07. Thermal Control

Introduction

There is a defined temperature range for all spacecraft components that must be met for optimal function and this regulation of temperatures that occur throughout a spacecraft in orbit is controlled by thermal management techniques. Following the high demand for small spacecraft in the last decade, miniaturized thermal management systems are required to ensure proper thermal control requirements are met. While traditional thermal control techniques have been demonstrated on larger spacecraft, these existing techniques may require additional development for miniaturization and testing for small spacecraft application. This technology will still be considered state of the art, however not at TRL 9 for small spacecraft applications. Table 7.1 is a list of the current state of the art passive thermal techniques applicable for small spacecraft.

Table 7.1: Passive Thermal Systems
Product Manufacturer Status
MLI Blanket Sheldahl, Dunmore,
Aerospace, Fabrication and
Materials, MLI Concepts Inc.
TRL 9
Paint AZ Technology, MAP, Astral
Technology Unlimited, Inc.,
Dunmore Aerospace
TRL 9
Sun Shields Sierra Lobo TRL 8
Flexible Thermal Straps Thermal Management
Technologies, Thermacore,
Technology Applications, Inc.,
Thermotive Technology
TRL 9 for metal straps, TRL
8 for composite straps
Thermal Louvers NASA Goddard Space Flight
Center
TRL 8
Deployable Radiators Thermal Management
Technologies, Kaneka
Corporation/JAXA
collaboration
TRL 6
Passive Heat Pipe Thermacore, Inc. and
Advanced Cooling
Technology, Inc.
TRL 6
Passive Thermal Louver NASA Goddard Space Flight
Center
TRL 8

State of the Art

Passive Systems

Passive thermal control requires no input power for thermal regulation within a spacecraft. This can be achieved using several methods and is highly advantageous to spacecraft designers, especially for the cubesat form factor, as passive thermal control systems are associated with low cost, volume, weight and risk, and have been shown to be reliable. The integration of Multi-Layer Insulation (MLI), thermal coating, heat pipes, and sunshades are examples of passive methods to achieve thermal balance in a spacecraft. The use of MLI and surface coatings has been a traditional thermal technique that has been used on nearly every spacecraft since early spaceflight, and there has not been any modification of these methods for small spacecraft application. Cubesats such as Pharmasat, GeneSat, O/OREOS, SporeSat, YamSat, Compass-1, DelfiC-3, and minisatellites Bird, SDS-4, FASTRAC, and PICARD have all used MLI and coating to assist thermal regulation. Thermally isolated structural joints are often used for small spacecraft thermal management, where multiple washers with low thermal conductivity are stacked between fasteners and joined surfaces to limit heat transfer via conduction in specific places.

Thermal Insulation and Coating

Thermal insulation acts as a thermal radiation barrier from incoming solar flux and also to prevent excessive heat dissipation. Typically used to maintain a temperature range for the electronics and battery during orbit, or more recently, for biological payloads, thermal insulation is typically in the form of MLI blankets but reflective tape has also been used. MLI is fairly delicate and drops drastically in performance if compressed, so using it outside of the small satellite that fits into a deployed (P-POD, NLAS) requires a lot of caution. Additionally, MLI blankets tend to drop efficiency as their size decreases and the specific way they are attached has a large impact on their performance. Due to this, MLI application does not perform as well for small spacecraft (CubeSat form factor) as they do on larger spacecraft. Surface coatings are typically less delicate and are more appropriate for exterior of a small spacecraft that will be deployed from a dispenser. Lastly, internal MLI blankets that do not receive direct solar radiation (sun light) can often be replaced by low emissivity coatings that perform identically in that context, using less volume and often cost less. Silvered tapes offer superior performance as efficient radiators, rejecting solar heat very effectively, but must be handled extremely carefully to maintain optical properties and don’t always bond well to curved surfaces.

