Abstract:
A passive thermal system for use in a satellite and other aerospace applications includes a container having a heat-pipe working fluid disposed in a first chamber and a Phase Change Material (PCM) disposed in a second chamber that substantially surrounds the first chamber. The first chamber contains a wick for transporting the heat-pipe working fluid. The exterior of the first chamber has fins, etc., that extend into the PCM for heat spreading and increased interface area.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to earth-orbiting communication satellites. 
       BACKGROUND OF THE INVENTION 
       [0002]    Communication satellites receive and transmit radio signals from and to the surface of the Earth. Although Earth-orbiting communications satellites have been in use for many years, providing adequate cooling for the thermally sensitive electronics components onboard such satellites continues to be a problem. 
         [0003]    There are two primary sources of heat with which a satellite&#39;s thermal systems must contend. One source is solar radiation. Solar radiation can be absorbed by thermal insulation shields or readily reflected away from the satellite by providing the satellite with a suitably reflective exterior surface. A second source of heat is the electronics onboard the satellite. The removal of electronics-generated heat is more problematic since such heat must be collected from various locations within the satellite, transported to a site at which it can be rejected from the satellite, and then radiated into space. 
         [0004]    The smaller the satellite, the more problematic heat rejection can be. The limited size and mass of a smaller satellite naturally limits the surface area available for radiators and thermal control. 
         [0005]    Heat pipes and phase change material (“PCM”) are two technologies that are commonly used in satellites to address thermal issues. A heat pipe is a closed chamber, typically in the form of tube, having an internal capillary structure which is filled with a working fluid. The operating-temperature range of the satellite sets the choice of working fluid; ammonia, ethane and propylene are typical choices. Heat input (i.e., from heat-generating electronics) causes the working fluid to evaporate. The evaporated fluid carries the heat towards a colder heat-output section, where heat is rejected as the fluid condenses. The rejected heat is absorbed by the cooler surfaces of the heat-output section and then radiated into space. The condensate returns to the heat input section (near to heat-generating components) by capillary forces to complete the cycle. 
         [0006]    A PCM is used to damp transient temperature extremes by storing heat when the thermal load is high and releasing heat when the thermal load is low. The PCM absorbs heat via the latent heat of fusion; that is the PCM melts. The heat is absorbed without an appreciable temperature rise. Conversely, a radiator, heat pipe, thermal strap, or other means is used to remove this absorbed heat, wherein the PCM refreezes. 
         [0007]    PCM modules are typically mounted near or on a heat source of interest. The amount of the PCM module&#39;s surface area that is exposed to the heat source is maximized to the extent possible. Heat storage performance is directly related to the interface area for heat transfer and the depth of the PCM in the module. As necessary, heat pipes are mounted to the PCM modules and/or heat source to transport the heat to a heat sink (e.g., radiator, etc.). 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides an improved passive thermal system by combining a heat pipe with a PCM in a single housing. The passive thermal system is particularly adapted for use in satellites. 
         [0009]    In accordance with the illustrative embodiment, a heat-pipe working fluid is disposed in a first chamber of container and a PCM is contained in a second chamber that substantially surrounds the first chamber. The first chamber contains a wick for heat transport, as per typical heat-pipe design. The exterior of the first chamber has fins, etc., that extend into the PCM for heat spreading and increased interface area. 
         [0010]    This synthesis of heat pipe and PCM provides the following benefits: (i) the proximity of the PCM to the dissipation source is improved in most if not all situations, (ii) interface area is typically significantly improved utilizing the ability of the heat pipe to spread (transport) heat, and (iii) improves thermal coupling to the heat sink (e.g., radiator panel) by keeping the low thermal conductivity PCM out of the direct thermal path between the heat source (i.e., electrical component) and the heat sink. 
