in another Heinlein story, he has a powerful laser set up in the centre of a crater and then rotated to burn a level rim on the inside so that an inflatable dome can be sealed directly to the rock and then pressurised.
with the moon, a lot of tunnelling was done while mining.
as the population grew due to marrages between convicts producing offspring, it makes more sense to utilise the unused space below ground than build on the surface.
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Originally Posted by chrispenycate  Mass driver, yes, and the source of one of the few technical errors I caught Heinlein in; they didn't require steel shells, any reasonable conductor (like aliminium, for example , which could easily be extracted from the lunar regolith) would have sufficed. |
but was it known, or even suspected, that there was aluminium on the Moon when he wrote the book?
and anyway, steel would work so it isn't technically an error.
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Originally Posted by chrispenycate And something coming in in free fall from lunar orbit would(if it weren't for air friction and the tendency to vaporise on the way down) arrive at more than ten kilometres/sec – that, for a few tens of kilogrammes is a lot of kinetic energy; baby dinosaur killer. No real advantage to chemical explosives after that, and they'd only render the missile more vunerable. |
considering the sizes of the containers used, I would estimate that there would have been several tons of rock per projectile, and if the ballistics were worked out well enough, the trajectory into Earth's atmosphere could be designed for maximum impact (although I wouldn't like to have to work out how much velocity is lost due to the atmosphere and what the impact speed would be, but I would expect to see a sizeable crater.
as for an advantage over chemical explosives, rock is cheaper, safer to handle and readily available