You Are At: AllSands Home > Science > Stealth technology
At a hangar in California in the late 80’s military and civilian personnel at the Skunkworks aircraft design site were watching a radarscope prior to a flight demonstration of a new aircraft that sat out on the tarmac. A bird flew down and rested on the plane. Inside the hangar a radar sweep was made of the plane. Looking at the scope, the men were amazed. The bird seemed to float in mid-air even though they new it was not flying. The radar signature showed that the bird was there, but the plane was not. This was a demonstration of stealth technology.

Project Harvey was the original name for a secret military endeavor, which was designed to investigate new technological possibilities to hide aircraft from enemy radar. In 1975 a US research agency began to study stealth technology. It was named after the James Stewart movie about an invisible six-foot tall rabbit. As its name suggests, stealth technology is aimed at flying without being seen or even heard.

Stealth technology was dramatically introduced to the public during the Gulf War. In all the combat missions that the fighters flew, not one was hit or damaged. So what is stealth? What problems must stealth technology overcome in order to be successful?

We can start by addressing some of the issues that stealth must be overcome. It must be hard to detect on radar, the hot emissions from the engines must be minimal, it must be quiet, its engines should not make contrails high in the cold atmosphere or produce lots of brown exhaust smoke, and finally, it should be hard to see with the human eye. These are not small problems.

The key features of Stealth

Unusual Design
Outer Paint
Reduce Heat Exhaust Signatures
Eliminate High Altitude Contrails
Eliminate Brown Exhaust

1. The design of the plane has to be different and unusual. The driving idea in this design is to cut down the size of the aircraft's radar image, called its "radar cross section (RCS).” This normally involves using radical design features and some nonmetallic materials. In the case of the Northrop B-2 bomber, all of the surfaces are curves with constantly changing radii. These scatter radar beams in all directions instead of back to the radar source. There are no right angles on the exterior of the design. All angles are acute or obtuse. To present the smallest possible number of strong reflections to radar, the back edge of the wing is jagged. And the unusual diamond shape is mathematically convenient for deflecting radar echoes away from the source.

In constructing stealth aircraft, even if the total external shape is optimized to provide excellent "stealth" values, it can be compromised if attention is not paid to small details such as surface gaps, compartment doors, engine and systems inlets, windows, and antennas. The most perfect "stealth" design would also be quickly compromised if anything such as fuel tanks, rocket launchers, or air-to-air missiles were mounted externally. For this reason, all fuel and weapons must be carried internally in special bays and all doors and access panels must seal very tightly. Weapons bay doors, which open in flight to release weapons, must operate rapidly and be designed to reduce the amount of radar reflection. When looking at a "stealth" aircraft closely, the joints between the ailerons and flaps are tapered to scatter radar energy. The irregular interiors of cockpits are also refined to reduce radar reflections. Even the windshields, which usually consist of multiple acrylic plastic layers with gold or indium oxide coatings in between, are not overlooked. The coatings reduce radar returns from the cockpit without significantly affecting the optical properties of the clear acrylic.

Engine inlets, which channel air into the engine compartments for combustion, are given special consideration in "stealth" aircraft. Because jet engine turbine blades can reflect radar energy, the inlet is designed to reduce the chances of this happening. Duct components often incorporate radar-absorbing materials and an S-shaped duct, for instance, can remove the engine from direct line-of-sight, thus reducing the amount of radar energy reflected back.

2. A second way of stopping radar reflections is by coating the plane with material that soaks up radar energy. These typically consist of carbon or a magnetic ferrite-based substance. The result is that the B-2 is reported to have the same RCS as a child's tricycle! Radar-absorbing paints tend to consist of liquid chemical bases mixed with carbon or iron oxide powders. Still other coatings come in the form of sheets impregnated with iron oxides. These sheets, which are used on the Lockheed F-117 and Northrop B-2, can be cut or shaped to fit the outside surface panels of the host aircraft.

The color of an aircraft obviously affects its visibility and the range at which it can be spotted. White or brightly colored aircraft tend to be more easily discerned at great distances, as do unpainted, polished, all-aluminum aircraft. Flat black paint is very effective in reducing visibility under a variety of conditions. Camouflage patterns can also reduce an aircraft's visibility when the pattern reflects the environment in which the aircraft is operating.

3. To reduce the hot engine emissions, or infrared signature, the engine exhaust is designed to mix efficiently with the surrounding cold air. The same solution also helps quiet the engines. Reducing smoke in the exhaust is accomplished by improving the efficiency of the combustion chambers. In the area of heat emissions, generally in the infrared part of the electromagnetic spectrum, the primary source is the engine exhaust. Considerable research in this area has produced new generation of jet engine exhaust nozzles and heat-dissipating systems and techniques. Among them is mixing low-temperature ambient air with the exhaust gases almost at the instant of exit at the back of the engine. This causes a rapid decrease in exhaust gas temperature and a reduction in the size of the exhaust plume. This in turn reduces the aircraft's infrared or heat signature, reducing its chances of being tracked by such devices.

4. Getting rid of contrails - that distinct white line in the sky caused by highflying jets - is a harder task. Originally, tests were made with a special acid that broke up water molecules in the exhaust, making the trail invisible, but the acid was corrosive and hard to handle. Chemical additives have been developed that reduce or eliminate vapor trail production by converting water condensation into ice crystals. Engine exhaust by-products have been almost eliminated in most modern engines through improvements in combustion efficiency and fuel chemistries.

Disadvantages of Stealth

Increased Weight
Inefficient lift
Aerodynamically unstable

1. Because the advantages of "stealth" technology outweigh the disadvantages, the latter are considered necessary evils when a "stealth" aircraft is built. Increased empty weights result from the addition of external radar-absorbent coatings, from engines that are buried inside the structure to reduce the amount of heat they generate, and from exotic and sophisticated engine exhaust nozzles that also help reduce heat generation. Finally, a full complement of heavy and space-consuming electronic countermeasures equipment, which is part of the aircraft's defenses, adds to its weight. All of these add weight that otherwise would not be an encumbrance; and weight equates to penalties in speed, range, and altitude performance.

2. Another problem with incorporating "stealth" technology into an aircraft is a wing shape that does not provide the optimum amount of lift. The resulting increase in drag reduces flight performance. "Stealth" shapes, such as the "faceting" found on Lockheed's F-117 "stealth" fighter, also tend to be aerodynamically destabilizing. This is brought under control only through the use of highly sophisticated computers that serve to electronically balance the aircraft in flight through its autopilot and control system.

3. Because range is reduced by many of "stealth's" physical constraints, "stealth" aircraft sometimes need to be bigger than designers would like in order to carry sufficient fuel. "Stealth" aircraft cannot be equipped with external fuel tanks because doing so would increase their radar reflectivity. All external protrusions, such as the edges of the landing gear doors, must be perfectly matched to prevent radar energy from being reflected. Because of this, the detail work entailed in the design and construction of a "stealth" aircraft is much more exacting than in conventional aircraft. Even the smallest oversight in panel matching can make a "stealth" aircraft vulnerable to radar. Such exacting requirements increase engineering, manufacturing, and maintenance costs of the "stealth" aircraft so that they are several times as expensive as conventional aircraft.

All of these modifications, however, hurt the plane's performance, adding weight, affecting aerodynamics, and altering the structure of the aircraft. The advantages of stealth technology must always be weighed against its disadvantages.