need help with 4.5 in rocket launcher

I think getting scale drawings for scratch building is possibly the most difficult job in scratch building.  Some genres are easier than others (there are quite a few sources for ships and planes).  You might try a military museum- explain what you are doing and ask to take measurements if they have one of the objects you are interested in.  Sometimes a google image search will yield scale drawings.  I am not familiar with the rocket launcher you are talking about.  Would it be a current model?  If so maybe a local National Guard unit might have one.  I made a large caliper set out of wood to make measurements when I find a subject I am interested in modeling, to go along with photos.  To avoid distortion due to perspective when doing reference photos, try to take pictures square on (perpendicular to) the major axes.