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The M864 is an American made 155 mm artillery shell. It carries a DPICM warhead and incorporates base bleed technology to increase its range. The projectile is capable of delivering 24 M46 and 48 M42 dual-purpose anti-materiel/anti-personnel sub-munitions at ranges out to 29 kilometers. Base bleed technology was developed to reduce the amount of base drag on a projectile, thereby increasing the achieved range. The drag is reduced by a gas generator located on the base of the projectile. Once ignited, the gas generator bleeds hot gas into the projectile's wake, which causes the flow of air at the base to be less turbulent. The decrease in turbulence reduces base drag, which typically accounts for 50 percent of total drag. The amount of thrust produced by the base burner unit is negligible and does not serve the same function as the rocket motor on a rocket-assisted projectile (RAP).[citation needed]
Key Information
The M864 projectile is not ballistically matched to any projectile currently in the inventory, but because of the similarity of the trajectories, firing data for the M864 can be determined from M549A1 firing data.[citation needed]
See also
[edit]Footnotes
[edit]- ^ The range of 18.3 mi (29.4 km) is achieved when fired from the 39‐caliber barrels of the M109A6 Paladin and FH70.[1]
References
[edit]External links
[edit]Design and Development
Origins and Predecessors
The concept of cluster munitions in artillery projectiles traces its roots to World War II efforts to disperse submunitions over wide areas for anti-personnel and anti-material effects, with early examples including German SD-2 bomblets and U.S. experimental dispersions, though these lacked the precision and dual-purpose design of later variants.[5] In the U.S., systematic development accelerated during the Cold War to counter massed armored formations, leading to the improved conventional munitions (ICM) program in the late 1960s, which emphasized cargo-carrying shells that release multiple bomblets mid-flight for enhanced area coverage compared to unitary high-explosive rounds.[6] The direct predecessor to the M864 was the M483 155mm projectile, first developed in the 1960s and fielded in the early 1970s as the U.S. Army's initial dual-purpose ICM (DPICM) round, containing 88 submunitions (primarily M42 anti-personnel and M46 dual-purpose bomblets) expelled via a mechanical time fuze to achieve both fragmentation and shaped-charge effects against infantry and vehicles.[6] An upgraded M483A1 variant, introduced around 1975, incorporated refinements such as improved submunition fuzing for better reliability and spin-induced arming, while maintaining the standard payload of 88 DPICM units, with maximum ranges limited to approximately 22-24 kilometers from 39-caliber howitzers due to aerodynamic drag.[7] These munitions addressed shortcomings in earlier 155mm shells, like the M107 high-explosive round, by providing suppressive fire over larger footprints—up to 20,000 square meters—but suffered from higher dud rates (2-5% per submunition) and range constraints that reduced effectiveness against deep targets.[8] The M864 evolved from these limitations, entering low-rate production in 1987 as an extended-range DPICM carrier, sacrificing 16 submunitions (to 72 total) to integrate a base-bleed unit that emits gas to reduce base drag and extend range by 30-40%—reaching 30 kilometers from 39-caliber systems and 40 kilometers from 45-caliber howitzers—while retaining the M42/M46 mix for compatibility with M483 logistics.[4][7] This iteration reflected U.S. Army priorities post-Vietnam for precision in area effects amid Warsaw Pact threats, though it inherited dud vulnerabilities that later drew scrutiny in reliability studies.[9]Engineering Innovations
The M864 155 mm projectile incorporates a base-burn gas generator unit at its rear, which ignites upon firing to expel low-velocity gases into the projectile's wake, thereby reducing base drag by up to 50% compared to conventional shells and extending maximum range to approximately 29 kilometers when fired from an M198 howitzer at standard charges.[4][10] This innovation, distinct from earlier base-bleed systems by utilizing a controlled burn of solid propellant rather than continuous gas emission, minimizes aerodynamic instability while maintaining spin stabilization through rifling, allowing reliable flight performance modeled via coupled gas dynamics and point-mass trajectory equations.[11] The projectile's warhead design advances dual-purpose improved conventional munitions (DPICM) by housing 72 submunitions—48 M42 and 24 M46 grenades—optimized for both anti-personnel fragmentation and anti-armor shaped-charge effects, dispersed via a mechanical time fuze that triggers an expulsion charge at predetermined airburst altitudes for broader area coverage.[4] This configuration improves over predecessors like the M483A1 by integrating the base-burn element without altering the standard 155 mm separate-loading compatibility, enabling seamless integration into existing artillery systems such as the M109 and M198 while enhancing lethality density over extended ranges.[12] Engineering analyses confirm the base-burn unit's efficiency through conservation of mass principles in gas flow, where chamber pressure and nozzle design dictate mass flow rates that sustain drag reduction throughout the trajectory, though susceptible to environmental factors like atmospheric density variations.[11] These features collectively represent a pragmatic evolution in artillery projectile aerodynamics and payload delivery, prioritizing range extension and munition scatter patterns for tactical flexibility.[13]Technical Specifications
