Directory of U.S. Military Rockets and Missiles
Appendix 4: Undesignated Vehicles
2.75-Inch Rockets
Copyright © 2003-2006 Andreas Parsch

Air-Launched 2.75-Inch Rockets

Unguided air-launched rockets of 2.75 in (70 mm) diameter were originally developed in the late 1940s by the NOTS (Naval Ordnance Test Station) at China Lake. The rockets were to be used as more powerful supplements and/or replacements for guns in both air-to-air and air-to-ground applications. Many millions of rocket rounds have been built so far, and the latest versions are still widely used by all U.S. armed services.

Designation Note: No formal designations are allocated to all-up 2.75-inch rockets. Instead, the rocket type is generally identified by the designation of the motor assembly, which is the main body of the rocket and includes nozzle and fins. The various warheads are typically usable with all available motors, and are presumably often fitted to the rockets in the field only shortly before actual use. Therefore it was apparently deemed unnecessary to assign designations to every specific combination of rocket and payload. In fact, the original edition of the current designation system for rockets and missiles explicitly excluded unguided line-of-sight rockets from the system.

MK 4 Mighty Mouse, MK 40

The 2.75-inch FFAR (Folding-Fin Aircraft Rocket) was originally developed by the NOTS as an air-to-air weapon to be used by interceptors against heavy bombers. A salvo of rockets was considered much more effective than a stream of cannon shells. The original rocket model was the MK 4, which was spin-stabilized and featured four flip-out fins around the nozzle. Fitted with a 2.7 kg (6 lb) HE warhead, it was widely used in the 1950s as the Mighty Mouse air-to-air rocket by USAF interceptors like the F-86D Sabre, F-89J Scorpion, F-94C Starfire and F-102A Delta Dagger. Total length of the MK 4 Mighty Mouse was 1.2 m (4 ft), and it weighed 8.4 kg (18.5 lb). Maximum range was around 6000 m (6500 yds), but effective range was more like 3400 m (3700 yds). The rocket's accuracy was relatively poor, because its speed and spin rate were too low to effectively counter gravity drop, cross winds and dispersion.

Photo: via Ordway/Wakeford
Mighty Mouse MK 4

The 2.75-inch rocket was soon adapted for air-to-ground use, and a variety of warheads were developed. These included the heavier M151 and M229 HE fragmentation, the WDU-4/A flechette, and various smoke warheads for target spot marking and/or incendiary effects (for a list of 2.75-inch rocket warheads, see table in MK 66 section below). The 2.75-inch rocket was also adopted by the U.S. Army and Marine Corps as a primary weapon for its armed helicopters. For better performance when launched from these slow platforms, the MK 40 motor was developed. It has a modified nozzle for a greater spin and therefore higher accuracy.

Except for the specialized installations in early high-speed interceptors, the MK 4/40 rockets were almost exclusively employed from multi-tube launch pods. A wide variety of these pods were used, and the most important were:

The LAU-series was generally used by the fixed-wing aircraft of the Air Force and Navy, while the M-series launchers were used by the Army's helicopters. Most (possibly all) of these pods could be used with either the MK 4 or the MK 40 rockets.

Photo: U.S. Army
M200 rocket pod

General Dymanics (Lockheed Martin, BEI Defense) MK 66 Hydra 70

The current 2.75-inch (70 mm) rockets are known as Hydra 70 rocket system, and use the MK 66 rocket motor. The latter was developed by the U.S. Army as a common replacement of the MK 4 and MK 40 for both fixed-wing aircraft and helicopters. The MK 66 is longer than the MK 4/40, uses an improved smoke-less propellant and has a completely new fin and nozzle assembly. The three fins are of the wrap-around type, and fit around the circumference of the rocket nozzle. Therefore the MK 66 is sometimes called a WAFAR (Wrap-Around Fin Aerial Rocket) instead of an FFAR. The MK 66 has a higher thrust and spin rate than the MK 4/40, increasing effective range and accuracy. The original MK 66 MOD 0 version was ready in 1972, but was not mass-produced. The first production versions were the MK 66 MOD 1 for the U.S. Army and the later MK 66 MOD 2 for the U.S. Air Force and Navy. The MOD 2 (development and full-rate production beginning in 1981 and January 1986, respectively) 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. The MK 66 MOD 3 is a HERO safe version of the MOD 1 for the Army. Development of the current MK 66 MOD 4, to be used by all armed services, began in 1991. It entered full-scale production in December 1999, and is the current standard motor for U.S. air-launched 70 mm rockets. It has internal changes, including new initiator and igniter, for further enhanced HERO safety.

