Skeptics once scoffed at the potential for human flight. Naysayers didn’t think early automobiles could make it across the United States. Which futuristic mode of transportation will revolutionize the world in decades to come? Wild idea contenders include vacuum tubes, underwater planes, space elevators and bicycles in the sky.

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John Hansman is an MIT professor of aeronautics and astronautics, heads the Institute’s Division of Humans and Automation and directs the International Center for Air Transportation. He’s climbed ice, flown planes, driven a high performance racecar, sailed boats and flown on numerous parabolic flights simulating zero gravity. Speaking while driving through New England, he offered his take on well-known futuristic transportation ideas.

A space elevator would likely use advanced cables based on nanotechnology to carry a car from Earth to a satellite or even the Moon.Bruce Irving

“Next floor, outer space” has long been the dream for space elevator proponents. Numerous space elevator concepts have been floated over the years. Recent contenders include Japanese construction company Obayashi Corp’s plan to drop cables from a spaceship in 2050 and LiftPort’s project to send smart climbing robots up to a high-altitude test platform. But the general idea is to create a structure that can take humans directly to space.

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“There are some concepts of what we call tethered satellites and there are some interesting things you can do with the orbital dynamics by building tethers and expanding them,” Hansman said. But even a tethered satellite would still use lots of energy. “There’s no free ride to space,” Hansman added.

Juliet Marine Systems' GHOST ship uses the principal of supercavitation, which involves surrounding an object with a bubble of gas, to pass stealthily through the water with little friction.Courtesy Juliet Marine Systems

The phenomenon known as supercavitation is best known in the military world. Cavitation -- an air or gas bubble forming around an object that’s moving underwater -- can be useful when those bubbles fall away from a vehicle. The drag on a ship or underwater vehicle gets greatly reduced.

“In some cases they do this by blowing bubbles and having the bubbles go below the surface,” Hansman said.

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Last year New Hampshire-based defense tech company Juliet Marine Systems developed a supercavitating watercraft called the Ghost marine platform combining stealth fighter aircraft and attack helicopter technologies for tracking targets above and below the water’s surface. Moving efficiently through the water still ends up being like moving through the air.

“It’s advanced for a boat or submarine,” Hansman said, “but probably not as fast as airplanes.”

A vacuum tube would involve an airless tunnel, which allows trains inside to go very fast.ET3

Vacuum tubes have been eyed to transport humans ever since American engineer Robert Goddard began a train that could travel through an evacuated tube around 1909. The RAND Corporation picked up the baton in the 1970s with its proposal for a very high speed transit or VHST system. Vacuum tubes are appealing because without air resistance, trains can go quite fast. The problem, as John Hansman pointed out, is that requires a lot of energy.

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“It’s very hard to have a tube that’s perfectly sealed that doesn’t have any leaks,” he said. If the system is built to anticipate leaks, that means making a system that constantly pumps the air out. And that costs money.

Elon Musk's Hyperloop concept is a solar-powered, city-to-city elevated transit system that would take passengers and cars from Los Angeles to San Francisco in 30 minutes.Courtesy Elon Musk

Entrepreneur Elon Musk, known for heading up SpaceX and Tesla Motors, first mentioned his Hyperloop transit concept last year. Since then it’s been shrouded in mystery, known only as “a cross between a Concorde and a railgun and an air hockey table.” On Aug. 12, Musk announced that the Hyperloop system would involve steel tubes containing aluminum pods that are mounted on skis made of a sturdy alloy called inconel. The skis get moved by air that's compressed and routed to them. Magnets will help give them an initial thrust.

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Hansman said a continuous flow loop of air that can take vehicles along a track is physically possible, but he doubted that building one made sense economically or from an energy standpoint. For his part, Hansman sees himself getting around in a super-efficient car, plane or even just a bike 10 or 20 years down the road. If we still have roads.

A magnetic launcher is a maglev system that propels vehicles at high speeds to shoot them out of the atmosphere.NASA

Picture a magnetic levitation system that’s so powerful it could shoot objects along a track and out of the atmosphere. American engineer Keith Lofstrom began proposing such a launch loop in the 1980s, and the concept has been picked up by others. Whether a rollercoaster-style cable system or a cannonball-type of launch, the common thread is getting high enough speeds to reach orbital velocity.

