ICES-2020-164

Thermal design, analysis and testing of Stood- Off Radiator Assembly

Stefan Herndler1 and Christian Ranzenberger-Stindl2 RUAG Space GmbH, Stachegasse 16, 1120 ,

Mark Grimminck3 Airbus Defence and Space Netherlands B.V., Mendelweg 30, 2333 CS Leiden, The Netherlands

and

Claudio Damasio4 European Space Agency, ESTEC, Keplerlaan, 1, 2201 AZ Noordwijk, The Netherlands

Solar Orbiter is a solar-heliospheric ESA mission investigating how processes on the Sun drive the properties and phenomena of the heliosphere. To achieve this, Solar Orbiter will perform 22 orbits around the Sun, with perihelions up to 0.28 AU and 1.02 AU as maximum aphelions, carrying a payload of six remote sensing instruments and four in-situ instruments. Five of the six remote sensing instruments (EUI, METIS, PHI, SPICE, STIX) have very stringent temperature requirements and are therefore mounted on the internal side of the spacecraft (S/C). To maximize efficiency of the radiators of these instruments it is necessary to thermally decouple the radiators from the S/C’s interior and structure to a maximum extent. Therefore, they are attached to the external sides of the S/C via insulating iso-static mounts, forming the so-called Stood-Off Radiator Assembly (SORA). The instruments are thermally coupled to the radiators by flexible thermal links and rigid conduction bars. For an improved thermal efficiency of the radiators in some cases encapsulated annealed pyrolytic graphite (APG) is used. Heat pipes are also used for heat transportation to allow for the geometric distribution of the separate radiators in the limited available volume. The SORA radiators are white-coated except for one, which uses optical surface reflectors (OSRs). In addition to an extensive testing and characterization on component level a sub-system verification was performed via a Thermal Balance (TB) test on a subset of the SORA containing all relevant technologies (iso-static mounts, flexible thermal links, conduction bars, encapsulated APG, white coating, OSRs, heat pipes and multi-layer insulation). The paper provides details about the thermal design of the SORA, the component and TB test campaign as well as the correlation of the TB test thermal model yielding verified modelling parameters for flight predictions of the SORA.

Nomenclature APG = Annealed Pyrolytic Graphite AU = Astronomic Unit CE = Cold Element EMC = Electro-Magnetic Compatibility ESD = Electrostatic discharge EUI = Extreme Ultraviolet Imager - Internal Remote Sensing Instruments

1 Systems Engineer, Thermal Systems, stefan.herndler@.com 2 Manager Engineering Space, Thermal Systems, [email protected] 3 Lead Engineer, [email protected] 4 Solar Orbiter Thermal System Engineer, Thermal Division (TEC-MT), [email protected]

Copyright © 2020 Stefan Herndler GAM = Gravity Assist Maneuver GFRP = Glass-Fiber Reinforced Plastic HE = Hot Element ISM = Iso-Static Mount ITO = Indium Tin Oxide ME = Medium Element METIS = Multi Element Telescope for Imaging and Spectroscopy, coronagraph - Internal Remote Sensing Instruments MLI = Multi-Layer Insulation OSR = Optical Surface Reflector PHI = Polarimetric and Helioseismic Imager - Internal Remote Sensing Instruments S/C = Spacecraft SLI = Single-Layer Insulation SORA = Stood-Off Radiator Assembly SPICE = Spectral Imaging of the Coronal Environment - Internal Remote Sensing Instruments STIX = X-ray Spectrometer/Telescope - Internal Remote Sensing Instruments TB = Thermal Balance TMM = Thermal Mathematical Model TP = Test Phase VDA = Vacuum Deposited Aluminum

I. Introduction HE mission of Solar Orbiter S/C is dedicated to solar and heliospheric physics. Solar Orbiter is an ESA-led T mission with strong NASA participation. It was selected as the first medium-class mission of ESA's Cosmic Vision 2015-2025 program. It will be used to examine how the Sun creates and controls the heliosphere, the vast bubble of charged particles blown by the solar wind into the interstellar medium. The science payload of Solar Orbiter comprises six remote sensing and four in situ instruments to gain new information about the solar wind, the heliospheric magnetic field, solar energetic particles, transient interplanetary disturbances and the Sun's magnetic field. Launched on February 10th 2020, the mission will provide close-up, high-latitude observations of the Sun. Solar Orbiter will perform 22 highly elliptic orbits – between 1.02 AU at aphelion and 0.28 AU at perihelion. It will reach its operational orbit just under two years after launch by using gravity assist maneuvers (GAMs) at Earth and Venus. Subsequent GAMs at Venus will increase its inclination to the solar equator over time, reaching up to 24° at the end of the nominal mission (approximately 7 years after launch) and up to 33° in the extended mission phase. To allow Solar Orbiter to withstand the very high solar flux at 0.28 AU, about 17500 W/m2 (i.e. about 13 solar constants), the sun-pointing S/C features a heat shield to provide a complete sun shielding of the platform itself and act as thermal insulation of the entire S/C including all equipment and parts of the structure within an 8º half-cone to cover both the operational modes of the satellite and maximum allowed off-pointing when the satellite enters safe mode.

