Patent Publication Number: US-2004050014-A1

Title: Passive aerial protection system

Description:
BACKGROUND OF THE INVENTION  
       [0001] On Sep. 11, 2001, terrorists destroyed the World Trade Center towers in New York City by suicidally flying fuel-laden airliners into them at high speed. It is widely believed that the towers&#39; ultimate demise resulted not so much from the impact of the aircraft but from the intense heat generated by the huge amounts of jet fuel they carried. The volatility of the fuel combined with its massive concentration in a relatively confined region resulted in an inferno that fatally weakened the towers&#39; structural members. Fireproofed steel loses half its strength upon reaching a temperature of 1,100° F., and the concentrated fireball of burning fuel probably subjected the towers&#39; structural members to much higher temperatures than that.  
       [0002] Whether primarily responsible for the collapse or not, the impact of the aircraft was heightened, too, by a deadly combination of impact force and concentration of that force. The towers were designed to withstand 100 mph winds, which would subject each story of the towers to distributed force of perhaps a hundred thousand pounds. But each tower suffered tremendous, perhaps fatal damage from the concentrated, bullet-like impact of an airliner&#39;s fuselage slicing into it “head on” at high speed.  
       [0003] After the September 11 terrorist attacks, the U.S. Nuclear Regulatory Commission admitted that it did not specifically contemplate attacks by the type of aircraft used by the terrorists. Conventional defense systems had targeted missiles and planes of a military aggressor, not civilian airliners piloted by suicidal terrorists.  
       [0004] The danger of similar attacks to occurring against nuclear installations, as well as government buildings, ammunition stockpiles, and other sensitive facilities, has been widely observed. Significant political and logistic difficulties arise with conventional active (i.e., “shooting”) defense against such attacks. What is needed, then, is a simple, passive way to protect sensitive facilities against the most devastating, concentrated effects of an aerial attack.  
       SUMMARY OF THE INVENTION  
       [0005] An aerial protection system according to various aspects of the present invention relies on the principle of disrupting horizontal flight of an attacking aircraft toward the facility it protects. By disrupting the aircraft&#39;s flight, the system helps reduce concentration of any impact and fire from the aircraft.  
       [0006] The system includes a number of support masts disposed about an area of terrain sufficiently large to enclose the facility, and a plurality of cable groups that each include a multitude of cables, i.e., more than just a plurality. The cables in each group are coupled to an adjacent pair of the support masts, extending substantially coplanar with the support masts. The cables are separated from each other at a spacing that is comparable to (and preferably slightly less than) a typical wingspan of an aircraft from a class of interest.  
       [0007] An airliner hitting all but the strongest cables at near-cruising speed is unlikely to be entirely stopped by those cables. Advantageously, however, the inventive system can substantially reduce damage to the protected facility by disrupting the aircraft&#39;s flight before impact. For example, when an aircraft hits one or more cables suspended in accordance with various aspects of the invention, it may lose a wing, yaw violently, veer off course, or any combination of those disruptive actions. In addition, the aircraft may well burst into flame or break up upon impact with the cable. As used herein, the term “disrupt” includes any action that alters the flight of an attacking aircraft in a way that reduces damage caused by its impact or prevents the impact altogether.  
       [0008] The effect of this disruption is to lessen the concentration of impact and fuel should the aircraft nevertheless strike the protected facility. If a wing separates from the craft, the wing and remaining portions of the aircraft can be expected to hit the facility at somewhat separated points, distributing the impact and separating the burning fuel from the impact point. The fuselage can be expected to have less damaging impact on the facility if it has been forced to yaw significantly from a bullet-like “head on” orientation. Concentration of fire inside the facility is substantially lower from an aircraft that has exploded or begun burning violently before hitting the facility than from one that slices into the facility with a full load of fuel waiting in its tanks. Remaining fuel would also probably strike the facility, but in a less concentrated way.  
       [0009] The above summary does not include an exhaustive list of all aspects of the present invention. Indeed, the inventor contemplates that the invention includes all systems and methods that can be practiced from all suitable combinations of the various aspects summarized above, as well as those disclosed in the detailed description below and particularly pointed out in the claims filed with the application. Such combinations have particular advantages not specifically recited in the above summary. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0010] Various embodiments of the present invention are described below with reference to the drawings, wherein like designations denote like elements.  
     [0011]FIG. 1 is a perspective view of an aerial protection system according to various aspects of the invention having four masts in a square arrangement.  
