It’s Invisible

By Eric M. Jones

IN THE MID-1980s, “Photonics Magazine” published a curious letter to the editor:

“Most people know that you can make a hologram of something that does not really exist. A few people know that you can have a hologram of an empty missile silo placed over a silo that has a missile in it to fool observation satellites. Few people know that you can create a reflective hologram of empty space. This works much better at microwave frequencies.”

... Like radar? I almost fell out of my chair. Who wrote this curious letter and why was it published? I mentioned it to a few aerospace people who refused to believe it until they looked it up. I assume the “Photonics” people got a visit from guys in black suits.

The history of making things invisible at light frequencies is a long one. Camouflage is an old technique, now made even better by using electronic displays so that what the observer sees is more-or-less a perfect copy of its background.

This technique of making things invisible has been touted even on TV shows like “Top Gear,” which equipped a van with flat-screen TV displays. This works a bit poorly, but like a monkey smoking a cigarette, or a dog walking on two legs, it’s surprising to see at all. The problem is that the electronic video display can produce an “invisibility mask” for only one angle and only one distance, and this usually means only one observer. But it still would work fine for situations where the “cloaked” vehicle is very far away from a single observer, or for situations where the “cloaked” vehicle can adapt the display to the observer, which we presume it can locate and adapt to, but still one at a time.

Of course Wonder Woman had her invisible jet, but optical invisibility was developed long ago.

Before WWI was over, Germany had experimented with a clear cellulose aircraft covering called Emaillit. This skin, along with silver-painted internal structures, made aircraft that were effectively invisible from the ground when the aircraft was over an altitude of just 300 meters. We’ll presume that’s a slight exaggeration. The allies, especially the French, had similar programs.

Russian Professor S. G. Kozlov’s “Invisible Aeroplane,” a Yakovlev AIR-4, apparently had its opaque parts painted white and/or covered with a mirrorlike amalgam or enamel. The aircraft’s structure was then covered with transparent “Rodoid,” (a French substance like tough cellophane also called “organic glass”). The solid structure was difficult to conceal and the whole skin tended to get soiled from the dirt runways and smoky exhausts pretty quickly, but the experiment was at least somewhat successful. Cellophane is still a pretty decent airplane covering for RC models.

Invisibility programs continued in the 1920s and 1930s between the wars, but was concentrated on warship camouflage. One of the more interesting approaches was to put diffuse lighting on ships that would duplicate the background lighting. The Canadians, who launched all those Liberty ships to England, had a big stake in this technology and invested considerable sums to accomplish ship invisibility. But because ships made a horrendous racket and spewed smoke, this was only partly successful.

The Canadians continued to experiment with Yahudi ship camoflage until after WWII.

One possible result of this idea as applied to aircraft was by McGill University’s Edmund Godfrey Burr who, in 1940, observed that an aircraft could be invisible if observed when its landing lights were on. This is counterintuitive as it was assumed that lights made an aircraft more visible, not less. But an aircraft is usually just a black silhouette against a lighter sky, and if the aircraft had enough lighting of the right intensity distributed on its front surfaces, it could be very hard to see from a distance. In modern times, aircraft use wig-wag lighting systems for this reason. The human visual system easily notices them. It turns out that birds notice them too, so aircraft bird-strikes can be reduced as well.

Thus was born top-secret Naval Project Yahudi in 1942-1943 to help counter the U-boat threat when the allies attacked them from the air. German submarines usually ran on the surface and only submerged when approaching their prey, so attacking aircraft could, with luck, come up on the German Unterseeboots recharging their batteries on the surface and drop bombs or depth charges on them before they could submerge deep enough to no longer be vulnerable. Only a fraction of a minute spelled the difference between the submarine escaping or not, so anything that could allow the attacking aircraft to get closer without being observed would be vital to the war effort.

Both a B-24 Heavy Bomber and a TBM-3D Torpedo Bomber were outfitted with forward-facing lighting. Experiments were run that showed the system worked fine, but the implementation of defensive centimeter radar on the subs changed the game and the project was shelved. (Yahudi remained secret until 1980.)

One additional note regarding the plan to outfit anti-U-boat patrol aircraft with invisibility systems is that the attacking aircraft, too, had reason not to get too close to a surfaced U-boat. They preferred to get to attacking position when the U-boat had just begun to submerge. Otherwise the U-boats’ anti-aircraft guns could damage or destroy the airplane ... very far from home in the middle of the very cold Atlantic Ocean.

During the Vietnam War a similar Yahudi invisibility approach was tried on F4 Phantoms. This project, called “Compass Ghost,” outfitted the big F4’s with nine high-intensity lamps and special paint to make the F4 less visible by thirty percent to the relatively small MiG-21s. No matter how clever it was, it wasn’t particularly effective (it worked only head-on) and was soon dropped. But the military on both sides continued to experiment with it.

The invention of compact radar allowed radar to be used on ships and planes and the electronic wars began. All warships and many planes carried radar, which allowed them to see the enemy in all weather conditions, from long distances, and at night. Optical invisibility was no longer a big concern. But because radar sent out a signal that bounced off the enemy, it simultaneously gave away the searcher’s position. Although having communication with a command post could tell you where the enemy was, being invisible to the enemy’s radar became a critical goal.

