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

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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 only four panels out of six, +Y, -Y, +Z, and –Z panel during its observation period. Additionally, of course, the same is true during all mission phases, because EQUULEUS should control its attitude to face SAP to the sun with this configuration. Regardless of which of these four panels facing the sun, the temperature of all components should be kept within the allowable Figure 5. The sun direction and the attitude of temperature range. The hottest and coldest environment of EQUULEUS. The red arrow illustrates the observation EQUULEUS during the observation phase are shown in direction, and the blue arrow illustrates the SAP normal Table 2. direction. The sun direction, which is the large orange The last constraint is coupled with the propulsion arrow, is invariant. system, AQUARIUS. As mentioned above, AQUARIUS utilizes water as a propellant, which enables to develop the system safer, faster and at a lower cost than other conventional propulsion systems. There are already some conventional water resistojet thrusters, which conduct liquid vaporization and vapor heating simultaneously in a single cavity such as constructed at a nozzle. In contrast, AQUARIUS separates the two processes: vaporize water at the vaporizing chamber, then to heat and accelerate the water vapor at a thruster head3. This separation simplifies water flow physics and increases the reliability of the system. Furthermore, because of vaporizing and heating processes, AQUARIUS can get the high specific impulse. With this propulsion system, thermal design of EQUULEUS must care about two things: freezing and large latent heat for vaporization. Because water is stored in the water tank except when thrusting, the temperature of the tank must be kept higher than 0 °C to avoid freezing water. Moreover, water requires a larger amount of latent heat for vaporization than other propellants. In order to earn thrust power, EQUULEUS has to provide adequate heat to and keep the temperature of the vaporization chamber. Especially, since first delta-V (DV-1) is the largest one through the mission2, the requirement of thrust value at the DV-1 phase is most severe and then, of course, the required electrical power at the DV-1 phase is largest. Taking these three constraints into account comprehensively, the purposes of thermal design of EQUULEUS are to keep the temperature of all components and minimize the heater wattage for the propulsion system under the three most severe environments. First and second ones are common environments shown in Table 2, which is the most thermally severe environments and where the thermal design should take care of the temperature of all components. The third one is characteristic, the most severe environment in terms of electric power consumption, which is the DV- 1 phase at 1.017 AU with one panel facing the sun (shown in Table 1). 1.017 AU is the furthest possible distance to the sun at the DV-1 phase. This distance depends on the launch date of SLS. Furthermore, the temperature of VAP should be kept at higher than 18.6 °C, which is enough temperature of the water vapor to achieve the required thrust power at the DV-1 phase. At the three most severe environments, the thermal design of EQUULEUS aimed to keep the temperature of all components within allowable temperature and to provide adequate power for all components including the water propulsion system for a successful EQUULEUS mission especially the DV-1 phase.

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Table 2. Thermal Environment of EQUULEUS during the observation phase. The most thermally severe environments at EML2 to conduct scientific observations The distance b/w EQUULEUS The number of panels Internal heat generation 푄 and the Sun 푑 푖푛 facing the sun 푄퐻 = 31.3 W The hottest 푑퐻 = 0.975 AU 푖푛 (DELPHINUS & CLOTH Two panels environment ⋅퐻 퐻2 2 (푃푠/푑 = 1437 W/m ) Observation mode)

퐶 퐶 The coldest 푑 = 1.025 AU 푄푖푛 = 26.4 W 퐶 퐶2 2 One panel environment ⋅ (푃푠/푑 = 1300 W/m ) (3-axis attitude control mode)

Table 1. Possible Thermal Environment of EQUULEUS at the DV-1 phase. The most severe environment in terms of electric power consumption The distance b/w The temperature The number of panels Operating Mode and phase EQUULEUS and the Sun of VAP facing the sun

퐷푉1 푑 = 1.017 AU 퐷푉1 Thrusting and Unloading Mode 퐷푉12 2 푇 = 18.6 °C One panel (푃푠/푑 = 1322 W/m ) during the DV-1 phase

