Abstract:
Systems and methods for aerial refueling are disclosed. In one embodiment, an aerial refueling system includes a first conduit portion moveably coupled to a second conduit portion and moveable relative to the second conduit portion along a longitudinal axis, and a force absorbing assembly operatively coupled to the first and second conduit portions. The force absorbing assembly includes a first absorber portion and a second absorber portion engaged with the first absorber portion. The first absorber portion is configured to compress when subject to a compression force having a longitudinal component at least approximately aligned with the longitudinal axis, the longitudinal component tending to urge the first conduit portion toward the second conduit portion and causing absorption of at least a portion of the longitudinal component by the first absorber portion until the longitudinal component reaches a first limit. The second absorber portion is configured to compress when the longitudinal component exceeds the first limit.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to aerial refueling, and more specifically, to systems and methods that reduce incidental impact forces exerted by a refueling boom during aerial refueling. 
     BACKGROUND OF THE INVENTION 
     Aircraft in flight are commonly refueled from a refueling aircraft. Many refueling aircraft use a system of fixed and extendable tubing, often referred to as a refueling boom, for refueling a receiving aircraft. Typically, an operator in the refueling aircraft controls the refueling boom into alignment with the receiving aircraft, either visually or with the assistance of camera equipment. The refueling boom typically has control surfaces (fins or airfoils) to allow the refueling operator to “fly” the refueling boom into engagement with a refueling receptacle on the receiving aircraft. A distal end of the refueling boom may be extendable to allow the operator to extend the refueling boom into engagement with the refueling receptacle. Examples of prior art aerial refueling systems include those systems described in U.S. Pat. No. 6,966,525 B1 issued to Schroeder, and U.S. Pat. No. 6,651,933 B1 issued to von Thal et al. 
     Although desirable results have been achieved using such prior art systems, there is room for improvement. For example, as the refueling boom is being positioned for insertion into the refueling receptacle on the receiving aircraft, the refueling boom may inadvertently contact portions of the receiving aircraft other than the refueling receptacle. Such inadvertent contacts may result in damage to the receiving aircraft and to the refueling boom. Existing devices for absorbing boom forces that may be exerted between the refueling aircraft and the receiving aircraft through the boom are typically configured to operate when such forces reach relatively high magnitudes (e.g. several thousand pounds of compression force) and are intended to avert potentially extreme or catastrophic events. Such existing devices do not alleviate damages that may result from relatively lower magnitude forces that result from relatively normal, incidental contacts between the boom and the receiving aircraft that typically occur in day-to-day aerial refueling operations. 
     SUMMARY OF THE INVENTION 
     Embodiments of systems and methods for aerial refueling in accordance with the present invention are configured to absorb compression forces experienced by a refueling boom. More specifically, embodiments of the present invention may advantageously absorb both relatively large compression forces (typically associated with potentially extreme events), as well as relatively smaller compression forces that result from normal, incidental contacts that occur in day-to-day aerial refueling operations. In this way, embodiments of systems and methods in accordance with the present invention may reduce operational expenses associated with repairs of aircraft and in-flight refueling assemblies, in comparison with prior art aerial refueling systems. 
     In one embodiment, an aerial refueling system includes a first conduit portion moveably coupled to a second conduit portion and moveable relative to the second conduit portion along a longitudinal axis, and a force absorbing assembly operatively coupled to the first and second conduit portions. The force absorbing assembly includes a first absorber portion and a second absorber portion engaged with the first absorber portion. The first absorber portion is configured to compress when subject to a compression force having a longitudinal component at least approximately aligned with the longitudinal axis, the longitudinal component tending to urge the first conduit portion toward the second conduit portion and causing absorption of at least a portion of the longitudinal component by the first absorber portion until the longitudinal component reaches a first limit. The second absorber portion is configured to compress when the longitudinal component exceeds the first limit. 
     In alternate embodiments, the first and second conduit portions form an internal passage, and at least one of the first and second absorber portions includes at least one of a coil spring disposed about the internal passage, a plurality of fluidic shock absorbers concentrically disposed about the internal passage, a plurality of springs concentrically disposed about the internal passage, a tubular resilient member concentrically disposed about the internal passage, and an inflatable member concentrically disposed about the internal passage. 
