Abstract:
Methods and apparatus are provided for a lightweight heat rejection system suitable for spacecraft applications. The apparatus comprises a manifold configured with an array of heat pipes in thermal contact with a manifold coolant. The heat pipes transfer the coolant heat to associated bumper/radiators external to the manifold. The bumper/radiators are fabricated from a lightweight thermally conductive foam material. The bumper portion protects the heat pipe from space debris and the radiator portion dissipates the heat transferred from the heat pipe through the bumper to the radiator portion. The foam bumper/radiator can be cast over the heat pipe in a relatively simple and economical manufacturing process.

Description:
TECHNICAL FIELD  
       [0001]     The present invention generally relates to heat rejection systems, and more particularly relates to a foam bumper and radiator configuration for a lightweight heat rejection system.  
       BACKGROUND  
       [0002]     A heat rejection system is typically used to remove excess heat from a power generating system. One type of heat rejection system, generally known as a pumped loop system, involves the use of a coolant medium circulated through a heat transfer duct in order to transfer heat from a power generating source to heat dissipating radiators. Another type of heat rejection system uses heat pipes rather than heat transfer ducts to transfer heat from a coolant medium to heat dissipating radiators.  
         [0003]     In certain types of high performance heat dissipation applications, such as spacecraft cooling for example, the weight of a heat rejection system can become a limiting factor in the overall performance capabilities of the spacecraft. Moreover, the heat transfer ducts or heat pipes in a spacecraft heat rejection system can be damaged by contact with MicroMeteoroid and Orbital Debris (MMOD), and are therefore typically protected by the inclusion of some type of shielding. Conventional types of protective shields generally complicate the fabrication process of the heat dissipating components while adding undesired weight to the system. In addition, conventional protective shields tend to reduce the thermal radiation efficiency of the heat dissipating components.  
         [0004]     Accordingly, it is desirable to provide a heat rejection system with MMOD protection that is both lightweight and thermally efficient. In addition, it is desirable to provide a fabrication process for the exemplary heat rejection system that is relatively straightforward and economical. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.  
       BRIEF SUMMARY  
       [0005]     According to various exemplary embodiments, devices and methods are provided for reducing the weight and improving the thermal conductivity of a shielded heat rejection system. One exemplary device comprises a manifold configured with a heat transfer passageway for carrying a coolant medium. One or more heat pipes are each configured with a heat input portion that is in thermal contact with the coolant medium within the manifold. The one or more heat pipes are each further configured with a heat output portion that is in thermal contact with an associated heat radiator. The heat radiator is configured in part as a protective bumper enclosing an exposed outer surface of the associated heat pipe, and the heat radiator is further configured with at least one heat-dissipating fin. The heat radiator protective bumper and the heat-dissipating fin(s) are fabricated from a foam material to provide thermal conductivity away from the heat pipe.  
         [0006]     One exemplary method of fabricating the heat radiator foam bumper and fin(s) comprises the steps of casting a foam material around the outer surface of the associated heat pipe, shaping the foam material to form a bumper around the outer surface of the heat pipe, and shaping the foam material to form at least one fin integrated into the outer surface of the foam bumper. The foam bumper is configured to provide a protective shield around the heat pipe, and the foam bumper and fin(s) are further configured to provide thermal radiation away from the heat pipe.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and  
         [0008]      FIG. 1  is an exemplary illustration of a spacecraft with a heat rejection system;  
         [0009]      FIG. 2  is an exemplary illustration of a heat pipe and radiator configuration;  
         [0010]      FIG. 3  is an exemplary illustration of heat pipe operation;  
         [0011]      FIG. 4  is an exemplary illustration of a heat pipe with standoff bumper; and  
         [0012]      FIG. 5  is an illustration of an exemplary embodiment of a foam bumper and radiator configuration. 
     
    
     DETAILED DESCRIPTION  
       [0013]     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description.  
