For a project for my world history class, I'm supposed to complete standards about the progression of technology and the development of rockets and nuclear warheads. My plan was to build three. One for the types of fuel during the cold war (still working on) 1 for between the cold war and the present ( solid mixture of potassium nitrate and other reactants.) and 1 for the current era. My question is fo the new era how you I make a standard model rocket compatible for liquid combustion. Since liquid hydrogen isn't the easiest to acquire, I was instead going to use liquid nitrogen to freeze the air around it and get some liquid oxygen. I can't seem to find a way to modify a model rocket without going over budget and taking too much time. Any and all help is appreciated and questions are more than welcome.