Will Plasma Revolutionize Aircraft Design



by Justin Mullins



London - Oct. 28, 2000



They can reduce drag, repel shock waves and make jet fighters vanish. Will plasmas start an aerospace revolution, or are they just another mirage?

To look at, the test vehicle suspended in the hypersonic wind tunnel is little more than a cone. But inside is a small device that could revolutionise the way aircraft fly, saving fuel and heralding a new age of travel.

It's a generator that sends a beam of microwaves upstream into the Mach 6 flow, ripping apart the gas ahead of the model so that it is flying through a plasma--a boiling mix of positive ions and electrons--rather than ordinary gas.

The experiment, at NASA's Langley Research Center in Hampton, Virginia, tests a ground-breaking idea developed by Russian researchers during the Cold War. They discovered that injecting a few ions into the flow around a high-speed craft can dramatically reduce the drag it experiences.

With less drag, supersonic airliners might become economically viable, while hypersonic missiles and aircraft flying at more than five times the speed of sound could travel farther on a single tank of fuel. And future generations of space shuttles might rely on plasmas to help them fly during re-entry, which is why NASA is interested.

But there are more clandestine applications. The way plasmas interact with radio waves around aircraft is causing more than a little excitement in the secret world of military aerospace research. Could they provide the ultimate invisibility shield for stealth aircraft? Other researchers have found that plasmas can dissipate shock waves from supersonic aircraft, stifling troublesome sonic booms.

There are even indications that plasmas might influence airflow at subsonic speeds. If that were the case, tiny plasma generators could replace control surfaces such as ailerons and flaps. Planes of the future might not need any moving control surfaces at all. There's no doubt about it: plasma is the height of fashion in aerospace research.

The trouble with fashion is that it is based on the whims of those who buy into it, and the amazing claims made for plasmas have the insidious smell of Cold War hype about them. So is plasma research destined to become the bell-bottoms of the aerospace world? Or is there some substance behind these claims--something that engineers can really build into aircraft of the future?

The story begins in the late 1970s, when Anatoly Klimov embarked on an unremarkable series of experiments at the Moscow Radio-Technological Institute, one of the Soviet Union's most secretive laboratories. His goal was to understand how shock waves behave in ionised gases, a topic of real interest to plasma physicists, for whom the phenomenon seemed rich in possibilities.

Hot and loud

But shock waves are also of interest to the aerodynamicists who design re-entry vehicles and hypersonic aircraft, for whom they are troublesome obstacles in the quest for speed. Shock waves slow vehicles down, cause terrific heating and create sonic booms. For these researchers, any suggestion that they can be reduced or modified is manna from heaven. Which is why the work of Klimov and colleagues at the Ioffe Institute in St Petersburg was so interesting.

One experiment by the Ioffe group involved firing a steel sphere the size of a walnut at 1 kilometre per second through a tube filled with argon gas at low pressure. Gas in a section of the tube was ionised to create a plasma, and the group filmed the shock wave around the sphere before and after it entered the plasma. To their surprise, they found that the difference was huge. Something--call it plasma magic--was forcing the shock wave to stand twice as far from the sphere as it would in an ordinary gas.

For plasma physicists this was intriguing, but what sent aeronautical engineers reaching for their slide rules was that the sphere somehow experienced less drag when it entered the plasma. The group found that this was not some minor effect: they measured a whopping 30 per cent reduction in drag. Aeronautical engineers usually struggle to shave fractions of a per cent off drag, so the results set their pulses racing.

Others in Russia even reported that plasmas could somehow reduce the drag in subsonic flows as well. With less drag, might it be possible to fly sluggish airliners far faster--perhaps even at supersonic speeds? Though nobody could explain the results, there was no doubt about their potential. What Klimov and the others had found was a way to revolutionise the design of hypersonic aircraft and missiles, perhaps gaining a crucial advantage in the arms race that was slowly crippling the Soviet Union.

The work was duly classified, and as the Soviet Union crumbled around him, Klimov and his colleagues began the long slog needed to understand the effect. When the Soviet Union collapsed in 1991, funding for these experiments dried up, forcing the Russians to woo foreign scientists. One of them was Ron McEwen.

