VASINR Engine

The Variable Specific Impulse Nuclear fusion Rocket, or VASINR engine, is a fusion engine developed by martian scientists in 2050. The overall architecture is still in use to this day, though much more refined over the last century. VASINR is a high-efficiency fusion torch drive that runs on deuterium-deuterium.
VASINR was developed at the peak of tensions between earth and mars. Earth's rule over mars was becoming more overbearing, mars wanted independence but didn't have the resources, mainly water, to support itself. VASINR allowed mars to reach for the asteroid belt and harvest water from there. Earth was days from declaring martial law on mars when shematics for the VASINR were gifted in exchange for total independence.