49th International Conference on Environmental Systems ICES-2019-30 7-11 July 2019, Boston, Massachusetts

Thermal Design and Analysis of the GUSTO Gondola

Robert F. Coker1 The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, 20723

NASA’s Galactic/Extragalactic ULDB Spectroscopic Terahertz Observatory (GUSTO) will untangle the complexities of the interstellar medium and map out large sections of the plane of our Milky Way galaxy and the Large Magellanic Cloud. Set for launch in 2021, the balloon gondola is currently under development. The mission is expected to stay in the air for over 100 days, the longest duration balloon mission ever out of McMurdo in Antarctica. This has resulted in challenging mission requirements and environments. This paper will present an overview of the thermal design and analysis of the GUSTO gondola.

Nomenclature COBE = Cosmic Background Explorer CONOPS = concept of operations CSBF = Columbia Scientific Balloon Facility Dec = declination DOY = day of year Fg = fraction of solar flux that reaches the ground FIRAS = Far Infrared Absolute Spectrophotometer 2 Fs = solar flux reaching earth on a given DOY (W/m ) 2 Fw = solar flux reaching the ground as a function of Fg and SEA (W/m ) IR = infrared MLI = multi-layer insulation PFPE = perfluoropolyether RA = right ascension SEA = sun elevation angle (degrees) SIP = Support Instrumentation Package SPV = South Polar Vortex

I. Introduction HE Galactic/Extragalactic Ultra-Long Duration Balloon (ULDB) Spectroscopic Terahertz Observatory T (GUSTO) is a National Aeronautics and Space Administration’s (NASA) Explorer Mission of Opportunity that is set to launch in 2021. It is led by the Principal Investigator (PI) Christopher Walker of the University of Arizona (UA). UA has partnered with The Johns Hopkins University Applied Physics Laboratory to provide day-to-day project management, mission systems engineering, and the balloon-borne gondola. GUSTO will be the first mission to utilize the 100-day flight potential of the Super Pressure Balloon (SPB), provided by NASA’s Balloon Program Office (BPO). GUSTO will serve as a Rosetta Stone for understanding interstellar gas in the Large Magellenic Cloud (LMC) and center of the Milky Way (MW). The mission will be the first to survey the full interstellar medium (ISM) gas cycle in order to understand star formation throughout cosmic time. Using over half a million different lines-of-sight (LOS), GUSTO will conduct [CII], [OI], and [NII] surveys of the MW and LMC at over hundreds of times better angular resolution and thousands of times better frequency resolution than the Far Infrared Absolute Spectrophotometer (FIRAS) on the Cosmic Background Explorer (COBE). Simultaneous observations of multiple emission lines permits discrimination of, for example, diffuse clouds from reflection nebulae and giant molecular clouds. The Stratospheric Terahertz Observatory (STO) and its follow-on STO-2 served as a pathfinder for GUSTO, establishing a gondola and instrument architecture. GUSTO will use additional flight heritage from the Balloon

1 Observatory Lead Thermal Engineer, Space Exploration Mechanical Engineering Group, The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723-6099.

Copyright © 2019 Johns Hopkins University Applied Physics Laboratory Observation Platform for Planetary Science (BOPPS). With a proven measurement approach and the ultra-long duration of the SPB, GUSTO can conduct these ISM observations of the MW and LMC at a fraction of the cost of a comparable orbital mission. GUSTO will launch on an ULDB from Antarctica and observe at a float altitude of more than 30 km for 100 days. The ballooncraft trajectory will be completely dependent on the winds at float altitude, so it can only be approximated based on historical wind data and simulations. Typically, the South Polar Vortex (SPV) produces a circular orbit for 10 to 20 days around Antarctica starting in mid-December. When the SPV breaks down in January, the resulting path of the ballooncraft becomes hard to predict. It could remain over the continent or drift as far north as -58º latitude by the end of the mission. In the former case, the mission never sees night, whereas in the latter case, it would see nights approaching 9 hours in length. Figure 1 shows two notional bracketing trajectories, with one, in blue, remaining over the Antarctic continent for the whole mission and the second, in red, nearly reaching South America by the end of the mission. The thermal environment changes significantly when over ocean compared to land. In addition, when conducting the MW survey, the sun will be on the forward side of the observatory, whereas when conducting the LMC survey, the sun will be on the back side of the Figure 1. GUSTO Trajectories. Two possible trajectories used for the observatory. GUSTO mission design. Asterisks denote mission days. Green shaded area To meet the science objectives, the covers the most likely ballooncraft paths for up to ~ 50 days of flight. instruments will be cooled by a liquid helium (LHe) cryostat. However, the entire payload will require over 1000 W of power. Thus, a liquid cooling loop using Galden’s D02 perfluoropolyether (PFPE) fluid has been designed. It will deliver waste heat to a radiator when the system is hot and redistribute it between components when cold. Combined with the potentially changing thermal environment, GUSTO presents a complex thermal challenge. This paper will present an overview of the thermal design and analysis of the GUSTO gondola and how it supports the payload thermally.

