Rocket propelled Grenades

[flano]

How do RPG's eliminate recoil in terms of momentum, force etc.
cheers

[Didwot]

Newtons laws of mechanics. Every action has an equal and opposite reaciton. The Action is the exhaust of the rocket flame at high velocity, the reaction is the missile flying forward. There is no recoil becaus there is nothing to recoil. In a gun there is a recoil because the action is the explosive expansion of the charge in the cartridge which both forces the gun backwards, and the bullet/shell forwards.

[Tim Baxter]

See this URL

http://science.howstuffworks.com/rpg.htm

When you fire a cartridge in a rifle, hand gun, ship's 12" gun etc the action and reaction are equal and opposite.  That is, if you fire a 50lb shell at 100mph the action of expanding gasses forcing the projectile up the barrel forces the projectile and the barrel in opposite directions but with the same inertia. So the recoil of the gun (weighing 5,000lb) will amount to 1lb at 1mph and so is easily absorbed by the gun mountings.  If the gun weighed only 5lb the recoil would be 10lb at 100mph, slightly more difficult for the mountings to handle.

The difference in the RPG is that the thrust from the rocket is allowed to pass through the launch tube (the exhaust gasses thrust against the air) and exhaust from the rear thus there is no reaction against the tube and so no recoil.

[Extrachilli]

Further to previous answers, RPGs, such as the RPG7 often seen toted around the world's trouble spots, don't completely eliminate recoil but reduce it drastically in a couple of ways. The main reduction comes from the main propulsion rocket not firing until the grenade has left the launch tube. The first stage may be likened to a gun bullet with the rocket leaving the tube at around 385 feet per second, before a piezoelectric fuze ignites the primer (a slow-burning gunpowder mix). This then ignites the rocket fuel in the sustainer motor taking the rocket to it's target at about 965 feet per second. The recoil of the initial launch is further reduced by the venturi shape of the breech at the rear of the launch tube. The venturi (a restriction in the tube, opening to a wider cone shape) aims to balance the flow of exhaust gases with the force of the exiting rocket and is common to some other systems such as the Carl Gustav anti-tank weapon.  

[Tim Baxter]

If the RPG7 is rocket launched how can it possibly recoil against its launcher?  Surely its expanding gasses are exhausd out of the rear of the launch tube?

[Extrachilli]

I'm sorry I've been away for so long! Maybe I didn't make it clear - the rocket does not actually fire until it leaves the launcher tube. What makes it leave the tube in the first place is a charge being fired, exactly like a bullet cartridge. The expanding gases from this first stage do indeed exhaust through the rear of the launch tube, after passing through the venturi (the familiar cone shape at the rear of the RPG-7).