reuse of the first stage. Constraints Extra weight Reusable stages weigh more than equivalent expendable stages. This is unavoidable due to the supplementary systems, landing gear and/or surplus propellant needed to land a stage. The actual mass penalty depends on the vehicle and the return mode chosen. Refurbishment After the launcher lands, it may need to be refurbished to prepare it for its next flight. This process may be lengthy and expensive. The launcher may not be able to be recertified as human-rated after refurbishment, although SpaceX has flown reused Falcon 9 boosters for human missions. There is eventually a limit on how many times a launcher can be refurbished before it has to be retired, but how often a launcher can be reused differs significantly between the various launch system designs. History With the development of rocket propulsion in the first half of the twentieth century, space travel became a technical possibility. Early ideas of a single-stage reusable spaceplane proved unrealistic and although even the first practical rocket vehicles (V-2) could reach the fringes of space, reusable technology was too heavy. In addition many early rockets were developed to deliver weapons, making reuse impossible by design. The problem of mass efficiency was overcome by using multiple expendable stages in a vertical-launch multistage rocket. USAF and NACA had been studying orbital reusable spaceplanes since 1958, e.g. Dyna-Soar, but the first reusable stages did not fly until the advent of the US Space Shuttle in 1981. 20th century Perhaps the first reusable launch vehicles were the ones conceptualized and studied by Wernher von Braun from 1948 until 1956. The Von Braun Ferry Rocket underwent two revisions: once in 1952 and again in 1956. They would have landed using parachutes. The General Dynamics Nexus was proposed in the 1960s as a fully reusable successor to the Saturn V rocket, having the capacity of transporting up to to orbit. See also Sea Dragon, and Douglas SASSTO. The BAC Mustard was studied starting in 1964. It would have comprised three identical spaceplanes strapped together and arranged in two stages. During ascent the two outer spaceplanes, which formed the first stage, would detach and glide back individually to earth. It was canceled after the last study of the design in 1967 due to a lack of funds for development. NASA started the Space Shuttle design process in 1968, with the vision of creating a fully