From:
http://science.howstuffworks.com/rpg3.htmFiring an RPG-7
The RPG operator or an artillary assistant takes a propelling charge (booster, in image below) and screws it onto the end of a warhead. Basically, this is a stabilizing pipe that has four stabilizing fins that are folded around it with two additional fins at its rear end. A cardboard container encases the back end of the stabilizing pipe. Inside the cardboard container, a squib of nitroglycerin powder is wrapped around the stabilizing pipe and a primer or charge of gunpowder is stuffed into the end of the stabilizing pipe.
The RPG operator or artillary person then takes this assembled artillery and loads it into the front end of the RPG launcher so that it lines up with the trigger mechanism.
After the RPG operator pulls the trigger, this is what happens:
The force of the built-up gases throws the grenade out of the tube at approximately 384 feet per second (117 meters per second). The abrupt acceleration of the grenade leaving the launcher triggers a piezoelectric fuze that ignites the primer (pyro-retarding gunpowder mixture). This then ignites the squib of nitro, thereby activating the rocket propulsion system (sustainer motor) to carry the grenade the rest of its trajectory.
A socket in the breach block alleviates recoil during firing. The exhaust gases exit to the rear of the launcher unit and the operator is free to immediately reload the weapon. In practice, however, no RPG operator would ever remain stationary and spend the time to reload; the launching flash and whitish blue-gray smoke provides a clear indication to the enemy of the RPG launcher's location. An effective, surviving RPG operator is one who quickly changes position and gets under cover.
