Thermal Design, Analysis, and Testing of GUSTO

ICES-2020-280

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 mission will be the longest duration scientific balloon mission launched out of McMurdo in Antarctica, staying in the air for 75 days or longer. GUSTO will use a fluid loop attached to a large heated radiator to address the challenging mission requirements and environments. This paper will present an overview of the thermal design, analysis, and testing of the GUSTO gondola and telescope.

Nomenclature BOPPS = Balloon Observation Platform for Planetary Science BPO = Balloon Program Office CDR = critical design review CFRP = carbon fiber reinforced panel 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 ISM = interstellar medium LHe = liquid Helium LMC = Large Magellenic Cloud lpm = liters per minute LOS = line of sight MLI = multi-layer insulation MW = Milky Way MPPT = maximum peak power tracker NASA = National Aeronautics and Space Administration PFPE = perfluoropolyether PI = principal investigator RA = right ascension SEA = sun elevation angle (degrees) SIP = Support Instrumentation Package SPB = super pressure balloon SPV = South Polar Vortex STO = Stratospheric Terahertz Observatory TVAC = thermal vacuum ULDB = Ultra-Long Duration Ballooning

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

Figure 1. GUSTO Trajectories. Two possible bracketing trajectories used for the GUSTO mission design. Asterisks denote mission days. Green shaded area covers the most likely ballooncraft paths for up to ~ 50 days of flight.

I. Introduction HE Galactic/Extragalactic Ultra-Long Duration Ballooning (ULDB) Spectroscopic Terahertz Observatory T (GUSTO) is a National Aeronautics and Space Administration’s (NASA) Explorer Mission of Opportunity that is set to launch at the end of 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 (JHU-APL) 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 in the 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] emission line 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 pathfinders for GUSTO, establishing a gondola and instrument architecture. GUSTO will use additional flight heritage from the Balloon 2 International Conference on Environmental Systems

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 SPB from Antarctica and observe at a float altitude of more than 30 km for up to 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 -50º 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 10 hours in length. Error! Reference source not found. shows two notional bracketing trajectories, with one, in blue, remaining over the Antarctic continent for the whole mission and the second, in red, reaching South America by the end of the mission at 75 days. The thermal environment changes significantly when over ocean in nighttime compared to over land in daytime. 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 observatory. To meet the science objectives, the instruments will be cooled by a liquid helium (LHe) cryostat. However, the entire payload will require nearly 1000 W of power. Thus, a liquid cooling loop using a perfluoropolyether (PFPE) fluid, D02, from Galden®, has been designed. It will deliver waste heat to a radiator when the system is hot and redistribute it between components and the heated radiator when cold. Combined with the potentially changing thermal environment, GUSTO presents a complex thermal challenge. The GUSTO Mission critical design review (CDR) was held in October of 2019; although significant schedule challenges exist, no thermal show-stoppers were found. This paper will present an overview of the thermal design and analysis of the GUSTO gondola and telescope. The presentation in Lisbon will include the results of upcoming thermal testing.

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. The gondola thermal design is 1 modelled with Thermal Desktop© and 2 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 100 m above the top of the gondola. The thermal environment3, is defined by a combination of air temperature, sky Figure 2. GUSTO Thermal Model. Geometry of the thermal model for infrared (IR) temperature, ground IR GUSTO, with the cradle (green), radiator (yellow,) and solar panels temperature, solar flux, and albedo. (various shades of pink) most evident.

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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, all convection is ignored at float (the validity of this assumption is checked occasionally; at night as the balloon drops in altitude, convection does result in a few degrees C additional cooling of the solar arrays, for example). The assumed values given in the rest of this section are taken from Ref. 3, updated with recent Columbia Scientific Balloon Facility (CSBF) data. Together, they bracket the thermal extremes that will likely be seen by the ballooncraft.

