I read this book when I was a kid and the last chapter had been ripped out - it's always bugged me. Anyway, the part I remember is there's this teenaged kid (an orphan maybe?) and he's taken in by an interplanetary trading ship. The ship is only equipped with slower-than-light missiles and has to run through pirate-infested space. The pirates use some kind of beam weapon to knock trading ships unconscious and then board and strip them. He's trained as a missile targeter who launches missiles to intercept the pirate - which he does successfully on one occasion.
Not much to go on, but I thought I'd ask