# Hydra 70

The Hydra 70 rocket is an American made 2.75-inch (70 mm) diameter fin-stabilized unguided rocket used primarily in the air-to-ground role. It can be equipped with a variety of warheads, and in more recent versions, guidance systems for point attacks. The Hydra is widely used by US and allied forces, competing with the Canadian CRV7, with which it is physically interchangeable.

## Overview

The Hydra 70 is derived from the 2.75-inch (70 mm) diameter Mk 4/Mk 40 Folding-Fin Aerial Rocket developed by the United States Navy for use as a free-flight aerial rocket in the late 1940s. The Mk 40 was used during the Korean and Vietnam wars to provide close air support to ground forces from about 20 different firing platforms, both fixed-wing and armed helicopters.
The main change made to produce the Hydra was the Mk. 66 motor which uses a new propellant that offers considerably more thrust, 1,335 pounds-force (5,940 N) (Mod 2/3) 1,415 pounds-force (6,290 N) (Mod 4). The fins of the Mk 40 flipped forward from the rear when the rocket left the launching tube, but in the Hydra they are curved to match the outside diameter of the rocket fuselage and flip sideways to open, which is referred to as WAFAR (Wrap-Around Fin Aerial Rocket) instead of FFAR (folding-fin aerial rocket). To improve stability during the time the fins are still opening, the four motor nozzles have a slight cant angle to impart a spin while the rocket is still in the launch tube.
Today, the OH-58D(R) Kiowa Warrior and AH-64E Apache Longbow, as well as the Marine Corps' versatile UH-1 Huey and AH-1 Cobra, carry the Hydra rocket launcher standard on its weapon pylons.

### Mk 66 rocket motor variants

| Designation | Description |
| Mk 66 Mod 0 | 70 mm (2.75 in) WAFAR universal motor; common motor for the GD Hydra 70 series of rockets; original prototype; for US Army |
| Mk 66 Mod 1 | Mk 66 variant; production variant; for US Army |
| Mk 66 Mod 2 | Mk 66 Mod 1 variant; HERO (Hazards of Electromagnetic Radiation to Ordnance) safe; for US Navy and US Air Force |
| Mk 66 Mod 3 | Mk 66 Mod 1 variant; HERO safe; Mk 66 Mod 2 for US Army |
| Mk 66 Mod 4 | Mk 66 Mod 2/3 variant; incorporates a Salt rod to reduce exhaust gases; for all services |
| Mk 66 Mod 5 | Mk 66 Mod 4 variant; Incorporates propellant venting during fast cook off |
| Mk 66 Mod 6 | Mk 66 Mod 4/5 variant; designed to reduce the tendency of secondary launch gases to combust in the parent aircraft’s engine, primarily with the AH-64 helicopter |

## Service

The family of Hydra 70 (70 mm) 2.75 inch rockets perform a variety of functions. 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 also includes smoke screening, illumination, and training warheads. Hydra 70 rockets are known mainly by either their warhead type or by the rocket motor designation, Mk 66 in US military service.

### United States

In the U.S. Army, Hydra 70 rockets are fired from the AH-64 Apache and AH-64D Apache Longbow helicopter variants using M261 19-tube rocket launchers, and the OH-58D Kiowa Warrior using seven-tube M260 rocket launchers. In the U.S. Marine Corps, either the M260 or M261 launchers are employed on the AH-1W SuperCobra and AH-1Z Viper, depending upon the mission. The M260 and M261 are used with the Mk 66 series of rocket motor, which replaced the Mk 40 series. The Mk 66 has a reduced system weight and provides a remote fuze setting interface. Hydra 70s have also been fired from UH-60 and AH-6 series aircraft in US Army service.
The AH-1G Cobra and the UH-1B "Huey" used a variety of launchers including the M158 seven-tube and M200 19-tube rocket launchers designed for the Mk 40 rocket motor; however, these models have been replaced by upgraded variants in the U.S. Marine Corps because they were not compatible with the Mk 66 rocket motor. The Hydra 70 rocket system is also used by the U.S. Navy, and the U.S. Air Force.

