Abstract:
A heat pipe structure is provided for facilitating temperature stability of heat generating devices residing in a spacecraft. The heat generating structure comprises wall tubing made of a low thermal expansion alloy and a low thermal expansion saddle joined together and embedded into composite panel face sheets or face skins with minimal to no coefficient of thermal expansion (CTE) mismatch. The saddle to composite radiating panel interface employs an adhesive as the joining material. The saddle to heat pipe interface is joined together employing a higher conductivity joining media, such as tin-lead solder, which improves the thermal performance of the heat pipe assembly and minimize the temperature drop across the interface.

Description:
This invention was made with Government support under Contract No.TMC96-5835-0094-03 awarded by the United States Air Force. The Government has certain rights in this invention. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to cooling of operating devices on spacecraft, and more particularly to a heat pipe structure with low expansion alloy heat pipes. 
     BACKGROUND OF THE INVENTION 
     Orbiting spacecrafts carry various devices (e.g., electronic devices) that generate unwanted heat during operation of the spacecraft. Additionally, the spacecrafts are subjected to intense environmental conditions that exasperate problems caused by the unwanted heat. Typically, the unwanted heat is removed by cooling devices referred to as heat pipe assemblies. Heat pipe assemblies transfer heat by conduction to one or more heat pipes which then convects the removed heat to radiating panels. In certain heat pipe designs, the radiating panels distribute the heat across the panel to maintain a uniform temperature across the radiating panels to provide isothermal control. In other heat pipe designs, the radiating panels extend outside the orbiting spacecraft such that the heat is radiated into ambient space. Variable conductive heat pipes vary the amount of conductivity, so that the heat remains generally constant. 
     In practice, the heat generating device is affixed to or within a heat pipe assembly having a heat absorbing host structure equipped with radiating panels and having at least one heat pipe embedded therein. The heat from the heat generating device vaporizes a working fluid in the heat pipe which is then condensed and the heat of the condensation conducted to the radiating panels. The embedded heat pipe removes heat from the heat generating device at its evaporator end and the vapors are condensed at its condenser end. The heat pipe assemblies can be fabricated using similar material for the host structure and the heat pipe to avoid problems caused by components having a different coefficient of thermal expansion (CTE) that could cause stress failures in the assembly. Limiting material selection is a compromise that affects the efficiency performance of the heat pipe, contributes to the weight of the spacecraft and is less optimum in terms of heat removal. 
     Current spacecraft design requires the use of lightweight materials possessing near-zero CTE such as composites in order to meet the reduced weight, thermal management, and precision pointing requirements. Composite radiator panels with embedded aluminum heat pipes lead to thermally induced stresses due to the dissimilar CTE of the aluminum and composites facesheets. This causes detachment or debonding of the aluminum heat pipe from the composite panel and/or fracture of the composite facings leading to failure of the thermal control system. Therefore, it is desirable to limit the use of materials with dissimilar thermal expansion coefficients and at the same time meet the preferred thermal and structural requirements of spacecraft construction that lend themselves to easy assembly and fabrication. 
     Thermal performance efficiency of a heat pipe panel is determined by how effective the heat is transferred from the heat source to the heat pipe inner wall. For a conventional extruded aluminum heat pipe, the heat has to travel through the panel skin, across the skin/heat pipe interface, and through the heat pipe extrusion wall to the working fluid, which provides the cooling. This is an efficient design, however, countless efforts have been made to accommodate aluminum heat pipes in composite panels with minimal success. The CTE mismatch between the aluminum and the composite skin is large (e.g., greater than 10 times) and during normal operation, this CTE difference has caused unwanted joint failure and/or panel skin failure. Design sacrifices have been made to provide a more flexible composite panel to support the CTE mismatch of these materials. 