B. Thermal Design Concepts To make whole EQUULEUS mission successful in terms of the temperature of all components and electric power consumption, we proposed two design concepts. The first concept is to average solar heat input to each panel of the spacecraft. Having the solar array paddles using the gimbal system, EQUULEUS only experiences the sunlight from the Y, Z direction, not from the X direction basically during all mission phases. With this configuration, we designed the product of the optical property 훼, surface absorptance, and the panel area to be thermally equal for these four panels. The equality of the spacecraft’s panels declines a thermal sensitivity to the uncertainty of launch date. The second concept is to utilize waste heat of a communication module as vaporization heat at the vaporization chamber of the water propulsion system. EQUULEUS can generate only 48 W power from the solar array paddles, which is not enough to run the other components if all of the required heat is supplied by a heater. To save the heater wattage supplied to the vaporization chamber under severe power constraint, we designed the heat flow to the vaporization chamber from the communication module, which consumes a large amount of power.

III. Thermal Design Results To design and validate the thermal configurations along the concepts mentioned above in detail, we conducted three numerical simulations. First, to decide the distribution of surface optical properties, one node analysis was conducted. Subsequently, to arrange the components and show an advantage of the proposed method, three nodes analysis was conducted. Finally, multiple nodes simulation on Thermal Desktop was conducted, which showed the thermal feasibility of temperature for all components.

A. One Node Analysis The proposed concept of the surface property is to make the products of 훼푛 and each panel area 푆푛 (⋅푛 describes which panel’s property in +Y, -Y, +Z, and -Z) to be thermally equal for the four panels which are likely to experience sunlight. To determine the surface property according to the concept, we conducted one node simulation. In this simulation, the product of 훼 and 푆 is replaced with a new parameter 푆′, which means the effective area to calculate the solar input, and set an approximation that the whole spacecraft is at the same temperature. To keep the temperature of all components within the allowable temperature range, the spacecraft temperature 푇 at one node analysis should be within the upper limit 푇푈퐿 and the lower limit 푇퐿퐿. By expressing the hottest temperature experienced by the spacecraft as 푇퐻 , and the coldest temperature as 푇퐶 , the constraints of the allowable temperature and the most thermally severe environments can be expressed as the following equations.

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′ 푃푠 퐻4 4 퐻 √2푆 2 − 휎휖푆푎푙푙(푇 − 푇푠 ) + 푄푖푛 = 0 (1) 푑퐻

′ 푃푠 퐶4 4 퐶 푆 2 − 휎휖푆푎푙푙(푇 − 푇푠 ) + 푄푖푛 = 0 (2) 푑퐶

푇퐿퐿 < 푇퐶 and 푇퐻 < 푇푈퐿 (3)

The hottest temperature 푇퐻 and the coldest temperature 푇퐶 can be calculated in Eq. (1) and (2), with the two ′ surface property parameters, 푆 (= 훼푛푆푛) and 휖. The two property parameters should be designed to satisfy the requirement of allowable temperature range expressed in Eq. (3). However, to minimize the required power during 퐷푉1 thrusting operation 푄푟푒푞 , we should minimize 휖 value and save the radiation of heat. As 휖 value decreases when 훼 value is fixed, the temperature of the spacecraft increases. That is why when the hottest temperature reaches the upper limit of the allowable temperature range, the 휖 value is minimized. By using Eq. (1), 휖 value can be expressed as follow:

′ 푃푠 퐻 √2푆 2+푄푖푛 휖 = 푑퐻 = 휉푈퐿푆′ + 휂푈퐿 (4) 푈퐿4 4 휎푆푎푙푙(푇 −푇푠 )

Here, 휉푈퐿 and 휂푈퐿 are constants defined by the boundary constraints such as the allowable temperature range. 휖 value can be expressed as a function of 푆′ or 훼. Furthermore, the required internal heat at the DV-1 phase at 1.017 퐷푉1 AU with one panel facing the sun (shown in Table 1), 푄푖푛 , can be calculated in the following equation.

