THE ivy-clad building on a hill overlooking the small town of Troy in New York State seems an unlikely venue for such a futuristic experiment. But appearances can be deceptive. The building houses several engineering laboratories belonging to Rensselaer Polytechnic Institute, and in its basement is one of the few wind tunnels in the world that can produce shock waves at more than 25 times the speed of sound.
The experiment which will begin here in the next few weeks, will test a device that its designers hope will control the airflow around aircraft and launch vehicles travelling at many times the speed of sound. Known as an air spike, it works by carving a path through the air with pure energy. A vehicle fitted with an air spike would experience little drag, dramatically improving its performance. Decreased friction would reduce the temperature of the craft’s skin, so it would not need the same degree of thermal protection as the space shuttle, for example. And because the airflow over the vehicle would be dictated by the air spike, it would not even need a streamlined shape. In future, air spikes could allow vehicles of almost any shape to fly at Mach 5 and faster.
The men behind the idea are Leik Myrabo, an aerospace engineer at Rensselaer, and Yuri Raizer, a plasma physicist at the Moscow-based Institute for Problems in Mechanics.
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In 1993, with funding from the Space Studies Institute in Princeton, New Jersey, they worked out how an air spike could be made to work. Last spring, Myrabo tested the idea for the first time at Mach 10 using the wind tunnel at Rensselaer. “The experiment was a huge success,” he says.
In the next series of experiments, Myrabo will make detailed measurements of the pressure and heat transfer around a disc protected by an air spike as it sits in a flow of air travelling at up to 25 times the speed of sound. Mach 25 is a magic number. It is the speed needed to escape from Earth into orbit and the speed at which returning spacecraft slam into the atmosphere. If the air spike can be made to work at this velocity, Myrabo believes that it could lead to a revolutionary new generation of lightweight, reusable launch vehicles that would resemble flying saucers and get their power from beams of microwaves or lasers. “The equipment to do it is available now,” he says. In the past, Myrabo has received funding from the US Air Force, the now-defunct Star Wars programme and NASA to develop the idea further. Now NASA is considering the idea as a way of launching satellites when the next generation of reusable spacecraft become obsolete some time in the 21st century.
But first Myrabo must perfect the air spike. The device works in a similar way to the sharp nose of a supersonic aircraft. As the aircraft ploughs through the air at high speed, the nose creates a conical shock wave. If the aircraft flies inside this cone it experiences less drag. But as the airspeed increases, the cone becomes narrower and it becomes more difficult to build an aircraft to fit inside.
One way round this is to make the nose longer but this creates other problems. For one thing, a long nose is heavy, and Concorde’s nose is so long that it obscures the pilot’s view of the ground. “During landing, the nose has to be lowered, which greatly complicates the design,” says Myrabo. Also, longer aircraft may not be able to use many of the world’s airports.
Blast waves
The air spike provides a potential solution to all these problems. The idea is to carve a path through the air by focusing a microwave or laser beam in front of the aircraft. At the focal point, the concentration of energy is high enough to rip electrons from molecules in the air to form a plasma. These electrons smash into other molecules stripping off still more electrons. The result is a chain reaction known as inverse bremsstrahlung and it unleashes an explosive force. “The pressure waves created by laser-induced detonations can reach thousands of atmospheres,” says Myrabo. “With microwaves, however, the result is a few tens of atmospheres, which may be enough for our purposes.” In practice, these detonation waves tend to travel up the beam towards the microwave or laser generator. “If you’re not careful, they can destroy your equipment,” says Myrabo. The way to prevent this is to use rapid pulses of energy that generate a series of blast waves.
In still air, each blast wave would travel away from the focal point in all directions. But Myrabo and Raizer reasoned that in a rapid airflow the successive blasts would form into a parabolic shock wave sweeping back from the focal point, with the size of the paraboloid dependent on the power of the air spike and flight speed. The parabolic shock wave offers less resistance to the flow of air than the conical shock wave created by a long, pointed nose. In addition, it should be possible to keep the size and position of the shock wave constant as the speed increases simply by boosting the power. “This is a major advantage,” says Myrabo.
Another benefit is the reduced friction with the atmosphere. Friction is a major problem for hypersonic vehicles because of the heating it causes. When a conventional spacecraft reaches Mach 25, the temperature of compressed air behind the shock wave can rise to about 8000 K. With an air spike, however, the shock wave need never touch the craft, so aerodynamic heating would be dramatically reduced.
Until last spring, these ideas were just theory. Then Myrabo decided to put the idea to the test. He reasoned that a welder’s plasma torch, which heats the air to 20 000 K, would create a similar plasma to a focused laser or microwave beam. A plasma torch would also be easier to set up inside the wind tunnel.
Ferocious environment
The tunnel is an extraordinary device, built by the American aerospace company General Electric in the late 1950s to test nose cone materials for ballistic missiles. It generates rapidly moving shock waves by suddenly releasing a pressurised “driver” gas into a pipe about 16 metres long and 10 centimetres wide. Inside this pipe is the test gas, and at its end is the narrow end of a cone-shaped nozzle. As the test gas is driven through the widening cone, it accelerates to many times the speed of sound, albeit for only a fraction of a second.
Mach 10 was fast enough to test the plasma torch. In this ferocious environment, Myrabo showed that a parabolic shock wave formed about the end of the torch where the plasma was being created. He also demonstrated how the spike could deflect the airflow around a blunt disc-shaped model, avoiding the enormous drag that it would otherwise have created.
