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REL’s Skylon spaceplane aims to take on SpaceX with a reusable rocket design

But will the idea fly?

Reaction Engines Limited

Aerospace engineers have dreamed of a spaceship that can launch like a plane, get to orbit, and land on a runway since the 1960s. A British company, Reaction Engines Limited, wants to make that dream a reality. REL’s sleek, winged spaceplane, called the Skylon, looks like something out of the retro-futuristic visions of old magazine covers.

The spacecraft is built to fly like a jet — at first The uncrewed spacecraft is built to fly like a jet until it gets to an altitude of about 92,000 feet at five times the speed of sound (3,800 miles per hour). Then rocket propulsion will shoot  the Skylon to orbit along with 15 metric tons of cargo. On return, it’s designed to glide down to a waiting airport, rather like the Space Shuttle.

According to a recent economic analysis by REL – but with some backstopping from independent consultancy London Economics –  Skylon can get a pound of mass to orbit for between $686 and $1,230 per pound, depending on how optimistic the forecast. This is comparable to SpaceX’s currently advertised rate of about $2,100 per pound for the Falcon 9 and $770 for the upcoming Falcon Heavy.

That would be a huge savings over the Space Shuttle, which was about $10,000, according to NASA. (One reason NASA’s estimate is so high relative to SpaceX or Skylon is not just that the Shuttle is expensive, but NASA has to reveal their total costs. SpaceX is a private company, so estimating what their actual costs are, as opposed to the price, is more difficult).

Yet there are still big hurdles. Nobody has made a combined jet-and-rocket design work before, let alone single stage to orbit. Designing a propulsion system that can do it has been one of the biggest obstacles – pure rockets or jet engines are one thing, but combining them is another. And then there’s building something that flies under conditions that give the most advanced designs a run for their money. Compared to the spaceplane, building a Concorde was easy.


Breathing Air

The secret to getting their spaceplane aloft is the Synergetic Air-Breathing Rocket Engine, or SABRE, a combination jet engine and rocket. Initially it "breathes" air, functioning the way a jet engine does: by igniting hydrogen fuel with the oxygen in the atmosphere. Once the air gets too thin, it simply switches to using an onboard tank of oxygen.

Ordinarily, a jet engine can't operate at Skylon's speeds Ordinarily a jet engine can’t operate at the speeds at which Skylon flies. The fastest jet plane ever flown was the SR–71 "Blackbird," which hits three times the speed of sound. Go much faster than that and the air coming into the engine compresses and heats up – and the engine cooks. The solution? Cool the air coming in.

"We are developing the key technologies for the SABRE engine," says REL’s managing director Mark Thomas. "The most important is the heat exchanger."

When air comes into the SABRE, it gets cooled down with liquid helium. The helium has itself been cooled via an exchanger that uses the liquid hydrogen fuel. Once the helium is done pre-cooling the air, it gets heated again by the combustion of the hydrogen and oxygen, and that energy drives the turbines in the engine. The combined mechanism saves weight and allows the engine to work from a resting start.

The company plans to test the engines this year; the tests will be on the ground, essentially firing them to see if they work as planned.

Mark Ford, head of propulsion engineering at the European Space Agency, says there’s no reason the SABRE shouldn’t work."We saw no technological or engineering showstoppers," Ford says. A 2011 report from ESA said the idea is feasible.

While engines are the most important part of the craft, other challenges still give experts pause. Heat is one. The Space Shuttle had to be covered with tiles because most metals wouldn’t handle the heat generated by re-entry. "We called it the crockery-covered spacecraft," said Ivan Bekey, a former head of the Advanced Concepts Office at NASA and now a private design consultant. "If you’re flying at 25 times the speed of sound then for a spaceplane the heat becomes a problem for the ascent as well."

"We called it the crockery-covered spacecraft."REL says it plans to have two layers of skin on the Skylon, separated by a small space. The outer layer will be made of ceramics, materials that have been in development for decades and advanced since the Shuttles were built, says Richard Varvill, technical director at REL. That will help insulate the craft as it zooms through the upper atmosphere.

For re-entry, Skylon won’t come down in the same way as the Shuttle. Varvill says instead of plunging into the atmosphere, Skylon will take a gentler approach. "It has a more efficient aerodynamic shape," he says, "with sharper leading edges on the wing. The overall heating is a lot less than the Shuttle, though local hot spots on Skylon need local cooling systems." The heating shouldn’t get to more than a few hundred degrees centigrade, as opposed to the 1,200 degrees (or about 2,300 Fahrenheit) that the Shuttle would experience.

