The high-speed bearings and rotating-shaft seals of turbopumps which supply extremely low temperature propellants such as liquid oxygen (LO2) and liquid hydrogen (LH2) to liquid rocket engines are very important parts. However, the operating conditions of turbopump bearings and shaft seals are characterized by poor lubrication because of the low viscosity due to oxidation (LO2) or reduction (LH2). Therefore, turbopump bearings and shaft seals must be highly durable and reliable under extreme operating conditions in LO2 and LH2 environment. As a means of reducing launch costs, reusable rocket engines are required for future space transportation systems, and the durability of bearings and shaft seals in reusable turbopumps must be much greater than that of currently expendable turbopump bearings and shaft seals. For example, the required life of the reusable turbopump bearings and shaft seals of the space shuttle main engine (SSME) is 7.5 hours, which is 15 times longer than that required for the expendable LE-7 engine of the Japanese H-II rocket. However, a serious wear problem has been reported for the turbopump bearings of the SSME and hybrid ceramic bearings are now used in an attempt to reduce serious bearing wear. In order to develop turbopump bearings and shaft seals, many tribological problems for extremely low tomperatures and high rotating speeds must be solved. The cryogenic tribological problems experienced in the development of the turbopump bearings and shaft seals of the LE-5 engine for the H-I rocket and the LE-7 engine for the H-II rocket are introduced in this report. Additionally, the performance of a self-lubricating hybrid ceramic bearing developed to increase bearing life is described.
The degradation of the inter-laminar shear strength (ILSS) of GFRPs was evaluated after electron irradiation at 77K and reactor irradiation at 20K. The GFRPs used in this study were prepared by varying matrix resins. Optical microscopic observation of the fracture surface was carried out to reveal the degradation behavior of the ILSS. The exposed fiber area of the fracture surface was found to depend on the absorbed dose. This suggests that a change in ILSS is induced by the interface failure between fiber and matrix as a result of the change in matrix resin.
Cable-in-conduit conductors (CICCs), composed of multi-strand superconducting cables cooled with supercritical helium, are essential to superconducting magnets that require large current capacities, low AC losses and high rigidity for fusion machines. A conductor cabled by several hundreds of superconducting strands with superficial chrome plating, oxide or CuNi result in the problem of unbalanced current distribution among the strands. This is caused by small differences in the self-inductance of each strand as well as an electrical resistance difference several times larger at the portion of electrical contacts between each strand and the current leads. This unbalanced current decreases the minimum quench energy (MQE) under the case of uniform current distribution. The MQE of CIC multi-strand cables under unbalanced current distribution is greatly affected by the electrical resistance among the strands. This means that the stability is influenced by the current-sharing process among strands. A higher margin of stability is induced by increasing current sharing and more rapid current transfer to the neighboring strands rather than thermal diffusion to helium. The most suitable parameter to govern this process is the impedance between strands. A simplified electrical circuit model simulating the current-sharing process among strands was proposed to estimate stability. The circuit parameters, that is the impedance, governing the sharing process were measured easily using short samples of CIC cables at 4.2K. The experimental results of stability tests show that the CIC has an extensive stability margin according to lower impedance between strands. We also confirm that the proposed method is applicable to estimate the stability of CIC.
Superconducting Nb3Al tapes with Nb sheath were prepared by the powder metallurgy method using the diffusion reaction between σ-phase (Nb2Al) and Nb powders. External heating using an electric furnace and ohmic heating by electrifying the tape specimens were compared to obtain the stoichiometric composition of A15 Nb3Al. During ohmic heating, control of the ultimate temperature was enabled by measuring the radiant temperature on the surface of the tapes. In the reaction between σ-phase and Nb powders, the reaction temperature, above which only the A15 phase is produced, was lowered by 100-200°C as compared with the reaction between Al and Nb powders. For the specimen obtained by ohmic heating and subsequent annealing, the highest onset Tc was 18.5K. Jc was 41A/mm2 at 20T and Bc2 was 27T. For the specimen prepared by ohmic heating, Tc increased by 2.0K after post annealing at 800°C for 10h.
A Bi2Sr2Ca1Cu2Ox (Bi-2212) superconducting solenoidal coil with a persistent current switch (PCS) has been newly designed and fabricated employing an Ag-sheathed Bi-2212 rectangular wire. It is the first superconductively jointed oxide superconducting coil. The operation behavior of the solenoidal coil at 4.2K has been evaluated with the PCS in off and on states. When the PCS was off, the solenoidal coil worked well, generating magnetic fields over 95% of the designed value. After the PCS was turned on, a magnetic field of about 0.01T was continuously generated for 100h and then the PCS was turned off. The residual resistance of the entire circuit is estimated to be below 1.6×10-11Ω from the relaxation rate of the magnetic field generated during the last 1, 900min (=31.7h) of operation. This resistance value reaches the practical level of commercial metallic superconducting magnets for magnetic resonance spectroscopy operated in the persistent current mode. This study has proved that practical use of the Bi-2212 solenoidal coil for persistent current mode operation is feasible.
In order to evaluate the cryogenic fracture toughness and the temperature rise of G-10 woven glass-epoxy laminates, plane-strain fracture toughness testing was performed with 0.4T compact tension specimens at room temperature, 77K and 4K. Testing was conducted in accordance with ASTM standard E399-83 and JSME standard S001 for determining the fracture toughness. Chromel-vs-Au/0.07% Fe thermocouples were used to measure the temperature rise near the crack tip. The effects of temperature, crack length and crosshead speed on the fracture toughness are examined. The damage morphology around the crack tip of tested specimens was characterized using scanning electron microscopy (SEM).