宇宙工学

The development of space is a great dream left to mankind. More than 30 years have passed since the world's first satellite was launched. Now, international telephone communication can be made via comsats and the image of clouds can be seen on television and in newspapers thanks to through a meterological satellites. As the power output of satellites and spacecraft increases, a thermal control system must be considerd for the recovery, transport, and disposal of waste heat. The change of phase of the working fluid results in lower electrical power requirements to operate pumps and gives a relatively isothermal sink temperature. Various system designs must be made to accomplish this two-phase thermal management capability. However, there is only little information available on two-phase flow pressure drop and heat transfer in reduced gravity environments. Here, the present status of investigation into two-phase flow in micro-gravity environments is introduced, with an emphasis especially on flow pattern.

The development of space is a great dream left to mankind. About 40 years have passed since the artificial satellite was launched for the first time in the world. At present, international telephone communication can be made through comsats and the image of clouds can be seen on television and newspapers through a meteorological satellite. With a greater power output in satellites and spaceships, the thermal control system must be considered for the recovery, transport, and disposal of waste heat. The change of phase of the working fluid results in lower electrical power requirement to operate pumps and gives a relatively isothermal sink temperature. Various system designs must be made to accomplish this two-phase thermal management capability. However, there is only little information available, as a restricting factor, on the fluid and heat transfer characteristics of two-phase flows such as pressure drop, flow pattern, and heat transfer in reduced-gravity environments. Here, the present status of investigation on gas-liquid two-phase flow under microgravity environments is outlined with a development of space radiator techniques for use in space.

In a gas-liquid two-phase flow, forces such as surface tension, momentum force and gravity affect on the interaction between dispersed phases such as droplets and bubbles, as well as gas and liquid continuous phases. Gas-liquid two-phase flow experiments under micro-gravity environment have been increasingly carried out in recent years using drop shafts, aircraft, and space stations, to evaluate the influence of these forces included in the constitutive equation utilized in numerical analysis. Here the drop shaft is a very useful tool for two-phase flow experiments, especially when microscopic measurement is required as in the confirmation of a constitutive equation for numerical analysis, because there is a very small remaining gravity of 10-4-10-5 g and no horizontal force. This report outlines examples of research being conducted in Japan on a gas-liquid two-phase flow under micro-gravity environment using a drop shaft, with the aim of confirmation of constitutive equation. Research on gas-liquid two-phase flow under micro-gravity environment using a drop shaft as in these examples is expected to be conducted increasingly in the future, shedding future light on the phenomena of gas-liquid two-phase flow.

Liquid propellant rocket engine is introduced, and discussed about the technology status. As a relation with Multiphase technology, a new rocket engine development approach is introduced.

H-IIB launch vehicle is the newest and the most advanced Japanese rocket which can transport 16.5 ton of cargo vehicle (HTV) to International Space Station (ISS) and up to 8 ton of payload into geostationary transfer orbit. H-IIB has been developed efficiently under the government-industry joint framework of JAXA and MHI. Its first launch mission to transfer the first HTV to ISS was successfully completed in September 2009 as scheduled. The efficient and quick development and success are based on the advanced technology accumulated in JAXA and Japanese companies through Japanese rocket development history.

Detailed numerical simulations of Diesel liquid jet sprays have been conducted to elucidate the liquid atomization mechanisms. The obtained flow field is very complicated, but the spray behavior can be understood physically. The jet head formation leads to vortex shedding and subsequent atomization from the head edge. The liquid core surface is also unstable due to gas boundary layer instability and atomization also occurs from here. The final process of droplet generation from a ligament is basically the same as the past research results, and can be explained by the capillary wave destabilization. Such findings can be used for droplet modeling for practical-scale simulations.

With the progress of human activities in space, the occasion to handle liquids in non-uniform acceleration or low-gravity is now growing. On the launch vehicles with liquid propulsion system, the dynamic acceleration during its powered ascent or ballistic flight makes it very difficult to control the position of propellants in the tanks. For the establishment of the technology for the management of liquid propellant in space vehicles, a numerical method, called ‘CIP-LSM’ (CIP based Level Set & MARS), was developed to simulate three-dimensional free-surface flows under various gravity conditions, which has been applied to clarify the dynamic behavior of liquid propellant in the tanks of launch vehicles.