### Common U.S. Mk 66 compatible launchers

| Designation | Description |
| M260 | 7-Tube LWL (LightWeight Launcher, 35 lbs, 15.9kg) |
| M261 | 19-Tube LWL (LightWeight Launcher, 80 lbs/ 36.3kg) |
| LAU-130/A | 19-Tube rocket launcher |
| LAU-131/A | 7-Tube rocket launcher |
| LAU-68D/A | 7-Tube LAU-68C/A variant; compatible w/ Mk 66 rocket motor; external thermal protection coating; launcher supports single and ripple firing |
| LAU-69D/A | 19-Tube LAU-61B/A variant; compatible w/ Mk 66 rocket motor; external thermal protection coating; launcher supports single and ripple firing |

### Accidents

In 2019, a 72-year-old Taiwanese man was killed after a discarded Hydra rocket which he had cut into with an electric saw exploded. He had believed it to be a length of pipe. The rocket had been caught in the net of a fishing vessel and then discarded by the crew ashore as scrap metal.

## Warheads

Hydra 70 warheads fall into three categories:
* Unitary warheads with impact-detonating fuzes or remote-set multi-option fuzes.
* Cargo warheads with air burst-range, with settable fuzes using the "wall-in-space" concept or fixed standoff fuzes.
* Training warheads.

### Fuzing options

| # | Designation | Description | Arming Range, Acceleration or Time |
| 1 | M423 | Nose Mount, Point Detonating for slow speed platforms (helicopters) | 47 to 102 yards (43 to 93 m) |
| 2 | M427 | Nose Mount, Point Detonating for high speed platforms | 197 to 466 yards (180 to 426 m) |
| 3 | XM436 | Air burst, Motor-Burnout Delay | |
| 4 | XM438/M438 | Nose Mount, Point Detonating | |
| 5 | M440 | Point Detonating | |
| 6 | Mk 352 Mod 0/1/2 | Point Detonating | |
| 7 | M429 | Proximity Air burst | |
| 8 | M433 | Nose Mount, Resistance Capacitance (RC) | SuperQuick (PD) 11 to 49 yards (10 to 45 m) Delay in 5.5 yards (5.0 m) increments including 3.3 yards (3.0 m) Bunker penetrating option |
| 9 | M439 | Base Mount, Resistance Capacitance (RC), Payload Discharging Pilot-Selectable | Discharges submunitions between 547 and 7,874 yards (500 and 7,200 m) (766 to 7,546 yards [700 to 6,900 m] on AH-1s) 27Gs |
| 10 | M442 | Air burst, Motor-Burnout Delay | Discharges Flare at 3,281 yards (3,000 m), 17-22 g required for arming |
| 11 | M446 | Base Mount, Air burst, Motor-Burnout Delay | |
| 12 | Model 113A | Base Mount, Air burst, Motor-Burnout Delay | |

