Reaction Engines

Reaction Engines Skylon

In aerospace, Skylon is a proposal by Reaction Engines Limited (managed by British rocket scientist Alan Bond) for a single-stage, turbojet-based, airbreathing orbital spaceplane.

The vehicle would be a hydrogen-powered aircraft that would take off from a conventional runway, and accelerate to Mach 5.5 at 26 km before lighting a rocket engine to take it to orbit. It would then release a 12-tonne payload, and reenter. The payload would be carried in a standard payload container, or a passenger module containing up to 60 passengers.

During reentry the relatively light vehicle would fly back through the atmosphere and land back at the runway, with its skin protected by a strong ceramic. The vehicle would then undergo any necessary maintenance and would be able to fly again within 2 days.

A fleet of vehicles is envisaged; each vehicle would be reusable at least 200 times. Costs per kilogram of payload would be below the current costs of launch (as of 2006), including the costs of R&D, with costs expected to fall much more over time after the initial expenditures have amortised.

Originally the key technology to build this aircraft did not exist - the required heat exchanger was about ten times lighter than the state of the art. However, research has now achieved the necessary performance. Currently no funding to fully develop and build the vehicle exists, but research and development work is nevertheless ongoing.

Single stage to orbit

Currently, all orbital spacecraft use multiple stages. Having multiple stages requires the jettisoning of parts of a launch vehicle during the flight to reduce weight—otherwise the vehicle would be too heavy to reach orbit. A vehicle that can fly to orbit without staging is known as single stage to orbit (SSTO).

Proponents of SSTO claim that staging causes a number of problems such as being difficult, expensive or even impossible to recover, reuse and reassemble the parts and therefore believe that SSTO designs hold the promise of reducing the cost of spaceflight.

The Skylon design aims to take off from a specially strengthened runway, fly into low earth orbit, reenter the atmosphere, and land back on the runway like a conventional aeroplane, without staging, whilst being fully reusable. Further, it aims to do this with a higher payload fraction than any existing multi-stage vehicle.

The engines

One of the significant features of the Skylon design is the engine, called SABRE. The engines are designed to operate much like a conventional jet engine at up to around Mach 5.5 (1700 m/s), 26 km altitude, and then close the air inlet and operate as a highly efficient rocket to orbital speed. (See for an independent analysis).

Operating a turbojet engine at up to Mach 5.5 is difficult. Previous engines proposed by other designers have been good jet engines but poor rockets. This engine design aims to be a good rocket engine, as well as being an excellent jet engine at all speeds. The problem with operating at Mach 5.5 has been that the air coming into the engine heats up as it is compressed into the engine, which can cause the engine to overheat and eventually melt. Attempts to avoid these issues typically make the engine much heavier (scramjets/ramjets) or greatly reduce the thrust (conventional turbojets/ramjets). In either case the end result is an engine that has a poor thrust to weight ratio at high speeds, and so the installed engine is too heavy to assist much in reaching orbit.

The SABRE engine design aims to avoid this by using some of the liquid hydrogen fuel to cool the air right at the inlet. The air is then burnt much like in a conventional jet. Because the air is cool at all speeds, the jet can be built of light alloys and the weight is roughly halved. Additionally, more fuel can be burnt at high speed. Beyond Mach 5.5, the air would still end up unusably hot, so the air inlet closes and the engine instead turns to burning the hydrogen with onboard liquid oxygen as in a normal rocket.

Because the engine uses the atmosphere as reaction mass at low altitude, it would have a high specific impulse, and burn about one fifth of the propellant that would have been required by a conventional rocket. Therefore, it would be able to take off with much less total propellant than conventional systems. This, in turn, means that it doesn't need as much lift or thrust, which permits smaller engines, and allows conventional wings to be used. While in the atmosphere, using wings to counteract gravity drag is more fuel-efficient than simply expelling propellant (as in a rocket), again reducing the total amount of propellant needed.

Differences from HOTOL

Skylon was based upon a previous project, HOTOL, that ended when funding was cut by the UK government.

One difference is in the undercarriage. To save weight, HOTOL was to have been launched from a sled. This was dispensed with for Skylon which would use a relatively conventional-looking retractable undercarriage. This is to be achieved by using high pressure tires on a specially strengthened runway with water cooled brakes. If problems were to occur just before takeoff the brakes would be applied to stop the vehicle, the water boiling away to dissipate the heat. Upon a successful takeoff, the water would be jettisoned, thus reducing the weight of the undercarriage by many tons. During landing, the empty vehicle would be far lighter, and hence the water is unneeded.

