The Hydra 70 rocket system is a family of unguided air-to-surface rockets. Basically, Hydra 70 refers to the MK 66 rocket motor with any warhead / fuze combination. The rocket motor was designed to provide a common rocket for helicopters and high-performance aircraft. These rockets are used to perform various roles, including anti-materiel, anti-personnel, air-to-ground suppression and illumination.
MK 66 MOD 0: the original version, which was not mass-produced.
MK 66 MOD 1: the first production version for the U.S. Army.
MK 66 MOD 2: the first production version for the U.S. Air Force and Navy. The MOD 2 development made the motor HERO (Hazards of Electromagnetic Radiation to Ordnance) safe. HERO safety prevents accidental ignition of the motor by stray electromagnetic fields like those emitted by a radar.
MK 66 MOD 3: the HERO safe version of the MOD 1 for the U.S. Army.
MK 66 MOD 4: this version has internal changes, including new initiator and igniter, for further enhanced HERO safety. It is used by all armed service branches.
M261: high-explosive MPSM (Multipurpose Submunition) warhead; contains nine M73 anti-personnel/anti-materiel bomblets, which are released in mid-air, and drag-retarded to fall vertically to the ground.
M264: red phosphorus smoke warhead.
M267: practice warhead for M261 (contains three M75 practice submunitions).
M274: practice warhead for M151 (contains a smoke signature charge).