BBC Science's Richard Hollingham considers X-34's rivals

The X-34 demonstrator is another step in Nasa's pursuit of cheaper access to space. The key is being able to re-use vehicles over and over.

When completed, the X-34 may be used to launch satellites but its main purpose is to test technologies that will be used in future, manned spacecraft, such as the X-33.

Testing composite materials is part of X-34's purpose

The X-34 is a single-engine rocket plane which will fly itself using onboard computers. It is much smaller the current Space Shuttle, measuring 18 metres (58 feet) long and 8m (28ft) wide.

The X-34 will launch from an airliner and travel up to eight times faster than the speed of sound, reaching altitudes of up to 76 kilometres (47 miles).

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The first tests after the rollout will simply involve the airliner carrying the X-34 mounted underneath. This is to allow the Federal Aviation Administration to approve the required modifications to the L-1011 airliner.

The larger X-33 will benefit from technology developed on the X-34

The next stage will be powered flights for the X-34. In these, the demonstrator will be carried aloft before igniting its rocket engine. Following the powered portion of flight, the unpiloted X-34 will land horizontally, initially on a dry lakebed and eventually on a runway.

There are 27 flights planned for the first year and these will aim to demonstrate the X-34's ability to fly again within 24 hours of its last mission and its ability to fly in bad weather.