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At a certain level, details on what comprises the major details of stealth technology are highly classified. So don't look for any scientific treatise on the subject anytime soon. However, a lot of material can be found in what are typically considered to be open sources - publically available. And of course examination of available photographs and personal inspection (while from afar) at an Air Show, can certainly help fill in some of the information thirsted for by fans of military aircraft. Also, a history of stealth in general is quite eye opening.
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| F-22 Raptor (ATF, Senior Sky) |
B-2 Spirit (Tacit Blue) |
F-117 Nighthawk (Have Blue, Senior Trend) |
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| B-1B Lancer (ATB) |
U-2 Dragon Lady (Oarfish) |
SR-71 Blackbird (Oxcart) |
Different written sources attribute exceedingly high degrees of stealth being attributd to masking of infrared signatures as well as radar signatures. Popular Science 1 (how open source can you get!) reports also the use of exotic chemicals to be inserted into the engine outlet gases to modify infrared signature as well as force water molecules in the exhaust plume to break up into much finer particles, thus reduce or even eliminate telltale contrails (white condensation seen high in the sky behind aircraft). According to The World's Great Stealth and Reconaissance Aircraft 2, the chemical used for this feat is chloro- fluoro-sulphonic acid, whatever that is. It does sound quite toxic if not corrosive, so one would imagine its use is prohibited except in combat or sensitive (covert?) recon missions, and probably rarely in either circumstance. Storage must be a wonderful thing to consider as well. And how about longevity of the metal surfaces this corrosive chemical comes into contact with?
The most impressive feature of stealth of course is the low
observability
(LO) on radar and infrared. We've discussed the engine situation,
however
the airframe itself draws immediate attention. Stealth aircraft use
deceptive
shapes to mask their signature. In the case of the B-2,
the smooth blending
of the main fuselage into the wing area, obviously produces a flying
wing shape when seen from above and below. However the rather apparent
stair-step
shape when seen from these positions also shows that edges of the wings
produce opposing reflections. One can only assume this aids in
cancelations
of reflected radar energies. The paint as well as the carbon composite
structures that make up the airframe surely reduce (or absorb) the
radar energies as well. From the front or rear, the B-2 shape
almost disappears visually. In fact, ironically, it appears to
look like a flying saucer. Hmm.
The F-117 airframe, being an early stealth design appears to have taken a brute force method -- it has a large number of faceted surfaces, not unlike a crystal. The facets are presumed to reflect radar energy away from the aircraft in any other direction than that of the radar emitter (i.e. the radar site or radar transmitter in an aircraft or air defense system). Regardless of the early design, the F-117 appears to work just fine -- ask Iraqi air defense controllers -- both wars! Over Kosovo, the F-117 was just as invisible -- even with Russian made "high technology" defense systems.
The stair-step, or its modification the "W" shape, are also found at numerous places on all of the stealth aircraft. For instance, in the forefront of every cockpit glass, there is a very apparent "W" shape. Presumably this reduces the radar energy reflected during a head-on pass to the radar emitter. The "W" shape is also found on landing gear doors, engine inlets or outlets, as well as other openings (permanent ducts or transient openings such as flaps or landing gear doors).
The B-2's shape is vastly different looking than most other aircraft in the worldwide military, as is each of the other stealth aircraft. The shape is so smooth and elliptical looking from the front and rear, it tends to look like a ovoid, with little overall physical cross section. And while the F-117 is certainly different looking, one notices immediately that it is as deceptive head on as the B-2. This is especially interesting as it is fascinating. The Lockheed manufactured F-117 uses mostly a faceted surface design, panels that are always at odd angles to the next panel, while the Northrup designed B-2 uses smooth blending. In either case, not only are the designs electronically hard to find, they are also hard to spot with the naked eye until it is far too late (as in overhead after bomb release).
Looking backward in time to predecessors of stealth technology, the SR-71 and the U-2/TR-1 aircraft featured the radar aborbing paint and composite carbon structures, as well as deceptive fuselage blending (the SR-71 is as mystical looking from the front as the B-2 or F-117).
In some contrast, the F-117 and the F-22 ATF both have a low height
triangle
appearance from the front. This physical cross sectional view ensures a
small signature from the "get-go", and low observability touches such
as
paint and materials, as well as little "W" shapes where straight lines
might have appeared, all tend to break up
the signature by absorbtion or redirection.
Anti-Stealth
Sltealth has worked well, up to an including attacks in Yugolsavia
(Bosnian War) and the combat zones of Iraq in two wars and the
Northern/Southern watch fiascos.
