7.0 Thermal Control Subsystem

MARS OR BUST, LLC

1.0 Thermal Control Subsystem

7.1 Requirements

The thermal subsystem is responsible for maintaining all equipment within proper operating temperatures under all mission environmental extremes. This involves collecting heat from or providing heat to all the subsystems and the crew of the habitat. Any excess heat will be dissipated to the Mars environment. The level 2 requirements for the Thermal Control Subsystem are given in Table 7-1 below:

Table 7-1: Level 2 Requirements

Requirement Number Requirement Description Source 7.1 Maintain a heat balance with all subsystems over all Martian Derived temperature extremes 7.2 Keep equipment within operating temperature limits Derived 7.3 Thermal subsystem must be autonomous DRM 3.3.1 7.4 Accommodate transit to Mars Derived 7.5 Auto-deploy and activate if inactive during transit Derived 7.6 Report status for communication to Earth at all times Derived 7.7 Mass shall not exceed 5000 kg Derived 7.8 Thermal protection system shall be provided by the Mars transit Derived vehicle

7.3 Must be autonomous. During un-occupied times and while crewed, the upkeep time of the thermal system must be minimized. This will allow the crew to focus on their mission. 7.5 Must auto-deploy and activate if it is inactive during transit. The habitat will not be occupied upon arrival, and must be able to set up and verify functionality before human crew arrives. The thermal system must auto-deploy and start up operations automatically upon arrival. This will ensure that the equipment has not frozen or melted before crew arrival. 7.7 Mass shall not exceed 5000 kg. DRM requirement was originally 550 kg, but after review of existing thermal system designs for habitats, this requirement was revised upwards to 5000 kg. 7.8 Thermal Protection System (TPS) shall be provided by transit vehicle. The thermal protection system protects the habitat while it enters into the Martian atmosphere. It was determined that this should be part of the transit vehicle. This will reduce the mass of the thermal system (TPS weighs between 800-1000 kg) and simplify the scope of this system for this project.

7.2 Overview of Design Process

The above requirements are derived from the level 1 requirements given by the DRM. From here, the basics of a thermal control system can be established. To begin with, the thermal system must collect all the heat from the various subsystems and crew. This heat must then be transferred to the outside of the habitat without compromising the pressurized structure. A schematic of the basic heat flow can be seen in Figure 7-1. Also

of 12 MARS OR BUST, LLC shown in this flow chart is the power input to the thermal system, the telemetry to the C3 system, and commands from the C3 system.

This heat, once transferred to the outside of the pressurized habitat is then rejected to the atmosphere of Mars. The approximate amount of heat that needs to be rejected to the atmosphere is proportional to the amount of power used by each system of the habitat (Larson, 2000). The peak heat load from the subsystems is approximately 30.75 kW, while the minimum heat load is 6.13 kW. This heat must be rejected over all the environment extremes on Mars.

Three scenarios were created to take these extremes into account. The most important scenario is the HOT- HOT conditions. This scenario occurs on the hottest Martian day, with all subsystems fully powered (at peak), and the highest metabolic load expected from the crew. This scenario will determine the size of the thermal system. The second scenario is the COLD-COLD conditions. This occurs on the coldest Martian day, with the minimal required equipment powered, and no crew in the habitat. This scenario determines which systems may need heaters to keep their equipment within operating limits. The final scenario is the transit phase. This occurs while the spacecraft is on the journey to Mars. The vacuum of space and the very cold temperatures are the controlling variables. The habitat’s radiators will not be used during this time. Instead, a heat exchanger will be connected to the transit vehicle system. It is assumed that this system has its own thermal system designed to accommodate the habitat.

