Helium-3 or He-3 is a light, non-radioactive isotope of helium with two protons and one neutron. It is rare on the Earth, and it is sought for use in nuclear fusion research. The abundance of helium-3 is thought to be greater on the Moon (embedded in the upper layer of regolith by the solar wind over billions of years), though still low in quantity (28 ppm of lunar regolith is helium-4 and from one ppb to 50 ppb is helium-3), and the solar system's gas giants (left over from the original solar nebula).
It is unknown when the Moon Nazis started to use Helium-3 in their technologies. By 2018 they have set up a large refinery several kilometers from their base and have at least 15 to 20 large towers full with He-3.
The main cannons of the Götterdämmerung and the reactors aboard the Nazi ships use He-3 as fuel.