The invention concerns a propulsion device for acceleration of a self- propelled engine, such as a missile or a rocket. The propulsion device comprises a body (1) equipped with a nozzle (2) and a block of propellant (3) having a longitudinal central duct (4) of variable section. The block of propellant comprises a downstream part (5) having a cylindrical duct (6) which has a circular section whose diameter is less than two thirds of the diameter of the throat of the nozzle (2) and an upstream part (7). The duct (8) of this upstream part (7) on the one hand has a diameter which exceeds the diameter of the duct (6) of the downstream part and is less than the diameter of the throat of the nozzle (2), and on the other hand is connected to the duct (6) of the downstream part by a portion in the shape of a truncated cone (9). The length of this upstream part (7) is on the one hand between two and four times the external diameter of the block of propellant and on the other hand less than the length of the downstream part (5), the total length of the block of propellant being more than six times the external diameter of the block of propellant.
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