Mark II Hanley-Messer DFR

The Mark II Hanley-Messer DFR is a type of human-manufactured fusion drive. s such as the were equipped with three Mark II reactors. The Mark II engines were obsolete by 2552, providing only a tenth of the power generated by modern reactors at the time. In order to fit the demands of Operation: RED FLAG, the Autumn was later refit with a significantly more efficient experimental reactor set.

Trivia
Though unconfirmed, "DFR" may be short for "deuterium fusion reactor", as UNSC starship reactors are known to use deuterium for fuel. This may suggest the drive utilizes deuterium-deuterium nuclear fusion reactions for power generation.