II. Description of Stood-Off Radiator Assembly Five remote sensing instruments installed within the platform (EUI, METIS, PHI, SPICE, STIX) are operational around closest approach, and at the minimum and maximum heliographic latitudes. Key to the mission success is the maintenance of the stringent thermal environment of the payload sensors: some instrument interfaces (called elements) have to be kept at temperature lower than -40°C or -50°C with a heat rejection capability of 5 W or more. In addition, payloads require accurate and adjustable alignment at integration using iso-static mounts (ISMs), which restrict the efficiency of heat rejection as the payloads are not directly mounted to the panel. The temperature distribution across the platform affects the thermoelastic distortion of the platform and thus the pointing accuracy of the payloads. An additional and important driving requirement is the mechanical interface load to the instrument feet. Those requirements drive the choice to control the thermal environment of these items primarily by stood-off radiators which are connected to the units via thermal links to avoid heat transfer from the S/C structure to the payload radiators.

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The stood-off radiators are thermally isolated from the S/C panel, providing two key advantages. Firstly as the radiators are a separate item to the panel the thermo-elastic distortion of the panel onto which the payload is mounted is reduced due to the warmer panel temperature. Secondly, the required radiator area will be smaller as heat leak to the radiator from the rest of the S/C is restricted. +Z The payload radiators are located on the –Y and +Z walls as +Y shown in Figure 1, with the +X wall carrying the heat shield, which is nominally sun-pointing throughout the mission. Those on –Y wall are +X in view of the solar array, which at the near sun distances operates at a E high temperature. Purpose of the radiators was to radiate to space the E C PIC heat dissipated by the remote sensing instrument, the solar flux S passing through the feedthroughs, the parasitic heat from the platform to the instruments themselves and to radiators, the IR flux from the solar array and last the small amount of solar flux reflected by the solar arrays. Thermal design effort was required to ensure the coldest E I H payload interface temperatures are achievable in these environments. EU The radiator location was optimized mainly taking into account the position of the relevant remote sensing instruments and the IR flux IX ST 1 and reflected solar flux coming from solar arrays. ME UI To minimize the impact of thermo-elastic distortion whilst E E I C ensuring that the instruments are well coupled to any heat rejection EU system, there was the need of a good thermal conduction pathway E H without hard mounting onto the S/C panels. This challenge was TIS ME addressed in the use of flexible thermal links and thermal conductive E2 bars. The use of flexible thermal links enables good thermal contact M E UI C E IS between hot elements and cold fingers of the instruments with the ET E M E radiators whilst the flexibility of the link allows for mechanical M I C TIS PH decoupling of the payload. This permits the payload to be mounted ME E iso-statically, making the design tolerant to any distortions in the S/C H HI panel beneath. As the METIS instrument already planned to use P flexible straps on the payload side of the element interfaces, the link between the element and the radiator needs to be rigid. This is to Figure 1. Stood-Off Radiator Assembly. minimize deflection at the element which would cause the METIS straps to flex. For this reason, conduction bars have been used.

III. Detailed Design of Stood-Off Radiator Assembly The radiator panels forming the SORA are nominally designed based on a traditional radiator design of a solid aluminum thermally functional structure in combination with a lightweight mechanical structure to provide sufficient stiffness with a minimum impact on overall mass. The panels are supported by highly thermally isolating glass-fiber reinforced plastic (GFRP) ISMs with a flexible blade design pointing to a nominal thermal center to minimize thermo-elastic loads within the structure. Mechanical decoupling/thermal coupling of the radiators to the instruments (EUI, PHI, SPICE, STIX) is typically made with Airbus Defence and Space Netherlands HiPeR flexlinks ensuring that only marginal mechanical loads are transferred to the instrument interfaces from thermo- elastic, mechanical dynamic or mounting misalignment loads. The METIS instruments are thermally coupled via rigid K-Core thermal links with further thermal and mechanical (de)coupling undertaken by the METIS instrument. In the case of geometric constraints on ideal radiator positioning, redundant aluminum grooved heat pipes, using ammonia as working fluid, have been implemented for heat transportation over longer distances. The thermally functional surfaces are nominally Solar White (Enbio) with a thermally conductive Indium Tin Oxide (ITO) coating covering the surface coating for electrostatic discharge (ESD) requirements. The exception to this is the SPICE CE radiator which uses optical surface reflectors (OSRs) in order to prevent an exceedance of its maximum temperature limit during the off-pointing case when the radiator is illuminated by the Sun.

