Sorry to spoil you fun, but Werfergranate 21cm was really used (and widely) by luftwaffe aircraft. Also other calibers were used.
WGr. 21cm
WGr. 32cm
I couldnt find any good english language sites telling about conversion, but here's one in german:
http://www.balsi.de/werfer.htm
[ July 08, 2003, 07:43 PM: Message edited by: illo ]