You might benefit from reading the following link and an extract from one of the authors below:
Michael H. Maggelet | October 10, 2013
The weapons involved at Goldsboro, Thule, Yuba City, etc., were in no danger of detonating. While many critics and well meaning individuals continue to harp on the “one switch” mantra, let’s take a look at what was required for the Mk 39 Mod 2 to produce a nuclear yield. All info is declassified and from DOD and DOE sources.
First and foremost, there is no safety switch in the cockpit. The device is known as the DCU-47 and it’s part of the Aircraft Monitoring and Control equipment (AMAC). Since we’re discussing Goldsboro, I’ll go into some detail and refute some myths also.
AMAC for the B-52 at Goldsboro (actually near Faro, NC) consisted of the DCU-47 and the T-249. The DCU-47 was located near the pilot’s seat and was a toggle operated switch designed to provide power on demand to the T-249. It was safety wired and sealed to prevent inadvertent activation and to detect tampering (per SAC regs). During the bomb run, the pilot would break the seal and open the cover, and toggle a switch for the forward or aft bomb. He had to keep the switch toggled during the entirety of the bomb run, otherwise power would be removed from the T-249 and the weapons would automatically safe.
The T-249 was located in the lower flight deck of the B-52 at the bomb/nav or weaponeer station. During pre-flight or in flight, the T-249 was used to monitor the safe status of the bomb (or safe the bomb, if required). The T-249 could not be used by itself to arm the bombs- that could only be done with the consent of the pilot using the DCU-47. It too was safety wired and sealed to prevent inadvertent operation and to detect tampering.
On a side note, I see mention of the SWESS system occasionally, it was nothing more than an emergency jettison system with mechanical and electrical interlocks which gave the crew the means to emergency jettison the bombs if necessary. It did not “automatically” drop weapons if the crew was disabled or other nonsense.
The Mk 39 Mod 2 had three safety switches, not one as is commonly stated. These consisted of the MC-772 Ready/Safe Switch which was visible through the bomb case and a glass window, the MC-788 Electrical Arming/Safing Switch in the bomb’s electronics cartridge, and the MC-732 Trajectory Arm Switch in the bomb tail subassembly.
The primary means of safing the bomb was through the AMAC controllers and MC-772, which was designed to be electrically rotated to “arm” by aircrew intent following a nineteen step checklist. The bombs internal thermal batteries could not be used to rotate the MC-772. Only aircraft power of 28 volts/3 amps for three continuous seconds, applied via the DCU-47 and T-249 and a special aircraft/bomb cable could pre-arm the weapons before drop. The MC-788 Electronic Arming/Safing Switch in the cartridge could only be operated via the MC-772.
During Goldsboro, the low voltage and high voltage thermal batteries (HVTB) activated after the pulse plugs were removed- that would also happen if the bomb accidentally fell during a ground accident (failure of the U-2 sling during loading, for example). The LVTB’s would activate, however no current could reach the X-unit due to the fact that the MC-772 and MC-788 were in the safe position, and the MC-732 did not sense a pressure change.
Since the bombs were torn from their U-2 racks during the Goldsboro accident, the pullouts were extracted and the LVTB and pulse generators activated. The MC-772, MC-788, and MC-732 prevented any current from reaching the X-unit (which could not be charged).
During the entire sequence of events, the MC-772 and MC-788 prevented any current from the LVTB, pulse generators, HVTB, closing of the baro’s and timers, and piezo’s from reaching the X-unit. Thus, the X-unit could not be placed into a condition to charge the saturable core transformers and the cold cathode tube spark gaps.
The low voltage current from the nose impact switches (piezo’s) could not bypass the R/S switches, and was insufficient to fire the bomb (since the X-unit was not charged).
The detonator bridge wires required very high voltage for initation, and this could only be supplied from the X-unit (with the high degree of precision and simultaneity required for implosion). There were numerous other steps required to place the Mk 39 into a condition to produce a nuclear yield, and without the charging of the X-unit that could not take place. The nuclear system was also one point safe, as shown by the impact of bomb 2 with no HE detonation.
As for claims that “stray voltage” could have resulted in armed bombs during breakup, that’s rather dubious given the complexity of the electrical system of the aircraft and bomb pullout cable. It was a neoprene covered cable (may have been shielded) with numerous pins and contacts, and was torn from the bomb during breakup. Could the cable have been sheared and voltage passed through the correct few pins for an adequate time period to rotate the MC-772? Highly unlikely.
There were several ground incidents during the Cold War where assembly crew smashed cables during weapons assembly, loads crews smashed connectors during loading, and testers failed. When aircraft power was applied, this resulted in activation of the LVTB or stepping of the R/S switch. It should be noted that the R/S is easily safed by manually rotating it to “safe” using a screw driver or pushing a detent (it cannot be manually armed, which would result in breaking contacts in the switch, safing the weapon). Some of the early weapons I worked on, the B43 and the B57, were “antiques” by today’s nuclear safety standards. Never lost a moment of sleep worrying about their safety.