Mk9

280mm Artillery Fired Atomic Projectile (AFAP)_{5.1, 6.1} used by the M65_8.1 cannon_2.1 was 54.8″ in long and weighed 802lbs_9.1 it was in service from 1952-1957_8.4

The shell weighs around 890lbs, fired with a muzzle velocity of 1700ft/s from a 280mm gun mounted on a 240mm carriage._3.2

Later adapted into the T-4 mine._1.1

Soon after the Navy successfully persuaded the Atomic Energy Commission (AEC) to develop the Mk8 weapon, the Army began lobbying for an atomic shell for a large artillery piece as an interim tactical nuclear weapon. However, its development was slow and opposed by both the Navy and the Air Force, who resented what they saw as Army infringement on their nuclear missions._10.1

With guided missile technology not mature enough for policy makers to arm them with nuclear ordnance, it was felt, that artillery field pieces could be used, at least until suitable guided missiles became available. Army Ordnance had developed a mobile gun of 240mm diameter, weighing 90 tons, which was soon to be proof-fired, and a conference was held with Los Alamos Scientific Laboratory (LASL) personnel November 10,1949, at which use of this gun as advocated. LASL stated that a gun-type nuclear device would fit inside a 240mm barrel, although concern was felt about the relatively inefficient usage of fissionable material in such as assembly. Another meeting held from the 16th to 17th January 1950, showed an increased LASL interest in the project, and Army Ordnance representatives discussed possible shell designs. One proposal was to develop a 240mm diameter shell, to fit the mobile gun, and another was to install a 280mm barrel on the 240mm mount, thus allowing the shell diameter to increase._3.5 It was suggested that Mk8 internal nuclear components be used. With hopes of the Mk9 becoming operational in June 1950 impossible due to other obligations of LASL, it was suggested that the Military assume primary responsibilities for development and testing of the shell. A shock-resistant initiator would be required, and LASL agreed to provide an appropriate design early in 1951._3.6

This shell would be a defensive or offensive weapon against infantry masses concentrated in front of the sector through which a breakthrough was planned. It was hoped that a nuclear artillery shell, fuzed for air burst, could destroy  the ability of these troops to stage an attack, or to defend the sector._3.5  In May 1950 the Joint Chiefs of Staff establish the military requirement for an artillery-delivered atomic weapon also assigning the Army Ordnance responsibility for developing the nonnuclear parts of the shell. Picatinny Arsenal, Dover, New Jersey, was placed in charge of design work. The shell was given an Army designation of T124, and a schedule was established that called for test firings of scale models in October 1950, full-scale firings in April 1951, and completion of design in April 1952.  The Santa Fe Operations Office requested Sandia, in a letter dated June 19, 1950, to provide any necessary design consultation and to assist in solving problems of storage, surveillance and handling. It was suggested that the project be placed under a committee having representation from Army Ordnance, Los Alamos, Santa Fe Operations Office, and Sandia. The Committee would allocate responsibilities, define weapon characteristics, and generally administer the program. The first meeting of this group, called the Button Coordination Committee, was held July 21, 1950. The shell was variously called T124 shell, Artillery-Fired Atomic Projectile (AFAP), and Button (possibly because it was predicted that the shell could be delivered “right on the button”)._3.6

The Button Committee worked to modify a gun-type weapon into a diameter suitable for use in an artillery-fired projectile. At first request, the Army wished to produce an atomic projectile for the T92 240mm self-propelled howitzer but LASL indicated that a 280mm bore would be required to meet their range and yield requirements for a 15Kt Mk9 weapon. A boosted Mk9 weapon with a 75Kt yield was also proposed but was found to be impractical for use by short-range battlefield artillery._8.2

The Ordnance Department then proposed a set of military characteristics that were considered in a Button Committee meeting of August 11, 1950, and accepted on a preliminary basis. The shell would weigh about 890lbs and be fired with a muzzle velocity of 1700 feet per second from a 280mm gun mounted on a 240mm carriage._3.2

The projectile would be able to withstand temperatures from -65°F to 160°F during storage and transportation, and operating-temperature limitations would be determined by the M65 itself. _3.2

The Button Coordination Committee was dissolved October 23, 1950, with Santa Fe Operations Office noting that the number and variety of weapons currently being developed necessitated the establishment of a uniform policy for assignment of responsibilities. The Mk9 was in full development but not yet scheduled for production, and Los Alamos and Sandia were requested to assume normal project responsibilities. Subsequently, Sandia took over control of the nonnuclear phases of the Mk9, and reassigned many of the design tasks to Army Ordnance. With the dissolution of the Button Coordination Committee, a TX-G or Gun Committee, with representatives from Los Alamos and Sandia as permanent members, was formed to assume the functions previously carried on by the Button Committee. This committee had control over all gun weapon designs, and initially met December 8, 1950._3.3

