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Tuesday, March 3, 2009

M28 120mm Atomic Battle Group Delivery System ('Davy Crockett')

The Davy Crockett Atomic Battle Group Delivery System was born of a time where the US Army felt it needed some 151,000 nuclear weapons for deployment in a protracted conflict with the Soviet Union. Of this total, 106,000 would be for tactical battlefield use, 25,000 for air defense of US Army units and installations, and 20,000 to support its Allies. All of this was predicated on the thought at the US Army would use an estimated 423 atomic warheads in a single day of intense atomic combat - not to include surface-to-air missiles.

This vision was never realized, but the US Army would field some 11,500 nuclear weapons from 1954 until denuclearization in 1992 - around 16% of the entire US nuclear arsenal.

The M29 155mm (Heavy) Davy Crockett Launcher is pictured tripod-mounted in the display photo above. The next couple photos are of the M28 120mm (Light) Launcher on it's tripod.

The 'heavy' and 'light' designations afforded the Davy Crockett Battle Group Weapon System came from differentiating how far the recoilless rifles lobbed the M388 Projectile - both used the same weapon - and not actually an indicator of destructive potential. The 'heavy' designation denotes the 155mm launcher's range improvement over the 120mm 'light' launcher.

Both Launchers could be demounted from their trucks or APC's and handled by a three-man team for tripod launch of the Davy Crockett projectile. The Davy Crockett Atomic Battle Group Weapon System is centered around the M388 279mm Projectile. Somewhat reminiscent of a watermelon with fins, the M388 was attached to a spigot (commonly called a piston too) that was inserted into the barrel of the M28 or M29 Launcher. The M388 Projectile itself never sat inside the Launcher barrel - that would make for one BIG gun tube! A propellant charge was inserted into the rear of the recoilless rifle barrel - and when fired - both the spigot / piston and the M388 Projectile were hurled downrange to target.

The M388 Projectile could carry either conventional explosives or a variable-yield W-54 atomic fission warhead - operator selectable explosive yield from 10 to 250 tons TNT. Minimum detonation range was an astounding 1000 feet - I can hardly imagine this regardless of the low atomic yield setting - see table below.

Maximum range of the M28 'Light' Launcher was 1.24 miles, and the M29 'Heavy' Launcher's maximum range was 2.5 miles.

The Davy Crockett artillery round was deployed to give US infantry squads the capability of destroying large units of tanks or infantry pockets on the front lines of the battlefield. This would have been an effective weapon system to employ against large formations of Soviet Armor or troop formations on the European Battlefield of the 1950's and early 1960's.

(Note: A variant of the W54 warhead, the B54, was used in the Special Atomic Demolition Munition (SADM). A nuclear land mine, the SADM was deployed in the Continental United States, Europe, South Korea, and Guam from 1964 to 1989. The M159 Atomic Demolition Munition (ADM) was a man-portable (two-man I'd think better stated) variant of the W54 warhead too. It weighed 150 pounds. The warhead developed for the Davy Crockett artillery projectile would be modified for application in the successful US Air Force's AIM-26A [originally designated GAR-11] Falcon air-to-air missile. The Nuclear Falcon's W-54 warhead had an explosive yield equivalent to 500 tons of TNT and supplemented the 1.5 kiloton Genie rocket on NORAD interceptors defending the Continental United States and Canada.)

Even though the explosive yield (selectable equivalent from 10 to 250 tons of TNT) of the Davy Crockett does not appear huge as compared to the nuclear weapons of today - consider this:
The lowest setting of the warhead is equivalent to 10 tons of TNT in destructive power. This is roughly three times as powerful as the ammonium nitrate bomb used against the Alfred P. Murrah building in Oklahoma City in 1995. With nuclear weapons, as the weapon yield gets larger, the flash injury range increases faster than the blast damage range - and the blast damage range grows faster than the prompt ionizing radiation range.

In very large weapons thermal burns can occur quite some distance beyond the blast injury range. Significant thermal burns injury, however, only occurs so close to the bomb that the blast itself would pulverize you.

Okay - so what does all that mean? With very small battlefield atomic weapons like Davy Crockett the flash effects upon detonation shrink to insignificance - with prompt radiation effects dominating the characteristics of the blast - Davy Crockett isn't a weapon deployed for a spectacularly big bang - which tended to negate its original purpose.