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That sounds like a nice concept but Hansman said he doesn’t think it really works. The reason spacecraft can go so fast is that they’re moving right up and out of the atmosphere. Employing a magnetic levitation or maglev system to accelerate a vehicle to 25,000 miles an hour would likely cause melting. “The vehicle,” Hansman said, “would burn up on the acceleration.”

Workers install solar panels over the tunnel that will power the train to run between Paris and Amsterdam.BRUNO FAHY/AFP/Getty Images

Back in 2009, an Arizona-based company called Solar Bullet LLC proposed a solar-powered bullet train that could go from Tucson to Phoenix in about half an hour. Today the group has turned into more of a think tank but it continues to campaign for the idea, which would involve building a solar array along the train’s route.

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In 2011, two miles along a European train route from Paris to Amsterdam got 16,000 solar panels to help power the Antwerp Central Station and give a little boost to the trains. Hansman pointed out that while solar energy is good, it’s currently not very concentrated. “In order to run a train, you have to collect solar energy from -- it depends where you are on the Earth -- an area the size of a county or two counties,” he said.

The Burevestnik 24 amphibian aircraft skims just about the surface of land or water.Bobylev Sergey/ITAR-TASS Photo/Corbis

Transportation systems have been seeking to incorporate an interesting aeronautical effect for decades. Known as “wing in ground,” the effect comes from the fact that wings become more efficient when they’re close to the ground. A number of organizations have proposed concepts with aircraft that never fly higher than 10 or 15 feet, Hansman said.

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That includes Boeing’s proposal for a large-capacity transport aircraft called the Pelican and Russian vehicles like the recent Burevestnik-24. This ground effect aircraft can skim over water and ice, and in the air reaches speeds up to 150 miles per hour. Such systems could potentially transport people and cargo in all kinds of tough conditions. “It’s not particularly fast but it would be great for carrying very, very big payloads,” Hansman added.

The Transition aircraft is a legal street machine that also flies.Terrafugia

“We’d all like to have George Jetson cars, right?” Hansman said. The idea is both seriously appealing and seriously challenging, though. Tackling that straight on is the Massachusetts company Terrafugia, which created a street legal aircraft called the Transition and announced the concept for a more automated hybrid version known as TF-X earlier this year.

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Other aircraft mashups include the EPFL Clip-Air concept for a flying train and the Horizon Project’s concept for an all-electric drone launched from maglev rails. “It’s hard to design something that’s both a good car and a good airplane,” Hansman cautioned. “If it were easy, we’d all be driving flying cars.” Still, he does think that in the next 20 years it will be much easier to buy a personal airplane where you can get in, press a button and the vehicle will fly you where you want to go.

In an Intelligent Transportation System, driverless cars tag along behind leading trucks. SARTRE

Road trains are a type of intelligent transportation system where cars and trucks join a line on the highway and get automatically driven in that chain until they need to turn off for various destinations. Currently the approach is being led by the European Safe Road Trains for the Environment project known as SARTRE for short.

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“When the highways get very crowded, if you can run the cars together automatically, you can get them closer together and safer,” Hansman said. Vehicles, just like stock car and bike racers, get more efficient when one is close behind another. “But you’ve got to make it work perfectly because if it screws up you’ve got a car crash.”

A human-powered monorail could serve future commuters best.Shweeb

New Zealand-based Shweeb first revealed the concept for a human monorail several years ago: a monorail infrastructure where travelers pedal recumbent bicycle-style vehicles along tracks. Designer and inventor Geoff Barnett built a prototype in Rotorua, New Zealand, where it functions as a fun velodrome racetrack and in 2010 Google awarded the company $1 million for R&D.

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This summer an adventure park opened in Mumbai that features a Shweeb monorail. Hansman considers human monorails fairly realistic compared to the others in this list. “I actually think the idea of more creative bicycle and human-powered things are going to make a lot of sense, particularly in cities that get more crowded,” he said.