black cigar shaped craft with smalles craft rising up to the bottom of it. those craft seemed to come and go

My grandfather and I witnessed what we thought was a UFO over Lake Michigan Just North of Gary Indian.  I was stationary and had several smaller craft coming and going from the underside of the craft.  It remained long enough for my grandfather to step into a dime store and purchase a small video camera.  He filmed the craft for several minutes.
The film has been lost.  However, it has always puzzled me as to what it actually was.
Could have been part of an air show stunt involving a blimp launching small planes?  I just thought it was unusual enough to mention, just to see if there were any reports from other people.
I'm not solid on the date it was the early 1970's and it was a clear bright day. we witnessed it from the small hotdog stand the my grandparents ran at the time.