Sandia assigned the nomenclature TX-9 to the project December 18th, 1950._3.3

Robert Schwartz of the Picatinny Arsenal, Dover, New Jersey, produced the preliminary design for the first atomic shell in 1949. In a private room at the Pentagon, he laid out the basic specifications for the T124 280mm (11-inch) shell in a period of two weeks, the first prototype projectile without nuclear components, was completed at Picatinny by the first quarter of 1951._10.2 The product of his design was a projectile that measured 54.5in long and weighed 890lbs. Samuel Feltman, Chief of the Ballistics Section of the Ordnance Department’s Research and Development Division championed the atomic projectile until its final approval by the Pentagon._8.1

The first public disclosure of the nuclear artillery shell was made in February 1951, at the close of Operation Ranger, by General J. Lawton Collins, U.S. Army Chief of Staff, who told reporters that the U.S. would soon have atomic artillery shells._10.3

Los Alamos Scientific Laboratory (LASL) designed it and Sandia Laboratory supplied an evaluation program, certification and developed a training program for gun crews. The Army Ordnance Corps produced the war reserve, training, and spotter shells, less their nuclear components, field handling equipment, production tooling, and packaging with Picatinny Arsenal developed the fuze._8.1

Tests of special initiators for the TX-9 continued in May 1951; the most likely candidate was the “Squab” derived from the wartime “Abner” initiators used on the Mk2 weapon. A tungsten alloy gun-barrel was used inside the projectile. Fissionable components were required to be interchangeable with components of the TX-8, TX-10, and TX-11 weapons, to provide flexibility in the election of weapons for use and to standardize component manufacture. This commonality requirement complicated development of nuclear parts, including the “Squab“._10.3

The Artillery Test Unit at Fort Sill, Oklahoma, carried out preliminary testing of telemetry equipped dummy nuclear and conventional rounds in the spring of 1953. The tests established a range of 31,400 yards for the T124 conventional shell and a range of 26,300 yards for the heavier atomic projectile. In 1958, the T124 atomic shells, designated M354, were remanufactured into T-4 Atomic Demolition Munitions and replaced on a round for round basis with an updated T315 projectile that contained a lighter Mk19 warhead. The new 600-pound shell had a range of 32,700 yards. The Army described the accuracy of projectiles fired from an M65 gun as four times more accurate than pre-war mobile artillery._8.3

The TX-9 contained more than 145lbs (60kg) of oralloy and its nuclear efficiency was less than 2%. Originally, the projectile portion of the nuclear mass was to have been located at the base of the shell, so as to preclude any movement toward the target rings when the shell was fired from the howitzer. However, this design featured very little tamper material at the target end in the ogive of the nose of the shell; in addition, a heavy tamper in the front of the projectile moved its center of gravity forward and ruined its ballistics._10.2

To solve the problem, John Taub, a LASL metallurgist, devised a dense tungsten carbide alloy which could not only withstand the shock of projectile launch, but also had the necessary nuclear properties to tamp the explosive assembly of the critical mass. LASL also suggested mounting the internal projectile in the nose of the shell, using accelerometer-triggered detents to lock the uranium projectile in place until the shell was out of the howitzer barrel and was no longer accelerating._10.2

In addition, a so-called “safety gate” was installed in the shell so that the propellant charge to blast the internal projectile into its target mass could not be fired until centrifugal force caused by the rotation of the shell “closed” the gate._10.2

Early production of Mk 9 Mod 0 started April 1952 with a full-scale test in Operation Upshot-Knothole, shot Grable on May 25th, 1953._{3.1, 4.1}

In preparation for firing, the M65 gun crew inserted four fissile target rings into a tungsten carbide tamper at the base of the shell and screwed two circumferentially-threaded, cone-shaped, beryllium-polonium-nickel-barium “Phoebe” initiators into a locking plate behind the target rings, with their tips protruding into the center of the target rings. They then enclosed the target rings to prevent movement and screwed the gun-barrel assembly into the enclosure. The breech of the gun barrel assembly was located in the ogive nose of the shell to optimize the barrel’s length. This arrangement maximized projectile velocity and provided room for the tamper that surrounded the target rings at the base of the shell. Next, the gun crew loaded the insert projectile, a powder charge in a perforated can, and a percussive detonator into the gun barrel. Two detents restrained the projectile from forward movement. The detents retracted when the shell began spinning after the gun crew fired it._8.3

The final act of assembly took place when a crewmember inserted an assembly composed of three MT-220 mechanical time fuzes into the shell’s tip. The gunner then sat astride the shell on the gun’s loading tray and set three dials corresponding to each fuze with a wrench. Initiation of the propellant charge was by means of a percussion detonator triggered by the time fuzes. As a safety measure, a “safety gate” prevented activation of the detonator until centrifugal force caused by the rotation of the shell closed it. The fuze assembly was not equipped for impact detonation._8.3

When the projectile was at its intended air zero point at the end of its trajectory, terminal fuzing fired a charge in the nose of the shell to drive the internal projectile (a solid cylinder) back into the four target rings at the base of the projectile._10.2