I'm no good with setvelocity, but I've toyed with it and well it won't look realistic. I'm assuming this rocket is not under its own power when flying which is the reason for the script in the first place. Your taking a missile and making it move in a straight line via the coordinates 10,0,20. A real rocket or missile might slowly go down or up or side to side with wind or a guidance system.
Maybe a more complicated script that makes minor adjustaments in flight to make it looks like its moving around a little.