].Each propellant pump is individually driven by a brushless DC motor featuring a speed of 40000 r/min and power of 37 kW.Since 2020, Beihang University, together with Guizhou Aerospace Linquan Motor, has developed several canned motor pumps for hydrogen peroxide and kerosene, with speeds up to 36000 r/min and power up to 75 kW [6][7][8][9][10].The pumps were applied to feed systems for both liquid rocket engines and hybrid rocket motors, and firing tests have been successfully performed.
Among the above application of electric pumps, canned motor pumps were adopted in some of the cases, i.e., in ref. [3,4,[6][7][8][9][10].Owing to elimination of dynamic seals, canned motor pumps are more reliable and compact and thus suitable for delivering risky or hazardous propellants in rocket engines.With the rotor immersed in the operating medium, additional water friction loss occurs and leads to temperature rise of the partial flow that passes through the air-gap.Although this issue has been studied in the field of reactor coolant pumps and oil & gas pumps, however, these pumps generally operate at speeds below 3000 r/min [11][12][13][14].As the speed comes to over 30000 r/min in propellant pumps for rocket engines, the water friction loss increases intensely and makes the major loss of the canned motor.Consequently, the medium in the cooling recirculation is faced with a greater risk of overheating.As a critical issue that affects safe operation of the propellant pumps, the internal temperature rise in high-speed canned motor pumps is yet to be investigated.This paper developed a high-speed canned motor pump for rocket engines, and tests on its internal temperature rise were conducted.Influencing factors including the configuration of the jet holes and the balance hole area were analysed.

Metrics of the electric pump
The electric pump is developed for delivering the oxidizer of a notional rocket engine.The oxidizer is assumed to be dinitrogen tetroxide or hydrogen peroxide, both of which are storable propellant with strong oxidizing properties.The former is toxic and corrosive, featuring a relatively low boiling point (21℃) at atmospheric pressure, while the latter is thermally sensitive and unstable.Thus, the operating temperature of the media should be carefully controlled.Both media have a density of approximately 1.4 kg/L.
The design point of the electric pump is obtained according to the requirements of the feed system, as shown in table 1.The speed is determined with a comprehensive consideration of efficiency, weight and technology readiness level, and the motor power is derived with the expected pump efficiency (approximately 65%).

Design of the electric pump
The specific speed of the pump is 23 (defined as ), which is suitable for a centrifugal pump.To reduce the requirement on suction pressure, an inducer is equipped with the centrifugal impeller.As for the electric motor, a permanent magnet synchronous (PMS) motor is adopted for better operating quality and higher efficiency than that of the brushless DC (BLDC) motor, although a more complex controller is needed.

Overall design.
As mentioned above, the medium to deliver is aggressive, which indicates that possible leakage of a shaft seal becomes a huge safety risk.Therefore, a hermitically sealed canned motor pump is adopted and the operating medium is confined without any dynamic seals.The stator is segregated from the medium by the stator can, while the magnets are protected and isolated by the rotor can which is assembled and welded to the shaft.The rotor is totally immersed in the medium during operation, and the medium that flows through the air-gap may cool the motor and lubricate the bearings.
Various forms of the cooling recirculation can be designed according to the properties of the medium and operating conditions, and a simple form is adopted here for normal temperature medium.A partial flow is branched off at the outlet of the centrifugal impeller, and flows through the seal ring clearance on its rear shroud.If there are balance holes on the impeller, a portion of the partial flow is diverted back to the inlet of the impeller while the other enters the cooling recirculation.If not, all medium passing through the seal ring clearance goes into the recirculation.Then the medium flows through the front bearing, the air-gap, and the rear bearing in sequence and finally returns to the suction side of the pump through the hollow shaft.The structural layout and the cooling recirculation are shown in figure 1.The impeller has a wear ring on the front and rear shroud each.The diameters of the wear rings are 42 and 48 mm respectively, and the clearances are both 0.1 mm.In the initial design, 6 balance holes with a diameter of 2 mm are set on the impeller, connecting the cavity behind the rear shroud and the suction eye.The modified impeller removes all balance holes, and otherwise it is identical to the initial design.The initial and modified impellers are shown in figure 2.  In the initial design, 5 jet holes are set on the nut that locks the inducer at the shaft end, enabling the partial flow to return to the suction side of the pump through the hollow shaft.One of the jet holes is axial with a diameter of 3 mm and the other four are nearly radial, of which the diameter is 2 mm.The modified design aims to reduce the impact of the main flow on the jet and lower the flow resistance, then the axial hole is eliminated and 6 radial holes with a larger diameter of 4 mm are configured.The initial and modified configurations of jet holes are shown in figure 3.

