Abstract:
A liquid-vapor separation system for use in a zero gravity environment for transferring and storing fluids, and particularly fluids that are primarily liquid, such as propellants, reactants and coolants, among others, has a low supply tank pressure and low pressurant gas requirement, which results in maximized capacity, reduced system weight and reduced cost. The temperature of a container provided as part of the present invention is decreased below the freezing point of residual liquid within the container. Non-compressible gas is thereafter vented and the liquid raised to liquid temperature. Gaseous contaminants are thereby vented and subsequent filling of the container is thereby maximized.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to fluid transfer systems, and more particularly to systems for transferring fluids in a zero gravity environment. 
     BACKGROUND OF THE INVENTION 
     In space (i.e., beyond the earth&#39;s atmosphere), the transfer of fluids, such as propellants and reactants, will be required to replenish space based propulsion and power generation systems. For example, re-supply of fuel to the International Space Station&#39;s (ISS) attitude propulsion system from a space shuttle may be provided. The ISS attitude propulsion system corrects for atmospheric drag and disturbances resulting from shuttle orbiter docking over the life of the space station. In addition to the ISS application, space based fuel storage depots with propellant transfer capability will be required to fuel reusable upper stages, as well as to fuel space based transfer vehicles in support of future human exploration and development of space (HEDS). 
     Fluid transfer, including for example, propellant and reactant transfer operation, requires reducing the pressure of a receiver tank through gaseous venting in order to achieve a high liquid fill level. Gaseous venting of a tank in a zero gravity space environment is difficult to achieve because the specific location of the liquid and vapor in the tank is typically not known. In order to provide gaseous venting in a zero gravity atmosphere, a separation of the liquid phase (i.e., liquid content) from the non-condensible pressurant gas (i.e., vapor) is required to prevent the loss of liquid propellant or reactant and maximize tank storage levels. In particular, liquid-vapor separation is needed to efficiently transfer and maximize storage of fluids, such as liquid propellants, in space. 
     Present liquid-vapor separation systems provide for propellant transfer in space using centrifugal force to separate the denser liquid phase from the lighter gas. The centrifugal force causes the liquid to move to the outside of a mechanism creating the force, where it is collected and returned to the tank, leaving the lighter gas in the center of the mechanism. The center gaseous core is subsequently vented outside the tank. These systems provide acceptable operation in low liquid quality applications (e.g., liquid droplets in vapor) wherein the majority of the volumetric flow is made up of gas. However, a centrifugal type system does not operate properly if the fluid comprises primarily a liquid. Further, in a zero gravity environment, problems arise in such systems, including the possibility of liquid phase moving to the vent line inlet, thereby making this type of liquid-vapor separation system ineffective. 
     Thus, a need exists for a liquid-vapor separation system for use in transferring fluids, and particularly fluids that are primarily liquid (e.g., propellants, reactants and coolants), in a zero gravity environment. 
     SUMMARY OF THE INVENTION 
     The present invention provides a system and method for separating the liquid and gaseous parts or phases of fluids (e.g., propellants, reactants and coolants), particularly fluids that are primarily liquid, in a zero gravity environment, through the freezing of the liquid phase. Generally, using a freezing process, liquid within a storage container (e.g., a tank) or similar device is caused to migrate to the storage container wall where the liquid is solidified. After the liquid is solidified, which is provided through the removal of heat from the storage container, non-condensible gas (e.g., pressurant gas) remaining in the storage container can be vented outside (i.e., to space) the storage container without loss of the liquid (e.g., frozen propellant, reactant and/or coolant). The present invention also provides for purging non-condensible gases from inside liquid acquisition screens and enables the filling of capillary devices with liquid. 
     Specifically, a zero gravity liquid-vapor separation system of the present invention is adapted for use in transferring and storing fluids, including for example, propellants that are primarily liquid, in a zero gravity environment. The zero gravity liquid-vapor separation system includes a heat exchanger for lowering the temperature of a container below a freezing point of a fluid (e.g., propellant, reactant and/or coolant) therein and a vent for use in removing non-condensible gas (e.g., pressurant) within the container when the fluid is below its freezing point. 
     Further, the heat exchanger may include a cooling loop with a radiator for cooling the fluid within the container, or, alternately, may include a plurality of louver type members for providing cooling. A diffuser may be used in combination with the vent for venting the container. A heater may be provided in combination with the container for heating the frozen fluid (e.g., frozen propellant, reactant and/or coolant) to a liquid temperature point after venting the non-condensible gas. 
     The present invention also provides a method of transferring a fluid (e.g., propellant, reactant and/or coolant) that is primarily or substantially liquid to a container having residual fluid therein in a zero gravity environment. The method includes lowering the temperature of the residual fluid below the freezing point of the residual fluid, venting the container to remove any non-frozen substances (e.g., non-condensible gases) remaining in the container, raising the temperature of the residual fluid to a liquid point of the residual fluid, and filling the container with additional fluid, such as, for example, a propellant. 
