Thermal Design and Validation for a 6U Cubesat EQUULEUS Under Constraints Tightly Coupled with Orbital Design and Water Propulsion System

Thermal Design and Validation for a 6U Cubesat EQUULEUS Under Constraints Tightly Coupled with Orbital Design and Water Propulsion System

49th International Conference on Environmental Systems ICES-2019-193 7-11 July 2019, Boston, Massachusetts Thermal Design and Validation for a 6U CubeSat EQUULEUS under Constraints Tightly Coupled with Orbital Design and Water Propulsion System Shuhei Matsushita1, Toshihiro Shibukawa1, Keidai Iiyama1, and Ryu Funase2 Intelligent Space Systems Laboratory (ISSL), The University of Tokyo, Bunkyo-ku, Tokyo, 113-8656, Japan EQUULEUS, a 6U CubeSat co-developed by the University of Tokyo and JAXA, will fly to a libration orbit around the second Earth-Moon Lagrange point (EML2), and conduct scientific observations such as detection of the lunar impact flashes. To realize these observations, thermal design of EQUULEUS faces three constraints coupled with the design of other subsystems. Firstly, because there is an uncertainty of the launch date, EQUULEUS will have to consider the variation of the sun direction at thrusting phases, which changes the heater wattage to achieve the required thrust value. Secondly, at the observation phase around EML2, EQUULEUS will also experience the sunlight from various direction. Lastly, the water resistojet propulsion system adopted for trajectory control and angular momentum unloading of EQUULEUS makes the thermal design more difficult in terms of freezing and the latent heat for vaporization. Under the constraints, we proposed the thermal design of EQUULEUS Engineering Model (EM), validated the design by numerical simulations and the thermal vacuum test. Nomenclature 퐶 = heat conductance 푑 = distance between the sun and spacecraft 2 푃푠 = solar constant (= 1366 W/m ) 푄푖푛 = internal heat 푄푙푎푡 = latent heat 푄푐표푛푠 = generated power not consumed as internal heat 푄푟푒푞 = total required power 푄푉퐻 = heater power at the vaporization chamber 푄 = generated heat at a component 푆 = area 푇 = temperature 푇푠 = the cosmic background temperature 훼 = surface absorptance 휖 = surface emissivity 휎 = Stefan-Boltzmann constant acronyms EM = Engineering Model EML2 = Earth-Moon Lagrange point 2 DV = Delta V SAP = Solar Array Paddle VAP = VAPorization chamber 1 Graduate Student, Department of Aeronautics and Astronautics, 7-3-1 Hongo, Bunkyo-ku, Tokyo 2 Associate Professor, Department of Aeronautics and Astronautics, 7-3-1 Hongo, Bunkyo-ku, Tokyo Copyright © 2019 The University of Tokyo 49th International Conference on Environmental Systems ICES-2019-193 7-11 July 2019, Boston, Massachusetts Subscripts Superscripts 퐶푂푀푀 = the communication module 퐿퐿 = lower limit 푉퐴푃 = the vaporization chamber 푈퐿 = upper limit 퐵푈푆 = bus components 퐶 = coldest environment 푃 = a panel 퐻 = hottest environment 퐷푉1 = the DV-1 phase I. Introduction QUULEUS (EQUilibriUm Lunar-Earth point 6U Spacecraft) is one of 13 CubeSats that will be delivered into E deep space by NASA’s SLS (Space Launch System) EM-1 (Exploration Mission-1) in 2020 and is jointly developed by the University of Tokyo and JAXA1. The spacecraft will fly to a libration orbit around the second Earth- Moon Lagrange point (EML2) and demonstrate the ability of trajectory control within the Sun-Earth-Moon region for the first time by a nano-spacecraft. EQUULEUS has also several scientific observation instruments. One of them is PHOENIX (Plasmaspheric Helium ion Observation by Enhanced New Imager in eXtreme ultraviolet), which will conduct the imaging of the Earth's plasmasphere by extreme UV wavelength. By observation at farther from the Earth than a low Earth orbit, PHOENIX will be able to get the overall image of the Earth’s plasmasphere. Another scientific instrument, named DELPHINUS (DEtection camera for Lunar impact PHenomena IN 6U Spacecraft), will observe the impact flashes at the far side of the moon from EML2 for the first time. A libration orbit around EML2 is the proper location to conduct these observations. The EQUULEUS trajectory overview is shown in Figure 1. The diameter of a designed libration orbit for EQUULEUS observation2 is about 80,000 km and altitude of it from the moon surface is around 40,000 km as shown in Figure 2, which is much larger than the moon diameter 3,474 km. The spacecraft at this libration orbit can observe the moon surface and the Earth alternately with slight attitude control. However, when EQUULEUS stationary observes the Earth and the moon surface around EML2, the sun direction toward EQUULEUS changes gradually. The temperature of instruments has to be kept in allowable operation temperature adapting the changes of the sun direction. From the viewpoints of operation and thermal design mentioned in detail later part of this paper, EQUULEUS adopts the configuration shown in Figure 4 and Figure 3. The observation direction