Numerical simulations were carried out to understand the transient behavior in ion extraction and acceleration for a variable-thrust ion thruster, thrust in which is controlled by changing the duty ratio of the ion extraction-on and extraction-off periods. The duty ratio is controlled by rapidly switching the accel grid voltage with a rising and falling time of several hundreds of nanoseconds. Calculation results showed that the thrust could be varied by the on/off control of ion extraction; however, the transient direct impingement of ions caused grid erosion. Thus, the effect of grid erosion on lifetime was evaluated for the planned on/off cycles, which showed that grid erosion was negligible compared to that by charge exchange erosion. In addition, the parametric analysis of the rising and falling times showed that the grid erosion is a function of the rising and falling time and it reaches a minimum at a rising and falling time of around 200 ns.
The effect of neutral gas injection from downstream side of electrodeless thrusters is numerically investigated using two-dimensional axisymmetric particle-in-cell simulations with Monte Carlo collisions algorithm (PIC/MCC) for charged particles, and direct simulation Monte Carlo (DSMC) for neutrals, where the propellant gas is injected from both the upstream and downstream sides of the source cavity. The analysis is performed for various ratios of the gas injection from the upstream and downstream sides, while maintaining the total gas flow rate of 30 μg/s. The PIC/MCC and DSMC numerical results show that the performance degradation induced by neutral depletion is inhibited and the plasma density peak shifts to the downstream side of the source when increasing the downstream gas injection. As a result, the downstream gas injection is effective for performance improvement by applying external magnetic fields. This tendency is in qualitative agreement with recent experiments.
Electrospray thrusters are simple propulsion systems, which can be operated on nanosatellites, and it is possible to reduce components of high pressure gas systems using ionic liquid as a propellant. However, evaluations of electrospray thrusters are difficult in experiments since the size of emitter cones is on the order of micrometers. Then, the authors performed Particle-in-Cell simulations of ions in the electrospray thruster and investigated ion beam extraction mechanisms in steady and transient flows numerically. The simulation results show that steady ion beam distributions were rarely affected by ion species. On the other hand, it was clarified that a jet affected the ion beam profiles. In addition, the authors evaluated propulsion performance of the electrospray thruster, which showed a thrust of about 64 nN per emitter and a specific impulse of about 6400 s. In the transient flow, the ion beam behavior showed rapid responses when applied voltages with or without a slew rate are assumed.
Carbon fiber reinforced plastic (CFRP) is considered to be one of the possible materials for reflectors of space-based astronomical observation systems because of its high specific elasticity and low coefficient of thermal expansion. However, it was demonstrated that non-negligible out-of-plane thermal deformation would be generated on CFRP reflectors. In this study, capability of a method of suppressing the out-of-plane thermal deformation was examined. In the proposed method, surface shape of the reflectors was controlled by applying external forces with linear actuators so that RMS error due to the thermal deformation was minimized. As a result, it was shown that the out-of-plane thermal deformation caused by fiber orientation error in manufacturing process could be suppressed significantly by the proposed simple method. Sensitivity analysis on location of actuators and input forces was also performed and it was seen that the location of actuators and the input forces need to be controlled strictly to realize high precision surface shape control of CFRP reflectors.
At Japan Aerospace Exploration Agency (JAXA), several sample return missions from deep space have lately been proposed. Since its reentry speed is higher than 10 km/s, flow conditions are not well known due to highly non-equilibrium phenomena within such speed range. A hyper-velocity shock tube (HVST) at JAXA is capable of producing such flows, and thus, in order to analyze hypersonic flows in HVST, we have developed an unsteady DSMC flow solver and carried out unsteady DSMC flow simulations for a 11.8 km/s test case. The computed electron number density was consistent with the estimated value from the VUV measurement in HVST, and the DSMC results show thermally non-equilibrium conditions in the downstream region.
