How Light Propulsion Will Work

How Light Propulsion Will Work

More than 20 years ago, the United States began to develop a missile defense system that was given the nickname "Star Wars." This system was designed to track and use lasers to shoot down missiles launched by foreign countries. While this system was designed for war, researchers have found many other uses for these high-powered lasers. In fact, lasers could one day be used to propel spacecraft into orbit and to other planets.

An early model of a laser-propelled lightcraft

To reach space, we currently use the space shuttle, which has to carry tons of fuel and have two massive rocket boosters strapped to it to lift off the ground. Lasers would allow engineers to develop lighter spacecraft that wouldn't need an onboard energy source. The lightcraft vehicle itself would act as the engine, and light -- one of the universe's most abundant power sources -- would be the fuel.

A lightcraft in action. The bright light you see is the air combusting under the rim of the craft.

The basic idea behind light propulsion is the use of ground-based lasers to heat air to the point that it explodes, propelling the spacecraft forward. If it works, light propulsion will be thousands of times lighter and more efficient than chemical rocket engines, and will produce zero pollution. In this edition of How Stuff WILL Work, we'll take a look at two versions of this advanced propulsion system -- one may take us from the Earth to the moon in just five and a half hours, and the other could take us on a tour of the solar system on "highways of light."

Laser-propelled Lightcraft

Light-propelled rockets sound like something out of science fiction -- spacecraft that ride on a laser beam into space, require little or no onboard propellant and create no pollution. Sounds pretty far-fetched, considering we haven't been able to develop anything close to that for conventional ground- or air-travel on Earth. But while it may still be 15 to 30 years away, the principles behind the lightcraft have already been successfully tested several times. A company called Lightcraft Technologies continues to refine the research that began at Rensselaer Polytechnic Institute in Troy, N.Y.

Photo courtesy Rensselaer As the laser pulses, it superheats the air until it combusts. Each time the air combusts, it creates a flash of light, as seen in this photo of a test flight.

The basic idea for the lightcraft is simple -- the acorn-shaped craft uses mirrors to receive and focus the incoming laser beam to heat air, which explodes to propel the craft. Here's a look at the basic components of this revolutionary propulsion system:

• Carbon-dioxide laser - Lightcraft Technologies uses a Pulsed Laser Vulnerability Test System (PLVTS), an offspring of the Star Wars defense program. The 10 kw pulsed laser being used for the experimental lightcraft is among the most powerful in the world.
• Parabolic mirror - The bottom of the spacecraft is a mirror that the laser beam into the engine air or onboard propellant. A secondary, ground-based transmitter, telescope-like mirror is used to direct the laser beam onto the lightcraft.
• Absorption chamber - The inlet air is directed into this chamber where it is heated by the beam, expands and propels the lightcraft.
• Onboard hydrogen - A small amount of hydrogen propellant is needed for rocket thrust when the atmosphere is too thin to provide enough air.

Prior to liftoff, a jet of compressed air is used to spin the lightcraft to about 10,000 revolutions per minute (RPMs). The spin is needed to stabilize the craft gyroscopically. Think about football: a quarterback applies spin when passing a football to throw a more accurate pass. When spin is applied to this extremely lightweight craft, it allows the craft to cut through the air with more stability. Once the lightcraft is spinning at an optimal speed, the laser is turned on, blasting the lightcraft into the air. The 10-kilowatt laser pulses at a rate of 25-28 times per second. By pulsing, the laser continues to push the craft upward. The light beam is focused by the parabolic mirror on the bottom of the lightcraft, which heats the air to between 18,000 and 54,000 degrees Fahrenheit (9,982 and 29,982 degrees Celsius) -- that's several times hotter than the surface of the sun. When you heat air to these high temperatures, it is converted to a plasma state -- this plasma then explodes to propel the craft upward.

Lightcraft Technologies, Inc., with FINDS sponsorship -- earlier flights were funded by NASA and the U.S. Air Force -- has tested a small prototype lightcraft several times at the White Sands Missile Range in New Mexico. In October 2000, the miniature lightcraft, which has a diameter of 4.8 inches (12.2 cm) and weighs only 1.76 ounces (50 grams), achieved an altitude of 233 feet (71 meters). Sometime in 2001, Lightcraft Technologies hopes to send the lightcraft prototype up to an altitude of about 500 feet. A 1-megawatt laser will be needed to put a one-kilogram satellite in low earth orbit. Although the model is made of aircraft-grade aluminum, the final, full-size lightcraft will probably be built out of silicon carbide.

This laser lightcraft could also use mirrors, located in the craft, to project some of the beamed energy ahead of the ship. The heat from the laser beam would create an air spike that would divert some of the air past the ship, thus decreasing drag and reducing the amount of heat absorbed by the lightcraft.

Microwave-propelled Lightcraft

Another propulsion system being considered for a different class of lightcraft involves the use of microwaves. Microwave energy is cheaper than laser energy, and easier to scale to higher powers, but it would require a ship that has a larger diameter. Lightcrafts being designed for this propulsion would look more like flying saucers (now we're really heading into the realm of science fiction). This technology will take more years to develop than the laser-propelled lightcraft, but it could take us to the outer planets. Developers also envision thousands of these lightcraft, powered by a fleet of orbiting power stations, that will replace conventional airline travel.

Photo courtesy NASA Microwave-powered lightcraft will rely on orbiting power stations.

A microwave-powered lightcraft will also utilize a power source that is not integrated into the ship. With the laser-powered propulsion system, the power source is ground-based. The microwave propulsion system will flip that around. The microwave-propelled spacecraft will rely on power beamed down from orbiting, solar power stations. Instead of being propelled away from its energy source, the energy source will draw the lightcraft in.

Before this microwave lightcraft can fly, scientists will have to put into orbit a solar power station with a diameter of 1 kilometer (0.62 miles). Leik Myrabo, who leads the lightcraft research, believes that such a power station could generate up to 20 gigawatts of power. Orbiting 310 miles (500 km) above Earth, this power station would beam down microwave energy to a 66-foot (20-meter), disk-shaped lightcraft that would be capable of carrying 12 people. Millions of tiny antennae covering the top of the craft would convert the microwaves into electricity. In just two orbits, the power station would be able to collect 1,800 gigajoules of energy and beam down 4.3 gigawatts of power to the lightcraft for the ride to orbit.

The microwave lightcraft would be equipped with two powerful magnets and three types of propulsion engines. Solar cells, covering the top of the ship, would be used by the lightcraft at launch to produce electricity. The electricity would then ionize the air and propel the craft for picking up passengers. Once it's launched, the microwave lightcraft used its internal reflector to heat the air around it and push through the sound barrier.

Once in a high altitude, it would tilt sideways for hypersonic speeds. Half of the microwave power could then be reflected in front of the ship to heat the air and create an air spike, allowing the ship to cut through the air at up to 25 times the speed of sound and fly into orbit. The craft's top speed peaks at around 50 times the speed of sound. The other half of the microwave power is converted into electricity by the craft's receiving antennae, and used to energize its two electromagnetic engines. These engines then accelerate the slip stream, or the air flowing around the craft. By accelerating the slip stream the craft is able to cancel out any sonic boom, which makes the lightcraft completely silent at supersonic speeds.