EVERY decade or so, enthusiasts dream up plans for yet another spaceplane—a craft that can take off from a runway like an aeroplane, fly up into orbit to deliver a payload, and then glide back to Earth for a runway landing. And then be ready to do the same again within a day or two. Nothing has ever come of them. Even so, the dream persists. What is not to like about a workhorse launch vehicle that jettisons none of its parts while climbing up to orbit and returns to Earth intact, ready to be refueled for yet another flight? Unfortunately, the complexity and weight inherent in such designs has always rendered them impractical. Simple, multi-stage rockets with expendable boosters and fuel tanks remain the cheapest way of putting satellites into orbit, or ferrying crew and cargo to a space station. Complexity extracts a particularly heavy toll. Towards the end of their life, NASA’s fleet of partially reusable Space Shuttles—which took off vertically while strapped to a pair of expendable solid-fuel booster rockets and an enormous fuel tank that was jettisoned before going into orbit—could carry a kilogram of payload into low-Earth orbit for $10,400. Meanwhile, the Falcon 9 two-stage rocket, designed by SpaceX, a private company, to do some of the heavy lifting since the Shuttles retired, can do the job for $4,100. SpaceX’s next-generation launcher, the Falcon Heavy, is expected to do it for $2,200. When conceived back in the late 1960s, the Shuttle promised a turnaround time of around 30 days. In the event, the entire fleet of six vehicles (two disintegrated in flight, claiming the lives of all crew members) managed little more than half a dozen flights a year between them, even fewer after the tragic accidents.

The Shuttle’s biggest problem was maintenance. The delicate silica tiles that protected the airframe from melting during re-entry had to be laboriously inspected, tested and replaced where necessary. The engines had to be pulled from the airframe and examined for hairline cracks and microscopic wear. The turbo-pumps had likewise to be removed and rebuilt in many instances. The fastest Shuttle turnaround ever was 53 days. Most went 180 days or more between launches. So much for reusability.

That has not deterred the space cadets one bit. The Pentagon recently announced plans for another reusable spaceplane known as the XS-1. This is being designed specifically to put satellites weighing up to two tonnes into orbit at short notice for around $5m a launch (ie, $2,500 per kilogram). According to Pamela Melroy, a former astronaut and now a deputy director at the Defence Advanced Research Projects Agency (DARPA), the XS-1 is to be a true “10X” vehicle—ie, able to fly ten times in ten days, at ten times the speed of sound (Mach 10) as well as being ten times cheaper at putting surveillance satellites in orbit than the US Air Force’s admittedly expensive Minotaur IV—a four-stage rocket based on a decommissioned MX intercontinental ballistic missile.

Yet, even if the XS-1 can be made to work as planned, it will still be no cheaper than SpaceX’s forthcoming Falcon Heavy launcher. Why this fascination with complicated, unproven spaceplanes rather than simple, well-tried rockets?

One answer is the unshakable belief by many in high office that, provided enough taxpayer dollars are thrown at a problem, clever engineers will find ingenious solutions. Turf wars also play a part. The Air Force has never forgotten what happened in the 1970s when it was forced to abandon its own manned space programme in favour of the Shuttle. When Shuttle flights were halted for over two-and-a-half years following the Challenger accident in 1986, the Air Force suffered badly.

Since then, it has been largely free to develop its own interests in space. Between them, DARPA and the Air Force Research Laboratory (AFRL) now have reams of technical data about launchers, re-entry vehicles, hypersonic aircraft, ramjets, scramjets, and everything in between the ground, the sky and beyond—thanks to unmanned experimental vehicles like the X-37 spacecraft, the X-51 hypersonic aircraft and HTV-2 test vehicle. By all accounts, the new XS-1 spaceplane picks up where the HTV-2 recently left off.