A. Iso-Static Mounts Two types of ISMs are designed to provide mechanical mounting for radiator panels (high type) and heat pipes (low type) to allow survival of launch loads, minimize thermo-elastic loads to the S/C during in-orbit operations and

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for conductive thermal decoupling from the S/C structure. The iso-static requirements for SORA were only achievable by implementation of a dedicated design using GFRP to a maximum extent. GFRP offers one of the best ratios of stiffness to thermal resistance of any material and a design was implemented to make a fully GFRP solution for the strict mechanical and thermal requirements of SORA. 1. ISM Mechanical Design and Verification The mechanical design has a number of elements that are applicable to its functional requirements. These include oriented glass fibers for high in-plane stiffness (relative to the ISM blade) and strength; a section profile for low out-of-plane bending stiffness; an optimized bending radius of the fibres in the direction change from blade to base; clamping washer for load distribution to minimize creep, as shown in Figure 2. These elements have been verified by test with the results shown in Table 1. The mechanical properties were characterized by component level test and verified by sub-system level mechanical qualification for four of the SORA panels during dynamic testing. Table 1. ISM mechanical test values. Load test Min. stiffness Min. ultimate strength High / Low High / Low In-plane shear 2215 / 4495 N/mm 4175 / 6187 N In-plane tension 21000 / 18250 N/mm 9500 / 17620 N In-plane buckling N/A 15000 / >16666 N Figure 2. GFRP ISM with Out-of-plane Shear 135 / 593 N/mm 14.1 / 6.54 mm deflection titanium washers for mechancial mounting. 2. ISM Thermal Design and Verification The thermal design with respect to the conductive elements of the ISM was to maximize the thermal length of non-conductive materials. In this respect the thermal path is entirely dominated by the GFRP materials, and metallic elements required for fasteners have a very low impact on the overall thermal performance. This has been verified by analysis where the titanium washer and nut washer are modelled as aluminum with only a small impact on the thermal performance. Testing was carried out with the hot side at +20°C and three separate cold side temperatures of -10°C, -40°C and -75°C, supplemented by a fourth data point for correlation with a change on the hot side at +50°C and the cold side at -50°C. 14 ISMs with radiative isolation as shown in Figure 3 were simultaneously tested in parallel to improve the accuracy of the test due to the extremely low thermal conductivity of the ISMs and hence its Figure 3. ISM with full radiative sensitivity to power dissipation. encapsulation applied. Internal The test setup was radiatively and conductively isolated from the (left), external (right). environment on the hot side in addition to the shroud being maintained at the hot side temperature of the setup. All potential radiative cavities with view factors to the cold side were covered. The results estimated from the component level test measurements are a radiative coupling (A) of 0.0000704 m2 and a conductive coupling of 0.0011 W/K and 0.00185 W/K for high and low ISMs, respectively.

B. Encapsulated APG Radiator Panel Due to the stringent thermal requirements and limited space for the SORAs the SPICE CE and EUI CE radiator panels required a higher efficiency to ensure sufficient heat dissipation. These two radiators were therefore designed with an encapsulated annealed pyrolytic graphite (APG) core (K-Core) technology from Thermacore. Two slightly differing designs were produced with the SPICE CE radiator requiring an APG core of 4 mm and EUI CE with a core of 2 mm. For mechanical reasons a minimum aluminum skin thickness of 1mm was required for both designs. Mechanical vias are designed through the APG core to provide additional mechanical performance to the radiator panels. As per the general design of the SORAs, the EUI CE and SPICE CE panels include a light weighted stiffener structure on the back side of each panel, and mounting interfaces for the ISMs which are nominally aligned to the flexlink thermal interface locations from the instrument.

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1. APG Core Panel Thermal Verification Both EUI CE and SPICE CE, shown in Figure 4, were verified through a critical technology qualification program where key aspects of the APG panels were tested including thermal performance pre and post environmental testing as well as inspection of permanent deformations in the panels. The tested panels were representative of the longest sections of the APG core which was considered the critical element in the panel design. Environmental testing included thermal cycling only as this also represented the highest mechanical load case due to the thermal expansion differential between the APG and the aluminum skin. 40 thermal cycles were made on the section panels starting from +80°C to -70°C and finally decreasing the lower limit to -150°C which exceeded the lifetime thermal cycling of the APG panels. The over testing was intentionally performed to attempt to force the APG to fail as it was assumed that critical failure could occur spontaneously. As no statistical sampling was possible the Figure 4. The EUI CE and SPICE CE overtesting criteria was to demonstrate a combination of safe life radiators from Thermacore with an APG and failure tolerant design. core for improved thermal efficiecy. Regardless, it was expected that the thermal performance would degrade during thermal cycling due to brittle nature of the APG with the potential of cracks and disjoints forming within the core. This was verified by test with intermitted performance verification showing that the degradation was mostly linear during the cycling, however no critical failure was demonstrated. A final degradation for EUI CE and SPICE CE was measured 15% / 20% at ambient and 7.5% / 8.8% at -50°C, respectively. 2. APG Core Panel Mechanical Verification Both the EUI CE and SPICE CE panels were tested as part of the mechanical qualification testing of the SORAs. These panels (along with the EUI HE and METIS CE panels) were vibrated in both sine, random and shock environments in their flight configuration. No degradation in mechanical performance (via subsystem resonance search both pre and post mechanical environments exposure) were measured. Final qualification via thermal balance (TB) test was carried out with no issues identified.

