Stealth Planes Still Have a Very Visible Problem: Contrails

Scientists haven’t yet figured out how to stop aircraft from producing these high-altitude water vapor trails.

Online military forums like SecretProjects went wild last year over a grainy, indistinct image of an aircraft. Basic digital enhancement showed a bat-winged craft unlike any known U.S. military plane, silhouetted against the blue sky. The consensus among defense media was that this mystery craft must be a top-secret RQ-180 stealth drone, used for spy missions over the most sensitive areas—like Iran, other parts of the Middle East, and areas close to China.

It was the second of three such photographs to emerge in the past few years. All three aircraft were given away by the same decidedly un-stealthy feature.

“I heard a faint aircraft noise and noticed a contrail straight above us,” Joerg Arnu, who witnessed the third mystery aircraft, told The Drive, a website focused on automotive culture and military matters.

That contrail—a cloud-like trail of water vapor produced by aircraft at high-altitude—led them straight to the mystery plane, like a long, white arrow saying “here I am.”

“It’s the stealth equivalent of walking out of the restroom, trailing toilet paper behind your shoe,” says Scott Lowe, a photographer who caught a rare image of a U-2 spy plane after noticing its contrail earlier last year.

Stealth technology dramatically reduced the radar and infrared signatures of aircraft that alerted air defenses to their presence. Previously, aircraft were most often picked up by radar at long range. Engineers have also developed a variety of techniques to eliminate contrails altogether. So why are some supposedly “secret” aircraft still leaving them behind?

Prepare for a dive into the world of aviation dark arts—of smoke and mirrors, acid, and lasers.

Contrails (or condensation trails) are visible for the same reason as your breath, or car exhaust, on a cold day. Warm, moisture-laden air mixes with cold, dry air and creates condensation. In the case of contrails, the condensation takes the form of tiny ice crystals. These form around tiny particles, mainly soot, in the engine exhaust.

Contrails first became a problem during WWII when the U.S. Army Air Forces’s massed bomber formations left broad swaths of contrail across the sky. German fighters could see the contrails from miles away, long before the planes themselves were visible, and learned to home in on them to make intercepts.

Technical wizards developed “chaff,” made up of tiny metallic strips, for planes to deploy behind them as reflective clouds. It helped to blind German radar, but contrails still remained visible. This made nighttime raids the preferred option. After the war, jets replaced piston engines; unfortunately, they left even more distinct contrails.

Pilots soon found that contrails could often be eliminated by changing altitude slightly, though the science behind this wasn’t fully understood until the 1950s.

“In theory there will always be drier air a few thousand feet above you,” says Adam Durant, CEO of SATAVIA, which produces contrail modeling and prediction software. This generally makes it easy to find a level where contrails do not form.

The problem was that pilots sometimes did not see they were leaving a contrail until it was too late and because of limited visibility behind them. This was literally a matter of life and death for the pilots of the CIA’s U-2 spy planes flying over Soviet territory. The pilots soon figured out a simple solution: fit the aircraft with a rear-view mirror outside the cockpit to give a view behind the aircraft.

Tests were carried out with “Article 349,” a U-2 specially modified to test a variety of stealth technologies including early radar-absorbing paint known as “black velvet” and a rear-view mirror. Details of the 1958 project were only released in 2003, and even then the reports were redacted, but it is apparent that U-2 makers Lockheed and the Air Force were involved in the assessment.

“It is Operation’s view that this installation is a valuable asset,” according to the CIA assessment on ‘Rear View Mirror.’ “The need will increase as time goes on, based on estimates of future Russian capabilities to intercept.”

Tests showed that the pilot could see a contrail when it was less than a mile long; it was hoped it might also be useful for spotting intercepting fighters. The external rear-view mirror became standard equipment and was installed on many subsequent versions of the U-2.

Meanwhile, Air Force engineers were looking at solutions which did not require the aircraft to change its flight path. They focused on the particles in the exhaust around which water droplets form.

“The number of ice crystals is pretty much dependent on the number of soot particles. If we were to reduce them, that would reduce the contrail,” says Dr. Marc Stettler, an expert on transport emissions at University College, London.