Canned motor design.
A canned PMS motor is designed, and parameters related to the cooling recirculation flow are introduced as follows.The inner diameter of the stator core and the outer diameter of the rotor magnets are 53 and 45 mm respectively, leading to a magnetic air-gap of 4 mm.The stator can is constructed of carbon fibre reinforced plastic (CFRP) with an engineering plastic liner, so eddy current loss is basically eliminated.The thicknesses of the stator can and the rotor can are 1.8 and 1.2 mm respectively, and an operating air-gap of 1 mm can be derived.The length of the rotor can is 144 mm, and the diameter of the shaft hole is 5.5 mm.
The stator can is designed with a dome at the rear to reduce the seals, and a separate bearing seat for the rear bearing is embedded in the can.A threaded hole is reserved at the centre of the rear dome as a connecting seat, so that pressure and temperature sensors can be connected to the stator can through a joint.
Rolling bearings are adopted for this high-speed application.The front bearing is the fixed-end and a three-point contact ball bearing is selected, while a deep groove ball bearing is mounted at the rear serving as the free-end.
The schematic of the stator can and the rotor is shown in figure 4.

Schemes of the canned motor pump
Three schemes of the canned motor pump were tested.In scheme A, the initial impeller with balance holes as well as the initial configuration of jet holes was adopted.In scheme B, the modified configuration of jet holes was adopted, while the impeller remained unchanged.In scheme C, the modified impeller with no balance holes and the modified configuration of jet holes were adopted.The schemes of the canned motor pump are listed in table 3.

Test system
The test was conducted with water for safety reasons.The test system consisted of gas pressurization system, liquid flow system, electrical system and measurement & control system.The gas pressurization system included a compressed air source (0.8 MPa), an inlet valve, and an exhaust valve.The liquid flow system included a tank, a valve at the tank outlet, a flowmeter, the canned motor pump, an adjusting valve, and a container.The electrical system included a DC power supply unit and a motor controller (inverter).The measurement & control system included a data logger and a host computer.The schematic of the test system is shown in figure 6.In addition to the pressure and temperature sensors installed on the inlet and outlet pipelines, two sensors were mounted at the rear of the canned motor pump to test the internal pressure and temperature.To make it clear, the parameters were measured where the cooling recirculation flow had passed through the bearings and the air-gap but yet to enter the shaft hole, as seen in figure 4. The arrangement of the sensors in the test system is shown in figure 7.

Sensors and meters
The range and accuracy of the sensors and meters in the test system are listed in table 4.

Results and discussion
External characteristics including head and outlet temperature rise, as well as operating parameters including internal pressure difference and internal temperature rise, were examined in the test.Data was processed in methods listed in table 5. pin, pout, pint -inlet, outlet, and internal pressure; Tin, Tout, Tint -inlet, outlet, and internal temperature; ρ -density of water at room temperature, 998 kg/m 3 ; g -gravitational acceleration, 9.81 m/s 2 .It can be seen that the head curves of schemes A and B are almost the same at both 16000 and 24000 r/min, and the relation between the curves at different speeds is subject to the similarity laws.It indicates that the configuration of the jet holes has little effect on the primary flow in the impeller.

