In today's age, if I started messing with liquid fuels, or built rockets over 6 feet tall, I would likely get harassed by local law enforcement (or more likely my neighbors would call for them), assuming I could even get the proper permits to be allowed to build the thing... permits to build something with my own two hands and then test it out on my friend's private property (a farm)? CRAZY, and wrong.
While that may draw some odd looks, there is a regulated process by which you can build that kind of thing. Building a six-foot tall rocket is not illegal (I have done it many a time, the largest I have built was a two-stage 11 foot sounding rocket, staging from a K1100T (about 256 times more powerful than a C) to a J1299L (about 128 times more powerful than a C)), and no permits of any kind are required to build it. You do need a certification (or to know someone with a certification) to buy larger propellants, which basically help you to be less of a hazard to others (because presumably if you have a Level 1 or 2 or 3 certification, you know somewhat what you are doing). You do know about the NAR, right?
In short, rocketry is not illegal, but you do need to know what you are doing so that you don't kill or injure people people.