Abstract:
A vehicle in free space or air, with external surfaces primarily fashioned from planar facets. The planar facets or panels are angularly positioned to reduce scattered energy in the direction of the receiver. In particular, radar signals which strike the vehicle are primarily reflected at an angle away from the search radar or are returned to the receiver with large variations of amplitude over small vehicle attitude changes.

Description:
TECHNICAL FIELD 
     The invention relates to the field of airborne vehicles or vehicles in free space and in particular, to vehicles configured to have a minimal radar cross section. 
     BACKGROUND OF PRIOR ART 
     When vehicles operate over enemy territory they are often, if not continuously, subjected to illumination by electromagnetic radiation, such as radar, the enemy objectives being the detection, location and destruction of such vehicles at the earliest possible moment. 
     Stealth vehicles of the prior art, while often being treated with antireflective coatings in an effort to reduce their vulnerability to detection, have nevertheless remained relatively detectable. This detectability is an inherent characteristic of the vehicle shape and, since vehicle shape has usually been determined by design criteria other than stealth, large radar cross sections result. Thus due to improperly shaped vehicles, radar cross section reduction has been only marginally successful. The success of such vehicles penetrating enemy territory can be significantly enhanced if radar detection ranges can be shortened or eliminated by reducing radar cross section which in turn reduces the signal at the radar receiver. 
     Accordingly, it is a general object of this invention to provide a vehicle whose external surfaces are configured to make such vehicles substantially invisible to radar by reducing the signal received below receiver sensitivity levels and/or clutter. 
     It is another object of the present invention to provide a vehicle whose surface configuration is designed so that search radar directed to detect its presence is provided with a response signal which has wide amplitude variation relative to vehicle attitude with respect to the illuminating radar. 
     It is a further object of the present invention to provide a vehicle having a substantial absence of curved surfaces in order to satisfy these objectives. 
     SUMMARY OF THE INVENTION 
     The desired stealth capability (i.e., low radar cross section is imparted to the vehicle of the invention through the use of a basic polyhedron shape, the respective surfaces of the vehicle being planar facets. These facets are arranged so as to present the illuminating source with high angles of incidence, thus causing the primary reflected power to be in a direction of forward scatter, i.e., away from the source. Thus, with the possible exception of minor regions, few rounded external surfaces exist on the vehicle. Facets and edges are also sometimes constructed partially or totally from, or are treated with, antireflective materials and surface current density control materials. The flat, facet surfaces, concentrate scattered energy primarily into a forward scatter direction, minimizing side lobe direction magnitudes. Thus, the tracking radar receives either small undetectable signals or only intermittent signals which interrupt continuous location and tracking ability. The desirable characteristics may be provided while also maintaining reasonable and adequate aerodynamic efficiency in the case of an airborne vehicle. Particular attention is given to the sweep angles and break angles for this purpose, minimizing drag. 
     The novel features which are believed to be characteristic of this invention, both as to its organization and method of operation, such as reducing in a vehicle the power scattered per unit solid angle in the direction of an illuminating source receiver; scattering power primarily in directions other than toward the illuminator, enhancing scintillation with large amplitude variations; and shaping the vehicle such that its facets are arranged with high angles of incidence and appropriate edge boundaries to suppress scattered side lobes in the direction of the receiver. These features will be better understood from the following description in connection with the accompanying drawings in which a presently preferred embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for purposes of illustration and description only and are not intended as a definition of the limits of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top perspective view of a typical vehicle configured in accordance with the teachings of the present invention; 
     FIG. 2 is a bottom perspective view of the vehicle; 
     FIG. 3 is a rear view of the vehicle; 
     FIG. 4 is a sectional view of typical surfaces and surface junctions of the vehicle taken along line 4--4 of FIG. 1; 
     FIG. 5 is a top view of a second vehicle configured in accordance with the teachings of the present invention; and 
     FIG. 6 is a side view of the second vehicle. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIGS. 1 through 4 a typical vehicle configuration designed and constructured in accordance with the teachings of the present invention is illustrated. The vehicle, indicated by the numeral 10, is shown to be generally polyhedron in shape and of a substantially delta-shaped configuration and includes a fuselage 12 having a cockpit region 14 with an appropriate windshield 16. The fuselage 12 structurally supports a pair of wings 18 and 20 which extend generally outwardly therefrom, preferably with a slight dihedral, substantially as shown in FIG. 3. Extending generally upward and inward from regions of intersection between the fuselage 12 and the wings 18 and 20 are a pair of &#34;vertical&#34; stabilizers 22 and 24. The inward tilt of the stabilizers 22 and 24 is considered to minimize radar cross section, since this configuration tends to hide, or mask, other elements. Movably affixed to the respective trailing edges or the wings 18 and 20 are elevens 26 and 28 for vehicle control. Similarly attached for movement to the trailing regions of the vertical stabilizers 22 and 24 are a pair of rudders 30 and 32 for vehicle control. On the sides of the fuselage 12 are a pair of air inlet cowlings 34 and 36 for the aircraft propulsion system (not shown), having in their respective inlets 38 and 40 a pair of inlet grids 42 and 44. The nose 46 of the vehicle 10 is preferably pointed to the maximum practical extent, generally as illustrated. It will also be noted that the leading edges 48 and 50, in the preferred embodiment, are common to both the fuselage 12 and the respective wings 18 and 20. The edges 48 and 50 are usually made as sharp as can be accommodated structurally, as are each of the other external edges on the vehicle 10. 
