Skylon challenges analyzed at the Space Review

Work by Reaction Engines on the Skylon concept stretches back more than two decades. Skylon had its origins in another SSTO RLV project called HOTOL by British Aerospace and Rolls Royce in the 1980s. When the British government declined to further invest in HOTOL in the late 1980s, a group of engineers led by Alan Bond created Reaction Engines to further the technology that had been planned for HOTOL.

The heart of Skylon is its engine, called the Synergistic Air-Breathing Rocket Engine, or SABRE. Housed in curved nacelles on the tips of stubby wings, the twin SABRE engines have air intakes that allow the vehicle to use atmospheric oxygen as oxidizer, with liquid hydrogen fuel, from takeoff to an altitude of 25 kilometers, at which point the vehicle is traveling to Mach 5.5. At higher altitudes SABRE becomes a more conventional rocket engine, using onboard liquid oxygen to accelerate the rest of the way to orbit. This approach greatly reduces the amount of liquid oxygen the vehicle has to carry, decreasing its takeoff weight, which in turn has other effects on the vehicle’s design.

There will be a test of a subscale version of the planned SABRE precooler, attached to a Rolls Royce Viper jet engine, but Longstaff said it will effectively test the technology that is essential to the engine. “It will be sufficient to verify the aerodynamics and thermodynamics and the frost control technology” designed to prevent water vapor in the atmosphere from freezing within the engine. “We bet the farm on this, so it better work.” (As this article was going to press, a spokesperson from Reaction Engines confirmed that the tests were scheduled to start this month, “a few months earlier than originally scheduled.”)

If all goes well the company plans to build a full-scale boilerplate version of the engine for ground tests by 2013 or 2014. A subscale Nacelle Test Vehicle, about nine meters long, will be flown at speeds of up to Mach 5 to verify the internal design of the engine and its aerodynamics. “By 2014 we would be ready to pass over the manufacturing drawings for the engine to a contractor, and we would have the analysis of the airframe completed,” Longstaff said. If that work goes as planned and the project is funded, he added, “we could fly about 2018 and we could be fully operational by 2020.”

Another challenge will be the business case for Skylon. The current Skylon C1 design can carry up to 10,275 kilograms to low Earth orbit (LEO), with a goal of 12,000 kilograms; Longstaff noted in April that it could carry about 7,000 kilograms to the orbit of the International Space Station and 3,000 kilograms to Sun-synchronous orbit. That suggests Skylon may be undersized to serve many existing portions of the commercial launch market, such as the launch of geosynchronous orbit (GEO) communications satellites that can weigh in excess of 6,000 kilograms, plus an upper stage needed for transfer from LEO to GEO.

“It’s been apparent that the goal of 12 tonnes to low Earth orbit, at 300 kilometers [altitude], is perhaps a bit small,” Longstaff said at Space Access. The company, he said, is looking at an enhanced version of the Skylon, designated the D1, using an upgraded version of the SABRE engine and an optimized trajectory that could carry 15 tonnes to LEO.

Reaction Engines has studied the inclusion of a passenger module in the Skylon cargo bay that could carry 30–40 passengers; a revised concept published in 2008 included room for 20 people for missions to ISS orbit.

Reaction Engines has projected the development cost at roughly that of the Airbus A380 superjumbo, reported to be about €12–14 billion (US$17–20 billion), although the ESA report cites a “pessimistic estimate” of the development cost, provided by a costing model developed by the company, of only $12.3 billion. In any case, that is a large sum of money, especially since company officials have indicated they plan to privately finance the vehicle’s development rather than government funding.

While company estimates put the per-flight operating costs as low as $9.5 million, that requires a flight rate of 70 missions a year; costs when the vehicle enters service will be on the order of $30–40 million a flight, according to the Reaction Engines web site. That could put the vehicle at a competitive disadvantage to some expendable vehicles. Assuming a capacity of 15 tonnes to LEO, initial Skylon costs would be $2,000 to nearly $2,700 per kilogram. By comparison, SpaceX’s Falcon Heavy, with a capacity of 53 tonnes to LEO and a projected cost of $80–125 million per launch, would come in at $1,500 to $2,350 per kilogram. Moreover, development of the Falcon Heavy is expected to cost a small fraction of the Skylon’s projected cost, and it could enter service as soon as 2013, years before Skylon will be ready in even the most optimistic scenario.

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