     Previous industry attempts have been made to construct a lightweight heat pipe radiator utilizing an organic matrix composite tube with an aluminum lined foil. However, the shortcomings to this approach are the poor through thermal conductance in the radial direction of the heat pipe that limits heat transfer, as well as the thermal stresses between the thin aluminum liner and the organic matrix composite tube. Prior art, such as “The Embedded Heat Pipe Structure”, U.S. Pat. No. 6,065,529, assigned to TRW, Inc., provided a heat pipe assembly formed of dissimilar CTE materials for the heat pipe and the panel structure by inserting a thermally expandable fluid between the aluminum heat pipe and the composite panel structure. However, a major shortcoming to this arrangement includes a difficult, different, and cost ineffective assembly process to the conventional heat pipe assembly procedures. 
     BRIEF SUMMARY OF THE INVENTION 
     The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended neither to identify key or critical elements of the invention nor delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later. 
     The present invention relates to a heat pipe structure for facilitating temperature stability of heat generating devices residing in a spacecraft. The heat pipe structure comprises tubing made of a low thermal expansion alloy (e.g., Invar) and a low thermal expansion saddle (e.g., carbon-carbon) joined together and embedded into composite panel skins with minimal to no CTE mismatch. The saddle to composite facesheet interface can employ the common approach of using adhesive as the joining material. However, the saddle to heat pipe interface is joined together by using a higher conductivity joining media, such as tin-lead solder, which improves the thermal performance of the heat pipe assembly and minimize the temperature drop across the interface. 
     The heat pipes and the joining interfaces can be plated to facilitate the joining of the saddle to the heat pipes, for example, by soldering. The increase of the thermal heat transfer capability across the heat pipe/saddle interface optimizes the performance of the heat pipe assembly at the saddle/pipe interface. Typical interface media tends to possess low thermal conductivity, thus, lowering thermal performance or the ability to transfer heat from the heat source to the heat pipe. Soldering the saddle to the heat pipe provides a substantially higher thermal conductivity joining material with over a 50 times improvement in thermal transfer properties across this interface compared to the conventional adhesive bond approach. Additionally, the interface is stronger and more robust than conventional adhesive bonding. 
     To the accomplishment of the foregoing and related ends, certain illustrative aspects of the invention are described herein in connection with the following description and the annexed drawings. These aspects are indicative, however, of but a few of the various ways in which the principles of the invention may be employed and the present invention is intended to include all such aspects and their equivalents. Other advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a spacecraft that includes a plurality of heat pipe structures in accordance with an aspect of the present invention. 
     FIG. 2 illustrates a perspective view of a portion of a heat pipe structure in accordance with an aspect of the present invention. 
     FIG. 3 illustrates a cross-sectional view of the heat pipe structure of FIG. 2 taken along lines A—A. 
     FIG. 4 illustrates a cross-sectional view of a heat pipe assembly in accordance with an aspect of the present invention. 
     FIG. 5 illustrates a methodology for fabricating a heat pipe structure in accordance with an aspect of the present invention. 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The present invention relates to an improved lightweight heat pipe structure for removing heat generated by one or more operating devices within a spacecraft. The heat pipe assembly employs lightweight components having similar, low CTE (i.e., less than 7.2 parts-per-million per degrees Kelvin (PPM/K)) with a high thermal conductivity interface. The heat pipe assembly includes a heat pipe formed from a low expansion metal alloy (e.g., iron-nickel based alloy) that is coupled to radiator panel face sheets or face skins via a saddle assembly. The saddle assembly is comprised of a lightweight (i.e., less than 2 grams per cubic centimeter (g/cm 3 )), high thermal conductivity (i.e. equal to or greater than 220 watts per meter-degrees-Kelvin (W/mK)), low CTE material (i.e., less than 7.2 PPM/K), such as carbon-carbon. The metal alloy heat pipe and joining interfaces of the saddle are plated (e.g., nickel, copper), by an electroplating process. The plated metal alloy heat pipe and the joining interfaces of the saddle can then be coupled by solder. 
     The standard saddle/heat pipe design utilizes an adhesive joint at the interface. Polymer based adhesives are inherently poor thermal conductors, thus blocking the efficient heat transfer from the heat source down through the high conductivity saddle and into the heat pipe. Solder alloys offer a 50 times improvement in thermal conductivity properties compared to conventional “high conductivity” adhesives. In addition, a solder joint provides a robust mechanical interface that offers improved thermal transfer, which increases performance and efficiency of the heat pipe assembly. 