A liquid oxygen supply system for a small-scale sounding rockets was developed. The sounding rokets are hybrid type employing a combination of plastics (PMMA or high density polyethylene) and liquid oxygen as propellants. Key points for the miniaturization were using no valve in the liquid oxygen feeding line and omitting precooling treatment of the line. Without precooling treatment, the liquid oxygen in the feeding line becomes multiphase flow until the temperature of the feeding line falls below the boiling temperature of the liquid oxygen. A characteristic time of multiphase flow duration was proposed to evaluate the duration a multiphase flow holds in the feeding line. Static firing tests showed that the multiphase flow ends within the half the characteristic time, showing that the liquid oxygen flow rate history without a precooling treatment is acceptable for an actual operation of the rocket motor. Finally, an impinging type injector was developed to remove a combustion instability, caused by a coupling of the combustion chamber pressure and the propellant (liquid oxygen) feed system.

MON3 is used as a propellant for the bipropellant thruster in the spacecraft propulsion. MON3 is Nitrogen Tetroxide (N2H4) with 3wt% solution of NO. Heat transfer characteristics with boiling of the MON3 have been measured. The critical heat transfer was identified when the superheat temperature was around 50 ℃ (degree C) under different flow parameters. Pressure vibration from 8 to 22Hz frequency was observed in the specific flow parameters during the boiling of MON3. These frequencies are in good agreement with that of combustion perturbations observed in the test injector which simulated the actual bipropellant thrusters.

Increase in the consumption of electricity and in the heat dissipation density from semiconductors will become a serious problem also in space. To develop thermal management systems for space platforms, fundamental data for their design is to be obtained through boiling and Two-Phase Flow (TPF) experiments onboard international space station (ISS). In addition to the acquisition of data for heat transfer, the clarification of flow and heating conditions, where gravity effects disappear, become an important objective of the experiment. An experimental setup was developed thorough the long-term discussion and design among JAXA, researchers and manufactures supporting the project, and was already transported to ISS. Objectives and situation of the present research and a desired direction of future experiments are described with reference to the existing data obtained from short-term microgravity experiments for flow boiling.

No less than 99.9% of visible matter in our universe is in the collisionless plasm state. Hence, it is essential for understanding of our universe to study collisionless space plasma. This paper introduces various basic theories with different approximations for space plasma. Various numerical methods for simulating space plasma with these basic equations are also described briefly.

Numerical simulations of plasma processes taking place near spacecraft require fully-kinetic modeling of plasmas, proper particle and field conditions at spacecraft surfaces, and sound parallelization strategy adapted for modern supercomputers. Some relevant techniques are presented, with particular focus on the use of the particle-in-cell approach, charge redistribution processed on conducting surfaces, and a robust load balancing algorithm for distributed-memory parallel computers. Example applications to emerging spacecraft-plasma interaction problems are presented, on spacecraft charging processes in non-stationary space environments as well as electron acceleration processes within microwave discharge chambers designed for ion thrusters.

Auroral breakup is the term used to describe a transient phenomenon in which brightness of aurora suddenly increases and the bright aurora expands rapidly. When the auroral breakup takes place, the magnetosphere and the ionosphere are highly disturbed. The ultimate cause of the auroral breakup is the Sun, but intermediate processes between them remained unsettled. The global magnetohydrodynamics (MHD) simulation developed by Prof. Emer. Takashi Tanaka is shown to reproduce well the sequence of the auroral breakup. Here, we show the intermediate processes leading to the auroral breakup on the basis of the results obtained by the MHD simulation.

