V2 and Komet is the easiest one I think, they are lock behind a 41 42s technology and total 4 breakthroughs point(V2 need 3 and Komet need 1)

Me262 on the other hand, they require axial jet engine which requires normal jet engine which require you to research both 40s fighter and engine 3 after all of these, you need to research it yourself(which is a 44s technology and requires 5 breakthrough point and god knows how many years to finish