     In another embodiment, a refueling boom assembly includes a base portion having a first passage configured to receive a fuel stream; an extendible portion having a second passage fluidly coupled to the first passage and configured to receive the fuel stream; and a compression absorber assembly operatively coupled to at least one of the base and extendible portions. The compression absorber assembly includes a first conduit portion moveably coupled to a second conduit portion and moveable relative to the second conduit portion along a longitudinal axis, the first and second conduit portions being configured to receive the fuel stream; and a force absorbing assembly operatively coupled to the first and second conduit portions. The force absorbing assembly includes a first absorber portion and a second absorber portion engaged with the first absorber portion. The first absorber portion is configured to compress when subject to a compression force having a longitudinal component at least approximately aligned with the longitudinal axis, the longitudinal component tending to urge the first conduit portion toward the second conduit portion and causing absorption of at least a portion of the longitudinal component by the first absorber portion until the longitudinal component reaches a first limit. The second absorber portion is configured to compress when the longitudinal component exceeds the first limit. 
     In still another embodiment, an aerial refueling aircraft includes a fuselage; a fuel tank disposed within the fuselage; and a refueling boom assembly operatively coupled to the fuselage and fluidly coupled to the fuel tank. The refueling boom assembly includes a base portion having a first passage configured to receive a fuel stream; an extendible portion having a second passage fluidly coupled to the first passage and configured to receive the fuel stream; and a compression absorber assembly operatively coupled to at least one of the base and extendible portions, the compression absorber assembly including: a first conduit portion moveably coupled to a second conduit portion and moveable relative to the second conduit portion along a longitudinal axis, the first and second conduit portions being configured to receive the fuel stream; and a force absorbing assembly operatively coupled to the first and second conduit portions and having a first absorber portion and a second absorber portion engaged with the first absorber portion. The first absorber portion is configured to compress when subject to a compression force having a longitudinal component at least approximately aligned with the longitudinal axis, the longitudinal component tending to urge the first conduit portion toward the second conduit portion and causing absorption of at least a portion of the longitudinal component by the first absorber portion until the longitudinal component reaches a first limit. The second absorber portion is configured to compress when the longitudinal component exceeds the first limit. 
     In another alternate embodiment, a method of aerial refueling comprises providing a first conduit portion moveably coupled to a second conduit portion and moveable relative to the second conduit portion along a longitudinal axis; providing a force absorbing assembly operatively coupled to the first and second conduit portions; absorbing a longitudinal component of a compression force using a first absorber portion of the force absorbing assembly, the longitudinal component being approximately aligned with the longitudinal axis and tending to urge the first conduit portion toward the second conduit portion; and after the longitudinal component exceeds a first limit, compressing a second absorber portion of the force absorbing assembly operatively engaged with the first absorber portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention are described in detail below with reference to the following drawings. 
         FIG. 1  is a side elevational view of an aerial refueling system in accordance with an embodiment of the invention; 
         FIG. 2  is an enlarged side elevational view of a portion of an in-flight refueling assembly of the aerial refueling system of  FIG. 1 ; 
         FIG. 3  is an enlarged, partial cross-sectional view of a boom force absorber assembly in accordance with an embodiment of the invention; 
         FIG. 4  is a partially-exploded side elevational view of a compressible assembly of the boom force absorber assembly of  FIG. 3 ; 
         FIG. 5  is a front elevational view of a load transfer plate of the boom force absorber assembly of  FIG. 3 ; 
         FIGS. 6 and 7  are enlarged side elevational views of in-flight refueling assemblies in accordance with additional embodiments of the invention; and 
         FIGS. 8 through 11  are side elevational views of boom force absorber assemblies in accordance with further embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates to methods and systems for aerial refueling that absorb compression forces experienced by a refueling boom. Many specific details of certain embodiments of the invention are set forth in the following description and in  FIGS. 1-11  to provide a thorough understanding of such embodiments. One skilled in the art, however, will understand that the present invention may have additional embodiments, or that the present invention may be practiced without several of the details described in the following description. 
     In general, embodiments of systems and methods in accordance with the present invention may advantageously absorb both relatively high magnitude compression forces, as well as relatively lower magnitude compression forces that result from relatively normal, incidental contacts that occur in day-to-day aerial refueling operations. Thus, embodiments of the invention may reduce damage to receiving aircraft due to incidental contacts between the refueling boom  114  and portions of the receiving aircraft  120 , such as the fuselage, cockpit windows, antenna, and other portions of the receiving aircraft  120 . Operational costs associated with repairs of aircraft and aerial refueling systems, and expenses associated with aircraft downtime, may thereby be reduced. 