The researchers found that one of the major contributors was sulfur trioxide, which resulted from the combustion of sulfur in the fuel, so they tried low-sulfur fuel mixes. Ultimately, the effect was not sufficient, but research continued for some years.

The same research revealed that there might be another way to deal with contrails by altering the fuel. Rather than preventing a contrail from forming by reducing sulfur, they increased the amount of sulfur so that there were even more particles in the exhaust. The idea was that this would change the size of droplets in the contrail to render it invisible.

According to a 1962 U.S. Air Force study, if the particle size could be reduced to less than half a micron, the contrail would appear as a blue haze rather than a white trail: “From any distance this blue haze would be substantially invisible because of the lack of contrast with the atmosphere.”

Researchers moved on to blowing sulfur dioxide directly into the air intake, but even this was not sufficient. Dr. Roger Teoh, who is exploring the impact of aviation on climate change at Imperial College, London, says that even huge increases in sulfur failed to have the desired effect. “Adding large amounts of sulfur only led to a very small reduction in contrail formation; and there could be unintended consequences,” says Teoh.

By 1961, the Air Force had achieved something amazing. Photographs of a demonstration with a four-engined B-47 Stratojet bomber show the engines on one side leaving a normal contrail as usual, but nothing is visible on the other side. The bomber had been fitted with a new system which injected chlorosulfonic acid into the exhaust. This achieved what the experiments with sulfur had failed to do: producing a contrail with tiny particles too small to see.

The technique was highly effective, but the contrail-suppression equipment added 400 pounds to the bomber, reducing the bomb load. In addition, the plane needed a supply of contrail-suppression chemicals equal to about two percent of the fuel, potentially adding another 2,000 pounds.

While there is no record of the technology being deployed on bombers, the ‘no-con’ system was fitted to Ryan Firebee drones flying reconnaissance missions over Vietnam and China. These small and fast jet-powered drones usually evaded observation, but sometimes they were given away by their contrails.

The acid injection system succeeded in keeping the small drones unseen but was unpopular for other reasons. Chlorosulfonic acid is extremely corrosive and damages engines, shortening their flying life. It is also highly toxic and hazardous to the ground crews.

When the B-2 Spirit bomber was being developed in the late 80s, it was initially fitted with a chlorosulfonic acid injection system similar to that on the Firebees. However, for reasons which have never been disclosed, this was never used.

The motive could have been environmental; there was a growing awareness that secretly spraying highly toxic chemicals from aircraft might attract criticism. This was even before the rise of the “chemtrail” conspiracy theories of the 90s, which accused the U.S. government of spraying mysterious chemicals from aircraft that left long-lasting contrails. There is no evidence that this theory was connected with actual contrail research—the whole aim of which was to prevent such trails from forming.

Air Force Secretary Edward Aldridge revealed that an alternative solution had been found at a 1989 press briefing on the B-2, but kept journalists guessing on what the new technology was. “The contrail problem has been solved, but I’m not going to tell you how,” said Aldridge.

There was much speculation that the solution was either a new fuel additive or a system of baffles to mix cold air with the exhaust (see below).

The Stealth Contrail SpyNoshir Gowadia was an engineer working on the stealthy B-2’s complex exhaust system. His design helped ensure that cold air was mixed in with the hot jet exhaust before it left the plane, to dilute the plane’s thermal trace and make it harder to spot with infrared imaging.Gowadia used his expertise to redesign jet nozzles with the aim of eliminating visible contrails. This involved ‘non-uniform flow field’— a region of turbulent mixing—which would spread out the water droplets so much that any contrail would be invisible to the human eye and other sensors. The USAF thought they had found a solution to the contrail problem and awarded Gowadia a contract to develop his concept into a finished product.However, in 2011 Gowadia was convicted of espionage—specifically, passing details of stealth exhausts to China—and sentenced to 32 years. The nozzle redesign project was discontinued, and it is not clear whether this technique can effectively eliminate contrails.

It was only years later that the real secret was revealed to be the PAS, or Pilot Alert System. Developed by sensor company Ophir, the PAS uses a form of lidar: it fires a laser beam back into the jet exhaust and measures the scattering of the light off ice particles. This can immediately detect when a contrail starts forming, warning the pilot to change altitude before it becomes visible.

