The US military’s X-37B space plane has blasted off on another secretive mission that is expected to last at least a couple of years.

Like previous missions, the reusable plane resembling a mini space shuttle is carrying classified experiments.

There is no-one on board.

The space plane took off aboard SpaceX’s Falcon Heavy rocket from NASA’s Kennedy Space Center on Thursday night, more than two weeks late because of technical issues.

It marked the seventh flight of an X-37B, which has logged more than 10 years in orbit since its debut in 2010.

The last flight, the longest one yet, lasted two and a half years before ending on a runway at Kennedy a year ago.

Space Force officials would not say how long this orbital test vehicle would remain aloft or what was on board other than a NASA experiment to gauge the effects of radiation on materials.

Built by Boeing, the X-37B resembles NASA’s retired space shuttles.

But they are just one-fourth the size at 9 metres long.

No astronauts are needed; the X-37B has an autonomous landing system.

They take off vertically like rockets but land horizontally like planes, and are designed to orbit between 240 kilometres and 800 kilometres high.

There are two X-37Bs based in a former shuttle hangar at Kennedy.

AP