In the late 1960’s H.H. Koelle of the Technische University Institut Fuer Raumfahrttechnik in Berlin devoted considerable effort to studying a reusable heavy lift launch vehicle. A good, well-illustrated report was put out in 1968 covering the design:
Entwurfskriterien fur groBe wiederverwendbare Tragersysteme (Design Criteria for Large Reusable Space Transportation Systems)
Note that the Neptun was *gigantic.* It was a two-stage ballistically recovered design, unusual in that rather than being circular in cross-section it was hexagonal. The individual propellant tanks were each the size of or bigger than the S-IC first stage of the Saturn V.
A number of payloads were proposed. One was a sub-orbital intercontinental passenger transport, The passenger “capsule” would land separate from the Neptun itself.
One of the more interesting payloads contemplated was a large Orion nuclear pulse vehicle, transported in two pieces (propulsion module in one launch and payload/pulse units in the other). Presumably this would be a NASA Orion hitching a ride on a West German booster; I suspect politics would have negated the likelihood of the West Germans developing a mass production line for nuclear explosives.
This fusion-powered interplanetary spacecraft is also a NASA design, dating from the early 1960’s.
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So “Project Neptune”, once a cheap “urban legend” around NASA conspiracies, existed anyway! Yet as a “german” project, not truly NASA and less american, with all that teutonic gigantomania, with lots of steel from the best of Krupp forges.
Great. But a question remains now. By looking at Koelle’s bio, this guy was almost no second to Kraft Ehricke. He was even director of “future projects” at NASA which may explain how the half-secret Orion came up in his Neptune. It seems he had some problems on holding this position and decided to return to Germany. But not just to Germany. It seems he took the chair of the just deceased Dr. Sanger, the grand-grandfather of the space-gliders.
Meanwhile it seems that it took time to “draw” Neptune. On the archive there is a second report:
Project Neptune – Design Criteria for Large Reusable Space Boosters. Final Report.
https://ntrl.ntis.gov/NTRL/dashboard/searchResults/titleDetail/N7117333.xhtml
So, was there a real Neptune in NASA? Was it killed, dropped into the trashcan?
A more recent design of the Neptune (page 25):
http://spacearchitect.org/pubs/Koelle-19970501.pdf