Stealth Geometry

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, while often being treated with anti-reflective coatings in an effort to reduce their vulnerability to detection, have nevertheless remained 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.

Initially, the desired stealth capability (i.e., low radar cross section) was imparted to the early F-117 stealth aircraft through the use of a basic polyhedron shape, the respective surfaces of the vehicle being planar facets. A computer program known as "Echo 1," allowed designers to predict the radar return. Significant reduction of aircraft RCS has traditionally depended upon external shaping and to a lesserdegree, material composition. Denys Overholser, responsible for leading the the RCS prediction program Echo 1 development team at Lockheed Skunk Works durng Have Blue lists the four most critical factorsin RCS reduction as being "shape, shape, shape and materials." Echo 1 was limited to calculations in only two dimensions; this led designers to a faceted design rather than a smooth, seam-less one. 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.

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 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.

Facets and edges are also sometimes constructed partially or totally from, or are treated with, anti-reflective 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 surfaces 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.

With the success of the F-117, the Air Force invested in other stealth systems, most notably the B-2 and the F-22. As stealth technology advanced the balance between signature and aerodynamics improved, so the novel facetting - the exclusive use of flat surfaces - of the F-117 was no longer necessary. Both the B-2 and the F-22 exhibit more refined aerodynamic shapes, even though minimum radar signature remains the primary design requirement.

Conventional rotorcraft turbine engine air inlet systems are usually designed, within the constraints of the overall aircraft mission requirements, for maximum ram recovery to maximize engine power available and for minimum surface area to minimize aircraft penalties for power for inlet anti-icing. These design goals typically result in short, forward facing, inlets with minimum turning.

The advent of low radar observability requirements has introduced additional requirements for inlet designs which are, to a degree, inharmonious with the desire for high performance and low anti-ice power. Direct line-of-sight to the engine front face must be avoided because turbine engine compressor vanes are highly visible to radar scanning devices. Avoiding this typically requires longer ducts with more turns which, consequently, decreases inlet pressure (head) thereby decreasing engine power available and increases inlet surface area thereby increasing anti-ice power requirements. In addition, newer structural materials used for lowering radar observability have increased heat transfer resistance relative to conventional materials which results in an increase in the anti-ice power required per unit area of duct surface to maintain the surface ice-free.

Thus, radar observability requirements have two adverse impacts on inlet anti-icing power requirements; the additional turning for line-of-sight requirements increases duct surface area; and new duct materials increase anti-ice power per unit surface area. The combined effect results in significant increases in the energy, either electrical or bleed air, that must be provided for inlet anti-icing. This increased energy requirement leads in turn to either an increased size, weight, and power penalty for larger aircraft electrical generators or to higher levels of engine bleed air energy extraction which results in direct reduction of engine power available. Either case results in a significant impact on aircraft performance due to the reduction in power available to the rotor.

In gas turbine powered aircraft, especially military aircraft, there are several advantages in placing the gas turbine engine within the aft portion of the aircraft fuselage. For example, there is less drag as compared with aircraft having wing suspended engine nacelles. In twin engine aircraft, there is less asymmetric yaw produced when one engine fails, as the engines are closer to the center axis of the aircraft. It is also advantageous in stealth aircraft to reduce the radar signature produced by the engine profile. Thus, there are many advantageous in having the engine completely integrated within the aircraft fuselage.

There are, however, disadvantages associated with internal engine placement. Foremost, it is necessary to route air through the fuselage from the front of the aircraft to the engine inlet in order to provide oxygen required for combustion. Problems also arise with overheating of the engine bay since the engine is encapsulated within the fuselage where heat is not readily dissipated. Engine components and other aircraft components located inside the engine bay can overheat and become damaged if the engine bay reaches too high of a temperature. Also, since the engine is in more intimate contact with the fuselage, vibration throughout the aircraft is more pronounced.

The majority of military aerial vehicles, such as combat aircraft, attack helicopters and the like, are typically equipped with an external airborne stores suspension, delivery and release system. External airborne stores are devices intended for external carriage, mounted on aircraft suspension and release equipment and may or may not be intended to be separated in flight from the aerial vehicle. External airborne stores typically include missiles, rockets, bombs, nuclear weapons, mines, torpedoes, detachable fuel and spray tanks, chaff and flare dispensers, refueling pods, gun pods, electronic countermeasure (ECM) pods, electronic support measure (ESM) pods, towable target and decoy pods, thrust augmentation pods and suspension equipment, such as racks, eject launchers, drop launchers and pylons. The external stores are detachably installed on the aerial vehicle via specific suspension points, typically referred to as hard points or weapon stations, which are distributed across the external surface of the aerial vehicle in such a manner as to provide for the best possible performance of the stores carried and for the aerodynamically most advantageous flight conditions.

In the current state of stealth technology a stealth aircraft carries stores internally to optimize the radar cross-section and the infrared profile of the platform as much as possible. It is highly probable that in the near future the technology will advance such that external payloads such as externally mounted stores could be substantially stealth-configured (using specialized structure geometry, radar absorbing construction materials and coatings). When such a technology becomes practical then stealth aircraft would be able to carry external stealth-configured stores without compromising the stealth characteristics of the platform.

Mitigation or attenuation of the radar cross-section of modern stealthy aircraft is a focus of concern and considerable research and development. Nearly all aspects of the aircraft which contribute to the overall aircraft radar signature is of interest. In this respect, there have been several prior art attempts to mitigate reflected radar emissions or signals associated with the multitude of panel fasteners disposed about the aircraft surface.

Large numbers of removable panels are typically disposed about the exterior surface of modern stealthy aircraft, such as access doors and engine bay covers and the like. The panels are typically formed of a metal alloy or composite and conform to the surface contour of the adjacent aircraft skin. A multitude of panel fasteners are required to be used for removably mounting the contoured panels. Such panel fasteners typically comprise a threaded shank and a fastener head. A driving slot or recess is formed in the surface of the fastener head. A mating driving tool engages the recess for fastener installation and removal. The fastener head further includes a conical countersink portion. When installed, the countersink portion of the fastener head is seated within a counterbore formed in the panel, in a position recessed relative to a local external surface plane of the panel. Such configuration allows the surface of the fastener head to be flushed-mounted with the surrounding portion of the panel.

Aircraft surface contour irregularities tend to increase radar reflection characteristics. Such discontinuities can occur at the circular interface or surface gap between the panel and the fastener head. As mentioned above, the fasteners may be initially flush-mounted with the surface of the associated panel via a countersink and counterbore arrangement. Such flush-mounting can be disrupted by a common practice of applying a radar absorptive material (RAM) to the panels. Thus, when the fastener is installed, a depression is formed in the surface contour which is above the fastener head the depth of the RAM material applied to the associated panel.

In addition to surface contour irregularities, variations between interfacing materials tend to increase the radar reflectivity. In this respect, when aircraft panels are exposed to electromagnetic energy used for radar detection, localized currents are distributed throughout the panels, which are typically metallic or highly conductive composites. Where these localized currents encounter a significant change in electromagnetic material properties, radar signals may be reflected therefrom. Such a significant change in electromagnetic material properties can arise where there is a void or lack of material, as in the case with the above-described depression above a flush-mounted fastener installed in a RAM coated panel.