Abstract:
This invention solves problems associated with prior-art soft-dock mechanisms by placing all active components of a soft-dock system on the chaser side of the mechanism, leaving the target side of the mechanism completely passive (i.e., requiring no power expenditure or self-actuated moving parts to operate). In particular, the active components are supported on the end of a flexible cable attached to the probe, or chaser, side of the device. These components act as a sort of spring-loaded “trap.” Once the end of the probe passes into a receptacle on the target side, the mechanism is triggered, engaging it in such a way that it can no longer be pulled out of the receptacle until it is reset. The soft-docking cable may be replaced with a rigid, semi-rigid or jointed post that is used to bring a capture mechanism into engagement with its corresponding receptacle or receiving structure. The magnetic end effector may also be implemented as an electro-magnet, which requires power to maintain the holding force, or a permanent magnet, which captures a target without power. The main target cone may be either a metallic cone: or a non-metallic cone constructed of fabric, plastic, or other flexible material.

Description:
REFERENCE TO RELATED APPLICATION 
   This application claims priority from U.S. Provisional Patent Application Ser. No. 60/416,138, filed Oct. 4, 2002; and is a continuation-in-part of U.S. patent application Ser. No. 10/286,192, filed Nov. 1, 2002, which claims priority from U.S. Provisional Patent Application Ser. No. 60/335,563, filed Nov. 1, 2001. The entire content of each application being incorporated herein by reference. 

   This invention was made with Government support under DAAH01-00-C-R012 and DAAH01-01-C-R015 awarded by the U.S. Army Aviation and Missile Command, with funding from the Defense Advanced Research Projects Agency (DARPA). The Government has certain rights in this invention. 

   FIELD OF THE INVENTION 
   This invention relates generally to spacecraft docking and, in particular, to a system of the type wherein all active components are disposed on a chase vehicle. 
   BACKGROUND OF THE INVENTION 
   There is interest in commercial uses of outer space, particularly earth orbit. This environment offers unique characteristics, such as zero gravity and unlimited vacuum, which may be useful to certain manufacturing processes. A major problem is the costs in launch and recovery of payloads. 
   One possible solution involves the use of a semi-permanent orbiting vehicle that is periodically serviced for refueling, resupply, and payload exchange. The most economical approach to such operations involves unmanned, unsupervised, autonomous rendezvous and docking vehicles. This requires capture and docking mechanisms which are simple and reliable, even in the presence of some misalignment. It would also be advantageous for the mechanism aboard the normally orbiting craft to be largely passive, thereby preserving power resources for other, perhaps more critical uses. 
   Existing spacecraft coupling structures typically take the form of compatible male and female devices, such as a conical seating platform on one vehicle and a docking adapter on the other for alignment and coupling of the spacecraft. Such structures typically absorb the relative kinetic energy between the two space vehicles upon engagement, and upon coupling, rigidly and securely interconnect the two spacecraft until their desired disengagement or decoupling. 
   In most applications, attachment is accomplished by remotely controlling one spacecraft on earth. The controlled vehicle typically includes an elongate probe or grappling arm for insertion into the conical seating platform in the other vehicle. U.S. Pat. Nos. 5,735,488; 5,364,046; 4,177,964; 4,195,804; 4,391,423; and 4,588,150, and Japanese Patent No. 226,497 are illustrative of such structures. 
   The apparatus described in U.S. Pat. No. 5,735,488 includes an elongate grappling arm extending from a first space vehicle. A pair of inflatable bladders are positioned about the grappling arm for engaging an inner surface of the combustion chamber of a second vehicle upon inflation, and a pair of rear bladders are positioned about the grappling arm for engaging an inner surface of the nozzle downstream from the combustion chamber upon inflation. This aligns the grappling arm and the rocket propulsion nozzle. A pressurized fluid source is provided on the first space vehicle for supplying fluid pressure to the inflatable bladders, and a fluid control valve manifold selectively controls the release of pressurized fluid to the bladders. In operation, the grappling arm is inserted into the rocket propulsion nozzle, and the control valves are actuated to first inflate the front bladders and thereby interconnect the grappling arm and the rocket propulsion nozzle. The rear bladders are subsequently inflated to align a central axis of the grappling arm with a central axis of the rocket propulsion nozzle. Inflation of the rear bladders provides an axial reaction load to balance the axial load provided by the front bladders. Attaching the vehicles in space may be controlled from the earth by activating the control valves to inflate the bladders. 
   According to U.S. Pat. No. 5,364,046, a largely passive capture mechanism disposed on a first spacecraft includes a concave cone section with the narrower interior end to admit a ball of a predetermined diameter. When tripped, a capture device restricts the diameter of passage for capture of the ball. In the release position passage for the ball is unrestricted. The capture device is preferably reset by the other spacecraft to release the ball. A docking mechanism disposed on the second spacecraft includes a convex cone section constructed to mate with the concave cone section, ball at the end of a cable and a boom. The cable may be extended from or retracted to the apex of the convex cone section. A rotary drive coupled to the convex cone section permits relative rotation of the spacecraft. The boom may be extended from or retracted into the second spacecraft. The spacecraft dock by directing the extended ball into the cylinder, where it is captured. The cable and boom retract to dock. The active docking mechanism releases and resets the capture device to undock. A pyrotechnic cutter disposed inside the boom can cut the cable for emergency release. 
   SUMMARY OF THE INVENTION 
   This invention solves problems associated with prior-art soft-dock mechanisms by placing all active components of a soft-dock system on the chaser side of the mechanism, leaving the target side of the mechanism completely passive (i.e., requiring no power expenditure or self-actuated moving parts to operate). In particular, the active components are supported on the end of a flexible cable attached to the probe, or chaser, side of the device. These components act as a sort of spring-loaded “trap.” Once the end of the probe passes into a receptacle on the target side, the mechanism is triggered, engaging it in such a way that it can no longer be pulled out of the receptacle until it is reset. 
   In an alternative embodiment of the invention, the soft-docking cable may be replaced with a rigid, semi-rigid or jointed post that is used to bring a capture mechanism into engagement with its corresponding receptacle or receiving structure. As a further alternative, the Harpoon latching cable end effector may be replaced with a magnetic latching device. The magnetic end effector can be either an electro-magnet, which requires power to maintain the holding force, or a permanent magnet, which captures a target without power. As yet a further alternative, the main target cone may be either a metallic cone, or a non-metallic cone constructed of fabric, plastic, or other flexible material. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates the Harpoon Capture Mechanism (Armed); 
       FIG. 2  illustrates the Harpoon Capture Mechanism Actuation Sequence; 
       FIG. 3  illustrates the Harpoon Capture Mechanism (Deployed); 
       FIG. 4  illustrates the ARD Soft-Dock Capture Mechanism; 
       FIG. 5  illustrates the ASDS Soft-Dock Capture Mechanism; 
       FIG. 6  illustrates the ARD Boom Extension System (Rack-and-Pinion); 
       FIG. 7  illustrates the ASDS Boom Extension System (Ballscrew); 
       FIG. 8  illustrates the ARD Cable Extension System (Linear Actuator); 
       FIG. 9  illustrates the ASDS Cable Extension System (Ballscrew); 
       FIG. 10  illustrates the ARD Head Rotation Indexing System; 
       FIG. 11  illustrates the ASDS Chaser-Side Indexing System Components; 
       FIG. 12  illustrates the ASDS Target-Side Indexing System Components; 
       FIG. 13  illustrates the ARD Boom Glide Components; 
       FIG. 14  illustrates the ASDS Boom Glide Components: 
       FIGS. 15   a-c  illustrates the rigid/semi-rigid/jointed post capture alternatives, respectively; 
       FIG. 16  illustrates the magnetic capture alternative components; and 
       FIG. 17  illustrates the fabric target cone alternative. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The invention will now be described in detail with reference to the Figures, using the following definitions:
     Chaser  10 . 1 —The half of a docking mechanism  10  that is attached to the satellite that is performing the servicing operation, or chase vehicle  1 ;   Target  10 . 2 —The half of a docking mechanism  10  that is attached to the satellite that is being serviced, or target vehicle  3 ;   Soft-Dock—The capture of the target vehicle  3  by the chase vehicle  1  by a method that imparts little or no force on the target vehicle  3 . This is in contrast to hard-dock which nominally involves a collision between parts of the chaser  1  and target  3  vehicles. Hard-dock procedures generally impart a great deal of force on the target vehicle  3 , which can push it away before the docking mechanism is fully engaged;   Harpoon—The end effector  12  used by the chase vehicle  1  to capture the target vehicle  3 ;   ARD  100 —The autonomous rendezvous and docking mechanism  100  described in U.S. Pat. No. 5,364,046; and   ASDS  10 &#39; —The autonomous satellite docking system  10 &#39; according to the present invention, which includes the Harpoon end effector  12 .   

   The operation of the Harpoon end effector  12  will now be described with particular reference to  FIGS. 1-3 . 
   The Components of the Harpoon End Effector 
   Docking Cable  14   
   The Harpoon end effector  12  is attached to a docking cable  14 , which is extended from the chaser side  10 . 1  of the docking mechanism  10 . This docking cable  14  comprises a series of flexible load-bearing wire ropes or cables  16  that are fixed to the Harpoon base part  18  and the attachment platform  20  (see  FIG. 9 ) of the chaser half  10 . 1  of the docking mechanism  10 . Inside these load-bearing cables  16  is a control cable  22  that, when pulled back by a retraction mechanism  24  located inside the chaser half  10 . 1  of the docking mechanism  10 , disengages and resets the Harpoon mechanism  12 &#39;. Outside of the docking cable  14  is a sheath  26  consisting of a tightly wound extension spring  26 &#39;. This sheath  26  both protects the inner cable components  16 ,  22  from abrasion or contamination and adds sufficient stiffness to the docking cable  14  to enable the docking cable  14  and sheath  26  to push the Harpoon end effector  12  into contact with a target receptacle  28  of a target vehicle  3 . 
   Outer Shell  30   
   The outer shell  30  of the Harpoon end effector  12  is the main structural component of the Harpoon mechanism  12 &#39; containing all of the moving parts thereof. The outer shell  30  also holds a series of twelve outer ball bearings  32  in tapered holes  34  that prevent the ball bearings  32  from falling out of the Harpoon mechanism  12 &#39; when the Harpoon mechanism  12 &#39; is in a deployed state. The forward portion  36  of the outer shell  30  is threaded on the inside to accommodate an inner shell  38 , and is rounded  40  to prevent snagging on the target receptacle  28  of the target vehicle  3  when the Harpoon end effector  12  is pushed into contact therewith. 
   Inner Shell  38   
   The inner shell  38  of the Harpoon end effector  12  is threaded into the outer shell  30 , thereby containing all of the interior components ( 32 ,  56 ,  60 ,  62 ). Referring also to  FIG. 5 , the forward aperture  42  of the inner shell  38  is adapted to allow a triggering pin  44  in the target receptacle  28  to slide inside the Harpoon mechanism  12 &#39;, depressing the trigger plunger  46  as the Harpoon end effector  12  is seated in the target receptacle  28  of the target vehicle  3 . 
   Trigger Plunger  46  and Trigger Spring  28   
   The trigger plunger  46  slides inside of the inner shell  38  and is held in place by a trigger retaining screw  50 , attached to the inner shell  38 , that can be adjusted for depth. A trigger spring  48  between the trigger plunger  46  and the inner shell  38  biases the trigger plunger  46  in an extended position forward  52 , as illustrated in FIG.  1 . In this position, the outer cylindrical surface  54  of the trigger plunger  46  holds a set of inner ball bearings  56  outward in a corresponding set of holes  58  in the inner shell  38 . 
   Inner Ball Bearings  56   
   The inner ball bearings  56  prevent an actuator sleeve  60  from sliding forward  52  in the Harpoon mechanism  12 &#39; until the trigger plunger  46  is depressed, after which, the inner ball bearings  56  are allowed to move radially inward toward the trigger plunger  46 , clearing the way for the actuator sleeve  60  to be pushed forward  52 . 
   Actuator Sleeve  60  and Actuator Spring  62   
   The actuator sleeve  60  is the main functional component of the Harpoon mechanism  12 &#39;. Referring to  FIG. 2 , after the trigger plunger  46  is depressed, a groove  64  therein aligns with the holes  58  in the inner shell  38  so as to receive the inner ball bearings  56  which then allow the actuator sleeve  60  to slide forward  52 , the motion of which is responsive to an actuator spring  62  within the Harpoon mechanism  12 &#39;, between the outer shell  30  and the actuator sleeve  60 , held in compression when the Harpoon mechanism  12 &#39; is armed. When the actuator sleeve  60  is pushed forward  52  by the actuator spring  62 , a ramped surface  66  of a groove  68  in the actuator sleeve  60  forces the outer ball bearings  32  outward until an outer cylinder surface  70  of the actuator sleeve  60  locks the outer ball bearings  32  in place. The control cable  22  is attached to the actuator sleeve  60 . Retracting the control cable  22  pulls the actuator sleeve  60  back, resetting the Harpoon mechanism  12 &#39;. 
   Outer Ball Bearings  32   
   The outer ball bearings  32  hold the Harpoon end effector  12  in the target receptacle of the target vehicle  3  once the Harpoon mechanism  12 &#39; is deployed therein. The outer ball bearings  32  are allowed to move inward again once the Harpoon mechanism  12 &#39; is reset, thereby enabling the Harpoon end effector  12  to separate from the target receptacle  28 . 
   Actuation of the Harpoon Mechanism 
   The Harpoon mechanism  12 &#39; is nominally kept in the armed configuration. The outer ball bearings  32  are allowed to move freely in and out of their deployed position, while the inner ball bearings  56  are held outwards by the trigger plunger  46  so as to prevent the actuator sleeve  60  from sliding forward  52  in its travel space. Referring also to  FIG. 5 , upon docking, the Harpoon end effector  12  enters the target receptacle  28  of the target vehicle  3  and slides through a capture ring  72  therein. The triggering pin  44  of the target vehicle  3  engages with the forward aperture  42  of the Harpoon end effector  12  and depresses the trigger plunger  46  against the trigger spring  48  sufficient to allow the inner ball bearings  56  to move into the a groove  64  in the trigger plunger  46 , thereby moving the inner ball bearings  56  out of the way of the actuator sleeve  60 , which is then pushed forward  52  in its travel space by the actuator spring  62 . The outer ball bearings  32  are then forced outward by the ramped surface  66  on the outside of the actuator sleeve  60 , and then locked in place once the outer cylindrical surface  70  adjacent the ramped surface  66  lies beneath the outer ball bearings  32 , thereby placing the Harpoon mechanism  12 &#39; in a deployed state. The capture ring  72  is adapted to enable passage of an armed Harpoon end effector  12  therethrough, but to prevent passage of a deployed Harpoon end effector  12 . Upon deployment, the outer ball bearings  32  are located forward of the capture ring  72  within the target receptacle  28 , so that the Harpoon end effector  12  is thereby captured by the capture ring  72 . 
   The Harpoon end effector  12  is retracted by pulling on the control cable  22 , which pulls the actuator sleeve  60  back in its travel space, allowing the outer ball bearings  32  to slide inward, after which the Harpoon end effector  12  is free to be released from the target receptacle  28  of the target vehicle  3 . Upon release, the Harpoon end effector  12  is pulled away from contact with the triggering pin  44  of the target vehicle  3 , which allows the trigger plunger  46  to snap forward  52 , pushing the inner ball bearings  56  outward again so as to engage with and retain the actuator sleeve  60  with the actuator spring  62  compressed, thereby re-arming the Harpoon mechanism  12 &#39;. 
   Differences Between ASDS and the ARD System 
   To assist in appreciating the ways in which the instant invention distinguishes over the prior art, a comparison will be made between the ASDS  10 ′, and ARD system  200 ′, described in U.S. Pat. No. 5,364,046. 
   Capture Mechanism 
   The ARD system  200 &#39;, depicted in  FIG. 4 , utilizes an active latching receptacle  202  on the target side  200 . 2  and a passive brass sphere  204  attached to a steel docking cable  206  on the chaser side  200 . 1 . (Although U.S. Pat. No 5,364,046 appears to describe the capture mechanism as being largely passive because the source of power or effort to actuate the capture mechanism is provided by the chase vehicle  1  to the target vehicle  3 , rather than originating in the target vehicle  3 , a mechanism is considered herein to be active if it requires a source of effort or power for the actuation thereof, regardless of the source of that effort or power). As the docking cable  206  and sphere  204  are extended from the chase vehicle  1 , they seat into the bottom of a main target cone  208  on the target side  202 . 2 , which provides for capturing the sphere  204  with the latching receptacle  202 . The sphere  204  is released from the latching receptacle  202  by a capture mechanism release motor  209 . The main drawback of this design is that it requires active components on both the chaser side  200 . 1  (docking cable  206  and boom extension and retraction systems  212 ,  218 ,  222 ) and the target side  200 . 2  (active latching and release systems  202 ,  209 ) of the docking mechanism  200 . 
   This problem was addressed in the design of the ASDS  10 &#39; by locating all of the active latching components on the chaser side  10 . 1  (see FIG.  5 ). An active latching Harpoon end- effector  12  is mounted on the soft-docking cable  14  (rather than a passive brass sphere  204  as used in the ARD system  200 &#39;), allowing the chaser side  10 . 1  of the docking mechanism  10  to carry out all the active processes of closing the distance to the target vehicle  3 , entering the target receptacle  28  of the target vehicle  3 , latching in the target receptacle  18 , retracting the docking cable  14  to bring the docking mechanism halves  10 . 1 ,  10 . 2  together in a hard-dock, and later releasing the docking mechanism  10  so as to provide for separating the chase  1  and target  3  vehicles. 
   Boom Extension Drive 
   The moving boom  210  of the ARD system  200 &#39; is driven by a rack-and-pinion gear system  212  (see FIG.  6 ). The rack  212 . 1  is attached to the boom body tube  214  and runs the entire length of the boom  210 . The pinion  212 . 2 , driven by a geared-down motor (not illustrated), is held in contact with the rack  212 . 1  to drive the boom  210  in or out of the mounting structure  216 . The main reason for this type of docking mechanism  200  was that the ARD system  200 &#39; was designed to withstand a positive hard-docking impact and the geared motor system was intended to absorb this impact without damaging the structure or the spacecraft it was mounted on. 
   The docking mechanism  10  of the ASDS  10 &#39; is adapted to provide for soft-docking with minimal force imparted to either side, so the geared motor system as used in the ARD system  200 &#39; was unnecessary and would have been inefficient. Instead, a ballscrew-driven boom drive actuator  74  is used, comprising a ballscrew  76  supported by first  78 . 1  and second  78 . 2  ballscrew mounts, and driven through a spider coupling  80  by a motor and gearbox system  82  attached to the aft end of the main docking boom  84 . The first ballscrew mount  78 . 1  is attached to the main dicking boom  84 , and the second ballscrew mount  78 . 2  is attached to a mounting structure  86  by which the chaser half  10 . 1  of the docking mechanism  10  is mounted to the chase vehicle  1 . The ballscrew  76  drives a ballscrew nut  88  on the ballscrew  76  between the first  78 . 1  and second  78 . 2  ballscrew mounts. The ballscrew nut  88  is attached to the mounting structure  86 . In operation, the ballscrew  76  is rotated by the motor and gearbox system  82  through the spider coupling  80 , The ballscrew  76  rotates freely within the first  78 . 1  and second  78 . 2  ballscrew mounts but reacts with the ballscrew nut  88  so as to cause the main docking boom  84  to translate relative to the mounting structure  86 . The ballscrew-driven boom drive actuator  74  provides for more extension and retraction force with a smaller motor and gearbox system  82  due to fewer losses in the system and a greater mechanical advantage (see FIG.  7 ). 
   Cable Actuator System 
   The cable actuator  218  of the ARD system  200 &#39; is simply a standard linear actuator  218 &#39; attached to the interior (aft) end of the docking cable  206 , which design is not space-rated and takes up a great deal of volume (see FIG.  8 ). 
   In contradistinction, the ASDS docking mechanism  10  uses a ballscrew-driven cable actuator  90  that can be placed almost entirely inside the main docking boom  84 . A single ballscrew  92  runs the length of the boom structure  84 &#39; and is supported therefrom by a plurality of ballscrew mounts  94 . The ballscrew is driven by a geared motor  96  at the interior (aft) end of the main docking boom  84 , and a ballscrew nut  98  on the ballscrew  92  between the ballscrew mounts  94  is operatively coupled to a cable shuttle  100 . The docking cable  14  is attached to the cable shuttle  100 , which is mounted on a linear rail  102  inside the main docking boom  84  to prevent the cable shuttle  100  from turning with the ballscrew  92  while allowing the cable shuttle  100  to move axially inside the main docking boom  84 , the range of motion of which is limited by an end-of-travel microswitch  103  (see FIG.  9 ). 
   Cable End Effector 
   The end effector  204 &#39; of the ARD system  200 &#39; is a brass sphere  204  on the end of the docking cable  206 . 
   In contradistinction, the ASDS docking mechanism  10  incorporates a Harpoon end effector  12  — which is active — at the end of the docking cable  14  so as to provide for having all the active components of the docking mechanism  10  on the chaser side  10 . 1  thereof (see FIGS.  1 - 3 ). 
   Mechanism Alignment System 
   The ARD system  200 &#39; provides for rotational alignment of the chaser  200 . 1  and target  200 . 2  sides of the ARD docking mechanism  200  after docking by a rotatable boom head  220  actively driven by a motor  222  mounted aft of the boom  210 . After the rotatable boom head  220  makes hard contact with the main target cone  208  of the target receptacle  224  of the target vehicle  3 , the rotatable boom head  220  is rotated to index the ARD docking mechanism  200  into proper rotational alignment (see FIGS.  10  and  13 ). The rotatable boom head  220  can be rotated ±180 degrees so as to compensate for any rotational misalignment. 
   With the advent of modern sensors and guidance, navigation and control (GN&amp;C) algorithms that are far more accurate at close range than similar systems during the time of the design of the ARD system  200 &#39;, ±180 degrees of alignment correction is no longer required. Therefore, the ASDS  10 &#39; is simplified by instead using a trio of guideposts  104 , attached to the mounting structure  86 , that are adapted to slide into matching receptacles  106  on the target side  10 . 2  of the docking mechanism  10  so as to provide for auto-alignment thereof. While this arrangement does not provide as great a correction range as the active indexed rotatable boom head  220  of the ARD system  200 &#39;, the larger range of rotational alignment correction is no longer required so that the ASDS  10 &#39; can be much simpler. A boom head  108  located on the forward  52  end of the boom is shaped so as to mate with a corresponding main target cone  110  on the target side  10 . 2  of the docking mechanism  10 , and incorporates a central opening  112  through which the Harpoon end effector  12  and docking cable  14  are extended (FIGS.  11  and  12 ). 
   Boom Extension Glide Mechanism 
   The moving boom structure  210 &#39; on the ARD docking mechanism  200  is supported from the mounting plate  226  by eight aluminum rollers  228  that lay in direct contact with the outside skin  230  of the boom structure  210 &#39;. Due to the appearance of some surface galling on the ARD boom structure  210 &#39;, later designs included a V-groove and track roller system, similar to that of the ARD docking mechanism  200  planned for orbit (see FIG.  13 ). 
   The ASDS docking mechanism  10  uses a series of linear ball-bearing guide shuttles  114  which cooperate with corresponding linear guides  116  so as to provide for rigidity of the system during testing. This was not intended to fly in orbit, as a space-rated version of the ball-bearing glides  118  used in the linear ball-bearing guide shuttles  114  does not currently exist in a practical form for use in the ASDS  10 &#39; (see FIG.  14 ). 
   In an alternative embodiment of the invention, the docking cable  14  may be replaced with a rigid  120 , semi-rigid  122  or jointed  124  post that is used to bring a capture mechanism  126  into engagement with a corresponding target receptacle  28  or receiving structure  128  (see  FIGS. 15   a-c ). The rigid post  120  design variant takes the form of a latching device  130 , such as the Harpoon mechanism  12 &#39; described above, on the end of a non-flexible member  120 &#39; that is moved toward the target vehicle  3 , either by actuated motion or by movement of the chase vehicle  1 , until it engages a target receptacle  28  of a target vehicle  3  and successfully creates a connection between the two spacecraft  1 , 3 . The semi-rigid post  122  variant takes the form of a slightly flexible, but stiff member  122 &#39; that is used in the same capacity as above, or of a rigid member  124 &#39; that is allowed to move in compliance with transverse motion. This might result from joints  132  placed at one or both ends of the rigid member  124 &#39; to allow it to align with the target receptacle  28  of the target vehicle  3 . 
   As a further alternative, the Harpoon end effector  12  may be replaced with a magnetic latching device  134 , which is generally utilized in the same manner as the Harpoon end effector  12 , but uses an attraction force between a magnetic end effector  136  and a strike plate  138  on the target vehicle  3 . The magnetic end effector  102  can, for example, be either an electro-magnet  136 . 1 , which requires power to maintain the holding force, or a permanent magnet  136 . 2 , which captures a target vehicle  3  without power. In the case of the permanent magnet magnetic end effector  136 . 1 , an electromagnet  140  in either the target vehicle  3  or magnetic end effector  136  itself is required to nullify the magnetic attraction for release, but power is only used to disengage in this version. 
   As yet a further alternative, the metallic main target cone  110  of the target vehicle  3  may be either a metallic cone  110 . 1  or a non-metallic cone  110 . 2 , for example, constructed of fabric, plastic, or other flexible material, supported by a shape-retaining ring  142  at the opening thereof. A non-metallic cone  110 . 2  would guide the end effector ( 12 ,  136 ) of a given docking mechanism  10  into a corresponding target receptacle in the same manner as the metallic cone  110 . 1 , but would represent a considerable reduction in mass and manufacturing complexity. 
   While specific embodiments have been described in detail in the foregoing detailed description and illustrated in the accompanying drawings, those with ordinary skill in the art will appreciate that various modifications and alternative to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limited as to the scope of the invention, which is to be given the full breadth of the appended claims and any and all equivalents thereof.