http://www.russianspaceweb.com/a4_team_moscow.html

http://www.astronautix.com/g/groettrup.html

https://military.wikia.org/wiki/Helmut_Gr%C3%B6ttrup

http://everything.explained.today/Helmut_Gr%C3%B6ttrup/

https://www.airspacemag.com/space/the-rest-of-the-rocket-scientists-4376617/

https://historyofinformation.com/detail.php?entryid=2698

https://link.springer.com/10.1007/978-1-4419-5906-5_281

Moonbound, by Jonathan Vetter-Vorm