Dunmore Aerospace corporation has produced MLI blankets for aerospace use since the early 1980’s and have since then participated in many US and European space missions [1]. While their MLI blankets have been used on small spacecraft missions, their recent developments with STARcrest Spacecraft Materials has engineered a SATKIT consisting of DE330, DE076, DM116, and DM100 MLI blankets for cubesat application. These materials are constructed from previously flown MLI, however the actual KIT has TRL 8. Dunmore also offers polyimide film tape and MLI tape designed to insulate wires and cables on a spacecraft or aircraft and has a TRL of 8 for small spacecraft.

The alteration of the optical characteristics (solar absorptance and emittance) of a surface material by applying matte paint is another passive method of thermal control. While black paint will absorb all incident heat, white paint limits how much heat is absorbed from the surrounding environment due to its low absorption/emittance ratio [2]. Tape is another known useful thermal coating resource; it is easy to both apply and remove, is relatively inexpensive, and has a longer usable lifetime than paint [3].

AZ Technology, MAP, Astral Technology Unlimited, Inc., Lord Techmark, Inc., Sheldahl, Akzo Nobel Aerospace Coatings manufacture thermal coating (paint and tape) for aerospace use that has been demonstrated on multiple small spacecraft missions. Some examples of small spacecraft using thermal coatings include Picard (150 kg) which used white SG12FD paint on the Sun pointing face and cubesat YamSat which had black paint applied inside of the spacecraft for temperature maintenance.

Sunshields

End view of Sun Shield on CryoCube-1 developed at Sierra Lobo
Figure 7.1: End view of Sunshield on CryoCube-1 developed at Sierra Lobo. Image courtesy of Sierra Lobo (2014).

The application of a sunshield, or sunshade, is common for spacecraft thermal control, although the implementation on a small spacecraft, especially for cubesat form factor, has been a recent addition for the improvement of thermal performance. Sierra Lobo has developed a deployable sunshield that will be flown on CryoCube-1, estimated launch in 2016. This sunshield can support a multiple month long duration lifetime and can provide temperatures below 100 K and below 30 K with additional active cooling [4]. Figure 7.1 displays the design of the sunshield used on CryoCube-1.

MLI Concepts, Inc. also has extensive expertise in the design and development of stainless steel and titanium heat shields that will not break down due to heat or other stress [5], though this technology has been demonstrated on larger spacecraft only.

Thermal Straps

Recent use of flexible thermal straps has become a convenient form of temperature control on small spacecraft as the required mass for the strap is limited with reduced stiffness between components. Flexible thermal straps can be applied to allow for passive heat transfer to a thermal sink and can be fitted to any particular length for design.

Thermal Management Technologies Aluminum thermal strap tes
Figure 7.2: Thermal Management Technologies Aluminum thermal strap test. Image Courtesy of Thermal Management Technologies (2015).

Thermal Management Technologies (TMT) has developed standard flexible thermal straps available in thin aluminum or copper foil layers or a copper braid; custom accommodations can be fabricated and tested for service [6], see Figure 7.2. While these straps have been tested, they have not been flown on any small spacecraft missions.

Thermal straps are also being manufactured in materials other than the traditional aluminum and copper. Thermacore has utilized k-Technology in solid conduction in the design of lightweight thermal k-Core straps that supply a natural conductive path without including structural loads to the system and have shown to have greater conduction efficiency compared to traditional aluminum straps in testing [7]. Here, the k-Core encapsulated graphite facilitates the heat dissipation in high-power electronics. This technology is fully designed and tested but has not flown on a small spacecraft.

Graphite Fiber Thermal Straps (GFTS)
Figure 7.3: Graphite Fiber Thermal Straps (GFTS). Image courtesy of Thermal Management Technologies (2015).

Technology Applications, Inc. has specialized in the testing and development of Graphite Fiber Thermal Straps (GFTS), with flight heritage on larger spacecraft missions (Orion and Spice). GFTS are known to be extremely lightweight and highly efficient and thermally conductive with unmatched vibration attenuation [8], see Figure 7.3. While this technology has not been demonstrated or tested on small spacecraft platforms, the capability for small spacecraft applications is still applicable. Thermotive Technology developed the Two Arm Flexible Thermal Strap (TAFTS) that is currently flying on JPL’s PRISM (Portable Remote Imaging Spectrometer) instrument. For space infrared cameras, there is a requirement for extremely flexible direct cooling of mechanically-sensitive focal plane. The design of TAFTS uses three “swaged terminals and a twisted section” that allows for significant enhanced elastic movement and elastics displacements in three planes, while a more conventional strap of the same conductance offers less flexibility and asymmetrical elasticity [9]. Infrared cameras have flown on small spacecraft missions, although the TAFTS design has not.

