VTVL

Vertical takeoff, vertical landing (VTVL) is a form of takeoff and landing for rockets. Multiple VTVL craft have flown. A notable VTVL vehicle was the Apollo Lunar Module which delivered the first humans to the Moon. Building on the decades of development, SpaceX utilised the VTVL concept for its flagship Falcon 9 first stage, which has delivered over four hundred successful powered landings so far.

VTVL technologies were first seriously developed for the Apollo program. By the '90s, development on large reliable restartable rocket engines made it possible to use the already matured technology for rocket stages. The first pioneer was the McDonnell Douglas DC-X demonstrator. After the success of the DC-X prototype, the concept was developed substantially with small rockets after 2000, in part due to incentive prize competitions like the Lunar Lander Challenge.

Starting in the mid-2000s, VTVL was under intense development as a technology for reusable rockets large enough to transport people. From 2005 to 2007 Blue Origin did a series of successful tests, first with the jet powered Charon demonstrator, later using the Goddard demonstrator. Small VTVL rockets were also developed by Masten Space Systems, Armadillo Aerospace, and others. In 2013, after the failure of stage recovery with parachutes, SpaceX demonstrated vertical landing on a Falcon 9 prototype after climbing 744 meters in the air. Later, Blue Origin (New Shepard) and SpaceX (Falcon 9), both demonstrated recovery of launch vehicles after return to the launch site (RTLS) operations, with Blue Origin's New Shepard booster rocket making the first successful vertical landing on November 23, 2015, following a flight that reached outer space, and SpaceX's Falcon 9 flight 20 marking the first landing of a commercial orbital booster roughly a month later, on December 22, 2015. Many launches of the SpaceX Falcon Heavy have included VTVL attempts for the two side boosters on each rocket. SpaceX is also developing a fully reusable rocket named Starship. Starship became the first launch vehicle to demonstrate the technology with both of its stages on its fourth test flight.

VTVL rockets are not to be confused with aircraft that take off and land vertically and use air for support and propulsion, such as helicopters and jump jets which are VTOL aircraft.

The technology required to successfully achieve retropropulsive landings—the vertical landing, or "VL," addition to the standard vertical takeoff (VT) technology of the early decades of human spaceflight—has several parts. First, the thrust is normally required to be vectored and requires some degree of throttling. However, a thrust-to-weight ratio of more than 1 is not strictly necessary.

The vehicle must be capable of calculating its position and altitude; small deviations from the vertical can cause large deviations in the vehicle’s horizontal position. RCS systems are usually required to keep the vehicle at the correct angle. SpaceX also uses grid fins for attitude control during the landing of their Falcon 9 boosters.

It can also be necessary to be able to ignite engines in a variety of conditions potentially including vacuum, hypersonic, supersonic, transonic, and subsonic.

The additional weight of fuel, larger tank, landing legs and their deployment mechanisms will usually reduce the performance of a soft landing system compared to expendable vehicles, all other things being equal. The main benefit of the technology is seen in the potential for substantial reductions in space flight costs as a result of being able to reuse rockets after successful VTVL landings.