FORCED AIR CONVECTION THERMAL SWITCH CONCEPT FOR RESPONSIVE SPACE MISSIONS 6 August 26 2 th AIAA/USU Small Sat Conference Andrew D. Williams Air Force Research Laboratory, Space Vehicles Directorate Kirtland AFB, NM Dr. Scott E. Palo University of Colorado, Department of Aerospace Engineering Sciences
Overview The Problem in a Nutshell Study Approach Forced Air Convection Thermal Switch Concept Hot Case Design Cold Case Design Summary of Subsystem Design Future Work
The Problem in a Nutshell Challenges - No clear ORS definition - Requirements vacuum Orbits? Payloads? Components? Pre-built satellites in a warehouse - Short tactical timelines Trade-offs - Mass is no longer THE driver - Time is the primary driver - Cost is the secondary driver Things we do know - Relegated to smallish satellites <4 kg - Must be fast and affordable Modular satellites How do you design a TCS under these parameters?
Study Approach Some system detail is required to begin TCS design Current ORS limitation is to 4 kg satellites Used this limitation to determine what capabilities a small satellite can provide - Developed a baseline low capability bus (LCB) - Developed a much higher capability bus (HCB) - LCB and HCB represent the upper and lower design bounds Evaluated all circular LEO orbits to determine the absolute worst hot and cold environments Literature search to investigate relevant technologies Developed busy, subsystem, and component models to compare passive conduction based approach to Forced Air Convection Thermal Switch (FACTS) approach
Summary of Bus Capabilities Subsystem Capability Nominal Power Size Low capability bus [kg] [W] [cm] Minimum capability required Qty Mass ADC -5 attitude control.3 8.5 3 x 24 x 2 TTC Mbs, S-band transmitter 2.8 7.4 9.8 x 9.6 x 7.2 NG 2 channel GPS receiver.2.8 7. x 4.5 x. CDH Plug-n-play USB architecture 5.2 5 34 x 25 x 2 PM 5 W, 3J array, PPT system 8.3 7.3 25 x 23 x 2 Structure Al Honeycomb Panels 2.5 n/a 27 x 4.5 x 7 xx 68. 47. 27 x 4.5 x 7 Propulsion Not applicable High capability system Higher capability system for more demanding missions Not the end all be all solution but the 8% solution Represents the upper bound of the design regime Subsystem for broad reaching utility Baseline entry level position Represents the lower bound of the design regime Capability Nominal Power Size [kg] [W] [cm] Qty Mass ADC. - attitude control 23.3 49.5 35 x 35 x 22 TTC 274 Mbs, Ku-band transmitter.6 64.4 25 x 25 x 5 NG 2 channel GPS receiver..8 7. x 4.5 x. CDH Plug-n-play USB architecture 5.2 5 34 x 25 x 2 PM 5 W, 3J array, PPT system 54.6 253 72 x 23 x 2 Structure Al Honeycomb Panels 38.6 n/a 52 x 4.5 x 7 xx 42 47.7 52 x 4.5 x 7 Propulsion Not applicable
FACTS Concept Forced Air Convection Thermal Switch (FACTS) Concept. Subsystems separated into hermetically sealed enclosures - Easy to swap out subsystems to provide different capabilities - Subsystem enclosure is isolated from base plate - Subsystems are pre-engineered - Short timeline thermal design is to balance subsystem heat loads at the subsystem enclosure level and not all of the components ADC PM C&DH C&DH ADC NG TTC PM2 NG TTC PM
FACTS Concept Forced Air Convection Thermal Switch (FACTS) Concept 2. Add a DC axial fan and a finned heat exchanger (HX) to the system - Fan is used to vary the heat transfer rates between the hot and cold cases - Finned HX significantly increases the heat transfer rate - Thermally isolated hermetic seal to minimize conduction through the - enclosure to the bus structure Minimize radiation by using low emissivity surface coatings Enclosure Legacy Interface Board Interstitial Material Processor Board Mounting Flange Finned HX Base Plate
