Mk 15/29

Mark 15, or Mk-15 was an American thermonuclear bomb of the 1950's. The first relatively light (3450 kg) thermonuclear bomb, production of Mark 15 began in 1955, in all, 1,200 were produced until 1957. The bomb was in service from 1955 to 1965. There were three modifications - Mod 1, Mod 2 and Mod 3.

The Mark 15 is usually described as a transitional design between a fission-based weapon and a thermonuclear weapon. Mark 15 had two stages (Teller- Ulam design ), using radiation implosion from the first stage ( Cobra ) fission reaction , to detonate the second stage. Unlike most modern thermonuclear bombs, HEU (highly enriched uranium) was used in the second stage of Mark 15. There was a thermonuclear core in which the reactions of thermonuclear fusion proceeded, but most of the energy was produced by HEU fission. The HEU division was enhanced by neutrons, which were the product of the synthesis reaction. Some later bombs in the fission stage used insets from depleted uranium, in which fission reactions were initiated due to the neutrons produced in the synthesis, but the bulk of the energy release (50% or more) accounted for the fusion reactions.

All three modifications are very similar - their weight is 3450 kg, diameter - 87 - 89 cm, length - 3.45 - 3.55 m.

1. Mod 1 corresponds to the prototype Zombie , the test of Castle Nectar . The power during the test was 1.69 Mt.
2. Mod 2 corresponds to the model TX-15-X1, the Redwing Cherokee test . The test yielded a power of 3.8 Mt. Redwing Cherokee was the first American thermonuclear bomb dropped when tested from an airplane.
3. Mod 3 also had a capacity of 3.8 Mt.

In the mid-1950s, the Mark 15 was developed as the combat part of the missile under the designation W15. The carrier was to be a cruise missile SM-62 Snark. In 1957, the program was closed, and Snark was equipped with a warhead W39. The W39 warheads and B39 bombs used a Mark 15 nuclear explosive. In testing, the experimental W39 devices were designated as TX-15-X3.

On February 5, 1958, during the training flight, the B-47 bomber lost a Mk 15 nuclear bomb in the sea near the coast of Georgia.

The Military Liaison Committee wrote to the U.S. Atomic Energy Commission April 23, 1953, stating that the Department of Defense hoped to be able to deliver large-yield weapons by high-performance fighter-bombers and guided missiles. A weapon proposal was requestedby September 1953, with the prospect of stockpiling any selected designs by late 1955. The Division of Military Application forwarded this letter to Los Alamos, noting that a preliminary statement concerning the TX-15 might help to clarify this design, which apparently had potentialities either as a tactical or strategicweapon, or both.

It was felt that this would havea diameter or 34.5 inches, a length of 145 inches, and a weight of 8000 pounds. The primary would be located aft of the secondary, provision would be made for parachute retardation, and a half-caliber flat nose would be used if this were proven ballistically acceptable. Field Command notified Sandia October 30, 1953, that early military authorizationof the TX-15 program was expected. The bomb should be carried internally in the Air Force's B-36, B-47, B-52 and B-66; and in the Navy's AJ-1, AJ-2 and A3D-1. The B-47 would be the most restrictive aircraft as far as length and fin dimensions were concerned. The bomb should be capable of either free fall or parachute-retarded drop, and investigation should be made of the design of a bluff shape to optimize drag.

A set of proposed military characteristics was released by Field Command November 13,1953. Inasmuch as some thought had been given to the possibility of making the TX-15 compatible with the Air Force's long-range strategic missiles, such as REDSTONE and SNARK, both a TX-15 Bomb and an XW-15 Warhead were prescribed. Limiting parameters included a diameter of 35 inches, a length of 120 inches, and a weight of 6500 pounds. The weapon would be required to withstand an altitude of 60,000 feet, temperatures from -65°F to +165°F, and an acceleration of ±10 gravities along the longitudinal axis. Automatic insertion and retraction of the primary capsule within a timecycle of 10 seconds would be possible at any time prior to release of the bomb, and during the missile trajectory except during high acceleration at launch or boost. The TX-15 should be able to resist, without damage, any forces created by catapulted takeoffs, arrested landings, or normal flight maneuvers. The bomb would be capable of being dropped free, fall and, if a high-drag shape was provided, consideration was to be given to the deletion of the drogue parachute. The ballistic properties of the bomb should be such that either free-fall or retarded trajectories would be predictable and reproducible. Releases were to be possible at all altitudes up to 60,000 feet and aircraft speeds of Mach 0.95. It was hoped that the power supply would have a storage life of at least 2 (and preferably 5) years, and require no preparation other than installation in the TX-15, if permanent storage in the weapon were not possible.

Sandia reported to the November 20, 1953 meeting cf the TX-Theta Committee that a hemispherical nose appeared to be the most stable in wind-tunnel tests. This change was accordingly made to the shape of the TX-15. Since a high-drag shape could not be provided without unduly increasing the weapon diameter, this would not be developed. No attempts would be made to test releases at 60,000-foot altitudes and speeds of Mach 0.95 until aircraft with this capability became available. The TX-Theta Committee issued a formal report on the TX-15 January 4, 1954. The bomb would be designed for internal carriage and with an option of retarded delivery. A barometric fuzing system would be provided, having a continuous height-of-burst adjustment and a radar-type proximity fuze for the near-surface burst. An automatic inflight insertion mechanism would be incorporated.

