TECHNICAL FIELD 
     The present invention relates generally to the safe storage and transfer of cryogenic fluids inside the cargo bay of a reusable launch vehicle, and more particularly to cryogenic fluid transfer systems for transferring supercritical cryogenic fluids to subcritical storage tanks in zero gravity environments, thus eliminating potential ignition hazards associated with cryogenic oxygen and hydrogen storage and management through a unique fluid transfer process in a space environment. 
     BACKGROUND OF THE INVENTION 
     Cryogenic fluids such as liquid oxygen (LO2) and liquid hydrogen (LH2) are widely used by the aerospace industry as propellants, reactants for power generation, life support systems, sensor cooling, and the like. Although launch vehicles, such as the Space Shuttle, use these cryogens routinely with on-board systems, the storage and handling of these cryogens has been discouraged for payloads due to serious safety issues arising from storage and handling of cryogens inside a closed payload bay compartment. This is due to the fact that reusable launch vehicles (RLV), such as the Space Shuttle, impose unique safety requirements on cryogenic payloads because the payload must be loaded with cryogens on the ground inside a closed compartment, and the RLV must return to the ground with the payload intact in case of an aborted mission. Because the LO2 or LH2 tank is inside a closed cargo bay, serious safety issues arise during loading or after an aborted mission touchdown from small leaks and post landing venting. The concern is that a small amount of hydrogen or oxygen leakage over an extended period of time, e.g., during loading, launch, or post-touchdown, can cause a buildup of hazardous gas concentrations which can result in a fire or catastrophic explosion causing the possible loss of the space vehicle and its crew. Safety issues associated with ignition and explosion can be eliminated if the payload does not have cryogens below altitudes where ignition/explosion can occur. For hydrogen and oxygen the safe altitude where ignition does not occur is above 160,000 ft. At this altitude the atmospheric pressure is too low to support ignition, and therefore hydrogen and oxygen behaves as inert fluids like nitrogen or helium. 
     Because RLV&#39;s, such as the Space Shuttle, contain cryogenic storage tanks for on-board power generation and life support systems, it is possible to transfer hydrogen or oxygen into payloads once the vehicle reaches a safe altitude where ignition hazards are eliminated and where there is sufficient time to completely dump and vacuum inert the payload storage tanks prior to landing. One source of cryogenic fluid is the supercritical storage tanks used to generate electrical power for the Shuttle. The Shuttle&#39;s supercritical storage system consists of LO2 and LH2 tanks located in the Orbiter vehicle and also additional storage tanks located on a palette inside the cargo bay referred to as the extended duration orbiter (EDO) tanks. To eliminate liquid acquisition devices that are typically needed in a zero gravity environment, the cryogenic LH2 and LO2 is stored at super critical pressures. Consequently, the fluid is stored as a single phase fluid with no liquid vapor phase. The supercritical pressure is maintained by adding electrical heat to the tank to offset the pressure decay from fluid expulsion. Because the cryogens are stored at supercritical pressures, fluid transfer to a subcritical cannot be done directly. 