         [0011]    Some embodiments in accordance with the present teachings provide: an apparatus comprising a passive thermal system, wherein the passive thermal system includes:
       a housing, wherein the housing has an inner chamber and an outer chamber, wherein the outer chamber substantially surrounds the inner chamber;   heat pipe working fluid, wherein the heat pipe working fluid is contained in the inner chamber; and   phase change material (PCM), wherein the PCM is contained in the outer chamber.       
 
         [0015]    Some embodiments in accordance with the present teaching provide: an apparatus comprising a passive thermal system, wherein the passive thermal system includes:
       heat pipe working fluid contained in an inner chamber; and   phase change material (PCM) contained in the outer chamber, wherein the inner chamber and the outer chamber are arranged so that heat is transferred between the heat pipe working fluid and the PCM.       
 
         [0018]    Some embodiments in accordance with the present teaching provide: a satellite comprising:
       a plurality of radiator panels that radiate heat to an external environment;   a first plurality of electronics components that are contained in a second plurality of containers; and   a passive thermal system, wherein the passive thermal system includes:
           (a) a housing, wherein the housing has an inner chamber and an outer chamber,   (b) heat-pipe working fluid, wherein the heat pipe working fluid is contained in the inner chamber,   (c) phase change material (PCM), wherein the PCM is contained in the outer chamber; and wherein:
               (i) one of the second plurality of containers is coupled to a first end of the passive thermal system; and   (ii) one of the radiator panels is coupled to a second end of the passive thermal system.   
               
               
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  depicts a satellite in accordance with the present teachings. 
           [0028]      FIG. 2  depicts an exploded view of portions of the satellite of  FIG. 1 . 
           [0029]      FIG. 3  depicts an improved passive thermal system for use in conjunction with the satellite of  FIGS. 1 and 2 , in accordance with the illustrative embodiment of the present invention. 
           [0030]      FIG. 4A  depicts, via a side view, the improved passive thermal system of  FIG. 3  in an arrangement for transferring heat from satellite electronics to a satellite radiator panel. 
           [0031]      FIG. 4B  depicts a cross-sectional view of the arrangement of  FIG. 4A  through the line A-A of  FIG. 4A . 
           [0032]      FIG. 4C  depicts a cross-sectional view of the arrangement of  FIG. 4A  through the line B-B of  FIG. 4A . 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    Embodiments of the present invention can be used for all types of satellites (e.g., LEO, GEO, etc.). Before addressing the specifics of the instant passive thermal system, a satellite in which such a system can be used is described. 
         [0034]    Satellite.  FIG. 1  depicts satellite  100  in accordance with the present teachings.  FIG. 2  depicts an “exploded” view of some of the salient features of satellite  100 . Referring now to both  FIGS. 1 and 2 , satellite  100  includes unified payload module  102 , propulsion module  114 , payload antenna module  122 , bus component module  132 , and solar-array system  140 , arranged as shown. It is to be noted that the orientation of satellite  100  in  FIGS. 1 and 2  is “upside down” in the sense that in use, antennas  124 , which are facing “up” in the figures, would be facing “down” toward Earth. 
         [0035]    Unified payload module  102  comprises panels  104 ,  106 , and  108 . In some embodiments, the panels are joined together using various connectors, etc., in known fashion. Brace  109  provides structural reinforcement for the connected panels. 
         [0036]    Panels  104 ,  106 , and  108  serve, among any other functionality, as radiators to radiate heat from satellite  102 . In some embodiments, the panels include adaptations to facilitate heat removal. In some embodiments, the panels comprise plural materials, such as a core that is sandwiched by face sheets. Materials suitable for use for the panels include those typically used in the aerospace industry. For example, in some embodiments, the core comprises a lightweight aluminum honeycomb and the face sheets comprise 6061-T6 aluminum. 
         [0037]    Propulsion module  114  is disposed on panel  112 , which, in some embodiments, is constructed in like manner as panels  104 ,  106 , and  108  (e.g., aluminum honeycomb core and aluminum facesheets, etc.). Panel  112 , which is obscured in  FIG. 1 , abuts panels  104  and  106  of unified payload module  102 . 
