Mark 17 / Mk 24
The Mark 17 was the first deliverable thermonuclear weapon - the first H-bomb that could be dropped from a plane. This massive bomb weighed 21 tons and could be carried in a B-36 after modifications were made to the bomb bay. Pilots who test-dropped the weapon reported that the plane rose hundreds of feet after the weapon was dropped, as if the bomb released the plane rather than the reverse. The aircraft, suddenly lightened of the heavy weapon, would soar hundreds of feet as the pilot's fought to regain control.
At 11 megatons the Romeo shot of 26 March 1954 was the third largest test ever detonated by the United States. The Runt I device in this shot was a proof test of the Mk-17 bomb. The Mk 17/24 Thermonuclear bombs were first stockpiled in April 1954 as Emergency Capability (EC-17) until fuzing and suitable parachute delivery was developed.
The Mark 17 steel casing was about 3.5 inches thick. The MK17 was 24 feet 8 inches long, 61.4 inches in diameter, and weighed between 41,400 and 42,000 lbs.; much of this was casing weight. The MK17 bomb was only four feet shorter than a POLARIS A-1 SLBM, but weighed half again as much. Approx 200 were made. Size wise, the Mk 17 was the largest nuclear weapon ever built by the United States. In combat a gigantic 64-foot diameter ribbon parachute was designed to slow down or retard the big bomb giving the delivery aircraft time to escape the deadly effects of the thermonuclear explosion.
The yield of the Mk 17 was in the megaton range - a megaton (Mt) being the explosive equivalent to one million tons of TNT. The bomb was at one time referred to as a Mk 17/24 because the same weapon case was used for both of these early weapons. The only differences between the Mk 17 and the Mk 24 were internal. This was the primary EWO weapon carried by the B-36 fleet during the 1950s. The bomb was carried in the two aft bomb bays, while a smaller 6,000 pound MK6 atomic bomb was carried in one of the forward bomb bays.
As more confidence was developed in the TX-16 design, it was assigned a nomenclature of TX-17. Sandia would be responsible for the design and production of the afterbody, fuze power supply, parachute and pertinent test andhandling equipment. The TX-14 baro fuze would be used in the early weapons butbe replaced by a proximity fuze as soon as possible. By using components and designs from other weapons, emergency-capability TX-17 and TX-24 Bombs were hand-produced by Sandia and stockpiled May 1954. The fuze from the now-discontinued TX-14 Bomb was used, although work was in progress on a proximity fuze.
The problem of handling a bomb that weighed slightly over 20 tons required considerable thought. Plans had been made to store up to nine bombs in each storage igloo, and this meant that the bombs would have to be moved to one side or the other of the igloo entrance so that three lines of the bombs could be established. In one proposed design, the bomb would be wheeled into the igloo on tracks running the length of the igloo. For transverse movement, the bomb was then lowered onto temporary tracks laid at right angles. Sandia designed a system consisting of four strongbacks, each having a pair of 8-inch-diameter wheels and 6-inch-wide tires, connected together longitudinally. The dolly could be moved into position by using a jackscrew to adjust the wheels to the proper angle, and a few men could easily push the bomb to the desired position.
Drop tests cf the parachute for the TX-17 started November 2, 1954 and continued until late 1955. Most of these drops were made from B-36 airplanes at Edwards. Air Force Base, with some being made at the Yucca Lake Range and the Salton Sea Test Base. Initially, it was felt desirable to provide a large escape distance. A 64-foot-diameter chute could be reefed to provide a 100-second time of fall, but became unstable when reefed to produce a 75-second descent. Eventually, the system was designed to contain a 5-foot guide surface chute, a 16-foot deployment chute, and a 64-foot main canopy, all released in stages.
By November 1954, all emergency-capability units had been modernized to Mk 17/24 Mod 0 status. The design did not differ in any important aspects from the TX-17, but the components were of certified stockpile quality. The Mk 17/24 Mod 0 Bombs thus became the first thermonuclear weapons to be stockpiled as a result of a regular production program. The weapon could be carried and dropped by a B-36, if the release was made at an altitude of 40,000 feet or higher. Lower altitude releases, into denser air, would produce high-shock loadings. The B-52 could not be used as a carrier due to its high speed, which also created high shock loadings at release. It was noted in the May 21, 1954 meeting of the TX-Theta Committee that heavy production rates had been authorized for Mk 17/24 Bombs in late 1954, and it was suggested that as many improvements as possible be incorporated prior to that date.