of PHOENIX and DELPHINUS is fixed to the +Y direction of EQUULEUS, and the solar array paddles are arranged to be rotatable along with X-axis using a gimbal mechanism. This configuration makes it possible to observe the Earth and the moon surface by the scientific instruments and to generate adequate power by the solar array paddles simultaneously. Figure 1. EQUULEUS trajectory in the Sun-Earth Figure 2. Close-up of EQUULEUS libration orbit rotating frame2. Illustrates the trajectory from the earth approach and science phase2. Illustrates the libration to the libration orbit for observation. There are four orbit and the position of the moon in the Earth-Moon delta-V operations. rotating frame Copyright © 2019 The University of Tokyo Figure 4. External view of EQUULEUS and axis Figure 3. Internal Configuration of EQUULEUS. We definition. Illustrates the observation direction of can find the configuration of components of AQUARIUS. scientific instruments DELPHINUS and PHOENIX. Furthermore, another feature of EQUULEUS is that EQUULEUS adopts the cold gas propulsion system, named AQUARIUS (AQUA ResIstojet propUlsion System)3 for trajectory control and angular momentum unloading. This propulsion system utilizes water as propellant, which is the most “green” propellant. One of the difficulties to develop the propulsion system is a large amount of heat to vaporize water. Due to the limitation of the power resource of a 6U CubeSat, we have to save the heater consumption. Following the overview of the thermal design of EQUULEUS engineering model (EM) by Koshiro et al.4, this paper arranged again and in detail the thermal design concept of EQUULEUS with these constraints: the change of the sun direction and the limitation of heater wattage. Then, to validate this thermal design concept, one-node analysis, three-nodes analysis, and multiple nodes simulation with Thermal Desktop were sequentially conducted. Finally, after thermal correlation using the thermal vacuum test results was conducted. II. Concept of Thermal Design A. Thermal Environment of EQUULEUS mission Under the orbital and operational constraints described as follows, EQUULEUS adopts the configuration shown in Figure 4 and Figure 3. Then, this section introduces the characteristics of EQUULEUS thermal environments during the whole mission. The first constraint is coupled with the orbital design and a launch window of the launcher, SLS. EQUULEUS is piggyback spacecraft and cannot decide the launch date of SLS. According to NASA’s study5, the launch window of SLS, which depends on the SLS performance, is a 10-11 days launch period per sidereal month, though the launch months is already roughly defined. Considering the uncertainty of launch date, the orbital design subsystem of EQUULEUS has already researched the design method of the transfer orbit to EML2 and proposed a solution of the orbit and proper thrust timings in some examples of launch date2. However, as the launch date changes, designed transfer orbit and thrust timings change, then the sun direction from the spacecraft at all phases, especially the thrusting phases, may also change. After all, the thermal design of EQUULEUS should make the thrusting phases successful under the uncertainty of sun direction. The second constraint is coupled with the orbital design especially during an observation phase at EML2. EQUULEUS will conduct several scientific observations, DELPHINUS and PHOENIX. The spacecraft will stay at the libration orbit around EML2 for about six months2, and change attitude slightly to observe the lunar impact flash by DELPHINUS and the Earth's plasmasphere by PHOENIX alternately. While EQUULEUS faces to the moon surface and the Earth from EML2, the relationship between the sun direction and the observation direction changes periodically. Figure 5 shows that the relationship between the red arrow (the observation direction) and the large orange arrow (the sunlight direction) changes as the moon moves. On the other hand, to generate adequate power continuously, solar array paddles (SAP) should always face the sun straightforwardly. To achieve these two requirements: observation direction facing the moon and the Earth and SAP direction facing the sun, EQUULEUS adopted the gimbal system solar array paddles configured to be rotatable along the axis perpendicular to the moon 3 International Conference on Environmental Systems orbit plane. Figure 5 shows the change of relationship between observation direction and SAP direction using the gimbal system. Whereas the SAP direction is always toward the sun, the observation direction gradually changes as the moon moves. For EQUULEUS, we set X- axis as the gimbal axis of solar array paddles, and +Y as the observation direction of DELPHINUS and PHOENIX as shown in Figure 4. As a result, with this characteristic configuration, EQUULEUS will experience sunlight on

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