         [0038]    Propulsion module  114  includes fuel tank  116  and propulsion control system  118 . The propulsion control system controls, using one or more valves (not depicted), release of propulsion gas through the propulsion nozzle (not depicted) that is disposed on the outward-facing surface of panel  114 . Propulsion control system is appropriately instrumented (i.e., software and hardware) to respond to ground-based commands or commands generated on-board from the control processor. 
         [0039]    Payload antenna module  122  comprises a plurality of antennas  124 . In the illustrative embodiments, sixteen antennas  124  are arranged in a 4×4 array. In some other embodiments, antennas  124  can be organized in a different arrangement and/or a different number of antennas can be used. Antennas  124  are supported by support web  120 . In some embodiments, the support web is a curved panel comprising carbon fiber, with a suitable number of openings (i.e., sixteen in the illustrative embodiment) for receiving and supporting antennas  124 . 
         [0040]    In some embodiments, antennas  124  transmit in the K u , band, which is the 12 to 18 GHz portion of the electromagnetic spectrum. In the illustrative embodiment, antennas  124  are configured as exponential horns, which are often used for communications satellites. Well known in the art, the horn antenna transmits radio waves from (or collects them into) a waveguide, typically implemented as a short rectangular or cylindrical metal tube, which is closed at one end and flares into an open-ended horn (conical shaped in the illustrative embodiment) at the other end. The waveguide portion of each antenna  124  is obscured in  FIG. 1 . The closed end of each antenna  124  couples to amplifier(s) (not depicted in  FIGS. 1 and 2 ; they are located on the interior surface of panel  104  or  108 ). 
         [0041]    Bus component module  132  is disposed on panel  130 , which attaches to the bottom (from the perspective of  FIGS. 1 and 2 ) of the unified payload module  102 . Panel  130  can be constructed in like manner as panels  104 ,  106 , and  108  (e.g., aluminum honeycomb core and aluminum facesheets, etc.). In some embodiments, panel  130  does not include any specific adaptations for heat removal. 
         [0042]    Module  132  includes main solar-array motor  134 , four reaction wheels  136 , and main control processor  164 . The reaction wheels enable satellite  100  to rotate in space without using propellant, via conservation of angular momentum. Each reaction wheel  136 , which includes a centrifugal mass (not depicted), is driven by an associated drive motor (and control electronics)  138 . As will be appreciated by those skilled in the art, only three reaction wheels  136  are required to rotate satellite  100  in the x, y, and z directions. The fourth reaction wheel serves as a spare. Such reaction wheels are typically used for this purpose in satellites. 
         [0043]    Main control processor  164  processes commands received from the ground and performs, autonomously, many of the functions of satellite  100 , including without limitation, attitude pointing control, propulsion control, and power system control. 
         [0044]    Solar-array system  140  includes solar panels  142 A and  142 B and respective y-bars  148 A and  148 B. Each solar panel comprises a plurality of solar cells (not depicted; they are disposed on the obscured side of solar panels  142 A and  142 B) that convert sunlight into electrical energy in known fashion. Each of the solar panels includes motor  144  and passive rotary bearing  146 ; one of the y-bar attaches to each solar panel at motor  144  and bearing  146 . Motors  144  enable each of the solar panels to at least partially rotate about axis A-A. This facilitates deploying solar panel  142 A from its stowed position parallel to and against panel  104  and deploying solar panel  142 B from its stowed position parallel to and against panel  106 . The motors  144  also function to appropriately angle panels  142 A and  142 B for optimal sun exposure via the aforementioned rotation about axis A-A. 