Skylon also uses a different engine design. This is partly due to patent and Official Secrets Act issues, but the SABRE engine is expected to offer higher performance.

Another issue that the Skylon design aims to circumvent was the intrinsically poor stability of HOTOL. The weight of the rear-mounted engine tended to make the HOTOL vehicle fly backwards. Attempts to fix this problem ended up sacrificing much of the potential payload that the HOTOL vehicle could carry, and contributed to the failure of the project. Skylon would solve this by placing the engines at the end of the wings closer to the center of the vehicle and thus moving the center of mass forward, ahead of the center of drag.

Vehicle structure

The overall shape of the fuselage of Skylon bears a resemblance to the similarly named futuristic structure, the Skylon at the Festival of Britain.

The vehicle design is physically big—82 m long and 6.3 m in diameter—mainly because it uses low-density liquid hydrogen as fuel.

The structure is primarily a spaceframe design which carries the weight of the tanks, and to which the skin is attached. In between the bars of the structure would be multiple layers of insulation.

The relatively large tanks required are kept very light by running them at low pressure. This size means that the vehicle would have an easier time during reentry compared to other vehicles, such as the Space Shuttle due to the low ballistic coefficient.

Due to the low ballistic coefficient, the vehicle would end up slowing down at higher altitudes where the air is thinner. The skin of the vehicle would only reach 1100 kelvin, and the extremely fragile tiles that the Space Shuttle thermal protection system employs would not be required. The Shuttle's fragile silica tiles are damaged even flying through rain, whereas the Skylon's proposed skin material is a much more durable reinforced ceramic. Due to a difficulty with turbulent flow around the wings during reentry, some parts of Skylon would be actively cooled.

The proportion of payload to takeoff weight (the payload fraction) would be more than twice that of normal rockets and the vehicle should be fully reusable (200 times or more).

Specifications

Skylon Specification:

Length: 82 m
Fuselage diameter: 6.25 m
Wingspan: 25 m
Unladen mass: 41,000 kg
Fuel mass: 220,000 kg
Maximum payload mass: 12,000 kg
Specific impulse: 2000 to 2800 s (20 to 27 kN·s/kg) atmospheric, 450 s (4.4 kN·s/kg) exoatmospheric
SABRE engine thrust/weight ratio: greater than 10

Comparisons with other vehicles

Compared to the Space Shuttle Orbiter plus the external tank but not the external boosters, the Skylon design is for a vehicle:

more than twice the length
just over 1/3 the payload
one third the takeoff weight
with an unladen weight about 40% that of the Orbiter and tank

The SR-71 Blackbird holds the official speed record for a turbojet powered aircraft. Compared to the Blackbird the Skylon design is for a vehicle:

more than twice the size
more than one and a half times as fast under turbojet power
more than three times the takeoff weight
with an unladen weight about a third higher than that of the Blackbird.

Economics

The estimated R&D cost was $10 billion in 1992 including building a small fleet of aircraft. This translated into a payload cost of roughly $3000/kg.

The initial estimated cost of a launch was conservatively estimated to be $40 million (in 1995 prices). This would be expected to drop at higher launch rates. The launch crew size was to be 200 and the turnaround time 2 days.

The Skylon was designed for low cost, to use largely present day materials and to represent a low development risk. The ultimate estimated cost of launching a Skylon vehicle was estimated to be as low as $5 to $10 million.

The aircraft was initially intended to be unmanned and to carry only standard aerocontainers, but it was intended that up to 60 passengers could be carried in a purpose built container once the vehicle was certified. It is likely that the vehicle would be economic for space tourism.

Government funding

Request for funding from the British government was undertaken in 2000, with a proposal that could have offered a large potential return on investment. The request was not taken up on at that time.

Ongoing work

Alan Bond's company Reaction Engines Limited in conjunction with Bristol University has been engaged in research, mainly covering the SABRE engine's heat exchangers; which have now been proven to be manufactureable.

Alan Bond is currently trying to build an actual working SABRE engine.

The complete Skylon project has a projected R&D cost of under $10 billion and an estimated program length of 7-10 years.

Reaction Research Limited is working on its design proposal, the Reaction Engines A2, for LAPCAT, an EU program investigating hypersonic intercontinental transport designs.