In 2018, practical broadcasting of 4K/8K ultra-high-definition television will start with 12-GHz band satellite broadcasting using right- and left-hand circular polarization in Japan. For 4K/8K satellite broadcasting reception, the Association of Radio Industries and Businesses standardized specifications of distribution equipment for home use. Therefore, we fabricated a dual-polarization receiving antenna and optical distribution equipment in accordance with the specifications. We measured the aperture efficiency and cross-polarization discrimination of the fabricated antenna, and the respective results indicated that they were more than 74% and 26 dB. We also confirmed that the fabricated optical distribution equipment was able to deliver 50 modulated signals with C/N degradation of 0.1 dB.
Active X-ray fluorescence Spectrometer (AXS) is designed and developed as an elemental analyzer to induce fluorescence X-rays from rock samples without radioactive sources for future lunar and planetary landing missions in Japan. Pyroelectric crystal is applied for X-ray generator because of its compact package, lightweight, low-power consumption, and radioactive source-free. As the X-ray emission intensity from Pyroelectric X-ray Generator (PXG) is not strong enough to be applied for space missions, we have been developing the PXG with an increased intensity to be used in future lunar and planetary landing missions. In future landing mission to the Moon, several putative landing sites are expected in the Procellarum KREEP Terrane and South Pole-Aitken Terrane, which are interesting for investigating lunar igneous activity and mantle composition, respectively. In this paper, the current status of development and science targets of AXS is reported and discussed.
Precipitable water vapor is a crucial geophysical parameter for numerical weather prediction (NWP) and climate study. Space-based microwave radiance measurements from channels near 23-GHz water vapor absorption and 19- and 37-GHz window channels have been utilized to retrieve precipitable water vapor in remote sensing applications. In this study, to retrieve precipitable water vapor over land, we used dual-polarized microwave observations (vertical and horizontal) at 19 GHz and 23 GHz from the Global Change Observation Mission-Water (GCOM-W)/Advanced Microwave Scanning Radiometer 2 (AMSR2). We found the polarization difference between brightness temperatures over land to be proportional to their land emissivity differences. The polarization difference also has an exponential dependence on the amount of precipitable water vapor and considerable spatial variability due to vegetation, soil moisture, and other surface conditions. The ratio of spectrally close channels and an assumption of their frequency independence with respect to land surface emissivity enables the utilization of their sensitivities to retrieve atmospheric water vapor over land. We used ground-based Global Positioning System (GPS) observations of precipitable water vapor over land to calibrate the retrieval algorithm. The accuracy of the retrieved precipitable water vapor was approximately 2.4 mm. Using this product, we obtained the precipitable water vapor distribution in desert areas along with their seasonal changes. Retrieval of precipitable water vapor over land has potential for producing accurate initial conditions for use in NWP by data assimilation and records of their long-term data will be useful in climate studies for developing a better understanding of global water changes.
We propose an imaging grating spectrometer suite with an agile pointing system and two-dimensional detectors to monitor global and regional greenhouse gas fluxes and target local emission. Three imaging spectrometers cover the nitrogen dioxide (NO2) band between 460 and 480 nm, the oxygen (O2) A band and solar-induced chlorophyll fluorescence (SIF) between 747 and 783 nm, and the weak carbon dioxide (CO2) and methane (CH4) absorption band at 1.6 μm with a spectral resolution of 0.1, 0.1, and 0.2 nm, respectively. We observe solar reflected spectra to retrieve column-averaged dry air mole fractions and cover entire emission areas such as mega cities, coal mines, and landfills with spatial resolution of 1 km.
