FWIW and maybe this is already known, but the Heinlein Archives http://www.heinleinarchives.net/upload/index.php contains a lot of correspondence between RAH and GHS. Heinlein was outraged when Harry was fired from his job at White Sands and wrote to newspapers and everyone in authority he could find.

I have some of the pieces from the archive, but it's all copyright and for profit. Not sure that I could even find the bits I have now.