Researchers at NASA are keeping the dream of the space frontier alive with advanced spacecraft designs that might eventually make space travel as common as air travel.

The Advanced Space Transportation Program develops futuristic projects -- some still a vague idea or merely an artist's rendering -- to chart the course of human spaceflight.

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Ideas germinating at Marshall Space Flight Center in Huntsville, Alabama range from the fantastic, such as antimatter propulsion, to the actual lab tests of air-breathing rocket engines and magnetic-rail launching systems.

The path won't be easy though. Many of the projects present daunting technological challenges, along with the search for funding.

During a recent experimental spacecraft symposium, Marshall's Pathfinder program manager, John London, summed up the difference between looking at a slide projected on a wall and actually building a new spacecraft.

"View graphs are easy, flight hardware in the air and space is tough," London said. "It's never as easy as it seems it's going to be."

While the space-shuttle fleet is likely to fly another 25 years and possibly beyond the 100-flight life expectancy for each orbiter, the space agency has developed milestones for developing future spacecraft.

Currently, we're in what NASA calls the first of four generations of reusable launch vehicles (RLV). First generation is based mainly on the stalwart of RLVs -- the space shuttle.

With each generation of spacecraft, the space agency hopes to make access to space cheaper, safer and more reliable.

The next three generations and their goals are:

• Increase safety of launch so that a loss of crew is one in 10,000.
• Reduce launch costs from $10,000 per pound to about $1,000.
• RLVs such as the VentureStar or another in development will supplement the shuttle fleet.

Third generation - 2025

• Reduce chance of crew loss at launch to one in 1 million.
• Reduce launch costs to hundreds of dollars per pound.
• Spacecraft are likely to be something like the Spaceliner 100.

• No difference between a spacecraft and commercial airliner
• Spacecraft are so safe that an escape system is not necessary.
• Advanced propulsion, including lasers, electric, antimatter and plasma

The majority of weight carried aboard any current spacecraft is the propellant and oxidizer needed to lift the craft up and out of Earth's atmosphere. Less weight means more payload or boosting the same payload into a higher orbit.

A Marshall Space Flight Center concept spacecraft of an air-breathing cargo ship.

Air-breathing rockets suck in oxygen from the atmosphere and therefore do not need to carry oxidizer. Marshall revived 1960s-era research in 1996 to investigate the possibility of these engines powering third-generation RLVs.

At liftoff, the air-breather rocket engine gets its initial flow of oxygen from specially designed rockets inside the craft's intake ducts. At twice the speed of sound, the rocket switches to atmospheric oxygen to burn the hydrogen fuel propelling the craft.

In May, engineers at the Rocketdyne plant in California conducted a ground-based test of an air-breathing engine for one hour

If the auto manufacturers can have "concept" cars, why can't NASA have concept spacecraft?

When 2025 rolls around, the third-generation RLV probably won't look like Spaceliner 100, but will likely incorporate the ideas and technology developed for the craft.

Though the current iteration looks like the über spy plane known as the SR 71 Blackbird, the similarities stop there.

The Spaceliner 100: Flying the friendly skies of 2025

This third-generation RLV takes off like a plane, mostly likely on a rail system, and is powered by air-breathing rockets and ramjets. Improved thermal tiles and coatings require little maintenance and are not affected by weather conditions such as rain.

Like a commercial airliner, the Spaceliner 100 operates from a "spaceport" where it picks up passengers and cargo, is maintained, launched and readied for another flight within hours of landing.

NASA's goal is to reduce launch costs to $100 per pound, making it a tempting choice for business travelers and people wanting the vacation of a lifetime.

Unlike all human spaceflight vehicles to date, Spaceliner 100 and other third-generation craft probably won't lift off vertically. One idea under development is using a magnetic-levitation rail system to fling the craft into space much like in the classic 1950s movie When Worlds Collide.

The rail system offers numerous safety and operational advantages. The track system uses rows of magnets to levitate the spacecraft and move it rapidly down the line. The track has no moving parts, uses electricity for power and offers a chance to abort the flight if problems develop.

Magnetic levitation tracks may someday replace the traditional vertical launch pad

A full-scale track proposed by Marshall to be built and operated at Kennedy Space Center would be about 1.5 miles (2.4 kilometers) long. It would accelerate a spacecraft to about 2 Gs for 9.3 seconds and reach speeds of 400 to 600 m.p.h. (640 to 960 kilometers per hour).

Researchers at Marshall are already testing 50-foot (15-meter) and 400-foot (120-meter) tracks at the center.

What does the future hold? It gets tough to predict, but researchers at Marshall say by then space travel will probably be as commonplace as airline travel is today.

The current bane of NASA's existence - funding - may stall or prevent a lot of these projects from leaving the drawing board. With enough time and corporate partners, this technology and the spacecraft spawned from it could become reality.

More exotic drives using electric, solar, plasma, ion and antimatter propulsion may have come to fruition by 2040 for long-duration spaceflight and the beginnings of interstellar travel.

One "concept" craft looks amazingly familiar to anyone who's seen a flying-saucer movie. In this case, microwaves beamed from a satellite power the flying disc.

Stay tuned. Hollywood may have gotten it right after all.