Some logical aspects of a digital computer for a space vehicle are described, and the evolution of its logical design is traced. The intended application and the characteristics of the computer's ancestry form a framework for the design, which is filled in by accumulation of the many decisions made by its designers. This paper deals with the choice of word length, number system, instruction set, memory addressing, and problems of multiple precision arithmetic. The computer is a parallel, single address machine with more than 10,000 words of 16 bits. Such a short word length yields advantages of efficient storage and speed, but at a cost of logical complexity in connection with addressing, instruction selection, and multiple-precison arithmetic.