We don't know much about rockets. I think the game will only have one type of rocket. The technologies increase the damage the rocket do, there is probably no way to defend against rocket attack but destroying the launch ramps. Rockets are thus expensive strategic bombers but they do not need air superiority so the make more sense for nations in a bad situation then for nations in a good situation.
You must research first rocket technology to be able to research the jet engine (Jet aircrafts can be considered wunderwaffe as they need alot of research and can only be gotten at the end of the game).
Then there is of course the nuke
