The concept of a “space ship” (or “rocket ship”) was further developed in twentieth century science fiction such as Flash Gordon, as a self-contained, presumably rocket-powered, unitized vehicle capable of reaching an extraterrestrial destination keeping its structure intact, and requiring only refueling, like an airplane. Real-world rocket technology did not make this possible; while the airplane requires an amount of fuel occupying a relatively small fraction of the total size and mass, the rocket requires an oxidizer in order to operate in the vacuum of space. It also cannot use atmospheric air as its propellant; this function is served by the high-volume and high-mass fuel and oxidizer. Also, the high amount of energy required to reach at least low Earth orbital speed requires an extremely high proportion of propellant to dry vehicle mass. Also, mid-twentieth century structural technologies made it impossible to construct a single set of propellant tanks capable of holding enough mass to reach the required velocity. Thus, expendable multi-stage launch vehicles were the necessary design choice when spaceflight began in the late 1950s. However, starting in the 1990s, developmental work began on such unitary single-stage-to-orbit (SSTO) space vehicles with projects like X-33, Roton, McDonnell Douglas DC-X, and Skylon. By 2020, most SSTO developmental projects had failed with the exception of Skylon, which continues development.

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