         [0014]     Various embodiments of the present invention pertain to the area of heat rejection systems in applications such as spacecraft cooling, where it is generally desirable to minimize system weight and to maximize the thermal conductivity of the cooling apparatus. Moreover, in space applications, it is also desirable to protect the cooling apparatus from damage by MicroMeteoroid and Orbital Debris (MMOD). A foam bumper and radiator fin configuration is proposed that is generally lighter in weight than conventional bumper and radiator assemblies, and that also provides protection from MMOD. In addition, the proposed foam configuration can improve the thermal conductivity of the cooling apparatus.  
         [0015]     A simplified illustration of one type of space vehicle  100  is shown in  FIG. 1 . In this example, a power source  102  provides power to a propulsion system  104  via a heat rejection system that includes a manifold  106  and a series of radiator panels  108 . Typically, manifold  106  functions as a conduit for a cooling fluid (not shown), such that heat generated from power source  102  is transferred through the cooling fluid in manifold  106  to thermally connected dissipating radiators  108 .  
         [0016]      FIG. 2  depicts one example of a manifold/radiator heat-dissipating configuration  200  that incorporates heat pipes to transfer heat from a manifold to heat dissipating radiators. A manifold  202  is generally configured to receive a cooling fluid  203  that passes through manifold  202 . An array of heat pipes  204  are typically integrated into manifold  202  such that the heat input sections of heat pipes  204  are contacted by cooling fluid  203  as it passes through manifold  202 . As will be described more fully below, the heat received by heat pipes  204  from cooling fluid  203  is typically transferred to the heat output sections of heat pipes  204 . In this example, each heat output section of heat pipes  204  is thermally connected to an associated radiating fin  206 , such that the heat is transferred into radiating fins  206 , from where it is typically radiated into space.  
         [0017]     The operation of a typical heat pipe  300  is illustrated in  FIG. 3 . A closed container  302  generally incorporates a wick structure  304  and a small amount of working fluid  306  that is normally saturated at operating conditions. Heat pipe  300  typically employs a boiling-condensing cycle, wherein a heat input  308  around the evaporation zone  310  of heat pipe  300  causes working fluid  306  to boil and to enter a vapor state with a latent heat of vaporization. The vapor generally moves through heat pipe  300  to a colder location (condensation zone  312 ), where it condenses back into working fluid  306 . The condensation process gives up the latent heat of vaporization as heat output  314  around the condensation zone  312  of heat pipe  300 . Capillary action of wick structure  304  acts to transport the condensate (working fluid  306 ) back to evaporation zone  310  of heat pipe  300 , and the process continues.  
         [0018]     Heat pipes can be designed as highly efficient heat transfer devices since the vapor pressure drop between the evaporation zone and condensation zone is typically very small. As such, the temperature losses between a heat source and the vapor, and between the vapor and a heat sink can be very small. Moreover, heat pipes can be used to transfer relatively large amounts of heat within relatively small, lightweight structures. Therefore, the combined features of efficient heat transfer and lightweight structure make heat pipes generally advantageous for use in heat rejection systems for spacecraft types of applications. It will be appreciated that an alternate heat transfer configuration can incorporate heat transfer ducts (pumped loop) rather than heat pipes in a heat rejection system. As such, the following discussion can pertain to a heat pipe or a heat transfer duct configuration, but will generally be referenced to heat pipes for clarity.  
