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JAXA's X-Ray Astronomy Mission ASTRO-H: Launch and First

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JAXA's X-Ray Astronomy Mission ASTRO-H: Launch and First 46th International Conference on Environmental Systems ICES-2016-157 10-14 July 2016, Vienna, Austria JAXA’s X-ray Astronomy Mission ASTRO-H: Launch and First Month’s In-Orbit Thermal Performance Naoko IWATA1 Japan Aerospace Exploration Agency, Sagamihara, Kanagawa, 252-5210 Tel: +81-50-3362-3592, Fax: +81-42-759-8068 E-mail: [email protected] Takashi USUI2, Akihiko MIKI3, Mizuho IKEDA4 NEC Corporation and Yoh TAKEI5, Atsushi OKAMOTO6, Hiroyuki OGAWA7, Tadayuki TAKAHASHI8 Japan Aerospace Exploration Agency In this study, the thermal performance evaluation results of JAXA’s X-ray astronomy mission ASTRO-H are presented. ASTRO-H was successfully launched on February 17, 2016 from Tanegashima Space Center using an H2A rocket. ASTRO-H houses four telescopes and six detectors. Each detector has its own radiator and heat pipes for heat dissipation. The most major mission will feature a soft X-ray spectrometer having four loop heat pipes (LHPs) for heat transport from two cryocoolers. Six fans have been mounted close to each cryocooler for ground cooling in the fairing just before the launch. Hard X-ray imagers (HXIs) are mounted on an HXI plate; this plate is expanded by 6.4 m via an extensible optical bench (EOB) in orbit to achieve the necessary focal length. The EOB was successfully expanded 11 days after the launch. Heat pipes appropriately functioned in orbit. The two LHPs for the compressors have been operated properly for more than one month Measured inflight temperatures agree well with predicted ones for an attitude condition. Nomenclature BOL = Beginning Of Life CFRP = Carbon Fiber Reinforced Plastic EOB = Extensible Optical Bench EOL = End Of Life FM = Flight Model FOB = Fixed Optical Bench HCE = Heater Control Electronics HXI = Hard X-ray Imager HXT = Hard X-ray Telescope 1 Researcher, Research and Development Directorate, Research Unit 2 2 Assistant Manager, Space Technologies Department, Space Systems Division 3 Manager, Space Technologies Department, Space Systems Division 4 Engineering Manager, Space and Satellite Systems Department, Space Systems Division 5 Assistant Professor, Department of Space Astronomy and Astrophysics, Institute of Space and Astronautical Science 6 Senior researcher, Research and Development Directorate, Research Unit 2 7 Associate Professor, Department of Space Flight Systems, Institute of Space and Astronautical Science, Senior Member AIAA 8 Professor, Department of Space Astronomy and Astrophysics, Institute of Space and Astronautical Science JAXA = Japan Aerospace Exploration Agency LHe = Liquid Superfluid Helium LHP = Loop Heat Pipe LP = Launch Pad MLI = Multilayer Insulation SGD = Soft Gamma-ray Detector SNT = Santiago Ground Station STA2 = Second Spacecraft Test and Assembly building SXI = Soft X-ray Imager SXS-XCS = Soft X-ray Spectrometer X-ray Calorimeter Spectrometer SXT = Soft X-ray Telescope TKSC = Tsukuba Space Center TVT = Thermal Vacuum Test TMM = Thermal Mathematical Model TTM = Thermal Test Model USC = Uchinoura Ground Station UT = Universal Time UVC = Under-voltage Controller VAB = Vehicle Assembly Building I. Introduction STRO-H observes black holes and clusters of galaxies using a set of instruments with the highest energy A resolution ever achieved and a four-decade range from soft X-rays to gamma rays.1 The development of ASTRO-H started in 2008. The flight model (FM) system integration test campaign, including a thermal vacuum test (TVT), was conducted in 2014 and 2015 in JAXA’s Tsukuba Space Center (TKSC). FM spacecraft was transferred to JAXA’s Tanegashima Space Center via land and sea. ASTRO-H was launched on February 17, 2016 using an H2A rocket after approximately two months of flight operation. Following JAXA’s custom, ASTRO-H was named after it was successfully launched. Its name, “Hitomi,” generally means “eye” in Japanese, specifically the pupil or entrance aperture of the eye. To avoid confusion, it is referred to as “ASTRO-H” in this study. ASTRO-H was injected into an approximately circular orbit of orbital height 575 km and inclination 31° (Fig. 1). As shown in Fig. Figure 1. Artist’s orbital ASTRO-H 2, any attitude within 0–30° relative to the y axis can be attained with in approximately circular orbit. a limit on the range determined by the incidence angle of sunlight on telescopes. ASTRO-H is a three-axis-stabilized spacecraft. The construction of ASTRO-H is shown in Fig. 3. There are four telescopes on the top of the fixed optical bench (FOB): two hard X-ray telescopes (HXTs)2 and two soft X-ray telescopes (SXTs). Two star trackers are also mounted on the top of the FOB to satisfy the need to precisely control directions for observations. Two science instruments for detecting soft X-rays, i.e., the soft X-ray spectrometer X-ray calorimeter