On the B-2 and F-117, anti-contrail systems inject chemicals into the exhaust stream to break water into droplets too small to be seen.).

An even more experimental active-camouflage system uses thin sheets of light-emitting polymer that glow and change color when charged. Different voltages cause the sheets to glow blue, gray white, or whatever shade is needed to match the sky. As an added advantage, the thin sheets are easy to apply to existing aircraft. "electrochromic" polymer has been developed at the University of Florida, and the Air Force is studying it as a way of

at which an air defense radar can track the aircraft by as much as 50 percent.

Such systems do not have to be perfect. The goal is not to build an invisible airplane, but to delay the visual detection of an aircraft for as long as possible. In fact, the most effective way of fooling either the eye or a missile may be to present it with an image that is difficult to interpret.

Using fast-changing electrochromic panels, the military is experimenting with "flickering skins" that could prevent missiles from locking onto their targets. In demonstrations at

sensors in several ways. The exhaust nozzles were flattened into slits, because a flat nozzle has a longer perimeter than a round plume, and the exhaust mixes more quickly with the cool air. Designers also developed paints containing compounds such as zinc sulfide, to suppress reflections from the airplane's skin. Paint cannot eliminate the heat generated by skin friction, but special coatings can change the "emissivity" of the surface; that is, the efficiency with which it transforms heat into infrared radiation. Low-infrared paints and coatings are now widely used on many aircraft. After years of research focused on the suppression of infrared and radar signatures, aircraft designers now appear to be giving more attention

applying a variable tint to the cockpit canopy of a fighter aircraft. In theory, such a coating could also be used over a white-painted skin to vary its color.

But what about concealing an airraft from an enemy flying above it! Defense contractors have told POPULAR SCIENCE that an even more exotic skin is being tested on two stealth aircraft at the high-security Groom Lake air base in Nevada. The skin is derived from an electromagnetically conductive polyaniline-based radar-absorbent composite material. It is optically transparent except when electrically charged, much like the LCDs used in laptop computers.

What makes this new material attractive is that it can change brightness and color instantaneously. Photo-sensitive receptors, mounted on all sides of the plane, read the ambient light and color of the sky and ground. An onboard computer adjusts thebrightness, hue, and texture of the skin to match the sky above the plane or the terrain below it.

The system is also claimed to make the aircraft even stealthier. The electrically charged skin dissipates radar waves, reducing the range

Groom Lake, engineers have turned the entire skin of an aircraft into a missile jammer.

A flickering skin could help aircraft hide from a new generation of missiles that use visual and infrared sensors to build an image of a target. Older heat-seeking missiles could be lured away from aircraft by decoys; hot flares ejected during flight. But the newer missiles use visual sensors to "see" the edges of an aircraft and distinguish its shape from that of a decoy. A shimmering skin confuses the missile's sensors by displacing or distorting the aircraft's image.

Engineers have also taken steps to reduce the heat signatures of military aircraft. In the1970s, infrared sensors had a much greater range than visual imaging systems. Infrared accordingly became the stealth designers' second priority, after radar.

Infrared sensors detect hot spots, such as engine exhaust or the wing's leading edges, which are heated by air friction. At closer ranges, sensors detect solar radiation glinting off curved surfaces or scattering from the skin more attention to visual stealth. Designers countered infrared

to visual stealth. Some basic physical problems still need to be solved. For example, even a very efficient lighting system requires a lot of energy to match the brightness of the sky, equivalent to several times the power absorbed by the fighter's radar.

Experts in the field of electrochromic materials caution that major technical hurdles have not vet been cleared in the unclassified world, and not for lack of interest: The building industry would love to see a practical, large area electrochromic film, because it could greatly reduce the energy needed to heat and cool buildings.

Electrochromic materials must not only be able to change color, but also to withstand sunlight and extreme weather, and continue operating through many switching cycles. The problems are compounded for a stealth aircraft, because the material must also be compatible with existing radar and infrared technologies.

This may well be the reason why, for now, visual stealth measures are confined to a few experimental aircraft and may stay that way for some time.