Mark 39 nuclear bomb
As the weapon now always contained all components needed to fire it, several safety systems were added to avoid inadvertent detonation, including safing pins that would hold in the arming rods, arming rods that required considerable force to pull, and a high-voltage safing switch to prevent detonation in the event that fire or extreme heat igniting the high-voltage batteries.
Difficulties arose that necessitated reworking the parachute arrangement as well as developing a different contact fuze (as the original piezoelectric crystals would not reliably operate at the relatively low impact speeds contemplated).
[10] The original Mark 39 Mod 2 used an MC-772 Arm/Safe switch to keep it from firing inadvertently should it be accidentally dropped from an aircraft at sufficient height and under the right circumstances to otherwise start its arming sequence.
[11] A consequence of the accident was that all Mark 39 Mod 2 weapons lacking Alt 197 were "red-lined" and removed from deployment until the change could be enacted.
[12] In the 1961 Yuba City B-52 crash a few weeks later, the Mark 39 Mod 2 bombs involved did have Alt 197 applied to them, but the low-voltage thermal batteries were nonetheless activated in one of the weapons despite the MC-1288 Arm/Safe switch being in the "Safe" position.
According to the Defense Atomic Support Agency, "post-mortem analysis indicates a probable cause of the activation of the low voltage thermal batteries of the one weapon was a cable short which permitted the energy from the MC-845 Bisch Generator to bypass the MC-1288 Arm/Safe Switch.
It is suspected that the MC-845 pulse resulted from the mechanical shock sustained upon impact and was passed to the MC-640 [thermal batteries] through one of the possible random short circuits.
On November 4, 1958, a B-47 with a Mark 39 Mod 1 (sealed pit) weapon on board crashed and exploded shortly after takeoff (for a training mission) from Dyess Air Force Base, near Abilene, Texas.
[15] On January 24, 1961, two Mark 39 Mod 2 nuclear bombs that were carried by a B-52 Stratofortress which broke up in the air and crashed near Goldsboro, North Carolina.
[16] On March 14, 1961 a B-52 carrying two Mark 39 Mod 2 (Alt 197) weapons crashed 15 miles (24 km) west of Yuba City, California.
The other had separated from the aircraft after impact, tumbled several times, and had its internal components ("the primary and most of its secondary") thrown out of its ballistic case.
[15] A Mark 39 Mod 2 casing is on display in the Cold War Gallery of the National Museum of the United States Air Force in Dayton, Ohio.
