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A spaceplane is a rocket plane designed to pass the edge of space. It combines some of the features of an aircraft and some of a spacecraft. Typically, it takes the form of a spacecraft equipped with wings.
DescriptionAn aerospace planes possess some differences with wingless rocket launch systems. Aerodynamic LiftAll aircraft utilize aerodynamic surfaces in order to generate lift. Typically the force of lift generated by these surfaces is many times that of the drag that they induce. The ratio of these forces (the Lift-to-drag ratio or L/D) varies between different aircraft designs but can be as high as 60 in high performance gliders but is closer to 7 for the aircraft configurations more typical of supersonic aircraft including aerospace planes. In practice a lift to drag ratio of 7 means that a thrust force equal to 1/7th of the weight of the aircraft is sufficient to support it in flight. This low thrust requirement significantly reduces the amount of fuel required to propel the aerospace plane in comparison to rocket launch systems which must provide thrust greater than the weight of the vehicle. A partially off-setting difference between these systems is that the aerospace plane would typically experience powered flight for much longer periods of time than a rocket. In addition winged vehicles need extra dry mass for the wings, and this penalises vehicles towards the end of the flight. Rockets are also able to use their high thrust at an angle which gives reasonable lifting efficiency when burning for orbit. Air breathing enginesA difference between rocket based and airbreathing aerospace plane launch systems is that aerospace planes designs typically includes minimal oxidizer storage for its propulsion. The airbreathing aerospace plane designs include engine inlets so its propulsion system uses atmospheric oxygen during combustion. Since the mass of the oxidizer is the single largest mass of most rocket designs (the Space Shuttle's liquid oxygen tank weighs 1,387,457 lb - more than one of its solid rocket boosters[1]), this provides a huge potential weight savings benefit. However, air breathing engines are usually very much heavier than rocket engines and the empty weight of the oxidiser tank, and this greatly offsets the overall system performance. ComplexityBecause aerospace planes must operate in harsh environments, utilize a number of different propulsion systems, and require more control systems (e.g. aerodynamic as well as thrust vectoring), they are typically far more complicated in design than equivalent rocket systems. In fact, just a comparision of a typical jet engine to that of a rocket engine gives some indicator of the difference in complexity of the engine components. Additionally most aerospace vehicles include also include rocket engines in addition to their 1 or more jet engine types. Harsh flight environmentThe flight trajectory required of airbreathing aerospace vehicles fly what is known as a 'depressed trajectory' which places the aerospace plane in the high-altitude hypersonic flight regime of the atmosphere. This environment induces high dynamic pressure, high temperature, and high heat flow loads particularly upon the leading edge surfaces of the aerospace plane. These loads typically require special advanced materials, active cooling, or both for the structures to survive the environment. Center of mass issuesA wingless launch vehicle has lower aerodynamic forces affecting the vehicle, and attitude control can be active perhaps with some fins to aid stability. For a winged vehicle the centre of lift moves during the atmospheric flight as well as the centre of mass; and the vehicle spends longer in the atmosphere as well. Historically, the X-33 and HOTOL spaceplanes were rear engined and had relatively heavy engines. This puts a heavy mass at the rear of the aircraft with wings that had to hold up the vehicle. As the wet mass reduces, the centre of mass tends to move rearward behind the centre of lift, which tends to be around the centre of the wings. This can cause severe instability that is usually solved by extra fins which add weight and decrease performance. Orbital spaceplanesThe orbital spaceplanes successfully flown to date, the United States Space Shuttle and the Soviet Buran, have used their wings to provide aerobraking to return from orbit and to provide lift to allow them to land on a runway like conventional aircraft. Both these vehicles are still designed to ascend to orbit vertically under rocket power like conventional expendable launch vehicles. Each of these vehicles has a much smaller payload fraction than a ballistic design with the same takeoff weight. This is primarily due to the weight of the wings - around 9-12% of the weight of the atmospheric flight weight of the vehicle.