boosters Horizontal (winged) Single or main stages, as well as fly-back boosters can employ a horizontal landing system. Examples are: Space Shuttle orbiter - as part of the main stage Venturestar - a project of NASA Space Shuttle's studied fly-back booster Energia II ("Uragan") - an alternative Buran launch system concept OK-GLI - another Buran spacecraft version Liquid Fly-back Booster - a German concept Baikal - a former Russian project Reusable Booster System - a U.S. research project SpaceShipTwo - a spaceplane for space tourism made by Virgin Galactic SpaceShipThree - a spaceplane under development for space tourism made by Virgin Galactic Dawn Mk-II Aurora - a spaceplane under development by Dawn Aerospace XS-1 - another U.S. research project RLV-TD - an ongoing Indian project Reaction Engines Skylon SSTO A variant is an in-air-capture tow back system, advocated by a company called EMBENTION with its FALCon project. Vehicles that land horizontally on a runway require wings and undercarriage. These typically consume about 9-12% of the landing vehicle mass, which either reduces the payload or increases the size of the vehicle. Concepts such as lifting bodies offer some reduction in wing mass, as does the delta wing shape of the Space Shuttle. Vertical (retrograde) Systems like the McDonnell Douglas DC-X (Delta Clipper) and those by SpaceX are examples of a retrograde system. The boosters of Falcon 9 and Falcon Heavy land using one of their nine engines. The Falcon 9 rocket is the first orbital rocket to vertically land its first stage on the ground. Both stages of Starship are planned to land vertically. Retrograde landing typically requires about 10% of the total first stage propellant, reducing the payload that can be carried due to the rocket equation. Landing using aerostatic force There is also the concept of a launch vehicle with an inflatable, reusable first stage. The shape of this structure will be supported by excess internal pressure (using light gases). It is assumed that the bulk density of the first stage (without propellant) is less than the bulk density of air. Upon returning from flight, such a first stage remains floating in the air (without touching the surface of the Earth). This will ensure that the first stage is retained for reuse. Increasing the size of the first stage increases aerodynamic losses. This results in a slight decrease in payload. This reduction in payload is compensated for by the