C. Conduction Bars Conduction bars are developed for and employed on the METIS instrument, like the encapsulated radiator panels, they make use of an APG core encased in aluminum. The rigid conduction bars, shown in Figure 5, are used as a connection to the METIS instrument, which offers a flexible strap as an interface. The conduction bars use a thicker core and aluminum skin than the radiator panels but the technology is otherwise similar. Thermal interfaces are made with the radiator perpendicular to the APG encapsulation for all rigid bars. The METIS CE and ME also have a perpendicular interface with the instrument flexible strap and the METIS HE has a parallel interface. 1. Conduction Bar Verification

Initial testing for the qualification of the conduction bars included four thermal- Figure 5. The METIS vacuum cycles followed by 96 ambient pressure thermal cycles. As expected, some CE/ME (left) and CE (right) degradation in performance of the rigid links was seen as a result of the thermal conduction bars. cycling. To assess the degradation further thermal-vacuum testing was carried out with intermittent performance testing, showing an asymptotic performance degradation of up to 12% occurring within the first 100 cycles. The thermal performance results are shown in Table 2. Rigid bar thermal performance testing results. Table 2 compared to requirements. The results show Rigid link Thermal conductance, Requirment, a conformant design for the METIS HE and partially W/K at -60°C W/K at -60°C conforming for METIS CE and METIS ME. The METIS CE/ME 2.0 ≥ 2.4 underperformance is covered by margins, which is METIS HE 1.7 ≥ 1.5 finally verified via the TB test without identification of issues. 5 International Conference on Environmental Systems

D. HiPeR Flexlinks The HiPeR flexlinks, shown in Figure 6, provide the thermal link between the instruments and the radiator panels in combination with mechanical decoupling. 11 flexlinks were provided with a combination of instrument/radiator interfaces as well as both single and double strap configurations. The flexlinks are made from flexible pyrolytic graphite sheets with an aluminum end fitting. As the pyrolytic graphite can shed particles the flexlinks are wrapped in a “cleansleeve” that prevents particulates from being released onto the sensitive Solar Orbiter instruments whilst still being able to vent air during the transition to vacuum during launch. Additionally the straps are wrapped in a single-layer insulation (SLI) sleeve of vacuum deposited aluminum (VDA) coated Kapton® to minimize the thermal coupling to the S/C Figure 6. SORA flexlink examples showing which is typically warmer than the flexible links. a single and double strap configuration. 1. Flexlink Environmental Verification The HiPeR flexlinks underwent a test program of mechanical and thermal performance qualification. Mechanical environment testing included vibration testing covering sine and random environments that envelope the conditions of the Solar Orbiter launch. Due to the high flexibility of the straps the flexlinks are generally insensitive to higher frequency vibration (>50Hz). An oversize in length prevents the flexlink from loading under tension during vibration allowing the instrument and the radiator to Table 3. Flexlink post qualification model testing vibrate in independent modes without risk of damage to thermal performance results. the flexlinks. Flexlink Thermal conductance, Requirment, Angular stiffness was only tested on the HiPeR W/K at -50°C W/K at -50°C flexlink prototype, as it showed a maximum angular stiffness of 0.0213 Nm/deg in the stiffest direction, which EUI CE1 2.13 ≥ 1.74 are two orders of magnitude below the requirement of EUI CE2 2.10 ≥ 1.82 lower than 2.5 Nm/deg. EUI HE 2.65 ≥ 1.83 Linear stiffness measurements were carried out on the EUI ME1 1.72 ≥ 0.89 qualification model flexlinks with measurements typically EUI ME2 1.13 ≥ 0.96 below 0.4 N/mm and the highest (EUI CE in Z direction) PHI CE 1.22 ≥ 0.97 of 1.1 N/mm, against a requirement of 10 N/mm. PHI HE1 1.24 ≥ 1.04 Thermal performance results of the component level PHI HE2 1.45 ≥ 0.98 qualification model testing show good compliance to the PHI HE3 1.46 ≥ 0.98 requirements with the exception of the SPICE CE and SPICE CE 0.96 ≥ 1.30 STIX CE flexlinks, see Table 3. The underperformances STIX CE 1.02 ≥ 1.17 are covered by margins, which is finally verified via the TB test without identification of issues.