### Common warheads

The most common warhead for the Hydra 70 rocket is the M151 "10-Pounder," which has a blast radius of 10 meters and lethal fragmentation radius of around 50 meters. The M247 HEDP warheads have similar penetration to the standard M72 LAW warhead (~300 mm of rolled homogenous armor).
| Designation | Description | Weight | Payload | Fuze Type | Fuzing options |
| M151 | High explosive (HEDP) '10 pounder' | 8.7 lb (3.9 kg) (w/o Fuze) | 2.3 lb (1.0 kg) Comp B-4 HE | M423 | 1,2,5,7,8 |
| M156 | White phosphorus munitions (WP) | 9.65 lb (4.38 kg) | 2.2 lb (1.00 kg) WP | M423 M429 | 1,2,6,7 |
| M229 | High explosive (HEDP); elongated M151 '17 pounder' | 17.0 lb (7.7 kg) (Fuzed) | 4.8 lb (2.2 kg) Comp B-4 HE | M423 | 1,2,6,7 |
| M247 | High-explosive anti-tank (HEAT)/high-explosive dual purpose (HEDP) | 8.8 lb (4.0 kg) | 2.0 lb (0.91 kg) Comp B HE | M438 PD | 4 (integral to warhead) |
| M255 | APERS (anti-personnel) warhead | | 2500 28 grains (1.8 g) flechettes | | 9 |
| M255E1/A1 | Flechette warhead | 14.0 lb (6.4 kg) | 1179 60 grains (3.9 g) flechettes | M439 | 9 |
| M257 | Parachute illumination | 11.0 lb (5.0 kg) | One M257 Candle (Flare) 1 million candela | M442 | 10 (integral to warhead) |
| M259 | White phosphorus (WP) | | | | 9 |
| M261 | Multi-purpose submunition (MPSM) | 13.5 lb (6.1 kg) | 9 M73 (Grenade) Submunitions | M439 with M84 electric detonator | 9 |
| M264 | Red phosphorus (RP) Smoke | 8.6 lb (3.9 kg) | 72 RP Pellets | M439 | 9 |
| M267 | MPSM Practice | 13.5 lb (6.1 kg) | Three Marking SMs, 6 Metal Weights | M439 with M84 electric detonator | 9 |
| M274 | Practice (Smoke) | 9.3 lb (4.2 kg) | 2 ounces (57 g) of potassium perchlorate and aluminum powder | M423 | 1 |
| M278 | Infra-red (IR) parachute illumination | 11.0 lb (5.0 kg) | One M278 IR Flare | M442 | 10 (integral to warhead) |
| M282 | Multipurpose penetrator warhead | 13.7 lb (6.2 kg) | 0.98 lb (0.44 kg) PBXN-110 | delayed | |
| Mk 67 Mod 0 | White phosphorus (WP) | | | | 1,2,6,7 |
| Mk 67 Mod 1 | Red phosphorus (RP) | | | | 1,2,6,7 |
| WTU-1/B | Practice | 9.3 lb (4.2 kg) | Inert | None | None |
| WDU-4/A | APERS warhead | 9.3 lb (4.2 kg) | 96 flechettes of unknown weight | | 12 (integral to warhead) |
| WDU-4A/A | APERS warhead | 9.3 lb (4.2 kg) | 2205 20 grains (1.3 g) flechettes | M405A2 | 12 (integral to warhead) |

## Mk 66 rocket motor technical data

* Weight: 13.6 pounds (6.2 kg)
* Length: 41.7 inches (1,060 mm)
* Burn time: 1.07 sec
* Average thrust (77 °F (25 °C)):
** 1,335 lbf (5.94 kN) (Mod 2/3)
** 1,415 lbf (6.29 kN) (Mod 4)
* Motor burnout range: 1,300 feet (400 m)
* Motor burnout velocity: 2,425 ft/s (739 m/s)
* Launch spin rate: 10 rps, 35 rps after exiting launcher
* Velocity at launcher exit: 148 ft/s (45 m/s)
* Acceleration:
** 60–70 g (initial)
** 95–100 g (final)
* Effective Range: 547 to 8,749 yards (500 to 8,000 m) depending on warhead and launch platform
* Maximum Range: 11,483 yards (10,500 m) under optimum conditions

## Precision guided Hydra 70

There are several design efforts to turn the Hydra 70 rocket into a precision guided munition (PGM) to produce a weapon with greater accuracy but at less cost than other guided missiles. These include:
* The BAE Systems Advanced Precision Kill Weapon System (APKWS) II
* U.S. Navy Low-Cost Guided Imaging Rocket (LOGIR)
* Lockheed Martin Direct Attack Guided Rocket (DAGR)
* The ATK/Elbit Guided Advanced Tactical Rocket – Laser (GATR-L)
* Raytheon TALON
* Forges de Zeebrugge SAL-Laser Guided Rocket (FZ275 LGR)
The APKWS was the first to be fielded in March 2012, and the TALON entered full rate production for the United Arab Emirates Armed Forces in September 2014.
The Turkish ROKETSAN Cirit is a similar missile compatible with 70 mm rocket launchers, but it was developed from scratch and doesn't use Hydra 70 components.

## Operators

* Australia − APKWS
* Colombia
* Croatia − Used on Bell OH-58D Kiowa helicopters
* Egypt
* India − APKWS
* Indonesia
* Iraq − APKWS
* Japan
* Jordan − APKWS
* Kuwait
* Lebanon − APKWS
* Netherlands − APKWS
* Nigeria − APKWS
* Pakistan
* Philippines − APKWS
* Qatar − APKWS
* Saudi Arabia
* Singapore
* Taiwan
* Thailand
* Tunisia − APKWS
* Ukraine − Used on Mi-24 gunships
* United Arab Emirates
* United Kingdom
* United States