Developmental History of Liquid Oxygen Turbopumps for the LE-7 Engine∗

Trans. Japan Soc. Aero. Space Sci. Vol. 44, No. 145, pp. 155–163, 2001

) ) ) ) By Kenjiro KAMIJO,1 Hitoshi YAMADA,2 Norio SAKAZUME3 and Shogo WARASHINA4

1)Tohoku University, Institute of Fluid Science, Sendai, Japan 2)National Aerospace Laboratory, Kakuda Research Center, Kakuda, Japan 3)National Space Development Agency of Japan, Tokyo, Japan 4)Ishikawajima-Harima Heavy Industries Co., Tokyo, Japan

(Received February 2nd, 2001)

The first stage of the H-2 rocket used a 110-ton thrust liquid oxygen, liquid hydrogen, pump-fed engine, the LE- 7. This engine required high-pressure and high-power liquid oxygen and liquid hydrogen turbopumps to achieve the two-stage combustion cycle in which the combustion pressure is around 13 MPa. Furthermore, it was very important to operate both turbopumps at higher rotational speeds to obtain a smaller, lighter-weight engine because the LE-7 had not low-speed, low-pressure pumps ahead of both the main pumps. The present paper shows the design, test results, and modifications that had been performed until a flight-type liquid oxygen turbopump for the LE-7 engine was completed. The liquid oxygen turbopump had been developed by the use of three models, that is, research, prototype, and flight models.

Key Words: Rocket, Turbopump, Inducer, Cavitation, LE-7

1. Introduction The research model was fabricated to clarify the major design parameters of the liquid oxygen turbopump related to The H-2 rocket, Japan’s previous expendable launch ve- the hydrodynamics and mechanical configuration. The pro- hicle, which was capable of placing a two-ton payload into totype model was developed to modify the defects that were a geostationary orbit, had been successfully operated in six found in the research model. The flight type model was pro- flights since its first flight in 1994. The seventh flight, how- duced by minor changes in the prototype model. ever, was unsuccessful because of the failure of the inducer of the liquid hydrogen pump. This failure was thought to be 2. Design of LE-7 Liquid Oxygen Turbopump caused by the superposition of some complicated phenom- ena in the inlet portion of the liquid hydrogen pump, which 2.1. Mechanical integration1) occurred mainly because of cavitation and backflow of the The major specifications of the LE-7 liquid oxygen tur- inducer. bopump is presented in Table 1. The three types of the tur- The first stage of the H-2 rocket used a 110-ton thrust liq- bopump are shown in Fig. 1. Since the rotational speed of uid oxygen, liquid hydrogen, pump-fed engine, the LE-7. To the turbopump was closely related to the weight of the first obtain high performance, a two-stage combustion cycle was stage of the H-2 rocket, a parametric investigation was car- employed in the engine. The LE-7 engine required high- ried out to optimize the relationship between the rotational pressure and high-power liquid oxygen and liquid hydrogen speed and inlet flow coefficient of the inducer.1) The liquid turbopumps to achieve the two-stage combustion cycle in oxygen tubopump for the LE-7 has some features in mechan- which the combustion pressure is around 13 MPa. Further- ical configuration. The simplification of the rotating assem- more, it was very important to operate both turbopumps at blies was especially emphasized in the design to avoid rotor higher rotational speeds to obtain a smaller, lighter-weight dynamic problems. engine because the LE-7 engine had no low-speed, low- The liquid oxygen turbopump consists of a main pump pressure pumps ahead of both the main pumps. The rota- and a preburner pump that are driven by a single-stage gas tional speeds of the liquid oxygen and hydrogen turbopumps turbine, as shown in Fig. 1. The main pump has a single- were 18,300 and 42,500 rpm, respectively. stage impeller with an inducer. A large flow rate and higher The present paper shows the design, test results, and mod- suction performance required an increased inlet diameter of ifications that had been performed until a flight-type liquid the inducer. Therefore the inducer and the main pump im- oxygen turbopump for the LE-7 engine was completed. The peller were arranged as shown in the figures. The guide liquid oxygen turbopump had been developed by the use of vanes between the inducer and the main impeller are useful three models, that is, research, prototype, and flight models. to support a housing for self-lubricated ball bearings. With the connection of the main and preburner pump impellers, an external diffusing passage was selected because the shaft c 2001 The Japan Society for Aeronautical and Space Sciences ∗Presented at 36th AIAA/ASME/SAE/ASEE Joint Propulsion Conference seal pressure of liquid oxygen would be lower than that of & Exhibit, Huntsville, Alabama, July 16–19, 2000. 156 Trans. Japan Soc. Aero. Space Sci. Vol. 44, No. 145

