John Houbolt, who has died aged 95, was an engineer and aeronautics expert with NASA whose theories on space travel were instrumental in helping President Kennedy keep his promise that the US would reach the moon before the end of the 1960s. Houbolt's efforts are largely credited with convincing NASA to focus on the launch of a module carrying a crew from lunar orbit rather than the alternatives such as a rocket carrying the crew all the way directly to the moon from Earth.
Houbolt was born in Altoona, Iowa, the son of immigrants from Holland, and grew up on a farm. He was always interested in rocket science and studied for a degree in civil engineering from the University of Illinois. He also earned a doctorate from the Swiss Federal Institute of Technology at Zurich.
He started the early part of his career with the National Advisory Committee for Aeronautics, NASA's predecessor, in Hampton, Virginia, in 1942. and also served in the Army Corps of Engineers.
In the 1960s, with the US/Soviet space race in full flow, the focus at NASA was on reaching the moon and 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.
As various possible techniques for reaching the moon were discussed, Houbolt argued that a lunar orbit rendezvous (known as LOR) would be less mechanically and financially onerous than building a huge rocket or launching a craft while orbiting the Earth. LOR, he insisted, was the only option to meet President Kennedy's challenge before the end of the decade.
Convinced he was right, Houbolt took the bold step of skipping the normal channels and writing directly to an incoming administrator in 1961. "Do we want to go to the moon or not?" he said in the letter. "Why is a much less grandiose scheme involving rendezvous ostracised 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."
In the end, Houbolt won the argument and his preferred technique was adopted for the mission in 1969 which landed successfully on the moon. The lunar module Eagle was carried by a Saturn V rocket, captained by Neil Armstrong.
After the success of the mission, Houbolt later left NASA to work in an aeronautical research firm in Princeton, New Jersey, before returning to NASA in 1976 as chief aeronautical scientist, presiding over the Shuttle and other projects.
He retired in 1985 but continued private consulting work, and is survived by his wife and three daughters.