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The exploration of the moon is within view today. If it may be assumed that Project Mercury in the U. S. A. and similar efforts by the U. S. S. R. will establish that man can exist for limited periods of time in space, then a trip to the moon requires mainly the design, construction and proving of a large rocket vehicle. In one concept of a manned lunar vehicle, the entire mission, the trip to the moon and the return, is staged on the earth's surface. A highly competitive technique is to stage the lunar mission by refueling in a low earth orbit. This would permit the use of a smaller launching vehicle but would require development of orbital rendezvous techniques. This paper presents a parametric study of vehicle size for the direct-flight manned lunar mission. The main parameter is the take-off thrust which is influenced by many factors, principally the propellants in the several stages and the flight trajectory. A close choice exists in the second stage where conventional and high-energy propellants are compared. The size of the final stage and hence the entire vehicle is governed mainly by the method of approach to the earth's surface, whether the approach is made at elliptic, parabolic or hyperbolic velocities. The various design choices are applied to an illustrative vehicle configuration.