Like other world powers of World War 2 (1939-1945), the Soviet Union evaluated and ultimately adopted a portable flamethrower weapon system. The series emerged from design work of the 1930s as the "ROKS" series ("Ranzewuj Ognemjot") and eventually encompassed the ROKS-2 (also "KS-2") and the simplified ROKS-3 marks. The line was in service from the period spanning 1935 to the end of the war in 1945 and was produced at various Soviet government factories from 1935 onwards.
As a man-portable, fire-projecting system, the ROKS integrated a set of backpack tanks with shoulder straps to be worn by the operator. The primary tank of the ROKS-2 was a large, flat, square-shaped unit seated atop a smaller, capsule-shaped component. The upper tank held 9 liters of fuel while the lower tank held the required nitrogen gas propellant. A delivery hose ran from the lower right of the upper tank to the awaiting projector unit which, itself, resembled a wooden-stocked service rifle. Indeed, the stock was taken from the Mosin-Nagant Model 1891/30 service rifle line and included a traditional underslung trigger group with the stock forming the grip handle. A telltale sign of the weapon was the large muzzle ignition device cover added to the business end of the projector which clearly gave the system away to discerning observers. The rifle component was also fitted with a shoulder strap for ease of transport when on the march.
In practice, the ROKS series proved as bulky in the field as their contemporaries of the period, weighing a hefty 50lb across the operator's back. Ranges reached out to about 27 yards with an extreme reach of 33 yards. Undoubtedly, the weapon was a ferocious anti-infantry device that could quickly clear dug-in enemy troops from tunnels and bunkers. However, its range was limited when compared to a conventional service rifle and its operator was made a clear target for enemy gunners and snipers - the operator almost always had to expose most of his body when firing, such was the bulky nature of this weapon. Firing duration would only last for up to eight seconds due to the limited fuel store which further limited the tactical flexibility of flamethrowing units as a whole. The primary fuel tank was usually shrouded over to help conceal the operator's true battlefield role. Flamethrower operators were seldom "lone wolf" units and were generally fielded alongside regular infantry for protection. While the tactical value of the flamethrower unit was limited by the stated ways, the psychological and physical results on the enemy were truly terrifying.
The ROKS-3 designation marked evolved flamethrowers with simplified construction practices. After the German invasion of June 1941, many Soviet factories in the West fell under the enemy onslaught and had to be rebuilt or relocated elsewhere out of harm's reach to begin manufacturing Soviet war goods. Having been a low-priority weapon prior to the war, the ROKS-2 eventually fell by the wayside which led to a more simplified production form emerging as ROKS-3. This instrument differed by its implementation of a traditional vertical "two-tank" cylinder arrangement. The fuel tank sat side-by-side with the propellant tank while the rifle-style projector arm was retained. All other functions remained the same and these weapons were pressed into service as they became available.
Some ROKS flamethrowers were known to have fallen into the hands of the Finns during the Soviets twin wars with their neighbor (the "Winter War" and the later "Continuation War"). These received the local Finnish Army designation of "liekinheitin M/41-r".