Thermal Louvers

Passive Thermal Louver on 6U cubesat Dellingr
Figure 7.4: Passive Thermal Louver on 6U cubesat Dellingr. Image Courtesy NASA Goddard (2015).

Thermal louvers have not yet been integrated on small spacecraft due to volume and power constraints. Full-sized louvers for larger spacecraft have high efficacy for thermal control, however their integration on small spacecraft has been challenging. Typical spacecraft louvers are associated with a larger mass and input power which both are limited on a small spacecraft. Goddard Space Flight Center has developed a passive thermal louver for small spacecraft and will be space demonstrated on a 6U cubesat, Dellingr, where 14 W has been shown to be the power dissipation. This louver design uses bimetallic springs for control of the position of the flaps; when heat in the spacecraft rises, there is expansion in the springs due to the bimetallic properties of the spring causing the flaps to open which alters the thermal radiation of the exterior surface. Similarly when the spacecraft cools and the flaps close, the exterior surface returns to the previous emissivity [10]. Figure 7.4 is a representation of the thermal lovers on Dellingr.

Deployable Radiators

TMT_radiator
Figure 7.5: TMT Conductive Hinge for Small Satellite Mode. Image Courtesy of Thermal Management Technologies (2015).

Similarly to thermal louvers, the utilization of deployable radiators on a small spacecraft is challenging due to the volume restrictions. Paint has been widely used to create a radiator-like surface, which has shown to be advantageous on smaller platforms. For a system that requires a large amount of heat dissipation regardless of incident thermal radiation intensity, a passive deployable radiator that is lightweight and simple in design would greatly enhance thermal performance.

TMT_radiator_model
Figure 7.6: TMT Deployable Radiator for Small Satellites. Image Courtesy of Thermal Management Technologies (2015).

Thermal Management Technologies is developing Thermally Efficient Deployable Radiators for small spacecraft that integrates an isothermal radiator surface with a high conductance hinge for high thermal efficiency [11]. This thermally conductive hinge allows for minimal temperature gradients between the radiator and spacecraft, as seen in Figure 7.5 and an illustration  of the deployable radiator can be seen in Figure 7.6. The radiating surface uses graphite composite material for mass reduction and increased stiffness, where the typical radiator uniformity is greater than 0.1°C W-1m-1.  This technology is currently in the development and testing phase [11].

The design of a flexible deployable radiator for small spacecraft was proposed, developed and tested by Shoya Ono and Hosei Nagano and colleagues from Kaneka Corporation and JAXA. This design can deploy or stow the radiation area depending on the environment temperature for proper heat dissipation control and has an overall volume of 0.5 x 360 x 560 mm and 0.287 kg total mass, see Figure 7.7. The fin is passively stowed and deployed by an actuator that consists of a shape memory alloy and bias spring.

Flexible_radiator_JAXA
Figure 7.7: Flexible radiator conceptual diagram. Image Courtesy of Ono et  al. (2015).

To increase both radiator size and the value of thermal conductivity, multiple layers of Kaneka Graphite Sheets (KGS) are used for the fin material. The rear surface of the fin is insulated with MLI to reduce the amount of heat dissipation under cold conditions. Testing for the deployment and stowing was conducted in a thermostatic chamber, and the thermal performance test was conducted under vacuum conditions, where it was shown that the half-scaled radiator dissipated 54 W at 60°C [12].

Heat Pipes

SDS-4_heatpipe
Figure 7.8: FOX flight model heat pipe developed at JAXA. Image Courtesy of Nakamura et al (2013).

