The HYDRA 70 (70mm) Rocket System is a family of 2.75" unguided rockets.The 2.75 inch Folding-Fin Aerial Rocket (FFAR) was originally developed by the US Navy for use as a free-flight aerial rocket in the late 1940s. Used during both the Korean and Vietnam wars, their role has expanded to include air-to-ground, ground-to-air, and ground-to-ground. The 2.75 inch rocket system has a rich history of providing close air support to ground forces from about 20 different firing platforms, both fixed-wing and armed helicopters, by all US armed services. When the requirements of this system were changed to a new air-to-ground role for fixed and rotary wing aircraft, new fuzing and warhead performance characteristics, as well as a modified motor for low speed aircraft became necessary. The HYDRA 70 family of rockets was designed to fill this role. The Hydra 70 rocket system is used by US Army Special Operations Forces, the US Marine Corps, the US Navy, and the US Air Force. The Hydra-70 rocket is fired from all armed Army Helicopters and the armed helicopters of most sister services. The rocket is also fired from many U.S. fixed wing platforms and is a major export munition to many allied nations. The Army's Hydra-70 PM at Rock Island, IL is assigned as the single item manager responsibile for meeting the rocket needs of all users.
The war reserve unitary and cargo warheads are used for anti-materiel, anti-personnel, and suppression missions. The Hydra 70 family of Folding-Fin Aerial Rockets (FFAR) also includes smoke screening, illumination, and training warheads. These rockets are used by rotary, wing, fixed and ground platforms. The most widely used application is on helicopters for air-to-ground engagements.
In the US Army, Hydra 70 rockets are fired from the AH-64A Apache/AH-64D Apache Longbow using M261 19-tube rocket launchers, and the OH-58D Kiowa Warrior and the AH-1F "modernized" Cobra using seven-tube M260 rocket launchers. The AH-1G Cobra and the UH-1B "Huey" used M200 19-tube rocket launchers. The Navy uses the 19 round LAU-61C/A and the seven round LAU-68 D/A rocket launchers. These reusable
launchers have an external thermal coating that greatly prolongs cook-off protection time.
Full production of these launchers began in June 1985.
To provide some stability the four rocket nozzles are scarfed at an angle to impart a slight spin to the rocket during flight. The modified motor provides increased stand-off range and reduced ballistic dispersion. The MK 66 rocket motor was designed to provide a common 2.75-inch rocket for helicopters and high-performance aircraft. Compared to the MK 40 motor, it has a longer tube, an improved double base solid propellant, and a different nozzle and fin assembly. Increased velocity and spin provide improved trajectory stability for better accuracy. The launch signature and smoke trail have been significantly reduced. The MK 66 Mod 1 is not hazards of electromagnetic radiation to
ordnance safe. It can be inadvertently ignited by electromagnetic radiation, especially by radio frequencies found aboard Navy ships. Both the Mod 2 and Mod 3 have HERO filters, and the Mod 2 filter may prevent the AH-1 rocket management system from inventorying. The Mod 1 is the standard motor for Army use as will be the Mod 3 when it is fielded.
The HYDRA-70/2.75 Inch Rocket System is managed by Project Management Office, 2.75-Inch (70mm) Rockets. This office is a new Project Management Office, effective 01 October 1997. The 2.75"/HYDRA Rocket Acquisition Team received the Secretary of the Army Award for Excellence for Systems Contracting at a ceremony on 14 December 1998. Currently, General Dynamics is the general contractor for the rocket system and Thiokol produces flare rockets.
Components
The HYDRA-70 Rocket System is the Army name for the improved 2.75 Inch Rocket System and includes the following items:
M260/M261 Light Weight Launchers (LWLs)
MK66 Rocket Motor, Mods 1, 2, & 3
M151 (10 lb) HE Warhead with M423 PD Fuze
M229 (17 lb) HE Warhead with M423 PD Fuze
M255A1 Flechette Warhead with M439 RS Fuze
M257 Illumination Warhead with M442 Fuze (delay)
M261 MPSM/HE Warhead
M264 Smoke Screening (RP) Warhead with M439 Fuze
M267 MPSM Practice Warhead
M274 Smoke Signature Practice Warhead
M278 IR Flare Warhead with M439 RS Fuze The M260 Light Weight Launcher is a seven-tube launcher.
Configuration Center of Gravity (In Inches from Front)
Weight (lbs)
Empty 35.2 35.2
M229/M423/MK66 (HE) 25.3 196.2
M151/M423/MK66 (HE) 32.9 202.5
M151/M433/MK66 (HE) 32.3 317.7
M257/MK66 (Illumination) 28.9 205.0
M264/MK66 (Smoke) 31.8 192.7
M261, M267/MK66 (MPSM) 28.4 225.6
M255A1/MK66 (Flechette) 28.4 225.6
M274/MK66 (Practice) 32.9 196.2
The M261 Light Weight Launcher is a nineteen-tube launcher.
