I am told by one of the Black Buck pilots that the last raid dropped the bombs safe because of confusion over the switchs labelled NOSE-TAIL/TAIL and VT/OFF. (edited)

As for bomb spacing I seem to remember times in the order of 0.24 and 0.45 seconds. There were shorter intervals but these would have resulted in a tight stick with bombs unnecessarily close and also in danger of jostle. There were also much higher intervals which would have been useful for mine laying.

The following text is my recollection of the development and use of the fuzing for 1000lb bombs in the Vulcan.

RAF Conventional bombs have exploder pockets in the nose and the tail. Initially pistols were fitted to each pocket namely a Number 76 Nose and a Number 75 tail. An alternative tail pistol was the Number 79 delayed action pistol. These were all ‘impact’ fused systems albeit the detonator could incorporate a delay, short in the case of the 76/75 and up to 48 hours or more for the No 79.

The fusing was initiated in by the selection of a two-way switch. The default (off) position was TAIL with the up position as NOSE and TAIL. This enabled airborne selection of impact fusing (nose) and tail (backup) or delay (tail) and nose inactive. If a bomb was fitted with nose and tail pistols it would automatically be armed if dropped by the bomb aimer or NBS. To drop them safe would require the use of Pilot’s Emergency Safe Jettison (PESJ). (edit)

The pistols were carried safe with a safety pin in the arming vane and a fusing lanyard. With the appropriate fusing selected an electro-magnetic fusing unit (EMFU) was activated which locked the fusing lanyard. When the bombs dropped the fusing lanyard would pull the safety pin from the pistol, the arming vane would rotate, and after a safe separation and fusing unit (SAFU) period the pistol striker would be free to start the explosive train.

In the 50s a new fusing system, the 900 series, variable time (VT) fuse was developed to allow air burst fusing. This fuse, with an integral detonator, was initiated in much the same way but also required an electrical pulse through a ‘Churchill’ plug. A second fusing switch was then added to the armament panel next to the Nose/Tail – Tail switch; this was the VT on/off.

To drop a stick of bombs with airburst function the switches would be set to Nose/Tail and the other to VT. To drop the stick with only Tail impact the VT would be left off. Now we get a problem.

A later multi-function bomb fuse, also IIRC in the 900 series, was developed for the tail fusing pocket. For this fuse to function the VT function had to be selected.

In the case of the Falklands the correct selection for an airburst would have been Nose/Tail and VT. For an impact detonation the selection would be Tail and VT. If the airburst fuse, say the 952, failed then the tail fuse should have worked. However whether or not a nose fuse was fitted, it would be essential to select VT. If VT was not selected then neither a nose nor a tail function would work.

Now I am speculating but I do not think there was any intention to drop the bombs in airburst mode. In that case only the tail fuse would have been fitted. Fusing selection should have been Tail and VT. [There is a possibility that it was necessary to select Nose/Tail and VT because of the VT wiring but it was 35 years ago and I may have memory fade]. (edit)

The stick that failed to explode would have caused as much chaos and disruption as the one that did. Bombs that do not explode may be delayed action with delays from 30 minutes to 48 hours or even possible 96 hours. As they failed to explode the first task would have been to identify the impact area and impose a quarantine over an area at least 1600 x 500 yards and that only after you had located bombs 1 and X. I imagine there would have been a shortage of volunteers doing the initial survey!