     The cryogenic storage tanks on the EDO pallet are typically tied into (i.e., in fluid communication with) both the fuel cells/life support systems and the pre-existing standard supercritical storage tanks in the orbiter, as shown in the configuration depicted in FIG.  1 . The EDO cryogenic fluid storage system  10  typically consists of a tank  12  having a fill port  14  and a vent port  16 . A conduit  18  from the vent port  16  branches off into another conduit  20  that leads to a relief valve  22  which in turn leads to a common relief line  24 . Conduit  18  also branches off into another conduit  26  which leads to a shutoff valve  28  which in turn leads to a conduit  30  which is in fluid communication with the orbiter cryogenic fluid storage system  32 . The orbiter cryogenic fluid storage system  32  typically consists of a tank  34  having a fill port  36  and a vent port  38 . A conduit  40  from the vent port  38  leads to a shutoff valve  42  which leads to a conduit loop  44  having a check valve  46  located therein. A conduit  48  from the shutoff valve  42  leads to a vent disconnect assembly  50 . Conduit  30  from vent port  16  is in fluid communication with conduit loop  44  and conduit  48 . A conduit  52  from fill port  36  leads to shutoff valve  54  which leads to a conduit loop  56  having a check valve  58  located therein. A conduit  60  from the shutoff valve  62  leads to a fill disconnect assembly  62 . A conduit  64  from the fill port  14  leads to a shutoff valve  66  which in turn leads to a conduit  68  which is in fluid communication with conduit  60 . In order to supply cryogenic fluid to the orbiter&#39;s fuel cells and life support systems, it is necessary to provide supply conduits from the two main sources of cryogenic fluid. The EDO cryogenic fluid storage tank  12  is provided with a conduit  70  which leads to a check valve  72  which in turn leads to a conduit  74  which is in fluid communication with the orbiter&#39;s fuel cells and life support systems. Likewise, the orbiter cryogenic fluid storage tank  34  is provided with a conduit  76  which leads to a check valve  78  which in turn leads to a conduit  80  (which ties into conduit  74 ) which is also in fluid communication with the orbiter&#39;s fuel cells and life support systems. 
     Therefore, there is a need for a system that permits the safe and efficient transfer of cryogenic fluids from supercritical storage systems to subcritical storage systems, especially in low g and/or zero-g vacuum environments. 
     The present invention provides for the safe transfer of LO2 or LH2 from the Space Shuttle supercritical tanks in a low g vacuum environment which enables cryogenic upper stages to be flown in the cargo bay of the Space Shuttle or second generation RLV. The cryogens that can be transferred to a payload cryogenic tank may be used to demonstrate long term cryogenic fluid management, power upper stages, and provide reactants for power generation, cool sensors or electronic equipment. 
     SUMMARY OF THE INVENTION 
     It is therefor an object of the present invention to provide a new and improved cryogenic fluid transfer system. 
     It is another object of the present invention to provide a new and improved cryogenic fluid transfer system for use in zero gravity environments. 
     It is still another object of the present invention to provide a new and improved cryogenic fluid transfer system for transferring a cryogenic fluid from a supercritical cryogenic fluid storage system to a subcritical cryogenic fluid storage system. 
     In accordance with one embodiment of the present invention, a cryogenic fluid transfer system for transferring a cryogenic fluid from a supercritical cryogenic fluid storage system is provided, comprising: 
     a first subcritical cryogenic fluid storage system for receiving the cryogenic fluid from the supercritical cryogenic fluid storage system; 
     a conduit for providing fluid communication between the supercritical cryogenic fluid storage system and the first subcritical fluid storage system; and 
     a heat exchanger assembly in contact with the conduit, the heat exchanger assembly located downstream of the supercritical cryogenic fluid storage system and upstream of the first subcritical fluid storage system; 
     wherein the heat exchanger assembly cools the cryogenic fluid expelled from the supercritical cryogenic fluid storage system prior to the cryogenic fluid being introduced into the first subcritical fluid storage system. 
     In accordance with another embodiment of the present invention, a cryogenic fluid transfer system is provided, comprising: 
     a supercritical cryogenic fluid storage system; 
     a first subcritical cryogenic fluid storage system for receiving the cryogenic fluid from the supercritical cryogenic fluid storage system; 
     a conduit for providing fluid communication between the supercritical cryogenic fluid storage system and the first subcritical fluid storage system; 
     a heat exchanger assembly in contact with the conduit, the heat exchanger assembly located downstream of the supercritical cryogenic fluid storage system and upstream of the first subcritical fluid storage system; 
     wherein the heat exchanger assembly cools the cryogenic fluid expelled from the supercritical cryogenic fluid storage system prior to the cryogenic fluid being introduced into the first subcritical fluid storage system; and 
     a source of pressurized inert gas in fluid communication with the first subcritical fluid storage system, wherein the source of pressurized gas permits the pressurization of the first subcritical cryogenic fluid storage system. 