     [0012]FIG. 2 is a perspective view of an aerial protection system according to various aspects of the invention having six masts in a hexahedral arrangement. 
    
    
     DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS  
     [0013] A system for protecting a facility against aerial attack according to various aspects of the present invention limits damage to the facility from an attacking aircraft by erecting a barrier around the facility using masts and cables. As may be better understood with reference to FIG. 1, for example, system  100  surrounds a nuclear facility  110  with cables suspended from a square arrangement of support masts  122 - 128  around facility  110 . As discussed above, and also below with reference to an example involving an attacking aircraft  150 , system  100  substantially reduces damage to facility  110  by disrupting the flight of aircraft  150  before impact.  
     [0014] Each adjacent pair of masts  122 - 124 ,  124 - 126 ,  126 - 128 , and  128 - 122  in system  100  supports a group of cables that extend substantially coplanar with the support masts of that pair. For clarity of illustration, FIG. 1 fully depicts just one of the four cable groups of system  100 , i.e., the one supported by adjacent masts  122 - 124 . That group includes a top supporting cable  130 , which suspends substantially vertical cables  142 ,  144 , and others not referenced by number in FIG. 1. Intermediate horizontal support cables (optional) are also shown, such as cable  160  in FIG. 1. Only the top supporting cables of the other groups in system  100  are depicted in FIG. 1. These are: for mast pair  124 - 126 , cable  132 ; for mast pair  126 - 128 , cable  134 ; and for mast pair  128 - 122 , cable  136 .  
     [0015] The masts may be arranged in any closed or semi-closed shape. A semi-closed shape takes advantage of terrain features such as a cliff adjacent to the facility, with the masts and cable groups shielding other directions. A closed arrangement surrounds the facility but may follow an irregular path, such as to avoid obstacles or other nearby property. A polygonal arrangement of support masts is one in which the masts define vertices of a polygon-shaped border, i.e., a closed border bounded by straight line segments. Each line segment of the border connects adjacent masts. Masts in a polygonal arrangement need not be connected by lines of any actual polygon, although it can be expected that such will often be the case. For example, the masts of exemplary system  100  (FIG. 1) are indeed connected by lines of a regular polygon, i.e., a square, in the form of top suspending wires  130 ,  132 ,  134 , and  136 .  
     [0016] The cables in each group are separated from each other by a spacing that is comparable to a typical wingspan of an aircraft from a class of interest, preferably a bit less than the typical wingspan. For example, cables  142 - 144  for mast pair  122 - 124  are separated from each other by about the wingspan of attacking aircraft  150 , and have about the same separation from other adjacent cables.  
     [0017] The length of a typical wingspan depends on the class of aircraft being considered, but is certainly no less than about 15 feet (for small pleaure aircraft) and no greater than about 150 feet (for commercial airliners). In exemplary system  100 , protected facility  110  is a nuclear plant that has enough structural integrity to withstand attack from small commuter aircraft. Thus, cables  142 ,  144 , etc. can be separated by a spacing comparable to the typical wingspan of a commercial airliner, for example somewhere between 100-200 feet. A fairly small spacing that is comparable to a typical wingspan (e.g., 50 feet) can be employed to virtually guarantee a collision between a horizontally attacking airliner and at least one cable. A fairly large spacing, still comparable to a typical wingspan (e.g., 150 feet), can be employed instead to maintain a reasonable likelihood of collision while minimizing the number of cables that need to be suspended.  
     [0018] The term “comparable” is generally employed herein to indicate that two dimensions are functionally equivalent, i.e., that no significant change in performance would be expected between systems that differ only in the dimensions considered comparable. For example, one-foot cable spacing would certainly not be considered comparable to 100-foot spacing in any sense of the word because cables so closely spaced would require a tremendous amount of support structure while offering no significant functional advantage over cables spaced 100 feet apart. But 75- or 125-foot spacing is clearly comparable to 100-foot spacing because the difference would entail no significant change in support structure or likelihood of collision. In accordance with a more particular aspect of the invention, the term “comparable” can be understood as simply indicating less than about a two-to-one difference between two measurements.  
     [0019] The spacing of principal concern is measured along a horizontal axis, because aircraft typically have a head-on cross-section that is much wider (through the wingspan) than it is high. Nonetheless, spacing along the vertical axis is preferably kept comparable to that along the horizontal axis to prevent attackers from rapidly rolling their aircraft to a sideways orientation to pass through the “net” of cables. In system  100 , for example, horizontal spacing between vertically suspended cables  142 ,  144 , etc. is about half the vertical spacing between top supporting cable  130  and a midpoint horizontal cable  160 .  