     Employing low thermal expansion materials eliminates the CTE variance and provides no structural design limitations or compromises. Additionally, by soldering the interface joint between the saddles and heat pipes, a significant increase in thermal performance is realized across this interface compared to adhesive bonding. This attribute makes this soldered heat pipe design thermally comparable to the baseline aluminum design. A lightweight heat pipe assembly can be constructed that meets the thermal transfer requirements of a heat pipe radiator panel. 
     In one aspect of the invention, a carbon-carbon saddle material is prepared by electro-plating a thin (˜0.002 inch) layer of nickel onto the joining surface. The nickel layer provides a compatible metallic surface for the solder alloy to adhere to the carbon-carbon saddle. Upon the subsequent soldering operation, as the solder flows and solidifies, it provides a mechanical lock between the saddle and the heat pipe. The solder material also provides a much higher thermal conductivity media between the two substrates, thus, providing better heat transfer across the interface than conventional adhesive bonding. 
     The near-zero coefficient of thermal expansion of the configuration allows the heat pipe assembly to be embedded within the graphite composite radiating panel skins or sheets without the formation of thermal stresses. Soldering the carbon-carbon saddle to the low expansion alloy heat pipe provides a much higher thermal conductivity material at the interface, and the interface will be stronger and more robust than conventional adhesive bonding. The solder joint provides over a 50 times improvement in thermal transfer across this interface compared to a conventional, high conductivity adhesive. By providing a higher thermal transfer coefficient from the carbon-carbon to the low expansion alloy heat pipe, thermal efficiency and overall performance is facilitated. 
     FIG. 1 illustrates a spacecraft  10  (e.g., a satellite) that includes a plurality of heat pipe structures. The spacecraft  10  includes a body housing  12  and a plurality of guidance fins  14  and  28  that facilitate steering and guidance of the spacecraft  10  during flight. A first heat pipe structure  16  resides in a lower portion of the body housing  12 . At least one heat generating device  18  (e.g., electronic device) is mounted to one side of a host structure (e.g., radiator panel) of the first heat pipe structure  16 . The first heat pipe structure  16  is operative to distribute heat generated by the at least one heat generating device  18  across the host structure, so that the at least one heat generating device  18  remains within a desired operating temperature range. The heat generated by the at least one heat generating device  18  conducts to at least one heat pipe associated with the first heat pipe structure  16 . The conducted heat causes a working fluid (e.g., ammonia) to vaporize at an evaporating end of the heat pipe. The working fluid is then condensed and the heat of the condensation transferred through a convection end of the heat pipe. The heat is then convected to the host structure through the convection end of the heat pipe. 
     A second heat pipe structure  20  resides along a sidewall of the body housing  12 . At least one heat generating device  22  is mounted to one side of a host structure (e.g., radiator panel) of the second heat pipe structure  20 . The second side of the host structure is mounted to a sidewall of the body housing  12 . The second heat pipe structure  20  is operative to distribute heat across the host structure that is dissipated into ambient space through the body housing  12 . A third heat pipe structure  24  is mounted to the fin assembly  28  that extends through and around the body housing  12 . At least one heat generating device  26  is mounted to one side of a host structure of the third heat pipe structure  24 . The third side of the host structure is mounted to the fin assembly  28 . The third heat pipe structure  24  is operative to distribute heat across the host structure that is dissipated into ambient space through the fin assembly  28  that extends outside the body housing  12 . 
     At least one of the first heat pipe structure  16 , the second heat pipe structure  20  and the third heat pipe structure  24  includes one or more metal alloy heat pipes (e.g., formed from an iron-nickel based alloy) soldered to saddle mounts residing within a host structure, such that the alloy heat pipes and the saddle mounts are fabricated from materials having similar, low CTE (i.e., less than 7.2 PPM/K). Additionally, the soldering interface provides high thermal conductivity for the heat pipe structure. It is to be appreciated that a single heat pipe structure or a plurality of heat pipe structures can reside within the body housing by a variety of different mounting configurations. In the example of FIG. 1, the first heat pipe structure  16  is an isothermally controlled structure, while the second and third heat pipe structures  20  and  24  are deployable radiators such that the heat is dissipated into space. 
