...how do we launch our nukes WITHOUT Presidential orders? Is seems to me that for deterrence to be credible, there must be a mechanism to authorize a retaliatory strike even if the President is taken out by the enemy in a sneak attack, or is otherwise isolated or out of communication and cannot launch on his own authority.

I would think a potential enemy would attempt to knock out the President first in order to delay a US counterstrike as long as possible. At any rate, there must be other fingers on the button other than POTUS, which suggests that POTUS alone may not have full authority. I know that units that are nuclear-capable, such as ships, missile silos, submarines, bomber wings, etc must receive authorization from more than one source, and that more than one officer in each command has the final say-so. For example, two launch officers must simultaneously turn launch keys from stations not in physical proximity to one another.

There are two contradictory C and C issues at work here.

1) You cannot let one rogue or deranged officer launch independently, and

2) In the event entire levels of command have been eliminated by an enemy sneak attack, the system must still be able to respond in retaliation.

Otherwise, the entire concept of "retaliation" is meaningless.

And what about related issues, such as targeting, or preemptive strikes, which might make selecting new targets or abandoning old ones, necessary. There's no point in taking out a silo that has already launched all its warheads, is there?