II. The GUSTO Thermal Environment The GUSTO mission, as is typical for balloon missions, has different phases: ground, ascent, commissioning, and science. The thermal environment for each phase is different. A hot and cold case is defined for each phase. Further, the thermal model uses a simplified concept of operations (CONOPS), one for when over land and one for when over water. A different CONOPS means a variable observing sequence and thus, for example, a different thermal load on the telescope as the pointing changes with respect to the sun. 1 2 The gondola thermal design is modelled with Thermal Desktop© and SINDA/FLUINT© . The geometry of the entire observatory as represented in Thermal Desktop is shown in Figure 2. Since it subtends a significant angle to the gondola, the balloon itself, not shown in Fig. 2, is also included in the model as an ellipsoid located over 100m above the top of the gondola. The thermal environment3, is defined by a combination of air temperature, sky IR temperature, ground IR temperature, solar flux, and albedo. The air temperature is used to determine natural and forced convection coefficients, with the former active on the ground and during ascent and the latter only active during ascent since the air pressure at float is less than 10 mbar. The assumed values given below in the rest of this section are taken from the reference3 and together bracket the extremes that will likely be seen by the ballooncraft.

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A. Ground Infrared (IR) Temperature The ground temperature for an Antarctic launch in late December of 2021 is taken to be between -10 ºC (cold case) and 5 ºC (hot case). The ground IR temperature that the balloncraft sees is assumed to drop linearly with altitude, reaching a constant -30 ºC for the cold case and -15 ºC for the hot case after 1 hr of ascent.

B. Sky IR Temperature The sky IR temperature is used to determine the diffuse sky IR flux that the balloncraft sees. On the ground, the sky IR temperature is taken to be -28 ºC (cold case) or -13 º C (hot case). It drops linearly with altitude, reaching a constant -270 ºC for the cold case and -245 ºC for the hot case after 3 hrs of ascent.

C. Albedo Over land, the albedo is assumed to be 0.8, whereas over water it is assumed to be 0.4. Thus, four cases are modeled: hot and cold over both land and water. Future work will use bracketing hot and cold values for land and water based on Columbia Scientific Balloon Facility (CSBF) data.

D. Solar Flux 7 The solar flux, Fs, that reaches Earth varies with the day of year (DOY) :

Fs = 1367 1 + 0.034 cos 2 . W/m2 (1) . DOY ∗ � ∗ � π 365 25��

The portion of this flux that reaches a flat surface on the ground, FG, depends on the weather factor, FW, and the sun elevation angle (SEA)3:

Fg = Fs Fw cos (90 SEA) . W/m2 (2) π ∗ ∗ � − ∗ 180�

FW is assumed to vary between 0.16 and 0.84. The final solar flux that is seen by the balloncraft during ascent is determined by linearly interpolating with time between the ground value, FG, at t=0 and the float value, Fs, at=3 hrs. At float, GUSTO will thus see the full solar flux, Fs.

E. Air Temperature The air temperature on the ground is assumed to be the same as the ground IR temperature. However, at the tropopause between 11 km and 18k m, the air temperature is taken to be -55 ºC (cold) or -45 ºC (hot). At float, the air temperature is taken as -20 ºC (cold) or +7 ºC (hot) during the day and -55 ºC (cold) or -45 ºC (hot) during the night. During ascent, the air temperature is linearly interpolated between these limits. The air pressure and density are exponential functions of altitude4. Together with the air temperature, they are used to determine the natural and forced convection experienced by the ballooncraft. Thermal Desktop’s “forced convection over a flat plate” and “isothermal natural convection over a vertical plate” methods5, are used with a surface scale height of 0.5 m. The relative air velocity for forced convection is assumed to be the ascent rate (=33.5 km / 3 hrs). In some cases the cold tropopause can be the thermal cold design driver for some components.

