1. Solar Orbiter EUS: Thermal Design Considerations Bryan Shaughnessy, Rutherford Appleton Laboratory 1 Solar Orbiter EUV Spectrometer Thermal Design Considerations Bryan Shaughnessy

2. Solar Orbiter EUS: Thermal Design Considerations Bryan Shaughnessy, Rutherford Appleton Laboratory 2 Basic Configuration Detector Assembly Aperture (100mm*100mm) Grating Length 1.4 m Width = 0.3m Slit Assembly Optical path Primary Mirror (100mm*100mm) Heat Stop Baffles Spacecraft Sunshield

3. Solar Orbiter EUS: Thermal Design Considerations Bryan Shaughnessy, Rutherford Appleton Laboratory 3 Basic Thermal Requirements Detector temperature < -60 deg C (target -80 deg C) Structure and optics: –Multilayer coatings (if used) are assumed to be a limiting factor. < 100 deg C assumed at present. Thermal Control System Mass TBD Thermal Control System Power TBD (minimise)

4. Solar Orbiter EUS: Thermal Design Considerations Bryan Shaughnessy, Rutherford Appleton Laboratory 4 Thermal Environment Distance From Sun AU Heat Flux W/m 2 Through Aperture, W 1.2 1.0 0.9 0.8 0.6 0.4 0.2 951 1370 1691 2140 3805 8562 34250 9.51 13.7 16.9 21.4 38.0 85.6 342.5 Cold case non operational Hot case non operational Start Up Hot Case operational Cold Case Operational

5. Solar Orbiter EUS: Thermal Design Considerations Bryan Shaughnessy, Rutherford Appleton Laboratory 5 Solar Thermal Loads at 0.2 AU 350 W Through Aperture 250 W 200 W Absorbed at primary 100 W 50 W 3 W 103 W Absorbed at baffles 47 W Absorbed at heat stop (‘focussed’) (Absorbing Optics/No Aperture Filter)

6. Solar Orbiter EUS: Thermal Design Considerations Bryan Shaughnessy, Rutherford Appleton Laboratory 6 Irradiance Profile at Primary Mirror (No Aperture Filter) 35 KW/m 2 30 KW/m 2 25 KW/m 2 20 KW/m 2 15 KW/m 2

7. Solar Orbiter EUS: Thermal Design Considerations Bryan Shaughnessy, Rutherford Appleton Laboratory 7 The Thermal Challenges Reject heat input to system of ~350W at 0.2AU –Filter at aperture? –Maintaining sensible temperatures/gradients within instrument –Getting heat to radiators (or to spacecraft cooling system) –Spreading the heat across the radiators Prevent heat loss when instrument is further from the Sun –Maintaining sensible temperatures within instrument –Minimising heat transfer to radiators (or to spacecraft cooling system) –Minimising power required for survival heaters

8. Solar Orbiter EUS: Thermal Design Considerations Bryan Shaughnessy, Rutherford Appleton Laboratory 8 Heat Rejection by Radiators Radiator heat rejection capability a function of: –Emissivity ~ 0.95 for black paint –Efficiency ~ 0.96 –View-factor to space ~ 0.95 How to transfer heat to radiator? Radiator (1.4 m x 0.3 m) TemperatureHeat Rejection K°C°CW/m 2 Watts 233 253 273 293 313 333 343 353 373 -40 -20 0.0 20 40 60 70 80 100 144 200 270 357 461 587 654 734 907 62 87 117 154 200 254 284 318 393

9. Solar Orbiter EUS: Thermal Design Considerations Bryan Shaughnessy, Rutherford Appleton Laboratory 9 Thermal Design Options Solar absorptivity of the optics: –High (i.e., SiC) – remove more heat from primary mirror –Low (e.g., gold coated) – remove more heat from heatstop – but likely restriction on coating temperature Coupling to radiators: –Fitted with heat pipes or loop heat pipes to distribute heat –Primary mirror connected to radiator via thermal straps and/or heat pipe evaporator. How to get high thermal conductance coupling? –Heat loss minimised during cold phases by: Louvers Temperature dependent coatings (major development programme required) Use of loop heat pipes Use of variable conductance heat pipes

10. Solar Orbiter EUS: Thermal Design Considerations Bryan Shaughnessy, Rutherford Appleton Laboratory 10 Loop Heat Pipe Concept Solar load LHP Evaporator Radiator (condenser) Flexible lines

11. Solar Orbiter EUS: Thermal Design Considerations Bryan Shaughnessy, Rutherford Appleton Laboratory 11 Loop Heat Pipe Concept Advantages: Control over amount of heat removal (reduce when further from Sun) Flexible couplings allow for pointing of primary mirror Technical Challenges: Selection of working fluid compatible with hot and cold environments ammonia: -40 °C →+80 °C methanol: +55 °C → +140 °C Freezing of working fluid in radiator during cold cases? Thermally coupling the primary mirror to the evaporator Redundancy? multiple lines to same evaporator, multiple evaporators?

12. Solar Orbiter EUS: Thermal Design Considerations Bryan Shaughnessy, Rutherford Appleton Laboratory 12 Thermal Model Predictions (Absorbing Optics/No Aperture Filter/Heat pipes or loop heat pipes to radiators)

13. Solar Orbiter EUS: Thermal Design Considerations Bryan Shaughnessy, Rutherford Appleton Laboratory 13 Detector Cooling Aluminium filtering blocks any remaining solar thermal loads Detector fitted in a thermally isolated enclosure: –Low emissivity shielding –Low conductivity mounts Dedicated radiator attached to detectors via a cold finger –Multistage to shield thermal loads from spacecraft sunshield? Electric heaters fitted for temperature control and outgassing operations

14. Solar Orbiter EUS: Thermal Design Considerations Bryan Shaughnessy, Rutherford Appleton Laboratory 14 THE END