"There's a mysterious type of crystal called "God Crystal" that can be found in both Gensokyo and the Moon World. This crystal is born from parts or blood of the gods that dropped on the ground in the ancient wars long long time ago, they then disintegrated into the ground, overtime, they formed with the elements and air around them to make crystals with high radiation level. There are two types of God Crystal, one is formed in the ground of Gensokyo, these crystals are very light, they have a blue color and emits a blue glow. They are totally safe to touch with bare hand, in fact people who held them said that they even felt like they are energized or those who had illness said that they don't feel sick anymore. It is used as an energy source for power plants in Gensokyo since it was discovered in 1965. The other type is formed on the Moon, there isn't much information about it but some did ended up in the dirt of Gensokyo for some reason, these crystals emit a blinding purplish pink and is extremely radioactive, 1 million time more radioactive than Radium and is extremely deadly, people who made contact with them died within 3 hours, it's still unclear what they are for, but with such radiation, it won't have any use soon."
-Gensokyo's materials book-

After the Legacy of Lunatic Kingdom event, Reisen's plane is upgraded into a space fighter jet with 2 massive fans and TVC, the plane no longer has a fuel tank instead, the plane "lives" with the Moon God crystal as its core, the crystal gives power to the entire plane including the engine and weaponry, since its energy is so large, the plane even has a supercharged mode that massively drains the energy of the crystal, but in return, unleases out a godly amount of firepower.

Alright, that's enough lore for just a crystal, free to use, disconnect the cockpit to drag it into your sub assemblies, it's weighs only 9kg, you can use it as a kind of fancy fuel tank... and you don't have to worry about running out of fuel anymore :)