Thermodynamics
I love some thermodynamics. It's one of the most interesting and useful things that I've learned. In fact I find myself doing thermo analysis on things too often.
Rockets are cool, jet engines are awesome, engines are complex. If you know some thermo, a hint of fluid dynamics, and some simple heat transfer you can calculate just about anything.
Projects
This is a Thermodynamics 2 project that i worked on. I think you can also see some thermo calculations I did on an HHCI design I put up on github... In fact I've got a script built in excel and in python to call into enthalpy tables and calculate each of the transitions.