Actually, this was one of the books that Heinlein deliberately chose *not* to write, IIRC. See the postscript to _Revolt In 2100_, "Concerning Stories Never Written."
Variable Star was *never* supposed to see the light of day.
interesting tidbit.
Great reference. If Heinlein "abandoned it", hre did so because he didn't like it. I hate this recent practice of resurrecting dead novelists anyway. Robert Ludlum, Lawrence Sanders, and a host of other long-deceased writers seem to have left stockpiles of "manuscripts" for no-name authors to "put the finishing touches on".