Yesterday I outlined some design constraints for a beachhead Moonbase. Unfortunately, I've been missing a crucial fact over the last couple of weeks; this is a good example of why these journal entries are a learning experience.
While looking at some data on lava tube sites that have already been identified, something struck me: all the lava tubes that have been located so far have been rille discontinuities, i.e. the lava tube has collapsed at each end of the possible lava tube, probably leaving a lot of debris that would hinder access to the tube. I then also finally realised that entirely uncollapsed tubes would have no way in - they would be completely underground. The fact is that even if a lava tube was found from orbit that looked absolutely perfect for inhabitation, it is most probable that the tube would be inaccessible.
So, in order to gain access to a lava tube some heavy plant would very likely be required: bulldozers, excavators and possibly tunnelling equipment, none of which is going to be on the cards for a beachhead Moonbase (although I have some ideas about how some of this stuff could be made available).
So, having established that a beachhead Moonbase cannot be assumed to be located in a lava tube, I'm back to square one: how to protect astronauts from solar particle events and cosmic rays while they clear access to a lava tube. Well, one way would be with a metre or so of regolith. But heavy equipment is needed to move the regolith on top of the base. I really don't like the thought of astronauts being unprotected so far from home: if a big solar flare occurred, they would be to far away to return before it hit, and could easily suffer a fatal dose of radiation.
Which came first, chicken or egg? I will need to ponder this further.