An ageing Airbus A300, currently used to train astronauts in zero-g flight, is about to find another use as the first stage of a new air-launched space vehicle.

The brainchild of Swiss Space Systems, the plan is for the jetliner to be capable of carrying a Dassault-designed reusable space plane called Soar on its back. On release, the vehicle will climb into suborbital space and launch a third orbital stage with a satellite payload of up to 250kg (550lb) into low Earth orbit (LEO) before gliding back to earth for a runway landing.

The project is just one of a number of new air launch concepts under development. Whether dropped from below, released from above, or dragged from the inside of a large transport aircraft or bomber, air-launched launch vehicles have many technical and operational advantages over their ground-launched brethren.

For example, flying shallow trajectories at altitude (sometimes using wings) can minimise drag and gravity losses. Launching at altitude lessens the compromise on a rocket's nozzle design and expansion ratio, improving its overall efficiency.

Starting from a high altitude also saves the energy needed to reach that level, while a carrier aircraft's speed can also make its own beneficial contribution to the initial launch velocity.

With no launch pad to refurbish, infrastructure costs can be minimised, as can weather delays. Likewise, reflected acoustic vibrations are negated. By flying due east - as close to the equator as possible - air launches can also get the best of the Earth's spin boost for low-inclination orbital attempts.

Alternatively, given the restrictions of some nations' geography, air launching can be an effective way of removing such limitations.

Orbital Sciences's three-stage solid-fuel Pegasus launch vehicle remains the only air-launched orbit-capable design to achieve operational status.

Most air launch concepts - and there have been almost too many to list - have either remained as paper projects or have become infinitely delayed as a result of technical problems or funding issues.

However, as Pegasus XL, and its Lockheed L-1011 TriStar carrier aircraft, draw towards the end of their careers, there has been renewed interest in using a new generation of air-launched vehicles to put payloads into orbit.

As old hands at air launching, Orbital Sciences is designing a rocket to be the launching element of Stratolaunch, an ambitious new project requiring construction of one of the largest aircraft ever built.

Its launch vehicle design is not yet finalised, but is thought likely to lean heavily on the basic aerodynamics of the Pegasus design. However, to remain competitive, the company will probably use liquid propellants over solid fuels, with a wing closer to the rear to control the ascent. The planned rocket is expected to be capable of launching 6,100kg into LEO, more than 10 times the payload capability of the Pegasus.

Space tourism operator Virgin Galactic publicly announced in July 2012 that it would be designing a small satellite launcher, dubbed the LauncherOne. This would be dropped by the company's carrier aircraft WhiteKnightTwo and be able to carry payloads of up to 225kg to low inclination LEO and up to 100kg to near-polar Sun-synchronous orbits.

Several customers have already signed for launches, and industry stalwarts Surrey Satellite Technology Limited and Sierra Nevada Space Systems are designing custom satellites for the launcher. Flights are expected to begin in 2016.

Not to be outdone, Virgin Galactic's main space tourism competitor Xcor intends to modify its Lynx II suborbital space plane with an external dorsal pod to carry a two-stage microsatellite launcher.

Boeing is in the middle of conducting an 18-month study of technologies for transporting small satellites in air-launched rockets under a $4.5 million contract from the US Defense Advanced Research Projects Agency called the Airborne Launch Assist Space Access.

In addition, the company is studying an air-dropped Small Launch Vehicle concept that uses two reusable air-breathing stages and a rocket-powered expendable stage to place 50kg payloads into orbit.

Constrained by direction limitations, Israel is again exploring the concept, re-examining the air launching of a Shavit 2 vehicle by dropping it from beneath a Boeing 747.

Israel is also interested in launching smaller 100kg satellites from F-15 fighter aircraft, using a launch vehicle based on the Rafael-produced Black Sparrow target missile.

Japan has been intrigued by air launch systems and started a technology development programme called Air Launch System Enabling Technology, examining methods of air launching satellite launch vehicles. The country has also conducted studies in "Nanolaunchers", small launch vehicles carried by fighter jets that are able to launch very small satellite payloads.

Some of the new round of air launch projects may yet prove too difficult to develop or require too much financing to proceed.

Nevertheless, innovative air launch concepts are now probably much more likely to reach fruition than in the past. This is because of a growing perceived need for rapid response launches, the availability of already-developed "stage zero" carrier aircraft and technological developments arising from suborbital space tourism operations.

Source: Flight International