Heat pipes are an efficient passive thermal transfer technology, where a closed-loop system transports excess heat via temperature gradients, typically from electrical devices to a colder surface, which is often either a radiator itself, or a heat sink that is thermally coupled to a radiator. Traditional heat pipes are cylindrical in shape, which was utilized on BIRD (92 kg), but there are also flat plates that are rectangular stainless steel tubing sandwiched between two aluminum plates and charged with a working fluid inside [13]. Small spacecraft SDS-4 (50 kg) successfully incorporated this flat plate design that was developed at JAXA, as seen in Figure 7.8. Although this technology has been applied on a 50 kg small spacecraft, additional fabrication and testing may be required for cubesat platform applications. For cubesat design, the TRL for passive heat pipes are TRL 6.

Active Systems

Active thermal methods for spacecraft thermal control rely on input power for operation, are associated with higher precision and have been shown to be more effective [14]. Typical active thermal devices include electrical resistance heaters, coolers or the use of cryogenic materials. Until spacecraft designers are able to miniaturize existing actively controlled thermal techniques, the utilization of active thermal systems in small spacecraft will be limited. Small spacecraft designers are keen to use active thermal systems for temperature sensitive devices (such as batteries, cameras and electronics). In such cases where a complete passive system is not sufficient for thermal management, electrical resistance heaters and coolers are attached to specific equipment to maintain operational temperatures. For the current state of the art in active thermal technologies applicable on small spacecraft, see Table 7.2.

Table 7.2 Active Thermal Systems
   Products Manufacturer Status
Electrical Heaters Minco Products, Inc. and All Flex Flexible Circuits, LLC. TRL 9
Mini Cryocoolers Ricor-USA, Inc., Creare, Sunpower Inc., Northrop Grumman and Lockheed Martin Space Systems Company TRL 7
Flexible and Enhanced Active Thermal Straps (FEATS) LoadPath TRL 7/8

Thermal Straps

Active thermal straps have been shown to increase thermal performance, especially in a design that is associated with high concentrated heat fluxes on the electronics. The advanced thermally conductive path on the strap supplies a reliable mitigation method for reducing hot spots while also limiting integration overhead and space. Load Path Aerospace Structures currently have Flexible and Enhanced Active Thermal Straps (FEATS) that are capable of heat dissipation up to 50 Wcm-2 and cooling capacity of 35 W [15]. While these have not yet flown on small spacecraft missions, they have been developed and tested for small spacecraft.

Heaters

On small spacecraft, electrical resistance heaters are typically used to maintain battery temperature during cold cycles of the orbit and are controlled by a thermostat or temperature sensor. 1U cubesats Compass-1, MASAT-1 and OUTFI-1 required an electrical heater attached to the battery in addition to the passive control for the entire spacecraft system to maintain thermal regulation in eclipses [16]. As biological payloads are becoming more common on small spacecraft, the biology have their own specified temperature maintenance requirements. NASA Ames biological nanosats (GeneSat, PharmaSat, O/OREOS, SporeSat, EcAMSat and Biosentinel) all utilize actively-controlled resistance heaters for precise temperature maintenance for their biological payloads with close loop temperature feedback to maintain the biology temperatures. Minco Products, Inc. manufactures flexible strip heaters equipped with polyimide insulation. These heaters are TRL 9 for small spacecraft missions.

Cryocoolers

There have been recent improvements in the cooling technologies for small spacecraft. Cryogenic coolers, or cryocoolers, are used on instruments or subsystems requiring cryogenic cooling, such as high precision IR sensors. The low temperature improves the dynamic range and extends the wavelength coverage. Further, the use of cryocoolers is associated with longer instrument lifetimes, low vibration, high thermodynamic efficiency, low mass and supply cooling temperatures less than 50 K [17]. Instruments such as imaging spectrometers, interferometers and MWIR sensors use cryocoolers to function at the extremely low temperatures required. Cryocube-1 will be the first cubesat mission that will perform cryogenic management tests (fluid location sensing, slosh characterization and cryogenic fluid transfer) on orbit in 2016. The 3U will carry gas onboard and will be passively cooled and liquefied using a cryotank developed at Sierra Lobo, Inc [4].

