I’d like to add some suggestions and thoughts:

For one, I’d prefer D – Deutschland (picture: map with Germany in the borders of 1941 or so) rather than R – Reich. This would leave R for something more spacy, like ‘Raumfahrer’ or ‘Raumschiff’, with a picture of a Reichsflugscheibe e.g.
And Y – Yankee looks like a funny idea, with the astronaut and the Stars and Stripes, but no German would call an American ‘Yankee’. They’d say ‘Ami’, so unless you’d want to drop ‘Adolf’ and shift it to A, it wouldn’t seem very fitting to me.
Another idea would be to change S to ‘Sonne’, so you’d have earth, moon and the sun and make an additional SS – SS (with the rune symbol). I think, this would not be too far fetched, since many German words have the double-s in them and German typewriters during the war often had a special key for the runic-SS (only used for when “die SS” was meant, of course). It could have developed to a special letter on the moon, maybe even replacing the ‘ß’.

I like C – Chemie (chemistry, picture: test tube) and
I – Industrie (industry, picture: steaming chimneys), as were suggested
because they would represent the industrial component and
Ä – Ähre (if you chose to keep ÄÖÜ), because it would represent the
agraric component…
And unless you’d use the suggestion to take XYZ as one, I’d suggest to go with
X – X-Strahlen (‘Röntgenstrahlen’, x-rays),
Y – Ytong (a light form of concrete, used for building buildings fast, picture: brick).