     In accordance with still another embodiment of the present invention, a cryogenic fluid transfer system is provided, comprising: 
     a supercritical cryogenic fluid storage system; 
     a first subcritical cryogenic fluid storage system for receiving the cryogenic fluid from the supercritical cryogenic fluid storage system; 
     a second subcritical fluid storage system for receiving the cryogenic fluid from the supercritical cryogenic fluid storage system or the first subcritical cryogenic fluid storage system; 
     a conduit for providing fluid communication among the supercritical cryogenic fluid storage system and the first and second subcritical fluid storage systems; 
     a heat exchanger assembly in contact with the conduit, the heat exchanger assembly located downstream of the supercritical cryogenic fluid storage system and upstream of the first and second subcritical fluid storage systems; 
     wherein the heat exchanger assembly cools the cryogenic fluid expelled from the supercritical cryogenic fluid storage system prior to the cryogenic fluid being introduced into the first or second subcritical fluid storage systems; 
     a source of pressurized inert gas in fluid communication with the first and second subcritical fluid storage systems, wherein the source of pressurized gas permits the pressurization of the first and second subcritical cryogenic fluid storage systems; and 
     a gaseous fluid source in fluid communication with the first and second subcritical fluid storage systems and the supercritical cryogenic fluid storage system. 
     Additional objects, advantages, and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various advantages of the present invention will become apparent to one skilled in the art by reading the following specification and subjoined claims and by referencing the following drawings in which: 
     FIG. 1 is schematic illustration of the fluid communication configuration of an extended duration orbiter pallet cryogenic fluid storage tank with the orbiter cryogenic storage tank and the orbiter fuel cell/life support systems, in accordance with the prior art; 
     FIG. 2 is partially broken away side elevational view of an extended duration orbiter pallet system in proximity to a payload tank containing two primary components of a cryogenic fluid transfer system, in accordance with one aspect of the present invention; 
     FIG. 3 is schematic illustration of the fluid communication configuration of a cryogenic fluid transfer system of the present invention with the extended duration orbiter pallet cryogenic fluid storage tank/orbiter cryogenic storage tank system, in accordance with one aspect of the present invention; 
     FIG. 4 is perspective view of an extended duration orbiter pallet system in proximity to an alternative embodiment of a cryogenic fluid transfer system, in accordance with one aspect of the present invention; and 
     FIG. 5 is schematic illustration of the fluid communication configuration of an alternative embodiment of a cryogenic fluid transfer system of the present invention and the extended duration orbiter pallet cryogenic fluid storage tank/orbiter cryogenic storage tank system, in accordance with one aspect of the present invention. 
    
    
     The same reference numerals refer to the same parts throughout the various Figures. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following discussion of the preferred embodiments directed to cryogenic fluid transfer systems for transferring supercritical cryogenic fluids to subcritical storage tanks or payloads in zero gravity environments are merely exemplary in nature, and are in no way intended to limit the invention or its applications or uses. 
     A first embodiment of the present invention includes a cryogenic fluid transfer system which is able to transfer a cryogenic fluid from a supercritical cryogenic fluid storage system to a subcritical cryogenic fluid storage system. The cryogenic fluid transfer system includes three basic components: a storage tank, a pressurization tank, and a heat exchanger, the function of all of which will be explained herein. Additional components such as conduits, valves, check valves, regulators, orifices, pumps, diffusers, as well as other auxiliary components not specifically mentioned or discussed, enable the cryogenic fluid transfer system of the present invention to function properly. 
     Referring to FIG. 2, there is shown an intended placement of the cryogenic fluid transfer system  100  of the present invention in proximity to the supercritical cryogenic fluid storage system  102  of the extended duration orbiter pallet  104 . It will be noted that the cryogenic fluid transfer system  100  of the present invention is depicted as being housed within a payload tank  104  (e.g., a satellite); however, it should be noted that the cryogenic fluid transfer system  100  of the present invention is not required to be contained within any type of structure. In this view, the cryogenic storage tank  106  and the pressurization tank  108  of the cryogenic fluid transfer system  100  of the present invention are clearly shown. 