Mars Environment ISRU Plant Legend Oxygen Nitrogen

CO2 Robotics/ Cabin Air Structures Trace Contam. Automation

H20 Vapor H2 Potable H 20 Non-Pot. H 20 Solid Waste Liquid Waste ISRU Food Thermal Telemetry Video Audio ECLSS Packet. Data TCP/IP C3 Command Heat Power EVAS Electric. Power

Nuclear Reactor Crew Crew Accommodations Mars Com Satellites Habitat Boundary

Thermal Subsystem Input/Output Diagram

Figure 7-1: Input/Output Diagram

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Using the basic scenarios and the input/output diagram of heat, a more detailed diagram of the thermal system was derived. This takes into account all the subsystems of the habitat, the various components of the thermal subsystem, and the structure of the habitat. The system is shown in Figure 7-2. There are five main components of the thermal subsystem. They are as follows: cold plates, fluid loops and piping, pumps, heat exchangers, and the radiators. The first component, the cold plates, collects heat from a source. This is then transferred into the internal fluid loop and pumped to a heat exchanger. This transfers heat into the external fluid loop, which is then cycled through the radiators and the heat rejected into the environment. As seen in the system schematic, there are cold plates attached to the avionics of the various subsystems of the habitat. Each plate receives cold fluid, controlled to be a specific temperature. From here, the fluid flows through the pumps and out to the heat exchangers. Here it is transferred across to the external fluid loops. There are four loops to provide redundancy. Fluid is pumped through the radiators, cooling it. The cold fluid then passes over the heat exchanger again, cooling the internal fluid loop.

Figure 7-2: Thermal System Schematic

7.4 Thermal Subsystem Design

From the functional diagram shown above, the design of each subsystem begins. The total heat load that the thermal system must be designed to handle is a function of the degradation and safety factor. Over time, the thermal system will lose capacity. An approximation of 1% per year was assumed, for a total of 15 years. Thus, the degradation increased the system size by 15%. A safety factor of 1.1 was also included. In table 7- 1, the breakdown of heat loads is shown. The ECLSS electronics produce 9.6 kW of heat, while the ECLSS air/heat exchanger produces 10.85 kW. This 10.85 kW includes the 3.5 kW from the humans, while the remainder is from Crew Accommodations equipment that vents heat to the habitat atmosphere. The remaining Crew Accommodations equipment either required a cold plate or uses a heat exchanger to cool water coming from the shower, laundry machine, etc. The structures heat gains/losses have yet to be determined. From Table 7-2 the total heat load is thus 30.75 kW * 1.15 * 1.1, or 39 kW. This was used to size the system.

7.4.1 Assumptions 1. Thermal heat load to be rejected is the same as the power load used by the habitat. 2. The highest human metabolic output for six crew members is 3.5 kW. 3. The safety factor and degradation factors are 1.1 and 1.15 respectively. 4. The TPS will be part of the transit vehicle. 5. The cabin air heat exchanger will be part of the ECLSS system.

7.4.2 Heat Collection The electronics that are used to run, control, and monitor the habitat produce a large amount of heat. To collect this heat and transfer it to the fluid loop and eventually to the radiators, cold plates are employed. A cold plate is a metal plate that is attached to the electronics that has cold fluid circulating through it. As it passes through, it collects the heat through conduction from the metal. The now hot fluid passes out and back into the system to be taken and transferred out of the habitat.

For a habitat, the size of a heat exchanger is a function of the heat exchanger’s capacity (Larson, 2000). The mass in kg is given as 17 + 0.25*capacity in kW. The volume in m3 is given as 0.016 + 0.0012*capacity in kW. A cold plate can be similarly sized with the mass in kg as 12*capacity in kW and volume in m3 as 0.028*capacity in kW. For each subsystem’s electronics, a cold plate is required. A summary of thermal component sizing can be found in Table 7-3.

Table 7-2: Thermal Systems Heat Loads HOT COLD Heat Load Heat Load (kW) (kW) ECLSS 9.6 3.53 ECLSS Air/HE 10.85 0 CCC 1.9 1.3 EVAS 0 0 Robotics & Auto 2 0 Mission Ops 0 0 Crew Accommodation 4.4 0

