Wondering how to tell when engine variants unlock.

So I've got a RSS/RO/RP-0 playthrough going. I've gotten way farther than what I had hoped -- just did my first flyby probes to Venus and Mars.

Anyway, I've gotten my tech tree to the point where I'll have all my Apollo parts available pretty soon. Next on my list is a Surveyor clone -- that is, I'm landing a probe on the moon. Now, I try to stick to what's thematically correct. So I'm trying to build an Atlas-Centaur to get the thing there (I have FASA installed). In the engine variant dialog, I see that I lack the tech for the LR89-NA-7.2, which is listed as what was used in the Atlas-Centaur. As this would have happened prior to unlocking all the Apollo parts, I'm left confused as to when these things will unlock.

Is there any documentation stating when engine variants (that is, those listed in the Engine GUI) are unlocked? I understand that RSS/RO/RP-0 is a work in progress, so maybe it's in the wrong node or something.

Also, when the heck do you unlock cryogenic procedural tanks? Service Modules only get you so far and all of the "cryogenic tanks" I see are listed as non-RP-0.