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47th International Conference on Environmental Systems -2017-210 16-20 July 2017, Charleston, South Carolina

Actively Controlled Loop Heat Pipes as a Human Spacecraft External Active Thermal Control System

Eugene K. Ungar1 NASA/Johnson Space Center, Houston, TX, 77058

Jentung Ku2 NASA/Goddard Space Flight Center, Greenbelt, MD, 20771

Actively controlled loop (LHP) systems have been proposed as for human space vehicles. The internal loop would flow through heat exchangers attached to the LHP . By controlling the LHP compensation chamber temperatures, the return temperature of the internal loop coolant could be controlled within the desired setpoint range. The present work is an analytical study of a spacecraft internal cooling loop/LHP system. Different arrangements of the LHPs are explored and a detailed analysis is performed on the configuration with the simplest control scheme. The operation of the control scheme is delineated, the required hardware complexity is explored, and the LHP system is compared qualitatively to the typical pumped external loop system.

Nomenclature CC = compensation chamber CHX = condensing HX = heat exchanger IFHX = interface heat exchanger LHP = loop heat pipe LMTD = logarithmic mean temperature difference TEC = thermoelectric cooler

I. Introduction ECENT United States human spacecraft (Space Shuttle, International Space Station, and Orion) have used dual R loop active thermal control systems1-5. In these systems a non-toxic internal coolant accepts cabin heat loads and transfers the heat to a low freezing point external flow loop where it is rejected to space. It has been proposed to use a system of loop heat pipes (LHPs) to replace the external flow loop. The internal loop would flow through heat exchangers attached to the LHP evaporators. The LHP condensers would be attached to the which would reject the to space. The internal flow loop/LHP system is explored in the present work.

II. Background Two loop active thermal control systems have been used on all US human spacecraft following Apollo. Figure 1 shows a simplified schematic of a two loop system. In these systems, a non-toxic -based fluid flows through an

1 Senior Thermal and Fluids Analyst, Crew and Thermal Systems Division, EC2, 2101 NASA Parkway, Houston TX 77058 2 Aerospace Engineer, Thermal Engineering Branch, Code 545, NASA/GSFC, 8800 Greenbelt Road, Greenbelt, MD 20771

internal thermal control loop to remove cabin heat loads. The internal loop connects with an external loop at an interface heat exchanger (IFHX). The external thermal loop uses a low freezing point fluid that flows through radiators and rejects the spacecraft waste heat to space. A radiator bypass valve controls the external fluid return temperature to maintain the internal fluid supply temperature within its specified limits. An internal bypass valve maintains the fluid to the cabin coldplates above cabin dewpoint. By their design, two loop systems include an external pump, accumulator, temperature control valve, and an interface heat exchanger. If this hardware can be eliminated, the system mass is reduced and its reliability might be increased. It has been proposed to use a system of loop heat pipes (LHPs) as an external thermal control system. LHPs are high capacity, high capillary pumping head devices6,7. They have been used as satellite thermal control systems for many orbiting spacecraft such as ICESat, Aura, OBZOR, Swift, ICESat-2 and GOES-R8-12. A LHP radiator system on a human spacecraft would have no moving parts. The internal loop would flow through individual heat exchangers attached to the LHP evaporators. The LHP condensers would be attached to the radiator. The LHP system could use passively controlled LHPs, but tighter temperature control of the returned fluid can be obtained using active control of the LHP compensation chamber (CC) temperatures. A LHP radiator system could be connected in parallel or in series. A parallel arrangement would have identical inlet and exit temperatures of Figure 1. Two Loop Active Thermal the internal fluid at each loop heat pipe. This would require active control Control System. of all the LHP compensation chambers to maintain setpoint. In addition, all the LHP condensers would be at the same temperature which would be lower than the internal coolant return temperature. A series arrangement would allow the upstream LHPs to operate at higher temperatures which would decrease the required size of the radiator array. The upstream LHPs would operate in an uncontrolled mode to reject the bulk of the waste heat. One LHP at an intermediate position would control the coolant to setpoint while downstream LHPs that were not needed for heat rejection would be turned off. A related system was considered by Van Velson et al.13. They studied a system where the internal fluid flowed through a series of LHPs but used a radiator bypass to control the fluid return temperature. The series arrangement of LHPs with CC temperature control is evaluated here.

III. System Overview A simplified schematic of the series LHP radiator thermal control system is shown in Figure 2. The IFHX and the external coolant loop are replaced by an array of LHPs. The internal coolant flows through a series of heat exchangers attached to the LHP evaporators. A more detailed schematic of the series LHP radiator is shown in Figure 3. Temperature are located at the inlet and exit of each LHP heat exchanger and on the CCs which are heated to control the LHP operation. Condenser heaters prevent the LHP from freezing, allowing the system to survive even under zero heat load. Evaporator heaters are also required for LHP startup as the heat flux provided by the warm internal coolant is not sufficient.