퐷푉1 퐷푉14 4 ′ 푃푠 푄푖푛 = 휎휖푆푎푙푙(푇 − 푇푠 ) − 푆 2 (5) 푑퐷푉1

Here, 푇퐷푉1 is the required temperature of the vaporization chamber to achieve the required thrust value. By 퐷푉1 defining the consumption power 푄푐표푛푠 that is consumed as communication radiation and heat to the water vapor at 퐷푉1 the thrust head instead of internal heat, and defining the latent heat 푄푙푎푡 that is required heat at VAP to achieve the 퐷푉1 required thrust power 4 mN, the total required generation power 푄푟푒푞 can be obtained as follows:

퐷푉1 퐷푉1 퐷푉1 퐷푉1 푄푟푒푞 = 푄푖푛 + 푄푙푎푡 + 푄푐표푛푠 (6)

퐷푉1 Finally, by using the expression of 휖 value shown in Eq. (4), the total required generation power 푄푟푒푞 can be obtained as follows:

퐷푉1 푈퐿 ′ 푈퐿 푄푟푒푞 = 퐴 푆 + 퐵 (7)

퐷푉14 4 퐷푉14 4 퐴푈퐿 = − 푃푠 + 푇 −푇푠 √2푃푠 , 퐵푈퐿 = 푇 −푇푠 푄퐻 + 푄퐷푉1 + 푄퐷푉1 (8) 퐷푉12 푈퐿4 4 퐻2 푈퐿4 4 푖푛 푙푎푡 푐표푛푠 푑 푇 −푇푠 푑 푇 −푇푠

Here, 퐴푈퐿 and 퐵푈퐿 are constants defined by the boundary constraints and Eq. (4). This equation shows that the total required power is a linear function of S′. As a result, in order to minimize the total required power, we should design the 훼 value as small as possible when 퐴푈퐿 is positive, or as large as possible when 퐴푈퐿 is negative. When it comes to EQUULEUS mission, parameters and the boundary constraints are defined as shown in Table 3, and 퐴푈퐿 is positive; therefore, the 훼 value of four panels (+Y, -Y, +Z, and –Z panel) should be as small as possible. Figure 6 shows the result of these formulations by the parameter settings of Table 3. The blue curve in Figure 6 illustrates the temperature of EQUULEUS under the coldest environment shown in Table 2, and the red line illustrates the required electric power during the DV-1 phase shown in Table 1. Even based on the one node analysis, the 훼 value should be small to minimize the required power.

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Table 3. Parameters and Boundary constraints for one node simulation. Parameter Value Notes 2 푆푛 0.04 m The area of Y panel 2 푆푎푙푙 0.29 m Total surface area Upper limit temperature 푇푈퐿 35.0 °C during observation by DELPHINUS Lower limit temperature of 푇퐿퐿 5.0 °C the water tank Required temperature of the 푇퐷푉1 18.6 °C vaporization chamber during the DV-1 phase Latent heat to achieve 4 mN 푄퐷푉1 13.1 W 푙푎푡 thrust Figure 6. The results of one node simulation. Blue Generated power not curve illustrates the temperature at the coldest condition 푄퐷푉1 3.6 W 푐표푛푠 consumed as internal heat and red line illustrates the required power at the DV-1.

B. Three Nodes Analysis To minimize the heater wattage consumed for vaporization at VAP during the thrust phase, the thermal conductances between components, especially around VAP, should be optimized. Our approach is to utilize the waste heat of the communication module for vaporization. To evaluate the concept briefly, three nodes simulation was conducted. With this analysis, EQUULEUS is modeled as three nodes as shown in Figure 7: the communication module (COMM), the vaporization chamber (VAP), and other components (BUS) excluding the solar array paddles. We introduced a panel node as the boundary. COMM has the largest amount of power consumption, and its allowable temperature range is large. On the other hand, other components have relatively small power consumption and a small allowable temperature range. To optimize the conductances between COMM, Figure 7. Three nodes thermal model of EQUULEUS. BUS, VAP and Panel, defined as 풙 = Illustrates BUS, COMM, and VAP node, and panel node T as the boundary condition [퐶1, 퐶2, 퐶3, 퐶4, 퐶5, 퐶6] , we formulated the optimization problem as follows:

퐷푉1 min 퐽(풙) = min 푄푉퐻 (9) 푥 푥 subject to:

퐻 푈퐿 푇퐵푈푆 ≤ 푇퐵푈푆 (10)

퐻 푈퐿 푇퐶푂푀푀 ≤ 푇퐶푂푀푀 (11)

퐿퐿 푈퐿 퐶푖 ≤ 퐶푖 ≤ 퐶푖 , 푖 = 1, 2,⋅⋅⋅, 6 (12)

Eq. (9) shows the cost function to minimize which shows that the heater wattage of VAP heater to achieve to provide adequate heat for VAP during the DV-1 phase. Eq. (10) ~ (12) describe the constraints of the temperature and 7 International Conference on Environmental Systems

the conductances. To solve this optimization problem, we introduced the assumption that thermal equilibrium states are satisfied. An example derived in BUS node is shown below. A similar equilibrium equation can be derived for all other nodes.

