It is necessary to clearly differentiate between infantry antitank weapon systems that are rockets, and those that are recoilless guns. Most frequently the majority of such weapons are described as rocket launchers, but this is glaringly inaccurate.
The well-known M136/AT4 single-shot weapon in wide- spread use by the U.S. Army is in fact a disposable recoilless launcher. The infamous RPG-7, despite firing rocket-assisted projectiles, is a reusable recoilless launcher. One supposes the key distinguishing feature is where the pressure, built up by propellant combustion, occurs, and where the pressure drop which produces the propulsive force occurs. In a rocket system, the propellant combusts entirely within the rocket itself and the pressure drop which produces the propulsive force occurs across the body of the munition. This may best be illustrated by considering the case of a rocket like that from an M72 LAW being ignited outside its launch tube. The rocket would travel just as far as if it were fired normally; the tube only provides for initial aiming, and does not contribute to the propulsive process. In a recoilless weapon, the launch tube is an integral part of the propulsive process, and incorporates a chamber for the propellant to burn at a relatively high pressure, and a nozzle to create a constriction that vents the high pressure gases rearwards, usually at an accelerated velocity, whose momentum is then used to balance exactly the momentum of the projectile leaving the muzzle. If one were to ignite the propelling charge of a recoilless round like a PG-2 or a PG-7 in the open, it would simply burn. The grenade would not go anywhere. Without a chamber to allow burning at a high pressure, no propulsive force is generated.
It’s also worthwhile to take a brief look at the physics of recoilless weapons. Simply put, recoilless guns work by expelling a projectile from the front in the usual manner, and a countermass out the back of the gun. The earliest recoilless guns were the Davis Guns of WW1. These used a central propelling charge to fire a projectile out of a forward-pointing barrel, and a solid countermass of equal weight out a rearward pointing barrel of identical length. While solid countermass recoilless guns have been in use since then, and a few still are, in most applications a solid countermass is a nuisance at best and a danger to one’s own troops at worst. Between the World Wars it was found that it was only necessary to match the momentum (mass times velocity) of the countermass to that of the projectile. Thus, a very light countermass, such as propellant gas moving at a very high velocity, can have a momentum equal to a heavy projectile discharged at a lower velocity.
Most recoilless guns using a propellant gas countermass feature a prominent nozzle or nozzles at the back of the weapon. These are sometimes called “venturis” (acceptable) or “blast cones” (incorrect); more on these below. But one can’t help but note that the Panzerfausts and the RPG-2 had simple straight-tube launchers with neither constricting orifices nor conical venturis. So how did these weapons function and qualify as “Recoilless,” without simply venting the propellant gases out the back at low pressure?
The answer lies in the fluid mechanics of compressible fluids. Most of us are aware that passing a fluid through a constriction will raise the velocity of the fluid. (Simply take your garden hose and constrict the water stream with your thumb, and watch how the water speeds up.) The higher the upstream pressure the greater the downstream velocity. But in gas systems, this only happens until a condition called choked flow is reached. At that point further increases in upstream pressure do not cause further increases in downstream velocity.
The result in a recoilless weapon is a rise in pressure sufficient to launch a projectile. While this principle is the basis for most recoilless weapons, in straight tube launchers it has substantial performance limitations. The outlet velocity of the propellant gases in a straight tube launcher remains subsonic. To achieve a choked flow situation quickly, very fast burning propellant is required; in the RPG-2 fine granular black powder is used. But this fast propellant in turn causes a rapid pressure rise in the area of the propelling charge. The maximum pressure must be limited to remain within the strength limits of the tube for function and safety. So there’s a limit to the weight of propellant that can be used, which in turn limits the mass of propellant gas available to form a countermass. As already mentioned, the gas velocity is limited in this system as well, the result is a relatively low available counterrecoil momentum. The final result of all these limiting factors is a very low muzzle velocity for a projectile of useful size. This was readily apparent in the early Panzerfausts, whose effective range was severely limited, at first to only 30 meters, by their low velocity and resultant highly curved trajectory
Since gas velocity and tube strength impose limits on available counterrecoil momentum, the only way to really improve this system’s performance was to add additional propellant (and thus additional gas for the countermass), and since increasing the charge attached to the projectile would only increase the local pressure to unacceptable levels, the only solution available was to apply the maximum operating pressure over a greater length of the launch tube by distributing the propelling charge. In the later, longer-range Panzerfausts this was achieved by adding a secondary propelling charge approximately in the middle of the launch tube. The charge at the base of the projectile was initiated in the usual way, and this in turn ignited the secondary charge, boosting muzzle velocity and thus range, up to 100 meters and more.