Fig. 1. Three models used in the development of the LE-7 LOX turbopump. Nov. 2001 K. KAMIJO et al.: Developmental History of Liquid Oxygen Turbopump 157

Table 1. Major speciﬁcations of LE-7 LOX turbopump.

Rotational speeds, rpm 20,000 Main pump Required NPSH,m 30 Mass flow, kg/s 229.1 Pressure rise, MPa 20.9 Efficiency, % 75 Preburner pump Mass flow, kg/s 43.8 Pressure rise, MPa 11.4 Efficiency, % 65 Turbine Power, kW 6,400 Gas inlet pressure, MPa 23.5 Pressure ratio 1.43 Inlet temperature, K 970 Efficiency, % 48.5

Fig. 3. Balance piston characteristics.

2.2. Major component design1) The major inducer design parameters are presented in Ta- ble 2. The inducer and its guide vanes were designed to use helical blades. The blade profile of the inducer consists of a straight line at the entrance and a circular arc. This inducer is characterized by a low flow coefficient that requires a small inlet angle. This angle requires a sharp leading edge to reduce blockage resulting from cavitation to achieve good suction performance. A large swept-back angle was necessary to reduce stress near the root of the blades. The inducer was machined from heat-resistant alloy (Inconel 718). Both the main and the preburner pumps have three- Fig. 2. Critical speeds of LOX turbopump. dimensional impellers designed by only straight lines, using a ruled surface method. This made it fairly easy to fabricate the impellers and to analyze flows through the blade pas- the internal crossover passage. sages. Figure 5 shows the main pump impeller. To minimize the overhang of a turbine rotor, a single- The turbine blade profile was designed by using the pre- stage gas turbine is employed at the cost of turbine efficiency, viously reported method. A partial admission nozzle the which results in smaller shaft vibrations than those in a two- research model employed was changed to a full admission stage gas turbine. The turbopump could be designed so that nozzle in the prototype model because cracks occurred at the nominal rotational speed is less than the second critical speed, as shown in Fig. 2, because the second critical speed has a mode in which the liquid oxygen pump impellers (in- Table 2. Design parameters of LE-7 main pump inducer. cluding the inducer) whirl. The axial thrust of the rotor assembly is regulated by a bal- Rotational speed N, rpm 20,000 NPSH ance piston mechanism as shown in Fig. 3. The pressure of Required , m 30.0 Suction specific speed S,m,m3/s, s−1 2.10 the balance piston cavity is controlled by two orifices formed Cavitation number σ 0.017 by a back-shroud of the main impeller and a casing. The Number of blades 3 turbopump uses a purge of high-pressure, low-temperature a Inlet flow coefficient φ1 0.083 gaseous hydrogen to prevent the turbine’s working gas (hy- a Outlet flow coefficient φ2 0.104 drogen rich hot gas) from entering the shaft seal system, as Inducer head coefficienta ψ 0.097 shown in Fig. 4. Self-lubricated ball bearings are cooled by Tip diameter Dt , mm 149.8 liquid oxygen that passes through filters with fine meshes set Inlet tip blade angle βt1, deg 7.50 in the coolant passages. Outlet tip blade angle βt2, deg 9.50

a Values for 1.07 times the quantity of nominal ﬂow. 158 Trans. Japan Soc. Aero. Space Sci. Vol. 44, No. 145