Configuration Center of Gravity (In Inches from Front)
Weight (lbs)
Empty 35.8 82
M229/M423/MK66 (HE) 27.2 660
M151/M423/MK66 (HE) 33.1 518
M257/M442/MK66 (Illumination) 29.0 542
M264/M439/MK66 (Smoke) 32.4 493
M261/M439/MK66 (MPSM) 28.5 598
M255A1/M439/MK66 (Flechette) 28.4 604
M274/MK66 (Practice) 32.9 516.0
The MK66 motors use a longer motor tube (than the older MK40/MK4), that is of a different aluminum alloy, and is assembled with a new fin and nozzle assembly. The fins are of a spring loaded, wrap around design, and are attached around the circumference of the single nozzle. The propellant grain is longer and of a different formulation than that of the MK40/MK4, however, the stabilizing rod and igniter are essentially the same design. The MK66 motors have a substatially higher thrust, 1335 lbs, and a longer range than the older motors. The current generation of the MK66 in use by the U.S. Armed Services are the MK66 MOD 3 for the Army, and the MK66 Mod 2 for the Air Force, Navy, and Special Operations Force. The MOD 3 incorporates a Hazard of
Electromagnetic Radiation to Ordnance (HERO)safe electronic RF filter in the igniter circuit. The RF filter is mounted onto the igniter can and allows the aircraft's direct current firing pulse to pass to the igniter squib, but absorbs and dissipates RF energy. The RF filter does not change the electrical resistance of the firing circuit. Electromagnetic Test Report SR-RD-TE-87-43 dated 8 May 1987 documents the HERO suitability of the MK66 MOD 3 motor. A brass EMR shield is used over the fin and nozzle to prevent the DC energy produced by the eletrical arcing encountered when loading motors into launchers in high intensity RF fields from igniting the motor. These shields are furnished with the MOD 3 motor and should be retained for unloading rockets. Due to the
shipboard concern of Foreign Object Damage (FOD), the other Armed Services use the MOD 2 motor. The MOD 2 contains a Dahlgren Bridge Assembly (DBA) for a HERO filter. The DBA is a wheatstone bridge designed to prevent the RF of concern from igniting the motor. It consists of two stainless steel wires and two copper core, stainless clad wires of proper resistance crimped together. The alternating current induced by the RF is shunted around the igniter squib. The MOD 2 motor uses a stabilizing rod that is hollow core that acts as a conduit for the igniter wires. The DBA increases the resistance
of the motor from 0.7 - 2.0 ohms to 2.3 - 3.0 ohms. This additional resistance can cause the fielded aircraft fire control to err during inventory of available rockets.
The MOD 4 motor is a new design that will become common to all Armed Services and eliminates the undesirable traits of the previous designs. The first deliveries of the MOD 4 motor will be in 1998. The purpose of the MK 66 MOD 4 Rocket Motor is to improve several safety (specifically E3) deficiencies of the MK 66 MOD 2 Rocket Motor. The MOD 4 motor incorporates a new initiator (MK 26 MOD 0), igniter (MK 311 MOD 0), and E3 filtering. As a result, the MK 66 MOD 4 Rocket Motor is HERO, 300 KV and 25 KV ESD, and EMP safe. The E3 filtering includes a capacitor under the nozzle and two low pass filters located on the igniter and in the initiator. Also, a natural spark gap also exists between the contact band and nozzle body. The bare MK 66 MOD 4 Rocket Motor does not function when exposed to 300,000 volts of Electrostatic Discharge (ESD).
During testing, as a worst case test, the U.S. Government forced the charge from the 300 KV ESD to hit the rocket motor contact band (firing contact). The filters reliably diverted the ESD pulse away from the initiator. The MK 26 MOD 0 Initiator, the MK 311 MOD 0 Igniter, and the MK 66 MOD 4 Rocket Motor pass 25 KV ESD (human generated
electrostatic discharge). The MK 66 MOD 4 Rocket passed HERO testing using the Apache and Blackhawk helicopters as the test platforms. These helicopters are considered to present the worst case for HERO testing. HERO safe certification exists in letter
DD/NSWC 8020 F52-RDD dated 13 April, 1995. EMP analyses on the MK 66 MOD 4 Rocket Motor shows it has an EMP factor of safety of over 35 million. This is because the EMP pulse is extremely short (3 microseconds). The short pulse does not provide enough energy to heat the MK 26 MOD 0 Initiator bridgewire.