The propulsion unit comprises an intermediate cylindrical ring (20) of the same dia. as that of the grenade (10) and equipped with a variable time delay, the ring and delay assembly being fixed to one end of the grenade's cylindrical body. The grenade also has a cylindrical propulsion base (22), of the same dia. as the intermediate ring, made with a recess for a charge and a hole for a percussion cap. The base also has a shoulder which is retained by the barrel of the grenade launching weapon. It is attached to the ring (20) by a suitable coupling and a thermo-shrink sleeve. The intermediate ring (20) is made by moulding or machining from a thermoplastic or a metal. It has cavities in its end face and a central projecting tube to receive the delay compound.