A Mk 7-type automatic inflight insertion mechanism was to be used, and thisredesign required some lengthening of the bomb nose. Sandia ran a wind-tunnelcheck, which showed that little effect on weapon stability resulted. The Mod 0 of the Mk 17/24 was not exactly a model of aerodynamic stability, as it oscillated with an amplitude of about 10 degrees during its fall, even with parachute retardation, but the nose, which was 2 inches longer, did not aggravate this effect.
Additionally, the TX-15 afterbody was changed at this time from an all-mètal design to one of composite metal and plastic, and this change was also made in the Mk 17/24 Mod 1. Cables that connected components in the afterbody to the primary in the nose of the weapon were routed through a plastic conduit having pressure connections to maintain the warhead case seal. Other case ports and joints, which had been closed with tape in the Mod 0, were permanently sealed. A hatch door was provided in the nose and was of a sealed type that could be opened to install the capsule at the time the weapon was loaded on the strike aircraft.
The Mk 17/24 Mod 1 was design-released December 1954 and appeared in stockpile March 1955. By September 1955, all the Mod 0's in stockpile had been converted to the Mod 1 configuration. In the meantime, work had been proceeding on the design of a proximity fuze, but many difficulties had been encountered.
Additionally, the Military - and especially the Strategic Air Command - had developed increasing interest in true contact bursts, to be used for cratering enemy air fields. Sandia thus decided to désign a true contact-burst fuze, and to apply this initially to the Mk 17/24 weapons. The initial approach was to design a contact fuze that would be effective against hard, flat targets, with the bomb striking in a nearly vertical position. Tests would then be made to discover how far the fuze could be extended for more severe impact conditions.
It was relatively simple to design contact fuzes for fission weapons, since these devices were covered with a light ballistic fairing that was able to sense the shock of impact and signal detonation before any real physical damage was done to the weapon. However, in thermonuclear devices, the heavy case would have to sense the impending shock and detonate just before actual impact took place. Discussions with Los Alamos led to the conclusion that the secondary reaction would occur if the primary reaction was completed, and it was decided to use a barium titanate crystal contact fuzing system developed for fission bombs. Sandia performed some sled tests in Area III, from which a design was evolved using two networks each containing two impact crystals, mounted on the nose of the bomb. A fast-firing gap-type X-unit was concurrently developed.
Further work on parachutes was necessary, in order to produce a design which would permit releases from B-52 bombers and result in a down-time of 75 seconds. Sandia placed an order on Wright Air Development Center for both standard and heavy-duty parachutes with diameters of 40 feet. The first phase of this testing program was completed January 19, 1955, after eight drops had been made with standard chutes. Test results showed that opening shocks were lower than expected. However, the requirement for carriage in the B-52 was canceled April 24, 1956, due to release troubles in which the suspension sling failed to retract properly. Subsequently, the Strategic Air Command decided to use a 64-foot-diameter chute and to place an operational restriction of 365 knots airspeed and 20,000-foot altitude on the weapon at release.
The Mk 17/24 Mod 2 Bomb was design-released June 1, 1955. The weapon contained a Mk 17 Mod 0 Fuze, with both contact- and air-burst capabilities. The bomb was 61.4 inches in diameter, 298 inches long, and weighed 42,000 pounds.
The weapon could be stored for 18 months under stockpile storage conditions, and for 15 days as a ready weapon. Efforts were under way to increase the latter figure to 30 days, to meet the desired ilitary characteristics. A 6-month capability was desired for the fuze, normally stored in its own sealed container, and the attainment of this goal appeared hopeful. Prior to dropping the weapon from the bomber, a safing switch was closed. Release of the weapon resulted in closure of the pullout switches, and weapon power was applied to the electrical system of the bomb. Parachute deployment was now initiated, if this option had been selected. At the selected arming altitude, an arming baro closed and started the charging of the X-unit. At the selected height for air-burst operation, the firing baro closed and detonated the bomb by firing the trigger circuit. The contact fuze acted as backup in the event of air-burst fuze failure, or it could be selected as the fuzing option.
About 25 percent of the Mk 17 stockpile was converted to the Mod 2 configuration. The work was performed at sites by the Military under Sandia supervision duringthe period June to August 1956. The program which would have resulted in the Mk 24 Mod 2 was canceled, and by October 1956 all Mk 24's had been retired in favor of the Mk 36 design. The Mk 17 Mod 1 weapons were retired between November 1956 and August 1957. The Mk 17 Mod 2's were retired between August and October 1957, since Mk 36 weapons were then entering the stockpile in large enough quantities to fully support military plans for weapons of this yield range. The Mk 36 provided a higher yield than the Mk 17 and at a much lesser weight.
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