Effect of the jet holes
A slight deviation occurs between the outlet temperature rise curves of schemes A and B, but their trends are identical.The outlet temperature rise is an indicator of the specific energy loss in the pump (including water friction loss of the rotor), but it is also affected by the heat transfer with the motor and the atmosphere.Taking account that the difference between the outlet and inlet temperature is quite small compared with the range of the sensors, it can be considered that the outlet temperature rises of schemes A and B are at a similar level.For scheme A, the internal temperature rise varies from 19 to 42℃ at 16000 r/min, and from 45 to over 100℃ at 24000 r/min (not depicted on the curve for exceeding the upper limit of the sensor).It is much higher than the temperature rise at the outlet, because only a small part of the medium enters the cooling recirculation.According to calculations with empirical formulas as well as preliminary results of numerical simulations, the water friction loss of the rotor is as high as 8~10 kW under the rated condition, while the copper loss and iron loss total less than 3 kW according to the motor design.Eddy current loss can also be neglected for the stator can made of CFRP and engineering plastic.From another perspective, the water friction loss heats the medium in the air-gap directly, but heat from the stator is first conducted to the stator can and then transferred to the medium through convection.The materials of the stator can feature low thermal conductivities, and the heat capacity of the stator is large.Thus, it is reasonable to consider the water friction loss as the major reason of the internal temperature rise, at least in a short-term operation as performed in the test.
Although the water friction loss varies with the flow rate in the air-gap, preliminary results of numerical simulations show that the variety is within a narrow range at a given rotating speed.Therefore, the internal temperature rise is mainly affected by the flow rate of the cooling recirculation.In the designed canned motor pump, the cooling recirculation is driven by the pressure difference generated by the centrifugal impeller.As a result, for a given configuration, the flow rate of cooling recirculation is determined by the head of the pump.As the flow of the pump increases, the head gradually decreases, leading to a weaker driving force on the cooling recirculation and thus a lower flow rate.With the water friction loss varying only in a narrow range, the internal temperature rise grows higher.The only exception occurs in scheme A when the relative flow rate reaches 130%, which is most probably caused by an undesired reverse of the cooling recirculation.The axial hole on the nut makes it possible for the fluid with high momentum at the inlet to overcome the jet and enter the shaft hole directly, and the low head of the inducer as well as the impeller under the large flow condition exacerbates the situation.
The temperature rise in scheme A is obviously too high even for water, not to mention the sensitive oxidizers.Results show that the modified configuration of jet holes mitigates the situation.For scheme B, the internal temperature rise varies from 13 to 35℃ at 16000 r/min, and from 36 to 70℃ at 24000 r/min.Although the internal temperature still exceeds 100℃ at the rated speed (32000 r/min), the effect of jet holes can be evaluated.By eliminating the axial hole and increasing the area of the radial holes, the internal temperature rise decreases by 30%~40%.The nut without axial holes denies the potential reverse of the cooling recirculation, and it can be seen that the internal temperature rise of scheme B increases monotonically as the flow of the pump increases.
Variation of the internal pressure difference can also demonstrate the effect of the modified configuration of jet holes.The jet holes locate downstream of the cooling recirculation, and the modified configuration lowers its flow resistance.In consequence the pressure drop at jet holes decreases, leading to a lower internal pressure difference and a higher flow rate of the cooling recirculation.
Due to the excessive internal temperature rise detected at 24000 r/min, operation at the rated speed was not performed with scheme A. The rated speed was reached with schemes B and C, which will be discussed in the following section.

Effect of the balance holes
Test results of schemes B and C are compared to analyse the effect of the balance holes.Results are shown in figure 9.
The head curves of schemes B and C are almost the same at all tested speeds, and are verified to be subject to the similarity laws again.The outlet temperature rise curves are also similar at each speed.
For scheme C, the internal temperature rise varies from 6 to 8℃ at 16000 r/min, and from 12 to 15℃ at 24000 r/min.A significant decrease of 68%~78% is obtained by removing the balance holes on the impeller.The rated speed of 32000 r/min is reached with an internal temperature rise of 20~36℃, which is acceptable for the sensitive oxidizers.As analysed above, with the increase of the flow, the head decreases and the internal temperature rise grows higher.As the speed increases, the curve of the internal temperature rise moves up.For similar conditions, the ratio of temperature rises is approximately the square of the ratio of speeds.
The results demonstrate that an adequate flow rate of the cooling recirculation is essential for a high-speed canned motor pump.The balance holes that link to the impeller suction eye serve as a path parallel to the cooling recirculation.The excessively large area of the balance holes on the initial impeller leads to unreasonably low flow resistance of the parallel path, and thus much of the flow that passes through the rear seal ring clearance is diverted back to the impeller inlet directly.As a result, the flow rate of the cooling recirculation is not enough to take the water friction loss away with an acceptable temperature rise.By reducing the balance hole area, a larger portion of the flow is allocated to the cooling recirculation, and the internal temperature rise is then lowered.
The internal pressure difference increases obviously when the balance holes on the impeller are removed, which also indicates a growth in the flow rate of the cooling recirculation.The modification of the impeller locates upstream of the cooling recirculation, and the characteristic of flow resistance remains unchanged.To match the greater pressure drop in the shaft hole caused by the higher flow rate of the cooling recirculation, the internal pressure difference increases correspondingly.