The last issue is where Skylon would land. The Space Shuttle landed on long runways, but it could theoretically have used commercial airports. Nineteen east coast airports were tapped for use if the Shuttle had to abort, among them Bangor, Miami, and Atlantic City. Skylon needs one 3.1 miles long (about five kilometers). Public runways that long aren’t common: there are two in China, two in Russia, and one under construction in Afghanistan. Another runway exists in France at a military base. In the US, runways that long are all on military bases, and the paved ones are all at Edwards Air Force Base.


Dyna Soar, artist's rendering, on an Atlas II (NASA/USAF)

(Dashed) Dreams of Flight

The aerospace landscape is littered with the corpses of failed and unfunded projects. Spaceplane attempts go all the way back to the two-stage X–20 Dyna Soar in the late 1950s. The program got cancelled in 1963. It was clear that rockets were simpler and cheaper to design and build; the Dyna Soar wasn’t going to be ready to launch an astronaut until the mid–1960s at best, while the Gemini program had already done it. The Air Force also didn’t have a clear need to be putting people in space.

Aerospace is littered with the corpses of failed and unfunded projectsIn the 1980s, the X–30, otherwise known as the National Aero-Space Plane, was designed under the auspices of the Defense Advanced Research Projects Agency – then-president Ronald Reagan mentioned it in the State of the Union Address as a possibility for hypersonic transport. The X–30 was a horizontal-launch design, which would reach a speed of about 18,500 miles an hour and achieve orbit. But after billions of dollars over nearly a decade, the program was cancelled in 1993, never having flown anything. In the 1990s there was the X–33 VentureStar, which would have launched vertically. That too was cancelled.

Only the Space Shuttle (and the USSR’s Buran on a test flight) have made it to orbit, and they both needed multiple stages and launched vertically, because vertical launches minimize the amount of atmosphere a spacecraft has to get through. The Shuttle was retired in 2011.

Other space agencies, such as the Indian Space Research Organization (ISRO), are looking at spaceplanes. That said, the Shuttle’s vertical-launch design is favored, though ISRO plans to push for a single-stage-to-orbit design in 2025. The next tests are slated for this year.

Ford, though, says the X–30 suffered from having to install separate engines for each stage of flight. "Some of the engines aren’t running at one time or another, and it’s effectively dead mass." The Skylon is different in that respect, as it combines the engines into a single unit.

Even so, SpaceX and Blue Origin are both working on reusable rockets to get orbit, and as conventional, vertical-launch vehicles, they don’t run into the problems that a spaceplane does. Blue Origin has soft-landed a rocket and SpaceX has also.


Artist's rendering of Skylon (REL)

Supply and Demand

REL will also have to raise a lot more money. Development costs could, by the company’s own estimates, easily hit the $12 billion mark. The company has raised a total of about $156 million from a combination of private and government funding – there’s a long way to go.

Skylon itself would be pricey to buildDemand for launches might be another issue. The problem with Skylon is the sheer number of flights you’d need to make it profitable, according to an analysis by Ashley Dove-Jay, an engineer at Oxford Space Systems. There’s only so many satellite communications companies, after all. And currently there’s no reason for travelers to go into orbit. (Virgin Galactic plans to pioneer the space tourism business and those trips will only be sub-orbital). In the meantime, SpaceX could provide cheaper access to space and bigger payloads per dollar. A similar problem plagued the Space Shuttle – every launch was far more expensive than the uncrewed rockets available, with estimates of per-flight costs at up to $1.2 billion. NASA underestimated the turnaround time for launches, and after the Challenger disaster, the Shuttle stopped taking commercial payloads altogether. That limited the market for Shuttle launches to the ISS, military, and science missions that required humans, a small piece of the launch market.

Skylon itself would also be pricey to build. A single Skylon’s price tag would approach that of a stealth bomber, and that isn’t something that many airlines are likely to pay.

Yet with all these challenges REL is confident. It thinks it can play a long game and that the technology will get developed. "We’ve made a lot of technical inroads," Varvill says. "And we’re competing with expendable rockets, a machine that is only used once."

"Reusability is the next big cost reduction," says Ford. And if the engines work it might spark the demand for access to space. He also likes it from a technical standpoint. "From an engineering perspective it’s an obvious solution to a problem," he says. Rockets, in that sense, are wasteful and inefficient. "I mean, what if someone said, ‘We’ll fly you to London, and only you and the seats will get there?"