A space rocket plunges earthward, twitching its steering fins and firing gas jets to stabilise itself. It looks for all the world like it is doomed. But as it nears the surface, the motors burn stronger and the rocket slows considerably, landing legs spring from its sides and, as the billowing smoke disperses, it's clear it has touched down upright – and in one piece.
It's a feat that was inconceivable only a decade ago. But after many spectacular failed attempts the Californian rocket maker SpaceX has in the last four months landed an orbital rocket stage four times – once on land in December 2015 at Cape Canaveral, Florida, and in April for the first time on a remote-controlled barge in the mid-Atlantic. And these were not dummy rockets: both were the 40-metre-high first stages of Falcon 9 rockets that had just launched commercial spacecraft into orbit.
By returning rocket stages to Earth for reconditioning and reuse, SpaceX's founder, billionaire Elon Musk, hopes eventually to make spaceflight as economical as commercial aviation. His point: airlines don't throw away a Boeing 747 after every flight, so why do it with spaceships?
Sci-fi has predicted reusable spacecraft for a century or more, and space engineers have experimented with the idea since the mid-20th Century – the partially reusable Space Shuttle is arguably the closest we’ve got. So why has reusability taken so long to be seriously considered?
First, it’s worth highlighting that SpaceX is not alone. Blue Origin, backed by Amazon chief Jeff Bezos, has launched and then landed its New Shepard suborbital space tourism rocket three times, each time venturing just beyond the edge of space, an altitude of 100km (61 miles).
And Virgin Galactic’s SpaceShipTwo will also fly on multiple suborbital trips. "Smaller vehicles which employ modern technologies can be much more reusable than the shuttle, and suborbital vehicles even more so," says Virgin CEO George Whitesides.
But SpaceX's landings are the greater technical feat. Getting a satellite to low Earth orbit means the rocket needs to be travelling at around 6,000 km/h (3,726mph), and to reach geostationary orbit 9,000 km/h (5,590mph) – before that first stage can be released and return to Earth.
"Suborbital vehicles fly straight up and return directly down," says Laetitia Garriott de Cayeux, an American space entrepreneur. "While that's difficult, the velocity at the top is zero before gravity pulls them back towards the Earth. So sub-orbital reusability is far from being as difficult as orbital reusability," she says.
So a short explanation for why reusable rockets haven’t arrived earlier is simply the technical difficulty. However, the idea of reusable spaceplanes dates back to before World War Two.
Before the Apollo programme, spaceplanes were thought to be the future of reusable spacecraft, says Roger Launius at the Smithsonian Institution's National Air & Space Museum in Washington DC. "The idea has been out there since the Buck Rogers and Flash Gordon science fiction comic strips in the 1920s and 1930s. Every single one of the spacecraft in those strips was a reusable spaceplane. So since before World War Two we always thought spaceflight would be like airplane activity."
After 1945, captured German rocket scientists revealed they had planned – but never built – a suborbital spaceplane, the Silverbird, with which the Nazis had hoped to bomb the US. A novel design feature was that it was shaped like a wing, so its shape helped add aerodynamic lift. This 'lifting body' idea was harnessed by the US Air Force in 1958 when it started work on a reusable winged spaceplane, the X-20 Dyna-Soar – but the Moon program saw that canned in 1963.
After the triumph of Apollo, Nasa went straight back to its reusable first love: the winged, reusable spaceplane
"The reusable spaceplane was thrown out of the window because of the space race, which was all about beating the Russians. At the time, the spaceplane was not sufficiently advanced technology for Moonshots – but the research and testing on ballistic capsules had already been done on ICBMs," Launius says.
"The reentry capsules they used for a nuclear warhead are basically the same as those you put an astronaut in. You just change the payload."
After the triumph of Apollo, however, Nasa went straight back to its reusable first love: the winged, reusable spaceplane called the Space Shuttle.
Five shuttle orbiters averaged 27 missions each; the star of the fleet was Discovery, with 39 missions. "So spaceplanes have a long history of reusability," says Mark Sirangelo, head of SNC Space Systems at Sierra Nevada Corporation in Sparks, Nevada.