     Thus, the present invention provides a system and method for transferring a fluid (e.g., primarily liquid propellant, reactant and/or coolant) in a zero gravity environment, such that the storage capacity of the receiving container is maximized. Further, venting according to the present invention provides for removing dissolved gas containments from the liquid and results in a gas free liquid prior to filling or refilling the receiving container. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
       
     FIG. 1 is a schematic diagram of a zero gravity liquid-vapor separation system constructed according to the principles of the present invention; 
     FIG. 2 a schematic diagram of another embodiment of a zero gravity liquid-vapor separation system of the present invention; and 
       
     FIG. 3 a schematic diagram a further embodiment of a zero gravity liquid-vapor separation system of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. Thus, although the application of the present invention as disclosed herein is directed to a system for transferring propellants, reactants and coolants in a zero gravity environment and described in connection with specific component parts for the particular application, it is not so limited, and other component parts and applications, including transfer and storage of different fluids are contemplated. 
     Generally, a zero gravity liquid-vapor separation system and method of providing the same constructed according to the principles of the present invention provides for the removal of heat from liquid residual inside a container (e.g., a tank), and subsequent venting of remaining non-compressible gas prior to container fill. Essentially, prior to fill operation, the temperature of a container, such as a tank (i.e., storage or receiver tank) is decreased (i.e., chilled) below the freezing point of liquid residual within the tank, and more preferably, substantially below the freezing point of the liquid residual, to insure that all of the liquid residual has solidified. 
     Thus, for example, storable propellants such as hydrazine, which has a freezing point of 32 degrees Fahrenheit and nitrogen tetroxide (NTO), which has a freezing point of 12 degrees Fahrenheit, a tank temperature of −50 degrees Fahrenheit is preferably provided to insure that liquid residuals are solidified. As another example, Monomehylhydrazine (MMH) has a freezing point of −63 degrees Fahrenheit and a tank temperature of preferably −100 degrees Fahrenheit is provided. It should be noted that because these temperature levels are relatively close to the freezing point of water, a simple Freon® cooling loop with a space radiator, cryocooler, or other device using cryogenic fluids, such as liquid nitrogen, helium, hydrogen, etc. may be used as described herein. 
     Referring now specifically to FIG. 1, and an exemplary construction of a zero gravity liquid-vapor separation system, such a system is shown generally therein and indicated by reference numeral  20 . The liquid-vapor separation system  20  provides for reducing the temperature of a container  22  (e.g., storage or receiver tank), or other suitable storage device, using a heat exchanger  24 , which in this construction is provided external to the tank  22 . As shown therein, the heat exchanger  24  includes a refrigeration system  40  having a radiator  42  that removes heat from a coolant (e.g., Freon®) traveling around the container  22 , which may be provided using, for example, a recirculating pump  44 . In particular, the recirculating pump  44  pumps the coolant through cooling lines  46 , or other suitable fluid transfer member, provided on the external wall(s) of the container  22 , to lower the temperature of the container  22  according to the present invention as described herein. Essentially, a closed loop refrigeration system is provided for cooling the container  22 . 
     In operation, a micro gravity or zero gravity environment will cause residual liquid within the container  22  to contact the wall(s) of the container  22  and lowering the temperature of the wall(s) of the container  22  below the liquid freezing point temperature of residual liquid therein will cause the liquid residual to freeze and solidify to the container  22  wall(s). Thus, the freezing process results in solidified residual liquid, such as, for example, solidified propellant, reactant and/or coolant, among others, adhering to the container  22  wall(s). 
     During the liquid cool-down process resulting in the freezing of the residual liquid, dissolved non-condensible gas, such as a pressurant (e.g., helium, nitrogen, etc.) used to compress the fluid (e.g., propellant, reactant and/or coolant) in the container  22  will also separate out of the solution (e.g., pressurant separates from propellant) as the liquid temperature is lowered. It should be noted that the separation of non-condensible gas (i.e., out-gassing from the liquid) provides for the removal of dissolved gas contaminants from the liquid fluid, resulting in a liquid fluid that is gas free prior to the start of a fill operation. This out-gassing virtually eliminates the build-up of non-condensible gas inside the capillary screens of the container  22  and related storage system. 
     The container  22  as shown in FIG. 1 includes a diffuser  26 , which in combination with a vent system having a vent valve  28 , provides for venting of the non-condensible gas while the fluid in the container  22  is in a frozen state. Preferably, in order to prevent liquid fluid from freezing on the diffuser  26 , which may prevent proper venting of the container  22 , the diffuser  26  is thermally isolated from the container  22  and includes a heater (not shown) to maintain diffuser temperature above the liquid freezing point. It should be noted that the vent system may be provided in any suitable manner according to the requirements of the particular container  22  or system. 
