1. Throw the Nikon away.
I shoot with Canon stuff...I had to
Aperture wide open depends on the lens. If you're on a super-fast lens, you may lose enough depth of field.
I tend to shoot in shutter priority. The fastest lens I'll shoot with is F/4 so that mode plus auto ISO usually works well. Exposure compensation usually ticks toward the positive side by 2/3-1 stop to help with bright skies. All depends on what you have in each frame.
Yes, servo focus mode with a smaller zone should help stay locked on.
Also, back up and use a longer lens. Camera movement speed upward is reduced; making for more success and not needing crazy high shutter speeds. Trying to be close equals frustration.
Practice. I still suck at it, but am getting better the more I shoot rockets...