1. USAFA Department of Astronautics I n t e g r i t y - S e r v i c e - E x c e l l e n c e Astro 331 Thermal Control Subsystem (TCS)—Intro Lesson 37 Spring 2005

2. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 37 2 TCS—Intro Objectives Objectives Objective 1. Understand primary purpose of thermal control system design and analysis. Objective 2: Know the driving requirements for TCS design Objective 3: Know the basic steps in the thermal design & modeling process Objective 4: Understand and be able to apply basic heat transfer calculations. Objective 5: Understand various option for S/C TCS technologies Objective 6: Understand S/C surface thermo-optical properties Reading SMAD 11.5

3. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 37 3 TCS Purpose: Maintain all elements within temperature limits for all mission phases Sense temp, communicate, process command Affects virtually every subsystem Example: Hubble Space Telescope (HST) Huge vibrations upon entering/exiting eclipse Caused by thermal gradient of solar panels → had to replace panels to solve TCS—Intro Background

4. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 37 4 Components have different temperature needs Some components more restrictive than others batteries, gyros, precision oscillators, low noise amplifiers… May yield very narrow S/C operating range, if entire S/C to be kept at same temperature, or may not be able to satisfy at all without special cooling / heating measures for some components Strategies Selection of components vs mission objectives Passive vs active thermal control TCS—Intro Driving Requirements

5. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 37 5 TCS—Intro Driving Requirements

6. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 37 6 TCS—Intro Driving Requirements

7. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 37 7 TCS—Intro Thermal Design & Modeling Steps 1) Specify mission heat sources and sinks 2) Design S/C thermal control system (coatings, etc.) 3) Define thermal nodes and networks 4) Determine thermal resistance properties between nodes 5) Solve nodal network using finite differential or finite element analysis using computer code 6) Goto Step 2 and Iterate as necessary 7) Test S/C to verify model 8) Goto Step 2 and iterate as necessary

8. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 37 8 TCS—Intro Review of Heat Transfer Methods Conduction (Fourier’s Law) Heat flow in a medium, generally solid Convection (Newton’s Law) Heat flow using stirring medium, liquid or gas May use gravity to stir passively

9. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 37 9 TCS—Intro Review of Heat Transfer Methods Radiation (Stefan-Boltzman Law) Electromagnetic (EM) energy through free space (mostly in the IR spectrum) Heat Flux

10. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 37 10 Passive thermal control Coatings: paints, mirrors Insulation: multi-layer insulation (MLI) blankets Alternating layers of aluminized Mylar and thin net Often use Kapton for innermost / outermost layers (stronger) Radiators: radiate waste heat to deep space Locate radiators on S/C side not exposed or only partially exposed to Sun or Earth Phase Change Devices: paraffin absorbs heat as it melts (latent heat of fusion) For use next to equipment with high, short bursts of power Thermal Isolators: isolate propellant lines, etc Placement of components TCS—Intro Thermal Design Options

11. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 37 11 TCS—Intro Thermal Design Options Active thermal control Heaters and Thermostats Louvers: modulate a radiator Heat Pipes Liquid near hot component evaporates Moves to cold end of pipe and condenses Wicking device or capillary action brings liquid back to hot end (active or passive) Hard to test in 1g Cold Plate: cooling fluid passes through plate Cryogenic systems: refrigerator (thermodynamic cycle) or vented gas Attitude Maneuvers

12. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 37 12 TCS—Intro Thermal-Optical Properties  = % energy emitted with respect to a perfect black body. Usually averaged over IR range  IR  =  absorptivity, % of incident radiation absorbed Usually averaged over the solar range  SOL.  SOL =0.0 for a mirror or perfect transparent.  = reflectivity, % reflected. 1.0 for perfect mirror, 0.0 for black body.  = transmissivity, % transmitted, 0.0 for black body or mirror.  = reflectivity  = absorptivity  = transmissivity

13. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 37 13 TCS—Intro Thermal Analysis Gray body Assume   over entire spectrum of interest. Most real objects can be treated as gray bodies if we restrict the wavelength under consideration, e.g. solar spectrum (0.3 – 3.0 mm) or IR (3.0 – 30.0 mm). Thus,  SOL   SOL,  IR   IR S/C absorb most energy in the solar spectrum and emit in the IR. So when we compare materials, we’re interested in the ratio,  SOL  IR

14. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 37 14 TCS—Intro Emissivity / Absorptivity vs Wavelength

15. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 37 15 TCS—Intro Radiation Properties of Materials

16. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 37 16 TCS—Intro Radiation Properties of Materials