Orbital Transfer Vehicle (OTV) Thermal Control

1. Orbital Transfer Vehicle (OTV) Thermal Control Ian Meginnis February 26, 2009 Group Leader - Power Systems Phase Leader - Translunar Injection OTV Power Systems OTV Thermal Control Ian Meginnis Power Systems

2. Electronics Board Thermal Control Hall Thruster Thermal Control Acronym Definitions: PCDU - Power Conditioning and Distribution Unit PPU - Power Processing Unit Batt - Battery DC - Direct Current Note: Not to scale Ian Meginnis Power Systems (Exhaust) Radiator Radiator Hall Thruster 2 Radiators Thruster Support DC/DC Converter PPU PCDU Batt Aluminum Heat Pipes with Ammonia Aluminum Mount

3. OTV Thermal Control Ian Meginnis Power Systems • Approximately 250W needs to be removed from electronics board • Heat conducts from devices to aluminum mounting plate • Heat pipes with ammonia transfer heat to 2 radiators outside OTV • Thermal System Mass: 11.1kg • Xenon needs to be stored at gaseous state • 5W heater maintains xenon to constant temperature of 300K • Thermal System Mass: 0.2kg • 490W needs to be removed from Hall thruster • Heat conducts from thruster to aluminum mounting plate • Heat conducts from plate to 2 radiators that radiate heat to space • Thermal System Mass: 3kg • Total Thermal Control System Mass: 14.3kg

4. Calculation of Electronics Board Thermal Control Ian Meginnis Power Systems • Aluminum (Al) Mounting Plate: • Max. operating temperature of components: 323K • Total area of plate: 0.22m2 • Thermal conductivity for Al: 236W/(m*K) • Density of Al: 2700kg/m3 • From conduction: Δx < AK(T1-T2) / q • Δx = thickness of plate • T1 = max. operating temperature = 323K • T2 = temperature of plate (assume constant at 300K) • q = heat dissipation from components = 250W • Δx < 4.77m (thickness of plate must be less than this)  selected to be 0.5cm to minimize mass • Mass of plate = 2700kg/m3 * (0.005m * 0.22m2) = 3kg

5. Calculation of Electronics Board Thermal Control(continued) Ian Meginnis Power Systems • Sizing of heat pipes • Mass of ammonia: • Latent heat of vaporization of ammonia: 1371kJ/kg • Mass = 0.25kW * 450sec / (1371kJ/kg) = 0.0821kg • Assumes ammonia boils in 7mins (450sec) • Mass of Al pipes: • Diameter of pipes: 2.54cm (OD); 2.44cm (ID) • Length of pipes: 3m • Mass = π[(1.27cm)2 – (1.22cm)2] * 300cm * 0.027kg/cm3 = 3.17kg • Mass of radiators • Area of radiators: A = q / (εσT4) = 1.44m2 • For aluminum with white paint (Z93): Emissivity (ε) = 0.92 • σ = 5.67E-8 J/(K4*m2*s) • q = 250W • T = boiling point of ammonia @ 1 atm = 240K • Mass of radiators = 2700kg/m3 * 1.44m2 * 0.002m = 7.8kg

6. Calculation of Hall Thruster Thermal Control Ian Meginnis Power Systems • Find q radiated to space from EP structure • Radiated power: q = AεσT4 = 491W • Hall thruster surface area: A = 3 * (0.25m*0.25m) = 0.188m2 • 3 sides available to radiate power on Hall thruster • For aluminum with white (Z93) paint: Emissivity (ε) = 0.92;Absorptivity (α) = 0.20 • T = max. operating temperature of Hall thruster = 473K

7. Calculation of Hall Thruster Thermal Control(continued) Ian Meginnis Power Systems • Need to dissipate 509W (1000W-491W) • Conduction transports heat from thruster to 2 radiators • Radiation dissipates heat from radiators • Area of radiators: A = q / (εσT4 – α*1300W/m2) = 0.216m2 • q = 509W • T = 473K • α*1300W/m2 = power input from sun • Mass of radiators = 2700kg/m3 * 0.005m * 0.216m2 = 2.92kg