I think you're thinking of the "Illudium Q-36 Explosive Space Modulator" that Marvin needs to blow up the earth. Do a google search on "explosive space modulator" and you'll find it. Check out this site:
http://members.home.net/kdpennock/marvin/marvin.htm
Search on Wave Motion Gun and you'll find the Star Blazers stuff, like at this site:
http://www.wizard.net/~jghiloni/links.html