Space plasmas are essentially collisionless. Kinetic energies of plasmas are transferred through wave-particle interactions. Since plasmas are dispersive media, plasma waves show various features depending on how and which plasma wave modes interact with particles. Plasma wave observations via scientific satellites have a key role in knowing physical processes in space. Plasma wave investigation systems on board satellites are dedicated to the observations of electric field and magnetic field components of plasma waves. They consist of sensors, receivers and an onboard digital processing unit. While dipole antennas are used for electric field sensors, search coil magnetometers or loop antennas are used for magnetic field sensors. Plasma wave receivers are a kind of sophisticated radio receivers. The recent plasma wave receiver is designed as a waveform receiver that can save its mass and size. In addition to waveforms, the waveform receiver provides frequency spectra which are calculated in the digital processing unit from the observed waveforms. The present paper introduces plasma wave observation by scientific satellites on the basis of the latest design of plasma wave investigation systems.

Two-phase flow loop cooling system is required for increasing heat transfer rate, heat transport distance, and cooling heat flux in thermal control of space structures. The understandings on thermo-fluid dynamics of gas-liquid two-phase flows with phase change under microgravity is necessary for the system design. To clarify the phenomena and verify the operation of two-phase flow loops under microgravity condition, two-phase flow loop experiments were conducted as a JAXA project named TPF experiment onboard the International Space Station (ISS). The loop was a pump-driven loop and the pressure was maintained by an accumulator. Perfluorohexane was used as the working fluid because of low latent heat of vaporization and moderate boiling point under the restriction of power input and for the safe operation. The loop had two kinds of heating section, such as a copper heating tube and a transparent glass heating tube, and a condenser. Flow observation sections made of transparent polycarbonate resin were installed just downstream of each heating tube in order to observe and measure the interface structure of boiling two-phase flow in detail. In this report, the design concept and system characteristics of the experimental loop and the results of the gas-liquid two-phase flow measurements in the flow observation section are presented.

This paper discusses the transient model of loop heat pipes in detail. A new model was constructed using an implicit method for fluid calculations and an explicit method for heat transfer calculations between pipe walls and radiators. In addition, the effectiveness of this model was verified using a loop heat pipe used for demonstration purposes on International Space Station. Good agreement was shown with the results of thermal vacuum tests. On the other hand, a comparison with the on-orbit test results revealed differences in vibration phenomena. It was concluded that this was because the heat input in orbit, such as solar heat input, was not accurately estimated.

Fluid behavior in microgravity is different from in ground gravity since surface tension, viscous force, and wetting are dominant in this condition. In propellant tank for artificial satellite and future on-orbit spacecraft, sloshing due to disturbance and settling behavior by change of acceleration have to be understood for design of propellant supply system and attitude control system. These fluid behaviors in microgravity condition are affected by a dynamic wetting significantly, therefore it is important to understand dynamic wetting which dominates fluid behavior. We observed transient fluid behaviors in cylindrical containers upon entering microgravity conditions created by drop tower facility, and effect of viscosity and diameter of container on fluid behaviors were investigated. CFD (Computational Fluid Dynamics) analyses were conducted and these results were compared with experimental results. Additionally, it was confirmed that numerical model considering dependence of contact line velocity in dynamic contact angle by empirical and theoretical methods provided more reasonable results. In addition, the sloshing behaviors inside the cylindrical containers were investigated under applied horizontal and axial accelerations using both microgravity experiments and CFD. As a result, it was found that under conditions of low excitation acceleration and high viscosity coefficient of the liquid, where the fluid movement is minimal, the experimental and numerical analysis results exhibit good agreement.

Long-term storage of cryogenic propellants such as liquid hydrogen and liquid oxygen is required for manned long-distance exploration of deep space. In propellant tank, evaporation from the gas-liquid interface is accelerated by sunlight and heat input from surrounding equipment, and the boil-off gas (BOG) generated from the liquid surface, it is caused pressure raise in the tank and could lead to tank destruction. Therefore, the development of Thermodynamic Vent System (TVS) to reduce BOG generation is urgently needed. An effective way to reduce BOG is to agitate the bulk liquid by mixing cold jet issuing from the bottom of the tank. Here, to estimate the jet-tip height, one-dimensional dynamic model is described. And the height of the jet-tip is investigated under various gravitational conditions, and the experimental results compared to the results obtained from the dynamic model.