Throughout his career at the Sowerby Research Centre near Bristol, part of BAe (formerly British Aerospace), McEwen had had dealings with the Russians. Now the doors of the former Soviet Union's most secret establishments had been thrown open. Interested parties from the West were keen to take a peek at the science inside. So in 1994, McEwen travelled to Russia charged with establishing links with Russian institutions, sifting through their wares and cherry-picking the technologies of most use to BAe. He would not be disappointed.

Rumours of the Russian work on plasmas reached him soon after his arrival. To a cautious scientist, the claims seemed outrageous. But a review of the scientific papers published in reputable Russian journals suggested that plasma magic was neither a confidence trick nor wild exaggeration. The work fired McEwen's and BAe's imagination.

If plasmas really could reduce drag, it might be possible to delay the onset of sonic booms, steer aircraft by applying plasmas selectively to different parts of the vehicle, and even reduce heating around hot spots on an airframe. Some researchers suggested that since ionised gases absorb radio signals, a plasma envelope would make missiles and aircraft practically invisible to radar.

At about this time, Britain's military research organisation, the Defence Evaluation and Research Agency, became aware of McEwen's findings in Russia and together with BAe, decided to test the Russian claims for itself.

Aerodynamicists believed that by ionising the air upstream of an aircraft, they were changing the medium through which the vehicle was flying. Plasmas can behave very differently to ordinary gases. The electrons and ions they contain act independently, creating regions of positive and negative charge that flow through the plasma as waves. This sets up large electric fields that interact in complex ways. Engineers have some experience using electric and magnetic fields to influence the way a plasma flows, but this takes huge amounts of energy and heavy magnets that are too big for an aircraft to carry.

Strike it rich

The great hopes raised by the Russian experiments were that the electric and magnetic forces at work inside a plasma might transform the aerodynamics seen in an ordinary gas, and that these exotic effects--the plasma magic--would explain all. "Plasma dynamics is much richer than ordinary gas dynamics. There is a lot more physics involved," says Sergey Nazarenko, a physicist who worked at the Moscow Radio-Technological Institute in the late 1980s and is now in Britain studying plasma drag reduction at the University of Warwick.

In 1996, Terry Cain, an engineer working at DERA's Farnborough research lab, travelled to Russia to meet Klimov and his colleagues and repeat their experiments at the Central Aerohydrodynamics Institute near Moscow. He decided to test streamlined bodies the size and shape of ice cream cones, and Klimov and his colleagues fitted them with on-board plasma generators. Placed in a supersonic wind tunnel, these generators could create plasmas upstream of the cone.

One device was fitted with a Tesla coil--essentially a circuit that generates voltages high enough to break down air over large distances. "It generated little streamers of lightning that propagated ahead of the model, and their remnants were blown back past the cone in the airflow," says Cain. "It was spectacular."

Crucially, these test bodies were far more aerodynamic than the spheres that Klimov had started with. It's one thing to reduce the drag on a sphere by 30 per cent but quite another to repeat the effect with a more aerodynamic shape. "You can get drag reduction for free with a proper nose cone," says Cain. Nevertheless, Cain was able to measure drag reductions of around 10 per cent. Not quite as spectacular as Klimov's results, but a figure worthy of further investigation.

For the designers of hypersonic vehicles, Cain's results were interesting, but by no means conclusive. Cain knew that the big question they would ask was whether the energy put into reducing drag would be better spent increasing thrust. "When you are adding energy to the flow, a decrease in drag is essentially the same as an increase in thrust. The distinction is really arbitrary," says Cain, who set about calculating a quantity known as the propulsive efficiency of plasma drag reduction. "I did an analysis and the result was marginal." The numbers just wouldn't make it worth building such a device into an aircraft, at least as far as the models tested at supersonic speeds were concerned.

The question remained, however, how the plasma achieved the drag reduction in the first place. Perhaps with a better understanding, plasma magic could become useful?

One of the most obvious suggestions was that plasma magic is simply the result of heat. In the 1960s and 70s, aerospace engineers experimented with forward-facing jets as a way to slow planetary probes as they entered an atmosphere. To their surprise, they found that the jets actually produced thrust in the direction of flight. The jets were heating the air and deflecting the flow away from the vehicle, effectively giving it a more streamlined shape. "With a forward-facing jet on a blunt body you can get drag reductions of a factor of 2 or so," says Cain.