The downside was that they had to be refurbished between launches – something that applies to SpaceX’s rockets too. While SpaceX has tested whether their rockets can return to Earth, it has yet to refly one. And that is the true test, says Launius. "If you can reuse any part of a space vehicle you are going to save money on the next launch. But if they have to tear it apart and completely refurbish it every flight you might as well build a new one."
Nasa also persevered with research on a raft of much smaller reusable spaceplanes that are now appearing in other guises. For instance, Nasa's X37 is now used by the US Air Force in the form of the X37B, an unmanned, rocket-launched spaceplane that performs extended secret military missions in low Earth orbit and which then flies home autonomously.
And Nasa's HL-20 spaceplane, developed in the late 1980s and 1990s as a potential space station lifeboat, is what has been acquired by Sirangelo's Sierra Nevada Corp and renamed Dream Chaser. SNC is converting the Nasa design into what Sirangelo calls the "strongest reusable vehicle" they can make. That has involved swapping out Nasa's metal alloy fuselage for an advanced lightweight composite one."
It will be significantly stronger and more capable of handling spaceflight stresses, pressures and temperatures," says Sirangelo. In addition to Nasa cargo flights, Dream Chaser – which can be launched on any modern rocket and land at any airport capable of receiving an Airbus A320 – is being considered by Esa and the German lab DLR for a variety of missions. These could include one to intercept and remove space debris, for instance.
Some truly bizarre designs have been suggested to boost reusability
While the spaceplane looks like being the most logical way to create a reuseable spacecraft, other more outlandish designs also saw the light. Take the Rotary Rocket Company's 'Roton', a pepperpot-shaped capsule tested in 1999 to avoid a problem that has dogged the hundreds of crew capsules that have returned to Earth since the space race began: they cannot land where they want to and are at the mercy of where parachutes carry them.
Rotary Rocket wanted to give a crew the chance to choose where their capsule would make a soft landing, leaving the vehicle more likely to be reusable. To do this, it would re-enter the atmosphere and once in thicker air deploy helicopter rotors embedded in its surface. Driven by rocket motors on the rotor tips they would deploy at a suitable altitude and the crew would navigate, helicopter style to a landing point of their choice: watch the Roton's atmospheric test vehicle below, with smoke and unburned fuel pouring out of the rotor rockets.
Unfortunately, the Rotary Rocket Company ran out of cash before their ship got very far. Nasa also once considered rotors, though not rocket driven ones, preferring sycamore-leaf-style autorotation – for its Orion capsule.
The Roton’s unconventional approach may partly live on, however, thanks to SpaceX . The seven-seat Dragon V2 crew capsule it's building for Nasa trips to the ISS aims ultimately to be reusable thanks to the eight rocket motors built into its exterior walls. These have two jobs: blasting the crew capsule clear if the rocket carrying it explodes during launch, and firing to provide a soft propulsive landing. Nasa's initial Dragon V2 landings will be conventional parachute drops into the ocean, with propulsive landings following.
American rocket firm United Launch Alliance – a joint venture of Boeing and Lockheed-Martin – is investigating how to jettison the large, expensive rocket motor at the base of its future Vulcan rocket and have it fly, on a parafoil, for a mid-air recovery snatch by an aircraft – the same way camera film canisters from spy satellites used to be recovered. ULA is also imagining a second stage that stays in orbit, waiting to be refueled for various tasks, such as satellite servicing.
There are many generations of reusable space vehicles to come – George Whitesides
In France, Airbus is investigating how the rocket motor at the base of the future Ariane 6 rocket could use wings and small jet engines – so it can fly back autonomously to an airport. And the Chinese Space Agency says it is planning reuse of its Long March rocket stages using multiple parachutes.
It all makes sense, says Whitesides. "There are many generations of reusable space vehicles to come. The pioneers like SpaceshipOne, Falcon9, New Shepard and the US Air Force X-37B will have descendants in many different forms, and they hold the promise of radically lower cost to orbit."
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