         [0019]     One potential disadvantage of a typical lightweight heat pipe structure is that it is relatively vulnerable to physical damage. In a spacecraft application, for example, there is the possibility of a collision between the heat pipes built into radiator panels and the previously mentioned MMOD. To protect a lightweight heat pipe from MMOD damage, a standoff bumper type of shielding has typically been mounted around the exposed portion of the heat pipe. One example of a typical standoff bumper configuration  400  is illustrated in the simplified diagram of  FIG. 4 , where a standoff bumper  404  is used to protect the otherwise exposed exterior surface of a heat pipe  402 . Typically, heat pipe  402  is fitted into a close-fitting sleeve (not shown) that structurally reinforces heat pipe  402 , but this type of sleeve is generally not adequate to protect heat pipe  402  from the impact of MMOD. Therefore, standoff bumper  404  is typically attached to the sleeve around heat pipe  402  to provide collision protection for heat pipe  402 . However, a conventional standoff bumper configuration such as  404  will typically add significant weight to the heat rejection system, which is generally disadvantageous for a spacecraft application. Moreover, a typical standoff bumper  404  can reduce the thermal radiation conductivity between heat pipe  402  and an external radiator, which is also generally disadvantageous for a spacecraft or similar application.  
         [0020]     In accordance with an exemplary embodiment of an improved heat pipe/radiator configuration  500  as illustrated in  FIG. 5 , a heat pipe  502  is covered with a foam material  504 . In this embodiment, foam material  504  is configured to function as a protective bumper for the exposed surfaces of heat pipe  502 . In addition, foam material  504  is typically integrated with foam material radiator fins  506  to form a foam material bumper/radiator ( 504 ,  506 ) for heat pipe  502 . Heat pipe  502  is typically enclosed within a structural sleeve (not shown for clarity) between heat pipe  502  and foam material  504 .  
         [0021]     Various types of foam material may be used for bumper/radiator  504 ,  506 , such as metal, carbon-carbon, ceramic, graphite, etc., and/or a combination of these materials. Foam materials such as these are generally available from commercial sources in a wide range of shapes and porosities. One commercial source is ERG Materials and Aerospace Corporation in Oakland, Calif. A distinguishing feature of foam material is the porosity of its open-celled structure, as in a honeycomb pattern, for example. The cavity-like pores of the foam surface typically increase the effective thermal emissivity of the foam material for a more efficient thermal radiation surface. As such, a low-density foam material can improve the thermal conductivity of bumper/radiator  504 ,  506  over that of a conventional material (e.g., aluminum) standoff bumper  404  in  FIG. 4 . That is, foam bumper  504  can conduct heat directly and relatively more efficiently from heat pipe  502  to radiator fins  506 . This increased heat transfer efficiency may allow a reduction in the length of fins  506 , which would generally be desirable for a spacecraft application. Also, the thickness of fins  506  may be increased against the surface of bumper  504 , thereby enabling a further improvement in heat dissipation effectiveness.  
         [0022]     Another advantageous feature of foam material is its lighter weight in comparison to conventional materials. As such, a foam bumper/radiator configuration ( 504 ,  506  in  FIG. 5 ) would typically provide a significant reduction in weight as compared to a conventional solid material standoff bumper and radiator fin configuration. Therefore, a lighter weight foam bumper and radiator can be advantageously configured for a spacecraft or other type of application where weight reduction is a desirable objective.  
         [0023]     In addition to providing improved thermal conductivity and lighter weight, a foam material bumper/radiator can be simpler and more economical to manufacture than a conventional material standoff bumper/radiator. For a conventional material such as a carbon-carbon weave, for example, a relatively complex manufacturing process is typically employed to create the bumper, radiator fins and heat pipe sleeve assembly. In contrast, an integrated foam bumper/radiator can be produced in a more straightforward manner by casting the foam over the heat pipe sleeve and then shaping the bumper and radiator fins to a desired geometry.  
         [0024]     Accordingly, the shortcomings of the prior art have been overcome by providing improved bumper and radiator embodiments for heat pipes or heat transfer ducts in a heat rejection system. The improved bumper and radiator embodiments are typically fabricated in an integrated configuration from a porous foam material that can be cast directly over a heat pipe or heat transfer duct. The resulting foam material bumper and radiator generally provide better thermal conductivity than a conventional material bumper and radiator assembly, and also generally provide a significant reduction in weight. A further benefit of the foam configuration is the relative ease of manufacture in comparison to conventional material assemblies.  
         [0025]     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.