Pros and Cons Advantages - Heat switch operation w/o the problems of conduction heat switches - Very lightweight; good reliability on the fan for short lifetimes - Higher heat transfer rates than conduction alone - Sealed enclosure reduces clean room requirements in depot environment - Thermal joint requirements are reduced - Modular approach; simply swap subsystems Disadvantages - Multiple single point failure - locations Added enclosure mass required to contain 5 psi internal pressure Nitrogen compatible hermetic seal Increased power requirements Adds complexity to the ADC
Hot Case Design Effect that Adding a Finned Heat Exchanger has on the Convection Coefficient Convection Coefficient [W/m^2-K] CDH Finned Heat Exchanger CDH Bare Aluminum Base Plate PM Bare Aluminum Base Plate 5 5 2 25 3 Flow Rate [CFM] Two rows of cm tall fins with a spacing of.5 cm Heat exchanger pushes flow into the turbulent regime 35 4
Hot Case Design The Effect that Adding a Finned Heat Exchanger to the Base Plate has on the Maximum Temperature of the CDH 42 With Finned Heat Exchanger Bare Aluminum Base Plate Maximum Temperature [K] 4 38 36 34 32 3 28 5 5 2 25 3 35 4 Flow Rate [CFM] Point of diminishing returns on the system Between and 5 CFM the heat transfer rate of the finned HX exceeds that of the base plate
Adding Fins to the Enclosure Heat transfer from the components can be increased by adding fins to the other walls of the enclosure - Takes advantage of conduction from the components to the enclosure - Additional fins increase the heat transfer surface area - Reduces importance of component thermal design 37 Finned Heat Exchanger on Base Plate Only Additional Fins on Other Enclosure Walls Maximum Temperature [K] 36 35 34 33 32 3 3 5 5 2 Flow Rate [CFM] 25 3 35 4
Cold Case Design Isolate enclosure - Maximizes switching - Minimizes heater power Interface conductivity - 5 W/m2-K better - W/m2-K adequate Joint configuration - O-ring or Teflon energized hermetic seal - Felt interstitial material - 5 mm Teflon spacer - Isolate bolts with Teflon sleeves and washers Enclosure Mounting Flange Interstitial Material Contact Resistance Material Resistance R= R= Base Plate Contact Resistance Subsystem LCB ADC CDH PM TTC HCB ADC CDH PM TTC AK int R= L AK AK int Heat Load [W] Surface Area [m^2] Power Joint Density Conductivity [W/m^2] [W/m^2-K] 8.5 3. 6.2 7.4.68.236.84.672.9 55.85 88.43.9 2.8 6.4.24 2.8 8.5 3 4.2 7.4.228.236.372.6 8.4 55.85 7.53 462.5 9.43 6.4 2.88 5.38
Subsystem Design Using FACTS The optimal flow rate is 5 CFM - Flow rate can be increased to 4 CFM to maximize heat transfer - Better approach is to reduce the enclosure interface temperature Bare aluminum enclosures used; heater power can be added to subsystems that are at the edge of the lower temperature limit Conservative conductance ratio of 69: using FACTS - Conservative because different contact areas between on and off - Actual performance is dependent on the base plate area Parameter Variable Value Units 5.8 FHE Area Multiplier CFHE Convection Coefficient at 5 CFM h 57.8 W/m^2-K Convection Coefficient at 4 CFM h 236 W/m^2-K Base Plate Thickness L.635 m Conductivity of Aluminum Kal 85 W/m-K KJ 435 W/m^2-K Base Plate Interface Conductivty
Future Work Design, prototype, and test a thermally isolative dry nitrogen hermetic seal The primary requirements for the seal are: - Provide a joint conductivity of 5 W/m2-K - Provide a hermetic seal - Maintain an internal pressure of atm in a vacuum - Be compatible with dry nitrogen - Survive the space environment for an absolute minimum duration of one month without significant degradation or failure Investigate lightweight enclosure designs that are capable of withstanding an internal pressure of atm in a vacuum with minimal deflection Build a prototype/mock CDH subsystem assembly and test it in a thermal vacuum chamber to validate thermal model
Questions???