At about the same time, Pavel Tretjakov, an engineer at the Institute of Theoretical and Applied Mechanics in Novosibirsk, Siberia, carried out a similar experiment using a blunt, cone-shaped model and a laser-induced air spike. He discovered that at Mach 2 the air spike reduced the drag on the cone by half – a huge amount for aerospace engineers who struggle to shave fractions of a percent off drag coefficients. “I was surprised that the Russians had got to use a laser first,” says Myrabo. “But at least they proved that it works.”
Now, Myrabo and Raizer want to improve their understanding of the airflow created by an air spike. At the Institute for Problems in Mechanics, Raizer has developed a computer model of the spike which he hopes to calibrate using the results of Myrabo’s next test run. Armed with an accurate computer model, the two will be able to simulate re-entry and the passage of hypersonic vehicles through the atmosphere without having to rely so heavily on wind tunnel tests.
Efficient spacecraft
But aircraft manufacturers need not wait for the development of air spikes to benefit from the idea. Electrically heated, lightweight plasma torches that are powerful enough to modify the airflow around aircraft are already available, says Myrabo. He adds that they could be fitted and tested on real aircraft within a couple of years.
Ultimately, Myrabo believes that the greatest benefits will come from vehicles specifically designed to use air spikes. He says that air spikes will allow launch vehicles to be much lighter than the present generation. Today’s vehicles, and those scheduled to take over from them early next century, are huge because they must carry an enormous volume of chemical fuel. A far more efficient method, Myrabo suggests, would be to beam microwave or laser power to the launch vehicle, where some would be focused to maintain the air spike and the rest would be converted into electricity to propel the craft into orbit. “We know how to design power converters that are infinitely lighter than the power source.”
Hand in hand with the development of the air spike, Myrabo has designed a new type of vehicle called a lightcraft to exploit the idea. The vehicle is shaped like a flying saucer about 10 metres across. It will be built from thin films of silicon carbide, which can withstand skin temperatures of up to 2000 K, and inflated with helium to provide buoyancy at low altitude and act as a coolant. The entire structure weighs only 630 kilograms.
During flight, the air spike controls the shock wave so that it shaves the rim of the vehicle. The position of the shock wave is critical for the type of propulsion Myrabo intends to use. The rim would be fitted with current-carrying electrodes and superconducting magnets, which would accelerate ionised air in the shock wave backwards, propelling the vehicle forwards. “In effect, the entire vehicle becomes the engine,” says Myrabo. This type of propulsion is known as magnetohydrodynamic (MHD) acceleration and it is being seriously studied in many countries including the US, Russia and Britain.
Most MHD programmes are secret because of the huge military significance of hypersonic vehicles. Scientists in Britain and the US refuse to talk openly about their own work but some details have emerged about a Russian programme to build a reusable space plane called Ajax which will be powered by an MHD accelerator. The man behind the project is Vladimir Fraistadt, an aerospace engineer at the State Research Enterprise of Hypersonic Systems in St Petersburg. He says the vehicle operates on liquid methane fuel and uses the high temperatures generated at hypersonic speeds to split this into carbon and hydrogen ions which then pass into the MHD engine. Because it is reusable, Ajax will be cheaper to operate than today’s launchers which carry cargo at a cost of up to $6000 per kilogram. Ajax will do it for $1000 per kilogram, says Freistadt, and may form the next generation of reusable launchers in Russia.
NASA has its own candidates for the next generation of reusable vehicles such as the Delta Clipper spacecraft (see “Space Clipper comes of age”, Â鶹´«Ă˝, 12 August 1995) but is has already begun to study the generation beyond this. Last year, the agency began the Highly Reusable Space Transportation programme to identify the key aerospace technologies of the future and how they should be developed. Myrabo’s work is high on the list. “The notion of using electromagnetic effects to propel space craft appears very exciting,” says John Mankins, who runs the $1 million project at NASA’s headquarters in Washington.
Myrabo has already mapped out how this generation of launchers will work. He envisages a system in which constellations of orbiting satellites would convert solar power into microwaves and beam them downwards to a lightcraft below. The lightcraft must have a large surface area to gather as much microwave power as possible, so a flying saucer shape is ideal. Myrabo’s lightcraft would use these “corridors of power” to sail in and out of the atmosphere at speeds of up to Mach 25. Once the microwave transmitters are built, each launch of a lightcraft would cost a fraction of today’s launches. “Perhaps a thousand times cheaper,” he says. Mankins agrees. “This generation of launchers will reduce launch costs to around $100 per kilogram,” he says.
In the nearer term, launching a satellite weighing a few kilograms would need a multi-megawatt microwave beam. “These are available now,” says Myrabo. What about the dangers of using such powerful beams in the atmosphere? Obviously, the area around a microwave boosting station would be restricted, says Myrabo. Similar restrictions already operate at most airports. He points out that each engine on a 747 jumbo jet generates 100 megawatts (0.1 gigawatts). “You wouldn’t stand in the exhaust, would you?” he says.
Mankins believes that Myrabo’s vision is still several decades away but concedes that some of the concepts might have near term applications. This is borne out by the huge interest that Myrabo’s work has generated. After the first set of experiments, Myrabo received hundreds of calls from scientists and aerospace companies all over the world who were keen to know more. Just how much more he can tell them, and whether such futuristic craft ever take to the air, will depend on his next set of experiments.
