The 2.75" Folding Fin Aerial Rocket (FFAR) :
The advent of jet engines for both fighters and bombers posed new problems for interceptors. With head-on speeds of 1,500 ft/s or more, the time a pilot had to find, target, and fire at incoming aircraft was almost non-existent. Wartime experience had shown .50 caliber machine guns were not powerful enough to reliably down a bomber, certainly not in a single volley, and heavy cannon did not have the range or rate of fire to ensure a hit. Unguided rocket weapons had been proven effective in ground-attack work during the war, and late in the war air-to-air rocket attacks on Allied bomber formations proved that volleys of rockets could be a potent air-to-air weapon as well. The FFAR was developed in the late 1940s by the US Navy Naval Ordnance Test Center and North American Aviation, based on the German R4M rocket (used by the Messerschmitt Me-262 and others).
FFARs were the primary armament of many USAF interceptor aircraft in the early 1950s, including the F-86D, F-89, and F-94C. They were also carried by the F-102 Delta Dagger to supplement its guided missile armament.
The Mighty Mouse ultimately proved to be a poor aerial weapon. Although it was powerful enough to destroy a bomber with a single hit, its accuracy was abysmal. Its spin rate was not high enough to compensate for the effects of wind and gravity drop, and the rockets dispersed widely on launch: a volley of 24 rockets would cover an area the size of a football field.
As a result, by the late 1950s it had been largely abandoned as an aircraft weapon in favor of the guided air-to-air missiles then becoming available. The Mk 4 found other uses, however, as an air-to-ground weapon, particularly for the new breed of armed helicopter. A volley of FFARs was as devastating as a heavy cannon with far less weight and recoil, and in the ground-attack role its marginal long-range accuracy was less important. It was fitted with a more powerful motor to become the Mk 40. The Mk 40 was a universal motor developed from the Mk 4 2.75 FFAR, and could be fitted with different warheads depending on the mission. Pods (typically carrying seven or 19 rockets) were created for various applications, and a wide variety of specialized warheads were developed for anti-personnel, anti-tank, and target-marking use. The FFAR has been developed into the modern Hydra 70 series, which is still in service.