What is the best major to study for a career in propulsion systems design?
While the question is certainly subjective, I wanted to help you out as much as possible. I spoke to a friend who works as an electrical engineer at NASA and asked him your question. Based on his advice and some of my own research on your behalf, the most direct route would be to pursue an aerospace engineering degree in a good university program - CIT, MIT, UM Ann Arbor, etc. (there are plenty of schools to choose from). Most aerospace engineering students will choose a specialty in either an area of design or even in a specific type of vehicle - many universities will allow you to specialize in propulsion - essentially tailoring your electives to include subjects like fluid dynamics, multivariate and non-linear controls, combustive thermodynamics, and turbomachinery. That said, my friend indicated that the field can be very competitive, and that many propulsion system designers probably did not begin their career doing that kind of work. Be prepared to be flexible and plan on playing the long game when it comes to reaching your dream job. Also, as is the case in any competitive industry, networking can be key. Be sure to find opportunities to meet people in your field, ask questions, and get to know others who can give you the inside track on openings and opportunities.
Hope this helps.