III. Gondola Thermal Design

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To keep the payload detectors sufficiently cold to meet their cryogenic temperature requirements, a LHe cryostat with dual vapor cooled shields and an external vacuum shell is used. The cryostat, built by Ball Aerospace Corporation, is mounted to the telescope with composite struts. The two vapor cooled shields are cooled via cryocoolers and require good thermal performance and hermeticity. The instrument insert is mounted to the top of the LHe tank. The cryostat LHe tank will hold 150 liters of LHe that will, over the 100 days of the mission, slowly boil off; it will likely be the limiter of GUSTO’s mission duration. Typically, due to the large change in albedo, the observatory hot case occurs over Antarctica near the start of the mission and the observatory cold case occurs during the night over the ocean near the end of the mission. This helps the cryostat in that boil-off of the LHe is reduced later in the mission with the lower thermal loads. However, once GUSTO drifts sufficiently far off the continent, the payload components need to be heated through the night, when solar power generation is unavailable. In addition, the CONOPS results in solar heating on various sides of the gondola. Together these make for a challenging thermal design: nearly 1000 W needs to be rejected by the gondola thermal system during the hot days over Antarctica, whereas nearly 1000 W of total heater power is needed to get through the cold ocean nights. And this needs to be done within the context of variable solar illumination and variable thermal dissipation loads on the payload components as a function of time. The solution is a PFPE fluid loop and a 625 W heater on a 4m2 radiator; heaters totaling an additional ~350 W

Figure 2. GUSTO Thermal Model. Geometry of the thermal model for GUSTO, with the cradle (green), radiator (yellow,) and solar panels (red) most evident.

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are needed on components, such as the avionics box and amplifiers, that are not on the fluid loop. The radiator, unlike for STO-2, is placed near the top of the gondola, so that the rest of the observatory reduces the radiator’s view factor to the earth, reducing its albedo and earth IR loads. Since the SEA never exceeds 40º, the horizontal radiator can still effectively radiate to space. Most thermally active components on the gondola, such as the avionics box, generate sufficient self-heating that wrapping them in multi-layer insulation (MLI) will get them through the night, but not so much heat that they over-heat over the continent. Other smaller gondola components, such as the motor amplifiers, need small individual heaters to get through the night. However, the battery box, with 12 large high- performance and 110 Ah capacity batteries that will store the over 3 kW generated by the solar arrays, has a narrower temperature range requirement and a more variable heating environment as the batteries charge and discharge. Thus, the batteries are on the fluid loop with the payload components. The payload components are put in six parallel paths on the fluid loop, permitting individually tuned flow rates for each path. The total flow rate will be 5-10 liters per minute (lpm), with the final value depending on the flow the pump can deliver with the pressure drop across the entire fluid loop; excess flow will be put in a bypass loop. The present design, with a 7 lpm flow rate, can transport over 200 W per ºC of temperature difference. This high heat transfer rate maintains the temperature gradient through the fluid itself to less than 10 ºC (see Figs. 5 and 6 below). The temperature of the PFPE fluid needs to remain less than 45 ºC, when its vapor pressure begins to increase and its viscosity begins to drop significantly6, impairing its ability to transport heat. It addition, 45 ºC is the lowest maximum temperature permitted by a component on the fluid loop, so the fluid should not be warmer than that. The payload components and the batteries will be attached to their own cold plates and the PFPE fluid will flow through piping in those cold plates; this is the thermal choke point for the design. The design of this interface is still in work for some components, but it varies from 1 to 50 W/ºC, depending on the size, power dissipation needs, and temperature limits of the component. The goal is to maintain the component temperatures to within a few ºC of the fluid temperature. The design works well except for some components that have limited piping path length availability on their cold plate. In these cases the temperature delta between the component and the fluid can exceed 15 ºC, but the design still closes due to the low fluid temperature compared to the high component temperature limits. The six parallel payload fluid loop lines combine before the fluid pump and accumulator. After the pump and accumulator, the loop then proceeds to the battery box and thence on to the radiator. In hot cases, the waste heat picked up by the fluid is then radiated to space. The fluid temperature never exceeds 30 ºC. In cold cases, the fluid loop delivers heat put into it by a heater on the back side of the radiator and the fluid temperature never drops below -5 ºC. Thus, it is both a heating and cooling system for the payload and batteries.

