SF short story I read as a kid in Galaxy or similar mag, I read it in late 60s/early 70s but story may have been older.
A guy is transported, via some means which I don't recall, into Asgard at the time just preceding the beginning of Ragnarok. He is an engineer or scientist and understands the principles and technologies of manufacturing atomic weapons and heavy artillery and instructs the Norse gods in how to create the weaponry, which is then used by the Norse gods to win Ragnarok.