     Referring to FIG. 3, there is shown a schematic illustration of the fluid communication configuration of the cryogenic fluid transfer system  100  of the present invention with the extended duration orbiter pallet cryogenic fluid storage tank/orbiter cryogenic storage tank system  102 , in accordance with one aspect of the present invention. It should be noted that this illustrative configuration may be significantly modified or altered without departing from the scope of the present invention. 
     Electrical heaters  200 ,  202 , respectively, are installed in the EDO and pre-existing standard supercritical fluid storage tanks  204 ,  206 , respectively, to maintain a supercritical pressure during the cryogenic fluid expulsion and transfer processes. In this illustrative illustration, a conduit  208  coming off from the main supply conduit  74  to the fuel cells and life support systems leads to a valve  210 . 
     After valve  210 , there is a conduit loop  212  containing an orifice  214 . After the conduit loop  212 , there is a line heat exchanger  216  in contact with the conduit portion downstream of the conduit loop  212 . Heat exchanger  216  (which can be a parallel flow concentric tube design, or other type of conventional heat exchanger) subcools the two-phase cryogenic fluid into a single phase cryogenic fluid. At the terminal portion of heat exchanger  216  furthest downstream of the extended duration orbiter pallet cryogenic fluid storage tank/orbiter cryogenic storage tank system  102 , there is a side conduit  218  leading to a generic vent  220 . Disposed on side conduit  218  is an orifice  222  and a valve  224 . Downstream of heat exchanger  216  is a conduit  226  which leads to an orifice  228  which is turns leads to a valve  230 , which is turn leads to a T-shaped conduit junction  232 . 
     A side conduit  234  leads to a valve  236  which leads to a conduit  238  that is in fluid communication with subcritical cryogenic fluid storage tank  106 . Downstream of T-shaped conduit junction  232  is a conduit  240  which leads to a valve  242  which in turn leads to a pump  244 . A conduit  246  from pump  244  is in fluid communication with subcritical cryogenic fluid storage tank  106 . A conduit loop  248  is disposed in conduit  246 . A valve  250  and orifice  252  are disposed within conduit loop  248 . Conduit  246  is surrounded by a temperature control assembly  254  (e.g., a heat exchanger) located inside of subcritical cryogenic fluid storage tank  106 . The top portion of temperature control assembly  254  includes a conduit  256  leading to the generic vent  220 . 
     Pressurization tank  108  (containing an inert gas such as helium) is also in fluid communication with subcritical cryogenic fluid storage tank  106 . A conduit  258  which leads to a check valve  260  which leads to a regulator  262  which leads to a valve  264  which leads to another conduit  266  which terminates inside subcritical cryogenic fluid storage tank  106 . At the terminus of conduit  266  there is located a diffuser  268 . 
     In operation, the space vehicle&#39;s subcritical storage tanks or payload tanks, as the case may be, are typically located in the payload bay of the space vehicle, and are pressurized with low pressure helium on the ground, i.e., prior to launch. Once the space vehicle is safely in orbit, the payload bay doors are opened and the helium contained within the subcritical storage tank is vented to the atmosphere. There is now a safe environment to transfer liquid hydrogen and oxygen from the existing onboard supercritical cryogenic hydrogen/oxygen supply or the EDO pallet. Once the payload bay doors are opened it is safe to transfer the cryogenic hydrogen/oxygen from the EDO pallet&#39;s supercritical cryogenic fluid storage tanks to the subcritical cryogenic payload. 
     After venting the tank pressure, the subcritical cryogenic fluid storage tank is initially chilled using a spraybar/heat exchanger assembly as described in U.S. Pat. No. 5,398,515. The chilidown continues until the vent temperature or the tank wall temperature is appropriate. A series of valves are used to control the flow of the supercritical cryogenic fluid from the supercritical cryogenic fluid storage tanks to the subcritical cryogenic fluid storage tanks. 