Multi-Static Radar
However, the anti-radar capablities of the physical approach
of structures and shape might also mean a synchronized, large area
array of radar emitters and
receivers
may be able to dramatically degrade the stealth capability, by
observing the
reflected signals from locations better than perhaps 45 degrees from
the
emitter. This is called multi-static radar array systems, and is
rumored to be under development in a number of higher technology
companies. In his recent military fiction novels, Dale
Brown describes such a system in
use. While
it might be irritating to the DoD that a fiction writer may be
describing a
means to lower the stealth capability, it should be obvious that a
scientific
analysis of even a Testor model of the F-117 would yield much in ways
to
counter the F-117 stealth technology. All admit that it is only a
matter of
time
before there is a much better detection against the F-117. But it has
also
been pointed out that the majority of radar systems over the next
twenty years
will still be of a type easily fooled by the Stealth Fighters design,
and that
most likely targets of stealth tactical penetration don't have the
encompassing
geographical area to effectively detect without simply being "flown
around". Mount radar emitters and receivers on every
cell phone tower, power
them by the cell phone company's power supply, and tell them you'll
take the tower under emminent domain if they squawk. Then using
some
high powered computers, analyze the signals. Use high power
computers to analyze what all the receivers see, and you should get
some interesting info on aircraft in the "corridors" created by the
line of sight intersections of the towers. Compare that data to
that of air-traffic-control radar and the stealth should stand
out. Oh, by the way, cell towers are above and down looking into
valleys between ridges, so you can find yourself solving the "flying
under the radar" problem for other attack aircraft.
Occlusion
Another approach, which on its face appears too simple to work, just
might! The idea is one of occlusion. Any astronomer can
tell you that in space, things that don't shine with bright light can
easily be found when they occlude another object which IS bright.
Of course you have to know where to look, "and therein lies the rub" as
our friend Shakespeare would have said. The problem is first
knowing where to look for a stealthy aircraft. However, if you
do, then having "before" picture of the background the craft is passing
in front of, will find it's radar absorbing/reflecting capabilities
creating an occlusion as the craft passes. In fact, the occlusion
will be moving at a nice clip as the craft's "shadow" will move across
the background such that a computer tracking system should be able to
note the movement quite easily. Assuming of course, you have a)
mapped the background, and b) have a system designed to look for
occlusions instead of reflections. This is not wholly
impossible. In fact, a pair of bright folks have published an
interesting paper on the subject at:
Phase Shifted/Multi-Path Non-Rejection/Passive Coherent Location (PCL) Systems
Yet another approach is less complex in terms of technology, but is
perhaps a brute force method which also requires some high power
computing. There are actually two similar ideas in this
arena. The first is to use radar mounted on cell phone towers,
the ubiquitous string of communications towers that are popping up
everywhere, even in what we might think of as underdeveloped
countries. Like China for instance. More on that
later. The idea being put forth is simple in concept, but
expensive and complex in implementation. Mount special receivers
on every cell phone tower, power them by the cell phone company's power
supply, and tell them you'll take the tower under emminent domain if
they squawk. Then using some high powered computers, analyze the
signals. What you are looking for is the subtle phase shift
created when an aircraft gets between you and a common radio or
television broadcast. In the early years of television we
saw this phenomonen -- ghosting that was simliiar to that seen in
multi-path reception -- same signal reflected off an aircraft (or a
mountain, neighbors house, etc.) which arrives late or out of phase
with the more direct signal.
There is a report that perhaps the Chinese are working on deploying
such a system, and perhaps upsetting the military analysts and planners
in Washington, D.C. 6
Popular Science 4 (such an odd place to find info on military wouldn't you think...however we are continually delighted to find well written and authoritive articles...usually shown NOT to be disinformation) recently (May 1997) printed an article on future stealth requirements in the visual and infrared spectrums.
The issue is daylight operation. The F-22 and the Advanced Strike Fighter (now the F-35) in development both are necessarily tasked with daylight operation. Therefore some methods for eliminating or at least reducing their visible light and infrared signatures are necessary. Cloaking Shields you ask? Nearly. The idea comes in several flavors.
The first is reflected infrared surfaces. The sun beating down on a wing surface can produce a rather long infrared event for an incoming heat seeking missile. With today's computer controls, this means a rather long period where the missile's intelligence can lock onto a portion of the aircraft, calculate an intercept vector, and refine its trajectory for impact. Given a number of these events, an aircraft with zero engine infrared is still doomed to intercept.
Enter visual/infrared masking. Using materials that can be
electronically
charged, their reflectivity to the visual spectrum can be
modified...kind of
like the LCD screen on a laptop computer or display on a watch. The
idea
then is to figure out where the sun is, and modify the reflectivity of
the
surface according to some well known rules about observability.