Fig. 4. Shaft seal system of LOX turbopump.

outlet of both pumps. The main pump of the research model showed a slightly higher head and efficiency than those of the prototype model. Six small holes were newly fabricated in the back-shroud of the main pump impeller of the prototype model to modify the characteristics of the axial thrust balance of the rotor assembly, which will be mentioned later. Since the holes increased the internal leakage, the efficiency and head decreased in the prototype model. Figure 9 shows the relationship between the turbine efficiency and the isentropic velocity ratio. The turbine efficiency was obtained by making use of the outputs of both pumps. The turbine with a full admission nozzle of the prototype model showed a slightly lower efficiency than that Fig. 5. Main pump impeller. of the turbine with a partial admission nozzle, which might be due to the reduction of blade height to 9.4 mm, from 15.8 mm. the roots of the turbine blades. The blades were subjected to cyclic loads at both ends of the partial admission nozzle arcs. 4. Modifications of the Turbopump Furthermore, although the blades and the disk were made from a solid material in the research model, the blades were 4.1. Regulation of axial thrust balance5,6) seperated from the disk in both the prototype and flight mod- Much effort was made to establish a balance piston system els to alleviate the stress resulting from pressure fluctuations in which the back-shroud of the main impeller is used as a and thermal shock at the start and cutoff of the turbopump. balance disk, as shown in Fig. 10. In the initial phase of development using the research model, the pressure in the bal- 2–4) 3. Hydraulic Performance ance piston cavity between the two orifices was much higher than predicted, which also caused a bigger balance piston The suction performance of the main pump was almost force than predicted. Figure 11 shows the test results with the same as the predicted one. Figure 6 shows the head co- the relationship between the rotational speed and inlet orifice + efficient curve of the inducer of the main pump. Crosses ( ) axial clearance (without balancing holes A). The inlet-orifice indicate values when the original inducer housing was used, axial clearance at the design rotational speed was small, and and circles ( ) represent values when the modified inducer a modification was required to increase the clearance. The was used. The original inducer housing caused the unstable balancing holes were newly fabricated at the back-shroud of head coefficient curve, which will be mentioned later. the main pump impeller to decrease the pressure in the bal- The overall performance of the main and preburner pumps ance piston cavity as shown in Fig. 10. The addition of bal- is shown in Figs. 7 and 8. Pump efficiency was estimated ancing holes A made the inlet-orifice axial clearance large by adiabatic efficiency, which was obtained by making use enough, as shown in Fig. 11. of the temperatures and pressures measured at the inlet and A great amount of time was necessary to find a cause of a Nov. 2001 K. KAMIJO et al.: Developmental History of Liquid Oxygen Turbopump 159

Fig. 6. Suction performance of LE-7 main pump inducer.

Fig. 9. Overall performance of turbine.

Fig. 7. Overall performance of main pump.

Fig. 8. Overall performance of preburner pump. Fig. 10. Balance piston details. 160 Trans. Japan Soc. Aero. Space Sci. Vol. 44, No. 145

Fig. 14. Displacement measurement probe.

