A Rocket usually means a Harmon Rocket which is a modified RV-4 that starts with the Vans kit and is modified with John Harmon's mods to put an IO-540 in it, clip the wings some and build up the area behind the canopy for aerodynamics. The other 'Rocket' is called an F1 Rocket. It is a fastbuild only kit developed my Mark Frederick. It is designed for the IO-540 and has either the clipped wings like a Harmon Rocket or the new "EVO" wing that Mark has developed. All 3 airplanes are AWESOME but have different stregth's and weaknesses. Go to
www.teamrocketaircraft.com for a more detailed explanation, pictures, etc. This is just a quick and dirty answer.