A. Ground IR Temperature The ground temperature for an Antarctic launch in late December of 2021 is taken to be between -10 ºC (cold case) and 3 ºC (hot case). The ground IR temperature that the balloncraft sees is assumed to drop linearly with altitude, reaching a constant -53.5 ºC for the cold case and -25.3 ºC for the hot case after 1 hr of ascent. Over water, the ground IR temperature is assumed to be -17.3 ºC (hot case) or -33.8 ºC (cold case). The ground IR temperature is used to determine the ground IR flux that the balloncraft sees; as such, it does not refer to the actual ground temperature when the ballooncraft is not on the ground.

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 -15 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. Over water, during the night the balloon drops in altitude, so the sky IR temperature is warmer at -254 ºC (cold case) or -230 ºC (hot case). The daytime sky IR temperature over water is the same as over land.

C. Albedo Over land (and over the snow- and ice-covered continent of Antarctica specifically), based on CSBF data, the albedo is taken to be 0.66, whereas over water it is taken to be 0.42. Thus, four cases are modeled: hot and cold over both land and water.

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

DOY+11 F = 1367 ∗ (1 + 0.034 ∗ cos (2π )). W/m2 (1) S 365.25

Interestingly, GUSTO’s nominal launch date is near the southern summer solstice, when Fs is maximum, while its termination date is near the southern fall equinox, when Fs reaches its minimum. Eqtn. (1) does not include the 2 0.35% or ~5 W/m solar variability due to the solar cycle; this is included in the model. The portion of Fs that 3 reaches a flat surface on the ground, FG, depends on the weather factor, FW, and the sun elevation angle (SEA) :

π F = F ∗ F ∗ cos ((90 − SEA) ∗ ). W/m2 (2) G S W 180

FW is assumed to be a constant 0.84, since lower values reflect bad weather when GUSTO would not likely launch. 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 -40 ºC (hot). At float, the air temperature is taken as -21 ºC (cold) or +5.7 ºC (hot) during the day. During ascent, the air temperature is linearly interpolated between these limits. Due to the lower altitude at night, the air temperature is taken to drop to -24 ºC (cold) or +0.8 ºC (hot). 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 4 International Conference on Environmental Systems

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 for all surfaces regardless of size (this is conservative since during ascent the concern is for the cold environment and this will underestimate convective cooling for e.g. the large solar panels). On the ground, this results in conductors of ~0.5 W/°C between the air node and each solar panel node, for example. The relative air velocity for forced convection is assumed to be the ascent rate (=33.5 km / 3 hrs); some Artic balloon mission data (e.g. SUNRISE) suggest that this approach underestimates the impact of forced convection by not including lateral differential air flow. For some components, the cold tropopause is the thermal cold case (see below).

III. Gondola Thermal Design 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 75 days of the mission, slowly boil off; it will likely be the limiter of GUSTO’s mission duration. Typically, in addition to the impact of Eqtn. (1), due to the large reduction in albedo from snow- and ice-covered land to open water, 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 (see below) results in solar heating on various sides of the gondola. Together, these make for a challenging thermal design: over 700 W needs to be rejected by the gondola thermal system during the hot days over Antarctica, whereas over 1300 W of total heater power is needed to get through long 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 4 m2 radiator; heaters totaling an additional ~350 W 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 so that wrapping them in 1” of insulating foam will get them through the night, but they do not generate so much heat that they over-heat when GUSTO is over the continent. Other smaller gondola components, such as the motor amplifiers, need small individual heaters to get through the night. The battery box, with 9 large high-performance and 82 Ah capacity Li-ion batteries by Saft that will store the over 3 kW generated by the solar arrays, has a narrower temperature range requirement than other gondola components and a more variable heating environment as the batteries charge and discharge. Thus, the batteries have been cold-biased with limited foam covering. They also have internal heaters to get through long nights; these high power heaters do not turn on unless the mission sees nights longer than 9 hours and even then only briefly. The payload components are put in five parallel paths on the fluid loop, permitting individually tuned flow rates for each path. The flow rate through the individual paths varies from 0.75 liters per minute (lpm) to 2.85 lpm. The total flow rate will be 5-10 lpm, with the final value depending on the final flight details of the payload. For a given system pressure drop, the pump delivers a fixed flow rate; excess flow will be put in a bypass loop. The CDR design, with a 7 lpm flow rate, can transport over 200 W per ºC of temperature difference that exists between the fluid inlet and exit. This high heat transfer rate maintains the temperature gradient through the fluid itself to less than 10 ºC at all times (see Figs. 5 and 6 below). The pump has over 30% head margin for the estimated 15 psi pressure drop across the entire fluid loop, so a larger flow rate to the payload can be used if needed. 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 never be warmer than that. The payload components 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 thermal coupling between the fluid and the cold plates varies from 1 to 40 W/ºC, depending on the fluid temperature, component size and internal piping, power dissipation needs, and temperature limits of the component. This maintains the component temperatures to within a

