91av

Washington DC

BEHIND the lenses of laser protection goggles, there is a glint of the future
in the eyes of Leik Myrabo. The scene is the once top-secret High Energy Laser
System Test Facility (HELSTF), in the deserts of White Sands Missile Range in
New Mexico. In front of Myrabo stands one of the world鈥檚 most powerful lasers,
its beam pointed at the sky. And in the beam鈥檚 path sits a strangely shaped,
silver vehicle that looks like a lemon squeezer the size of a rugby ball. This
is a lightcraft, a revolutionary launch vehicle designed to ride into space on a
shaft of laser light.

On 5 November last year, Myrabo tested his lightcraft outdoors for the first
time. The tests, funded jointly by NASA and the US Air Force, were a huge
success. The lightcraft rose to a modest altitude of 15 metres in 5.5 seconds
and then dropped into a net when the laser was switched off. In December, the
lightcraft reached 20 metres in only 4.9 seconds. And the tests continue.

Big plans

Myrabo has big plans for his invention. With each test he shoots the vehicle
slightly higher. If all goes according to schedule, he will have launched the
spinning craft to an altitude of 1 kilometre within 18 months. With a more
powerful laser, the lightcraft could reach space. A lightcraft launch vehicle
would carry almost no fuel, a major proportion of the weight of conventional
rockets. As a result, they could put their payloads into orbit at a fraction of
the cost of conventional launchers.

Lightcraft have been on the drawing board for several years ( 鈥淩ider on the
shock wave鈥�, 91av, 17 February 1996, p 28) and are unlike
anything that has flown before. They are hollow and machined from aluminium only
fractions of a millimetre thick. Each craft weighs less than 50 grams, lighter
than a rugby ball and only slightly bigger. Myrabo chemically mills the metal to
make the ship even lighter. The lightcraft are quite delicate, Myrabo says.
鈥淭hey can get damaged when they land so we鈥檝e started making them in
苍耻尘产别谤蝉.鈥�

The prototype lightcraft carry no fuel at all. Instead, they convert a series
of brief pulses of energy from a laser on the ground into a propulsive force.
This is hugely significant. A large percentage of a conventional rocket鈥檚 mass
at take off is fuel. This has to be stored and carried during the flight. It is
converted into thrust by complex and expensive engines which are generally used
only once .

By contrast, lightcraft engines are cheap and simple. The prototypes work
without any moving parts and are entirely reusable. The engine consists of a
ring-shaped mirror at the rear end of the vehicle that reflects and focuses the
laser light into a ring-shaped combustion chamber. At the focal point, the
concentration of energy is high enough to rip electrons from the molecules to
form a plasma.

The process is called inverse bremsstrahlung and it occurs with explosive
force. The pressure wave it creates can reach thousands of atmospheres and
temperatures of 30 000 kelvin. It is this pressure wave that pushes against the
vehicle driving it forward. What鈥檚 more, there are no toxic by-products. 鈥淲e鈥檙e
using a completely clean propellant鈥攁ir,鈥� says Myrabo, a professor of
engineering physics from Renssalaer Polytechnic in New York State who is working
with the propulsion directorate of the US Air Force Research Laboratory at
Edwards Air Force Base in California. After each explosion, cool air rushes into
the engine, ready for the next pulse of energy.

Each flight begins by spinning the lightcraft to around 6000 revolutions per
minute as it rests above the laser. This rapid rotation keeps the craft stable
during the flight by the same principle that keeps a spinning top upright. Then
the laser bursts into action, generating some 20 pulses per second. The infrared
light is invisible, but the explosions of plasma behind the lightcraft glow
brightly and generate a noise like machine-gun fire. Slowly, the spinning
vehicle rises into the air, floating atop a cushion of light.

The flight ends when the craft hits a black plywood board mounted at the top
of the test stand. The board absorbs any laser light that might otherwise spill
into the sky and damage the sensors on passing satellites. 鈥淔ree鈥� flights will
require clearance from the North American Aerospace Command, a US and Canadian
military organisation monitors satellites. 鈥淓ventually, we鈥檒l be given time
slots when there is nothing overhead. Then we will fly freely,鈥� says Myrabo.

Perhaps the most critical part of the experiment is the laser. It must supply
its energy in short pulses. Each pulse must be powerful enough to trigger
inverse bremsstrahlung and must be repeated many times a second. Thanks to the
American Star Wars programme in the 1980s, HELSTF is well equipped with powerful
lasers that can be precisely controlled and carefully aimed at moving targets.
The laser Myrabo uses is the most powerful of its kind in America. Known as the
Pulsed Laser Vulnerability Test System (PLVTS), it generates a rapid series of
short infrared pulses lasting only 18 microseconds each. One pulse has the
energy of 450 joules. And the PLVTS can generate 20 of them every second. This
gives an average power of 10 kilowatts.

Apart from reducing the weight, the only way to improve lightcraft
performance is by increasing the pulse rate of the laser while reducing each
pulse duration. This has two consequences. It allows Myrabo to control the
lightcraft more precisely: the longer the interval between each pulse, the
greater the chance that the vehicle could drift out of the beam. Short pulses
also increase the efficiency of the engine, whereas longer pulses create
explosions that burn partly outside the engine and so contribute little or no
thrust. Later this month, Myrabo plans to reduce the pulse duration.

