Space exploration by humans and robots benefits from optimization of many systems. Design of cryogenic rocket systems, typically using liquid oxygen and liquid hydrogen or liquid methane, is crucial to do as well as possible so as to be able to deliver the space vehicle to where the exploration and scientific research can occur. Warming of these chilled propellants shortens mission lifetimes, drives up cost, and, when poorly controlled, prohibits us from even attempting some missions. This research focuses on delivering much-needed improvements to how we model the heat transfer processes, such as warming and the use of chillers, with cryogenic propellants. Unique ground-based experiments along with detailed computer modeling of evaporation, condensation, motion, and heat transfer in the liquid and vapor phases are combined to open up the details of how fast these propellants move heat and mass around the fuel tanks in space by evaporation and condensation. This will permit designers to better optimize spaceflight systems to enable new missions, increase affordability, and reduce risk.