푄퐵푈푆 + 퐶1(푇푃 − 푇퐵푈푆) + 퐶4(푇퐶 − 푇퐵푈푆) + 퐶6(푇푉퐴푃 − 푇퐵푈푆) = 0 (13)

In addition, the generated heat at VAP, 푄푉퐴푃, is the value of the VAP heater wattage 푄푉퐻 minus the latent heat for vaporization 푄푙푎푡. Finally, by setting the parameters shown in Table 4, optimization was conducted with nonlinear programming, and the results are shown in Table 5. This result shows that COMM and VAP should be strongly coupled to each other (10 W/K), while both of them insulated from BUS and the panel thermally. To implement this concept, we divided the communication module into two parts, sandwiched the vaporization chamber between them, and adopted a thermal filler between the communication modules and the vaporization chamber to couple to each other thermally. Table 4. Parameter settings for three nodes optimization problem. Table 5. Results of optimization at three Parameter Value Parameter Value nodes simulation. 퐻 퐷푉1 Parameter Value [W/K] 푄퐵푈푆 19.5 W 푄퐵푈푆 14.5 W 퐻 퐷푉1 퐶1 5.3 푄퐶푂푀푀 11.8 W 푄퐶푂푀푀 11.8 W 퐻 퐷푉1 퐶2 0 푄푙푎푡 0 W 푄푙푎푡 13.1 W 퐷푉1 퐶3 0.24 푇푉퐴푃 18.6 °C 퐻 퐷푉1 퐶4 0 푇푃 20.0 °C 푇푃 12.0 °C 푈퐿 푈퐿 퐶5 10 푇퐵푈푆 35.0 °C 푇퐶푂푀푀 50.0 °C 퐿퐿 푈퐿 퐶6 0 퐶2−6 0 W/K 퐶1−6 10 W/K 퐿퐿 퐶1 5 W/K

C. Multiple Nodes Simulation by Thermal Desktop Finally, to design the thermal implementation of EQUULEUS in more detail, multiple nodes simulation on Thermal Desktop was conducted. Based on the results of previous simulations, we made the Thermal Desktop model of EQUULEUS, which has over 200 nodes and is shown in Figure 8. In order to verify whether the temperature of all components are settled within the allowable temperature range and the consumption power at the DV-1 phase is less than the required electrical power, we conducted the simulations at the most severe environments shown in Table 2 and Table 1. Although the surface properties of Figure 8. Thermal Desktop model of EQUULEUS. All four panels are designed homogeneity in the one node components in EQUULES are modeled as solid blocks, analysis, we should consider the difference between the surfaces or cylinders. four panels to each other in the multiple nodes analysis. This is why we set 12 thermal simulation environments: four directions of the sun in each thermal environment (the hottest environment, the coldest environment, and at the DV-1 phase). The result of multiple node simulation by Thermal Desktop is shown in Figure 9. The red polyline illustrates the highest temperature experienced by each component of EQUULEUS, and the green polyline illustrates the lowest temperature experienced by each component. Moreover, to simulate the total power consumption at the DV-1 phase, we simulated the required heater wattage at the vaporization chamber when the temperature of VAP is fixed at the required temperature to achieve the required thrust value at the DV-1 phase. By setting the VAP node as a boundary node, we calculated the heater wattage. The most severe sun direction at the DV-1 phase is the +Z direction, in which the total required heater power is calculated at 46.8W. This result satisfies to be below the maximum generable power by the solar array paddles. According to these simulation results, we designed the thermal environments and thermal configuration of EQUULEUS and developed the Engineering Model (EM) of EQUULEUS.

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Figure 9. Result of Thermal Desktop model simulation. The blue bars illustrate the operating temperature range. The names of components are listed below.