       FIG. 1  is a side elevational view of an aerial refueling system  100  in accordance with an embodiment of the present invention. In this embodiment, a refueling aircraft (or tanker)  110  is equipped with an in-flight refueling assembly  130  that includes a refueling boom  114 . The refueling boom  114  is configured to be guided into alignment with a refueling receptacle  126  of a receiving aircraft  120 . The refueling boom  114  includes a base portion  115  and an extendable portion  116  that may be extended into engagement with (and retracted from) the refueling receptacle  126 , fluidly coupling the refueling aircraft  110  with the receiving aircraft  120 . 
     In some embodiments, the refueling aircraft  110  is a KC-135 Stratotanker, or a KC-767 Global Tanker Transport Aircraft, manufactured by The Boeing Company of Chicago, Ill. Alternately, the refueling aircraft  110  may be any suitable refueling aircraft, including an automated refueling aircraft such as the experimental F/A-18A “tanker” aircraft operated by the NASA Dryden Research Center, or any tanker aircraft that partially or fully satisfies the specifications of the KC-X Aerial Refueling Tanker Aircraft program conducted by the U.S. Department of the Air Force, or the Future Strategic Tanker Aircraft program conducted by the Royal Air Force of the United Kingdom, or any other suitable type of manned or unmanned aerial refueling aircraft. 
       FIG. 2  is an enlarged side elevational view of the in-flight refueling assembly  130  of  FIG. 1 . In this embodiment, a gimble assembly  132  is coupled to the refueling boom  114 , enabling a controller  111  to controllably adjust the position of the refueling boom  114 . In some embodiments, the refueling boom  114  may be guided into alignment with the refueling receptacle  126  by adjusting one or more airfoils  118  disposed on the refueling boom  114 . The controller  111  may be a human operator, or alternately, may be an automated or semi-automated control device that includes one or more processors (or other computer devices) configured to adjustably control the position of the refueling boom  114 . The control device may further include input and output devices such that an operator of the in-flight refueling system  100  may monitor and override the operation of the controller  111 . The controller  111  may be in communication with the airfoils  118  via various devices and methods suitable for controlling the airfoils  118 , including hydraulic lines, electromechanical devices, electronic or wireless connections, or any other suitable control devices. 
     As further shown in  FIGS. 1 and 2 , the in-flight refueling assembly  130  includes a boom force absorber assembly  140  coupled between the gimble assembly  132  and the refueling boom  114 . The boom force absorber assembly  140  is configured to alleviate damages that may result from relatively lower magnitude forces that result from relatively normal, incidental contacts between the refueling boom  114  and the receiving aircraft  120  that typically occur in day-to-day aerial refueling operations. 
     More specifically,  FIG. 3  is an enlarged, partial cross-sectional view of the boom force absorber assembly  140  in accordance with one embodiment of the invention. In this embodiment, the boom force absorber assembly  140  includes a first housing  142  having a first end  144  configured to be coupled to the gimble assembly  132 , and a second housing  146  having a second end  148  configured to be coupled to the refueling boom  114 . The first and second housings  142 ,  146  are aligned along a longitudinal axis  145  and moveably coupled such that the second housing  146  may move axially along the longitudinal axis  145  relative to the first housing  142  (or vice versa) as depicted by double-headed arrow M. For example, in some embodiments, a portion of the second housing  146  slidably engages into the first housing  142  (or vice versa). The first and second housings  142 ,  146  are also configured to define an internal passage  147  that extends through the boom force absorber assembly  140 . The internal passage  147  fluidly couples the refueling boom  114  with the gimble assembly  132  or one or more other components that are, in turn, fluidly coupled to a fuel tank  115  ( FIG. 1 ) within the refueling aircraft  110 . 
     In this embodiment, the boom force absorber assembly  140  also includes a compressible assembly  150  having an insertion spring  152  disposed in the second housing  146 , and a primary spring  154  disposed in the first housing  142 . A load transfer plate  156  is disposed between the insertion spring  152  and the primary spring  154 . As best shown in  FIG. 5 , in one embodiment, the load transfer plate  156  includes an approximately flat ring-shaped member  158  having a central aperture  160  disposed therethrough. The central aperture  160  is configured to receive the internal passage  147 . Layers  162  of relatively low friction material may be applied to the inner and outer edges of the ring-shaped member  158  to reduce wear due to friction as the ring-shaped member  158  moves within the first housing  142  (or second housing  146 ) along the longitudinal axis  145  (arrow M). 