In a Davy Crockett fission explosion, the range for lethal exposure actually extends some distance beyond the blast injury range - which placed the operators in peril. At one setting above its minimum, a 20 ton TNT explosive blast yield for Davy Crockett results in the following types of damage:

500 210
400 500
300 1,350 2.5
200 4,600
150 10,300 7

The maximum range that exposed troops would experience 3rd degree flash burns from the Blast Center is 90 meters (297 feet).

Examples of Significant Radiation Effects:

200 REM causes sterility, and increased cancer risk, temporary immune system suppression. In our example this dosage occurs at a little under 500 meters (1,650 feet) from ground zero.

600 REM is considered a fatal dose - 50% fatality rate. This dosage occurs just inside 400 meters (1,320 feet) from blast center. At the 20 ton TNT yield setting, this would become the minimum distance the operators would want to select for setting the Davy Crockett projectile to detonate.

1000 REM is 100% lethal - perhaps a soldier might make it back to a rear area or Stateside before death. 1000 REM will kill those solders such exposed within two weeks. Why did I say Stateside? This dosage occurs outside 300 meters from blast center - which is also the minimum detonation range (also meaning the shortest detonator timer setting option) of the Davy Crockett. However, the selection of the minimum detonation range when firing the Davy Crockett in this example would spell certain death for the weapon operators - even at this low setting. This is the edge of Davy Crockett's lethal zone. The weapon's operators could be evacuated and transported back Stateside for treatment, but ultimately would perish.

To protect the operators of the Davy Crockett Atomic Battle Group System, the explosive yield would have to be dialed down to shorten the lethal prompt ionizing radiation range enough for their safety. Naturally, doing that decreases the destructive potential of the weapon - defeating the purpose of deploying it in the first place.

5000 REM incapacitates a soldier immediately. A "walking death" occurs temporarily allowing a brief recovery phase for a few hours, then the soldier relapses. Death occurs in a couple of days. This amount of dosage occurs inside 200 meters (a short 660 feet) from Blast Center - pretty much where the enemy troops or a small platoon of enemy vehicles would be concentrated. Even with armor shielding on their vehicles the 4,600 REM dosage level is a sure killer.

At 10,000 REM the soldier would keel over and die inside of several hours - if not immediately - as the radiation dosage attacks the central nervous system. This occurs a little more than 150 meters (495 feet) from Blast Center - again optimum for knocking out a platoon of armored vehicles or an enemy troop pocket.

The Blast Effect:

In this example's setting of 20 tons TNT - the resulting 7 PSI is enough to destroy (or significantly damage) any civilian structure inside 150 meters from Blast Center. This blast level will surely knock a soldier off his feet - but serious injuries more likely would occur from flying or falling debris. If the troops are caught in an open field (with little material to create projectile damage or local blast magnification effects) there actually might be little to no blast injuries suffered by the enemy troops. Simply Duck and Cover drills might be effective in warding off mass injuries.

In comparison, 4 PSI blast damage is caused by winds traveling some 160 miles per hour. This will destroy unreinforced wood and brick homes and some heavier constructed buildings. Unless scoring a direct to near-miss hit with their Davy Crockett round, the resulting 7 PSI blast level is not terribly significant in counting on knocking out large formations of armored vehicles - during that time or now. In the tight combat formations of , say a platoon of US M1 Abrams tanks, one Davy Crockett lobbed in the middle of a 'wedge' could be effective in killing the armored soldiers in their tanks at the 20 ton TNT setting - consider the resulting 10,000 REM dosage (occurring around 150 meters from Blast Center) - though no significant blast damage might be expected to other M1 Abrams tanks in accompanying that 'wedge' in combat formations. This would be the same if Soviet motorized troops were the target.

This is also assuming the Davy Crockett operators could hit a moving target - which they could not do with any degree of certainty.

Our example used the lower settings for the Davy Crockett Projectile. If so employed on a Battlefield Europe at the height of Cold War fear, one can imagine hundreds of these artillery shells lobbed across a wide front to halt the advance of a determined Soviet armored thrust. However, hurling hundreds of these things around isn't the best thing to do militarily speaking as need would dictate turning up the setting to the maximum explosive yield available - imperiling the friendly troops on the front lines. Knowing this shortcoming, the Davy Crockett would quickly be reduced to a pretty expensive one-shot-one-kill weapon against perhaps a handful of tanks or APCs in effort to maximize its effects. It likely wouldn't have been used in this fashion either - as at maximum ranges it proved inaccurate. Both the M28 and M29 recoilless launchers proved inaccurate in test firings.

