7.7 Technologies

Frances Zhu

7. Thermal Control

7.7 Technologies

There are various heat transfer technologies that can be used to implement conduction, radiation, and even convection. We’ll survey mechanisms used in spacecraft to achieve these thermal interactions.

Highlighted Thermal Design Components of the New Horizon’s Spacecraft. By the end of this section, you should be able to recognize these various components and understand their function in the system. Image by Matt C. Bergman.

New Horizons Spacecraft System Diagram. Image by Matt C. Bergman.

Suggested Reading

New Horizons Thermal Control System

Passive

Passive components do not consume any electricity or need any control logic to achieve their full function. We’ll review different passive heat transfer technologies, common use cases, and how to select a particular technology per your use.

Materials and Coatings

Selecting structural materials or coating surfaces varies the ratio of absorptivity and emissivity $\tfrac{\alpha_{visible}}{\epsilon_{infrared}}$ , which toggles the equilibrium temperature of the spacecraft. We described this phenomenon in the Radiation subsection of the Fundamentals of Heat Transfer section but here it is again for convenience:

Material	$\alpha(\lambda_{visible})$	$\epsilon(\lambda_{infrared})$	Ratio
Aluminum	0.09	0.03	3.00
White paint	0.2	0.92	0.22
Black paint	0.92	0.89	1.03
Silver Teflon	0.08	0.8	0.1
Aluminized Kapton	0.38	0.67	0.56

These materials can be leveraged to achieve general warming or cooling effects. A ratio greater than 1 promotes warming and a ratio less than 1 promotes cooling. Some common strategies:

Use Silver Teflon to minimize solar absorption but max emissions (e.g., for telescope mirrors)

The multiple light paths are taken by radiation impinging on a second-surface mirror oriented at an angle ν to the incoming solar radiation. By Youngquist. Image courtesy of NASA.

Use black paint to maximize energy transfer, both absorption and emission (interior)

Black heat shield on ESA’s Solar Orbiter at IABG in Ottobrunn, Germany in October 2019. ESA – S. Corvaja. Image courtesy of ESA.

Summary of properties of selected commercial black thermal control paints. Image courtesy of Acktar.

Use metals (e.g., Aluminum) to minimize both absorption and emission (instrument sun shield)

Technicians at Astrotech Space Operations in Titusville, Fla., work on the backside of the MESSENGER spacecraft, mating it with the Payload Assist Module, the Boeing Delta II third stage, below. The white panel seen here is the heat-resistant, ceramic cloth sunshade that will enable MESSENGER to operate at room temperature. Image by NASA.

White paint: State-of-the-art research conducted by Youngquist at Kennedy Space Center uses a very fine NaCl (Salt) powder as a thin coating, called “Solar White”, to theoretically reach temperatures as low as 50 Kelvin even in direct sunlight. Previously, one only thought that cryocoolers (active cooling systems) could reach cryo temperatures but this method is completely passive. The downside is that “Solar White” is in the early phases of transitioning from research to broad use.

Optical testing with NaCl. The image on the left shows finely ground NaCl, along with a 20 mm diameter, 1 mm-thick pressed disk of NaCl powder. The image on the right shows the ability to reflect this thin layer of NaCl powder. Image courtesy of NASA.
Combine these surfaces to get almost any $\tfrac{\alpha(lambda_{visible})}{\epsilon(lambda_{infrared})}$ you want

Suggested Reading

Cryogenic Selective Surfaces

Heat Shunts and Straps

Heat shunts, heat straps, heat sinks, and radiators are all passive methods to displace heat from one location to another. The aforementioned list of components ranges from smallest to largest in size.

Where it is preferable to deliver heat to the air, parts with higher internal thermal impedance should be used. One example is OARS-XP series (figure 5) in which the element is lifted away from the PCB. The corresponding thermal image (figure 6) shows that the solder joints remain at around 110°C even when the element temperature exceeds 200°C. This format will minimize heating of the PCB and can better take advantage of forced air cooling is provided. Image by Electro Pages.

“Electrically insulating but thermally conductive “heat shunt” components are attached to PC boards along with regular electronic components to improve heat dissipation. The heat shunts are typically a small bar of thermally conductive ceramic with spaced-apart metal mounting pads on the ends for soldering to the PC board. Their shape is similar to standard electronic components for placement by automatic machinery and they extend, for example, from a transistor collector contact pad on the PC board to an adjacent ground lead having holes plated through to the metal backplane of the PC board in contact with the heat sink” [US Patent 4941067].