     The compressible assembly  150  is advantageously configured to absorb relatively high magnitude compression forces (e.g. several hundred or several thousand pounds of compression force) typically associated with potentially extreme or catastrophic events, as well as relatively lower magnitude forces (e.g. up to and including a couple of hundred pounds of compression force) that result from relatively normal, incidental contacts between the refueling boom  114  and the receiving aircraft  120  that typically occur in day-to-day aerial refueling operations. In the embodiment shown in  FIG. 3 , this is accomplished by having different spring constants for the primary and insertion springs  154 ,  152 . For example, in some embodiments, the insertion spring  152  is configured to absorb relatively lower magnitude compression forces, and the primary spring  154  is configured to absorb relatively higher magnitude compression forces. 
     More specifically, in some embodiments, the insertion spring  152  is configured to absorb up to approximately 100 pounds of compression force, corresponding to an insertion and extraction force design criteria of a Universal Aerial Refueling receptacle presently used on many types of modern military aircraft. Similarly, in some embodiments, the primary spring  154  is configured to absorb approximately 3000 pounds, a force magnitude conventionally used as a design criteria for abnormal, relatively extreme compression forces within the refueling boom  114 . Of course, in alternate embodiments, the insertion spring  152  and the primary spring  154  may be configured to absorb other magnitudes of compression forces corresponding to different circumstances and designs of aerial refueling receptacles, and correspondingly different anticipated compression forces. 
     In an alternate embodiment, the insertion spring  152  may be replaced with two or more springs. For example,  FIG. 4  is a partially-exploded view of an embodiment of the compressible assembly  150  wherein the insertion spring  152  has been replaced by a first spring  151  and a second spring  153 . Another load transfer plate  156  is disposed between the first and second springs  151 ,  153 . In this embodiment, the first spring  151  may be configured to absorb compression forces between a lower limit (e.g. approximately 100 pounds) and an upper limit (e.g. approximately 1000 pounds) of compression force, and the second spring  153  may be configured to absorb a relatively smaller magnitude, such as up to a lower limit at which the first spring  151  begins to compress (e.g. up to 100 pounds). In one specific embodiment, the first spring  151  is configured to absorb between approximately 100 pounds and 720 pounds of compression force, and the second spring is configured to absorb up to approximately 100 pounds of compression. Of course, in further embodiments, either the primary spring  154  or the insertion spring  152 , or both, may comprise a plurality of springs. Also, in the embodiment shown in  FIG. 4 , the second spring  153  may be configured to approximately match an insertion forces of a standard refueling receptacle  124 , while the first spring  151  may be configured to provide a relatively small amount of additional spring force that may be needed in case the extendible portion  116  of the refueling boom  114  does not latch with the refueling receptacle  124 . In such a case, the first spring  151  maintains contact between the refueling boom  114  and the refueling receptacle  124  in a practice referred to as “forced refueling.” 
     In operation, the controller  111  may lower the in-flight refueling boom  114  to await the rendezvous of the receiving aircraft  120  with a position substantially aft and below the refueling aircraft  110 . The controller  111  may maintain the position of the in-flight refueling boom  114  relative to the refueling aircraft  110  while awaiting the approach of the receiving aircraft  120 . As the controller  111  guides the refueling boom  114  into engagement with the refueling receptacle  126  of the receiving aircraft  120 , incidental contacts between the refueling boom  114  and portions of the receiving aircraft  120  create relatively lower magnitude compression forces within the in-flight refueling assembly  130  which are absorbed by the insertion spring  152  of the boom force absorption assembly  140 . 
     Once the refueling boom  114  is in a position suitable for engagement with the refueling receptacle  126 , the controller  111  may extend the extendable portion  116  of the refueling boom  114  to engage the refueling boom  114  with the refueling receptacle  126 . Assuming an approximately linear alignment between the refueling boom  114  and the refueling receptacle  126 , in those embodiments in which the insertion spring  152  is configured to approximately match an insertion force design criteria of the refueling receptacle  126 , the insertion force necessary to insert the refueling boom  114  into the refueling receptacle  126  may be approximately absorbed by the insertion spring  152 , creating a “zero force” insertion condition within the in-flight aerial refueling assembly  130 . 