     [0020] Another exemplary system  200 , which may be better understood with reference to FIG. 2, employs a hexagonal arrangement of support masts  222 - 227  and a cable group arrangement with varied horizontal spacings. Support masts  222 - 227  are embedded at their bases in concrete blocks for strength and stability. For example, mast  222  is embedded at its base into block  232 , which extends some distance below ground level as illustrated.  
     [0021] System  200  also employs a number of partially buried concrete anchors, of which FIG. 2 illustrates only structures  252  and  253  for clarity. Each one of the anchors is disposed between an adjacent pair of the support masts. There may be several bases between each or any mast pair. Advantageously, each anchor also connects to some of the cables of the cable group for its corresponding mast pair, adding to the cable array without the need for additional support masts.  
     [0022] In system  200  and any other case where horizontal spacing varies significantly, the spacing is considered to be the maximum spacing of cables within an effective area of the cable group. For example, the horizontal spacing of cables in system  200  is indicated in FIG. 2 by the dimension mark “S.” This horizontal spacing is found along a horizontal axis between cables intersecting at the top of mast  222  and vertical midpoint cable  242 , which is tensioned by a midpoint anchor  252 .  
     [0023] A support mast according to various aspects of the invention includes any generally tall, slender structure suitable for (1) sustaining the significant weight of the heavy structural cables it must support and (2) at least partially sustaining the significant sideways impulse of an impacting aircraft. A support mast need not remain intact and vertical during an aircraft attack, because the breaking resistance and inertia of the mass is likely sufficient to counteract the impact force enough to disrupt the aircraft&#39;s flight. If desired, however, a mast can be designed to remain intact after such an attack, for example by ensuring that its strength exceeds that of the cables between it and an adjacent mast. Structure integral to the mast (e.g., composite materials, wide-diameter steel) can provide such strength, as can opposing guy wires, truss members, etc.  
     [0024] In accordance with various aspects of the invention, cables are generally flexible, extremely slender, tension-bearing structures that can sufficiently withstand impact from a wing or other member of an oncoming aircraft to disrupt flight of that aircraft. Such structures are typically made up of hundreds or even thousands of twisted wire strands, but can also consist of or include other structural material. While a cable need not remain intact during an aircraft attack, the more resistance it can provide to impact, the more damage it can do to the aircraft and thus more significantly disrupt its flight. Cables can be of any suitable type. For example, the design, fabrication, and implementation of cables can be in accordance with any suitable combination of the disclosures found in U.S. Pat. No. 4,473,915 to Finsterwalder; U.S. Pat. No. 4,557,007 to Daiguji; U.S. Pat. No. 4,216,636 to Cordel; and U.S. Pat. No. 3,967,421 to Dufossez. The detailed description portions of the aformentioned patents are incorporated herein by reference, including any documents and drawing figures referenced therein.  
     [0025] A cable group of a system according to various aspects of invention includes a multitude of cables. (As used herein, the term “multitude” simply means “three or more.”) For example, the cable group of mast pair  122 - 124  in system  100  (FIG. 1) includes a multitude of vertically suspended cables (seven) and some horizontal cables in a rectangular-hole configuration. In system  200  of FIG. 2, the cable groups of mast pairs  222 - 223  and  223 - 224  both include a larger multitude of cables (twelve) in an irregular-polygon configuration.  
     [0026] In operation, system  100  of FIG. 1 passively protects facility  110  against aerial attacks. In an illustrative example of an aircraft flight-disrupting method of the invention, attacking aircraft  150  (also illustrated in FIG. 1) is disrupted in its suicidal flight toward facility  110  by vertically suspended cable  142 . At the point depicted in FIG. 1, a wing  152  of aircraft  150  strikes cable  142 . The resulting impact then tears wing  152  from the fuselage of aircraft  150  and ignites fuel inside wing  152 . The fuselage of aircraft  150  rolls and drops violently due to lack of lift from wing  152 , and veers off its intended course toward facility  110 , striking the facility with a glancing blow rather than a direct one. Meanwhile, wing  152  disappears into a fireball that never reaches the interior of facility  110 , burning up in mid-air and outside the non-flammable exterior wall of facility  110 .  
     [0027] The result of this exemplary flight disruption is significantly reduced damage to facility  110 . Fuel from wing  152  never reaches the interior of facility  110 . Structure and remaining fuel of aircraft  150 , which may or may not ignite before impact, penetrates less into facility  110  (if at all) because the glancing blow imparts a far less concentrated impact on facility  110  than a direct one.  