Figure 2. Loop Heat Pipe Radiator Thermal Control System.

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Each LHP is controlled independently using the following algorithm: T T CC evaporator  If the upstream fluid temperature is above the setpoint range the LHP will attempt to control the T T CC evaporator downstream temperature to the setpoint by controlling the CC temperature, T T CC evaporator  If the upstream fluid temperature is within the setpoint range, the LHP T T will be turned off by heating the CC CC evaporator and the condenser heaters will be enabled. T T CC evaporator IV. Control Algorithm For the purposes of this study, the loop T setpoint temperature is taken as 4°C to allow for temperature and control Figure 3. Loop Heat Pipe Radiator. at the cabin heat exchanger. The setpoint control band is taken as ± 0.5°C. So, as long as the return temperature of the fluid remains between 3.5 and 4.5°C, the loop will be considered to be at setpoint. The control algorithm summarized above is detailed in Figure 4. Periodically the control system will check that each LHP is in the correct state. For each LHP :  If the upstream temperature is greater than 4.5°C the control system will;  Turn on the LHP if it is not already operating.  Attempt to control the downstream temperature to be between 3.5 and 4.5°C by 1. turning the CC heater on if the downstream temperature is below 3.5°C, 2. turning off the CC heater if the downstream temperature is above 4.5°C, 3. or maintaining the CC heater state if the temperature is within limits.

Figure 4. Loop Heat Pipe System Control Algorithm. 3 International Conference on Environmental Systems

 If the upstream temperature is less than or equal to 4.5°C the control system will  Turn off the LHP by heating the CC (if the LHP is currently operating) and activate the condenser heaters to prevent freezing.

V. LHP System Analysis An analysis was undertaken to explore the operation of the LHP system. The radiator inlet temperature and relative coolant mass flow rate were calculated over a range of operating conditions assuming:  5 kW maximum total heat load,  30°C maximum coldplate outlet temperature,  4°C setpoint temperature,  16°C mix temperature upstream of the coldplates (exceeding the dewpoint),  Condensing heat exchanger load consisting of; • 250 W heat load, • 860 W sensible plus latent metabolic load, • 10% of the coldplate loads. The CHX and coldplate heat loads were calculated over a total heat load range of 1.25 to 5 kW using the assumptions listed above. At the lowest heat load of 1.25 kW, virtually all of the waste heat is acquired at the CHX. At higher heat loads, most of the waste heat is acquired by the coldplates. The heat load distribution between the coldplates and the cabin heat exchanger over the range of total heat loads is shown in Figure 5. With knowledge of the coldplate heat load, the temperature leaving the coldplates is calculated. This is the radiator inlet temperature which is plotted in Figure 6. The bypass Figure 5. Heat Load Distribution. flow required to maintain the temperature upstream of the coldplates above dewpoint is then calculated. The radiator flow, shown in Figure 7, is the difference between the pump flow and the bypass flow. At low heat load, the internal bypass flow must be increased to maintain the coldplate inlet temperature above the cabin dewpoint. This decreases the radiator flow. With knowledge of the radiator flow rate and inlet temperature, the performance of the LHP radiator system was assessed with the following assumptions:  The logarithmic mean temperature difference (LMTD) from the internal fluid to the radiator at full LHP load was taken to be 10°C. The LMTD was scaled for lower heat loads. This temperature drop Figure 6. Radiator Inlet Temperature. encompassed all the thermal resistances from the spacecraft internal coolant to the LHP saturation temperature.  The radiator area associated with each LHP was taken to have the capability to reject 1 kW. This is a simplifying assumption since the warmer LHPs can actually reject somewhat more heat than the colder LHPs. This also includes an implicit assumption that the environment is constant. In fact, changes in the environment would change the heat rejection capability for each LHP radiator section.

Figure 7. Radiator Flow.

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The internal coolant and LHP temperatures at full load are plotted in Figure 8. The figure shows the internal coolant temperature as it flows through the LHP array. The temperature of each section of the radiator is uniform. The compensation chamber temperatures were calculated at steady-state over a range of heat loads to explore the operation of the system. A chart showing the temperatures of all five LHP CCs as a function of heat load is contained in Figure 9. The graph is most easily understood by examining the performance of a single LHP. Figure 10 shows the compensation chamber temperature for LHP 3. At heat loads of less than Figure 8. Temperatures in the Radiator. 2 kW, the waste heat is rejected by the upstream heat pipes. LHP 3 is not needed and its CC is heated to 6°C to disable operation. At heat loads above 2 kW, the upstream fluid temperature exceeds 4.5°C and LHP 3 becomes operational. Between 2 and 3 kW its CC temperature is adjusted to maintain the downstream fluid temperature at setpoint. The LMTD of the coolant/LHP interface increases as the heat load increases. At lower heat loads, the CC temperature is higher because a lower LMTD required to transfer the smaller amount of waste heat. At higher heat loads, the CC temperature must decrease to accommodate the increased heat load. At system heat loads in excess of 3 kW, LHP 3 is carrying its full 1 kW heat load. The CC temperature is no longer controlled and the LHP temperature floats with the internal coolant temperature.