IV. Validation Analysis of Thermal Test and Thrusting Test

A. Thermal Vacuum Test of Engineering Model After developing EQUULEUS EM, the thermal vacuum test was conducted to verify the thermal design and implementation of EQUULEUS. Since EQUULEUS cannot emit water vapor at this thermal vacuum test, we focused only on the thermally severe environments shown in Table 2. We aimed to arrange the hottest and coldest environments using infrared heating panels and conducted additional thermal correlation modes so that the thermal conductances around thermally critical components such as the battery, the water tank, and the vaporization chamber can be estimated. The configuration of the thermal vacuum test is shown in Figure 10 and Figure 11. At the hottest condition, the infrared heat panels facing to +Y and -Z panels of EQUULEUS are heated, and at the coldest condition, the infrared heat panels facing to +Z panel are heated. Since there are some differences from the desired configuration in orbit, we mainly focused on verifying the design of internal thermal conductances of EQUULEUS.

Figure 10. Configuration model of the thermal Figure 11. Real configuration of the thermal vacuum vacuum test of EQUULEUS EM. Shows the Thermal test of EQUULEUS EM. Shows the EQUULEUS EM, Desktop model for simulation and correlation infrared heating panels and electric cable harnesses.

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B. Final Verification Analysis Finally, the correlation analysis between the results of the thermal vacuum test of Engineering Model and the simulation results of the Thermal Desktop model was conducted. We improved the Thermal Desktop model to set the temperature difference between the results of the thermal vacuum test and the simulation results of the Thermal Desktop mode within ± 5 °C. The main improvements of the simulation model or the thermal design are described as follows: Y panel nodes were more finely divided in order to interpret the temperature gradient along the Z-axis at the coldest mode (the sunlight from +Z direction), and the thermal conductances around the DELPHINUS nodes were rearranged. After that, the result of the simulation of the hottest environment and the coldest environment is shown in Figure 12. This figure shows that the temperature of almost all components are within the range, but, the highest temperature of DELPHINUS exceeds the highest allowable temperature. That is because the thermal conductances around DELPHINUS cameras are smaller than expected. To avoid operating DELPHINUS at the temperature beyond the allowable temperature, we have to redesign the thermal configuration or make the operation strategy. Currently the project is considering changing the communication mode to lower power consumption mode at the DELPHIUS operation mode, but it is still on the way. Additionally, after correlation to the result of the thermal vacuum test, we recalculated the total required power at the most severe condition at the DV-1 phase, and the value resulted to be 46.1 W. This required power is lower than the simulation result before the correlation, and of course satisfies the generable power of the solar array paddles.

Figure 12. Result of Thermal Desktop model simulation after correlation to the thermal vacuum test.

V. Conclusion EQUULEUS is a 6U deep space CubeSat, which will fly to EML2 and conduct scientific observations. To achieve this mission, the thermal design of EQUULEUS has to consider three constraints. Because of the uncertainty of the launch date and the observation orbit design, EQUULEUS will experience sunlight on only four panels out of six. Moreover, EQUULEUS adopts the water propulsion system for trajectory control and momentum unloading. That is why the spacecraft has to care freezing and large latent heat for vaporization. Under these constraints, we found the solutions of thermal design: to make the surface properties of four panels homogeneous and to utilize the waste power from the communication module to vaporize the water at the vaporization chamber. In order to validate the concepts and achieve the optimal design, we performed the thermal analysis as follows: one node analysis to decide the surface properties, three node analysis to obtain the arrangement of components, and finally multiple node simulation by Thermal desktop to validate and refine the results of previous analyses. After that, along the thermal design, we developed the Engineering Model of EQUULEUS and conducted the thermal vacuum test to verify the design. After the correlation between the results of the thermal vacuum test and the simulation results by parameter tunings, we finally obtained the detailed thermal model. Using this thermal model, we could show that the temperature of almost all components will keep within the allowable temperature during the whole mission period, however, except of DELPHINUS. We are now considering some strategies for this problem, but it is still on the way. 10 International Conference on Environmental Systems

Acknowledgments We would like to thank I. Mase and K. Yamaguchi (NESTRA) for their suggestions for the thermal design and great help and operations to conduct the thermal vacuum test of EQUULEUS Engineering Model. We also like to acknowledge the continuous support of M. Nakano (WEL Research), and the advice of the EQUULEUS orbital design of Y. Kawabata.

References

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