     [0028] According to particular aspects of the invention, the following additional structures and configurations illustrated in FIGS. 1 and 2 can be advantageously included.  
     [0029] EMP NETTING—In addition to structural cables, system  200  includes conductive netting between adjacent masts and as a “roof” over facility  210 . (For clarity, FIG. 2 only illustrates netting between masts  226 - 227  and a fragment of “roof” netting.) The netting comprised of a multitude of electrically conductive wires. The wires are mechanically connected to the support masts (e.g., masts  226 - 227  in exemplary system  200 ) or, in a variation, structural cables of the system&#39;s cable groups, or both. The wires are sufficiently distributed around facility  210  and spaced close enough to each other to form an electromagnetic shield around facility given a cutoff frequency of interest. For example, if shielding of spectral content up to 100 MHz is desired, the maximum separation between conductive wires should be significantly less than about 75 cm, which is a quarter wavelength in free space at that frequency.  
     [0030] RAISING AND LOWERING SYSTEM—an advantage of employing suspended cables as a barrier is the ability to raise and lower the cable groups as desired, such as for maintenance or cable replacement. In a variation of system  100 , for example, masts  122 - 128  support top supporting cables  130 - 136  via end structures (not shown) that can move up and down the masts. Any suitable type of end structure can be employed, for example an anchor movably mounted on a vertical track. Conventional pulley and winch systems can assist in raising and lowering cables, as a group or one at a time. Other alternatives include providing underground vertical housing tubes for the masts and raising and lowering the masts themselves from and into the tubes. A raise-lower variation of system  100  preferably also includes a container (not shown) between each mast pair, which can house cables of that mast pair&#39;s cable group in a lowered configuration. Such a container can be, for example, a lined trench in the ground between adjacent masts or a trough-like structure at ground level that can double as a perimeter wall.  
     [0031] GROUND-LEVEL CABLES—System  100  further includes ground-level cables to help disrupt travel of ground-based vehicles. (The groups of ground-level cables illustrated in FIG. 1 are referenced with numerals  170  and  172 .) Advantageously, and unlike a fixed fence or wall, cables  170 ,  172  can be raised and lowered along with the other cables of system  100  in the variation discussed above. A gap, shown between  170  and  172 , can allow traffic to pass, such as through a guardpoint, for access.  
     [0032] Public Notice Regarding the Scope the of the Invention and Claims  
     [0033] The inventor considers various elements of the aspects and methods recited in the claims filed with the application as advantageous, perhaps even critical to certain implementations of the invention. However, the inventors regard no particular element as being “essential,” except as set forth expressly in any particular claim.  
     [0034] While the invention has been described in terms of preferred embodiments and generally associated methods, the inventors contemplate that alterations and permutations of the preferred embodiments and methods will become apparent to those skilled in the art upon a reading of the specification and a study of the drawings. For example, a system can employ just three supporting masts in a triangular arrangement. As another example, many more than the six supporting masts employed in system  200  of FIG. 2 can be employed.  
     [0035] Additional structure can be included, or additional processes performed, while still practicing various aspects of the invention claimed without reference to such structure or processes. For example, contact explosives can be placed at lengths along the suspended cables to promote explosion of an attacking aircraft and reduce the chances of the aircraft impacting the protected structure or, if it should, further reduce the resulting fire intensity inside the structure.  
     [0036] Accordingly, neither the above description of preferred exemplary embodiments nor the abstract defines or constrains the invention. Rather, the issued claims variously define the invention. Each variation of the invention is limited only by the recited limitations of its respective claim, and equivalents thereof, without limitation by other terms not present in the claim.  
     [0037] In addition, aspects of the invention are particularly pointed out in the claims using terminology that the inventors regard as having its broadest reasonable interpretation; the more specific interpretations of 35 U.S.C. § 112(6) are only intended in those instances where the terms “means” or “steps” are actually recited. As one example, the phrase “typical wingspan” indicates a wingspan having a length representative of wingspans typically encountered, not some precise average or median statistic.  
     [0038] The words “comprising,” “including,” and “having” are intended as open-ended terminology, with the same meaning as if the phrase “at least” were appended after each instance thereof. A clause using the term “whereby” merely states the result of the limitations in any claim in which it may appear and does not set forth an additional limitation therein. Both in the claims and in the description above, the conjunction “or” between alternative elements means “and/or,” and thus does not imply that the elements are mutually exclusive unless context or a specific statement indicates otherwise.