I've thought further about the problem of actually gaining access to a lava tube, and assuming it's the plan to build a long-term base in a lava tube, mission planners have two options:
- Send a surface scouting mission - probably a rover - to a potential site, to ascertain whether the chosen lava tube is accessible. If not, a different site could be investigated, or a hardened shelter and excavation equipment could be sent with a crew to dig their way into the tube.
Once an accessible site is found or access to a tube is cleared, send kit for setting up a base inside.
Advantages: You don't go to the expense of sending a hardened shelter and heavy excavation equipment unless you have to.
Disadvantages: For each site, an extra launch is necessary for a rover just to scout out the local area. Also, the project will take longer to get off the ground because of having to wait for the results of the rover's survey.
- Assume lava tube access will be blocked, and send a hardened shelter, excavation equipment, and stuff for populating a tube immediately. If access is blocked, the crew will clear it, and set up a base.
Advantages: doesn't require a surface scouting mission in advance, so quicker to implement.
Disadvantages: the extra mass of a hardened shelter and heavy plant has to be sent every time.
I expect that, at least initially, financial constraints will dictate that a beachhead will be a hardened shelter near to a lava tube, and that equipment for either gaining access to the lava tube or for establishing an inflatable-modules-covered-with-regolith base would come later, if the lava tube is inaccessible. Of course, if the lava tube is accessible immediately, then that's an added bonus.