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Space Shuttle Concepts illustration showing late 1960's designs, part of the Phase A-prime process.

The baseline design at the bottom represents a composite of features from different individual designs that met the Manned Spacecraft Center's (MSC) basic requirements spec. All of these were based on a two-stage design, with both stages manned and capable of a horizontal landing. This design had evolved out of a series of evolutionary studies during the Phase A process, and NASA planners became concerned that the resulting designs were too focused on the two-stage concept.

So as part of an A' series, they asked for more radical solutions. Some of the results are shown as the upper five concepts in this image. The two designs directly above the baseline concept were variations on the basic concept; on the right the fly-back booster has been replaced by two large booster rockets running down the side of the fuselage, while on the left the orbiter has been replaced by a conventional disposable rocket on top of the baseline booster.

In the middle-right is the North American DC-3 concept developed by Maxime Faget's team at the Marshall Space Flight Center (MSFC). This was largely a replacement of the MSC's delta-wing design with straight wings, which offered better performance and far more flexibility to change the position of the wing in the case of the almost unavoidable changes in center of gravity. In spite of never being a serious contender for the Shuttle program, the team was able to secure considerable funding, developing free-flight models and creating extensive design materials which were found in publications well into the 1970s.

At the top are the two most radical concepts, the Lockheed Star Clipper on the right and the Chrysler SERV on the left. The Star Clipper (and the somewhat similar McDonnell Douglas proposal, Tip Tank) were stage-and-a-half designs using a single powered orbiter combined with disposable drop tanks. SERV was the most radical design by far, a single-stage-to-orbit (SSTO) vehicle using an advanced engine design capable of completely automated missions. In this illustration SERV is shown with a fly-back shuttle on top, the MURP.

The various designs are not shown to scale. DC-3 is somewhat larger than the baseline design, and SERV is larger than any of them - the MURP was roughly the size of the DC-3 orbiter, which provides some sense of scale.