Thermal 2 1.3 Structures TBD TBD TOTAL 30.75 6.13

7.4.3 Heat Transfer After the heat is collected by the cold plates and sent into the system, it is then transferred to the exterior fluid loop. To do this, a heat exchanger is employed, along with pumps to move the fluid through the pipes and valves. A heat exchanger uses two fluids flowing past each other (separated by pipes) to transfer heat (Janna, 1998). Thus, the interior fluid warms the exterior fluid. The heat then can be rejected to the environment. Pumps can be sized with the mass in kg as 4.8*loop capacity in kW, the power required in watts as 23*loop capacity in kW, and the volume in m3 as 0.017*loop capacity in kW. Plumbing and valves add 15% to the total active system mass, while fluids add 5%. For the interior loop of the thermal system, water was chosen. For safety reasons, if the interior fluid loop were to leak, a non-toxic fluid will not harm the crew. Additionally, water has a high heat capacity. However, water cannot be used in the exterior fluid loop as it will freeze. After examining a few possible fluids to use, ammonia was chosen. It is easier to pump by a factor of 6 to 16 than hep t a n e , to l u l o l , and f r e on ( Messerschmid, 1999) . I t al s o has a highe r hea t capac i t y than ei t h e r of tho s e f l u i d s . The total mass, power, and volume of the heat transfer components can be seen in table 7-2.

7.4.4 Heat Rejection After the exterior fluid passes through the heat exchanger, it is pumped out to the radiators. The radiators are coated with a highly emissive coating. As the radiators warm up, they begin radiating heat into the environment. For radiators to work well, they must operate at a temperature significantly higher than the environment. An operation temperature of 290 K was chosen to accomplish this. This was based on the hottest day on Mars being about 263 K. The first step in determining the radiator size is to determine the area of the radiators required to dissipate the 39 kW of heat. Using equation 7-1, an area of 391.9 m2 is required

(Cengel, 1998).- Q A = 4 4 [7-1] s eh(Tr - Tr ) Where:  is the Stefan-Boltzman constant, 5.67e-8 W/(m2K4),  is the radiator efficiency = 0.85,  is the emissivity of the radiator = 0.9, Tr is the radiator operating temperature = 290 K, and Te is the environment temperature = 263 K (Larson, 2000).

The mass and volume of the radiators can be found from the area required to dissipate this heat. Two-sided deployable radiator panels were chosen. This was for two reasons, one being the requirement that the habitat be autonomous and self-deploy and activate upon arrives, the second being to reduce the mass and actual area of the panels. A two-sided deployable radiator panel has a mass in kg given by 8.5 kg/m2. Its volume is 0.06 m3/m2. The total mass, power, and volume of the radiator components can be seen in Table 7-3. Table 7-3: Thermal System Mass, Power, and Volume Hot Hot Cold Cold

Mass Volume (kg) (m^3) Power (W) Power (W) Radiators 3330.9 23.5 0.0 0.0 Heat Exchangers 80.2 0.2 0.0 0.0 Pumps 1120.3 2.5 1789.3 1241.4 Cold Plates 444.5 1.0 0.0 0.0 Page 5 of 12 MARS OR BUST, LLC

Instruments 248.8 Plumbing and Valves 746.4 Fluids 248.8 0.0 0.0 Totals 6219.8 27.2 1789.3 1241.4

7.4.5 Instrumentation Temperature sensors will be used to measure the inlet and outlet fluid temperatures of each cold plate, heat exchanger, and radiator panel. Flow rate sensors will also be placed at the inlet and outlet of each pump, the outlet of each cold plate, and the outlet of each radiator. External cameras will be used to monitor dust levels on the radiator panels. Instruments add approximately 5% to the active thermal system mass.

7.4.6 Operations Most thermal subsystem tasks are automated. Crew intervention will only be required for periodic equipment inspection and maintenance and recovery from emergency conditions. Radiator deployment and repositioning will be handled by the robotics and automation subsystem. Refer to Table X.X in the mission operations subsection for a detailed breakdown of anticipated operational requirements.

7.5 Verification of Requirements The following paragraphs detail how the thermal control subsystem design addresses the requirements described in the overview. The information presented below is summarized in Table 7-4. The thermal control subsystem design meets most of our requirements at a functional level. Two areas that need special attention during the next design iteration are improving heat load estimates and finding ways to reduce mass. Improved heat loads, especially the cold-cold scenario and the structural heat balance, will result in more accurate subsystem sizing and architecture. The total system mass overrun should be evaluated to determine if mass savings are possible through the use of alternative designs or technologies or if the thermal control subsystem’s mass allocation needs revision.