Fig. 11. Inlet-orifice axial clearance. higher pressure in the balance piston cavity than expected. It was concluded that the disagreement between the measured and predicted pressure was due to the annular grooves with a row of fastening bolts that were set on the wall of the casing, as shown in Fig. 10. The grooves suppressed the rotating velocity of the fluid and made the pressure higher than without grooves. The grooves functioned the same as a swirl breaker. However, it was also confirmed that the balance holes had another function to relieve the balance piston cavity pressure during a stop of the turbopump, which was performed in a short time and produced excessive pressure in the balance piston cavity because of the evaporation of liquid oxygen. Furthermore, the axial clearance of the inlet orifice at the Fig. 12. Groove geometry. design rotational speed was increased by the use of a kind of swirl breaker that consisted of many radial grooves fabricated on the casing of the front shroud (Figs. 10 and 12) by which the pressure in the balance piston cavity greatly decreased as a result of the suppression of flow rotation. Fig- ure 13 shows the effect of the grooves on the balance piston characteristics. In that figure, Q0, q0, r2, Tp, u2, and ρ are the main pump flow rate, balance piston leakage, radius of main pump impeller, pump axial thrust, tip velocity of main pump impeller, and density of pump fluid, respectively. The grooves were very effective in increasing the inlet orifice axial clearance. 4.2. Supression of rotating cavitation3,7) When the original inducer housing was used, it exhibited the unstable head coefficient curve presented by crosses in Fig. 6. Remarkable head degradation was present near the cavitation number, σ = 0.02–0.04. As shown in Fig. 15, the inducer also produced a supersynchronous shaft vibration and an amplitude jump of synchronous vibration at the same range of cavitation numbers, which were measured by a displacement measurement probe shown in Fig. 14. In par- ticular, both the largest head degradation and the amplitude jump of synchronous vibration in Fig. 15 occurred simulta- neously, that is, at the same cavitation number, σ = 0.027. From a comparison of the facts mentioned above, a former Fig. 13. Effect of grooves on balance piston characteristics. Nov. 2001 K. KAMIJO et al.: Developmental History of Liquid Oxygen Turbopump 161

Fig. 15. Spectrum analysis of main pump impeller displacements (inducer housing A).

Fig. 16. Details of inducer housing. Fig. 17. Main pump impeller displacement in LE-7 engine test. a) original inducer housing, b) modified inducer housing. report of rotating cavitation, and the experimental investigation of hydrodynamically induced shaft forces with an in- the tip leakage flow cavitation of the inducer. Although inducer, it was concluded that the shaft vibration was caused creasing the tip clearance was fairly effective in decreasing by the rotating cavitation that occurred in the inducer of the the amplitude of the supersynchronous vibration, it could main pump. not completely distinguish the vibration. A suction ring, It was conjectured that rotating cavitation might be closely which is usually used to regulate the back flow at the inducer related to the tip leakage flow cavitation of an inducer from inlet, was also very effective in suppressing the supersyn- the visual observations. Some efforts were made to influence chronous shaft vibration. Sometimes the vibration was com- 162 Trans. Japan Soc. Aero. Space Sci. Vol. 44, No. 145

inducer upstream housing, did not exhibit the dented part caused by the rotating cavitation. It was also comfirmed that the modified inducer upstream housing was very effective in suppressing the supersynchronous shaft vibration in the LE-7 engine test, as shown in Fig. 17. This device (inducer housing C) was applied to the flight model turbopump, since it had no durability problems.