IV. Thermal Verification of Stood-Off Radiator Assembly Subsequent to component level testing a SORA TB test is performed to demonstrate that the SORA thermal design conforms to the specified performance requirements and to provide data for the correlation of the SORA thermal mathematical model (TMM) supporting to SORA development process. Verification of the SORA thermal performance is ultimately performed using the correlated SORA thermal model. The TB test is performed on a selected SORA subset featuring all types of thermal control components of the SORA as mounted on the Solar Orbiter S/C. The test components are equipped with thermocouples (TC), focused on monitoring the main thermal path from the simulated instrument to and across the radiator panels. Heaters are installed to simulate heat dissipation of operational instruments, to maintain components within temperature limits, to emulate the radiative environment and to prevent unintentional heat losses. Monitoring, control and data acquisition was performed using equipment provided by the test facility. The TB test sequence featured phases enveloping the SORA hot operational and cold non-operational cases of the Solar Orbiter mission; an additional intermediate phase has been defined to ensure sufficient information for the correlation of the SORA TMM.

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A. Objectives of Thermal Balance Test The objective of the SORA TB test was to achieve a verification of the SORA assembly. Up to the point when the SORA TB test took place, the components forming SORA were only tested separately from each other. It was essential to test the SORA in a flight-representative configuration to minimize risks for the Solar Orbiter flight model test campaign. The key objectives of the SORA TB test were:  Verification of the suitability of the SORA thermal design.  Functional verification of SORA components in assembled status, close to maximum and minimum temperatures as well as in vacuum.  Enabling correlation of the SORA TMM and providing data on the sensitivity to parameter changes. For example, how sensitive the SORA is to variations in the dissipation at the instrument interface or to the heater power on the radiator panel.  Verification of the SORA TMM by comparing test results with the thermal model results. In this way, the correlated thermal model can be used, with confidence, to predict the in-flight operation of the SORA.

B. Description of Tested SORA Configuration To achieve the key objectives of the SORA TB test it is not necessary to test the entire SORA; hence, a subset only was tested. In addition this has the added benefits of reducing the cost of the test and shortening the schedule considerably. The tested SORA subset has been chosen to ensure all technologies are adequately tested to enable the flight thermal model to be correlated and verified. The SORA subset selected for the TB test comprises the following flight representative radiator panels:  SPICE CE – K-core APG panel with OSR radiator and flexlinks  EUI CE – K-core APG panel with Solar White coated radiator and flexlinks  EUI HE – aluminum panel with Solar White coated radiator, heat pipes and flexlinks  METIS CE – aluminum panel with Solar White coated radiator and conduction bars These panels all include flight representative ISMs, MLI, heaters, cabling and electro-magnetic compatibility (EMC) shields. Each panel is tested individually but is mounted on a single support panel, which represents the S/C mounting panels. Support panel mounts connect the support panel to the test adapter fixed to the vacuum chamber. In the test configuration shown in Figure 7 the MLI and the support panel have been removed to show the full extent of the radiators, the heaters, the heatpipes and the ISMs. The flight representative MLI used in the TB test was as follows:  S/C MLI attached to SORA facing side of support panel, including areas covered by radiator panels  MLI on the rear of the radiator panels  ISM SLI  Flexlink and conduction bar SLI  EMC shield  Skirts, including areas covering fronts of radiator panels for attachments and trimming The test MLI used in the TB test was as follows:  MLI on the rear of the support panel, including covers for S/C internal environment boxes and support panel mounts  Harness MLI Figure 7. SORA subset S/C side.

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C. Test Support Panel (MGSE) The SORA TB test configuration is mounted on a single support panel, which represents the S/C mounting panels. The support panel, shown in Figure 8, is connected to the test adapter, fixed to the vacuum chamber, by three support panel mounts. The support panel was held at a constant temperature controlled by heaters. Boxes mounted on the underside of the support panel were also temperature controlled by heaters to simulate the internal S/C environment around the instrument interfaces of the flexlinks and the conduction bar. Guard heaters were installed to compensate for any resulting heat loss from the support panel mounts and harness exiting the facility. Furthermore, the support panel was installed to be in a horizontal position to guarantee for an operation of the heat pipes in a representative manner, which was monitored by a tilt sensor.

