User:Jackehammond/sandboxes-high low system

The High-Low system also referred to as the "High-Low Pressure system", the "High-Low Propulsion System" and "High-Low projection system", which has been referred to by most publications as "employing some kind of the High-low System" ; is a method for cannons and antitank launcher designs where there is a smaller high-pressure chamber in which the propellant is stored. And when propellant is ignited, the higher pressure gases are bled out through vents (or ports) at reduced pressure to a much larger low pressure chamber to "push" the projectile forward. With the High-Low System a weapon can be designed with reduced or nil recoil. And with the High-low system, besides a great reduction in recoil, the weight of the weapon and its ammunition is significantly reduced; and manufacturing cost and time of production are drastically lower than standard cannon or other small arm weapon systems firing a projectile of the same size and weight. Also, there is far more efficient use of the propellant, unlike earlier recoilless weapons, where most of the propellant is expended to the back of the weapon to counter the recoil of the projectile being fired forward.

World War II History and Description
In the last years of World War II the Germans became desperate for antitank weapons. Large antitank cannons firing high velocity projectiles were the best option, but were expensive to produce and required a well trained crew and lacked mobility on the battlefield once emplaced. Antitank rocket launcher and recoilless rifles while much lighter and simpler to manufacture gave the gunners position away, were not as accurate as the antitank cannons and with the recoiless rifles used a huge amount of propellant to fire the projectile, with estimates of between only one-fifth to one-ninth of the propellant gases being used to push the projectile forward. The German military asked it defense industry for ideas, for an antitank weapon that was between the standard high velocity cannon and the cheaper rocket and recoilless infantry antitank weapons. And what ever ideas were presented had to be far more efficient in the use of propellant as the German war industry was at maximum production capacity of propellant for cannons.

In 1944 the German defense firm Rheinmetall-Borsig came up with a completely new concept for propelling a projectile from cannon, which while not recoilless, had greatly reduced recoil and drastically reduced the manufacturing cost. There concept was called Hoch-und-Niederdruck System which roughly translates to "High-Low Pressure System". With this system, only the very back of the cannon's breach had to be re-enforced against the high firing pressures.

Rheinmentall designed an antitank cannon using their "High-Low Pressure System" that fired a standard general purpose HE 81mm mortar bomb, that had been modified to an antitank round with a shape charge. . Normally a mortar bomb can not be fired from a cannon, because their thin walls can not take the stress on firing. The special 8.1cm round was mounted on a rod which was fixed to a round steel plate with 8 holes in it. A shear pin held the round to the rod. The round and the plate were fitted at the mouth of a cut down cannon shell casing which contain the propellant bags. On firing the pressure would build up in the shell casing, which along with the re-enforced breach, acted as the "High Pressure Chamber" and bled out the steel plate holes at half the pressure to the thinner walled cannon barrel which acted as the "Low Pressure Chamber". Unlike the standard cannon, where the propellant basically "kicks" the projectile out the barrel with an almost instant acceleration to maximum muzzle velocity, the Rheinmentall concept "shoves" the projectile out the barrel at a constantly increasing muzzle velocity. There is recoil, but no where near the recoil of the 5.5cm and 7.5cm antitank cannons being used at that time period by the German Army, which required a heavy constructed carriage, and both a hydraulic recoil mechanism and muzzle break to contain the massive recoil on firing, only a muzzle brake was required on the new lighter weight cannon design. The only major draw back was its 750 meters maximum range, but this was offset by an armor penetration of 140mm and not tell tale back blast. The German Army ordered the Rheinmentall cannon into production, as the 8cm Panzer Abwehr Werfer 600 (PAW 600) Only about 250 were produced before the war ended. None were reported to have seen combat.

Post World War II History and Description
The Allies had captured and examined the PWK 600 but little interest was taken in the new system developed by the Germans. The first example of a type of High-Low System after World War II was by the UK with the Limb antisubmarine weapon, which launched depth charge like projectiles. The Limbo was a development of the World War II Squid which while effective was limited by a set range 275 meter. With Limbo, the range could be varied between 336 meters to almost 1000 meters, by opening and closing vents, varying the pressure of the propellant on firing.



The most widely and well known use of the German developed principle of World War II, was by the US Army with the introduction of the M79 grenade launcher shortly before the Vietnam War. The M79 fired a 40mm shell which contained a standard fragmentation grenade with a modified fuze. The cartridge casing contained a heavy cup shape "High Pressure Chamber" in the bottom. On firing the propellant builds up pressure till it breaks through the copper cover, venting out to the "Low Pressure chamber (see two illustration). The US Army referred to their high-low system as the High-Low Propulsion System.  Along with a heavy rubber pad on the M79 butt stock recoil forces were kept in bounds for the infantry man using the weapon.  The M79 was later replaced by the M203 which mounts under the assault rifle.  Later the US Army developed a higher velocity 40mm round using their High-Low Propulsion system for use by heavier machine gun type grenade launchers used on vehicles and helicopters.  Today, besides the US military, the 40mm grenade family is extremely popular and in use by armies world wide and man variants of it in production by countries other than the US, with one reputable reference publication, needing almost a dozen pages to list all the versions and nations producing 40mm grenade ammunition. (see two illustration at right)

Shortly after the Vietnam War ended, the Russians introduced 30mm grenade launcher that used the High-Low Principle, but with a twist on the US Army developed 40mm grenade. The Russian 40mm grenade launcher fits under the assault rifle and fires a case less projectile that is muzzled loaded. Instead of having a case, the high-pressure chamber is located on the rear of the projectile with ten vent holes, with the launcher barrel acting as the high-pressure chamber.

Besides the previously mentioned family of popular 40mm grenades, developed before the Vietnam War and after, the only other major use of a high-low system was by the Swedish firm of FFV in the development of the Miniman one man infantry antitank weapon which was developed in the late 1960s. The Miniman was simpler and cheaper than even what the World War II designers could have imagined. Inside the rocket looking launcher tube, is the HEAT antitank projectile attached by a break away bolt to a alloy aluminum tube with ports drilled in in it that acts like a kind of high-pressure chamber. The launch tube, in which it is mounted acts at the high-pressure chamber. When the propellant is ignited in the aluminum tube, gases escape through the ports and is allowed to build up in the launch tube, to the point of almost causing a recoil and then the break away bolt snaps allowing the projectile to move forward. Unlike the other high-low systems gases are allowed to escape to the rear of the launch tube, to achieve a total recoilless effect.

Also, while very little is known, in the 1950s the Russian Army developed 73mm cannon for wheeled armored reconnaissance vehicles that fired a munition very similar to the original World War II German concept. But it was never introduced into service, and instead the Russians developed a 73mm cannon low velocity cannon that fired a rocket projectile that was ejected by a small charge.