     In the event that a greater-than-nominal compression force is experienced by the refueling boom  114 , the insertion spring  152  may become mostly or completely compressed, and the greater-than-nominal compression force acting through the load transfer plate  156  may begin compressing the primary spring  154 . For example, if the insertion spring  152  is configured to absorb up to 100 pounds, and a compression force of 500 pounds is experienced within the refueling boom  114 , the insertion spring  152  may become mostly or completely compressed, and the compression force of 500 pounds may begin compressing the primary spring  154 . In this way, the boom force absorption assembly  140  may be configured to absorb both relatively high magnitude compression forces (typically associated with potentially extreme events), as well as relatively lower magnitude forces that result from relatively normal, incidental contacts that occur in day-to-day aerial refueling operations. 
     After the refueling boom  114  is engaged with the refueling receptacle  126 , in-flight aerial refueling may be performed. More specifically, the controller  111  may cause fuel to flow from the fuel tank  115  within the refueling aircraft  110  through the boom force absorber assembly  140 , through the refueling boom  114 , and into the refueling receptacle  126  of the receiving aircraft  120 . During the refueling process, the boom force absorber assembly  140  may continue to absorb compression forces that may arise within the in-flight refueling assembly  130  due to the relative movement between the refueling aircraft  110  and the receiving aircraft  120 . When refueling of the receiving aircraft  120 , the controller  111  may disengage the refueling boom  114  from the refueling receptacle  126 , and the refueling process is completed. 
     Embodiments of the present invention may provide significant advantages over the prior art. For example, because the boom force absorbing assembly  140  is configured to absorb both relatively high magnitude compression forces, as well as relatively lower magnitude forces that result from relatively normal, incidental contacts that occur in day-to-day aerial refueling operations, the in-flight refueling assembly  130  may result in less damage to the receiving aircraft  120 . Incidental contacts between the refueling boom  114  and portions of the receiving aircraft  120 , such as the fuselage, cockpit windows, antenna, and other portions of the receiving aircraft  120  in the vicinity of the refueling receptacle  126 , may be absorbed by the boom force absorption assembly  140 , resulting in reduced repair costs and reduced downtime of the receiving aircraft  120 . Furthermore, safety of the in-flight refueling assembly  130  may be improved because the boom force absorber assembly  140  absorbs not only relatively high magnitude forces, but also relatively lower magnitude forces. 
     It will be appreciated that various embodiments of methods and systems for in-flight refueling in accordance with the present invention may be conceived, and that the invention is not limited to the particular embodiments described above. For example, the boom force absorber assembly  140  may be located at any suitable position to absorb compression forces exerted on the refueling boom  114 , and is not limited to the particular position described above and shown in  FIG. 2 . 
       FIG. 6  is an enlarged side elevational view of an in-flight refueling assembly  230  in accordance with another embodiment of the invention. In this embodiment, the boom force absorber assembly  140  is spaced apart from the gimble assembly  132  and is situated along the length of about refueling boom  214 . More specifically, the boom force absorber assembly  140  is disposed between the base portion  115  and the extendable portion  116  of the refueling boom  214 . Alternately,  FIG. 7  shows an in-flight refueling assembly  250  in which the boom force absorber assembly  140  is positioned at a distal end of the refueling boom  264 . Specifically, in the embodiment shown in  FIG. 7 , the boom force absorber assembly  140  is disposed at the distal end of the extendable portion  116  of the refueling boom  264 . Of course, in further embodiments, the boom force absorber assembly  140  may be positioned at any location along the refueling boom, including anywhere along the length of the base portion  115 , or the length of the extendable portion  116 . 
     Various embodiments of boom force absorber assemblies may also be conceived in accordance with alternate embodiments of the present invention. For example, referring again to  FIG. 3 , in an alternate embodiment, the roles of the primary and insertion spring&#39;s  154 ,  152  may be reversed. The primary spring  154  may be configured to absorb relatively lower magnitude compression forces associated with incidental contacts between the refueling boom  114  and the receiving aircraft  120 , and the insertion spring  152  may be configured to absorb relatively higher magnitude compression forces associated with potentially extreme conditions. Thus, incidental contacts that occur during normal day-to-day aerial refueling operations will compress the primary spring  154 , and compression forces (overload or secondary forces) which exceed the relatively lower magnitude forces associated with incidental contacts will either partially or completely compress the primary spring  154  and will also begin to compress the insertion spring  152 . In this way, such an alternate embodiment of the boom force absorbing assembly may be configured to absorb both relatively high magnitude compression forces, as well as relatively lower magnitude forces that result from normal, incidental contacts that occur in aerial refueling operations. 