5. Other Problems

We experienced a very curious phenomenon in the initial phase of the development of the LE-7 LOX turbopump. Three types of shaft vibrations appeared in the tests of the turbopump alone in almost the same operating conditions.8) One was a supersynchronous shaft vibration resulting from a rotating cavitation that was already described in the previous section. Figure 18 shows that only a supersynchronous shaft vibration occurred and only a subsynchronous shaft vibration occurred in the almost the same operating conditions. Furthermore, the supersynchronous shaft vibration appeared just after the subsynchronous shaft vibration had almost dis- appeared concomitant with the decrease of the inducer inlet pressure. The subsynchronous shaft vibration had not appeared when the modified inducer upstream housing was employed. Later, an analysis was performed to clarify the causes of the subsynchronous shaft vibration. It was concluded that it was caused by cavitation surge.8) Furthermore, an analyti- cal study of the flow instabilities of turbomachines indicated that the rotating cavitation and the cavitation surge occur in almost the same operating condition.9) In the initial phase of development, the vanes of a turbine nozzle that were attached to a turbine manifold casing with welding were separated from the casing, and a large amount of leakage of turbine working fluid from the inlet to the outlet of the nozzle occurred, which resulted in the large decrease of turbine output. This defect was improved by an increase Fig. 18. Fourier analysis of LE-7 LOX turbopump shaft vibrations. of the welding area between the tips of the vanes of the turbine nozzle and the manifold casing. Many other minute improvements were performed to in- pletely eliminated, but the ring was not applied to the flight crease the reliability and durability of the LE-7 liquid oxygen model turbopump because it required many tests to confirm turbopump, such as adding a bypass conduit to increase the its durability. coolant for the turbine-side self-lubricated ball bearings. The influence of the inducer upstream housing diameter on the supersynchronous shaft vibration was investigated. 6. Concluding Remarks Some relationship was found between the inducer upstream housing diameter and the amplitude of the supersynchronous The liquid oxygen turbopump of the research, prototype, shaft vibration. We obtained a very interesting relation of and flight models for the LE-7 engine had been fabricated, the inducer housing dimensions represented by the follow- tested, and modified from 1986 to 1993. Although the tur- ing equation, which almost completely extinguished the su- bopumps attained almost the hydraulic performance they persynchronous vibration, that is, the rotating cavitation. were expected to, regarding mechnical performance some efforts should be made for the flight type turbopump with D ≥ D + 2C = D + (D − D ) (1) 1 2 2 2 2 t enough reliability and durability. Some modifications were where C2 is the tip clearance and D1, D2, and Dt are de- required to achieve the axial thrust balance of the rotor as- noted in Fig. 16. The head coefficient curve presented by sembly. The turbine blades had to be changed from inte- circles in Fig. 10, which was obtained by using the modified grally machined blades with a disk to a fire-tree blade at- Nov. 2001 K. KAMIJO et al.: Developmental History of Liquid Oxygen Turbopump 163 tachment to reduce the thermal stress at the blade roots dur- 4) Kamijo, K., Yoshida, M. and Nagao, T.: Performance Evaluation of ing engine cutoff. With regard to the supersynchronous shaft LE-7 High-Pressure Pumps, J. Propul. Power, 10 (1994), pp. 819– vibrations, we had to start by clarifying their cause, which 826. 5) Kurokawa, J., Kamijo, K. and Shimura, T.: Axial Thrust Behavior in was found to be a rotating cavitation that occurred in the in- LOX Pump of Rocket Engine, J. Propul. Power, 10 (1994), pp. 244– ducer of the main pump. The vibrations were suppressed by 250. a simple modification of the inducer upstream housing. 6) Shimura, T., Yoshida, M., Hasegawa, S. and Watanabe, M.: Axial Trust Balancing of the LE-7 LOX Turbopump, Trans. Japan Soc. Aero. Space Sci., 38 (1995), pp. 66–76. References 7) Tsujimoto, Y., Kamijo, K. and Yoshida, Y.: A Theoretical Analysis of Rotating Cavitation in Inducers, ASME J. Fluid Eng., 115 (1993), 1) Kamijo, K., Hashimoto, R., Shimura, T., Yoshida, M., Okayasu, A. pp. 135–141. and Warashina, S.: Design of LE-7 LOX Turbopump, Proceedings of 8) Watanabe, M., Yamada, H., Yoshida, M., Komatsu, T. and Kamijo, the 15th International Symposium on Space Technology and Science, K.: Rotor Vibrations of Turbopump due to Cavitating Flows in In- 1986, pp. 347–355. ducer, Proceedings of (ASME) FEDSM99, 1999 ASME/JSME Fluids 2) Kamijo, K., Yoshida, M., Watanabe, R., Hashimoto, Ohta, T. and Engineering Division Summer Meeting, July 18–23, San Francisco, Warashina, S.: Development Status of LE-7 LOX Turbopump, Pro- California, 1999. ceedings of the 16th International Symposium on Space Technology 9) Tsujimoto, Y., Kamijo, K. and Brennen, C.: Unified Treatment of and Science, Sapporo, 1988, pp. 281–288. Flow Instabilitites of Turbomachines, J. Propul. Power, 17 (2001), 3) Kamijo, K., Yoshida, M. and Tsujimoto, Y.: Hydraulic and Mechan- pp. 636–643. ical Performance of LE-7 LOX Pump Inducer, J. Propul. Power, 9 (1993), pp. 819–826.