D. Heaters The TB test setup is instrumented with 39 prime and 23 redundant heaters in 25 heater circuits. All heaters are covered with MLI to minimize parasitic heat flows. The heater control and Figure 8. Support panel S/C side. data acquisition is performed using equipment provided by the test facility. The used heaters are grouped in six categories:  Instrument interface heaters (5 prime and 5 redundant in 5 circuits). These are test heaters to simulate the heat dissipated by the instruments. The power of these will be adjusted during the test to ensure the correct temperature at the interface for each phase of the test.  Panel heaters (6 prime and 6 redundant in 5 circuits). These are used during flight to maintain components within allowed temperature limits and are flight representative heaters (types and powers) mounted to the same positions as for flight.  Support panel heaters (16 prime and 0 redundant in 4 circuits). These are test heaters able to bring the support panel to the maximum temperature of the S/C mounting panel to simulate the maximum S/C internal environment temperature for conductive (via the ISMs) and radiative (via the S/C MLI) coupling.  S/C environment heaters (5 prime and 5 redundant in 5 circuits). These are used to simulate the S/C internal radiative environment around the thermal links and are attached to the boxes on the bottom of the support panel.  Support panel mount heaters (3 prime and 3 redundant in 2 circuits). These are attached to the support panel mounts to guard against any conductive heat loss from the support panel.  Harness heaters (4 prime and 4 redundant in 4 circuits). These are used to guard against any heat loss from the harness that exits the chamber.

E. Thermocouples The TCs used in the TB test are intended to serve three purposes:  To measure temperatures on the test components for each steady state phase to enable correlation of the thermal model,  To monitor temperatures of each equipment to ensure they do not exceed their respective temperature limits,  To enable control of the environment temperatures. The TB test setup, excluding the vacuum chamber, was monitored by 182 type T (Copper-Constantan) Teflon coated special class TCs with an accuracy of ± 0.5°C. All TCs were attached by 1mil Kapton® tape and covered by Cho-Foil® tape, depending on the surface finish of the component below black Kapton® tape was additionally applied on top. TC monitoring and data acquisition was performed using equipment provided by the test facility.

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103 TCs are used to monitor temperatures along the main thermal paths from the instrument interfaces of the thermal links to and across the radiator panels. 22 TCs are allocated to the ISMs, 37 TCs to the simulated S/C environment and 20 TCs to the verification of the guard heaters.

F. Vacuum Chamber The SIMDIA space simulation test facility in Airbus Toulouse is ideally sized to perform heat balance, thermal vacuum as well as verification and reliability tests on equipment, sub-systems and mini-satellites. The test facility consists of a horizontal cylindrical vessel of 3 m diameter and 3 m room depth, with a front door, thermal shrouds, a pumping system, a solar simulator and the corresponding data acquisition system.

V. Stood-Off Radiator Assembly Thermal Balance Test The TB test sequence features phases enveloping the SORA hot operational and cold non-operational cases of the Solar Orbiter mission. An additional intermediate phase is defined to ensure sufficient information for the correlation of the SORA TMM. No solar simulation case is performed, since it is not needed for model correlation.

A. Description of Test Phases In order to adequately correlate the thermal model, a number of test cases are chosen to cover the full range of the flight operation of the SORA. For correlation, specific operational or non-operational cases are not necessary but just the full range of temperatures. Therefore, the following TB test phases are established:  The test phase TP2 is aimed to represent the hottest case when the S/C is sun-pointing at 0.28 AU but protected by the heat shield and all instruments are operating.  The test phase TP3 in an additional case with intermediate temperature level simulated at the instrument interface to better correlate the flexlink/conduction bar/heat pipe interfaces, which are critical to the performance of the SORA.  The test phase TP4 is aimed to represent the coldest case when the S/C is in hibernation at 1.47 AU. All the instruments are switched off, the S/C is at its lowest dissipation level and the radiator panels are kept above their minimum non-operational temperature by heaters. The other test phases are not relevant for the correlation, but serve specific purposes such as pump down (TP0), bringing the simulated S/C environment to the required temperature (TP1) or bringing the test setup and the chamber back to ambient temperature (TP5). TP1 TP2 TP3 TP4 TP5

In Figure 9, a timeline of the TB test 80

C ° sequence is shown. For the test phases TP2 to 60 TP4 the chamber shroud is at -160°C and the simulated S/C environment at +50°C. In TP2 the 40 simulated instrument heaters are set to reach the 20 maximum non-operational temperature limits for all instrument interfaces. For TP4 the flight- 0 representative panel heaters are set to keep the -20 SPICE CE instrument interfaces above the heater switch-on -40 temperatures of the non-operational instruments. EUI HE -60 EUI CE The exception to this is METIS CE where the METIS CE

heater switch-on temperature when the tempearture, interface Instrument -80 instrument is operational is defined, as the 0 10 20 30 40 50 60 70 80 90 operational temperature range is lower than the Test duration, hours non-operational temperature range. In TP3 the Figure 9. TB test timeline. simulated instrument heaters are set to reach instrument interface temperatures intermediate to the ones in TP2 and TP4.

B. Success Criteria and Deviations The following success criteria were established during preparations for the TB test:  All of the TB test steady state and transient phases shall be performed satisfactorily.  No visible degradation of the SORA hardware of interfaces shall be observed following completion of the test.

