Paris (CNN) -- It turns out supersonic aircraft are just like buses: You wait years for one, and then two come along at once.

Almost eight years after Concorde touched down for the last time, plane manufacturers and designers at the Paris Air Show have unveiled rival "son of supersonic" concepts.

The first, aircraft manufacturer EADS's Zero Emission Hyper Sonic Transport (ZEHST), will be powered by biofuel made from seaweed, carrying passengers above the Earth's atmosphere and dramatically cutting flight times.

EADS says the plane's three engine types -- conventional jets, rockets and supersonic "ramjets" -- will allow it to speed at Mach 4, around 5,000 kmh.

That would cut the journey time from Paris to Tokyo from its current 11 hours to less than 2.5 hours.

There is a catch: The plane is not due to take-off until 2050.

The second, SonicStar, is billed by its creators Hyper Mach as "the future in flight." It will fly at twice the speed of Concorde, linking New York and Dubai in two hours, 20 minutes -- about the time it would take to watch just one in-flight movie -- and allow passengers to fly around the globe in under five hours.

Hyper Mach CEO Richard Lugg said the plane would make "the other side of the world feel like it's just down the road."

Lugg told reporters he had been inspired as a youngster watching the maiden voyage of Concorde, and had made it his "life's work" to come up with the next generation of hypersonic aircraft.

Lugg says the jet will reach speeds of up to Mach 3.6 -- twice the speed of Concorde -- and fly at 18,300 meters, high enough for passengers to see the curvature of the earth.

It also promises reduced emissions and low noise. New technology means it will not create the "sonic boom" its predecessor was known for.

"This is being done with an eye to the future, but it has its feet firmly rooted in solid scientific research," Lugg insists.

And importantly, he also believes it will be ready long before ZEHST: Hyper Mach is aiming to get SonicStar airborne by June 2021.

Like Concorde, though, flying on SonicStar will be out of the reach of most holidaymakers and business travelers: Lugg says it will seat just 20 passengers in "VVIP luxury accommodation."

But experts have sounded a note of caution: These are not the first technologically complex concept planes -- and they may not be the last to end their days stuck on the drawing board.

"Good luck to them, but I'm very skeptical," said Murdo Morrison, editor of Flight International magazine.

He added: "The costs of designing something like this from scratch are astronomical, and even if they can get it to the prototype stage, that's not even half the battle, it's maybe 10%. Aerospace is littered with companies that went bust once they went into production.

"The science exists, we know planes can fly at supersonic speeds: Fighter aircraft do it, Concorde did it -- the technology is there, but the problem is making it work commercially.

"If it was easy, if it was possible, one of the big manufacturers would have done it already."