Only a fantasy novel by a mechanically minded science fiction author could be this dull and literal-minded. It is as if every instinct that enforced verisimilitude, or at least plausibility, in his science fiction oeuvre led Heinlein to over-explain and literalise every potential marvel in 'Glory Road'.
This is a shame, really. I read 'Glory Road' as a teenager and brought away an impression of its action and brisk pacing, and the sense of the adventurer's road across uncountable imaginary realms is powerful and evocative. But, on rereading, I'm left only with a dash of perfunctory action and a great deal of trans-dimensional expository dialogue, sandwiched with cultural-relativism lite.
There's none of the light touch that Fritz Leiber brought to pulp fantasy adventures, and far too much effort extended in making the wonderful seem believable, as though Heinlein had decided to write a fantasy to show that he could, and used all the science fiction techniques at his disposal to show that he couldn't.