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One thing we've puzzled over if whether or not to have the forward Lunar Transfer Vehicle radiator bulkhead enclose the docking adapter. If there was a last-minute abort from a missed approach during assembly, or an unintentional side thrust when the two spacecraft separate, we wouldn't want to bang the end of the airlock module into the forward radiator structure. So a bit of margin would be in order.
On the other hand, the docking interface is spring-loaded and the intial assembly probably will be done with some sort of Remote Manipulator System (RMS) rather than flying two vehicles together (that is, "berthing" rather than "docking" in NASA parlance) so the probability of a collision is very low.
Even if the crew had to abort the descent and wound up flying the habitat back to the LTV in lunar orbit, they wouldn't dock. They'd go EVA to get back into the LTV, abandoning the habitat in lunar orbit. They can't bring it home; there's not enough fuel to push that heavy mass back to Earth orbit.
So there's no strong reason not to have the forward service ring enclose the berthing adaptor. The benefit of doing so is that it allows us to orient the LTV so that the berthing adaptor, which is the crew's EVA entry hatch once the hab and airlock have separated.
Looking at the TLI stack from the "top" (that is, looking toward the side with the windows), I imagine it like this:
=====
----------------- | | -----------------------------------
~ 30 ft | | | | ~ 10 ft | ~ 30 ft
|----- ------ -| |
Lunar || Airlock || | LTV | Tanks
Base || || | Crew | Engines
Habitat |------------------ -| Module | External Equipment
| * | |
----------------- ------------------------------------
The asterisk (*) shows the separation plane between the two vehicles. We'll need to add some sort of structure to the Spacehab's front door to accommodate the berth adaptor, but it's fairly thin; maybe only 6" to 9" thick. On the landing-stack side, we have the airlock's outer hatch (as opposed to its inner hatch or side hatch). On the orbit-stack side, we have the docking adaptor's hatch.
The docking adaptor's hatch would be recessed inside the docking adaptor ring, to leave room for the petals and attachment structure.
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