Warmatrix

A rocket propelled grenade (RPG) is an explosive grenade that is fired by a rocket. The term is usually applied to small hand held weapons, not large missiles.
Though both grenades and rockets were invented as early as the Middle Ages, they were not combined until the mid 20th century, because for a long time the double explosive combination was just as dangerous to friendly troops as the enemy. That started to change when the Second Industrial Revolution brought new materials and production methods. In 1883 CE the first 'shaped charge' was developed by Max von Foerster in Germany.
A shaped charge has explosives encased in a hollow warhead. When it hits its target, the explosive detonates and the hollow cone concentrates the outgoing energy straight forward, creating a hypersonic stream of melted metal that is able to punch through thick walls or armor. Though temperatures at the time of the explosion soar, the energy of the attack is kinetic, not thermal. Early shaped charges could penetrate steel to a depth of around 1½x - 2x the diameter of the warhead; modern ones achieve 7x - 10x. This makes them much more powerful than traditional solid armor-piercing shells.
Shaped charges were ignored by the military for several decades, then suddenly were rediscovered in the middle of World War II. The steady increase of armor thickness of the tanks during that conflict left the infantry almost powerless against them. An answer was found in small, portable rocket launchers mounted with shaped charge warheads, creating what the Soviets called rocket propelled grenades. The USA developed the bazooka; Britain the PIAT; Germany the panzerfaust and panzershreck. Suddenly infantry, armed with the new weapons, was able to take on tanks, though only at close range, as early RPGs had low accuracy. Nonetheless in rough terrain and urban environments they provided a deadly anti-tank defense.
The most famous RPG is the Soviet RPG-7, introduced in 1961 CE, which has become something of a blueprint for other models all over the world. It is just under a meter long, weighs about 8.5 kilograms loaded. Its maximum effective range is 500 meters for stationary targets; 300 for moving ones. The RPG-7 is a re-usable launcher; some other models are disposable weapons. Some blast off their rockets with a conventional gunpowder charge first and fire the rocket when the missile is already underway. As spinning hampers the effectiveness of RPGs, the trajectory of most missiles is not stabilized by rotation like with ordinary bullets, but by tailfins. Despite the fins RPGs cannot match the accuracy of conventional bullets and thus remain limited to short and medium ranges. Some warheads detonate on impact, others after a preset amount of time, or both. Not all RPGs are anti-tank weapons. Instead of shaped charges ordinary grenades can be used as warheads, effective against 'soft' targets, like infantry.
Shaped charges are not only used with RPGs, but also in long range unguided rockets, torpedoes, guided missiles, cluster bombs, mines and other weapons. In their anti-tank roles they are usually called High Explosive Anti-Tank (HEAT). Tanks designers have responded to the HEAT threat by equipping tanks with cage, composite and reactive armor. In turn HEAT missiles have become equipped with multiple charges, each designed to penetrate a different layer of armor. The weapons race between tank and anti-tank missiles continues to this day.
The storm of arms developments have left the simple, hand-held RPGs no longer powerful enough to take out tanks, except when they hit weak spots. But because of their low cost and their deadliness against armored personnel carriers, helicopters and other lightly armored vehicles, they remain popular among both irregular forces and professional armies.