4C-PURE OFHC Copper Thermal Straps. Image by Thermal Space.

“Thermal straps are often used when heat (thermal energy) needs to be transferred and a large temperature gradient cannot be tolerated between two or more discrete locations (interfaces) that are either:

positionally fixed but not well defined relative to each other
positionally fixed and well defined relative to each other, but it is necessary to decouple transported mechanical energy (vibration, shock)
not always positionally fixed relative to one another over time

Wide variety of potential applications for thermal straps in-ground, aeronautical, and aerospace systems. Image by Thermal Space.

Thermal straps in various forms, from simple or crudely constructed linkages to high-precision components with well-defined interfaces, have been used to manage heat flow in many applications from the commercial electronics and automotive industries to our most advanced scientific, military, defense, and space systems. Sometimes a thermal strap may be as basic as a coiled tube, bunch of wires, or stack of foils without a solid attachment terminal. In other configurations, the terminal or end fitting is welded, brazed, soldered, bonded, or otherwise attached to the flexible section of the strap to provide a more direct and robust mounting capability. The appropriate configuration for any thermal strap really depends upon what thermal management system it is going to be integrated with” [Thermal-Space].

Room temperature thermal conductivities of typical thermal strap materials. Image courtesy of Thermal Space.

Heat shunts on PCBs are like heat straps that stay on the board. Heat straps are a larger component that can transfer heat through conduction from hot components on a board to the primary structure, like the aluminum spacecraft frame. Heat straps differ from heat sinks as heat straps transfer heat and do not absorb or radiate heat away.

Heat Pipes

“A heat pipe is a heat-transfer device that combines the principles of both thermal conductivity and phase transition to effectively transfer heat between two solid interfaces. At the hot interface of a heat pipe, a liquid in contact with a thermally conductive solid surface turns into a vapor by absorbing heat from that surface. The vapor then travels along the heat pipe to the cold interface and condenses back into a liquid, releasing the latent heat. The liquid then returns to the hot interface through either capillary action, centrifugal force, or gravity, and the cycle repeats” [Wikipedia].

Thermacore copper water heat pipes cooling satellite board. Heat Pipes for Space Applications by Dr. Xiao Yang.

“Heat pipes and loop heat pipes are used extensively in spacecraft, since they don’t require any power to operate, operate nearly isothermally, and can transport heat over long distances. Ammonia is the most common working fluid for spacecraft heat pipes. Ethane is used when the heat pipe must operate at temperatures below the ammonia freezing temperature” [Wikipedia].

Iber Espacio’s Superficial Heat Pipes can be formed into Axial Grooved Heat Pipes and Arterial Heat Pipes. Image by Iber Espacio.

Heat Sinks and Radiators

We eliminate the unwanted heat that is given out by these electronic devices through heatsinks, which operate through conduction and radiation. What I Learned About Heatsinks Using Thermal Simulation by MENTOR, A Siemens business.

“A heat sink is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is dissipated away from the device, thereby allowing regulation of the device’s temperature. A heat sink is designed to maximize its surface area in contact with the cooling medium surrounding it”; in space, the medium is the space environment. “Heat sink attachment methods and thermal interface materials also affect the die temperature of the integrated circuit. Thermal adhesive or thermal paste improves the heat sink’s performance by filling air gaps between the heat sink and the heat spreader on the device. A heat sink is usually made out of aluminum or copper” [Wikipedia].

A fan-cooled heat sink on the processor of a personal computer. To the right is a smaller heat sink cooling another integrated circuit of the motherboard. GFDL 1.2. Image by Flag Staff Fotos.

Heat sinks are evaluated upon the following design factors: thermal resistance, material, fin arrangements, conductive connection, and surface color. Thermal resistance is defined as temperature rise per unit of power, analogous to electrical resistance, and is expressed in units of degrees Celsius per watt (°C/W). Thermal resistance is the reciprocal of thermal conductance:

thermal conductance = $\tfrac{kA}{L}$ , measured in W⋅K−1.

thermal resistance = $\tfrac{L}{kA}$ , measured in K⋅W−1.

Where $k$ is thermal conductivity, $A$ is the area, and $L$ is the thickness

Experimental values of thermal conductivity. Data from en: List of thermal conductivities. For en: Thermal conductivity Experimental values. Image by Grzegorz Knor.

For a constant geometry, the thermal conductivity is the way that we toggle the amount of heat transferred. Heat sinks are valued for transferring thermal energy, so we will pick materials that have higher values of thermal conductivity. To balance mass and cost, we typically use aluminum or copper.