This gradual approach is important, says Franklin Mead, co-director of the
programme with Myrabo and a researcher with the USAF鈥檚 advanced propulsion
group. 鈥淓very experiment with the lightcraft gets us more engineering
information,鈥� he says.

For the craft to climb higher, the pulses must be shorter still, about a
microsecond each. The laser would have to fire 1000 pulses a second, and each
would have to be more powerful than PLVTS. Such lasers are not in use in the US
today鈥攂ut they have been in the past.

A 150-kilowatt pulsed laser now lies in pieces in crates at HELSTF. Called
Driver, it was once part of a larger laser built in the 1970s capable of firing
megawatt pulses. Pulling Driver out of mothballs and converting it for use with
the lightcraft will cost around $500 000, a small fraction of what it
would cost to build a the laser from scratch. If Myrabo can generate the
funding, it could be ready in less than a year.

This ready availability of materials is part of the project鈥檚 beauty, says
Mead. 鈥淲e鈥檙e not inventing anything new,鈥� he says. 鈥淩ocket propulsion, lasers,
pointing and tracking鈥攊t鈥檚 all there. It鈥檚 just a matter of putting them
into this package.鈥�

Reaching space will require more advanced lightcraft. As the vehicle
accelerates to supersonic and hypersonic velocities, it compresses the air in
front forcing it through the engine inlets
(see diagram). 鈥淰ery little air is
needed so the inlets are tiny,鈥� says Myrabo. Ahead of the craft a shock wave
will build up and this must be kept well away from the inlet since it would
reflect off the inner surfaces in the engine causing havoc. The lightcraft鈥檚
nose is carefully shaped to deflect the shock wave away. 鈥淭his is a complicated
subject but one that is well understood,鈥� says Myrabo.

While this might be true of the current research, Myrabo and Mead will have
to rely on new technology if lightcraft are ever to reach space. For a start,
steering the vehicle will be tricky. Myrabo does not bother with the short
flights he is making now since they are stabilised by the craft鈥檚 spin. But in
the future, he and Mead will have to change the angle of the thrust to steer.
This could be done by changing the position of the laser on the cone at the back
of the craft or by firing microthrusters.

Of course, Myrabo can rely on air as a propellant only for the first few tens
of kilometres of the journey into space while the spacecraft is in the
atmosphere. Beyond that, the spacecraft will have to carry its own propellant
such as liquid hydrogen or nitrogen which will be pumped into the combustion
chamber ready for laser detonation. Myrabo reckons that the a kilogram of
propellant could put a lightcraft in orbit. This would be about equal to the
weight of the empty lightcraft.

Light but strong

Myrabo鈥檚 vision is to use a megawatt laser to put in orbit a spaceship about
a metre in diameter that weighs only 1 kilogram and carries a kilogram of
propellant. Making a vehicle of this size and weight will be difficult. The
trick is to find a material that is light and strong but that can also withstand
the tremendous temperatures and pressures generated in the engine. 鈥淭hey will
need to get away from aluminium,鈥� says Timothy Knowles, president of Energy
Science Laboratories (ESL) in San Diego, California, which has pioneered a
different approach.

Instead of aluminium, ESL has developed a way of producing thin carbon shells
which are strong and heat-resistant. Knowles is tight-lipped about the details
but says the technique involves heating a thin layer of resin until it turns to
a carbon-based film. With the right resin and the correct technique, the
resulting film can be surprisingly strong. Coated with metal, the carbon film
can even act as a mirror inside the engine.

With such a vehicle and at the rates of acceleration that will be possible
with a megawatt laser, Myrabo reckons a lightcraft will need a distance of 800
kilometres to reach orbital velocity. This would require a lift off trajectory
of about 30 degrees to the ground and would lead to an eventual orbit some 200
kilometres above the surface.

The lightcraft would have to carry a new generation of ultralight sensors
into orbit, since no more than 10 per cent of the ship鈥檚 weight can be devoted
to payload鈥攍ess than 100 grams. Myrabo does not see this as a problem.
鈥淓lectronics don鈥檛 weigh very much,鈥� he says. 鈥淲e could easily carry a small
hard drive, some memory, a sensor and a communications package.鈥� Parts of the
spacecraft could even have dual uses. For example, the engine mirror could
double as a telescope, a receiver or a way of focusing signals towards the
ground.

Jonathan Campbell, manager of beamed energy propulsion at NASA鈥檚 Marshall
Space Flight Center, is impressed with the results so far. 鈥淲e鈥檝e proven that
keeping our propulsion system on the ground actually works,鈥� he says. 鈥淭he
promise is that we can put things into orbit more cheaply than with conventional
谤辞肠办别迟谤测.鈥�

Conventional rocketry also began modestly. The first liquid-fueled rockets
were built by the American inventor Robert Goddard in 1926 and many tests were
carried out at Roswell, New Mexico, not far from HELSTF. Although his first
rocket reached a height of only 12.5 metres, Goddard鈥檚 work heralded the space
age. Today, liquid-fuelled rockets ride regularly into space. If Myrabo and Mead
have their way, lightcraft will not be far behind.