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Reaction Engines Ltd. is hanging its future hopes on the Skylon, a space vehicle they hope to be the first successful single stage to orbit spaceplane ever built. Many have tried to achieve this goal before, but to no avail. What makes Skylon so unique is a number of technologies rolled into a simple, straightforward airframe.
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The SABRE engines are remarkable in their simplicity, yet achieve enough additional performance margin to achieve the stated objectives. As mentioned in the previous Skylon article, the propulsion system operates in two modes - a “ducted rocket” using air as an oxidizer below Mach 5.5, and a conventional rocket at Mach 5 plus. The truly revolutionary aspect of the design is how it achieves this performance, especially via a subsonic inlet. More well-known engine designs such as the ramjet and supersonic combustion ramjet require exotic materials to cope with the elevated temperatures within the engine. To circumvent this, the SABRE slows the air to subsonic speeds, increasing the temperature as it does so. It then relies on active cooling of the inlet spike to reduce the temperature to a range that can be handled without the extensive use of exotic materials. If successful, this approach would allow hypersonic performance without the technical challenges and cost associated with scramjets. While the heat exchanger is complex, our technical understanding of conventional thermodynamics is much more complete than our understanding of high-temperature aerothermodynamics.
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Skylon employs an airframe much more closely resembling a rocket or the venerable Lockheed F-104 Starfighter rather than the waverider and lifting body designs that have been employed by the lion‘s share of hypersonic concepts.. The fuselage is long and proportionally narrow with a delta wing mounted near the midsection and a vertical tailplane, with the engines mounted at the wing tips, while the structure is a carbon fibre-based composite. Like the engines, this approach is extremely straightforward and remains within the realm of experience, though there are thermal protection system impacts associated with having the wings extend past the Mach cone in high-speed flight.
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Thermal Protection System
Perhaps the most challenging aspect of designing a SSTO space vehicle is that of developing a thermal protection system that can withstand the rigors of ascent into space and the even harsher environment of atmospheric entry. While the Shuttle’s TPS is a remarkable feat of engineering, it is also very fragile and expensive. Skylon’s lower wing loading allows it to employ less exotic materials, a decision that decreases the initial cost as well as well as the long-term support costs associated with refurbishing the TPS after each use. Skylon’s approach employs a silicon carbide reinforced ceramic “aeroshell” separated from the structure by titanium insulation. This system makes refurbishment and replacement of damaged panels easy and straightforward. Since the wings extend outside the Mach cone at high speed and thus experience high heating rates, the thermal protection also uses hydrogen in an active cooling strategy, circulating cold hydrogen through the hottest parts of the structure, then venting it overboard.
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Is It Worth It?
It would be easy to say that the project is worthwhile purely from a symbolic sense and as a means of gaining knowledge about the hypervelocity flight environment. However, Skylon stands to do far more than that if it gets off the ground. The approach that Reaction Engines has taken from the beginning is one of simplicity, starting with current technology and trying to determine what is needed from there rather than starting with a goal and trying to invent the technology to get there. In both mission performance and logistics, it holds the potential to usher in a new era of spaceflight. If successful, Skylon will have proven that practicality can win over bleeding edge technology, and that just might pave the way for others to make it to the High Frontier.
Skylon - A Conventional Approach To A Spaceplane?
Sometimes, good ideas don't die- they manage to be refined until they become a workable, finished product. Reaction Engines' Skylon may prove to be one more good idea whose time will eventually come.