The PAS was certainly an improvement over the U-2’s rear-view mirror, but what Air Force planners really wanted was to be able to fly without any risk of contrails forming in the first place.

Changing altitude works because contrails only form in particular conditions of temperature and humidity. German scientist Ernst Schmidt made the first steps towards a scientific understanding of the process in 1941, and in 1953 Herbert Appleman of the American Meteorological Society developed a precise formula for contrail formation. Known as the Schmidt-Appleman criterion, this can be neatly expressed as a graph of temperature and humidity: to prevent contrail formation, just avoid the area mapped out in the middle of the graph.

Air Force planners have used the Schmidt-Appleman Criterion to develop increasingly sophisticated software models to predict where contrails will form. By 1998, the Air Force assessed its JETRAX software as 84 percent reliable at determining whether contrails would appear on a flight path. Planners can reroute stealth missions to prevent leaving tracks in the sky.

While military contrail-prediction software has always been kept under wraps, there has been a surge in developments in the commercial sector. The reason: climate change.

While some contrails fade out quickly, others spread out to form high-altitude cirrus clouds, which have a significant warming effect. In fact, the warming effect from cirrus contrails is actually greater than that of the CO2 from burning aviation fuel. Removing contrails would make flying less harmful to the planet.

“Contrails account for 59 percent of the climate impact of air travel. That’s equivalent to 1.8 billion tons of CO2 a year,” says Durant. DECISIONX:NETZERO is SATAVIA’s planet-wide atmosphere model, driven by AI and fed with commercial meteorological data. The key, appropriately enough, is cloud computing, which makes intensive calculation affordable. This allows the system to divide the globe up into cells five-kilometers square, stacked sixty deep.

“We use complete global scale weather data sets to drive a physics-based model of atmospheric dynamics which shows us the probability of generating a contrail on any given route,” says Durant.

While most weather models concentrate on what’s happening at ground level, SATAVIA’s looks at aircraft cruising altitude and applies contrail-formation algorithms. Crucially, by showing conditions at sixty different altitudes it allows the flight plan to avoid contrail risk.

Durant notes that while this will require some efforts in air traffic management, a small number of flights produce the most damaging, long-lasting contrails. He says most of the benefit could be gained by rerouting just 5 percent of flights.

After a successful pilot scheme with airline Etihad to test the software in practice, the company is refining its model into a commercial product. Durant is not aware of anything like it in the commercial world, but the military, with its massive computing power, may well have something comparable.

There may be other developments in this field which are not public. A 2014 patent by engine maker Rolls Royce links a sensor similar to PALS to an engine control system. The patent claims that by altering the efficiency of the engine, the exhaust can be changed to prevent contrails from being formed. Rolls Royce declined to discuss this or its other work in this area, such as an outlandish plan to zap the exhaust with microwaves to prevent ice crystals from forming.

“Generally, a more efficient engine can slightly increase contrail formation because air in the exhaust leaves the engine at lower temperature,” says Teoh. “Therefore, reducing contrail formation can only be achieved by lowering the engine efficiency, which most likely comes at a cost of increasing the fuel consumption.”

Teoh also notes that new types of engine combustors may also dramatically decrease the amount of a soot in the exhaust by ensuring that fuel is fully burned before it reaches the exhaust. “The latest ICAO Aircraft Emissions Databank, a publicly available dataset, shows that different combustor types can significantly reduce the number of soot particles by up to four orders of magnitude,” says Teoh. This would represent a factor of ten thousand, which might be enough to eliminate visible contrails.

Spy planes may still leave contrails in places where they are not trying to stay hidden—hence Lowe’s lucky picture of that U-2. “Without a contrail or perfect light, the U-2 is invisible,” says Lowe. “I never would have spotted it otherwise.”

But in the case of the RQ-180 pictures, you have to ask why the same supposedly super-secret aircraft left highly visible contrails three times in a row, each time in broad daylight over a populated area? Once might be explained by accident, twice would suggest a failure to learn, but three times starts to look deliberate.

The bottom line is that we’re seeing the contrails, which are leading us to the aircraft, because they want us to. That line in the sky is a deliberate pointer. As to why that should be, and what is really being kept hidden—that’s another mystery.

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