Heat sink types: Pin, Straight, and Flared Fin. Image by DTC.

In general, the more surface area a heat sink has, the better it works. In reality, pin fin heat sink performance is significantly better than straight fins when used in their intended application where the fluid flows axially along with the pins rather than only tangentially across the pins. “Placing a conductive thick plate as a heat transfer interface between a heat source and cold flowing fluid (or any other heat sink) may improve the cooling performance. It is shown that the thick plate can significantly improve the heat transfer between the heat source and the cooling fluid by way of conducting the heat current in an optimal manner” [Wikipedia].

Rectangular, an inline heatsink with an attached transparent fan. Image by Semiconductor Engineering.

Like the surface coating subsection, similar concepts apply for the heat sink: “Matte-black surfaces will radiate much more efficiently than shiny bare metal. A shiny metal surface has low emissivity. The emissivity of a material is tremendously frequency-dependent and is related to absorptivity (of which shiny metal surfaces have very little)” [Wikipedia].

One element of which is the large, square black radiator visible at center, one of two that will be installed—is shown undergoing thermal testing at NASA Goddard Space Flight Center in Greenbelt, Maryland in late February. Image courtesy of NASA.

“Radiators come in several different forms, such as spacecraft structural panels, flat-plate radiators mounted to the side of the spacecraft, and panels deployed after the spacecraft is in orbit. Whatever the configuration, all radiators reject heat by infrared (IR) radiation from their surfaces. The radiating power depends on the surface’s emittance and temperature. The radiator must reject both the spacecraft waste heat and any radiant heat loads from the environment. Most radiators are therefore given surface finishes with high IR emittance to maximize heat rejection and low solar absorptance to limit the heat from the Sun” [Wikipedia].

Panels and radiators (white square panels) on ISS after STS-120. Image courtesy of NASA.

Insulation

Radiative insulation enlists the use of thin sheets, typically made from mylar or Kapton with surface coatings, to isolate panels underneath from solar radiation. This extra layer of separation between the sun and the panel creates a different equilibrium experienced by the panel. The panel reaches equilibrium with radiation from the sheet and from itself reflected from the sheet. The sheet reaches equilibrium with radiation from the sun and panel, and from itself reflected off-panel. Not extend this idea to multiple layers and we’ve got Multi-Layer Insulation: the most method of thermal control in space.

Multi-Layer Insulation (MLI) uses multiple insulation layers to cut down on radiative transfer; “in its basic form, it does not appreciably insulate against other thermal losses such as heat conduction or convection. MLI gives many satellites and other space probes the appearance of being covered with gold foil which is the effect of the amber-colored Kapton layer deposited over the silver Aluminized mylar” [Wikipedia]. This surface insulation is a highly effective means of insulation and can act additionally as a defense against space dust/particulate impacts. A problem that can occur during manufacturing is the physical connection between insulated components, creating conductive leak paths, analogous to short-circuiting a thermal circuit.

Closeup of Multi-layer insulation from a satellite. The metal-coated plastic layers and the scrim separator are visible. CC BY-SA 2.5. Image by Dantor.

To estimate the effective emissivity $\epsilon_{eff}$ of MLI, you need to know the emissivity of coating of the sun-side coating $\epsilon_{sun}$ , cold-side layer $\epsilon_{cold}$ and mylar $\epsilon_{mylar}$ [ThermalEngineer]:

$\epsilon_{eff} = \tfrac{2n}{ \epsilon_{mylar}} - n - 1 + \tfrac{1}{\epsilon_{sun}} + \tfrac{1}{\epsilon_{cold}}$

Where n is the number of mylar layers and

$\epsilon_{mylar}$ = 0.03

Effective emissivity with respect to a number of layers. Image by Thermal Engineer.

“Even if conduction between the layers is ignored, adding additional layers to achieve the required effective emittance is impractical due to weight and packaging considerations. Furthermore, since the effective emittance is a function of the reciprocal of the number of layers, adding additional layers becomes increasingly less effective with the number of layers. In order to achieve the required effective emittance, the shield needs to be configured such that energy is allowed to escape to space from between the layers before reaching the cold-side layer. As the example case in the figure above shows, this approach allows the number of layers to be greatly reduced. It is this approach of enabling the loss of energy out from between film layers which provides the required low effective emittance and makes it possible to passively cool to very low temperatures” [ThermalEngineer].

The golden areas are MLI blankets on the Mars Reconnaissance Orbiter. Image courtesy NASA.

