Will Plasma Revolutionize Aircraft Design?
Source: New Scientist
London - 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 revolutionize 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.
This article appeared in the Oct 28 issue of New
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by Justin Mullins
October 28, 2000 Issue