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few ºC of the fluid temperature, 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. On the cold side, however, this results in a colder fluid temperature than has flown previously (~-15 ºC vs ~-30 ºC), requiring a thermal delta qualification for the pump and fluid loop (see below). Alternatively, a higher radiator heater temperature set point and heater power could have been used, but the increased power needs were deemed unacceptable. The five parallel payload fluid loop lines combine before the fluid pump and accumulator. After the pump and accumulator, the loop then proceeds 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 does not drop below -15 ºC except briefly during ascent. Thus, the fluid loop is both a heating and cooling system for the payload.

IV. Model Predictions 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; a ground model is run for six hours to get the initial conditions at the start of ascent Table 1. GUSTO Hot 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 Cold 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 32 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] 32.0 33.5 33.5 33.5 33.5 32.0 32.0 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”

(the length of time GUSTO may sit “on the pad” before launch is highly uncertain, but six hours achieves a quasi- steady state. Table 2 lists the notional CONOPS for the end of the mission over the ocean. This corresponds to the “cold water” case. These are the two cases discussed in this paper. 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

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Figure 3. Daytime Thermal Block Diagram. Schematic ot of the loads on the fluid loop while over the continent 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. 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. Figures 3 and 4 show the hot and cold thermal block diagrams, respectively, for two snapshots in time. The environment loads, both on and through the radiator, change with time; the GUSTO thermal system, due to its larger thermal mass and constant motion with respect to the sun, is never in steady state. In both cases shown here, the radiator is losing heat (~10 W) 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 at the end of night over the ocean and illustrates the inefficiency of putting the heater on the radiator; nearly half of that power is lost directly to radiation rather than going into the fluid loop. Note that although no losses are assumed between components of the fluid loop (e.g., when flowing from the payload to the pump), not all the component thermal loads enter the loop, since some of a component’s thermal load can, even with good insulation, be radiated rather than conducted into the loop

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Figure 5. GUSTO Fluid Loop Hot 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 Cold Predicted Temperatures. Plot of the temperature of the fluid loop for the “cold water” case at various points in the system. 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 at 2 hrs. Note the “cold land” case (not shown) has an even colder fluid temperature during ascent. Also evident in Fig. 5 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 8 International Conference on Environmental Systems