     As with almost every aerospace system, it may be desirable to reduce the weight of the boom force absorber assembly.  FIG. 8  shows a side elevational view of a boom force absorber assembly  340  in accordance with another embodiment of the invention. In this embodiment, the boom force absorber assembly  340  includes a first conduit  342  having a first flange  344  configured to be coupled to the gimble assembly  132 , and a second conduit  346  having a second flange  348  configured to be coupled to the refueling boom  114 . The first and second conduits  342 ,  346  are coupled such that the second conduit  346  may move axially along the longitudinal axis  145  relative to the first conduit  342  (or vice versa) as depicted by double-headed arrow M. An internal passage  147  extends through the first and second conduits  342 ,  346 . 
     As further shown in  FIG. 8 , the boom force absorber assembly  340  also includes a compressible assembly  350  having an insertion spring  352  disposed about the second conduit  346 , and a primary spring  354  disposed about the first conduit  342 . A load transfer plate  356  is disposed between the insertion spring  352  and the primary spring  354 , and is configured to slideably move along the longitudinal axis  145  (arrow M) over one or both of the first and second conduits  342 ,  346 . As described more fully above, the compressible assembly  350  is advantageously configured to absorb relatively high magnitude compression forces typically associated with potentially extreme or catastrophic events, as well as relatively lower magnitude forces that result from relatively normal, incidental contacts between the refueling boom  114  and the receiving aircraft  120  during aerial refueling operations. The boom force absorber assembly  340  may be lighter than other, previously-described embodiments (e.g. assembly  140  of  FIG. 3 ) because the first and second housings  142 ,  146  have been eliminated. 
     In further embodiments, one or both of the coil springs  352 ,  354  of the compressible assembly  350  may be replaced (or augmented) with other types of compression absorbing devices. In the embodiment shown in  FIG. 9 , for example, a boom force absorber assembly  440  in accordance with another alternate embodiment includes a compressible assembly  450  having a plurality of shock absorbers  452  disposed about the second conduit  346  and coupled between the second end  348  and the load transfer plate  356 . The plurality of shock absorbers  452  may be concentrically disposed about the second conduit  346 . 
     In some embodiments, the plurality of shock absorbers  452  are equi-distally spaced about the second conduit  346 . The shock absorbers  452  may be pneumatic, hydraulic, magnetic, or any other suitable type of shock absorbers. In some embodiments, the shock absorbers  452  may be of a passive type, such as the type typically found in an automotive suspension system. Alternately, the shock absorbers  452  may be coupled to a fluid supply  457  via one or more supply lines  459 , and may be actively controlled by a control component (e.g. the controller  111 ). 
     In one particular aspect of the embodiment shown in  FIG. 9 , the boom force absorber assembly  440  is configured such that the plurality of shock absorbers  452  cooperatively absorb the relatively lower magnitude forces that result from normal, incidental contacts between the refueling boom  114  and the receiving aircraft  120 , and the primary spring  454  absorbs the relatively higher magnitude compression forces that are greater than those caused by normal, incidental contacts (e.g. of the type associated with extreme or catastrophic events). In an alternate aspect, the roles of the shock absorbers  452  and the primary spring  454  are reversed such that the plurality of shock absorbers  452  absorb the relatively higher magnitude compression forces, and the primary spring  454  absorbs the relatively lower magnitude compression forces associated with normal, incidental contacts. 
       FIG. 10  shows a side elevational view of a boom force absorber assembly  340  in accordance with another embodiment of the invention. As in the embodiments described above with respect to  FIGS. 8 and 9 , the boom force absorber assembly  540  includes a first conduit  342  having a first flange  344  configured to be coupled to the gimble assembly  132 , and a second conduit  346  having a second flange  348  configured to be coupled to the refueling boom  114 . The first and second conduits  342 ,  346  are coupled such that the conduits  342 ,  346  may move axially relative to each other along the longitudinal axis  145  (arrow M). The internal passage  147  extends through the first and second conduits  342 ,  346 . 
