Hydra 70 is the name associated with the family of 2.75-inch (70 millimeter) rockets. Hydra 70 refers to the Mark 66 rocket motor with any warhead/fuse combination. 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. Figure 5-5 shows the M66 rocket motor.

MK 40 rocket motors are no longer produced for the Army. Inventories for training were expected to be exhausted in FY 93. An unknown quantity are held in war reserve stockage. Table 5-5 shows rocket motor comparison data extracted from TM 43-0001-30.

M260 and M261 launchers are required to fire the MK 66 rocket. They have reduced system weight and provide remote set fuse interface capabilities. The M158A1 and M200 launchers are not compatible with the MK 66 rocket motor.


 Rocket motor comparison data



MK 66

MK 40

Length without warhead

41.7 inches

39.3 inches

Weight before firing

13.6 lbs.

11.0 lbs

Motor burn time (77F)

1.05 - 1.1 sec.

1.55 - 1.69 sec.

Average thrust

1,300-1370 lbs.

720 lbs.

Average spin rate

9 - 10 rps

1 rps

Motor burn out

1280 feet (397 m)

1460 feet (445 m)

Velocity at motor burnout

2425 fps

1965 fps

Maximum range at QE 43
degrees (MPSM warhead
ground launch)

10,425 meters

8,080 meters



a. M151 High Explosive. The M151 HE is an antipersonnel, anti-material warhead and is traditionally referred to as the "10 Pounder." The bursting radius is 10 meters; however, high velocity fragments can produce a lethality radius in excess of 50 meters. The nose section is constructed of malleable cast iron that is threaded to receive the fuse. The base section is constructed of steel or cast iron and is threaded so that it can be attached to the rocket motor. The base section and the nose section are welded (brazed) together. Total weight of the loaded, unfused, warhead is 8.7 pounds, of which 2.3 pounds is composition B4. The M151 can be used M423, M429, and M433 fuses.

b. M274 Smoke Signature (Training). This training rocket provides a ballistic match for the M151 HE warhead. The casing is a modified WTU-1/B with vent holes or blowout plugs. A modified M423 fuse mechanism is integral to the warhead. A cylindrical cartridge assembly is in the forward section of the casing; it contains approximately 2 ounces of potassium perchlorate and aluminum powder that provides a "flash, bang, and smoke" signature. The M274 weighs 9.3 pounds.

c. M261 High-Explosive Multipurpose Submunition.

(1) The MPSM warhead provides improved lethality against light armor, wheeled vehicles, materiel, and personnel. It has a plastic nose cone assembly, an aluminum warhead case, an integral fuse, an expulsion charge, and nine M73 submunitions. The primary warhead fuse, M439, is remotely set with the ARCS, MFD, or RMS to provide range settings (time of flight) from 500 meters to approximately 7,000 meters. On the AH-1, the RMS is programmable only from 700 meters to 6,900 meters.

(2) Initial forward motion of the rocket fuse timing. The expulsion charge is initiated at a point before and above the target, approximately 150 meters, depending on the launch angle. The submunitions are separated by ejection, and arming occurs when the ram air declarator deploys. The RAD virtually stops forward velocity and stabilizes the descent of the submunition. An M230 omni-directional fuse with an M55 detonator is used on each submunition and is designed to function regardless of the impact angle.

(3) Each submunition has a steel body that has a 3.2-ounce shaped charge of composition B for armor penetration. The submunition is internally scored to optimize fragments against personnel and materiel. Upon detonation, the shaped charge penetrates in line with its axis and the submunition body explodes into high velocity fragments (approximately 195 at 10 grains each up to 5,000 feet per second) to defeat soft targets. The fused weight of the M261 is 13.6 pounds.

(a) Approximate target area coverage. Figure 5-6 shows the approximate target area coverage of one M261 warhead. At shorter ranges, the RAD takes longer to overcome momentum, increasing dispersion. As range increases, the rocket loses momentum, increasing the effectiveness of the RAD. This increased effectiveness reduces submunition drift and ground dispersion. Forestation, other vegetation, and natural or man-made structures within the target area may cause the submunition to detonate or land in a dispersion pattern other than the one shown in Figure 5-6.

(b) Probability of impact angle. Aerodynamic forces affecting sub-munitions during vertical descent may prevent them from landing upright (0 degrees off center). Sixty-six percent of the time a submunition will land 5 degrees off center; 33 percent of the time a submunition will land 30 degrees off center.

(c) MPSM lethality potential. Each M73 HE submunition has a shaped charge that can penetrate in excess of 4 inches of armor. A submunition that lands 5 degrees off center has a 90-percent probability of producing casualties against prone, exposed personnel, within a 20-meter radius. A submunition landing 30 degrees off center has a 90-percent probability of producing casualties within a 5 meter radius.

d. M267 MPSM Smoke Signature (Training). The M267 MPSM training warhead operationally, physically, and ballistically matches the M261. Three M75 practice submunitions and six inert submunition load simulators take the place of the nine HE submunitions in the M261 warhead. Each practice submunition contains approximately 1 ounce of pyrotechnic powder. An M231 fuze with an M55 detonator is used with practice submunitions.

e. M257 Illumination. The M257 illumination warhead provides one million candlepower for 100 seconds or more. It can illuminate an area in excess of 1 square kilometer at optimum height. A deployed main parachute descent is approximately 15 feet per second. An M442 integral fuze provides a standoff range of approximately 3,000 meters with the MK 40 motor and approximately 3,500 meters with the MK 66 motor. The weight of the M257 is 10.8 pounds, of which 5.4 pounds is magnesium sodium nitrate.

f. M229 High-Explosive. The M229 HE warhead is currently in the inventory. An elongated version of the M151, it is commonly referred to as the "17 Pounder." The M229 filler consists of 4.8 pounds of composition B4 and has the same fuses as the M151. Its unfused weight is 16.4 pounds.

g. M156 White Phosphorous (Smoke). The M156 is primarily used for target marking and incendiary purposes. It ballistically matches the M151 and is of similar construction. Filler for the M156 is 2.2 pounds of WP with a .12-pound bursting charge of composition B. The approximate weight of the fused warhead is 9.7 pounds. The M156 uses M423 and M429 fuses.

h. M247 High-Explosive. The M247 is no longer in production; however, some of these warheads may still be found in war reserve stockage. With a shape charge for an anti-armor capability, the M247 employs a cone shaped charge like that of the M72 LAW. The point initiated detonating fuse (M438) is an integral part of the warhead. The weight of the M247 is 8.8 pounds, of which 2.0 pounds is composition B.

i. M255E1 Flechette. The M255E1 flechette warhead, which contains approximately 1,180 60-grain hardened steel flechettes, is in limited production. It is designed for use with the M439 fuse and has possible air-to-air as well as air-to-ground application. Figure 5-7 shows all current production warheads.



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