IV. Model Predictions

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Table 1. GUSTO Thermal Model. Nominal CONOPS for the beginning of mission, defining the “hot land” analyzed thermal case. Launch & MW GPS MW GPS Observation Name Pre-launch Commissioning LMC LMC Ascent l_336 l_336 Start Date 23-Dec-21 24-Dec-21 24-Dec-21 25-Dec-21 25-Dec-21 25-Dec-21 26-Dec-21 Start Time [UT] 21:00 3:00 6:00 6:00 19:00 23:00 2:00 Phase Duration [h] 6 3 24 13 4 3 4 Altitude [km] 0 0 – 33.5 33.5 33.5 33.5 33.5 33.5 Lat -78° -78° -78° -78° -78° -78° -78° Gondola position Long 167° 167° 167° 167° 167° 167° 167° RA Telescope Telescope 16h 06m 05s 16h 32m 59s 05h 38m 34s 16h 32m 59s 05h 38m 34s Target Stowed Stowed DEC -52° 28’ 34” -48° 16’ 27” -69° 04’ 48” -48° 16’ 27” -69° 04’ 48” Vertically Vertically

Table 2. GUSTO Thermal Model. Nominal CONOPS for the end of mission, defining the “cold water” analyzed thermal case. Note this case is always at float, but at an altitude of 33.5 km during the day and 28 km during the night. LMC 30 MW Gal. MW GPS MW GPS Observation Name MW GPS l_46 W31 GPS l_190 Doradus Center l_190 l_238 Start Date 2-Mar-22 2-Mar-22 2-Mar-22 2-Mar-22 2-Mar-22 2-Mar-22 3-Mar-22 Start Time [UT] 5:00 9:00 13:00 17:00 20:00 22:00 0:00 Phase Duration [h] 4 4 4 3 2 2 5 Altitude [km] 28.0 33.5 33.5 33.5 33.5 28.0 28.0 Gondola Lat -58° -58° -58° -58° -58° -58° -58° position Long -49° -49° -49° -49° -49° -49° -49° RA 05h 38m 38s 19h 17m 00s 18h 21m 26s 17h 47m 10s 6h 09m 01s 6h 09m 01s 07h 40m 31s Target DEC -69° 05’ 42” 11° 43’ 00” -20° 12’ 20” -28° 48’ 32” +20° 17’ 09” +20° 17’ 09” -22° 12’ 11”

Table 1 lists the notional CONOPS for the beginning of the mission. This also corresponds to the “hot land” case and includes the 3 hour ascent. Table 2 lists the notional CONOPS for the end of the mission. This corresponds to the “cold water” case. These are the two cases discussed here. Of particular note are the variable right ascension (RA) and declination (Dec) of the observed targets, resulting in significant changes in the thermal load as the gondola and telescope slew around with respect to the sun. An orbit that includes the changing altitude, latitude, and longitude was used in Thermal Desktop. The geometry of the gondola is attached to one tracker and the geometry of the observatory is attached to another, such that together they move the gondola’s azimuth and the observatory’s elevation as a function of time to match the CONOPS given in Tables 1 and 2.

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Figure 3. Daytime Thermal Block Diagram. Schematic ot of the loads on the fluid loop while over the continen at 51 hrs of flight time.

Figure 4. Nighttime Thermal Block Diagram. Schematic ot of the loads on the fluid loop at the end of 9 hrs of night. Figures 3 and 4 show the hot and cold thermal block diagrams, respectively, for two snapshots in time. The environment loads, both on and out of the radiator, change with time; the GUSTO thermal system is never in steady state. In both cases shown here, the radiator is losing heat (~10W) and thus cooling. The hot case is over Antarctica and conservatively assumes the batteries are charging; in practice, the batteries will likely be constantly fully charged when in constant daylight. The cold case occurs over the ocean and illustrates the inefficiency of putting the 7 International Conference on Environmental Systems

heater on the radiator; nearly half of that power is lost directly to radiation rather than going into the fluid loop. If overall power becomes a system issue, future designs might move the radiator to downstream of the radiator. Note that though no losses are assumed between components of the fluid loop (e.g., when flowing from the payload to the pump), not all the component loads enter the loop, since a load node can also transfer heat to or from its own box. The temperature of the fluid loop in these hot and cold cases at the location of each component on the loop, as well as the inlet and exit to the radiator, are shown in Figs. 5 and 6, respectively. In Fig. 5 the cold tropopause is evident in the middle of the ascent. Also evident is the change in loop temperature as the telescope moves between observing the LMC, the MW, and then the LMC again at hours 40 to 50. In Fig. 6, the radiator heater goes on and off all night (hours 18 to 26.5); the heater continues cycling, with an overall duty cycle of less than 65%, for a few hours after night ends to keep the temperature of the whole system above limits. At all times the gradient through the loop is less than 10 ºC.

Figure 5. GUSTO Fluid Loop Predicted Temperatures. Plot of the temperature of the fluid loop for the “hot land” case at various points in the system.