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 All the necessary data sets (power and temperature measurements) shall be stored securely and easily available to enable TMM correlation.  The temperature stability criteria for each steady state phase shall be met as follows: o The temperature gradient over a 5 hour period is less than 0.1°C per hour for 100% of the working TCs of the SORA test pieces measurements. o The temperature gradient over a 5 hour period is less than 0.1°C per hour for 95% of the working TCs of the simulated S/C environment measurements. o The temperature gradient over a 5 hour period is less than 1°C per hour for 100% of the working TCs of the guard measurements. o The stability of the chamber shroud temperature is less than ±1 K per hour.  The rate of temperature changes during the transient phases shall not exceed 20°C/min.  The target temperatures for the instrument interfaces and the simulated S/C environment shall be controlled within ±2.5°C.  Minimum/maximum temperature limits of sensitive equipment, e.g. heat pipes, shall not be exceed. The following deviations were experienced during test conduction:  The stability criteria were violated for multiple SORA pieces and in multiple test phases, but only by a maximum deviation of +0.1°C, which is considered marginal and without any effect on the subsequent correlation activities.  Some of the envisaged target temperatures were not reached. For the correlation the measured temperatures were used, thus this deviation has no effect on the subsequent correlation activities.  The temperature limit of +80°C was exceeded on SPICE CE flexlink instrument interface during one hour of the transient phase from TP1 to TP2 by a maximum of roughly +10°C. The exceedance was deemed not critical, as the maximum reached temperature was well below the qualification limit.  The harness guard heater temperatures for METIS CE and EUI HE were not set to the correct temperatures in TP3. The effect on the average radiator temperatures was estimated to be within the measurement tolerance and thus without any effect on the subsequent correlation activities.  Based on their measured temperatures two TCs were found to be detached from their item. The TCs were prime and redundant TC for a position on the support panel. Thus, information on the temperature of the position was lost, but, as the support panel was not a critical test piece, this had no effect on the subsequent correlation activities. Based on the general test conduction, considering the experienced deviations and the thereby unaffected subsequent correlation activities, the TB test was declared successful.

C. Test Results The test data were post processed with a focus on the calculation of the final temperatures of test phases TP2, TP3 and TP4 that were used for the subsequent correlation activities. The temperature measurements of TP2 each TC during the last half (Inst. I/F heaters) hour of the defined stability TP2 period of the relevant test TP3 phases were taken to calculate (Inst. I/F heaters) the average temperatures, TP3 standard deviations and giving TP4 (Panel heaters) minimum and maximum values. These final TP4 temperature readings of all TCs for TB test phases TP2, TP3 and TP4 as well as the Figure 10. Heater powers over TB corresponding power inputs test phases. formed the data set for the correlation activity. Figure 11. Instrument interface Figure 10 and Figure 11 show the heater powers and the resulting temperatures over TB test phases. instrument interface temperatures for TB test phases TP2, TP3 and TP4.

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VI. Stood-Off Radiator Assembly Model Correlation The correlation of the thermal model was performed based on the results gained from TB test phases TP2, TP3 and TP4. Aside from the TB test data set, formed by final temperature readings of all TCs and power values, the filled out TB test procedure and extensive photo documentation were crucial inputs to the correlation activities.

A. Correlation Quality Criteria In preparation of the correlation activities, correlation quality criteria are established to evaluate the status of the model correlation. In each correlation step the model results of each SORA radiator and its associated parts are separately compared to the valid measurements of the TB test. The results of these comparisons are used as indicators for the quality of the model correlation. The following correlation quality criteria were used for the evaluation, indicating when correlation of model and test results is satisfactory:  The difference of each measured to calculated temperature shall be less than ±3°C.  The average of temperature differences shall be less than ±2°C.  The standard deviation of temperature differences shall be less than ±3°C.  The differences of temperature differences of the following critical equipment shall be less than ±3°C. o Simulated instrument heater to thermal link o Thermal link instrument interface to radiator panel interface o Thermal link radiator interface to radiator panel o Along maximum distance on radiator panel  The difference of measured to calculated instrument heater power shall be less than ±5%. The correlation activities shall be declared successful when the results delivered by the model fulfill all the correlation quality criteria for each SORA radiator and its associated parts in all the relevant test phases.