spectrometer (SXS-XCS)3 and soft X-ray imager (SXI)4, are mounted on the base plate, which is an octagonal plate of 3-m diameter. Eight side panels are mounted on the base plate, one per edge. The side panel on the +X-direction edge shown in Fig. 2 is designated as side panel #1, and remaining panels are designated as side panels #2–#8 in a counterclockwise direction from #1. Soft gamma-ray detectors (SGDs)5 are symmetrically mounted on the outer sides of side panels #1 and #5. Hard X-ray imagers (HXIs)6 are mounted on the HXI plate; this plate is expanded to 6.4 m in orbit via the extensible optical bench (EOB) to achieve the necessary focal length. The Figure 2. Coordinate system of ASTRO-H. 2 International Conference on Environmental Systems total length of ASTRO-H before the launch is 8.2 m and that in orbit is 14 m. Six solar panels are mounted on the outer side of side panel #3. Total power generation is below 3500 W. Total mass is below 2700 kg. After the launch and deployment of the solar array paddle, bus components and mission instruments, including the cooling system of the SXS-XCS, were powered up, and then, the operational test of the SXS was conducted. Finally, the EOB was successfully expanded and the critical operation phase was completed in February 29, 2016. II. Thermal Design To achieve desired science objectives such as high- energy resolutions and sensitivities, the most important requirements for the thermal control system are as follows: 1) The minimization of the thermal distortion of the spacecraft structure to satisfy the need to precisely control the direction for a stable observation of celestial bodies along the optical axes of telescopes. 2) Science instruments should be maintained within Figure 3. Construction of ASTRO-H. their required temperature range. All plates, including the base plate, three plates of the FOB, and side panels, are made of aluminum honeycombs and carbon fiber reinforced plastic (CFRP) skins with a very low coefficient of thermal expansion. Truss tubes of the FOB are also made of CFRP. The entire structure above side panels is covered with multilayer insulation (MLI) to isolate the FOB from the external thermal environment. Furthermore, the exterior surfaces of each plate and truss tube of the FOB are covered with aluminized polyester film to minimize radiation coupling among adjacent components. These features are designed to minimize the temperature gradients of the FOB. The exterior of the EOB is covered with a one-sided aluminized polyimide film to minimize its temperature gradients. This is because the pointing accuracy of an HXI would be strongly impacted by thermal distortion in the EOB. The film is folded before the launch and expanded accordion style with the EOB in orbit. Most bus components are mounted on side panels. The components’ heat is dissipated by radiators (silverized Teflon) placed on the exterior of side panels. Each instrument has a surface of high IR emissivity (black paint) to equalize the internal temperature. For the same purpose, the interior surfaces of side panels are bare CFRP skins. All sensors, i.e., SXS-XCS, SXI, SGD, and HXI, have their own radiators for heat dissipation. Heat is transported via heat pipes using ammonia (NH3) as the working fluid. All heat pipes are dual channels or two single channels for redundancy. The heaters attached on the bus components, mission instruments, and heat pipes to satisfy their temperature requirements are controlled by Heater Control Electronics (HCE) except for the heaters controlled by mission instruments. Two heaters and temperature sensors are attached on a specified part to be controlled for redundancy. The set point of the redundant heater and temperature sensor is lower than that of the primary heater and temperature sensor so that only the primary heaters turn on normally. Besides, more than 50 temperature sensors are also attached on the bus components, mission instruments, and panels just to measure their temperature. Three HCEs are mounted on ASTRO-H, one is used only for the bus components and mission instruments on HXI plate and other two are used for those mounted on the part other than HXI plate. The former is defined as HXI-HCE and Figure 4. SXS-XCS thermal control system. the latter are defined as HCE-A and 3 International Conference on Environmental Systems HCE-B. Exceptionally, one temperature sensor connected to HCE-A is mounted on HXI plate to measure its temperature during EOB extension as all the electronics including HXI-HCE are planned to be turned off at that time. Total heat dissipation from SXS-XCS, which has six cryocoolers mounted on the exterior of the liquid superfluid helium (LHe) dewar, is 290 W. Figure 4 shows the SXS-XCS thermal control system. Approximately one-third of total heat dissipation is radiated from the LHe dewar surface to deep space, and remaining heat is transported to SXS-XCS’s radiator via heat pipes.
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