Solar Orbiter EUV Spectrometer

1. Solar Orbiter EUV Spectrometer Thermal Design Progress Bryan Shaughnessy

2. Summary • Progress and current status • Developing thermal design concepts for trade-off • Thermal Background • Thermal Concepts • Conclusions

3. z Aperture (approx 100mm*100mm) Primary Mirror (100mm*100mm) Optical path Heat Stop -X Width = 0.31m Grating Length 1.4 m Slit Assembly Detector Assembly Height = 0.108 m Basic Configuration

4. Initial 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 < 3.5 kg • Thermal Control System Power TBD (minimise)

5. Thermal Environment (Excludes solar input from outside of the observed region) Cold case non operational Hot case non operational Start Up Cold Case Operational Hot Case operational

6. The Thermal Challenges • Reject heat input to system of ~340W at 0.2AU • Maintaining sensible temperatures within instrument • Getting heat to radiators • 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 • Minimising power required for survival heaters • Overall challenge: achieving the above with sensible mass/power budgets.

7. Radiator Surface Area • Heat output via radiator(s) mounted on the +Z surface • Radiator heat rejection capability a function of: • Emissivity ~ 0.95 for z306 black paint • Efficiency ~ 0.96 • View-factor to space ~ 0.95

8. Basic Thermal Concept • Solar absorptivity of the optics: • High (i.e., SiC) – remove more heat from primary mirror • Low (e.g., gold coated) – remove more heat from structure – but likely restriction on coating temperature • Coupling to the main radiator: • Various options being considered in the thermal trade-off • Fitted with heat pipes or loop heat pipes to distribute heat • Primary mirror and structure connected to radiator via thermal straps and/or heat pipe evaporator. Development programme needed to attached heat pipe evaporators to SiC structure or optics. • 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

9. Radiator (~1.4 m x 0.31 m) at ~ 80 deg C ~ 340W LHP Evaporator Primary Mirror at ~ 100 – 120 deg C Loop Heat Pipe / Absorbing Optics Concept • Technical Challenges: • Selection of working fluid compatible with hot and cold environments (ammonia: -40C →+80C; methanol: +55C → +140C) • Thermally coupling the primary mirror to the evaporator

10. Basic Thermal Concept (cont) • Detectors: • Dedicated radiator attached to detectors via a cold finger • Detector fitted in an enclosure to thermally isolate it from the warm structure

11. Strap to Radiator with heater Low K (mylar) MLI High K (Aluminium) Detectors Detectors Anodized Thermal Screen Detector Supports (isolation) Detector Thermal Control Internal VDA

12. Conclusions • The EUS instrument presents an extremely challenging thermal design problem • Work is ongoing to investigate a number of thermal design options • Initial indications are that the mass of the thermal control system will exceed 3.5 kg (e.g., radiators, heat pipes, heaters, redundancy, etc)

13. Future Work • Consider options for reducing heat load into the instrument, e.g. • Shutter • Instrument rastering • Filters • Complete trade-offs and identify potential thermal designs (together with mass budgets, margins, hardware/suppliers, development programmes, etc) • Identify if a spacecraft level thermal control system should be considered