     In the embodiment shown in  FIG. 10 , the boom force absorber assembly  540  includes a compressible assembly  550  having a resilient member  552  disposed about the second conduit  346 , and an inflatable member  554  disposed about the first conduit  342 . The resilient member  552  may be an approximately tubular member, and may be formed of any suitable resilient material, including rubber or other elastomeric materials, fiberous materials (e.g. cotton or wood-based materials), or any other suitable materials or combinations of materials. A load transfer plate  356  is disposed between the resilient member  552  and the inflatable member  554 , and is configured to slideably move over one or both of the first and second conduits  342 ,  346  along the longitudinal axis  145  (arrow M). The inflatable member  554  is coupled to a controllable fluid supply  557  via one or more supply lines  559 , and may be actively controlled by a control component (e.g. the controller  111 ). 
     The compressible assembly  550  is configured to absorb both relatively high magnitude compression forces typically associated with potentially extreme events, as well as relatively lower magnitude forces that result from relatively normal, incidental contacts during aerial refueling operations. More specifically, as the controller  111  guides the refueling boom  114  into engagement with the refueling receptacle  126  of the receiving aircraft  120 , incidental contacts between the refueling boom  114  and portions of the receiving aircraft  120  create relatively lower magnitude compression forces which are absorbed by the resilient member  552  of the boom force absorption assembly  540 . 
     In the event that a greater-than-nominal compression force is experienced by the boom force absorption assembly  540 , the resilient member  552  may become mostly or completely compressed, and the greater-than-nominal compression force acting through the load transfer plate  156  may begin compressing the inflatable member  554 . As the pressure within the inflatable member  554  increases, a fluid medium within the inflatable member  554  (e.g. pneumatic or hydraulic) may be expelled from the inflatable member  554 . In some embodiments, the fluid medium may be expelled through the supply line  559  and into the fluid supply  557 . Alternately, the fluid medium may be expelled from a pressure relief valve  555  fluidly coupled to the inflatable member  554 . Thus, the inflatable member  554  absorbs the greater-than-nominal compression force. Once the compression forces on the boom force absorber assembly  540  are relieved, the inflatable member  554  may be re-inflated by the fluid supply  557  via the supply line  559  (e.g. by the controller  111 ) to a nominal operating pressure for continued aerial refueling operations. 
       FIG. 11  shows a boom force absorber assembly  640  in accordance with yet another embodiment of the invention. In this embodiment, the boom force absorber assembly  640  includes a first housing  642  having a first end  644  configured to be coupled to the gimble assembly  132 , and a second housing  646  having a second end  648  configured to be coupled to the refueling boom  114 . The first and second housings  642 ,  646  are moveably coupled and are configured to move axially relative to each other along the longitudinal axis  145  (arrow M). The internal passage  147  extends through the first and second housings  642 ,  646 . 
     As further shown in  FIG. 11 , the boom force absorber assembly  640  includes a compressible assembly  650  having a plurality of first springs  652  circumferentially disposed about an inner peripheral surface of the first housing  642 , and a plurality of second springs  654  circumferentially disposed about an outer peripheral surface of the second housing  646 . A load transfer plate  656  is disposed between the plurality of first springs  652  and the plurality of second springs  654 . The load transfer plate  656  is configured to slideably move over one or both of the first and second housings  642 ,  646  along the longitudinal axis  145  (arrow M). 
     As in the previously-described embodiments, the boom force absorption assembly  640  is configured to absorb both relatively high magnitude compression forces typically associated with potentially extreme events, as well as relatively lower magnitude forces that result from relatively normal, incidental contacts during aerial refueling operations. More specifically, as incidental contacts between the refueling boom  114  and portions of the receiving aircraft  120  create relatively lower magnitude compression forces, such incidental forces are absorbed by the plurality of second springs  654 . In the event that a greater-than-nominal compression force is experienced, the plurality of second springs  654  reach a first limit and become mostly or completely compressed. The greater-than-nominal compression force acting through the load transfer plate  156  then begins to compress the plurality of first springs  652 . The plurality of first springs  652  may be configured to absorb greater-than-nominal compression forces up to a second limit. In some embodiments, the second limit is at least one order of magnitude greater than the first limit. 
     While preferred and alternate embodiments of the invention have been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of these preferred and alternate embodiments. Instead, the invention should be determined entirely by reference to the claims that follow.