Solar Orbiter EUV Spectrometer

1. Solar Orbiter EUV Spectrometer Thermal Design Considerations Bryan Shaughnessy

2. Spacecraft Sunshield Aperture (100mm*100mm) Primary Mirror (100mm*100mm) Baffles Optical path Width = 0.3m Length 1.4 m Slit Assembly Grating Heat Stop Basic Configuration Detector Assembly

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. Cold case non operational Hot case non operational Start Up Cold Case Operational Hot Case operational Thermal Environment

5. 103 W Absorbed at baffles 100 W 250 W 200 W Absorbed at primary 50 W 3 W 47 W Absorbed at heat stop (‘focussed’) Solar Thermal Loads at 0.2 AU 350 W Through Aperture (Absorbing Optics/No Aperture Filter)

6. Irradiance Profile at Primary Mirror(No Aperture Filter) 20 KW/m2 15 KW/m2 25 KW/m2 30 KW/m2 35 KW/m2

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. 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?

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. Radiator (condenser) Flexible lines Solar load LHP Evaporator Loop Heat Pipe Concept

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. Thermal Model Predictions (Absorbing Optics/No Aperture Filter/Heat pipes or loop heat pipes to radiators)

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

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