     Once the subcritical cryogenic fluid storage tank is sufficiently chilled, the cryogenic fluid flows from the supercritical tank, by opening any number of appropriate valves (at this point the single phase supercritical cryogenic fluid becomes a two phase subcritical cryogenic fluid), and passes through a heat exchanger, which subcools the cryogenic fluid so that it recondenses into a single phase cryogenic fluid. The single phase, subcooled cryogenic fluid is then spray injected into the subcritical cryogenic fluid storage tank. A vapor-cooled shield inside the subcritical cryogenic fluid storage tank continues to receive subcooled cryogenic fluid from the heat exchanger. The subcooled cryogenic fluid wraps around the walls of the subcritical cryogenic fluid storage tank to continue the cooling process. Details concerning the spray bar and the vapor shield are also described in U.S. Pat. No. 5,398,515. This process subcools the cryogenic fluids from the supercritical cryogenic fluid storage tank and maintains a very low temperature cryogenic fluid in the subcritical cryogenic fluid storage tank. Flow of the cryogenic fluid continues until a desired level is reached in the subcritical cryogenic fluid storage tank. 
     During the time when the cryogenic fluid is transferred from the supercritical cryogenic fluid storage tank to the subcritical cryogenic fluid storage tank, a small fraction of the cryogenic fluid is vented through an appropriate valve and sent overboard through a vapor cooled shield flow control orifice and vent line. 
     The pressurization tank is used to pressurize the subcritical cryogenic fluid storage tank to the operating pressure. The pressurant (e.g., helium gas) comes from the pressurization tank through a regulator and isolation valve and then in through an internal diffuser. 
     Referring to FIG. 4, there is shown a perspective view of an extended duration orbiter pallet system  300  in proximity to an alternative embodiment of a cryogenic fluid transfer system  302 , in accordance with one aspect of the present invention. In this view, the subcritical cryogenic fluid storage tank  304  and pressurization tank  306  are supplemented with an additional subcritical cryogenic fluid storage tank  308  (also referred to as a receiver tank) as well as a gaseous fluid storage tank  310  (e.g., hydrogen or oxygen), both of which are in fluid communication with all of the major components of the cryogenic fluid transfer system  302 . 
     Referring to FIG. 5, there is shown a schematic illustration of the fluid communication configuration of an alternative embodiment of a cryogenic fluid transfer system  302  of the present invention and the extended duration orbiter pallet cryogenic fluid storage tank/orbiter cryogenic storage tank system  300 , in accordance with one aspect of the present invention. It should be noted that this illustrative configuration may be significantly modified or altered without departing from the scope of the present invention. 
     Electrical heaters  312 ,  314 , respectively, are installed in the EDO and pre-existing standard supercritical fluid storage tanks  316 ,  318 , respectively, to maintain a supercritical pressure during the cryogenic fluid expulsion and transfer processes. In this illustrative illustration, a conduit  320  coming off from the main supply conduit  74  to the fuel cells and life support systems leads to a set of two valves  322 ,  324 , respectively. 
     After the second valve  324 , there is a conduit loop  326  containing an orifice  328 . After conduit loop  326 , there is a line heat exchanger  330  in contact with the conduit portion downstream of the conduit loop  326 . Heat exchanger  330  (which can be a parallel flow concentric tube design) subcools the two-phase cryogenic fluid into a single phase cryogenic fluid. At the terminal portion of heat exchanger  330  furthest downstream of the extended duration orbiter pallet cryogenic fluid storage tank/orbiter cryogenic storage tank system  300 , there is a side conduit  332  leading to a generic vent  334 . Disposed on side conduit  332  is an orifice  336  and a valve  338 . 
     Downstream of heat exchanger  330  is a conduit  340  which leads to an orifice  342  which is turns leads to a valve  344 , which is turn leads to a T-shaped conduit junction  346 . A side conduit  348  leads to a valve  350  which leads to a conduit  352  that is in fluid communication with subcritical cryogenic fluid storage tank  304 . 