         [0045]    Member  150  of each y-bar  148 A and  148 B extends through opening  152  in respective panels  104  and  106 . Within unified payload module  102 , members  150  connect to main solar-array motor  134 , previously referenced in conjunction with bus component module  132 . The main solar-array motor is capable of at least partially rotating each member  150  about its axis, as shown. This is for the purpose of angling solar panels  142 A and  142 B for optimal sun exposure. In some embodiments, the members  150  can be rotated independently of one another; in some other embodiments, members  150  rotate together. Lock-and-release member  154  is used to couple and release solar panel  142 A to side panel  104  and solar panel  142 B to side panel  106 . The lock-and-release member couples to opening  156  in side panels  104  and  106 . 
         [0046]    Satellite  100  also includes panel  126 , which fits “below” (from the perspective of  FIGS. 1 and 2 ) panel  108  of unified payload module  102 . In some embodiments, panel  108  is a sheet of aerospace grade material (e.g., 6061-T6 aluminum, etc.) Battery module  128  is disposed on the interior-facing surface of panel  126 . The battery module supplies power for various energy consumers onboard satellite  100 . Battery module  128  is recharged from electricity that is generated via solar panels  142 A and  142 B; the panels and module  128  are electrically coupled for this purpose (the electrical path between solar panels  142 A/B and battery module  128  is not depicted in  FIGS. 1 and 2 ). 
         [0047]    Satellite  100  further includes omni-directional antenna  158  for telemetry and ground-based command and control. 
         [0048]    Disposed on panel  108  are two “gateway” antennas  160 . The gateway antennas send and receive user data to gateway stations on Earth. The gateway stations are in communication with the Internet. Antennas  160  are coupled to panel  108  by movable mounts  162 , which enable the antennas to be moved along two axes for optimum positioning with ground-based antennas. Antennas  160  typically transmit and receive in the K a  band, which covers frequencies in the range of 26.5 to 40 GHz. 
         [0049]    Convertor modules  110 , which are disposed on interior-facing surface of panel  106 , convert between K a  radio frequencies and K u  radio frequencies. For example, convertor modules  110  convert the K a  band uplink signals from gateway antennas  160  to K u  band signals for downlink via antennas  124 . Convertor modules  110  also convert in the reverse direction; that is, K u  to K a . 
         [0050]    In operation of satellite  100 , data flows as follows for a data request:
       (obtain data): requested data is obtained from the Internet at a gateway station;   (uplink): a data signal is transmitted (K a  band) via large, ground-based antennas to the satellite&#39;s gateway antennas  160 ;   (payload): the data signal is amplified, routed to convertor modules  110  for conversion to downlink (K u ) band, and then amplified again;   the payload signal is routed to payload antennas  124 ;   (downlink): antennas  124  transmit the amplified, frequency-converted signal to the user&#39;s terminal.
 
When a user transmits (rather than requests) data, such as an e-mail, the signal follows the same path but in the reverse direction.
       
 
         [0056]    Passive Thermal System.  FIG. 3  depicts a cross-sectional view of passive thermal system  370 . 
         [0057]    Passive thermal system  370  comprises housing  372 , which, in the illustrative embodiment, includes wall  374  and wall  376 . Wall  374  is dimensioned and shaped to couple to heat sink/source  396 . In the illustrative embodiment, heat sink/source  396  is a radiator panel, such as radiator panels  104 ,  106 ,  108 ,  112 , etc. As a consequence, heat sink/source  396  is functioning as a heat sink. Also, since radiator panels are relatively flat, wall  374  is flat as well. 
         [0058]    Internal wall  380  extends from wall  374  towards wall  376 . Wall  380  has a curved shape that generally mirrors the shape of wall  376 . Outer chamber  390  is defined between wall  376 , wall  390 , and portions of wall  374 . Inner chamber  386  is defined within wall  380 . 