Photo: U.S. Army Photo: General Dynamics
Hydra 70 rocketHydra 70 system

The original production of the MK 66 rockets was done by BEI Defense. In the mid-1990s, a follow-on production contract was awarded to Lockheed Martin Ordnance Systems, which was sold to General Dynamics in 1999. Current prime contractor for the Hydra 70 rocket system is GD's Armament and Technical Products division.

The current standard U.S. Navy launchers for MK 66 rockets are the LAU-61C/A 19-round and LAU-68D/A 7-round pods. All earlier versions (up to LAU-61B/A and -68C/A) could be used only with the older MK 4/MK 40 motors. Both launchers are thermally protected and support single and ripple mode firing. The USAF's pods for the MK 66 are the 7-round LAU-131/A and 19-round LAU-130/A, and the U.S. Army's current 70 mm LWLs (Light Weight Launchers) are the M260 (7-round) and M261 (19-round). All other 2.75" rocket pods are effectively no longer in use, presumably because they are not compatible with the MK 66 motor.

Photo: U.S. Army
Hydra 70 system

A wide selection of warheads, most of which were originally developed for the MK 4/40 FFARs, is available for the MK 66 Hydra 70 rocket. These include:

Drawing: General Dynamics
Hydra 70 warheads

The following table lists the basic characteristics (length, weight) of Hydra 70 rockets with the warhead/fuze combinations currently used by the U.S. Navy:

Warhead Warhead Type Fuze Options Length Weight
M151Anti-PersonnelM423, M427, MK 352138.2 cm (54.39 in)10.4 kg (22.95 lb)
M156Smoke (White Phosphorus)M423, M427, MK 352140.0 cm (55.13 in)10.5 kg (23.25 lb)
M257Illumination (visible)M442186.1 cm (73.25 in) 11.1 kg (24.45 lb)
M278Illumination (IR)
MK 67 MOD 0Smoke (White Phosphorus)M423, M427, MK 352140.0 cm (55.13 in) 8.5 kg (18.75 lb)
MK 67 MOD 1Smoke (Red Phosphorus)M427, MK 352146.8 cm (57.79 in)10.2 kg (22.52 lb)
WDU-4A/AFlechetteModel 113A142.9 cm (56.25 in)10.4 kg (22.95 lb)
WTU-1/BPractice(n/a)140.4 cm (55.30 in)10.5 kg (23.10 lb)

Hydra 70 rockets were used in large numbers in all recent American armed conflicts. They can be fired from essentially all close-support aircraft, but are primarily used by armed helicopters. Production of MK 66 rockets is continuing at General Dynamics under the APKWS (Advanced Precision Kill Weapon System) program. New launcher and motor developments for Hydra 70 are briefly mentioned in the section about other Hydra 70 developments.

Lockheed/BEI ARS

In 1991, the U.S. Army and Navy issued a request for proposals for the ARS (Advanced Rocket System), which was to be a 2.75-inch rocket to replace existing Hydra 70 and 5-inch Zuni rockets. The ARS requirements called for a rocket to propel a 4.5 kg (10 lb) warhead to a speed of at least 1000 m/s (3280 fps) and an effective range of at least 10000 m (11000 yds). The rocket was to comply with Navy standards for insensitive munitions. Multiple interchangeable types of warhead were to be used, together with an inflight-programmable fuze.

A development contract was awarded to Lockheed Missiles and Space and BEI Defense in July 1992, and full-rate production was at that time expected for 1997. However, the ARS program was cancelled in 1995, after the Army had already pulled out in 1994.

General Dymanics APKWS / BAE APKWS II

In 1996, the U.S. Army formulated a requirement for an APKWS (Advanced Precision Kill Weapon System) to close the gap in capability and cost between the unguided Hydra 70 rockets and the sophisticated AGM-114 Hellfire anti-armour guided missile. The Army needed a small and accurate weapon against non-hardened point targets especially in environments with a high risk of collateral damage, like e.g. in urban warfare. To fulfill the APKWS requirement, a guided development of the Hydra 70 rocket (also known as LCPK (Low Cost Precision Kill) 2.75-inch rocket) was selected. This missile will use the MK 66 motor with a new warhead/guidance section assembly, and will therefore be instantly compatible with all existing 70 mm rocket launchers in the U.S. inventory.

The initial variant of General Dynamics' APKWS was to use the M151 warhead combined with a low-cost semi-active laser seeker and small forward flip-out wings for flight control. The weapon was expected to have an accuracy of better than 1 m (3.3 ft) CEP.

