Houbolt died on Tuesday at a nursing home in Maine of complications from Parkinson's disease, his son-in-law Tucker Withington confirmed yesterday.
As NASA describes on its website, while under pressure during the US-Soviet space race, Houbolt was the catalyst in securing US commitment to the science and engineering theory that eventually carried the Apollo crew to the moon and back safely.
His efforts in the early 1960s are largely credited with convincing NASA to focus on the launch of a module carrying a crew from lunar orbit, rather than a rocket from earth or a spacecraft while orbiting the planet.
NASA describes "the bold step of skipping proper channels" that Houbolt took by pushing the issue in a private letter in 1961 to an incoming administrator.
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"Do we want to go to the moon or not?" Houbolt asks.
"... Why is a much less grandiose scheme involving rendezvous ostracized or put on the defensive? I fully realize that contacting you in this manner is somewhat unorthodox, but the issues at stake are crucial enough to us all that an unusual course is warranted."
Houbolt earned degrees in civil engineering from the University of Illinois at Urbana-Champaign. He earned a doctorate from the Swiss Federal Institute of Technology at Zurich in 1957.