Food production at the lunar base or during the manned Mars mission has recently gained significant attention. Plants such as zinnia, lettuce, thale cress, wheat, and barley have been cultured for a long duration in space. Although rice is considered a staple food for most people in the world, little research has been conducted on rice cultivation in space. Rice grains are nutrient-rich with a high content of carbohydrate, protein, and dietary fiber. Moreover, rice is a high-yield crop, and rice grains have a long shelf life. Rice symbolizes the rice-eating culture of Japan. Thus, rice should be considered as a viable Japanese food source for space travelers. A previous report introduced a new variety, Kozonosumika, as super-dwarf rice that could be cultured in space. This rice variety is one-fourth as tall as normal varieties with approximately 20 cm plant height. We investigated its taste characteristics to determine its viability as staple food in space. Glutinous Kozonosumika and non-glutinous Hosetsuwaisei (super-dwarf varieties) and Nipponbare were cultured for sensory tests. Kozonosumika was found to be superior to Hosetsuwaisei in taste. Therefore, Kozonosumika can be a promising candidate for space cultivation because of its short height and high protein content.
Space environment in geospace changes depending on the solar wind and high energy particles originated from the Sun. It is well known that the satellite anomaly sometimes occurred because of the dynamical variations of the space environment. To understand the current and future conditions of space environment, which we call 'Space Weather Forecast', is one of the quite important activities for safety and security operations of the spacecraft. On the other hand, the detailed information of spacecraft anomaly, possibly related to space disturbances, cannot be available in usual. The risk of the individual satellite depends not only on the space environment, but also on the satellite charging condition, which depend on the structure and materials of the satellite body. Therefore, it is not easy for the satellite operator to judge the risk of the satellite based on the space environment information only. To improve this situation, we start developing specialized information for the nowcasting and forecasting space environment for each satellite, and estimate the risk of satellite anomaly by combining information of space environment and that of satellite materials with a charging model. To seek this approach, the risk of satellite charging will be estimated based on the prediction of space environment using the case study of MICHIBIKI satellite, which is on the quasi-zenith orbit. As a first step, relationship among the space environment data and surface charging data obtained from MICHIBIKI satellite, and space environment data obtained from global magnetospheric MHD simulation are examined. Since the MHD simulation calculate temperature and density as single MHD fluid, an empirical relationship between simulation and observation needs to be made for obtaining the estimated temperature and density of electrons and ions. Initial comparison is introduced in this paper.
The 6 MV tandem accelerator system at the University of Tsukuba become available for a period of time at the end of 2015 when the beam line system for evaluating devices for space use was installed by the parties involved at the University of Tsukuba and the Japan Aerospace Exploration Agency (JAXA). The ion beam profile (uniformity, beam flux, heavy ion energy) was preliminarily evaluated, and use of the accelerator system for radiation testing regarding space applications was confirmed.
The understanding of cable dynamics is one of the most important issues for the design and construction of a space elevator. We propose a new method for constructing the space elevator by simultaneously deploying cables upward and downward from the main space station maintained in GEO and balancing both cables. The analysis is performed using our originally developed cable dynamics model. The model is a two-dimensional lamped mass model. In the model, the tether is modelled by mass points, which are connected to each other by a spring and a damper, and are pulled out per assumed length. In this study, we first analyze the cable dynamics and stress for assumed deployment conditions of the cables. We then obtain the total impulse necessary to keep the main station in GEO and control the cable speed during deployment so that it does not exceed the maximum stress of the cable. The results are also compared with the preceding method for the same conditions. The initial result shows that with the proposed cable-deployment method, the total propellant mass necessary to control cable dynamics while maintaining the station in GEO can be decreased by about one-third compared with that necessary to raise the main station to higher orbit and to control cable dynamics as in the preceding cable-deployment method.
For supersonic mixing enhancement in scramjet engine, streamwise vortices and its breakdown seem to be the most advantageous. We expect that streamwise vortices break down and develop turbulent mixing field as going the downstream. In order to increase the mixing performance, it is important to introduce streamwise vortices which have larger circulation, and to enhance breakdown of them. In the present study, we examined supersonic mixing field, which is generated by ramp injector which has swept ramp angle, by Schlieren flow visualization, oil flow, and stereoscopic PIV method. It was confirmed that strength of streamwise vortices becomes stronger and they break down rapidly as the swept ramp angle decreases.