         [0059]    Heat pipe fluid  388  is contained in inner chamber  386 . Typical heat pipe fluids include ammonia, ethane, propylene, etc. The phrase “heat pipe fluid” is defined for use in this disclosure and the appended claims to mean a fluid that, under the conditions of its use, is intended to change phase between a liquid and a vapor. As is well known to those skilled in the art, heat pipes include a wick structure, the purpose of which is move, via capillary action, the heat pipe fluid (when in liquid form) through the length of the heat pipe. Wick structure  384 , which is disposed in inner chamber  386 , is used for the same purpose. In the illustrative embodiment, wick structure  384  comprises a plurality of projections  385  extending inwardly from wall  380 . The projections extend the length of inner chamber  386 . A variety of wick designs are known in the art and any of such designs may suitably be used in conjunction with the present invention. 
         [0060]    PCM or phase change material  392  is contained in outer chamber  390 . The term “PCM” is defined for use in this disclosure and the appended claims to mean a fluid that, under the conditions of its use, is intended to change phase between a solid and a liquid. Any of a variety of materials can be used as PCM  392 , as is appropriate for the heat load and materials of construction. Typical materials suitable for use as PCM  392  include paraffin or salt hydrate. Fins  382 , which extend outwardly from wall  380 , project into outer chamber  390  and into PCM  392 . The purpose of fins  382  is to increase the heat transfer surface of wall  380  to maximize, to the extent possible, the surface area of the interface between wall  380 /fins  382  and PCM  392 . 
         [0061]    Because PCM  392  is a (very) high viscosity fluid, the heat it receives (from wall  380 /fins  382 ) will not transfer well therein. As a consequence, there will be a smaller temperature gradient over the length of the fin and between the fin and immediately surrounding PCM  392 . It will therefore be important to taper fins  382  such that they are thicker at their base (nearest wall  380 ) than at their tip. This will help to maintain a temperature gradient across fins  382  (because with relatively less mass at the tip than at the base, the tip will cool more quickly than a relatively thicker one). In light of the present disclosure, those skilled in the art will be able to design and build fins  382  suitable for their intended purpose, as discussed above. 
         [0062]    It is desirable to minimize the temperature gradient in PCM  392  between the exterior surface of wall  380  and the interior surface of wall  376 . For the reasons previously discussed, fins  382  should therefore extend well into outer chamber  392 . Based on various considerations, in some embodiments, fins  382  extend 40% or more of the distance between exterior surface of wall  380  and interior surface of wall  376 . In some other embodiments, fins  382  extend 45% or more of the distance between exterior surface of wall  380  and interior surface of wall  376 . And in some yet further embodiments, fins  382  extend 50% or more of the distance between exterior surface of wall  380  and interior surface of wall  376 . 
         [0063]    Housing  372  is coupled to the heat source/heat sink  396  via interface material  394 . The primary function of interface material  394  is to minimize, to the extent possible, the thermal resistance between housing  372  and heat source/heat sink  396 . As a consequence, the interface material should be characterized by a high thermal conductivity, an ability to form a thin bond line, and little or no tendency to form voids over the operating life. With respect to “high” thermal conductivity, a heat transfer coefficient greater than about 500 W/(m 2 K) is desirable. Although the coupling between housing  372  and heat source/heat sink  396  can be supplemented by mechanical fasteners, it is important for interface material  394  to adhere well (e.g., even contact, no voiding, etc.) to both coupled surfaces to keep thermal resistance as low as practical. 
         [0064]    In some embodiments, interface material  394  is room temperature vulcanized silicone (RTV). Other suitable materials for use as interface material  394  include, without limitation, pressure-sensitive adhesives, film adhesives, gaskets, and epoxy. Of course, interface material  394  must be compatible with the material of construction of heat source/heat sink  396  and housing  372 . In the illustrative embodiment, heat source/heat sink  396  is a radiator panel, which is typically formed of aluminum, and housing  372  comprises aluminum, which is typically compatible with the candidate interface materials mentioned above. 
         [0065]      FIG. 4A  depicts, via a side view, arrangement  400  wherein passive thermal system  370  is configured to transfer heat from satellite electronics  401  to satellite radiator panel  402 .  FIG. 4B  depicts a cross-sectional view of the arrangement of  FIG. 4A  through the line A-A and  FIG. 4C  depicts a cross-sectional view of the arrangement of  FIG. 4A  through the line B-B. 