Table 7-4: Requirement Verification Summary

Report No. Requirement Description Design 7.1 Maintain a heat balance with all subsystems over all Sized thermal subsystem based on maximum anticipated heat load, Martian temperature extremes considered minimum heat load 7.2 Keep equipment within operating temperature limits Cold plates gather heat from equipment for transport and dissipation

7.3 Thermal subsystem must be autonomous Telemetry and command interface with C3 subsystem 7.4 Accommodate transit to Mars Internal fluid loop collects heat during transit and transfers the heat to the Mars transit vehicle for dissipation 7.5 Auto-deploy and activate if inactive during transit Radiators will auto deploy or be deployed with rovers, external fluid loop pumps will be activated by C3, other equipment operates during transit 7.6 Report status for communication to Earth at all times Design includes sensors that are monitored by the C3 subsystem

7.7 Mass shall not exceed 5000 kg Approximate thermal subsystem mass is 6,220 kg 7.8 Thermal protection system shall be provided by the Mars Design does not include the thermal protection system transit vehicle

7.1 Must maintain a heat balance with all subsystems over all Martian temperature extremes. The largest anticipated heat load was used to size the thermal control subsystem. This maximum heat load included the maximum anticipated heat output from the habitat’s subsystems and the maximum amount of

of 12 MARS OR BUST, LLC metabolic heat generated by 6 crewmembers. The structural parameters necessary to estimate heat exchange between the Martian environment and the habitat were not available. Heat exchange with the environment is expected to be small compared with the other heat loads. The thermal team is not certain of the direction of heat flow with the environment because it will depend on the thermal properties of the habitat’s final structural materials, the number and placement of windows, airlocks and hatches and the habitat’s location on the planetary surface. In order to account for the environmental heat load and equipment degradation, the maximum heat load was multiplied by a safety factor of 1.1 to obtain the final maximum heat load for thermal subsystem sizing. The thermal control subsystem will only be able to maintain the habitat’s heat balance if the heat load does not exceed the maximum calculated heat load.

The cold-cold situation was not thoroughly analyzed because the thermal team did not receive detailed information concerning the minimum heat load until very late in the design process. Electrical equipment will generate heat during all mission phases. More research is required in this area, but the thermal team believes that the heat generation and retention during the coldest operating conditions will keep the habitat’s temperature above 4° C. Heaters for equipment that cannot handle temperatures down to 4° C should be included in the subsystem designs.

7.2 Must keep equipment temperatures within operating limits. The thermal control subsystem design provides heat dissipation to handle the maximum anticipated heat load as described in requirement 7.1. Additionally, cold plates have been allocated for the subsystems to prevent equipment from overheating. Heating requirements for equipment that cannot handle a 4° C minimum temperature should be handled within the subsystem designs.

7.3 Must be autonomous. During regular operations, the thermal control subsystem functions autonomously. The thermal control subsystem works with the C3 subsystem to automatically maintain the habitat’s thermal environment. The thermal control subsystem sends sensor data to the C3 subsystem for monitoring. In response, the thermal control subsystem receives and acts on commands from the C3 subsystem. Any problems with the thermal control subsystem will be reported through the C3 subsystem’s caution and warning functionality. Human intervention will be required for periodic equipment maintenance and inspection as well as during emergency conditions.

7.4 Must accommodate the transit to Mars. The habitat’s thermal control subsystem will operate during transit to Mars. During this time, the habitat’s thermal control subsystem will collect heat generated by operational equipment and transfer it to the transport vehicle’s thermal control subsystem. The transport vehicle’s thermal control subsystem must accept and dissipate the habitat’s heat load. The thermal team does not have a good estimate of the transit heat load at this time. Any heating required during transit should be handled in the subsystem designs.

7.5 Must auto-deploy and activate if it is inactive during transit. Everything except the radiators and the external fluid loops will operate during transit. The radiators will either auto deploy or be deployed by rovers. The external fluid loops will not require set-up after landing. Radiator and external fluid loop activation will occur when the external fluid loop pumps are activated by the C3 subsystem either automatically or in response to a command transmitted from Earth.

7.6 Must report its status for communication to Earth at all times.

Sensors are included in the thermal control subsystem design. Sensors will report the thermal control subsystem’s status to C3 for monitoring and periodic transmission to Earth. The C3 subsystem will handle all communications with Earth.

