One year ago, SpaceX chief executive Elon Musk stood up in Washington DC's National Press Club and formally declared his intention to build a fully reusable launch vehicle topped by a fully reusable capsule.

By that time it was clear from documents filed with the US Federal Aviation Administration that a vehicle called Grasshopper - essentially a Falcon 9 fuel tank system with a Merlin 1D engine and struts - would serve as a technology testbed for a reusable first stage.

Grasshopper has now made its first hop - all 6ft - after dozens of engine burns at SpaceX's McGregor, Texas facility.

The cargo version of the reusable Dragon capsule has made a first dock with the International Space Station and successful returned to Earth, evidently unscathed. Two of the three reusable stages have reached flight test which, while no guarantee of success, does lend credence to Musk's vision.

The missing piece of the puzzle is also the hardest: a reusable second stage. Because of speed and re-entry heat, second stages are not usually considered reusable. The level of shielding necessary - they re-enter at, or near, orbital space - does not come cheap.

The only reusable second stage to date is the recently-retired Space Shuttle, really a satellite that powered itself into orbit.

While reusable, the Shuttle required extensive and expensive refurbishment after each flight, at odds with Musks' goal to reduce launch costs.

No reusable first stages using retropropulsion have ever powered an object into space, much less economically. Several testbeds exist, notably owned by Masten Space Systems and Armadillo Aerospace. Only the McDonnell Douglas DC-X, a prototype of a vertical-landing orbital launch vehicle, and Blue Origin's New Shepherd can lay claim to practical flight tests of an orbital launch vehicle using retropropulsion.

DC-X was cancelled not long into its flight-test regime, and Blue Origin lost its test vehicle as it rocketed upwards of 40,000ft (12,200m) at Mach 1.0.

Certainly retropropulsion is not technically impossible, but it is very difficult, as Masten and Armadillo will attest having both lost test vehicles in recent months. The big challenge is in making it economical. While Musk has both the drive and development money, a large part of making reusability economical is high launch rates.

One year ago, Musk said: "If you look at the cost of the Falcon 9 rocket," - $54 million, according to the SpaceX website - "the cost of the fuel and oxygen and so forth is only about $200,000. So obviously if we can reuse the rocket, say a thousand times, then that would make the capital cost of the rocket launch only about $50,000."

The assumption of 1,000 launches for a reusable rocket is very, very optimistic. The most made by any reusable craft was Space Shuttle Discovery, which made 39 flights in 27 years.

SpaceX is nothing if not ambitious, and while it has made stunning progress on many of its programmes, it has not always done so in its advertised timeline.

Source: Flight International