Figure 6. GUSTO Fluid Loop Predicted Temperatures. Plot of the temperature of the fluid loop for the “cold water” case at various points in the system. 8 International Conference on Environmental Systems

The thermal design and analysis of the cryostat has been done by Ball; here, for the gondola thermal design, the outside cryostat vacuum shell is treated as a hollow passive component. However, the gondola will determine the boundary conditions for the cryostat analysis and that environment will thus drive the mission lifetime. Figs. 7 and 8 show gondola model results for the cryostat for the hot and cold cases, respectively. Again, as the gondola and telescope move with respect to the sun, the temperature distribution around the cryostat shell changes. In this

Figure 7. Predicted Temperatures for the GUSTO Cryostat. “Hot land” case for the cryostat outer vacuum shell.

Figure 8. Predicted Temperatures for the GUSTO Cryostat. “Cold water” case for the cryostat outer vacuum shell. 9 International Conference on Environmental Systems

geometrically simple model that does not include, e.g., the ribs of the cryostat vacuum shell, the temperature gradient across the shell approaches 50 ºC at times. However, the area-weighted average temperature never exceeds the requirement of +50 ºC. Thus, the cryostat is expected to meet the 100 day mission lifetime requirement. The complicated geometry of the entire observatory, as illustrated in Fig. 2, combined with the varying sun location, results in non-trivial view factors both between components and to the sun and space. For example, the carbon fiber reinforced panel (CFRP) cradle upon which the primary mirror rests and to which the cryostat struts attach, is hollow and only partially closed out, so it needs to be part of the external radiation group. The low thermal conductivity of CFRP can result in hot and cold spots on the cradle. Since the resulting thermal distortion can impact the primary performance, this was limited with white paint over most components, including the shades and the cradle. Other than individual boxes, the GUSTO model has no closed out regions, so all radiatively active surfaces are part of the external radiation group; that is, they radiate to space. Fig. 9 shows the temperature of the entire observatory at a snapshot in time for the hot case. The sun is shining nearly directly on the left set of solar panels, so they reach nearly 100 ºC. The top inner far side of the telescope sun-shade is warm as well, since it too is nearly directly seeing the sun; note the inner surface of the sun-shade is painted black. On the gondola base, the large box of the CSBF-supplied Support Instrumentation Package (SIP) is relatively warm for similar reasons; although the gondola baseplate is open below the SIP to facilitate its radiation to the earth, its view of cold sky is limited. Meanwhile, the radiator is operating efficiently at 20 ºC, maintaining the temperatures of the payload components well below their temperature limits.

Figure 9. GUSTO Predicted Temperatures. “Hot land” case at 18.5 hours after launch.

V. Conclusion In preparation for GUSTO’s mission critical design review (CDR) in July of 2019, a detailed thermal analysis of the observatory has been conducted. A PFPE fluid loop, in conjunction with a large radiator and a large heater, are sufficient to maintain the payload components within their temperature limits throughout the hot days over

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Antarctica and the cold nights over the ocean. Leveraging off previous missions, the overall thermal control system, including a 150 liter LHe cryostat, is expected to enable the ground-breaking science of GUSTO’s 100 day mission.

Acknowledgments The GUSTO mission is supported by the NASA Explorer Program under contracts to the University of Arizona and The Johns Hopkins University Applied Physics Laboratory. The authors acknowledge GUSTO Principal Investigator Chris Walker, at the University of Arizona, and the GUSTO Project Manager, Pietro Bernasconi, at The Johns Hopkins University Applied Physics Laboratory, for their support in the preparation and presentation of this paper.

References 1Panczak, T. D. and Ring, S. G., “RadCAD: Integrating Radiation Analysis with Modern CAD Systems,” Proceedings of the 26th International Conference On Environmental Systems, Monterey, CA, USA, p. 961375, (8-11 July 1996). 2Lucas, S., “RadCAD: Validation of a New Thermal Radiation Analyzer,” Proceedings of the 27th International Conference On Environmental Systems, SAE International, Lake Tahoe, NE, USA, p. 972442, Jul. 1997. 3Johnson, D. L. (ed.), “Terrestrial Environment (Climatic) Criteria Guidelines for Use in Aerospace Vehicle Development, 2008 Revision,” NASA TM-2008-215633, 2008. 4Sissenwine, N., Dubin, M., and Wexler, H., “The U.S. Standard Atmosphere, 1962”, JGR, Vol. 67, No. 9, Aug. 1962. 5Guyer, E. C. (ed.), “Handbook of Applied Thermal Design”, Taylor & Francies, Hamilton Printing, Castleton, N.Y., 1999. 6http://www.matweb.com/search/datasheet.aspx?matguid=014b0b4c179042ac8c894191418a23ad 7David Gilmore, “Spacecraft Thermal Control Handbook, Volume 1”. The Aerospace Corporation 2002.

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