7.7 Mass shall not exceed 5000 kg. The thermal control subsystem mass is estimated at 6220 kg, about 1200 kg more than the revised requirement.

7.8 Thermal protection system shall be provided by the Mars transit vehicle. A thermal protection system is not included in the thermal control subsystem design as it is assumed to be provided by the transit vehicle system.

7.6 Failure Modes Effects Analysis

The primary failure that can potentially occur in the thermal subsystem is insufficient cooling of the radiators, from which four different possibilities can occur. The four possibilities are insufficient flow rate, cooling plate failure, instrumentation failure and heat exchanger failure.

The first major problem is insufficient flow rate, which can arise from pump failure, valve failure and pipe failure. Pump failure can occur from insufficient power of clogged or broken pump. The next is pipe failure with can occur from leaks in the pipes, clogged pipes from residue buildup, or frozen pipes from insufficient heat. The next is valve failure, which can occur from a jam, cracks, insufficient sealing, a valve open failure, or a valve closed failure. The second major problem is cooling plate failure, which can arise from leaks in the system, clogs in the piping, overheating, or frozen lines in the plate. The third major problem instrumentation failure can be caused by faulty readout or complete failure of 1 or more sensors. The fourth major problem is heat exchanger failure, which could be caused by clogs in the piping leaks, or insufficient heating. A broken heater or a broken thermal control valve can cause insufficient heating. A breakdown of the FMEA is shown in Figure 7-3.

InsufficientInsufficient CoolingCooling

InsufficientInsufficient CoolingCooling Plate Plate InstrumentationInstrumentation HeatHeat Exchanger Exchanger FlowFlow rate rate FailureFailure FailureFailure FailureFailure

FaultyFaulty PumpPump Failure Failure PipePipe FailureFailure ValveValve FailureFailure LeakLeak CloggedClogged ReadoutReadout

InsufficientInsufficient CompleteComplete FailureFailure LeakLeak JammedJammed CloggedClogged LeakLeak PowerPower ofof Sensor Sensor

CloggedClogged Pump Pump CloggedClogged CrackedCracked OverheatedOverheated InsufficientInsufficient HeatHeat

NotNot Sealed Sealed FrozenFrozen FrozenFrozen (Failed(Failed Open)Open)

InsufficientInsufficient InsufficientInsufficient FailedFailed ClosedClosed HeatHeat HeatHeat

Figure 7-3: FMEA Breakdown

7.7 Technologies Here is a brief introduction into some of the technologies that are available today or in the future that may be used in the final thermal system design.

7.7.1 Heat Exchangers Currently, research is being conducted towards developing heat exchanger tubing made of oxide dispersion strengthened alloys (ODS) with enough circumferential creep strength for long term use as heat exchanger tubing in high temperature extremes. The researchers also plan to produce adequate connecting joints for the tubing, establish bending strain limits, establish high temperature corrosion limits and generate data for heat exchanger designers to use. This program will consist of a team effort between one material-producing company, one boiler manufacturing company, one national laboratory, two universities, and one non-profit welding research organization. The successful outcome of this project will result in innovative developments that allow the reliable use of ODS alloys for heat exchanger tubing, as well as a variety of applications previously not possible with metallic materials. The high performance heat exchangers to be developed in the years to come may be necessary to heighten energy conversion efficiencies and reduce maintenance requirements. Both of these design attributes will be essential throughout the mission lifetime of the Mars habitat (United States Department of Energy, 2000).

7.7.2 Cold plates

The depicted bimetallic cold plate (below) transfers heat from electrical power equipment to the external ammonia cooling loop by radiation through intermeshing aluminum fins. This technology is currently available, and a similar device may be implemented on the Mars habitat. The Active Thermal Control System DDCU cold plate absorbs heat from the DC to DC Converter Unit (DDCU) electronics on the ISS truss. The ATCS Main Bus Switching Unit (MBSU) cold plate accepts heat from the electronics on the ISS truss. Both cold plates contain a brazed stainless steel ammonia flow path, which is adhesively bonded to an aluminum fin array. The aluminum fin array interfaces with a mating fin array on the electronics heat source plate, providing a large fin surface area to accept the electronics heat via radiation. This provides a predictable and repeatable thermal interface in the vacuum of space, also an important factor should a repairable failure occur and the system is briefly opened to the Mars environment (Hamilton Sundstrand, 2003).

