The increase in hypervelocity launcher velocities, the tendency for models to become more fragile and the use of sabot techniques has made it increasingly more important to reduce peak accelerations during the launch phase. At the same time a high acceleration must be maintained to enable a launcher barrel of reasonable length to be used. This paper considers the theoretical design of a launcher to give constant acceleration to the model. A similarity solution is found for the launcher barrel and the solution is extended to the first stage of the launcher. Departure from ideal gas laws is also discussed and the theory is extended to include the effect of molecular volume. The motion of the driving piston in the first stage is considered in detail and techniques to ensure that the piston motion results in constant base pressure on the model are presented.