VI. Discussion The analysis demonstrates that controlling the compensation chamber temperatures of a LHP radiator array would allow the setpoint of the spacecraft coolant to be maintained. The control algorithm has theoretically well-defined state points for each LHP over the range of heat loads.

Figure 9. Compensation Chamber Temperatures over A. Implementation the Range of Heat Loads. The thermal environment of a spacecraft is not constant and can change rapidly if the spacecraft is in a planetary orbit. Also, the system heat loads change over time. To maintain the coolant return temperature within limits during transients, the CC temperature must be able to be changed quickly. biased compensation chambers with heaters would not allow the CC temperatures to decrease rapidly, leading to a temporary loss of setpoint control during transients. The LHPs can respond much more rapidly to changes in the operating condition if the compensation Figure 10. LHP 3 Compensation Chamber Temperature over the Range of chamber temperatures are Heat Loads.

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controlled using thermoelectric coolers (TECs)14-16. However, the incorporation of TECs will make the system more complex. Each CC-mounted TEC will require a thermal strap to connect the CC to the LHP evaporator and a bi-polar power supply to enable both heating and cooling. As the heat load and environment change, the LHP in the array that is controlling the coolant return temperature can shift upstream or downstream. Loop heat pipes can be turned off rapidly by heating the CC, but turning on LHPs will require use of the startup heaters. The new controlling LHP must complete the startup process before it can begin to actively control the coolant temperature. The time needed to complete the startup process will allow the coolant return temperature to temporarily exceed its setpoint limits. Two loop systems must be supplied with waste heat to avoid radiator freezing. Typical ratios of high to low load are 3:1 to 5:117. The condenser heaters allow a loop heat pipe radiator to survive zero heat load as long as power is available. Of course, if heaters are added to the radiator of a two loop system, it would have the same capability.

B. Comparison with Two Loop Systems The mass, reliability, and technological maturity of LHP systems must also be evaluated if they are to be considered as a replacement for two loop systems. LHP systems do not require the pumps, accumulators, flow control valves, and heat exchangers that an external temperature control loop does. For a 5 kW spacecraft such as Orion, deleting these external thermal control system components would save approximately 125 kg18. A typical IFHX in a two loop system has a LMTD of 5°C or less. Because the LHP evaporators have an order of magnitude less area than an IFHX, they have a significantly higher LMTD. In addition, the radiator temperature in a two loop system decreases monotonically with the fluid temperature. A LHP radiator would have the stairstep temperature profile shown in Figure 8. These factors cause a LHP radiator to operate at a lower average radiator temperature and reject less heat per unit area. Assuming an average internal loop temperature of 17°C and a design environment temperature of -60°C, the required radiator area of a LHP system with a LMTD of 10°C is 10% greater than that of a two loop system with an IFHX LMTD of 5°C. This requires an additional 8 kg18 of radiator for a 5 kW spacecraft such as Orion. The 10% increase in radiator area is a separate issue and is important if the real estate available for heat rejection area is limited. The LHP radiator requires reliable sensors and heaters to achieve successful operation. Each loop heat pipe requires an upstream and downstream temperature (although LHPs can share sensors). Each requires a thermoelectric device, a startup heater, and a condenser heater for freeze protection. Successful operation will likely require redundancy in the temperature sensors, TECs, startup heaters, and condenser heaters. Accounting for the additional radiator mass and the mass of the TECs, sensors, heaters, and their wiring, it would be expected that a LHP system would save approximately 100 kg of mass over a two loop system. Two loop systems have 35 years of flight heritage. They have proven to be highly reliable and require limited redundancy for successful operation. Loop heat pipes provide electronics cooling on numerous spacecraft and also have been proven to be highly reliable. However, when integrated into a human spacecraft coolant loop the LHP system has a low technological maturity. A summary comparison of the two systems types is contained in Table 1.

Table 1. LHP System and Two Loop System Comparison.

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VII. Conclusion A feasible concept of operation has been presented for an actively controlled LHP external thermal control system. Controlling the compensation chamber temperatures on the loop heat pipe array would allow the coolant return temperature to be controlled. The loop heat radiator system is lower mass than a traditional two-loop system because it requires no apump, accumulator, interface heat exchanger or control valve. However, it requires a 10% larger radiator, would have transient setpoint excursions following heat load or environment increases, and is of significantly lower technical maturity. These factors must be included in the consideration of LHP radiator systems for human spacecraft.

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