Conclusions
A high-speed canned motor pump for liquid rocket engines was designed and fabricated.Tests were conducted with water to examine the internal temperature rise, and the effects of jet holes and balance holes were analysed.
The results demonstrated high water friction loss at high speeds, and stressed the necessity of an adequate flow rate of the cooling recirculation in a high-speed canned motor pump.Therefore, related configurations and parameters should be carefully designed, otherwise the propellant in the cooling recirculation would be overheated.
Compared with the initial configuration of jet holes, the modified one which removed the axial hole and enlarged the radial holes decreased the internal temperature rise by 30%~40%.The modified configuration reduced the impact and the flow resistance, leading to a larger flow rate of the cooling recirculation and a lower internal pressure difference.The potential reverse of the cooling recirculation was also avoided.
The balance hole area had a significant effect on the internal temperature rise, which decreased by 68%~78% when the balance holes were removed.When balance holes existed, a part of the fluid which passed through the rear seal ring clearance was diverted back to the suction eye of the impeller, and the balance hole area determined the portion of the fluid allocated to the cooling recirculation.The reduced balance hole area led to an increase in the flow rate of the cooling recirculation and a decrease in the internal temperature rise.However, the variation in the balance hole area would also vary the pressure distribution on the rear shroud, and thus the axial thrust of the rotor would be affected.This issue will be further studied in the future work.
With modified configurations of jet holes and balance holes, the designed canned motor pump was able to operate under the rated condition with an acceptable internal temperature rise.Therefore, the design of the high-speed canned motor pump was verified, and further simulations and tests can be conducted on this basis.

Figure 1 .
Figure 1.Structural layout of the electric pump.

Figure 4 .
Figure 4. Schematic of the stator can and the rotor.2.2.4.Canned motor pump assembly.The pump and the canned motor designed above are fabricated and assembled.The canned motor pump assembly is shown in figure 5.

Figure 6 .
Figure 6.Schematic of the test system.

Figure 7 .
Figure 7. Sensor arrangement in the test system.
Test results of schemes A and B are compared to analyse the effect of the jet holes.External characteristic curves (Head & Out.Temp.Rise vs Flow) are shown in the left of figure 8 and operating parameter curves (Int.Press.Diff.& Int.Temp.Rise vs Flow) are shown in the right.

Figure 8 .
Figure 8. Test results of the canned motor pump (schemes A and B).

Figure 9 .
Figure 9. Test results of the canned motor pump (schemes B and C).

Table 1 .
Design point of the electric pump.

Inducer Centrifugal Impeller Pump Casing Front Bearing Motor Housing Stator Can Rotor Stator (Core & Winding) Back Cover Cooling Recirculation Front Bearing Seat Rear Bearing Seat Rear Bearing
The inducer features a variable pitch with a conical hub and cylindrical blade tips.The centrifugal impeller is a closed impeller with 3D twisted blades, and splitter blades are configured.A single spiral volute with a trapezoid cross section is adopted.Main parameters of the designed pump are shown in table 2.

Table 2 .
Main parameters of the pump.

Table 3 .
Schemes of the canned motor pump.

Table 4 .
Range and accuracy of the sensors and meters in the test system.

Table 5 .
Data processing methods.