Sumitaka Tachikawa

A variable-emittance radiator based on a metal–insulator transition of (La,Sr)MnO3 thin films

Variable-emittance radiators based on the metal–insulator transition of (La,Sr)MnO3 thin films have been developed. The emittance property of the films was evaluated from infrared reflectance spectra; that is, the (La,Sr)MnO3 thin films show low emittance at low temperature but high emittance at high temperature. Moreover, the emittance property significantly changes at the metal–insulator transition temperature, where the material changes from a highly reflective (i.e., low emissive) metal to a less reflective (i.e., high emissive) insulator. The (La,Sr)MnO3 thin films fitted on a spacecraft surface can, therefore, be used to automatically control the emissive heat transfer from the spacecraft without the need for any electrical power. The developed radiators also greatly reduce the weight and production cost of the thermal control devices. The dependence of the emittance property on film thickness reveals that 1500-nm-thick films can be used for variable-emittance radiators.

Radiative and optical properties of La1-xSrxMnO3 (0 ≤ x ≤ 0.4) in the vicinity of metal-insulator transition temperatures from 173 to 413 K

Radiative and optical properties of polycrystalline La1−xSrxMnO3 (0≤x≤0.4) in the vicinity of the metal–insulator transition are presented. The temperature dependence of the total hemispherical emittance εH of La1−xSrxMnO3 was measured by the calorimetric method in the temperature range from 173 to 413K. It was confirmed that εH showed unexpected variation as a result of changes in the hole concentration (x). Especially in the case of La0.825Sr0.175MnO3, εH remains high above the transition temperature TC due to insulator-like behavior; on the other hand, it decreases sharply below TC because of metallic behavior. The spectral reflectance was measured by FT-IR in the wavelength range of 0.25 to 100 μm at room temperature. The optical constants were calculated by Kramers–Kronig analysis of the spectral reflectance data. An insulator-like character of the optical properties appears at lower Sr2+ doping levels while a metallic one exists at higher Sr2+ doping levels.

Variable Thermal Emittance Radiator Using Metal-Insulator Phase Transition in La1-xSrxMnO3

A variable-emittance radiator device, made of thin and light ceramic tiles, has been developed for thermal control applications on spacecraft. The ceramic material used is La1-xSrxMnO3 with a perovskite structure, and shows a phase transition from ferromagnetic metal to paramagnetic insulator at around 290 K (Tc). This device automatically controls a spacecrafts temperature without electrical or mechanical instruments. Below the Tc, the device is metallic with a low thermal emittance of 0.3, and above the Tc, it becomes insulative with a high thermal emittance of 0.7. For the ceramic tiles, two different fabrication processes were studied to reduce the cost and weight; one is a conventional ceramic wafer process and the other is a thick film process on zirconia substrates. Total thickness of the ceramic tiles obtained is less than 70 µm and the weight is 450 gr/m2.

Thermophysical Properties of High-Thermal-Conductivity Graphite Sheet and Application to Deployable/Stowable Radiator

This study proposes a passively deployed radiator for use as a new thermal control device for small satellites. This radiator can control the amount of heat dissipation by varying its heat rejection area depending on the temperature. This radiator consists of a fin, a baseplate, and an actuator. First, thermal diffusivity, specific heat capacity, total hemispherical emissivity, and solar absorptance of graphite sheets, which are a part of this radiator fin, were measured. From these measurements, it was determined that the thermal conductivity of the graphite sheet varied from 950 to 1490  W/m K, the total hemispherical emissivity varied from 0.22 to 0.31, and the solar absorptance was 0.66. Second, the performance of the radiator was calculated via thermal analysis as a function of size and thickness, and a half-scaled test model of this radiator was designed. Third, the test model was fabricated and its thermal performance was tested under vacuum conditions. The testing showed that the thermal dissipati...

