The ultimate free ride is, maybe, three years away

After centuries of playing around with rockets, man finally seems about to embark on what could be the ultimate free ride - sailing through space using sunbeams for fuel.

His ''ships'' will be giant sails of thin, aluminized plastic film that deflect the pressure of light beams. The result: a giant, glittery kite as large as a football field spiraling around the solar system.

The idea of solar sailing is not new. The US National Aeronautics and Space Administration (NASA) rejected such a proposal in 1977. But sufficient research has now been carried out to convince supporters, many of them federal engineers, that solar flight is indeed possible, if not imperative to further interplanetary exploration.

After NASA rejected the solar sail in favor of other types of advanced propulsion, its future fell into private hands, specifically the World Space Foundation (WSF). The nonprofit, Pasadena-based corporation is tapping industry and foundation support.

Already one on-ground deployment test has been conducted and a second is scheduled for later this year.

While engineers in France and Czechoslovakia are working on similar space sailing theories - the French even challenging the United States group to a moon race - so far no one but the WSF has any flight-ready craft.

By 1986 the US group hopes to launch the first sailing craft 1,000 miles out into Earth's orbit. Success of that mission could spawn further involvement by private industry as well as a voyage to the moon - a relaxed trip of more than two years' duration - that supporters say is just the tip of solar sailing's potential as low-cost interplanetary transportation.

''Solar sailing is very sound engineering; success of the World Space Foundation's test flight could very well demonstrate its future feasibility,'' says Dr. Louis Friedman, a former NASA engineer and now executive director of the Planetary Society, a public-interest organization also based here.

Architects of the solar sail project say the significance of solar-powered lightships lies in their ability to outperform both traditional rockets and satellites. While chemically propelled rockets have a short life span and satellites are restricted in their orbital movements, solar sails surpass both obstacles with ease. By attaining high speeds, albeit over longer periods of time, solar-powered craft could conceivably travel for years, moving from one orbit to another. By using no fuel except sunlight, the solar sail will outlast even high-performance electric rockets, supporters say.

''The whole advantage of the sail is that you don't have to take propellant along, and you can move from one orbit to another,'' explains Chauncey Uphoff of NASA's Jet Propulsion Laboratory (JPL) in Pasadena. ''With any type of rocket propulsion, once the fuel is spent, it's done. If you have a sail and the sun is providing momentum, as long as there is light you will accelerate. It may be slow, but it's persistent and it does keep on going.''

Supporters say such a space transportation system not only will be lower in cost than current methods, thus encouraging private industry's space exploration efforts - estimates for solar sail construction run into the millions, but savings estimates run to the billions - but also will be reusable.

Enthusiasts envision solar-powered lightships hauling cargo, collecting soil samples from asteroids and planets, even carrrying freight to Mars for use by future astronauts.

Robert Staehle, a JPL engineer and WSF president, describes the solar sail's significance in terms of pure frontier development.

''Solar sails use what space has plenty of - light and vacuum - to create a low-cost, reusable space transportation,'' Mr. Staehle says. ''The American West wasn't developed until private industry got involved and utilized fuels that were available in that frontier.''

Wood-burning locomotives chugging across the West and clipper ships sailing the Atlantic are the ideological forerunners of solar sailing, according to Staehle.

Yet the sails of space have advantages that seagoing ships never had. At sea, fickle winds start, stop, and constantly shift, and a boat hull fights the constant ''drag,'' or friction, of the water. In space, there is no drag on the sail. Estimates of solar sail speeds reach into the hundreds of thousands of miles per hour. Slower off the starting block than typical rockets (and initially needing rocket propulsion to break from Earth's atmosphere), solar sails can eventually surpass rocket speeds - speeds essential to breaking free of Earth's orbit and for interplanetary voyages of long duration.

''When you burn a rocket, the amount of propellant it takes to add a certain amount of speed goes up exponentially with the amount of speed you want to add, '' explains Mr. Uphoff.

In other words, a one-ton rocket can carry a one-ton cargo to a kilometer (five-eighths of a mile) per second in a few minutes. But it will eventually coast when its fuel runs out. A two-ton rocket cannot double the speed of the craft, because the extra fuel weight would burden it.

Conversely, a solar sail would take several days to attain such speeds, but the acceleration would be constant. After a few years, a large solar sail would be adding speed at a rate of 86 kilometers every second. A chemical rocket would require almost the mass of the earth to achieve that much acceleration, according to WSF engineers.

When compared with Earth-bound aerodynamic principles, it seems incongruous that a spacecraft several miles wide and less than a hair's-breadth thick can achieve supersonic speeds. Yet, the principle behind the venture is as basic as electromagnetism - a theory evinced by Scottish physicist James Clerk Maxwell in the late 1800s indicating that energy, including light energy, exerted a pressure much like that of matter.

When that law is linked with the planetary momentum theories of Sir Isaac Newton, the sun-powered mirror concepts of two Soviet engineers, and the shiny liner of potato chip bags known as Mylar, the result is the solar sail.

