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Count down.

For the half-plane, half-missile X-15, the fantastic rocket-powered aircraft, the X-15 is designed to be blasted to a speed of 4,000 miles an hour, in just 90 seconds before its rocket fuel burns out.

It's 1960 and Richard Dimbleby is commentating on a test flight of the hypersonic X-15 at five times the speed of sound.

These were a series of flights in the early days of the US Maned Space Program,

but it wasn't just about getting into space.

There were also ambitions to usher in an era of hypersonic civil aviation, getting passengers

to the other side of the world in just a few hours.

Fifty years on, we can put people on the moon, but we can't fly from London to Sydney in four hours.

I'm Gareth Mitchell and in this edition of Discovery from the BBC World Service, I'm asking

why hypersonic flight is so hard. Paul Bruce is an aerodynamics

expert in the aeronautics department at Imperial College London. He says that physics

starts doing some very strange things when you go beyond the sound barrier.

Mac number is the key, and that's the ratio of the speed something moves to the speed of sound in air.

When you go below Mac one, so flying slower than the speed of sound and then

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