Performance Evaluation of New Thermal Insulation System with Polyimide Foams

Spacecrafts to go to inner or outer planet need to increase the insulation performance and also to evaluate its thermal performance precisely in order to design spacecrafts effectively. Our goal is to develop a new insulation system for spacecrafts instead of the traditional Multi Layer Insulation (MLI). Due to its high heat resistance, light weight, and high resistance to space environments, the porous super insulation polyimide foams (PI-F) is expected to progressively advance the spacecraft thermal insulation technique. The PI-F is based on the polyimide resin. In order to design the new thermal insulation system with PI-F, it is essential to clarify the thermal performance with combined phenomena of conductive and radiative heat transfer in PI-F. Heat transfer of new insulation with polyimide foam is basically decreased by using low conductive material instead of decreasing the radiative heat transfer. New MLI with polyimide foams has been constructed and its insulation performance was evaluated. As a result, we confirmed its insulation performance will be better than that of traditional MLI.

Investigation of Microwave Attenuation by Solid Rocket Exhausts

In rocket flights, ionized exhaust plumes from solid rocket motors may interfere with RF transmission under some conditions. In order to clarify the important physical process involved, microwave attenuation and phase delay due to rocket exhaust plumes were measured during sea-level static firing tests conducted on two types of full-scale solid propellant rocket motors. The measured data were analyzed by comparing them with numerical results such as flowfield simulations of exhaust plumes and by employing a detailed analysis of microwave transmission by using a frequency-dependent finitedifference time-domain (FD2TD) method. The results revealed that either the line-of-sight microwave transmission through ionized plumes or the diffracted path around the exhaust plume mainly affects the received RF level, which depends on the magnitude of the plasma RF interaction. For the actual launch vehicle flight, the transmission process is dominated by the diffraction effect so that we applied a two-dimensional diffraction theory to analyze the communication between a vehicle and a ground station. The attenuation levels estimated using diffraction theory agree with the data recorded in-flight.

Preliminary Design and Test Results of a New Flexible Thermal Control Mirror

The basic construction of thermal control mirrors, also known as Optical Solar Reflectors (OSRs) or Second Surface Mirrors (SSMs), consists of a thin glass substrate with a rear surface silver coating. It has a high total hemispherical emittance due to the glass, but a low solar absorptance from the highly transmitting glass and highly reflecting metallic surface. This paper will describe a new type of thermal control mirror, Controlled Optical Surface Film (COSF). Although the function of the COSF is the same as the conventional OSRs, the construction is different. The COSF consists of a polyimide film (UPILEX-S) for substrate and multi-layer coating on it. While the solar radiation is reflected on the rear surface silver coating in case of the OSRs, it will be reflected on the front surface multi-layer coating in case of the COSF. The preliminary design will be described and the preliminary test results of a comprehensive space qualification are also reported showing that the COSF is stable under high radiation fluence in space.

Variable Thermal-Emittance Radiator Using La1-xSrxMnO3 Thick Film on PSZ Substrate

We have developed a variable-emittance radiator device, made of thin, and light ceramic tiles, for thermal control applications on spacecraft. The ceramic material used is La1-xSrxMnO3 with a perovskite structure, which shows a phase transition from a ferromagnetic metal with a low thermal emittance of 0.35 to a paramagnetic insulator with a high thermal emittance of 0.75 at around 300K. This device automatically controls the temperature of a spacecraft without requiring electrical or mechanical instruments. The fabrication process for the ceramic tiles was designed to reduce production costs and weight. The tiles are less than 70-microns thick and weigh 450g/m 2 . Both a conventional type of ceramic wafer and a thick film type were developed and tested for their durability under various environmental and radiation conditions. The results showed no degradation. These variable emittance radiators made of ceramic tiles have been used for the MUSES-C spacecraft designed to probe asteroids, and are also scheduled to be used for the INDEX spacecraft designed to observe the earth.

A Variable-Emittance Radiator Based on a Metal-Insulator Transition of (La,Sr)MnO 3

Variable-emittance radiators based on the metal-insulator transition of (La,Sr)MnO 3 have been developed. The emittance property of the material was evaluated from infrared reflectance spectra; that is, (La,Sr)MnO 3 shows low emittance at low temperature but high emittance at high temperature. Moreover, the emittance property significantly changes at the metal-insulator transition temperature, where the material changes from a highly reflective (i.e., low emissivity) metal to a less reflective (i.e., high emissivity) insulator. The (La,Sr)MnO 3 thin-films fitted on a spacecraft surface can, therefore, be used to automatically control the emmisive heat transfer from the spacecraft without the need for electrical power. The developed (La,Sr)MnO 3 thin-film radiator also greatly reduces the weight and production cost of the thermal control devices.