Even before the turn of the century, it was common knowledge among scientists that light, as a type of electromagnetic radiation, exerted a force. Later, Albert Einstein demonstrated that matter and energy were equivalent, indicating that a flow of photons (the smallest units of light) could exert pressure similar to the flow of air molecules against a sail. For example, the force of sunlight on a football-size mirror exerts the approximate weight of a marble.

By the 1920s two Russian engineers, Konstantin Tsiolkovsky and Fridrikh Tsander, published the first known descriptions of such a sun-powered mirror. While the effect of light on a mirror is obliterated on Earth's surface, in the vacuum of space the force of sunlight beating on giant reflectors made of Mylar-like material could send these solar panels on voyages to the moon, comets , and beyond - at rocketlike speeds.

US space officials remain less than sanguine about the proposed flight. Jerome Millin, manager of the NASA Space Energy Conversion Office, says the agency chose to support electrical propulsion rockets as ''less costly and more flexible'' than the ''large, cumbersome, passive'' solar sail.

But supporters here point to three new technologies that boost the solar sail's probable success: improved fabric; deployment; and attitude, or steering, control.

''The first two we have demonstrated with on-ground deployment of a half-size prototype,'' explains Staehle. ''The third technology, the steering mechanism - we have done enough theoretical analysis to have a pretty good idea of how that will work in space.''

The flight sail, which will require nearly $2 million and six to eight months to construct, will consist of a series of aluminum-coated plastic film panels seamed together and trimmed into a rough kite-shape, approximately 100 feet on a side. Each panel, thinner than a human hair and 15 inches wide, produces a lightweight but extraordinarily tough surface that resists both puncturing and tearing.

The main sail ingredient is Kapton, a Du Pont polymer similar to Mylar, but far more resilient to the heavy levels of radiation found in space.

Experts estimate that a Kapton solar sail could survive 20 years of space travel before requiring refurbishing or repairs. The unique panel construction limits damage from potential holes in the sail - possibly caused by a meteorite or other debris whizzing through it.

''It's a lot like a canvas sail,'' says project director Mark Bergam. ''If you shot a bullet through a regular sail, it would still deflect wind. We think the solar sail will work much the same way.''

Of greater concern to project members has been the deployment, or unfurling, of the sail. While no space test has yet taken place, a half-size sail was successfully deployed on the ground last year.

During in-flight deployment, the sail, carefully folded and wrapped, will be housed in a giant, tablet-shaped capsule roughly three feet high and six feet in diameter and weighing 600 pounds. The craft, jettisoned from an already orbiting spacecraft such as the shuttle, will move into a higher orbit - the sail must be at least 1,000 miles above the surface of the earth to escape the atmospheric pull - where it will blow off its metal sail covers and slowly unfold in about three hours. Two metal spars, each about 70 feet long and coiled inside the capsule like a carpenter's rule, slowly unwind by means of a tiny motor extending the folded sail. Then, like a plastic rain hat that unfolds when pulled crosswise, the sail unpleats fully when two additional spars drive out perpendicular to the first two.

Even at 1,000 miles above Earth, the sail, like Skylab, would be as bright as Venus and visible to the naked eye, according to Uphoff.

By far the most unknown aspect of the project is the steering - a procedure that, despite computer modeling, cannot be fully tested until in flight. Yet the basic premise remains similar to that of an airplane wing: Manipulating less than 10 percent of the surface reorients the entire craft. In the case of the solar sail, the steering flap is a pair of triangular-shaped vanes at the tips of the sail and a small mast in the center of the craft. When all three devices are maneuvered by an onboard computer monitored via radio from the ground, the sail will rotate much like a pinwheel.

''By a combination of maneuvers, we can get the sail into any orientation we need in respect to the sun,'' says Staehle. Bergam adds: ''With a chemically propelled rocket, you get almost an instantaneous reaction. But the push on a solar sail is very gentle, so the reorientation of the vanes must be gentle. Turning 40 degrees from start to stop takes something like 20 minutes. But we've got time to make these maneuvers. Each orbit around the earth takes about 11 hours.''

The sail, like any other spacecraft, moves only in an orbital pattern, controlled by the gravity of a particular planet. To move from one orbit to another, the craft must begin to spiral, using sunlight and gravity as its ''wind'' and ''keel'' to push away from the planet.

''The effect of gravity on the sail is dominant to the effect of the solar pressure on the sail,'' Mr. Bergam explains. ''What we'll be trying to do is take a little bit of that solar pressure, and a little bit at a time work against the gravity, changing our position."

Because the strength of solar pressure depend's upon the sail's proximity to the sun, some observers have voiced caution over solar sailing's limited effectiveness for intestellar space travel. Even project engineers admit that light-powered vehicles will be most successful when exploring comets, asteroids, and those planets nearest the sun. Indeed, the original NASA plan proposed launching a solar craft specifically to rendezvous with Hally's Comet.

Nevertheless, some scientists perceive ever-widening horizons for solar sails.

One engineer has proposed using giant lasers in space to further the sail's reach. Others have explored the possbility of superlightweight sails--some actually constructed in space of pure aluminum molecules, others punctured with a plethora of microscopic holes, but all with the hope of sending the giant kites spiraling on and out beyond even the reaches of Pluto.