The Davy Crockett would probably be best described as America's first (albeit tiny) enhanced radiation battlefield weapon - like of modern-day Neutron Bomb fame - as its greatest destructive radius against personnel came in prompt radiation effects, and not blast effects. If lobbed within a few dozen meters of a single tank the blast effect would only then become significant. The concept of Davy Crocketts flying all over a battlefield proved inconceivable - even for a nuclear thirsty US Army of that time.

The M388 Projectile weighed 76 pounds (the W-54 warhead coming in at 51 pounds of this total). The projectile was 31 inches long and 11 inches at its widest point. Some 2100 were produced and the weapon was deployed from 1961 to 1971 - even though President John F. Kennedy reportedly ordered its removal in 1963.

Davy Crockett was the smallest and lightest implosion-type fission bomb ever deployed by the United States. The M388 Projectile was time-fused. It did not carry sophisticated altitude sensors for detonation. The troops had to compute time-of-flight from the launcher to the target zone. During practice firings in Europe, inherent inaccuracies in the recoilless launchers themselves help bring about the end of deployment of the Davy Crockett. Not insignificant to mention, the Davy Crockett also did not have a recall feature. Once fired, it was committed downrange to detonation, and could not be self-destructed.

Operation: SUNBEAM explored weapons effects tests for low-yield tactical nuclear explosions - in which the Davy Crockett Weapon System was tested. This was a part of the larger IVY FLATS Exercise. IVY FLATS was a exercise simulating a battle between a large enemy armored force advancing upon a smaller US force consisting of artillery pieces, not capable of halting the enemy advance. US Army Atomic Battle Group forces were to arrive on scene, and would setup both the M28 and M29 Davy Crockett weapons systems, against the simulated enemy. The actual detonation of the W-54 warhead and firing of a M388 Projectile equipped with the W-54 warhead round would be included in this exercise - along with some 1000 soldiers.

This test shot did, indeed, decimate the opposing enemy force, in simulation.

The first detonation of a Davy Crockett warhead was in the LITTLE FELLER II weapons effects test, taking place before the battlefield exercise with live soldiers (LITTLE FELLER I). LITTLE FELLER II took place on 7 July, 1962. This was a surface burst of the W-54 Davy Crockett warhead, which was suspended three feet above the ground on cables. The plutonium warhead's diameter was just short of 11 inches, was over 15 inches long, and weighed 50 pounds. The explosive yield in this test was equal to 22 tons of TNT. Below is a photograph of the code named "SMALL BOY" test shot (LITTLE FELLER I) - the second Davy Crockett detonation. Historical records don't state why LITTLE FELLER II occurred before LITTLE FELLER I.

LITTLE FELLER I saw the M388 Projectile launched by a five man team from a M29 155mm 'Heavy' Launcher mounted on a M113 APC, 17 July, 1962. The M388 Projectile in this weapons effects test traveled a distance of 1.7 miles. It carried an altitude trigger setting, set to detonate the Projectile at a height about 20 feet above ground. It is reported to have detonated at a height of 40 feet above the surface. The explosive yield generated in this test was equal to 18 tons of TNT. As part of this test, some 26 minutes after detonation a company of soldiers from the 4th Infantry Division and a platoon of tanks entered the shot zone to complete the simulated battlefield maneuvers.

This would be the last atmospheric test detonation at the Nevada Test Site. Attorney General Robert F. Kennedy and General Maxwell D. Taylor were in attendance to observe this detonation.

The Davy Crockett Atomic Battle Group Weapon System was carried by several vehicles during its deployment. The M38A1D Willys MD Jeep was the first mount for the M28. Later, M151A1D MUTTs replaced the Jeeps in this role, and carried both the M28 and M29 Launchers. Both the M38 and M151 4x4 trucks were originally recoilless rifle carriers - field modified into transporter / launchers for the Davy Crockett. When so equipped, their designations changed from 1C to 1D. The Davy Crockett could also be setup on its tripod and mounted in the cargo beds of the M35 Deuce or M54 5-ton Cargo Trucks readily.

Below is a photo of the M116 APC mounting the M29 Launcher. Though not deployed with US Army forces, the US Navy and US Marine Corps did field this APC - thus they could have been equipped with the Davy Crockett.

Below is a photo of the M29 as mounted on the M113 APC. Note the number of Ready Rounds carried inside the M113 - it would have proven a formidable weapons system have the deficiencies of the Davy Crockett had been worked out. The M29 Launcher mounted in the same position that the standard 106mm recoilless rifle launcher did on the rear right corner of the hull roof.

Pictured above is the M29 155mm (Heavy) recoilless launcher.

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