Projectile Components
The M864 is a 155 mm separate-loading artillery projectile featuring a forged steel body designed to contain and protect its payload during flight. The body includes an ogive section housing an ejection or burster charge, which functions to rupture the casing and expel submunitions rearward upon fuze initiation.[2] The projectile accommodates 72 dual-purpose improved conventional munition (DPICM) submunitions, comprising 48 M42-series and 24 M46-series grenades, arranged in a stacked configuration within the main cavity.[1][2] At the forward end, the M864 employs a mechanical time fuze (MTF) or electronic time fuze (ETF), which replaces a universal lifting plug prior to loading and is programmed for height-of-burst detonation typically between 300 and 400 meters above the target to optimize submunition dispersion.[1][2] The aft section incorporates a base-bleed unit, consisting of a gas generator assembly that sustains a controlled efflux of hot gases to reduce base drag and extend maximum range beyond that of standard ballistic projectiles. This unit contains approximately 2.6 pounds of hydroxyl-terminated polybutadiene (HTPB)-based propellant, ignited by the main propelling charge during firing.[1][2] The overall design emphasizes compatibility with 155 mm howitzers having a minimum barrel length of 24 calibers, with the base-bleed mechanism providing aerodynamic stability without significantly altering spin or trajectory characteristics.[1]Submunition Payload
The M864 155mm projectile incorporates a cargo payload consisting of 72 dual-purpose improved conventional munitions (DPICM) submunitions, specifically 48 M42 series grenades and 24 M46 series grenades.[2][4][14] These M42/M46-type submunitions are older grenade-like bomblets used in cluster munitions, with the M42 featuring a stabilizing ribbon for descent and arming. These submunitions, each weighing approximately 135 grams for the M42 and 185 grams for the M46, combine shaped-charge warheads for armor penetration with fragmentation effects for anti-personnel lethality.[2][15] Upon expulsion from the carrier shell via a base-ejection system at a predetermined altitude over the target area, the submunitions scatter across a wide footprint, typically covering an area of several hundred meters in diameter depending on burst height and dispersion parameters.[4] The M42 submunitions detonate on impact using a point-initiating, base-detonating fuze, delivering a high-explosive anti-tank (HEAT) jet capable of penetrating up to 200 mm of rolled homogeneous armor while producing lethal fragments.[15] In contrast, the M46 submunitions feature an added drogue parachute that orients the grenade downward, induces spin during descent to enhance fragmentation patterns, and incorporates a delay element in the fuze for improved top-attack effects against vehicles.[14][2] This payload configuration enhances area-denial capabilities compared to unitary high-explosive rounds, with the mix of M42 and M46 types optimizing coverage against both point and dispersed targets such as troop concentrations or vehicle formations.[4] The submunitions employ Composition A5 explosive filler, approximately 58 grams per M42 unit, to achieve dual-mode lethality without reliance on external sensors.[15] Production of the M864 with this payload began entering U.S. Army service in 1987, following testing that validated the submunition dispersion and terminal ballistics.[4]Performance Metrics
The M864 155mm projectile incorporates a base-bleed system that reduces aerodynamic drag by approximately 50% during supersonic flight and eliminates it in the subsonic regime, resulting in a range extension of about 23% compared to inert or non-bleed equivalents. Maximum range reaches 28,180 meters when fired with Charge 8S (M203A1 propellant), while Charge 7WB (M4A2) yields 17,180 meters; these figures assume standard atmospheric conditions and compatible howitzer tubes such as M185 or M284. Muzzle velocities vary by charge, attaining 807 meters per second with Charge 8S and 550 meters per second with Charge 7WB, enabling compatibility with propelling charges including M3A1, M4A2, M119 series, and M203A1.[16][11] The projectile weighs 102 pounds and measures 36.23 inches in length with fuze, delivering a payload of 72 dual-purpose submunitions (48 M42 anti-personnel/anti-material grenades and 24 M46 anti-armor grenades) via an expulsion charge of 105 grams of M10 propellant. Each submunition contains 30.5 grams of Composition A5 explosive, contributing to a total filler weight of 4.81 pounds per projectile, with the base-burner unit fueled by 2.6 pounds of HTPB-AP propellant. This configuration provides over 200% greater area coverage than the predecessor M483A1 projectile, primarily due to the extended range and optimized dispersal pattern upon burst at height.[16][4]| Propelling Charge | Muzzle Velocity (m/s) | Maximum Range (m) |
|---|---|---|
| 7WB (M4A2) | 550 | 17,180 |
| 8S (M203A1) | 807 | 28,180 |