A block of aerogel in a hand. Image courtesy of NASA.

Another type of insulation is called aerogel. “Aerogel is a synthetic porous ultralight material derived from a gel, in which the liquid component for the gel has been replaced with gas without significant collapse of the gel structure. The result is a solid with extremely low density and extremely low thermal conductivity. NASA used an aerogel to trap space dust particles aboard the Stardust spacecraft. The particles vaporize on impact with solids and pass through gases, but can be trapped in aerogels. NASA also used aerogel for thermal insulation of the Mars Rover” [Wikipedia].

The Stardust dust collector with aerogel blocks. Image courtesy of NASA.

Properties of Silica Aerogel compared to Silica Glass. Image courtesy of NASA/JPL.

Suggested Reading

Aerogel: Mystifying Blue Smoke

Radioisotope Heater Unit

Radioisotope heater units (RHU) are small devices that provide heat through radioactive decay, much like radioisotope thermoelectric generators (RTG) but without the step of converting this heat into electrical energy. Because RHUs do not need components to convert heat to electricity, RHUs are more compact than RTGs. RHUs give off heat continuously somewhere between several decades and up to centuries, given the gradual degradation of the radioactive isotope.

^{RHU Pull-apart Animation. Video courtesy of NASA.}

“An RHU contains a Pu-238 fuel pellet about the size of a pencil eraser and outputs about 1 Watt of heat. (The entire RHU is about the size of a C-cell battery.) Some missions employ just a few RHUs for extra heat, while others have dozens. NASA missions enabled by radioisotope heater units” [NASA]:

Apollo 11 EASEP Lunar Radioisotope Heater – contained two 15W RHUs
Pioneer 10 & 11 – 12 RHUs each
Voyager 1 & 2 – 9 RHUs each
Galileo – 120 RHUs (103 on orbiter, 17 on atmospheric probe)
Mars Pathfinder Sojourner Rover – 3 RHUs
Cassini – 117 RHUs (82 on orbiter, 35 on Huygens Titan probe)
MER Spirit & Opportunity Rovers – 8 RHUs each

Radioisotope Heater Units have been critical for providing heat to keep some spacecraft warm enough to accomplish their missions, including the battery-powered Galileo and Huygens probes and the two solar-powered Mars Exploration Rovers. Image courtesy of NASA.

Active

Active heating and cooling systems require electricity and control logic.

Heaters

“Heaters are used in thermal control design to protect components under cold-case environmental conditions or to make up for the heat that is not dissipated. Heaters are used with thermostats or solid-state controllers to provide exact temperature control of a particular component. Another common use for heaters is to warm-up components to their minimal operating temperatures before the components are turned on” [Wikipedia].

“The most common type of heater used on spacecraft is the patch heater, which consists of an electrical-resistance element sandwiched between two sheets of flexible electrically insulating material, such as Kapton. The patch heater may contain either a single circuit or multiple circuits, depending on whether or not redundancy is required within it” [Wikipedia]. These heaters are typically mounted to surfaces of components through a mounting adhesive or through some external adhesive [ProHeatInc].

Standard cartridge heater anatomy. Image by Maoyt.

“The cartridge heater is often used to heat blocks of material or high-temperature components such as propellants. This heater consists of a coiled resistor enclosed in a cylindrical metallic case. Typically a hole is drilled in the component to be heated, and the cartridge is potted into the hole. Cartridge heaters are usually a quarter-inch or less in diameter and up to a few inches long” [Wikipedia].

Louvers

“Louvers are active thermal control elements that are used in many different forms. Most commonly they are placed over external radiators, louvers can also be used to control heat transfer between internal spacecraft surfaces or be placed on openings on the spacecraft walls. A louver in its fully open state can reject six times as much heat as it does in its fully closed state, with no power required to operate it. The most commonly used louver is the bimetallic, spring-actuated, rectangular blade louver also known as Venetian-blind louver. Louver radiator assemblies consist of five main elements: baseplate, blades, actuators, sensing elements, and structural elements” [Wikipedia].

Louver Assembly. Image by Matt C. Bergman.

“Louvers are used to dissipate excess heat, typically from running too many instruments. Louvers are located on the belly side of the spacecraft. Resembling Venetian blinds, the louvers will radiate heat when open and reflect heat when closed. As a rule of thumb, louvers reject six times as much heat in the open position as it does in the closed position. These thermal louvers actuate if the internal temperature exceeds 25°C” [Bergman].

New Horizons with MLI Installed with Louvers circled in red. Image by Matt C. Bergman.