Creare developed an Ultra Low Power (ULP) cryocooler, a single-stage turbo-Brayton cryocooler that operates between a cryogenic heat rejection temperature and the primary load temperature. Components include a cryogenic compressor, a recuperative heat exchanger, and a turboalternator, where the continuous flow nature of the cycle allows the cycle gas to be transported from the compressor outlet to a heat rejection radiator at the warm end of the cryocooler, and from the turboalternator outlet to the object to be cooled at the cold end of the cryocooler [18], see Figure 7.9. This cryocooler is designed to operate at cold end temperatures of 30 to 70 K, loads of up to 3 W, and heat rejection temperatures of up to 210 K by changing only the charge pressure and turbo machine operating speeds. This technology has competed testing and fabrication (TRL 7).

Creare__UPL_cooler
Figure 7.9: Configuration of primary mechanical UPL cryocooler components from Creare. Image Courtesy of Creare, Inc. (2015).

Ricor-USA, Inc. developed the K562S, a rotary Stirling mini micro-cooler, that has a cooling capacity of 200 mW at 95 K and 300 mW at 110 K that has been used in several small gimbals designed for military applications. Ricor also developed K508N a Stirling ½ W micro cooler, that has cooling capacity 500 mW at 77 K and 700 mW at 77 K that is suitable for use on a small spacecraft, see Figure 7.10 and 7.11 for both mini coolers [19]. These coolers are TRL 7 for small spacecraft applications.

Rico_K562S__crycooler
Figure 7.10: Ricor-USA K562S Mini-cooler. Image Courtesy of Ricor-USA (2015).
Ricor_K580
Figure 7.11: Ricor-USA K508N 1/2 W Micro Cooler. Image Courtesy of Ricor-USA (2015).

Sunpower, Inc. developed the CryoTel DS1.5 Stirling Cryocooler featuring a dual-opposed-piston pressure wave generator and a separate cold head to minimize exported vibration and acoustic noise and has a nominal heat lift of 1.4 W at 77 K using 30 W power with a 1.2 kg mass [20]. Sunpower also offers MT-F, mini-cooler that has a nominal heat lift of 5 W at 77 K, using 80 W power with a total mass of 2.1 kg, see Figure 7.12 and 7.13 for both coolers. While the MT-F technology has been successfully demonstrated in applications such as High Temperature Superconductivity (HTS) filters, high altitude balloons, refrigeration, germanium detectors, IR detectors, radio telescopes and laser diode cooling , it has not been applied to a small spacecraft mission.

CryoTel-DS-1_5
Figure 7.12: CryoTel DS1.5 1.4 W Cryocooler. Image Courtesy of Sunpower, Inc. (2015).
CryoTel-MT-F
Figure 7.13: CryoTel MT-F 5 W Cryocooler. Image Courtesy of Sunpower, Inc. (2015).

 

Northrop Grumman designed a Micro Pulse Tube cooler that is a split configuration cooler that incorporates a coaxial cold head connected via a transfer line to a vibrationally balanced linear compressor, see Figure 7.14. This micro compressor has been scaled from a flight proven TRL 9 high efficiency cooler (HEC) compressor. The cooler has an operational range of 35 to 40 K and a heat rejection temperature of 300 K, using 80 W of input power, has 750 mW refrigeration at 40 K and a total mass of 7.4 kg [21].

LM_TRL6_Microcryocooler
Figure 7.15: Lockheed Martin TRL6 Microcryocooler. Cryocooler photograph provided courtesy of Lockheed Martin Corporation.
NorthropGrumman__PT_minicooler
Figure 7.14: Flight design PT microcooler and its flight configuration with attached reservoir tank. Image Courtesy of Northrop Grumman (2015).

Lockheed Martin Space Systems Company engineered a pulse tube micro-cryocooler, a simplified version of a Stirling cryocooler, consisting of a compressor driving a coaxial pulse tube coldhead, see Figure 7.15. The unit has a mass of 0.345 kg for the entire thermal mechanical unit, and is compact enough to be packaged in ½U of a cubesat [22]. The microcooler design underwent qualification testing at TRL 6 and is compatible for small spacecraft missions.

