Abstract: A method and apparatus for initiating a ram acceleration of a projectile that is at rest. A projectile (34) is positioned in a starting chamber (14) of a launch tube (12). Starting chamber (14) is either filled with a gas at a relatively low pressure or is substantially evacuated. A wave reflection disk (42/42') is positioned a short distance behind the projectile. Downstream of the starting chamber are a plurality of segments, including a first segment (16), which is filled with a combustible gas mixture at a substantially higher pressure than that in the starting chamber. The first segment is separated from the starting chamber by a pair of thin membranes (20a and 20b). These membranes have a characteristic burst pressure that is about midway between the differential pressure in the first segment and the starting chamber.