This paper describes the performance of a proposed bipropellant thruster using nitrous oxide (N2O) and dimethyl ether (DME). Conventionally, nitrogen tetroxide and hydrazine are used in thrusters for spacecraft, but requires gas treatment systems in the ground tests due to toxicity and reactivity to materials. Hence, we proposed to apply N2O and DME to a bipropellant thruster in order to develop a compact and eco-friendly bipropellant thruster. N2O and DME, which are liquefied gas, are neither toxic nor reactive to materials, and require no gas treatment system in the ground tests. Furthermore, the combustion chamber can be downsized because gaseous injection can eliminate the evaporation time. The application vapor pressures of 6.4 MPa (N2O) and 0.6 MPa (DME) at 25 °C to self-pressurization would eliminate the necessity of pressurant to simplifies the feeding systems. In this study, a 0.4-N class N2O/DME bipropellant thruster with two types of injector was prototyped, and tested in a vacuum chamber to clarify the dependence of injector design on characteristic velocity (C*) efficiency. Using an injector consisting of showerhead and subsequent 15-mm orifice, the thruster yields combustion chamber pressure of 0.27 MPa, with corresponding C*efficiency of 68.3 % at O/F = 3.5.
An Extensible Optical Bench (EOB) for a X-ray satellite (ASTRO-H) had a length of 6.4m in extended configuration. Although the same type of extensible mast was used in Space Radio Telescope (Halca) in 1997, the tip mass was quite different in the case of ASTRO-H. Due to the tip mass of 150 kg, the natural frequency of EOB was less than 1Hz in the extended configuration. ASTRO-H was launched on Feb. 17, 2016, and the EOB was extended on Feb. 28, 2016, successfully. However, because the vibration of EOB occurred during the extension, the extension operation was carried out over four passes intermittently. When the amplitude of induced vibration excessed the predefined threshold, we stopped the extension, then stayed until the vibration was damped. In this paper, the induced vibration during extension and its mechanism are reported. Through simulations, it is confirmed that one of the major causes of the vibration is a periodic change of gap between mast and canister at the root of EOB.
In this paper, we propose a novel singularity avoidance/passage logic based on inverse kinematics for steering a variable-speed double-gimbal control moment gyro (VSDGCMG). Two solutions generating the command torque are obtained using the inverse kinematics for a VSDGCMG system. The proposed method basically selects the solution close to the current wheel momentum and gimbal angles. A transition to another solution is conducted to reduce the attitude error in the vicinity of the singular state based on an evaluation function that corresponds to the approximate torque error and explicitly includes gimbal rate limitations. Its function results in a decision with small attitude error when compared with a previous method that uses just an approximate gimbal angle without the gimbal-rate limitation. Furthermore, the proposed method generates more precise torque closer to the reference torque than the previous method when avoiding the singular state. The effectiveness of the method is demonstrated through numerical simulations.
In this paper, anomaly detection that is configured as a combination of state observer and Mahalanobis-Taguchi (MT) method is proposed for real time fault detection of rapid and dynamic phenomena such as rocket engine operation. Real time anomaly detecting is recognized as one of the most important elements to realize advanced reusable space transportation system. Conventionally, bottom-up type anomaly detecting logic based on failure mode and effect analysis (FMEA) is usually used for this purpose, however, it requires large amount of time and labor. The proposed method can improve this process. In the present method, error values between calculated ones through rocket engine simulator constructed on autoregressive moving average model and extended Kalman filter (EKF) and measured ones are standardized with existing normal operation data of the rocket engine so as to compute Mahalanobis' distance, which expresses degree of anomaly. We performed engine hot firing tests in simulated anomaly conditions. The obtained data was processed with the present method, and the simulated anomaly in the tests was detected as expected.