Air launch to orbit
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Air launch to orbit is the method of launching rockets at altitude from a conventional horizontal-takeoff aircraft, to carry satellites to low earth orbit. It is a follow-on development of air launches of experimental aircraft that began in the late 1940s. This method, when employed for orbital payload insertion, presents significant advantages over conventional vertical rocket launches, particularly because of the reduced mass, thrust and cost of the rocket.
Air launching is also being developed for sub-orbital spaceflight. The Ansari X Prize $10 Million purse was won by a team led by Burt Rutan's Scaled Composites, launching the SpaceShipOne from the purpose-built White Knight carrier aircraft.
The principal advantage of a rocket being launched by a high flying airplane is that it need not fly through the low, dense atmosphere, the drag of which requires a considerable amount of extra work and thus mass of propellant. Higher densities at lower altitudes result in larger drag forces acting on the vehicle. In addition, thrust is lost due to over-expansion of the exhaust at high ambient pressure and under-expansion at low ambient pressure; a fixed nozzle geometry cannot provide optimal exhaust expansion over the full range of ambient pressure, and represents a compromise solution. Rockets launched from high altitude can be optimized for lower ambient pressure, thus achieving greater thrust over the entire operating regime.
Propellant is conserved because the air-breathing carrier aircraft lifts the rocket to altitude much more efficiently with the use of engines that do not require on-board storage of an oxidizer. This allows the launch system to conserve a significant amount of mass that would otherwise be reserved for fuel, reducing the overall size. A larger fraction of the rocket mass can then include payload, reducing payload launch costs. It is also possible to make use of higher-impulse fuels precluded from surface launches due to their toxicity, such as those containing beryllium or fluorine.
Air launch to orbit offers the potential for aircraft-like operations such as launch on demand, and is also less subject to launch-constraining weather. This allows the aircraft to fly around weather conditions as well as fly to better launch points, and to launch a payload into any orbital inclination at any time. Insurance costs are reduced as well, because launches occur well away from land, and there is no need for a launch pad or blockhouse.
An additional benefit of Air launch to orbit is a reduced delta V needed to achieve orbit. This results in a greater payload to fuel ratio which reduces the cost per unit mass to orbit. To further leverage the Delta V advantage, supersonic air launch to orbit has been proposed.
According to Aviation Week and Space Technology, air launch to orbit is limited by aircraft size. Additionally, airplanes may generate large lateral forces which could damage payloads.
Air launch systems
- Under development
- Virgin Galactic LauncherOne
- XCOR Aerospace Lynx Mark III
- Orbital Sciences Pegasus II – contracted design/build for Stratolaunch Systems
- CubeCab 
- ARCASPACE
- Generation Orbit Launch Services - contracted for NASA NEXT
- Swiss Space Systems SOAR
- NASA Armstrong Flight Research Center Towed Glider Air-Launch System
- Vulcan Aerospace 75-percent-scaled Dream Chaser crew-carrying spaceplane with rocket by Orbital Sciences
- OREL (proposed by Ukraine)
- Sura (proposed by Ukraine)
- Abandoned projects
- Norris, Guy (15 February 2015). "Design Space". Aviation Week and Space Technology (Volume 177 Number 2).
- Bergin, Chris (2013-05-25). "Stratolaunch and Orbital – The Height of Air Launch". NASASpaceFlight.com. Retrieved 2013-05-24.
- "Technologies". Retrieved 2015-12-01.
- Leone, Dan (November 26, 2013). "Startup Generation Orbit Launch Service Bets Big on ‘Small Space’".
- Diller, George (September 30, 2013). "NASA Awards First CubeSat-Class Launch Services Contract".
- Gebhardt, Chris (2014-11-26). "SNC, Stratolaunch expand on proposed Dream Chaser flights". NASASpaceFlight.com. Retrieved 2014-11-27.
- Russia, Kazakhstan to develop unique space system: "Ukrainian experts moved to develop the Svityaz system based on the An-225 Mriya (Dream) Cossack jumbo transport plane and the Zenit-2 rocket", "The Ishim complex will include two MiG-31I aircraft, a three-stage launch vehicle on a streamlined store between engine nacelles, as well as an Ilyushin Il-76MD Midas surveillance plane."