Spaceplane

A spaceplane is an aircraft 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, and may be airbreathing or be purely rocket based.

To date, only pure rocket spaceplanes have succeeded in reaching space, although several have been carried up to an altitude of several tens of thousands of feet by conventional aircraft before release.

Description

An aerospace plane features some differences from rocket launch systems.

Aerodynamic lift

Main article: Lift (force)

All 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. It can be as high as 60 in high performance gliders, but is usually closer to 7 or less for typical supersonic aircraft configurations 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 carry the weight of an 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 penalizes 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.

Propulsion

Rocket engines

All spaceplanes to date have used rocket engines. Rockets are the only kind of engines that work in space. Due to the circularising burn necessarily being done in space orbital spaceplanes require rocket engines for at least that portions of the flight.

Air breathing engines

A difference between rocket based and airbreathing aerospace plane launch systems is that aerospace plane designs typically include 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, at takeoff, 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), 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 since, unlike oxidiser, this extra weight must be carried into space it greatly offsets the overall system performance.

Types of air breathing engines proposed for spaceplanes include scramjet, liquid air cycle engines, precooled jet engines, pulse detonation engine and ramjets.

Complexity

Because airbreathing 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 comparison of a typical jet engine to that of a rocket engine gives some indicator of the difference in complexity of the engine components.

However modern combined cycle airbreathing engines, such as Rocket Based Combined Cycle or Scramjets have minimal number of parts, far less than turbopump fed rocket engines, which burn rocket fuel in a closed turbine to power propellant pumps for the main combustion chamber. The X-43 and X-51 experimental test vehicles utilize such advanced propulsion. While earlier spaceplane designs typically had to depend on multiple types of engines on board, this is not necessary with modern technologies.

Harsh flight environment

The 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.

However, even rocket powered spaceplanes can face a harsh thermal environment as the vehicle typically flies at high speed.

Center of mass issues

A 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, however aircraft normally require horizontal stabilizers, either in the tail or ahead of the wings as canards, and modern fly by wire technology allows computers to maintain level flight even under such states of dynamic instability. Most delta winged aircraft in existence today depend on canards for such stabilization and, in the case of military aircraft, already deal with significant CG shift issues after dropping ordinance and expending large quantities of jet fuel, and such aircraft have their engine mass in the rear of the vehicle.

Overall weight

A vertically-launched rocket forms the shape of a cylinder stood on end. This structure can be made very light and strong. A horizontally-launched spaceplane approximates a cylinder on its side. This structure experiences greater bending forces, so must be strengthened. This makes it heavier, requiring advanced materials and design techniques to reduce weight. For example Burt Rutan of Scaled Composites recently patented a method of gluing the fuel tank directly to the vehicle skin, saving the weight of fasteners while also stiffening both parts.

Flown spaceplanes

Orbital spaceplanes

The 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. This significantly reduces the payload size, but the reusability is intended to offset this disadvantage.

Suborbital spaceplanes

Other (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 was capable of entering orbit, and both began independent flight only after being lifted to high altitude by a carrier aircraft.

Scaled Composites and Virgin Galactic unveiled on December 7, 2009, the SpaceShipTwo space plane, the VSS Enterprise, and its WhiteKnightTwo mothership, "Eve". SpaceShipTwo will carry two pilots and six passengers on suborbital flights once it completes flight testing some time in 2012.

XCOR Aerospace signed a $30 million contract with Yecheon Astro Space Center to build and lease its Lynx Mark II spaceplane, which takes off from a runway under its own rocket power but reaches the same altitude and speed range as SpaceShipOne and SpaceShipTwo due to the fact that Lynx is propelled by higher specific impulse fuels. Lynx also carries only a pilot and one passenger, although tickets are expected to be around half those quoted for Virgin Galactic services.

Atmospheric reentry

Because spaceplanes designed for sub-orbital trajectories do not reach orbital speed, they do 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.

Single stage to orbit

Future orbital spaceplanes may take off, ascend, descend, and land like conventional aircraft, providing true single stage to orbit or SSO 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.

The problems of overall weight described above are at their worst for SSO operation.

Other designs

United States STS Space shuttle concepts circa 1970s

Various 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 Mitchell Burnside Clapp proposed a single stage to orbit peroxide/kerosene spaceplane called "Black Horse". 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.

Germany

The Deutschland-1 orbital space plane flight, funded by West Germany, included over 7 tons of German science research equipment

After the Sänger-Bredt RaBo and Silbervogel of the 1930s and 1940s, Sänger worked for time on various space plane projects, coming up with several designs for Messerschmitt Bölkow Blohm such as the MBB Raumtransporter-8. In the 1980s, West Germany funded design work on the MBB Sänger II with the Hypersonic Technology Program. Development continued on MBB/Deutsche Aerospace Sänger II/HORUS until the early 1990s, when it was canceled in favor of the Ariane rocket, despite predicted cost savings of up to 30 percent. Also in the 1980s, Germany funded its own Space Shuttle mission known as Spacelab D-1 using the U.S. Space Shuttle. Although superficially similar to other U.S. flights, it was actually funded by West Germany with German controlled mission science. Deutschland 1 (D-1), with corresponding NASA designation STS-61-A, marked the first German astronauts in space not in a guest program, the first manned German space mission, and the only non-U.S. funded Space Shuttle mission. A D-2 mission was planned for 1988, but delayed until 1993; Deutschland 2 marked the first manned German space mission after German reunification (NASA designation STS-55). The Daimler-Chrysler Aerospace RLV was a much later small reusable space shuttle design.

Unconfirmed reports

The 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 also

• Atmospheric reentry
• Rocket
• Single stage to orbit
• Spaceflight

External links

• http://www.astronautix.com/craft/uraeptor.htm Encyclopedia Astronautica article on Uragan / Zenit