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.[citation needed]
Air launch to orbit also works well as part of a combination launch system such as a reusable air-launched single stage to skyhook launch vehicle powered by a rocket or rocket/ramjet/scramjet engine.
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.[1]