The Fin-Folding Aerial Rocket (FFAR) is a 2.75 in (70 mm) diameter unguided rocket weapon commonly used by U.S. military aircraft. It was originally intended as an air-to-air weapon to allow interceptor aircraft to shoot down enemy bombers with greater range and effectiveness than machine guns or cannon, and later developed for air-to-ground use.

The advent of jet engines for both fighters and bombers posed new problems for interceptors. With closing speeds of 1,500 ft/s (457 m/s) or more for a head-on interception, the amount of time available for a fighter pilot to successfully target an enemy aircraft and inflict sufficient damage to bring it down was vanishingly small. Wartime experience had shown that .50 (12.7 mm) 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 the Luftwaffe had shown 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).

The original Mk 4 FFAR was about 4 ft (1.2 m) long and weighed 18.5 lb (8.4 kg), with a high-explosive warhead of about 6 lb (2.7 kg). It had four fins that flipped out on launch to spin-stabilize the rocket. Its maximum effective range was about 3,700 yards (3,400 m). Because of its low intrinsic accuracy, it was generally fired in large volleys, some aircraft carrying as many as 104 rockets.

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 Mk 4 was dubbed "Mighty Mouse" in service, after the popular cartoon character (whose battlecry was "Here I come to save the day!").

The Mighty Mouse was to prove 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 true 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 4/40. 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.