     As previously mentioned, a primary feature of the invention is that the complete outward facing surface area of the vehicle 10, and each of its identified components, is characterized as being faceted. For example, as seen in FIG. 1, the upper portion of the nose section 46 comprises three flat surfaces, namely, side surfaces 52 and 54 and top surface 56. Similarly, the wing 18 includes a multiplicity of facets upon its upper surface, namely, a leading facet 58, an inner facet 60, a top facet 62 and an end facet 64. The wing 20 is constructed as a mirror image of wing 18, the facets not being identified. The rearward portion of the fuselage 12 includes side facets 66 and 68 and an upper rearward facet 70 connecting them. The windshield 16 is also constructed from a plurality of faceted segments which are not individually described. The cross section of the vertical stabilizers 22 and 24 is of generally diamond shape, as indicated at 72 in FIG. 1. The inlet cowlings 34 and 36 have side panels 74 and 76 angled inward and rearward, with the upper panels thereof coincident with upper rearward facet 70 which terminates at a point 78 at the rear of the vehicle 10. 
     The underside of the vehicle 10 is similarly constructed of a plurality of facets, the primary ones of which are the wing and fuselage facets 80 and 82. A bottom rearward facet 84 terminating at point 86 is connected to the facets 80 and 82, each being oriented at a discrete angle with respect to each of the others. The presence of a minimum number of large facets on the bottom surface of the vehicle 10 greatly enhances the low radar cross section of the vehicle 10. The exhaust port of the vehicle 10 is generally indicated by the numeral 88 and is shielded by facet 84 from radar and infrared detection by the extension of facet 84 beyond facet 70 and point 78. 
     Since the radar cross section normal to each edge is relatively high, it is desirable that the vehicle be designed with as few such edges as possible. It is also desirable that those edges which are included be oriented, as are the above described surfaces, to place higher cross section values into sectors where minimum radar cross section is not required. 
     Although it is not considered possible to totally eliminate the radar cross section of a flyable vehicle, it is possible with the vehicle of this configuration to so reduce or disguise its detectability that the cross section vulnerability to detection is insignificant. 
     It will be recognized that the surfaces, as described, can be customized for the vehicle mission, depending upon such factors as the vehicle altitude and azimuth from known radar installations. This can be accomplished by designing the angles of the various surfaces to provide minimum reflectivity under the conditions extant, with the radar cross section being determined by a computer. The vehicle can be further designed in relation to the anticipated direction of the threat, as, for example, from the ground or from the air or from the direction of the nose or tail, and whether the radar signals are expected to be high frequency or low frequency. 
     The angles of the tail surfaces, with an inward tilt therebetween, enhance the ability of the vehicle to display a minimum radar cross section while retaining the ability to function with reasonable aerodynamic efficiency. 
     It is sometimes desirable in designing this vehicle to further decrease any reflection of a radar signal by applying to some or all of its surfaces and some edges, radar absorber, such as are currently used on state-of-the-art insurgency vehicles. As little of such material as possible should be utilized, however, since it is heavy and, therefore, detrimental to the flight performance of the vehicle. Reflective surfaces such as engines, stores and the like normally found on aircraft, are either enclosed within the fuselage of the vehicle or are otherwise contained interiorly of the facets. 
     Since it is desirable, for the reasons discussed above, that the vehicle incorporate air inlets of highly canted configuration, a particular operational difficulty is encountered, i.e., the ability to capture a significant amount of air in a sharply canted engine air inlet, such inlet configurations being represented by the inlets 38 and 40. Grids capable of providing a high percentage of air capture, i.e., directing the air into the inlets 38 and 40 rather than permitting it to bypass those inlets as would be normal in configuration of this character, are represented by inlet grids 42 and 44. 
     Such inlet grids 42 and 44 also possess the desirable feature of having a low radar cross section. 
     A second embodiment of a low radar cross section, faceted vehicle is illustrated in FIGS. 5 and 6 and indicates the breadth and flexibility of designs which may be evolved utilizing the teachings of the present invention. In this embodiment, the vehicle 102 is provided with a single vertical stabilizer 104, much in the nature of a standard aircraft stabilizer. The vehicle 102 includes a multiplicity of facets, none of which are individually identified, but each of which is designed in accordance with the principles set forth with respect to the above-described vehicle 10. It will also be recognized that this vehicle may either be manned as a piloted vehicle, or that the cockpit region 26 identified with respect to vehicle 10 may be eliminated or that the vertical stabilizer 104 may be eliminated and replaced by a thrust vector control system such as used in missiles and spacecrafts. In such event, the vehicle is provided with appropriate radio controls or such other system as may be necessary to achieve its guidance. 
     Having thus described the invention, it is obvious that numerous modifications and departures may be made by those skilled in the art; thus, the invention is to be construed as being limited only by the spirit and scope of the appended claims. 
     INDUSTRIAL APPLICATION 
     The vehicle is useful in tactical endeavors where it is desired to keep detectability at a minimum.