Figure 1: A bimetallic cold plate produced by Hamilton Sundstrand

7.7.3 Radiator Panels The Heat Pipe Radiator System onboard the International Space Station was developed by Swales, Inc. and is among the most technologically advanced of its type in the field. The purpose of this radiator system is the rejection of heat from various electronics modules located in the center segment of the ISS truss structure. Specifically, the system consists of one central heat pipe radiator, fourteen transport pipes, and seven equipment plates, each a machined aluminum plate structure with four or six heat pipes (Swales, 2003).

The Loop Heat Pipe Deployable Radiator for GEO spacecraft was developed for Space Systems / Loral. Its purpose is to augment spacecraft heat rejection capability by adding deployable radiators that interface with the exterior of the body–mounted heat pipe radiators. These were some of the deployable radiators studied for theoretical integration into the habitat (Swales, 2003).

Figure 2: One radiator used aboard the International Space Station

7.7.4 Liquids Liquid ammonia was chosen over several other several other common heat exchanger liquid solutions primarily from heritage concerns. It is currently being used onboard spacecraft and on the International Space Station. Liquid ammonium also has the lowest fouling rate 0.2286 J m K/s of any solution currently used in heat exchanger processes (Process Associates of America, 2003).

7.8 Future Considerations There are many more aspects of the thermal system and habitat that must be examined in greater detail. Some of the major hurdles to be examined are the radiator panel mass, heat rejection method, power system thermal loads, structures thermal loads, and landing site environments.

7.8.1 Radiator Panel Mass The radiators weigh in at 3331 kg, well over half the mass of the thermal system. New technologies are needed for deployable radiations in a gravity environment are needed. Optimization of the radiator design will also reduce the mass of this component.

7.8.2 Heat Rejection Method Although radiation was the only heat method examined here, there are many other options, which may greatly reduce the mass of the thermal system. The convection of heat by the Martian atmosphere from the structure could possible be very large on windy days and will reduce the need for large radiators. Possibly using conduction to the soil of Mars could reduce mass. The day and night cycle of Mars may allow for the use of a heat sink inside the structure. This would store heat produced during the day and then release it at night when the radiator panels would be much more effective. A smaller panel would then be required. Finally, possibly phase change material could be used. During the night, the material would freeze. Throughout the day, heat from the habitat would slowly melt the material. Then again at night, the material would be frozen again.

7.8.3 Liquid Solutions Pure liquid forms are rarely used in most liquid-based heat exchanger systems. Similar to vehicular radiator loops, solutions of various liquids are typically used. Switching from a pure ammonia loop to the more common ammonia/water solution may save weight and still remain above the freezing point. An in-depth analysis of chemical properties and solution percentages needs to occur before implementing the change. Page 11 of 12 MARS OR BUST, LLC

7.9 References

Cengel, Y. Heat Transfer: A Practical Approach. McGraw Hill, Inc., Nightstawn, NJ: 1998.

Hamilton Sundstrand. Space/Planetary Habitat Active Thermal Control System Cold Plates. 2003. 1 December 2003 < http://www.hsssi.com/Applications/SpaceHabitat/Cold_Plate.html>.

Janna, W. Design of Fluid Thermal Systems. PWS Publishing Company, New York, NY: 1998.

Larson, W. and Pranke, L. Human Spaceflight: Mission Analysis and Design. The McGraw-Hill Companies, Inc., New York, NY: 2000.

Messerschmid, E. and Bertrand, R. Space Stations: Systems and Utilization. Springer-Verlag, Berlin: 1999.

Process Associates of America. Typical Heat Exchanger Fouling Resistances. 2003. 1 December 2003 .

Swales Aerospace. ISS Thermal Management Systems. 2003. 1 December 2003 .

United States Department of Energy. DOE Takes 2nd Step Toward Ultra-Clean Energy Plant. 3 August 2000. 10 December 2003 .