     FIG. 2 illustrates a perspective view of a portion of a heat pipe structure  40  in accordance with an aspect of the present invention. The heat pipe structure  40  includes a plurality of metal alloy heat pipe tubes  46  extending longitudinally through the heat pipe structure  40 . The plurality of heat pipe tubes  46  are disposed within associated saddle assemblies  48  separated by a core  50 . The saddle assemblies reside between a top radiating panel skin  42  and a bottom radiating panel skin  44 . A plurality of operating devices  52  (e.g., electronic components) are affixed to the top radiating panel skin  42 . The saddle assemblies  48  can extend the entire length of the heat pipes  46  or be comprised of individual blocks in a spaced apart relationship. The heat pipes  46  reside underneath the operating devices  52  and are operative to conduct heat from the operating devices  52 . A working fluid (e.g., ammonia) within the heat tubes vaporizes from the heat of the operating devices  52  and convects the heat to an end (not shown) away from the operating devices  52 . The heat can also be distributed across the top and bottom radiating panel skins  42  and  44 , so that the operating devices  52  remain within an acceptable operating temperature range. 
     FIG. 3 illustrates a cross-sectional view of the host structure of FIG. 2 taken along lines A—A. The core  50  is formed of honeycomb material that separates the heat pipes from one another by a predetermined distance. The honeycomb can be formed from small sheets of aluminum (e.g., {fraction (1/1000)} inch thick). Each heat pipe  46  is formed from a low CTE metal alloy, such as INVAR  36 . INVAR  36  is an iron-nickel alloy which has a rate of thermal expansion approximately one-tenth that of carbon steel at temperatures up to 400° F. (204° C.). INVAR  36  is approximately 36% nickel with the balance being iron or steel. INVAR is trademarked by STE. AME. DE COMMENTRY FOURCHAMBAULT ET DECAZEVILLE CORPORATION of France. INVAR has a CTE of about 0.9 PPM/K to about 3.6 PPM/K. Alternatively, the metal alloy can be KOVAR a nickel-iron alloy with blends of copper and tungsten (Cu—W) or copper and molybdenum (Cu—Mo). KOVAR is a trademark of WESTINGHOUSE ELECTRIC &amp; MANUFACTURING COMPANY. The metal alloy heat pipe is plated with nickel or copper and soldered to a top saddle portion  54  and a bottom saddle portion  56  of the saddle assembly  48 . The top saddle portion  54  and the bottom saddle portion  56  are comprised of a low CTE graphite, such as carbon-carbon or carbon-foam. Carbon-carbon has a CTE of about 1.0 PPM/K and a thermal conductivity of up to 400 W/mK. 
     The joining interfaces of the top saddle portion  54  and the bottom saddle portion  56  are formed of semi-cylindrical recesses that mate with the cylindrical shape of the heat pipes  46 . The joining interfaces are also nickel plated to facilitate soldering of the top and bottom saddle portions  54  and  56  to the heat pipe  46 . The top saddle portion  54  and bottom saddle portion  56  are fixed to the top radiating panel skin  42  and the bottom radiator panel skin  44 , respectively, employing adhesive. The top and bottom radiating panel skins  42  and  44  can be comprised of graphite composite facesheets that possess a similar low CTE as the heat pipe  46  and the saddle assembly  48 , thus, forming an embedded heat pipe radiator panel with low thermal expansion characteristics for overall dimensional stability and high thermal dissipation capabilities. Alternatively, the top and bottom radiator panel skins  42  and  44  can be formed from beryllium and carbon-carbon or other forms of graphite. Separating walls  58  isolate the heat pipes  46  and saddle portions  48  from the core  50 . 