Of course the old "standard" radar and infrared masking paint and
materials works pretty good already.
In many cases, low to medium altitude can also be fixed by using, I know it sounds weird, bright lights. The idea here is to use lamps to lighten the undersurface of the aircraft so that it offsets the shadowy effect of a darkened underneath. How much light? I would imagine that is probably pretty well classified, even if it is technology over 50 years old.
Has Scotty come back in time to teach us how to do all this? Hardly. In fact, according to Popular Science (I know, I know), some of these ideas are owed to research performed during World War II. According to Sweetman and Douglass, a 1943 project called Yehudi was designated to better able Navy maritime patrol aircraft to sneak up on U-boats. The idea was that if the area couldn't prevent its being detected than at least the technology could reduce the overall warning time, an important factor that left the U-boat on the surface longer, thus more susceptible to surface attack by the Navy planes. The idea was revived in the mid 1960s during the Vietnam war, when similiar lights were used on the underside of the F-4 phantoms (Compass Ghost). According to the Popular Science 1 article this (and a color change...see below) reduced visible detection significantly.
The other flavor of visual masking is simply by color. Ever wonder why the Navy paints all their aircraft Gray? So they match their pretty gray ships? Nope. An aircraft at low to medium altitudes tends to be a black dot against the background of the sky. Ever been to an air show and had trouble following the path of a fighter as it does a vertical swoop to 40,000 feet. Chances are it was an aircraft painted a medium gray. The gray, when combined with light scattering at low to medium altitudes ensures about as low observability as can be possible without some other very high tech trickery. As mentioned earlier, an F-4 Phantom, Compass Ghost, was painted a light blue on top and bright white on the bottom. When combined with bright lamps this aircraft was seen to reduce visible detection by 30 percent.
Okay, so we've gone from fancy materials to color and lights to offset the visibilty of the underneath of the aircraft. What about from above.
Speculators Steve Douglass and Bill Sweetman (Popular Science 1) have come up with another idea...this one is closer to the cloaking devices of Star Trek. Imagine having a normally white or light gray surface. Now electrically, change the color of that surface so that it matches that below it. Looking from above, the surface appears to match the grays, browns and soft whites of the terrain below. Fly over forest, and the surface takes on a green like hue. A cloudy day, add clouds to match what sensors see underneath and the aircraft becomes a chameleon and dissappears. Science Fiction? Perhaps. Take a look at your color screen on your laptop, and it may not seem so far feteched all of a sudden. The Popular Science 1 article talks about this as well, using special materials in the aircraft skin that are adjusted by computers...picking the best reflectivity to match the altitude and overhead lighting situation. This is not a new idea, in fact several military fiction writers have already come up with the idea, in one particular instance having the aircraft continually modifying top and bottom like a magician's mirror box making the aircraft totally invisible - Dale Brown again.
Back to the underneath. How about adjusting the color of the gray to light blue of the undercarriage of the aircraft according to the level of sunlight beating down on the top surface? In this way, the aircraft's undercarriage provides the perfect blend for its particular altitude (Popular Science 1 postulizes that this could even result in altitude adjustments for the auto- pilot to better match limited color changes with overhead lighting).
If this all sounds far-fetched, imagine what a B-2 would have done
to the minds
of the pilots in Korea or Vietnam. Think about the poor Russians when
the SR-71 went streaking across their heavens. They must have thought
they were
seeing UFOs. For that matter, if Saddam Hussein had never seen or heard
of
a Stealth Fighter, imagine his feelings when his subordinates began
reporting
invisible aircraft dropping 2000 pound bombs right in the windows of
his
command and control centers? The future is here, folks!
1 Popular Science, May 1997, pgs. 54-59. Hiding in Plane Sight - Stealth Aircraft Own the Night, Now They Own the Day, Steve Douglass and Bill Sweetman, copyright, 1997 [MILNET #70]
2 The World's Great Stealth and Reconaissance Aircraft, [MILNET #56]
3 Air & Space Magazine, May 1997, pgs. 18-27, The Invisible Men, Bill Sweetman. [MILNET #78]
4 Air & Space
Magazine, August 1996, pgs. 50-55, The (Tacit) Blue Whale.
[MILNET #78]
5 Anti-Stealth Technology, December 15, 1998, Cyrus Mehta and Andeshir Mehta
6 China's Anti-Stealth Capability, StrategyPage.com, April, 2004.
For further reading, MILNET recommends: Skunk Works, Ben Rich and Leo Janus, Copyright 1994, Ben R. Rich, Little, Brown, and Company (Canada) Limited, paperback ISBN 0-316-74300-3, hardback ISBN 0-316-74330-5.