Pumped Fluid Loops

Suggested Reading

Mechanically Pumped Fluid Loops for Spacecraft Thermal Control: Past, Present & Future Past, Present & Future

Pumped Fluid Loop. Mechanically Pumped Fluid Loops for Spacecraft Thermal Control: Past, Present & Future by Pradeep Bhandari.

A pumped fluid loop is a system that circulates a working fluid, via routed tubing, to any part of a spacecraft’s structure. “The key difference between traditional means of T/C and the use of mechanically pumped fluid loops lies in the connection between the thermally controlled components and the heat loss surface (radiator). The connection is convective instead of conductive or radiative. Fluid flowing through tubes connected to the two sets of surfaces (source/sink) convectively picks up the heat (source) and dissipates it (sink). A mechanical pump is the prime mover of the fluid. This is the closest one comes to a true thermal bus where we can both pick up and reject heat simultaneously and automatically at multiple locations” [Bhandari].

Radioisotope Power System Heat Rejection System for MSL (Cruise). Mechanically Pumped Fluid Loops for Spacecraft Thermal Control: Past, Present & Future Past, Present & Future. Image by Pradeep Bhandari.

“Mechanically Pumped Fluid Loops (MPFL) are most useful for spacecraft thermal control when heat pickup/rejection capacity, control of this capacity, testability, and/or mechanical integration are driving factors. Advantages when compared with traditional spacecraft thermal control technologies:

Scalability of heat rejection capacity
Ability to accept and reject heat at multiple locations
Flexibility in locating heat-dissipating equipment
Adaptability to late changes in spacecraft design

Any of the following causes could lead to partial or complete failure of the thermal control system. Possible cons and preventive measures include:

Leaks – Leaks through mechanical joints or corrosion of tubing/components
- Use well-qualified fittings
  - Vibration/thermal
  - Accumulator sized to accommodate nominal leak rates
Pump failure – Long-term operation of pumps could degrade their performance or lead to their complete failure
- Use redundant pumps
Clogged filter – Filters used to guard small passages in pumps against particles that could clog
- Use well-qualified and sized filters
- Use check valves to automatically bypass filter in flight

We’ve seen limited use in robotic space missions over the past 30 years due to reliability concerns, but are increasingly being looked at to solve complex thermal control problems” [Bhandari].

Cryocoolers

Cryocoolers are refrigerators that reach cryogenic temperatures, typically used in conjunction with instrument payloads. For example, by necessity, JWST’s Mid-Infrared Instrument “MIRI’s detectors are a different formulation (Arsenic-doped Silicon (Si:As)), which needs to be at a temperature of less than 7 kelvin to operate properly. This temperature is not possible on Webb by passive means alone, so Webb carries a “cryocooler” that is dedicated to cooling MIRI’s detectors” [NASA].

Schematic diagram of a split-pair Stirling refrigerator. The cooling power is supplied to the heat exchanger of the cold finger. Usually, the heat flows are so small that there is no need for physical heat exchangers around the split pipe. CC BY-SA 3.0. Image by Split Sterling.

“In most cases, cryocoolers use a cryogenic fluid as the working substance and employ moving parts to cycle the fluid around a thermodynamic cycle. The fluid is typically compressed at room temperature, precooled in a heat exchanger, and then expanded at some low temperature. The returning low-pressure fluid passes through the heat exchanger to precool the high-pressure fluid before entering the compressor intake. The cycle is then repeated” [Wikipedia]. The component that needs to be cryogenically cooled is attached to the cold finger or cold space to conduct heat away from the component.

The cooling device for the Mid-Infrared Instrument, or MIRI, is one of the James Webb Space Telescope’s four instruments. The MIRI requires a lower operating temperature than Webb’s other instruments, the cryocooler accommodates this requirement. Image: NASA/JPL-Caltech.

In recent years, cryocoolers have gotten smaller and smaller and entered the COTS realm, making them exciting possibilities for small satellite missions in the future. Northrop Grumman has made a 900-gram pulse micro-cooler on the ground for night vision and a 3.8 kg space-qualified version [Raab and Tward]. “The Thales Cryogenics LPT9310 pulse tube cooler has undergone comprehensive characterization and flight qualification tests at the Jet Propulsion Laboratory (JPL) to determine its suitability for future cost-capped NASA flight missions” [Johnson et al.]. The Hawai’i Space Flight Laboratory is flying an AIM SF-070 cryocooler on their future Hyperspectral Thermal Imager (HyTI) mission [WestCoastSolutions].

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