Of the dozens of proposals for spaceplanes over the years, only five have ever flown—the X-15 rocket plane in the early 1960s, the Space Shuttle from the 1980s onward, the Soviet-era Buran, and more recently Virgin Galatic’s SpaceShipOne and the X-37. All were given a leg-up (either by a rocket booster or by a mother-ship releasing them from high altitude) before firing up their own rocket motors to take them into orbit, or at least the edge of space. None could take off horizontally from a runway, though all could glide back to Earth and land like an aircraft.

It would be ideal, of course, if such a vehicle could take off horizontally like an aircraft as well as land like one—and to do so without using an expendable booster or second-stage rocket engine. This “single-stage-to-orbit” feat can (just about) be performed by a rocket engine. Unfortunately, the amount of liquid oxygen needed to burn the fuel as the vehicle clawed its way up through the atmosphere would present such a weight penalty that precious little payload could be taken along for the ride.

The answer is to use an air-breathing engine for the ascent through the atmosphere, and then to fire up a rocket motor to achieve the burst of hypersonic speed needed to escape the pull of gravity. As air-breathing engines, such as the turbo-jets used in aircraft, get their oxygen from the atmosphere, they obviate the need to carry large amounts of oxidiser for the ascent through the atmosphere. The downside is that air-breathing engines are much heavier than rocket motors plus their oxidiser tanks. That extra dead-weight has to be carried into orbit and paid for in terms of less payload.

But what if a motor could combine the air-breathing virtues of a jet engine with the thrust of a rocket? That is the principle behind the hybrid Sabre engine developed by Reaction Engines of Culham in Britain, with UK government backing and European Space Agency blessing. The brain-child of Alan Bond, chief engineer and co-founder of Reaction Engines and a former aero-engineer at Rolls-Royce, the Sabre engine is destined for a spaceplane called Skylon—widely considered at present to be the only design capable of flying up to orbit under its own power.

Skylon’s predecessor, a space launcher called Hotol (short for horizontal take-off and landing), was canceled by the British government in 1988. The vehicle had serious stability problems and the hybrid engine was in an early stage of development. Since then, the stability problems have been licked—by mounting the engines on the wings rather than in the tail. Meanwhile, the hybrid engine has gained game-changing improvements.

Today, the Sabre engine resembles a conventional turbo-jet with a rocket motor stuck in the exhaust. Actually, the two halves are heavily dependent on one another. The jet engine uses the rocket’s combustion chambers and bell-shaped nozzles, while the rocket motor relies on the jet’s turbo-compressor machinery. Making the big components do double duty minimises the weight penalty of having effectively two separate sources of thrust.

On the runway, the engine sucks in air, pre-cools it massively before compressing and burning liquid hydrogen in it to deliver more than enough thrust to get the vehicle airborne and into its climb. As it gathers speed in the upper atmosphere, the air becomes too thin to feed the turbo-jet. The engine then switches to rocket mode, and starts using liquid oxygen from an on-board tank (rather than the atmosphere) to burn the fuel.

The key to the engine’s dual personality is an annular heat-exchanger at the front of the engine. The highly protected heat-exchanger design is reckoned to be 100 times lighter than anything else on the market. The company claims it can cool air traveling at Mach 5 from over 1,000ºC to -150ºC in a hundredth of a second. Without such a heat-exchanger, a single-stage-to-orbit craft would find it difficult, literally and figuratively, to get off the ground.

Will Skylon ever fly? The odds have to be stacked against it. Realistically, it will take at least £250m ($400m) to get the hybrid engine into production. So far, the UK government has provided only £60m of launch aid, with the private sector expected to chip in the rest. Beyond that, the cost of the airframe could easily amount to the same again or more. No way would the British taxpayer stand for that. It is also difficult to see investors being so far sighted.

Skylon’s best hope is that, down the road, the European Space Agency comes to view the Skylon with its remarkable Sabre engine as the only credible way to compete with the new generation of spaceplanes promising to put satellites in orbit for a disruptive $2,000 per kilogram or so. Even with the upgrades approved last December, Europe’s Ariane 5 will remain stuck in an industry that is suffering from serious over-supply, with prices that are three or four times too high.