Drawing: General Dynamics
APKWS M151 warhead

It was initially planned to field APKWS in 2001/02, but significant delays due to lack of funding have pushed this date further into the future several times. In September 2002, the APKWS program was redefined to cover the full range of MK 66 rockets, and therefore also included all unguided rockets of the Hydra 70 family. The first operational guided APKWS rocket was to be the M151 variant (also known as APKWS Block I), but the guidance section was expected to be compatible with the other warheads as well.

In April 2005, the Army eventually cancelled the General Dynamics guided APKWS program, because of poor test results. The competition has since been reopened under the label APKWS II. The only restriction was that the weapon must still be based on the Hydra 70/MK 66 rocket. Competitors in the APKWS II program were Lockheed Martin, Raytheon and a BAE Systems / Northrop Grumman / General Dynamics team. In April 2006, the BAE-led team was selected as winner by the U.S. Army, and received a 3-year contract for further development, testing, and initial production of the APKWS II missile. BAE's design is a laser-guided missile using a so-called Distributed Aperture Semi-Active Laser Seeker (DASALS). Externally, it is similar to General Dynamics' original APKWS design.

Photo: BAE/General Dynamics

Other Hydra 70 Developments

The Naval Air Warfare Center, Weapons Division (NAWCWPNS) is also developing a guidance system for the Hydra 70/APKWS under the LOGIR (Low-Cost Guided Imaging Rocket) program. LOGIR integrates a midcourse guidance set (derived from the DAMASK (Direct Attack Munitions Affordable Seeker) program) and an IIR (Imaging Infrared) terminal seeker with the Hydra 70 rocket motor. Additionally, NAWCWPNS cooperates with DARPA (Defense Advanced Research Projects Agenxy) to test a new so called "biomimetic" seeker for the Hydra 70.

The Naval Surface Warfare Center and the U.S. Army are developing a new 70 mm rocket launcher under the SMARt (Smart Munition/Advanced Rocket) program. The new launcher is planned to integrate all functions to control the next-generation APKWS rockets (target acquisition, fuzing options, firing sequence), and to provide a digital control interface to the carrying aircraft. Current Hydra 70 launch pods provide only limited means to select firing and fuzing options in flight. The SMARt rocket pod would replace all types of 70 mm pods currently in service with the U.S. armed forces.

Research efforts are currently under way to determine means to increase range and accuracy of unguided Hydra 70 rockets by improving the MK 66 motor. Options include a dual-thrust (boost/sustain) motor for higher range, and a redesigned nozzle/fin assembly for higher initial spin and therefore less dispersion. Another problem with the current MK 66 motor is secondary combustion of exhaust gases, which lead to firing restriction on the AH-64 Apache helicopter to prevent engine surges. The MK 66 MOD 6 motor (no information is available about a MOD 5) incorporates internal changes which greatly reduce this problem. The MOD 6 has been tested and was planned to enter production in 2004.


Note: Data given by several sources show slight variations. Figures given below may therefore be inaccurate!

Data for MK 4, MK 66:

 MK 4MK 66
Length (motor only)*99.8 cm (39.3 in)105.9 cm (41.7 in)
Weight (motor only)*5.0 kg (11.0 lb)6.2 kg (13.6 lb)
Diameter70 mm (2.75 in)
Speed600 m/s (2000 fps)1000 m/s (3280 fps)
Range6000 m (6500 yds)10000 m (11000 yds)
PropulsionSolid-fueled rocket; 3.3 kN (750 lb) for 1.3 s Solid-fueled rocket; 6.7 kN (1500 lb) for 1.1 s
*Note: Total length and weight depend on warhead; see main sections for data.

Main Sources

[1] Christopher Chant: "World Encyclopaedia of Modern Air Weapons", Patrick Stephens Ltd., 1988
[2] Norman Friedman: "World Naval Weapons Systems, 1997/98", Naval Institute Press, 1997
[3] Norman Friedman: "US Naval Weapons", Conway Maritime Press, 1983
[4] Navy Training System Plan for the Consolidated Rocket Systems (N88-NTSP-A-50-9801/A), August 2000 (formerly public, but access now restricted to authorized persons only)
[5] Website
[6] Hydra 70 - 2.75 inch Rockets
[7] Frederick I. Ordway III, Ronald C. Wakeford: "International Missile and Spacecraft Guide", McGraw-Hill, 1960
[8] Norman J. Bowman: "The Handbook of Rockets and Guided Missiles", Perastadion Press, 1963
[9] Eric Hawley: MK 66 Rocket Motor/Helicopter Compatibility Program, March 2003
[10] "Complementary enhancements under way for Hydra-70 rocket", article in Jane's International Defense Review, July 2003

Back to Directory of U.S. Military Rockets and Missiles, Appendix 4

Last Updated: 28 April 2006