         [0066]    Satellite electronics  401  is representative of any of a number of different electronics systems that are onboard satellite  100  for various purposes. All such electronics typically generate heat that needs to be expelled from the satellite. Satellite radiator panel  402  is representative of radiator panels  104 ,  106 ,  108 ,  112 , etc., of satellite  100 , as shown in  FIGS. 1 and 2 , which can be used to expel the heat generated by satellite electronics  401 . 
         [0067]    As depicted in  FIG. 4A , satellite electronics  401  is disposed near a first end of passive thermal system  370  and radiator panel  402  is disposed near a second end thereof. Passive thermal system  370  is coupled to satellite electronics  401  and radiator panel  402  via interface material  394 , previously discussed (see  FIG. 3  and also  FIGS. 4B, and 4C ). Satellite electronics  401  generates heat, Q, which is collected and transported by passive thermal system  370  to radiator  402 , where heat Q is rejected to space. 
         [0068]    Passive thermal system  370  operates as follows. Heat pipe fluid  388  collects the heat generated from satellite electronics  401 . Fluid  388  is selected such that it evaporates at a very low temperature. For example, saturated ammonia, which is a typical heat pipe material, evaporates at −33° C. PCM  392  typically undergoes phase change (liquid/solid) at a considerably higher temperature, usually in the range of about 20 to 60° C. as a function of the material. As a consequence, most of the heat, Q, collected by passive thermal system  370  transfers to heat pipe fluid  388  in inner chamber  386 . Heat pipe fluid immediately begins to evaporate, transferring heat across inner chamber  386  at near sonic speed. In practice, inner chamber  386  can be considered an isothermal environment because heat transfer is so effective and fast in this temperature range. 
         [0069]    The effect of this rapid heat transfer is to increase the surface area term, A, in the thermal conductance expression [1] between heat pipe fluid  388  in inner chamber  386  and PCM  392  in outer chamber  390 : 
         [0000]        G=h×A    (1)
       where:
           G is the thermal conductance;   h is the heat transfer coefficient; and   A is the contact area.
 
When thermal conductance, G, is large, heat transfers more readily into PCM  392 , such that the PCM is more effective and permitting a larger quantity PCM to be available for use.
   
               
 
         [0074]    It is notable that some heat transfers directly into the PCM from the heat source. However, the typical PCM (e.g., hydrated salt, etc.) is a very poor heat conductor such that there will not be much heat transfer along the length of PCM  392 , especially in a prior art arrangement wherein heat pipe fluid  388  is not present. 
         [0075]    As PCM  392  absorbs heat from heat pipe fluid  388 , it liquefies. As PCM  392  melts, the temperature of heat pipe fluid  388  will plateau. If and when all of PCM  392  melts, the temperature of heat pipe fluid  388  will begin rising again. In preferred embodiments, a sufficient amount of PCM  392  is present in outer chamber  390  so that the PCM never completely melts. 
         [0076]    If the temperature of heat pipe fluid  388  never reaches the phase change temperature of PCM  392 , then no heat storage will occur in the PCM. In such a case, passive thermal system  370  behaves like a conventional heat pipe. 
         [0077]    Once satellite electronics  401  stops generating significant quantities of heat, and to the extent that PCM  392  has stored (via the latent heat of fusion), the PCM slowly releases the stored heat back into heat pipe fluid  388  in chamber  386 . The heat pipe fluid then exchanges heat with radiator  402 , where it is radiated to space. 
         [0078]    Thus, PCM  392  is analogous to a large capacitor, storing energy until it can be released to ground. And it provides a safety net, ready to damp any temperature rise of heat pipe fluid  388 , preventing the fluid from exceeding temperature limitations. 
         [0079]    It is to be understood that the disclosure describes a few embodiments and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.