     FIG. 4 illustrates a cross-sectional view of a heat pipe assembly in accordance with an aspect of the present invention. The heat pipe assembly includes a wick  70  extending through the center of the heat pipe  46 . The heat pipe  46  includes an outer wall  66  that extends around the circumference of the heat pipe  46 . A plating layer  68  surrounds the outer wall  66  of the heat pipe  46 . It is to be appreciated that the plating layer  68  only needs to cover areas that interface to the top saddle portion  54  and the bottom saddle portion  56 . The top saddle portion  54  includes a first side with a concave semi-cylindrical recess portion  76  that provides a joining surface that mates with at least a portion of the circumference of the heat pipe  46 , and a second side that has a generally planar surface  80  that provides a joining surface for mating to a radiating panel skin. The bottom saddle portion  56  includes a first side with a concave semi-cylindrical recess portion  78  that provides a joining surface that mates with at least a portion of the circumference of the heat pipe  46 , and a second side that has a generally planar surface  82  that provides a joining surface for mating to a radiating panel skin. 
     The concave semi-cylindrical recess portion  76  of the top saddle portion  54  includes a plating layer  64  and the concave semi-cylindrical recess portion  78  of the bottom saddle portion  56  includes a plating layer  62 . The plating layers  62 ,  64  and  68  facilitate soldering of the heat pipe to the top saddle portion  54  and the bottom saddle portion  56 . The plating layers  62 ,  64  and  68  can be formed from a plating material such as nickel or copper. A soldering layer  72  couples the top saddle portion  54  to the heat pipe  46  and a soldering layer  74  couples the bottom saddle portion  56  to the heat pipe  46 . The soldering layers  72  and  74  can be solder plated to the heat pipes  46  and/or joining surfaces of the top and bottom saddle portions  54  and  56 , and the heat pipe assembly heated in an oven to bond the heat pipes  46  to the top and bottom saddle portions. Alternatively, the heat pipe  46  can be soldered to the joining surfaces of the top and bottom saddle portions  54  and  56  by hand or other common processes. 
     In view of the foregoing structural and functional features described above, a methodology in accordance with various aspects of the present invention will be better appreciated with reference to FIG.  5 . While, for purposes of simplicity of explanation, the methodology of FIG. 5 is shown and described as executing serially, it is to be understood and appreciated that the present invention is not limited by the illustrated order, as some aspects could, in accordance with the present invention, occur in different orders and/or concurrently with other aspects from that shown and described herein. Moreover, not all illustrated features may be required to implement a methodology in accordance with an aspect the present invention. 
     FIG. 5 illustrates a methodology for fabricating a heat pipe structure in accordance with an aspect of the present invention. The methodology begins at  100  where metal alloy pipes are plated, for example, by employing a plating process. The metal alloy pipes can be fabricated from an iron-nickel alloy metal such as INVAR or KOVAR having high temperature capability and a low CTE. The metal alloy pipes can be plated with nickel and/or copper to facilitate soldering. At  110 , saddle portions are fabricated and the joining surfaces are plated, for example, employing nickel electro-plating. The saddles can be formed of a composite graphite having a low CTE, such as carbon-carbon. The saddle joining interfaces are formed of a semi-cylindrical recess portion that mates with at least a portion of the circumference of the metal alloy pipe. The recess portions are plated to facilitate soldering of the saddle joining interfaces to the metal alloy pipes. At  120 , the metal alloy pipes and the saddle joining interfaces are then soldered by plating at least one of the metal alloy pipes and the saddle joining interfaces with a solder layer, and baking in an oven to bond the saddle joining interfaces and metal alloy pipes. A variety of different soldering techniques can be employed to couple the metal alloy pipes to the saddle joining interfaces. The methodology then proceeds to  130 . 
     At  130 , the saddles are bonded to a first radiating panel face skin or face sheet at space apart locations. The saddles can be bonded to the first radiating panel skin by an epoxy resin. At  140 , a core material is provided between the heat pipes and saddles in the space apart location to separate the heat pipes and saddles. Alternatively, the core material can be placed in the space apart locations prior to the saddles being bonded to the first radiating panel skin. The core material can be a honeycomb configuration of aluminum sheets bonded to the first radiating panel skin employing an epoxy. At  150 , a second radiating panel skin is bonded to the other side of the saddles via an epoxy to complete the construction of the heat pipe structure. 
     What has been described above includes exemplary implementations of the present invention. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims.