Abstract:
A crossing device aids in crossing from a near side to a far side of an obstacle. It comprises a lead assembly that is projected to the far side of the obstacle and that includes an anchor assembly and a pulley. The anchor assembly has a base, a launch tube that is secured to the base, a spike that is housed within the launch tube, and a propellant charge. The anchor assembly is anchored on the far side of the obstacle by remotely initiating the propellant charge. The explosion force drives the spike through the launch tube and the base, into a landing surface. A bridge is connected to the pulley and is pulled to span across the obstacle. The bridge, lead assembly, and anchor may be collected on the far side for additional uses.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 USC §119(e) of U.S. provisional patent application 61/981,930, filed on Apr. 21, 2014, which is herewith and which is hereby incorporated by reference in its entirety. 
    
    
     GOVERNMENTAL INTEREST 
     The invention described herein may be manufactured and used by, or for the Government of the United States for governmental purposes without the payment of any royalties thereon. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates in general to the field of ground unit tactical movement. Specifically, this invention relates to a collapsible, foldable, and portable bridge (or ladder) for mobile use, to aid in crossing generally horizontal obstacles or gaps in urban and rural terrains, and in improving security and safety. 
     BACKGROUND OF THE INVENTION 
     Soldiers in a small group of four or more people who are operating in a combat environment, might find the need to remain mobile while carrying various loads, including their gear and weapons. It is foreseeable that, during their operation, the soldiers will encounter horizontal obstacles, such as canals, rooftop-to-rooftop, gaps, or minefields, that require a crossing device to assist them in avoiding the obstacles. 
     Crossing devices of various designs have been proposed. The following publications illustrate exemplary designs for crossing devices and associated auxiliaries. U.S. Pat. No. 6,062,621 generally describes a collapsible grappling hook that has a hook on the end. It is spring loaded and opens when thrown. When the grappling hook is thrown, it springs open and locks in place. 
     U.S. Pat. No. 7,062,811 generally describes a collapsible bridge that uses two triangle supports with two rails to provide the main structure over the gap. The rails are then overlaid with transverse girders that lock into place. The supports on the side and the girder rail system are collapsible. The girders are also stackable and the track is foldable. 
     U.S. Pat. No. 5,904,025 generally describes a method to reinforce a 90-degree intersection in a structural frame. Four triangular cross section beams come together to form a shear-resisting member of the frame structure. The intersection is formed by cutting and folding the three faces of each beam to increase surface area in contact with the other beam. 
     In general, the tactical advantages of quick gap crossing outlines two types of crossing: hasty and deliberate. There are nine subtasks possible to successfully cross a gap: Plan, Reconnoiter, Prepare, Deploy Assets, Prepare Assault Site, Secure Far Shore, Provide Crossing, Cross Force, and Reconstitute. These steps are detailed and explained. Communication standards are recorded for recon purposes, and methods of recon are also detailed. Command and control for offensive and retrograde operations are described. 
     This article applies to our work because our function is to perform hasty crossings. Our product will be involved in the steps from prepare to reconstitute. Also, this article outlines certain features to profile the gap to be crossed—such as condition of access/egress points, location and condition of existing crossing sites, reinforced obstacles, bank height, slope, and soil stability—which could influence the deployment of our device. Methods of reconnaissance may prove useful. 
     The report by Hornbeck, et al., “Trilateral Design and EST Code for Military Bridging and Gap-crossing Equipment,” May 2005, available at http://www.dtic.mil/dtic/tr/fulltext/u2/a476390.pdf, generally describes the building code for the United States Military Bridging and Gap-Crossing Equipment. It discusses necessary material parameters, load parameters, size requirements, and safety parameters. It also discusses the United States Military system for rating bridges by Military Load Class. It discusses the necessary parameters of a military footbridge as well as commonly used materials. 
     Commercial ladders are also available, such as the three-way extension ladder found online at: http://www.ladder-guy.com/ladders/; and the Xtend and Climb telescoping ladder found online at: http://besttelescopingladder.com/xtend-climb-785p-aluminum-telescoping-ladder-type-i-professional-series.php. 
     While the foregoing conventional crossing devices provided a certain level of utility, there still remains an opportunity to provide a new gap crossing device that provides optimal features in term of portability, stackability, compactness, light weight, extension span, rapid deployment, reusability, durability, and ability to reliably support the weight of the soldiers, their gears, and their weapons (e.g., approximately 350 pounds). The new gap crossing device should be amenable for use in military and civilian applications. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the foregoing concerns and presents a new collapsible, foldable, flexible, cost effective, portable gap crossing device, also referred to herein as a bridge or a ladder, for mobile use, to aid in crossing generally horizontal obstacles in urban and rural terrains and in improving security and safely. The gap crossing device is lightweight, small in volume, and is capable of supporting a load of approximately 350 pounds. 
     More specifically, the gap crossing device is sufficiently compact and light to be readily portable during small team dismounted movements. Due to the variable and rugged environments in which soldiers are traveling, the gap crossing device is designed to be durable. Also, due to the security concerns associated with crossing linear danger areas, the gap crossing device is further designed to be rapidly deployable to help reduce security concerns by making linear danger areas crossings as fast as possible. 
     In a preferred embodiment, the gap crossing device can have a span of approximately twenty feet. It is also able to cover a completely flat obstacle or slightly banked obstacle with a height variation of approximately ten feet. It weighs between approximately five pounds and twenty pounds, and occupies a volume between approximately one cubic foot and five cubic feet. 
     The gap crossing device uses a unique anchoring mechanism. Its method of use can be separated into three distinct phases: anchoring, employment, and redeployment. The anchoring phase relies on both near and far side placement. However, one of the benefits of the gap crossing device is it allows natural surroundings to be utilized. The near side anchoring can be based off mobile spikes emplaced by the user, or it can use natural anchoring points such as columns or trees. The far side emplacement can also be flexible. The gap crossing device can employ a hand-thrown system that will drive a far-side anchor point into the material. It uses a propellant to drive the anchoring point, namely a spike, into the target material. 
     The anchor spike will allow for penetration into a range of material in which the pullout strength will remain relatively consistent based on the penetration depth. The penetration depth increases in softer materials and decreases in harder materials. The harder materials will have higher pullout strength per unit length, allowing sufficient pullout strength regardless of material. 
     However, even the far side anchoring mechanism can be adaptable to the surroundings. For example, the anchoring mechanism can be manually emplaced. The far side anchoring mechanism uses a pulley effect based off a loop in the lead-wire connected to the anchor. Once the anchoring phase is complete, the employment phase commences. 
     The employment phase of the gap crossing device uses the flexible bridge design to carry the load across the gap. The bridge design uses parallel rungs placed equidistant apart against connecting steel wire in order the bear the point loads. It is pulled across a loop in the lead wire. It is then clamped against itself in order to create tension in the bridge. 
     In a preferred embodiment, the gap crossing device uses a standard ratchet strap on the near side. This allows the flexible body to become tensioned against both the near and far side anchors. The near side will utilize either a wide-based column support or employ spaced anchoring points. The far side of the flexible body uses modifiable horizontal spreaders that reduce twisting along the longitudinal axis in the latter half of the bridge. The tension in the bridge also minimizes lateral motion and allows unburdened personnel to cross on their feet rather than crawling. After the personnel have crossed, the bridge can either remain as a permanent fixture or redeploy. 
     The redeployment phase salvages the bulk of the gap crossing device and allows for the system to reengage at a different location, if necessary. The redeployment phase uses a clasp device that releases the tension in the flexible bridge. This allows the body to be pulled in and used again. The clasp uses a torsion spring to maintain the tension within the system. After all personnel have crossed, the torsion spring can be disengaged remotely from the far side releasing the gap crossing device. The anchor emplacements will be consumable but the bulk of the weight of the gap crossing device can be used in future missions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in, and constitute part of this specification, illustrate embodiments of the invention and together with the description, serve to explain the principles of the invention. The embodiments illustrated herein are presently preferred, it being understood, however, that the present invention is not limited to the precise arrangements and instrumentalities shown, wherein: 
         FIG. 1  is an exploded view of a gap crossing device according to a preferred embodiment of the present invention, illustrating its four main components: a lead assembly, a cable, a collapsible bridge, and a near side anchor assembly; 
         FIG. 2A  is an exploded view of the lead assembly of  FIG. 1  that includes a far side anchor assembly, a tether cable, and a pulley; 
         FIG. 2B  is an assembled view of the far side anchor assembly of  FIG. 2A , showing the placement of the various components therewithin; 
         FIG. 3  is view of the cable of  FIG. 1 , shown secured to a tensioning ratchet strap and a redeployment clasp that form part of the near side anchor assembly; 
         FIG. 4  is a view of the gap crossing device shown during an anchoring stage; 
         FIG. 5  is a view of the gap crossing device shown during an employment stage; 
         FIG. 6  is a view of the gap crossing device shown during a redeployment stage; 
         FIG. 7  is a view of the bridge shown in a fully deployed condition; 
         FIG. 8  is a view of the bridge shown in a collapsed condition; 
         FIG. 9  is a side view of one of the rungs of the bridge of  FIGS. 7 and 8 ; 
         FIG. 10A  is a perspective view of a first anchor assembly that forms part of the lead assembly of  FIG. 2 , shown provided with a cap; 
         FIG. 10B  is a perspective view of the anchor assembly of  FIG. 10A , with the cap removed; 
         FIG. 10C  is a perspective view of a second anchor assembly shown provided with a cap; 
         FIG. 10D  is a side view of the cap of the second anchor assembly of  FIG. 100 ; 
         FIG. 10E  is a side view of a base that forms part of the second anchor assembly of  FIG. 10C ; 
         FIG. 11A  is a side view of a first preferred embodiment of a far side anchor or spike that forms part of the gap crossing device of  FIG. 1 ; 
         FIG. 11B  is a side view of a second preferred embodiment of the far side anchor or spike that forms part of the gap crossing device of  FIG. 1 ; 
         FIG. 11C  is a side view of a third preferred embodiment of the far side anchor or spike that forms part of the gap crossing device of  FIG. 1 ; 
         FIG. 11D  is a side view of a fourth preferred embodiment of the far side anchor or spike that forms part of the gap crossing device of  FIG. 1 ; 
         FIG. 12A  is a perspective view of a topper that forms part of the anchor assembly of  FIG. 100 ; 
         FIG. 12B  is a top view of the topper of  FIG. 12A ; 
         FIG. 12C  is a side view of the topper of  FIGS. 12A and 12B ; and 
         FIG. 13  is a view of the bridge shown provided with a spreader assembly. 
     
    
    
     Similar numerals refer to similar elements in the drawings. It should be understood that the sizes of the different components in the figures are not necessarily in exact proportion or to scale, and are shown for visual clarity and for the purpose of explanation. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to  FIG. 1 , it illustrates a new gap crossing device  100  according to a preferred embodiment of the present invention. The gap crossing device  100  presents numerous features and advantages, amongst which are the following: portability, stackability, compactness, light weight, long extension span, rapid deployment, reusability, durability, ability to reliably support the weight of the soldiers, their gears, and their weapons (e.g., ability to support a load of approximately 350 pounds). The new gap crossing device  100  is amenable for use in military and civilian applications, such as law enforcement, fire departments, emergency relief agencies, humanitarian aid and rescue operations, back country hiking and mountaineering, and other similar activities. 
     The gap crossing device  100  generally includes four main components: a lead assembly  110 , a cable  130 , a bridge (also referred to herein as ladder)  150 , and a nearside anchor assembly  170 . 
     With further reference to  FIGS. 2A and 2B , the lead assembly  110  is the component that is thrown as a projectile from a near side of an obstacle  400 , such as a river ( FIG. 4 ), to the far side of the obstacle  400 . The lead assembly  110  generally includes a far side anchor assembly  111 , a wire pulley tether  270 , and a pulley  280 . The far side anchor assembly  111  includes a steel base  200 , an aluminum topper (launch tube)  210 , a far side spike  220 , a pack of propellant  230 , a remote actuation device  240 , and a plastic cap  250 . 
     Upon assembly, the plastic cap  250  is glued to the aluminum topper  210  in order to allow the propellant pack  230  (e.g., packed powder charge) to eject the plastic cap  250  that cannot withstand the force of the blast, while introducing the far side spike (or anchor)  220  into the far-side surface. The propellant-based spike  220  uses geometry and a low center of gravity to land upright when thrown. The propellant is then actuated using, for example, command wire actuation to cause the far side spike  220  to be driven into the ground. The far side spike  220  penetrates multiple different materials from wood to soil, and can hold, for example, a load of approximately 350 pounds. Through testing, it has been determined that the pullout force required to remove the far side spike  220  from wood exceeds the force required to hold the flexible bridge  150  while being crossed, validating the design. 
     The aluminum topper  210  screws into place in the steel base  200  by means of one or more screws (not shown). The purpose of the heavy steel base  200  is to orient and land the far side anchor assembly  111  in the proper position when the gap crossing device  100 , including the pulley  280 , is thrown to the far side of the obstacle  400 . 
     The propellant pack  230  is stored between the plastic cap  250  and a head  221  of the far side spike  220 , providing an upward force on the cap  250  and a downward force on the far side spike  220  when blown. Due to restrictions upon availability of the propellants, gunpowder is a viable choice for the solid propellant pack  230 . A preferred propellant is a smokeless shotgun powder due to its rapid burn rate. 
     The remote actuation device  240  may be housed, for example, within an internal chamber of the cap  250 . The remote actuation device  240  enables a user (e.g., a soldier) to remotely initiate the propellant pack  230 , from the near side of the obstacle  400 . 
     The wire pulley tether  270  is secured at one end to a neck  222  of the far side spike  220 . The wire pulley tether  270  is crimped to make a loop  272  that is smaller than the head  221  of the for side spike  220 , in order to be tightly secured to its neck  222 . The wire pulley tether  270  extends through an opening  202  formed in the bottom of the steel base  200 , to connect to the pulley  280 . The pulley  280  provides connection to the cable  130 . An alternative to the use of the pulley  280  is a plain loop  274  at the terminal end of the wire tether  270 . 
     As further illustrated in  FIGS. 3 and 4 , the cable  130  connects the lead assembly  110  to the bridge  150 . The bridge  150  is initially on the near side of the obstacle  400 , and will need to be extended toward the lead assembly  110  that was previously anchored on the far side of the obstacle  400 , in order to breach the gap between the near side and the far side. 
     In use, the cable  130  is looped through the pulley  280  halfway. Once the lead assembly  110 , including the pulley  280 , is thrown to the far side of the obstacle  400 , the bridge end  300  and the tie end  305  of the cable  130  remain on the near side, completing the anchoring phase as illustrated in  FIG. 4 . 
     With further reference to  FIG. 5 , the employment phase starts with the user applying a pulling force to one of the cable ends, such as the tie end  305 , which in turn pulls the bridge end  300  of the cable  130  toward the far side. Since the bridge end  300  of the cable  130  is tied to one end  152  ( FIG. 1 ) of the bridge  150 , the pulling force also drags the bridge  150  toward the far side. Once the bridge  150  is extended to the desired length across the obstacle (or gap)  400 , the tie end  305  of the cable  130  is tied down on the near side for support. 
     One or more near side spikes  415 ,  416 , which form part of the near side anchor assembly  170 , are then stabilized in the near side surface, ground, or available elements found in the environment (e.g., a tree trunk). In a preferred embodiment, the near side spikes  415 ,  416  are stabilized by applying a downward force to the spikes  415 ,  416 , which secures the near side end  155  ( FIG. 1 ) of the bridge  130 . The near side spikes  415 ,  416  are able to hold the bridge  150  in tension while a load is applied at mid-span, normal to the walking surface. 
     With reference to  FIGS. 3, 4, 5, and 13 , the near side anchor assembly  170  may further include a commercially available tensioning mechanism. One proposed design incorporates a tensioning ratchet strap  330  and a redeployment clasp  340 , which attach to the end of the steel wire  1320  ( FIG. 13 ) that spans the bridge  1300  using loops in the wire  1320 . The tensioning mechanism can be attached at any rung  700  of the bridge  1300  before use or during use if necessary, and then either detached after use or stored, still connected to the bridge  1300 . The bridge  1300  tends to sag if not fully tensioned. Proper tension in the wire  1320  allows for a more easily traversable bridge  1300 . 
     During the employment phase of  FIG. 5 , the tensioning ratchet strap  330 , and the redeployment clasp  340  remain at the near side of the obstacle  400 . It should also be noted that the deployed bridge  150  could be locked at any desired length, depending on the width of the obstacle  440 . 
     The redeployment phase is illustrated in  FIG. 6 . The tensioning ratchet strap  330  is used to reduce lateral instability and cable sage. The redeployment clasp  340  is used to release the tension on the cable  130  and the bridge  150 , in order to enable the complete redeployment of the bridge  150  from the far side of the obstacle  400 , to the near side. The redeployment clasp  340  can be remotely activated from the near side of the obstacle  400 . 
     With further reference to  FIGS. 7, 8, 9 , the bridge  150  is constructed of a plurality of strong rungs  700  that support the intended loads, and of two cable sides  705 ,  710  that provide stability to the rungs  700 . In one design, the flexible bridge  150  spans a horizontal obstacle of approximately 20 feet. The flexible bridge  150  is approximately 25 feet long and weighs approximately 14 pounds. It is primarily composed of two ⅛th-inch wire side ropes  705 ,  710  and twenty aluminum rings  700 . 
     Each rung  700  is preferably hollow and cylindrically shaped, with each end including two diametrically opposed holes  720 ,  725  for securing the rungs to the side ropes (or cables)  705 ,  710 . The side ropes  705 ,  710  are preferably made of steel. 
     The near side of the bridge  150  is secured to a ratchet strap  330  for tensioning the bridge  150 , and a clasp  340  for redeployment. The far side of the bridge  150  can utilize a trapezoidally shaped aluminum pipe and wire rope to stabilize the bridge  150  and counteract twist. 
     As it has become clear from the foregoing description, one of the main benefits of the gap crossing device  100  is that the hand thrown lead assembly  110  replaces the need for a soldier to leave the security of the rest of the unit and expose himself or herself to hazards while navigating the obstacle  400  unaided, to secure the far side. Additionally, the gap crossing device  100  does not rely on a tree or other environmental structures to secure the bridge  150  to the far side of the obstacle  400 , making it usable in versatile environments, not merely wooded areas. 
       FIGS. 10A and 10B  are representations of the anchor assembly  111  according to a first embodiment of the present invention. As descried earlier, the anchor assembly  111  generally includes the base  200 , the topper  210 , and the cap  250 . In this embodiment, the cap  250  provides a mushroom cover along the exposed length of the topper  210 , over the hollow base  200 . This allows the center of gravity of the anchor assembly  111  a greater vertical moment with respect to the side of the base  200  when laid horizontally. 
     The topper  210  includes a hollow launch chamber  1000  that extends along the length of the topper  210 , to provide a launch path to the far side spike  220 . The launch chamber  1000  is open at it upper end  1010  and its bottom end  1020 . The upper end  1010  allows the entry of the far side spike  220  to the launch chamber  1000 , while the bottom end  1020  enables its exit through an opening  1070  ( FIG. 10E ) at the bottom of the base  200 . 
     While the topper  210  has been described earlier as being composed of aluminum, it should be understood that the topper  210  might be made of a different material, including but not limited to high-strength plastic. Alternatively, the topper  210  might be made of light weight material, with the launch chamber diameter accommodating a metallic conduit  1015 . 
     While the launch chamber  1000  can be axially disposed relative to the topper (or launch tube)  210 ,  FIG. 10B  illustrates an alternative embodiment wherein the launch chamber  1000  is disposed at an angle relative to the central axis of the topper  210 . In this alternative design, the far side spike  220  will travel along the slanted launch chamber  1000  to reach the far side surface at an angle, allowing for a higher pullout force. A higher pullout force allows more tension in the system, and therefore more weight to be supported. 
     The design of the anchor assembly  111  uses the moment and center of gravity principles to enable the anchor assembly  111  to land in the desired vertical (or slanted) position, so that far side spike  220  is propelled with optimal penetration force. The difference in densities between the parts of the anchor assembly  111 , and the dimensions of the steel base  200 , shift its center of gravity very close to the bottom of the base  200 , allowing the anchor assembly  111  to upright itself. 
       FIGS. 10C, 10D, 10E  illustrate a second anchor assembly  1030  shown provided with a new cap  1033 . In this design, the base  1035  of the cap  1030  is widened and the mass of the cap  1033  is reduced by perforating the cap  1033  with at least one circular cutout  1037 . This allows a lesser counter moment on the base  200 , helping in restoring moment to the vertical position. The circular cutouts  1037  are designed to reduce the weight of the cap  1033 , lowering the center of gravity of the anchor assembly  1030  and allowing the anchor assembly  1030  to land upright. The dotted lines mark the grooves on the inside of the base  200  allow the shaft  210  to screw into the base  200  and the cap  1030 . The opening  202  in the base  200  allows a fuse to run through the base  200  for detonation. 
     Considering now the various designs of the far side spike with respect to  FIGS. 11A, 118, 11C, 11D ,  FIG. 11A  is a side view of a first preferred embodiment of the far side anchor or spike  220  that forms part of the anchor assembly  111 . The far side spike  220  is preferably made of tungsten, though other suitable material may alternatively be used. 
     The far side spike  220  generally includes the head  221 , the neck  222 , and a body  1100 . The body  1100  is formed of a forward (or nose) section  1110  and a main section  1115 . Both sections  1110 ,  1115  are generally conically shaped, so the diameter change at the interface between these two sections  1110 ,  1115  creates a lip  1111 . The lip  1111  has a sufficient circumferential surface area to increase the pressure surface area with the penetrated ground (or surface), in order to increase the pullout force. 
     The neck  222  of the far side spike  220  includes a through opening  1118  to engage the tether cable loop  272  ( FIG. 1 ). The largest diameter of the far end spike  220  fits precisely within the launch tube in the topper  210 . The openings and surfaces of the far end spike  220  are cut or drilled to within ten thousandths of an inch. 
       FIG. 11B  illustrates yet another far side spike  1140  according to the present invention. The far side spike  1140  is generally similar in design and construction to the far side spike  220 , with the exception that the far side spike  1140  includes a plurality of conical body sections, for example three conical body sections  1150 ,  1151 ,  1152 . The present invention is not limited to the illustrated three conical body sections  1150 ,  1151 ,  1152 , and another number of body sections is contemplated within the scope of the present invention. The three conical body sections  1150 ,  1151 ,  1152  have different conical dimensions so that their intersection surfaces form a plurality of lips  1154 ,  1155  that further increase the pressure surface area with the penetrated ground as well as the pullout force. 
       FIG. 11C  illustrates still another for side spike  1160  according to the present invention. The far side spike  1160  includes the head  221 , a neck  1062 , a nose section  1161 , and a main body  1166 . The neck  1162  is not cylindrically straight, as illustrated in  FIGS. 11A and 11B . Rather, the neck  1162  has an arcuate outer surface in order to allow for penetration into softer materials such as wood. 
     The main body (or shaft)  1166  has an X-shaped cross-sectional area in order to increase the surface area contacting the surface material, therefore increasing friction holding the spike in place. A slight twist is added to the cross section to allow for an increase in pullout. The nose  1161  allows for a sharp surface area, which allows for increased fluid pressure to increase the penetration, velocity and pullout strength. 
       FIG. 11D  illustrates yet another far side spike  1180  according to the present invention. The far side spike  1180  includes the head  221 , the neck  222 , a nose  1180 , an intermediate shaft  1181 , and a conical body section  1182 . The conical body section  1182  extends from the neck  1118  and decreases in diameter to match the diameter of the intermediate shaft  1181  that has a smaller diameter than the conical body section  1182 . When manufactured, the intermediate shaft  1181  is extruded suddenly from the conical body section  1182 , to increase the pullout strength of the material, with the sharp angle of the extruded cut providing more resistance during pullout. The nose  1180  is conical and terminates in an ogive shaped tip  1184 . A lip  1190  is formed by the interface of the intermediate shaft  1181  and the nose  1180  to increase the required pullout force. 
     With reference to  FIGS. 12A, 12B, 12C , they illustrate a topper (or launch tube)  1210  that may be used in the anchor assemblies  111  and  1030  of  FIG. 10A, 10B, 10C . The topper  1210  is modified to accommodate the tether cable  270  to the far end spike  2210  this end, the topper  1210  is formed with a hollow launch chamber  1222  that is generally cylindrically shaped with a substantially circular cross-section. In this specific embodiment, the hollow launch chamber  1222  has been modified to include a smaller channel  1225  along its length, through which the tether cable  270  is run. The main function of the channel  1225  is to allow room for the tether cable  270 , which is attached to the spike. 
     A ballistics test analysis of the gap crossing device  100  was undertaken, including testing at ranges that provided data used to determine the ballistics characteristics of the propellant drive far end spike  220 . At these ranges, the spike penetration thickness, spike penetration angle, free recoil displacement, charge size, and muzzle velocity were measured through different means. 
     The Demarre equation was used to determine impact velocity. The standard form of the Demarre equation is utilized to predict armor penetration thickness, as follows: 
     
       
         
           
             
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     Newton&#39;s second law was used to determine the pressure in the barrel, as follows: 
     
       
         
           
             
               
                 
                   
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     Newton&#39;s second law was also used to determine the maximum barrel stress along the radial and longitudinal axes. 
     
       
         
           
             
               
                 
                   
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     Free recoil analysis was used to determine recoil velocity and maximum spike velocity. 
     
       
         
           
             
               
                 
                   
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                       r 
                     
                   
                 
               
             
             
               
                 
                   
                     
                       Spike Maximum Velocity: 
                     
                     ⁢ 
                     
                       V 
                       
                         m 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         a 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         x 
                       
                     
                   
                   = 
                   
                     
                       V 
                       recoil 
                     
                     
                       
                         ( 
                         
                           
                             ( 
                             
                               0.5 
                               * 
                               
                                 m 
                                 c 
                               
                             
                             ) 
                           
                           + 
                           
                             m 
                             p 
                           
                         
                         ) 
                       
                       
                         m 
                         r 
                       
                     
                   
                 
               
             
           
         
       
     
     Based on this and other analyses, the gap crossing device  100  is provided with a four-part construction, as described earlier. The heavier base  200  will be constructed of 1045 steel. The topper  220  will be constructed of aluminum, with a plastic cap  250  that is made for example with a 3D printer. The cap  250  is retained in place with screw threads and designed to detach upon detonation. The topper  210  may be made of plastic material and houses a steel barrel or conduit  1015  ( FIG. 10B ). The anchor assembly  111  attaches to a pulley or a pulley system  280 . In turn, the pulley  280  is connected to the bridge  150  by means of a strong steel leader or tether cable  270  that loops around the far end spike  220 . Prior to the initiation of the propellant pack  230 , the far end spike  220  is house within the launch chamber  1222 . 
     The far end spike  220  can be designed to penetrate concrete by constructing it of 1045 steel. The rungs  700  stretching across the bridge  150  are preferably constructed of lightweight PVC, a composite, or aluminum, depending on testing performance. 
     The anchor assembly  111 , including the propellant pack  230  are capable of creating a spike muzzle velocity that is sufficient to penetrate concrete. Achieving an adequate penetrative depth is imperative for ensuring a reliable anchor support that is capable of bearing the required minimum 350 pounds force. In order to accomplish this, the powder charge of the propellant pack  230  is capable of producing a 150 m/s muzzle velocity, with the assumption that the compressive strength of concrete is 30 MPa, which is a relatively high magnitude for typical man-made structures. 
     First, the energy of the far end spike  220 , immediately following the actuation was calculated using the following equation: 
     
       
         
           
             E 
             = 
             
               
                 1 
                 2 
               
               ⁢ 
               m 
               ⁢ 
               
                   
               
               ⁢ 
               
                 v 
                 2 
               
             
           
         
       
     
     where m is the mass of the far end spike  220  and v is its velocity. With a mass of 0.15 kilograms, the spike can obtain 1244.64 foot-pounds of energy. Using this value for the spike energy, the penetration depth in concrete was solved using the equation: 
     
       
         
           
             
               e 
               d 
             
             = 
             
               
                 m 
                 ⁡ 
                 
                   ( 
                   
                     
                       E 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       c 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       p 
                     
                     
                       f 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       c 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         d 
                         3 
                       
                     
                   
                   ) 
                 
               
               + 
               b 
             
           
         
       
     
     where m and b are dimensionless values derived from a linear equation. Ecp is the spike energy, fc is the compressive strength of concrete, and d is the spike diameter. Assuming CRH=3.0, then m=0.0941 and b=4.129. This equation asserts that a 1045CR steel spike can achieve a depth penetration of 5.28 inches, which is enough to bear the load and allow adequate tension of the entire bridging system. This depth ensures that as long as the reactionary forces do not cause the anchor assembly  111  to be ejected upward, the penetration of the far end spike  220  in concrete will be adequate to support the required load. 
     An analysis of the rungs  700  was also undertaken. The load imparted by the soldier&#39;s foot onto the rung  700  of the bridge  150  is modeled as a point load. This is a conservative assumption because it increases the shear and moment caused by the load. 
     The analysis of the rungs  700  included testing each rung separately for failure, by attaching each end of the test specimen to a looped cable pinned to the test apparatus. This essentially created a pin-roller support to test for ultimate strength in bending. Increasing incremental loads were added to the rung  700  until a final load exceeding approximately 350 pounds, applied centrally, was verified. Each specimen was tested at a length of 16 inches. This also introduced a factor of safety when the length is shortened to 15 inches. 
     An iterative code was then generated that took account of the maximum deflection, maximum shear, and bending failure. The code checked these three modes of failure against a set outer diameter of aluminum and returned the thinnest rung thickness that would not fail under these constraints. Each test also output an estimated volume and weight, so that the outer diameter could be adjusted by the programmer to optimize these factors. 
     The following three equations were used to check the properties of geometry and material strength versus maximum allowable deflection, shear failure, and failure in bending, respectively. 
     First equation: 
               I     δ   ⁢           ⁢   Allow       =       P   *     L   3         48   *   E   *     δ     m   ⁢           ⁢   a   ⁢           ⁢   x                 
where I is moment of inertia, P is load applied (350 pounds), L is Length of the member (e.g., 15 inches), E is the modulus of elasticity, δ max  is the selected maximum allowable deflection.
 
     Second equation: 
               τ   Allow     =         4   *   P       2   *     A     c   ⁢           ⁢   s           *         r   out   2     +       r   out     *     r     i   ⁢           ⁢   n         +     r     i   ⁢           ⁢   n     2           r   out   2     +     r     i   ⁢           ⁢   n     2                 
where τ is shear stress, A cs  is cross sectional area, r out  is the outer radius, and r in  is the inner radius.
 
     Third equation: 
               I     σ   ⁢           ⁢   Allow       =       P   2     *     L   2     *       r   out       σ     m   ⁢           ⁢   a   ⁢           ⁢   x                 
where σ max  is the yield stress of the material, which is used to find the maximum load that does not produce permanent deformation.
 
       FIG. 13  illustrates a bridge  1300  according to another embodiment of the present invention. The bridge  1300  is similar in design and construction to the bridge  150  of  FIG. 7 , with the exception that the bridge  1300  is provided with a spreader assembly  1310  that limits the twisting of the bridge  1300  along the longitudinal axis. The spreader assembly  1310  generally includes two horizontal spreaders  1315 ,  1316  that are secured to the forward rung  1330 , and that are tensioned by means of a wire or cable  1320 . 
     The spreaders  1315 ,  1316  are generally similar in design and function and therefore only one spreader  1315  will be described in more detail. The spreader  1315  is preferably made of the same material as the rungs  700 , including the forward rung  1330 . As an example, the spreader  1315  may be made of an aluminum pipe with the same gauge as that of the rungs  700 . The spreader  1315  is secured to one end of the forward rung  1330  by means of a larger gauge elbow  1341  that fits snuggly onto the end of the forward rung  1330 . Similarly, the spreader  1316  is secured to the opposite end of the forward rung  1330  by means of an elbow  1340  which is similar in design and construction to the elbow  1341 . 
     Upon assembly, the two spreaders  1315 ,  1316  branch out into a Y-shaped configuration to provide a wider support area to the bridge  1300 , and thus improve its stability against twisting. The tips or forward ends of the two spreaders  1315 ,  1316  are tensioned and kept in the Y-shaped configuration by means of the wire  1320 . In this regard, the wire  1320  spans across the open Y-shaped configuration of the two spreaders  1315 ,  1316  and is secured in this position by any known or available method. As an example, a clasp  1333  may be added to connect the two end of the wire  1320  between the spreaders  1315 ,  1316 . In addition, the wire  1320  may be run entirely through the rungs  700 . 
     When it is desired to stow the bridge  1300 , the ends  1360 ,  1361  of the wire  1320  are released and the tension on the two spreaders  1315 ,  1316  is relaxed, allowing them to collapse and to be tucked alongside the rungs  700  in a backpack or another storage container. 
     It is to be understood that the phraseology and terminology used herein with reference to device, mechanism, system, or element orientation (such as, for example, terms like “front”, “back”, “up”, “down”, “top”, “bottom”, “forward”, “rearward”, and the like) are only used to simplify the description of the present invention, and do not alone indicate or imply that the mechanism or element referred to must have a particular orientation. In addition, terms such as “first”, “second”, and “third” are used herein and in the appended claims for purposes of description and are not intended to indicate or imply relative importance or significance. 
     It is also to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. Other modifications may be made to the present design without departing from the spirit and scope of the invention. The present invention is capable of other embodiments and of being practiced or of being carried out in various ways, such as, for example, in military and commercial applications.