A propulsion system to provide silent firing of a mortar weapon using standard and nonstandard mortar tubes or launch assemblies has been invented. The system utilizes a precision rupture disc mechanism to control combustion and energy release to effect consistent operation. The gases generatedd by combustion of the propellant charge are contained by the rupture disc which insures uniform and efficient propellant combustion. After rupture of the disc at a prescribed pressure, the projectile is accelerated by the expanding gases through the action of a piston on the warhead. The gases are retained within the projectile after accelerating the projectile, preventing release of high pressure gas and resulting in a silent launch condition.In a first embodiment the warhead and piston moves first and then the heavier cylinder is moved. In a second embodiment the cylinder moves with the warhead and picks up the lighter piston. This takes less force than the first embodiment, permitting a lighter charge to achieve the same flight range for the warhead.Another improvement permits the same amount of explosive for charge 0 and charge 1 ranges. This is done by varying the size of the free volume between the piston head and the top of the cylinder which drives the warhead in flight.