The possibility of TX-15 missile compatibility was raised again in the March 26,1954 meeting of the TX-Theta Committee. The SNARK was to be operational in early 1958, but could only carry a warhead weighing up to 7000 pounds. The G-26 NAVAHO would be available in late 1957 and could carry only 4000 pounds, but a G-38 follow-on NAVAHO, operational in 1960, would be able to carry 7800-pound warheads. The REDSTONE, which would be available in late 1957, could carry a warhead of 6900 pounds and, by sacrificing some range, a warhead of 7800 pounds. Since all these applications were some time in the future, the Committee decided that emphasis would continue on the bomb program. Report SC3390, Proposed Ordnance Characteristics for the TX-15 Weapon, was discussed at the June 30, 1954 meeting of the Special Weapons Development Board.

The TX-15 was 34-1/2 inches in diameter, 130 inches long, and weighed 7500 pounds. Pullout switches closed at release of the bomb from the aircraft, and battery power started operation of the inverters. After the safe-separation interval timer completed its cycle, the X-unit could be charged by closure of the arming baro. When the detonation altitude was reached, the firing baro closed, conecting the output of the X-unit to the detonators. The weapon would be stored in a completely assembled condition, less power supply and primary capsule. Since the program was urgent, and since any prospective missile carriers were still far in the future, the TX-15 was restricted to bomb application. Field Command suggested that the weapon case be sealed, to provide protection against its environment, and Sandia provided internal seals for all case joints. The afterbody protected the fuzing and firing components.

The Mk 15 Mod 0 Bomb was design-released in October 1954. Four major changes had been made since the Proposed Ordnance Characteristics had been issued. Radar antennas were installed in the bomb fins, where they gave good coverage, not only straight ahead of the bomb but to the side. This caused detonation if the Mk 15 dropped close to the side of a building. Subsequent component testing for temperature, humidity, pressure, vibration, shock, dust and salt spray showed that all components, with some few exceptions, could satisfactorily withstand these environments. In the few exceptions, it was decided that the item would never experience the condition prescribed. Environmental tests were performed on six complete weapons, and included cycles of artic, desert and tropical conditions. These tests were successfully passed, as were various dynamic tests. Fly around tests were conducted in a B-47 to detect any adverse vibrational frequency ranges. Drop-tower tests simulated the maximum loads on the bomb caused by carriage in various aircraft. Theweapon was dropped from a height of 20 inches onto a concrete platform, propelled down a ramp and into a wall, and subjected to standard railway humping tests. The full-scale drop program included 11 ballistic and 22 fuzing and firing drops. To create extreme release conditions, two drops were accelerated by JATO boosters.

Consideration, again turned to the problem of developing a missile warhead, and a thorough discussion was held in a joint meeting of the TX-Theta and TX-N Committees on January 6, 1955. The design was still too heavy, but it was felt that perhaps 800 pounds might be shaved off the weight by reducing the thickness of the aluminum case. Some attention was given to carriage of the XW-15 on the F-101 aircraft. The project would have required a streamlined shape for external carriage known as Shape 96. However, the program was later canceled. Meetings had meanwhile been held at Redstone Arsenal to discuss installation of a warhead in the REDSTONE missile. Development requirements for the installation of a Mk 15 Warhead in the Pod of the B-58 Hustler airplane were established and, during 1955, the program was alternately canceled and revived. By the end of the year, however, it had been decided to delete all applications of the Mk 15 Warhead, and to use the Mk 39 weapon.

Meanwhile, work had been proceeding on the development of true contact fuzing forthe Mk 15 Bomb, with several possible methods being studied. The use of probes, both fixed and extendible, was discarded, as it was found that too much of the weapon area was left insensitive to impact. A design using a double shell, having laminated layers of insulator and contact material which would crush on contact, was found to be overly sensitive to anti-aircraft fire. A low-burst proximity fuze, operating in the range of 1 to 25 feet above the target, appeared feasible, but would require a long development period. The most practical method appeared to be the use of barium titanate crystals which, under pressure, produced a pulse of energy. Development of this device resulted in good reliability and high performance. Thermal-cell batteries would replace the nickel-cadmium units, and required no preparation or maintenance. A proposal was made that this new fuze be applied to Mk 15 weapons that would have only a bomb capability (at that time, the warhead application was still being considered). Requirements for these programs had been generated by a letter from the Secretary for Defense.

The Military Liaison Committee had meanwhile become concerned that a lightweight bomb would require quite different handling equipment than that provided for the Mk 15 Mod 0, and suggested that new nomenclature be assigned. Thus, on December 2, 1955, the Division of Military Application redesignated the Mk 15 with contact fuzing; and thermal batterry alone would be the Mk 15 Mod 1 Bomb. This latter program was later canceled when it was found that the production complex could not deliver enough critical components to support all the using programs. A trajectory arm switch was added to the fuzing system toprevent power from reaching the X-unit until the bomb had experienced a normal release. These changes were incorporated in the Mk 15 Mod 2 Bomb which entered stockpile in March 1957.