Be wary of believing any account when it comes to nuclear weapons procedures. There is much misinformation. Changes happen all the time. Some of the public stories have been adopted strait from fiction. The letters have been the subject of so many spy thrillers they are now more myth than functional.
The very concept of the letters presupposes that sub crews have full control over their missiles, that they can fire them based on instructions in a hand-written letter. That does not mesh with the safeguards surrounding other weapons that require external command/codes prior to launch.
It's also fairly well documented that UK Trident sub warheads do not have Permissive Action Links - the crews of the subs do indeed have the ability to launch without receiving any codes.
The justification for this is pretty simple - timing. In the event of a launch from a likely enemy (the Soviet Union in the bad old days) there simply wasn't enough time to guarantee that a code be transmitted before weapons bursting.
> The very concept of the letters presupposes that sub crews have full control over their missiles, that they can fire them based on instructions in a hand-written letter. That does not mesh with the safeguards surrounding other weapons that require external command/codes prior to launch.
The whole point of the submarine component of the strategic triad is to be the ultimate guarantor of MAD by presenting the capacity for launching a retaliatory strike in the event of sudden destruction of the highest command authorities (and, potentially, the land-based components of the triad), so it is absolutely plausible that submarines have a looser set of controls than bombers and land-based missiles because otherwise there would be no point in having them.
The very concept of the letters presupposes that sub crews have full control over their missiles, that they can fire them based on instructions in a hand-written letter. That does not mesh with the safeguards surrounding other weapons that require external command/codes prior to launch.