Watch a Nasa animation of the X-33

The new vehicles will also be able to undertake far more frequent launches than is possible with current technology.

Officials at Nasa, giving a status report on the experimental X craft, said a series of flight tests on three vehicles would begin in summer of 2000.

Nasa's Gary Payton and Pathfinder project manager John London outline future benefits

The X-33 craft will undergo suborbital flights next summer. Another craft, called the X-34, will undergo engine and structural tests in 2000, leading up to a series of flights. And a third craft, the X-37, would start experimental flights in 2002, once early development work had been completed.

Expendable rockets

Putting a pound (454 grams) into orbit using current rockets or the space shuttle costs $5,000 to $20,000, said Nasa's Gary Payton. The new generation of space launchers would reduce this to about $1,000 a pound.

The X-34 is a technological testbed (Nasa)

He said it was time to look beyond expendable rockets that take weeks to prepare for launch.

"The X-33, X-34 and X-37 will prove in flight the technologies that can remove the cost and reliability barriers to space flight. Instead of five flights in 68 days being a new world record, five flights in 21 days will be routine."

The X programme is partly funded by private industry.

Lockheed are involved in the X-33, which is designed to reach orbit using just one engine not several different stages. It is a 21-metre-long (69 ft), wedge-shaped craft that can take off vertically and then lands like an airplane after a gliding descent from orbit.

Launching from spaceports

The goal of the X-33 program is to build a spacecraft that can be launched every seven days by as few as 50 people from simple spaceports that could be located virtually anywhere. It will start a series of 15 test flights next summer.

Boeing is backing the X-37 project

The X-34 will be used as a test vehicle for much of the technology on the X-33 and later spacecraft. The X-34 is a robot rocket that will be launched at high altitude from a converted airliner. Some tests will involve unpowered drops to a landing, but later the X-34 will rocket up to 76 kilometres (250,000 feet) at eight times the speed of sound before returning to Earth.

The X-37, being built by Boeing, will be carried into orbit by the space shuttle, fly to higher orbits and then land like an airplane. About 40 technologies will be tested in two flights by the craft starting in 2002.

Nasa also has the X-38 in development, which will act as the "lifeboat" for the new International Space Station.

Shuttle launches delayed

Nasa says the dates for the next three space shuttle missions are now unknown because of time-consuming electrical safety checks. The delays mean Discovery's mission to repair the Hubble Space Telescope, due to fly 14 October, may not now go until late October or November.