Lockheed Martin

Introduction

Stealth or “Low-Observable” (LO) aircraft technology is not new. Efforts to create a stealth aircraft have been made since World War II. It was only a few years ago though, that a stealth bomber took part in full-scale operations and gave an important tactical advantage to their pilots. For an aircraft to be stealth it has to be stealthy in six disciplines: “Radar, Infrared, Acoustic, Visual, Smoke and Contrails”. The most important of these disciplines is radar because it is by far the most powerful means of detecting and tracking a target. Creating a stealth supersonic aircraft, as F-22 Raptor has shown, proved to be a difficult task and involved new innovating stealth technologies. These technologies, which are also integrated in the new F-35 Joint Strike Fighter, will be briefly discussed in the following report.

The principle of Radar Cross Section (RCS) reduction

When radars were first used, radar engineers thought that the range an aircraft could detect a target was only related to the target’s physical size. Soon they realized that this was wrong. They found out that the range was related to the energy reflected from a target. So they came up with the concept of the Radar Cross Section. RCS compares the energy reflected from a target with the energy reflected from a sphere, with cross-sectional area of one square meter.

A flat surface has an extremely large RCS if it is normal to the radar beam. As the plate is tilted or canted away from the beam in one dimension, its RCS decreases sharply: reflectivity is reduced by a factor of 1,000 at a cant angle of 30 degrees. But if the surface is both canted and swept back-the RCS reduction can be realized at an 8 degrees angle. This is how they did manage to built an “invisible” aircraft: by constructing it entirely out of flat surfaces, canted and rotated away from the arrival angle of any radar beam.

Shape

Denys Overholser, when asked what makes an aircraft stealth, said: “Shape, shape, shape and materials”. Overholser, although an electric engineer, is the man who had the idea of the “flat plane” as a stealth attribute. It is well known that a stealth aircraft also incorporates other kind of technologies that augment aircraft’s “invisibility”, like Radar Absorbent Materials (RAM). But an aircraft with a “wrong design” is impossible to become stealth, and that is why old designs cannot “upgrade” their stealth capabilities. The LO are the dominant attributes of an aircraft design, that affect its configuration, external lines and internal arrangements.

The most important stealth-driven feature of the F-35 is that it caries a 5,000-pound internal weapon load. The fuel is also carried in tanks inside the fuselage and the wings, and this is why the JSF is not a small aircraft. It had to be scaled up to make room for both. The importance of being stealth in all phases of a mission (inbound-outbound) forbids carrying disposable external fuel tanks, something usually done with other fighters.

Another LO characteristic of the JSF is that it has rounded edges where its flat canted surfaces meet. This can be easily seen in figures 2-4. This way the radar waves that hit against these edges are deflected away from their source instead of reflected back to it. Another thing that can also be seen in figures 2-4 is that all edges are aligned (see red lines). For example the trailing edges of the wing are aligned with trailing edges of the horizontal stabilizer and with upper and lower inlet lips on the opposite side of the airplane. The wing and tail leading edges are aligned with each other. The idea is that it is better to add energy to the spikes in the radar signature at right angles to each of the sharp edges than to add more spikes, even small ones, which increase the probability of detection.

Related with this technique is the way the JSF’s apertures are treated in order to avoid electromagnetic discontinuity and unacceptable RCS spikes. All apertures, including landing gears, maintenance panels, weapon bay doors, antennas, and the cockpit opening, are designed to follow the primary alignments. Where this was structurally difficult, the edges where serrated so that the aperture is basically square.

An also very important stealth feature is the JSF’s inlet design. Compressor’s face, with its rotating blades, is a major RCS contributor. Modern radars not only can detect a plane because of the blades but also identify the target by using a technique called Jet Engine Modulation (JEM). As you can see on Figure 1, engine position and inlet design totally hide the engine from radar view. JSF’s inlet features a shallow smooth bump on the body side, positioned between upper and lower inlet lips that slant forward, away form the body (Figure 3). This creates a pressure rise, and the result is that the boundary layer is gently diverted to either side.

Engine’s exhaust is a pain from the LO’s point of view and one of the most difficult tasks. Pratt and Whitney developed a Low-Observable Axisymmetrical Nozzle (LOAN), which is incorporated in the JSF USAF and Army versions. The system was thoroughly tested on an F-16. It derived from the Raptor’s exhaust as a simplified version without thrust vectoring. The exhaust system is also “hidden” from many angles of view by the vertical and horizontal stabilizers.

The cockpit of a conventional aircraft is one of the largest sources of radar signature. Pilot’s helmet, displays and the seats contribute to this. The best way to “hide” them from the radar is to block radar’s energy. JSF’s canopy is made of polycarbonate material. It has a metallic coating and is smoothly blended into the aircraft shape to minimize reflections.

Materials

On the F-35 several special materials are used, including Radar Absorbing Materials (RAM), Radar Absorbing Structure and Infrared (IR) Topcoat. Unlike the F-117, which was totally coated with 2,000 pounds of RAM, these materials are more selectively used on the JSF. Lockheed Martin developed paint-type RAM which is applied around the edges of doors and control surfaces. RAS is used on the body, wing and tail edges. For the application of this paint robots will be used, like the CASPER (Computer Aided Spray Paint Expelling Robot) system used for F-22 and the Have Glass II program used for painting 1,700 F-16s with RAM. Robots are essential because they can reach confined areas, as the inlet ducts, and can work without stepping on the aircraft.

These materials comprise ferromagnetic particles, embedded in a high-dielectric-constant polymer base. The dielectric material slows down the wave and the ferromagnetic particles absorb the energy. These coatings are also designed in a way that the small reflection from the front face of the absorber is cancelled by a residual reflection from the structure beneath it. This is not an easy procedure, and it makes RAM design much more tricky than most people believe.

JSF’s entire airframe is also painted with a camouflage topcoat that suppresses IR.

Radome-Radar

JSF’s uses a band-pass resonant radome and is one of its most complex structural components. The Northrop Grumman APG-81 has what is called Active Electronically Scanned Array (AESA). It comprises a fixed planar structure carrying more than 1,000 “transmit-receive modules”, each of them one tiny solid-state radar. Apart from its state of the art detection capabilities, it also contributes to the RCS reduction since it is tilted slightly upwards, deflecting away any possible reflection to any possible receiver.

An aircraft’s identity can easily be betrayed by its radar’s emissions. AESA though, uses many low-probability of intercept (LPI) techniques. For example, it is possible to reduce peak power adaptively, as the target gets closer. The radar power is decreased rapidly to a point where an intercept receiver cannot detect it. AESA can search simultaneously with multiple beams, many small sectors, reducing probability of detection. It also has the capability to confuse interception systems by varying almost every characteristic of its signal between two consecutive pulses.

Conclusion

While maneuverable enough, the F-35 JSF is not designed to be a champion as a dogfighter. Wing loading, thrust-to-weight ratio, armament layout, and cockpit design point to this fact. JSF is a state-of-the-art stealth aircraft. It incorporates every known LO technology. The most probable way to detect it is by its weapons’ noise of impact. It is designed to be the most lethally invisible aircraft ever made.

Lockheed Martin

F-35 JOINT STRIKE FIGHTER

A “Low Observable” Approach

In Partial Fulfillment

Of ASCI-603 Aircraft and Spacecraft Development

by Major Vassilios A. Evangelidis

Hellenic Army Aviation

Introduction

Shape

Materials