hours after night ends to keep the temperature of the whole system above limits. At all times the gradient through the loop is on the order of 5 ºC. This is consistent with the nearly 700 W payload thermal load that the 200 W/ºC fluid needs to transport. The thermal design and analysis of the cryostat has been done by Ball; here, for the gondola and payload 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 geometrically simple model that does not Figure 7. Hot Predicted Temperatures for the GUSTO include, e.g., the ribs of the cryostat vacuum shell, Cryostat. “Hot land” case for the cryostat outer vacuum the temperature gradient across the shell exceeds 50 shell. ºC at times. However, the area-weighted average temperature never exceeds the requirement of +50 ºC. Thus, the cryostat is expected to meet the 75 day mission lifetime requirement. The oscillating nodes evident in Figs. 7 and 8 correspond to the cryostat pumpout heater going on and off; the pumpout has a higher cold temperature limit than the cryostat shell itself. 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. To avoid unphysical spikes in temperature for small thermal mass components such as the secondary trusses, computationally expensive ray convergence criteria were needed. 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 over 90 ºC. The smaller panels are CSBF solar arrays; one is required in each cardinal direction of the observatory so as to be able to generate power in case of loss of azimuth control. The top inner far side of the telescope sun-shade in Fig. 9 is seen to be 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 less than 20 ºC, Figure 8. Cold Predicted Temperatures for the maintaining the temperatures of the payload GUSTO Cryostat. “Cold water” case for the cryostat components well below their temperature limits. outer vacuum shell.

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Table 3 lists the operational temperature limits, model predictions, and resulting thermal margin for all GUSTO components. The results presented here and elsewhere in this paper are based on the Observatory CDR thermal model. Due to mass issues, the gondola structure has been modified post-CDR; these updates will be discussed in Lisbon. The temperature limits, as per APL guidelines, should hold 10 °C margin on both the hot and cold sides. On the cold side, if heaters are present with a duty cycle less than 75%, this margin can be reduced to 5 °C. As Table 3 shows, we have more than 10 °C margin for most components during the entire mission, with the following exceptions. The solar arrays have been used in previous Antarctic missions and their upper temperature limit is very conservative, so their 5 °C margin is not a concern. The roll motor cold margin could be improved with a heater if deemed necessary. The fluid and pump have small margins that will be addressed by delta qualification thermal tests of the fluid loop to be conducted this summer. The accumulators need a heater since the fluid is insufficiently thermally coupled to the entire plastic accumulator to keep its interface temperature above its cold limit. The science stack and backup battery have flown on previous Antarctic missions and are only simplistic notional components in the system model. The SIP is the responsibility of CSBF. The cryostat is an average bulk temperature, so it does not require a 10 °C margin. The results of the “cold land” case are not shown in Table 3, but if launch occurs in a colder than usual environment and the tropopause is particularly cold, a number of components will briefly see colder temperatures than are listed in Table 3. In particular the LOS video multiplexor and transmitter and the fluid loop itself (due to the radiator) will be close to their low temperature limits. The former are non-essential hardware, while the latter can be addressed by “pre-heating” the radiator heater on the ground (by manually turning on the radiator heater) if necessary. There have been many design changes since PDR8, but most have been structural, with minimal impact on the thermal design. As University of Arizona has continued testing on the payload (outside the scope of this paper), the thermal loads of the flight hardware have been found to be lower than initially assumed. To reduce overheating concerns, the maximum peak power trackers (MPPTs) were moved from the back of the solar arrays to the gondola structure. The most significant thermal change was the removal of the batteries from the fluid loop. This was made possible by changing to a new battery type that has internal heaters and a wider temperature range than those used on STO-2. The boxes on the bottom deck were moved around as a result of the battery and structure changes. Additional detail and fidelity has been added to the thermal model as the observatory design has matured. Testing of an engineering model of the fluid loop will be done this spring, followed by “flat-sat” testing of the gondola hardware. The results of these tests will be discussed in Lisbon. However, the assembled payload thermal vacuum testing will not be done until early in 2021. Additionally, due to the size of the entire observatory, there will be no observatory level thermal vacuum testing.

V. Conclusion In support of GUSTO’s Mission CDR in October 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 Antarctica and the cold nights over the ocean. With large capacity batteries and large solar arrays to fully charge them even while doing science, sufficient power is available to enable heaters to be used to get the gondola components, which are not on the fluid loop, through nights as long as 10 hours. 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 75 day mission. If the SPV is stable for the mission duration, the observatory may be brought down over land and thus be recoverable .