On the Horizon

Traditional thermal control technologies for spacecraft will not always be able to be immediately integrated into small spacecraft platforms. As mentioned in the introduction of this chapter, the technology that is demonstrated on larger spacecraft may need to be altered slightly for small spacecraft compatibility and will not be automatically TRL 9. This section discusses some technology that is being proposed and developed for small spacecraft thermal control, and is not ready for space flight.

Thermal Straps

Thermotive has developed Pyrovo Pyrolytic Graphite Film (Pyrovo PGF) thermal straps that have already flown in optical cooling applications in high altitude cameras and avionics and are planned to be used in several upcoming space flight instruments in 2016. Pyrovo PGF straps use pyrolytic graphite wrapped in a HEPA filter-vented 4 ?m thick aluminized Mylar blanket and have no exposed graphite. The specific thermal conductivity of this material has shown to be 10x better than aluminum and 20x better than copper, as seen in Figure 7.16 [23]. While these straps have not flown in space on a small spacecraft mission, they are planned to be included on several upcoming space flight instruments in 2016 (TRL 6).

PyrovoPGF Material Comparison__Thermotive
Figure 7.16: Thermotive Pyrovo PGF Material Comparison. Image Courtesy of Thermotive (2014).

Deployable Radiators

Thermotive is researching the design of a deployable passive radiator for hosted payload instruments and cubesats, Folding Elastic Thermal Surface (FETS). Originally conceived as a thermal shield and cover for a passive cooler (cryogenic radiator) on JPL’s MATMOS mission, this proposed concept is being modified as a deployable radiator for small spacecraft use and has TRL 4/5 [23].

Storage Units

TMT_TSU
Figure 7.17: CubeSat Thermal Storage Unit. Image Courtesy of Thermal Management Technologies (2015).

Thermal storage units can be used in various applications for passively storing thermal energy for component protection or for future energy use . Thermal Management Technologies is developing a phase-changing thermal storage unit (TSU) design that considers desired phaseshades-change temperatures, interfaces, temperature stability, stored energy, and heat removal methodologies, see Figure 7.17. A complete fabrication of this device will allow the user to control temperature peaks, stable temperatures and/or energy storage[24]. Active Space Technologies also has storage units under development that integrates online design support and high cryogenic enthalpy. Both technologies are at TRL 5 for small spacecraft use.

Fluid Loops

LM_JT_micro_compressor
Figure 7.18: JT Circulator. Circulator photograph provided courtesy of Lockheed Martin Corporation.

A pumped fluid loop is capable of achieving heat transfer between multiple locations via forced fluid convective cooling. Mechanically pumped fluid loops are not of interest to small spacecraft engineers as they are associated with high power consumption and mass. Lockheed Martin Corporation is developing a circulator pump for a closed cycle Joule Thomson cryocooler, see Figure 7.18. With an overall mass of 0.2 kg, it can circulate gas as part of a single-phase or two-phase thermal management system using 1.2 W of electrical power and can manage around 40 W of spacecraft power as a single-phase loop, or several hundred Watts of spacecraft power as part of a 2-phase loop [25]. This design is TRL 3.

Conclusion

As thermal management on small spacecraft is limited by mass, volume and power constraints, traditional passive technologies, such as MLI, paints, coatings and metallic thermal straps, still dominate thermal design. Active technologies, such as thin flexible resistance heaters have also seen significant use in small spacecraft, including some with advanced closed-loop control. Technologies that have to date only been integrated on larger spacecraft are being examined, designed and tested for small spacecraft platform application. Passive louvers and sun shields have been proposed and developed for small spacecraft and will tentatively fly in 2016 (Dellingr and CryoCube-1). Deployable radiators and various types of composite thermal straps have also been fabricated and tested for small spacecraft utilization in the past few years and are offered from numerous vendors. Technology in active thermal control systems has started expanding to accommodate volume and power restrictions of a smaller spacecraft; cryocoolers are being designed to fit within 0.5U volume that will broaden small spacecraft ability to use optical sensors and imaging spectrometers. Thermal storage units are being developed that will better control amount of heat dissipation as well as storing energy for future use.

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