     Downstream of the T-shaped conduit junction  346  is a conduit  354  which leads to a valve  356  which in turn leads to a pump  358 . A conduit  360  from the pump  358  is in fluid communication with subcritical cryogenic fluid storage tank  304 . A conduit loop  362  having a valve  364  and an orifice  366  are disposed in conduit loop  362 . Conduit  360  is surrounded by a temperature control assembly  368  (e.g., a heat exchanger) located inside of subcritical cryogenic fluid storage tank  304 . The top portion of temperature control assembly  368  includes a conduit  370  having a valve  372  which eventually leads to generic vent  334 . 
     Pressurization tank  306  (containing an inert gas such as helium) is also in fluid communication with subcritical cryogenic fluid storage tank  304 . A conduit  374  which leads to a check valve  376  which leads to a regulator  378  which leads to a valve  380  which leads to another conduit  382  (which leads to valve  383 ) which terminates inside subcritical cryogenic fluid storage tank  304 . At the terminus of the conduit  382  there is located a diffuser  384 . 
     The cryogenic fluid receiver tank  308  is also in fluid communication with cryogenic fluid storage tank  304 , as well as pressurization tank  306  and gaseous fluid tank  310 . A conduit  386  from conduit  320  leads to a valve  388  which leads to an orifice  390  which in turn leads to a check valve  392 . A conduit  394  from check valve  392  branches into a conduit  396  which leads to a valve  398  which leads to a conduit  400  which in turn leads to conduit  332 . 
     The other branch of conduit  394  leads to a conduit  402  which is in fluid communication with gaseous fluid storage system  310 . Downstream of conduit  402  is a check valve  404  which leads to a regulator  406  which in turn leads to a valve  408  which eventually leads to conduit  382 . Conduit  382  then leads to valve  410  which leads to conduit  412  which terminates inside the cryogenic fluid receiver tank  308 . At the terminus of conduit  412  there is located a diffuser  414 . 
     A side conduit  416  from conduit  360  leads to an orifice  418  which leads to a valve  420  which leads to a conduit  422 . Conduit  422  leads to a T-shaped conduit junction  424 . A conduit  426  is in fluid communication with the subcritical cryogenic fluid receiver tank  308 . A conduit loop  428  having a valve  430  and an orifice  432  are disposed in conduit loop  428 . Conduit  426  is surrounded by a temperature control assembly  434  (e.g., a heat exchanger) located inside of subcritical cryogenic fluid receiver tank  308 . The top portion of temperature control assembly  434  includes a conduit  436  having a valve  438  which eventually leads to generic vent  334 . 
     On the other side of T-shaped conduit junction  424  there is a conduit  440  having a valve  442  leading to a pump  444 . A T-shaped conduit junction  446  branches off into a conduit  448  which is in direct fluid communication with subcritical cryogenic fluid receiver tank  308 . The other branch of T-shaped conduit junction  446  leads to a conduit  450  which leads to a valve  452  which leads to an orifice  454  which eventually leads to generic vent  334 . 
     The operation of the alternative embodiment of the cryogenic fluid transfer system  302  of the present invention is similar to the first embodiment, with the difference lying in the fact that a greater number of transfer options and scenarios are available. For example, the use of two subcritical cryogenic fluid storage tanks allows for one or both tanks to be alternatively filled and/or vented, depending on the space vehicle&#39;s particular needs. Additionally, the use of a pressurization tank and a gaseous fluid storage tank, both being in fluid communication with both of the subcritical cryogenic fluid storage tanks, as well as the supercritical cryogenic fluid storage system, allows for greater flexibility and versatility regarding filling and venting operations. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification and following claims.
A system for providing cryogenic sub-critical liquid oxygen and liquid hydrogen to storage tanks and payloads contained inside the payload bay of a space vehicle is described. The system provides for transferring cryogenic fluid from a supercritical storage tank to a subcritical storage tank or payload in a zero-g environment, wherein the H 2 and O 2 ignition hazard has been eliminated due to the low vacuum pressure operating environment. The system includes an external heat exchanger, for example, a parallel flow concentric tube design, and a temperature control system for re-condensing the two-phase transfer fluid expelled from the supercritical storage system into a single phase, sub-cooled cryogenic fluid which is then introduced into and stored within the subcritical storage tank.