The RPG-2 uses a rather more ingenious solution, with the black powder propelling charge subdivided into 6 increments by means of cardboard tubes and discs, the latter with flash holes to foster ignition. The primer in the base of the PG-2 grenade ignited the first increment, which burns rapidly, creating pressure and pushing the remaining four increments back down the tube. After a few inches of travel, the second increment is fully ignited, then the third, fourth, fifth, and sixth. The result is high pressure over a greater length of the tube, rather than merely at the base of the projectile, and a greater volume of propellant gases for both propulsion and countermass.
But even with this technique, there were limits to the performance of a simple straight tube launcher. One attempt at improvement was the Yugoslav M57 launcher, which incorporated a partial solid countermass in the form of a quantity of sand. However, this represented at best an incremental improvement over the simple straight tube launchers.
While the PG-2 grenade performed well enough in terms of armor penetration (and remains a threat to all but the most modern armored fighting vehicles), the weapon’s fatal flaw was its very primitive recoilless launcher. While its simple cylindrical tube was easy and cheap to fabricate, its lack of a chamber and nozzle, and resultant low pressure combustion, as described above, severely limited the velocity of the PG-2. This made range estimation very important at all but the shortest ranges. Additionally, the low velocity meant a longer time of flight. Both factors limited hit probability against both stationary and moving targets. An interim solution was found in the RPG-4. While this fired purely ballistic grenades, which resembled PG-2s albeit with increased standoff for the shaped charge, the 45mm diameter launch tube incorporated a larger diameter chamber and a venturi, or nozzle, at its rear end, causing it to greatly resemble its successor the RPG-7. The RPG-4, developed in the late 1950s, was not produced in quantity.
A brief description of the physics of the RPG-4 and RPG-7 is in order. These weapons incorporate the features seen in other, larger recoilless systems: a chamber of larger size than the propelling charge, and a convergent-divergent nozzle incorporating a constriction and a divergent (outlet diameter larger than inlet diameter) conical section. The constriction sets up the choked flow condition described above, albeit without the need to use a very fast propellant. The conical divergent nozzle in turn accelerates the propellant gases to supersonic speeds. Thus, in this type of system, there is much more counterrecoil momentum produced, both from burning more propellant over a longer time and from expelling the gas countermass at a much higher velocity. In the case of the RPG-7, this allowed the launching of projectiles heavier than PG-2s at velocities in some cases approaching double those of the older system. The RPG-7 recoilless launcher, introduced in 1961, reverted to the 40mm tube diameter of the RPG-2 while retaining the chamber and nozzle design of its precursor the RPG-4.
A brief observation on the function of its ammunition will be made in the hope of finally putting to rest the absurd “rocket propelled grenade” name. The RPG-7’s principal munition is one of a series of PG-7 antitank grenades. All of these incorporate a rocket motor. As the RPG-7 is a recoilless launcher, the PG-7 may be considered a rocket-assisted projectile; calling it a rocket would be incorrect. The reason for adoption of a recoilless-launched, rocket-assisted antitank munition of greater-than-average complexity is simple: it increases the munition’s velocity and thus reduces its trajectory and time of flight, without imposing any additional penalties on either the launcher or its user. The result is greatly improved hit probability against both moving and stationary targets.
In a 1970s study the US Army deemed the RPG-7 the best solution to hitting armored vehicles out to 300m. Better than pure rocket systems and better than pure recoilless systems.