He witnessed two tests with a rocket-driven railroad car in Burgwedel near Hannover and then joined Albert Püllenberg's group of amateur rocketeers.

This was the first man-made vehicle to reach space based on a then-current definition of 50 miles in altitude (see Kármán line for relevant background).

Dannenberg then became Walter Riedel's deputy and headed the crash effort to finalize production drawings of the V-2, the world's first ballistic missile, used by the Nazis to bomb London.

"[5] After the end of World War II, Dannenberg was brought to the United States with 117 other German specialists under Operation Paperclip to Fort Bliss, Texas.

[6] Most members of the group performed calculations and designs of future advanced launch vehicles with longer ranges and greater payloads.

About 30 members trained the U.S. Army and the support contractor General Electric to launch V-2s at the White Sands Proving Ground.

[1] At that time, rocket pioneer and former [7] SS major[8] Wernher von Braun decided not to start their own rocket engine development, but to purchase an engine from North American Aviation (NAA) that was being developed by Dannenberg's former boss, Riedel, who had previously left the team to join NAA.

In 1960, Dannenberg joined NASA's newly established Marshall Space Flight Center as Deputy Manager of the Saturn program.

[1] He received the NASA Exceptional Service Medal in 1973 for successfully initiating development of the largest rocket ever built, the Saturn V, which took the first human beings to the moon.