So Cain and a growing number of scientists in Britain, the US and Russia began to consider the possibility that the only effect the plasmas were having was to heat the air, a bit like forward-facing jets. "The fact that you can add heat to a flow field and modify it was no revelation," he says.

In the late 1990s, researchers in the West set out to discover whether there was more to plasma magic than heating. They used computers to simulate an early version of Klimov's experiments in which a shock wave travels through a tube and meets a region of ionised gas.

The experiment was a good one to do because it simplified the geometry. Seen from the side, this was essentially a one-dimensional experiment in which the physics would be simple. Nevertheless, when Klimov originally carried out the experiment he had seen all sorts of complex behaviour: when it entered the plasma, the shock wave accelerated, stretched and even split in two. He and his colleagues reasoned that nothing in ordinary one-dimensional gas dynamics could explain the results. Therefore something else--the plasma magic--must be to blame.

But simulations of this experiment by groups at Princeton University in New Jersey, Imperial College in London, and by Nazarenko at Warwick tell a different story. "The effect was not one-dimensional but two-dimensional, possibly even three-dimensional," says Nazarenko. "It's a very complex situation."

One of the effects the Russians failed to identify was that the plasma is hottest in the middle of the tube. The shock wave moves faster in the hotter gas, making it bow outwards. "When viewed from the side this can look as if the shock has split in two," he says.

According to simulations, the shock wave also creates vortices that interact in a complex way. Richard Hillier an aeronautical engineer at Imperial College even simulated the way a shock wave appears to split when it enters a hotter gas. "The Russians assumed it was a one-dimensional effect and were unable to explain what they saw. But if you recognise that it's a two- or three-dimensional effect, the mystery disappears and you're left with a horribly complex experiment," says Cain. "It was just a two or three-dimensional experiment not properly explained, and that was a bit disappointing."

So is plasma magic nothing more than a complex illusion explained by conventional heating? Not according to Biswa Ganguly, a physicist at the Air Force Research Laboratory of the Wright-Patterson Air Force Base in Dayton, Ohio, who has been carrying out his own version of the shock tube experiments. He believes that in certain kinds of plasmas--"non-equilibrium" plasmas in which the energy of the electrons is far higher than usual--the electric field can significantly heat the gas in specific areas of the flow. He has even measured the effect in his lab. "The local heating is up to six times greater than you'd expect from ordinary gas dynamics," he says. There is nothing conventional about this effect, which is just the kind of thing that the proponents of plasma magic would expect to see.

Despite the controversy over the mechanism, the signs are growing that plasmas could still play a role in the future of aircraft and missile design. Military researchers in the US are investigating ways of using plasmas to absorb radar signals, in the hope that a plane enveloped in one would disappear from radar screens. Meanwhile, a number of Russian groups have reported drag reduction at subsonic speeds, a phenomenon that would have much wider application than supersonic and hypersonic effects. A cut in drag of 1 per cent means you can increase an airliner's payload by about 10 per cent, or it could simply fly farther or faster--perhaps even to supersonic speeds. Just imagine the effect this could have on cash-strapped airlines.

Yet while Russian researchers continue to publish data measured at subsonic speeds, BAe, DERA, NASA and the US Air Force Research Laboratory only admit to having repeated the experiments for supersonic and hypersonic craft. If this is true, it's a remarkable oversight. More likely, work on plasmas at subsonic speeds continues in secret. Simon Scott, a researcher at BAe's Sowerby Research Centre, admits to at least one plasma-based project but says that it is still at the pre-patenting stage, which prevents him from revealing more.

More significantly, the Arnold Engineering Development Center at the Arnold Air Force Base in Tennessee has a number of ballistic ranges and wind tunnels that are being modified to put plasma-assisted models through their paces. "We're charged with anticipating future testing capabilities, and a number of organisations have shown interest," says Tom Best, who heads the applied technology directorate at the centre. Exactly who these organisations are and what they plan to test is not something Best is willing or able to reveal.

We may not have to wait long to find out. The new test facilities will be up and running within the next 12 months. Either the new labs are a huge waste of time and money, or the American military knows something we don't.