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 author acknowledges GUSTO Principal Investigator Chris Walker, at the University of Arizona, and the GUSTO System Engineer, Pietro Bernasconi, at The Johns Hopkins University Applied Physics Laboratory, for their support in the preparation and presentation of this paper.

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Model Results (ºC) Temperature Limits (ºC) Margin Component cold water (min) hot land (max) min max min max Solar Arrays -70 100 -70 95 30 5 MPPTs* -45 85 -53 52 12 33 PDU* -40 75 -34 40 6 35 Batteries* -40 70 -31 26 9 44 MTU (and Az Motor)* -55 50 -49 26 6 24 Az Motor Amp* -50 85 -36 40 14 45 Az Wheel -100 100 -67 32 33 68 Roll Motor Amp* -50 85 -36 42 14 43 Roll Motor -55 50 -50 41 5 9 Roll Wheel -100 100 -50 43 50 57 Accumulators* -45 55 -40 41 5 14 Elevation Motor -55 65 -40 32 15 33 Elevation Motor Amp* -50 85 -24 59 26 26 Elevation Encoder* -40 85 -34 35 6 50 Ethernet Hub -40 85 -11 61 29 24 Latch -70 60 -52 46 18 14 Mass Slider Motor* -40 65 -34 54 6 11 Mass Slider Controller* -40 70 -34 46 6 24 IMU* -40 75 -34 40 6 35 Magnetometer* -40 85 -34 40 6 45 Gyro Converter* -40 85 -12 73 28 12 Star Camera Head* -40 85 -34 45 6 40 Star Camera Computer* -40 85 -4 72 36 13 Star Camera Converter* -40 85 -28 63 12 22 Avionics* -40 75 -30 47 10 28 LOS Video Multiplexor -40 85 -50 39 50 46 LOS Video Transmitter -40 85 -51 33 49 52 Pump‡ -30 90 -27 42 3 48 Pump Converter* -40 85 -29 51 11 34 Radiator* -70 90 -18 26 52 64 Baseplate -100 100 -51 56 49 44 Gondola Structure -100 100 -57 61 43 39 QCL Cryocooler‡ -40 60 -5 34 35 26 OVCS Cryocooler‡ -40 60 0 37 40 23 Backend Electronics‡ -30 60 -8 33 22 27 Frontend Electronics -40 65 -23 55 17 10 LO Boxes‡ -30 50 -9 30 21 20 SLED‡ -30 50 -4 34 26 16 MicroLambda Boxes‡ -40 60 -12 31 28 29 Power Distribution Boxes -40 60 -12 33 28 27 Cryostat Struts -100 93 -45 62 55 31 Optics Box -40 70 -27 42 13 28 OVCS Cyrocooler Controller -40 70 -11 58 29 12 QCL Cyrocooler Controller -40 70 -20 52 20 18 Cradle -100 93 -41 54 59 39 Primary Mirror -55 50 -27 22 28 28 Secondary Mirror -100 100 -30 46 70 54 Secondary Covers -100 100 -55 60 45 40 Secondary Motor* -30 75 -17 51 13 24 Sunshade -100 100 -62 81 38 19 Trusses -100 93 -51 69 49 24 Cryo Shell (average) -60 50 -42 47 18 3 SIP -40 55 -27 51 13 4 Star Shades -100 100 -62 64 38 36 Fluid -25 45 -15 27 10 18 Mass Slider Weight -100 100 -42 62 58 38 Primary Mounts -100 100 -30 30 70 70 Cryostat Mounts -100 100 -38 41 62 59 Science Backup Battery -40 70 -36 38 4 32 Science Stack -40 50 -36 36 4 14 Pumpout -40 70 -20 53 20 17 Table 3: Temperature limits, model predictions, and resulting margin for all GUSTO components (in °C). Some components have heaters (*) and some are on the fluid loop (‡).

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

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