     In operation in accordance with the present invention, the container  22  is preferably first chilled below the freezing point of residual fluid (i.e., liquid) within the container  22  using the heat exchanger  24 . More preferably, the temperature of the container  22  is reduced to a level substantially below the freezing point of the residual fluid (i.e., liquid) therein. Following chill-down of the container  22  to a temperature below the freezing point of residual fluid (e.g., propellant, reactant and/or coolant) therein, venting of the container  22  using the vent valve  28  is provided. Essentially, the vent valve  28  is operated from a closed to an open position to allow venting of non-compressible gas to the atmosphere (e.g., space). It should be noted that because the liquid residual is solidified and remains attached to the container  22  wall, the pressure level of the container  22  can be reduced to space vacuum. Further, the elimination (i.e., a purging and venting) of all of the non-condensible gas (e.g., pressurant) from the container  22  prevents potential gas accumulation inside the liquid acquisition screen channels. Purging of gas from the liquid acquisition system provides proper subsequent liquid re-fill of the zero gravity surface tension acquisition system. 
     After completion of the venting process (i.e., removal of non-compressible gas), the container  22  is locked (i.e., vent valve  28  operated to a closed position) and the temperature is increased (i.e., heat allowed to return to the container  22 ) to the liquid temperature of the residual fluid. In one embodiment, heating of the container  22  is provided gradually through normal environmental heat leak. In an alternate embodiment, an accelerated heating process is provided using wall heaters (not shown) provided to the container  22 , which typically already exist in, for example, storage containers in connection with which the present invention may be constructed (e.g., propellant, reactant and/or coolant storage containers). 
     After the venting of the non-condensible gas and increasing the container  22  temperature, the container  22  remains at a low pressure equal to about the vapor pressure of the residual liquid fluid. A fill process may now be performed, with the container  22  provided at a low pressure (i.e., less than 15 psia) and only vapor in the ullage space (i.e., space unoccupied by liquid residual). In operation, the container  22  is filled using a fill system in combination with a fill valve  30 . Such a fill system may be provided in any suitable manner according to the requirements of the particular container  22  or system. Fluid (e.g., liquid propellant, reactant and/or coolant) is provided through the fill valve  30  in a known manner. It should be noted that as a result of the low pressure level of the container  22 , the pressure used to fill the container  22  is typically between about 20 and 50 psig, thus reducing the amount of pressurant (e.g., helium) needed. 
     With respect to implementing the present invention in connection with a container  22  for use in a zero gravity embodiment, alternate constructions are contemplated. For example, and as shown in FIGS. 2 and 3, different systems for reducing the temperature of the container  22  may be provided. As shown in FIG. 2, a liquid-vapor separation system  20 ′ having an internal heat exchanger  32  comprising may be provided for cooling fluid (e.g., propellant, reactant and/or coolant) within the container  22 . As shown in FIG. 3, a liquid-vapor separation system  20 ″ having louver type members  34  on the external wall(s) of the container  22  may be provided. 
     With respect to the liquid-vapor separation system  20 ′ shown in FIG. 2, in operation, the internal heat exchanger  32  reduces the temperature of gases (e.g., gaseous pressurant) in the container  22 , and through heat transfer, indirectly reduces the temperature of the container  22  wall(s) and the temperature of liquid fluid therein. With respect to the liquid-vapor separation system  20 ″ shown in FIG. 3, the louver type heat exchanger having the louver members  34  provides for direct radiation loss of heat to the atmosphere (i.e., space), and thereby lowers the temperature of the container  22  and the liquid fluid therein. It should be noted that the container  22  having louver type members  34  does not require a radiator, recirculating pump and use of a coolant fluid as in the heat exchangers shown in FIGS. 1 and 2. The louver type members  34  essentially provide insulation of the container  22 . In operation, the louver type members  34  may be moved to an open position as shown in FIG. 3 to radiate heat and thereby reduce the temperature of the container  22 . After the liquid fluid within the container  22  is frozen, the louver type members  34  may be moved to a closed position to provide insulation. 
     Further, it should be noted that alternate liquids may be used as a coolant, such as, for example, liquid nitrogen, hydrogen, helium, etc. It also should be noted that when using cryogenic fluids, the liquid-vapor separation system  20  may be modified, including, for example, removing the radiator  42 , which is no longer needed, as the cryogenic fluid may be vented outside the system after use (i.e., after passing through the cooling lines  46 ). 
     Thus, the present invention provides a system and method for transferring propellant in a zero gravity environment to thereby maximize the fill capability of a receiver container. Further, removal of non-compressible gas virtually eliminates any contaminants in the system. 
     Although the present invention has been described in connection with a specific container having particular component parts for lowering and raising the temperature of the container and the contents therein, it is not so limited, and the present invention may be provided in connection with other containers having different component parts. For example, the type of systems used for venting the container and filling the container may be provided according to the requirements of the specific application or system. Further, cooling and heating of the container may be provided in any suitable manner as required by the particular application or system, including, for example, the configuration of the particular container. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.