A former soviet scientist recently mailed his daughter a journal of his work on UFOs before the fall of the soviet union:
https://www.youtube.com/watch?v=-LSUrmfl46E&list=PL6PrA6lo8rJLRExhMvX6wKyNzy0hr_QM3
He includes drawings of the craft. Dimensions. The materials it was made of. Pages of mathematical formulas related to how it works…
He mailed the journal just a few months before he died.
Translation of the Book: https://www.engineeringinfinitybook.com/
Direct link to preview of book translation: https://www.yumpu.com/en/document/read/70789164/engineering-infinity-earths-first-interstellar-blueprint
I guess I’m not totally sure if this is legitimate or not, but I think it is very interesting.