Radar absorbing layers were added on submarines, frequently on snorkels and conning towers. Ships were too big to shield effectively, and aircraft couldn’t carry much radar absorbing technology but could be constructed with “low-visibility” to radar planned from the beginning. One of the techniques used on both sides was to return to building aircraft from wood or composites, and then adding absorptive layers or paints.

The world’s first stealth fighter prototype flew on February 14, 1945: the Horten Ho-IX/Go-229. It was a twin-jet powered flying wing with a fuselage built largely of wood that contained a radar absorbing mix of sawdust, charcoal, and resin. Its flying wing configuration would help keep it stealthy. The Horten had only a twenty percent reduction in its radar cross section, which is not as great as was hoped. But the Germans also developed a radar-absorbing paint called Chimney Sweep that was a thick carbon-laden mixture which eventually became the basis of the U.S. "Ironball" paint used on the U-2, and other “stealth” aircraft and ships.

[An early attempt at achieving stealth, at left, the
Linke-Hofmann R.I. heavy bomber had an all-wooden structure covered in “Cellon” plastic transparent material by the Germans during WWI. The material degraded rapidly in sunlight.]

Wikipedia has an amazing discussion of this if you want to make a “Stealth” vehicle of your own.

Modern techniques of making aircraft invisible include electrically ionizing the air in front of the aircraft. This is called “Plasma Stealth.” Radar waves simply get absorbed by the plasma and never return. In the mid-1960s, the SR-71 is believed to have done this with massive electron-beam guns inside its engine inlet cones. In theory, the entire aircraft could have been concealed, but in practice the SR-71 was pretty stealthy already and this was an attempt to conceal only its engines. It is known this was built and tested, but not implemented, because it was very near the end of the SR-71 program. Besides, the SR-71’s major defense was simply its speed, and psychologically there was always a “because fuck you, that’s why” joyousness about the whole program, so being invisible to radar was not critical. By the end of the program, the SR-71 had been fired at by missiles, some say, as many as 4,000 times, and the most the pilot ever had to do was knock the throttles up a tad. When firing at the SR-71 cruising above 25,000 meters (some say its operating ceiling was 35,000 meters), the air is so thin that the maneuvering capability of any missile is practically nonexistent, even in a head-on shot. The SR-71 also employed very sophisticated ECM (Electronic Countermeasures).

There is a curious story of the development of stealth aircraft like the F-117. A couple early crashes were instantly followed by the military fencing off the crash site and picking up even the tiniest flakes of material, even vacuuming up the desert sand for loose particles. So tight was the security regarding the stealthy material with which it was covered. My guess is it was that microwave hologram of empty space.

***

Yes, well, then there’s the Romulan Cloaking Device that achieves invisibility. Movies and other works of fiction often galvanize the public to develop devices, or even whole design genres. Millions have been spent to create a working hoverboard from “Back to the Future.” There are Star Trek tricorders in the works, too. The “Invisibility Cloak” seems to be one of these hopeful devices. Several groups have designed so-called invisibility cloaks, usually based on flexible display screens, microprocessors and cameras. Sometimes they throw the word “hyper” or “quantum” into the product name. Usually they are overreaching trying to get funding; no actual devices have arrived to be purchased ... as far as anyone but the CIA knows.

There have been some recent developments of “Cloaking Devices.” Of course, there are differences in optical, infrared, and electromagnetic (anti-radar) devices. The primary difference is that human beings see in the optical, and they don’t emit anything much, so all the perceived image comes from whatever is reflected from the object and the background that is blocked by the object. Infrared is nearly the same, but we can’t see it.

Radar detection systems, on the other hand, all depend on the observer emitting electromagnetic energy towards the target of interest. This energy is then reflected back to the radar receiver. So historically, if you are in the target vehicle, to make yourself invisible to radar requires merely deflecting or absorbing the energy beamed at the target.

Absorbing or deflecting radar might be pretty simple except that the radar wavelengths can differ by a factor of 100,000:1 whereas visible wavelengths vary only by a factor of 2:1. So the physics that might work for one radar system might not for another.

But today there is a more advanced stealthy electronic warfare. Modern U.S. fighters can use an associate’s radar so they don’t have to emit anything. The radars have numerous features such as frequency hopping, beam shaping, pulse operation, minimum power for the job, etc. For example, when only one of your squadron detects an enemy fighter, it can be displayed on all of your squadron members’ displays. It is only necessary that the radar of one member of your squadron watch an adversary via a narrow, low-power beam which can be switched from plane to plane among your squadron. They also actively screw with the opponent-radar’s electronics. The whole electronic chessboard can therefore be synthesized in the computer. And the game is on. If your fighter detects a target, another fighter in a better position can launch a missile at it, even if it never detected the target itself. This is not your father’s warfare. The whole enemy squadron can be destroyed without the enemy having seen anything on their radar-detecting apparatus.

Or you can just do what the Klingons and Romulans do, move your spacecraft ahead or behind in time by the tiniest fraction of a picosecond and it’ll be completely invisible. Or? 

Further Reading

Secret Projects.
AIR-4—the Transparent Biplane.
Stealth.
Horten Ho IX (Gotha Go 229).
The First Stealth Aircraft.
Radiation-absorbent material.
Plasma Stealth.
HyperStealth.

Eric M. Jones is the Associate Editor and co-founder of “Perihelion.” He is a design engineer, consultant, entrepreneur, and pilot, working in the experimental aircraft community, NASA, space transportation companies, and the ISS.