[citation needed] This significantly reduces the payload size, but the reusability is intended to offset this disadvantage. Suborbital spaceplanesOther (suborbital) spaceplane designs use the vehicle's wings to provide lift for the ascent to space as well, in addition to the rocket. As of June 21, 2004, the only such craft to reach space have been the X-15 and SpaceShipOne. Neither of these craft were capable of entering orbit, and both began independent flight only after being lifted to high altitude by a carrier aircraft. NASA and Boeing are currently developing unmanned orbital spaceplane technologies as a low-cost alternative to expendable launch vehicles for satellite launches (see X-34, X-37, X-40A) Atmospheric reentryBecause spaceplanes designed for sub-orbital trajectories will not need to reach orbital speed, they will not need the kinds of thermal protection orbital spacecraft require during the hypersonic phase of atmospheric reentry. The Space Shuttle thermal protection system, for example, protects the orbiter from surface temperatures that could otherwise reach as high as 3,000 °F (1,650 °C) -- well above the melting point of steel.[2] Single stage to orbitFuture orbital spaceplanes may take off, ascend, descend, and land like conventional aircraft, providing true single stage to orbit capability. Proponents of scramjet technology often cite such a vehicle as being a possible application of that type of engine, however pure rocket and turbojet designs have also been proposed and may be easier to design and build. Other designsVarious types of spaceplanes have been suggested since the early twentieth century. Notable early designs include Friedrich Zander's spaceplane equipped with wings made of combustible alloys that it would burn during its ascent, and Eugen Sänger's Silbervogel bomber design. Winged versions of the V2 rocket were considered during and after World War II, and when public interest in space exploration was high in the 1950s and 60s, winged rocket designs by Wernher von Braun and Willy Ley served to inspire science fiction artists and filmmakers. The USAF invested some effort in a paper study of a variety of spaceplane projects under their aerospaceplane efforts of the late 1950s, but later ended these when they decided to use a modified version of Sänger's design. The result, X-20 Dyna-Soar, was to have been the first orbital spaceplane, but was cancelled in the early 1960s in lieu of NASA's Project Gemini and the U.S. Air Force's Manned Orbiting Laboratory program. The Rockwell X-30 National Aero-Space Plane (NASP), begun in the 1980s, was an attempt to build a scramjet vehicle capable of operating like an aircraft and achieving orbit like the shuttle. It was cancelled due to increasing technical challenges, growing budgets, and the loss of public interest. The Multi-Unit Space Transport And Recovery Device (MUSTARD) was a concept explored by the British Aircraft Corporation (BAC) around 1964-1965 for launching payloads weighing as much as 5,000 lb. into orbit. It was never constructed. The British Government began a project known as HOTOL whose ultimate goal would have been a spaceplane, but the project was cancelled due to technical and financial issues. The lead engineer from the HOTOL project has since set up a private company dedicated to creating a similar plane called Skylon with a different combined cycle rocket/turbine precooled jet engine called SABRE . This vehicle is intended to be capable of a single stage to orbit launch and if successful would be far in advance of anything currently in operation. In 1994 a design for a single stage to orbit peroxide/kerosene spaceplane called "Black Horse"[3]. This was notable in that it took off almost empty and underwent mid-air refuelling before burning for orbit. The X-33 was a prototype made as part of an attempt by NASA to build a SSTO hydrogen fuelled space plane that failed when the hydrogen tank design proved to be unconstructable in the planned way. The Roton was an unusual attempt to build a space plane. Several configurations were evaluated ranging from a helicopter ascent to a pure rocket ascent; all landings were to use a helicopter landing system. It failed due to funding issues. Unconfirmed reportsThe March 5, 2006 edition of Aviation Week & Space Technology published a story purporting to be "outing" a highly classified US military two-stage-to-orbit spaceplane system with the code name Blackstar, SR-3/XOV among other nicknames. The alleged system, using an XB-70-like first-stage mothership, capable of mach 3, is said to launch an upper-stage wave-rider spaceplane capable of carrying small payloads and crews near to or into orbit or on skip-diving flights, ostensibly for reconnaissance and other missions, achieving surprise that cannot be attained by satellite. There has been considerable controversy over this story and its claims. The Soviet Union supposedly developed the spaceplane Uragan in the 1980s. Intended as a follow-on to Spiral and a smaller sibling to Buran, the project was cancelled in 1987, a year before the first Buran flight. The project has never been confirmed by Soviet or Russian authorities, however. See alsoReferences
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