Researchers at Reaction Engines Ltd. in the U.K. have designed a reusable, unmanned spaceplane that could take off like an airplane, fly into low-Earth orbit carrying 15 tons of cargo, launch or off-load that cargo, then fly back to Earth and land on a runway. The spaceplane, called Skylon, could then be quickly readied for another flight into space.
The key to making Skylon reusable is its two Sabre engines. The slightly curved engines take in air as they accelerate the spacecraft from take-off to Mach 5 at about 15 miles high. By burning oxygen taken directly from the atmosphere on this first part of the journey, Skylon eliminates the need to carry an additional 250 tons of liquid oxygen, thus reducing take-off weight and the volume of oxygen carried internally.
Skylon, an unmanned, reusable spaceplane, is being designed at Reactions Engines Ltd in the U.K. If all goes well, it could be flying by 2020.
The engines’ heat exchangers or pre-coolers play a critical role in this first leg. They cool incoming air travelling at up to Mach 5 from 1,800°F to -238°F in less than 0.01 sec., and remain frost free at that frigid temperature. This means the heat exchangers displace 400 MW of heat energy yet weigh less than 1.25 tons.
The exchangers serve two purposes. The first is cooling incoming air so that it can be compressed before it enters the combustion chamber. By cooling the air, engineers at Reaction Engines won’t have to size the compressor and turbomachinery to handle temperatures that would melt most metals. Secondly, the pre-coolers heat helium which is used to drive fuel pumps and other engine machinery.
Skylon will carry two Sabre engines which will take oxygen out of the atmosphere as at up to Mach 5. But at higher altitudes, they will switch to rocket mode using liquid oxygen stored onboard.
The pre-coolers, as engineers at Reaction Engines call them, are only 1% the weight of standard “lightweight” exchangers. Weight was reduced by using thin walls to separate hot and cold fluids, coupled with manufacturing techniques that strongly bond these fine nickel-alloy structures. In the current pre-coolers, for example, 31 miles of 1-mm-wide tubing with walls only 27 microns thick are bonded together to withstand pressures greater than 2,170 psi at temperatures ranging from 1,800 to -238°F.
The Sabre engines transition to rocket mode, burning onboard oxygen as Skylon goes into orbit at Mach 25.
While Skylon is climbing or descending through the atmosphere, moving aerodynamic surfaces provide control. The tail fin handles yaw, twin delta canards control pitch, and ailerons along the entire trailing edge of both wings provide roll control. When the engines slip into rocket mode, control transtitions to the gimballed combustion chambers. Then, once in space, an array of reaction thrusters take over, providing pitch, yaw, and roll control.
The current design calls of for a spacecraft 270-ft long, 20.5-ft in diameter with an 82-ft wingspan. It will mass out at 41,000 kg unloaded and its maximum take-off mass will be 275,000 kg.
There are two reasons for Skylon’s distinctive curved engines and nacelles. The front is angled down about 7° compared to the body and wings so the air intake points directly into the oncoming airstream during flight. At the same time, the body and wings will be canted up a bit so the angle of attack can create lift. The rear of the engine also curves down to get the rocket thrust pointing through the spacecraft’s center of mass. It’s a coincidence that the rear angle is also 7°.
The spaceship’s fuselage and wing structure is made of plastic reinforced with carbon fibers. Fuel (liquid hydrogen) and liquid oxygen is stored in three aluminum tanks suspended within the fuselage so they can move to accommodate thermal and pressurization changes. The 0.5-mm outside shell is fiber-reinforced ceramic that can withstand the heat when speeding through the atmosphere. This shell is also corrugated for added stiffness.
Skylon takes off like an airplane, supported by a retractable set of landing gear. But with a take-off weight of 275 ton, which includes 150 tons of LOX and 66 tons of liquid hydrogen, the runway would have to be heavily reinforced to withstand the high wheel loads.
The precooler/heat exchanger transforms hot inlet air into cooler air so the engine is lighter and more efficient. The heat exchanger also heats helium so that it can power fuel turbopumps and other ancillary equipment.
The spacecraft has a cargo bay that measures 15-ft in diameter and 40.4 ft long. It can handle payloads with expendable launchers as well as standard aero transport containers that have an 8-ft2 cross section and can be 10, 20, 30, or 40 ft long.
Once operational and certified, Skylon will be able to launch several small satellites mounted in a small payload carrier rack. It will also be able to put telecommunication satellites into geostationary orbits by having them equipped with upper-stage propulsion units. These upper stages can be collected for reuse. A special personnel and cargo container will fit in the cargo bay, letting up to 30 people or a combination of people and cargo be sent into orbit. (The spacecraft can remain operational on orbit for up to seven days, a parameter limited by the amount of fuel available for reaction thrusters.) People and cargo can be transported to the space station, flown to another point anywhere on Earth in four hours, or housed in space to conduct experiments. The cargo bay can also be used to carry modules or infrastructure for future space stations, space telescopes, or large satellites.
The European Space Agency has looked at the Skylon and its Sabre engines and concluded it can be built using today’s technology if engine development continues as planned. To that end, the EAS has contributed to an $8.5 million project to develop and test the engine. It’s estimated a total of $12 billion and eight years will be needed to build an operational Skylon spacecraft.