  Specific energy (i.e. energy per unit mass) is one of the most fundamental
and consequential properties of a fuel source. In this work, a systematic study
of measured fusion cross-sections is performed to determine which reactions are
potentially feasible and identify the fuel cycle that maximizes specific
energy. This reveals that, by using normal hydrogen to breed deuterium via
neutron capture, the conventional catalyzed D-D fusion fuel cycle can attain a
specific energy greater than anything else. Simply surrounding a catalyzed D-D
reactor with water enables deuterium fuel, the dominant stockpile of energy on
Earth, to produce as much as 65% more energy. Lastly, the impact on space
propulsion is considered, revealing that an effective exhaust velocity
exceeding that of deuterium-helium-3 is theoretically possible.