B. Correlation Activities Before the actual correlation activities could be started, the mapping of TC and heater positions to nodes needed to be updated to the final positions used during the TB test. Furthermore, the temperatures for some nodes that could not be equipped with TCs needed to be calculated based on interpolation of the temperature readings of two or more TCs. The actual correlation activities started by identifying the largest deviations of the thermal model from the TB test results and which of the three relevant test phases were affected. Based on these two parameters the priority with which a certain deviation of the thermal model has to be handled is decided, with higher priority the larger the deviation and the more affected test phases. Naturally, these priorities changed over the course of the correlation activities as deviations tended to decrease. As these deviations depend on different model parameters, changes in different parameters may all lead to a reduction of the deviation. Therefore, based on information from a sensitivity analysis of the thermal model, changes of parameters with higher sensitivity were favored over changes of lesser influential parameters. The major changes to the thermal model during the correlation activities were:  The emissivity of the Solar White coating used on all the SORA radiator panels except for SPICE CE was increased by 4%. The increase can be explained by the measurement uncertainty of the emissivity value, which was originally used to keep the modelling approach conservative.  The modelling of the EMC shield needed to be improved by adding thermo-optical properties and changing its in-plane conduction.  The in-plane conduction of some radiator panel nodes needed to be adapted to represent originally not modelled details of the radiator panel in connection with the thickness of aluminum.  The in-plane conductance of the APG radiators had to be reduced by 9%. This reduction can be justified by the uncertainty of the thermal conductance test of APG radiator samples, which were the base of the originally used values.  The thermal link interface areas to the instruments as well as to the radiator panels needed to be corrected. Changes of some contact areas are quite considerable with up to ±25%.  The conductance of the thermal links had to be changed by ±10%. These changes are justified by the uncertainty of the thermal conductance test of the thermal links.  The conductance of the interfiller used for thermal coupling of the simulated instrument heater blocks to the thermal links and of the thermal links to the radiator panels or the heat pipes needed to be increased

11 International Conference on Environmental Systems

by up to +150%. This huge increase results from the fact, that the data basis for this value was only a manufacturer data sheet and the value originally used in the model was again decreased to keep the modelling approach conservative. Only after the thermal model already showed good results and the potential of plausible parameter changes was exhausted, parasitic heat fluxes were taken into account. During the correlation activity, two parasitic heat fluxes with significant impact on the quality of the correlation were identified.  A parasitic conductive coupling of the support panel to the thermal link instrument interfaces via the TC and heater wires was introduced into the model. A similar parasitic coupling will also be present due to thermistor wires on Solar Orbiter S/C.  A parasitic radiative coupling of the bolts clamping the APG layers in the instrument-side endfitting of the flexlinks to the S/C internal environment was introduced into the model. Because of the result of the correlation, these bolts were covered with low emissivity material on the hardware implemented for Solar Orbiter S/C. During the final stage of the correlation activities, an anomaly of the temperature measurement on the EUI CE 2 simulated instrument heater in test phase TP4 was found. In test phase TP4, the simulated instrument heater is turned off and the heater on the radiator panel is turned on to keep the panel above its allowable minimum temperature. Temperature measurements show only a very small temperature difference from the EUI CE radiator panel to the EUI CE 2 flexlink instrument interface of only 0.9°C. At the same time, there is a temperature difference between the flexlink instrument interface and the simulated instrument heater of 11.4°C. As the TC on the simulated instrument heater shows this drastically higher temperature, the only plausible explanation is that the TC is only loosely connected to the part it should measure and is highly influenced by the surrounding simulated S/C internal environment, which is at +50°C. This influence was also present during test phases TP2 and TP3, but there the corresponding heater was turned on and the temperature delta was smaller. In test phase TP4, the corresponding heater was turned off, but the TC did not cool down as far as it should have, due to its coupling to the simulated S/C internal environment. The temperature measurement on the EUI CE 2 simulated instrument heater is thus neglected for model correlation of test phase TP4. After the above described modifications of the thermal model were performed, the results delivered by the model fulfilled all the correlation quality criteria for each SORA radiator and its associated parts in all the relevant test phases and thus, the correlation activities were declared successful.

C. Concluding Flight Prediction Based on the correlated thermal model the steady state thermal cases were verified including a sensitivity analysis to check the margins and the transient flight predictions were performed for the following cases:  Hot operational case, with the S/C at a distance of 0.28 AU to the Sun with end of life thermo-optical properties and instruments turned on.  Cold operational case, with the S/C at a distance of 1.20 AU to the Sun with end of life thermo-optical properties and instruments turned on.  Cold non-operational case, with the S/C at a distance of 1.47 AU to the Sun with begin of life thermo- optical properties, instruments turned off and the radiator panels kept above their minimum non- operational temperature by heaters. The results generated in the above cases were heater consumed powers and duty cycles as well as instrument interface minimum and maximum temperatures. The results are compliant with the requirements and no need of design modifications is identified.

VII. Conclusion The themal design of SORA was successfully implemented and its critical technologies ISMs, encapsulated APG radiator panels, rigid conduction bars and flexible thermal links tested on component level. A TB test was performed on a selected SORA subset featuring all types of thermal control components of the SORA, providing data for the correlation of the SORA TMM supporting to SORA development process. The adequacy of the SORA thermal design was ultimately demonstrated using the correlated TMM for flight predictions, showing that all the requirements in all the mission phases are fulfilled.

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