[citation needed] During Titan II ICBM development, it had been found that the first stage turbopump gearbox was prone to total failure caused by resonant vibration in the idler gear.

[citation needed] There was also a potentially serious problem with the turbopump bearings which led to more design changes, however the odds of failing on a Gemini launch were slim to nil since GLV boosters used specially selected and tested bearings, in addition the turbopumps would be "hot fired" as part of prelaunch checks[citation needed] Combustion instability in the second stage engine was also a concern although that too had only been witnessed in static firing runs.

[citation needed] The most significant issue in man-rating the Titan II was resolving problems with resonant vibration known as "pogo" (since the action was said to resemble that of a pogo stick) that could produce g-forces sufficient to incapacitate astronauts,[citation needed] but the Air Force were not interested in helping NASA with a problem that did not affect the ICBM program and could potentially delay it, or require major modifications to the design.

However, Martin-Marietta argued that the pogo problem could be fixed fairly easily, and also the Air Force began to develop more of an interest in man-rating the Titan II due to the proposed Manned Orbiting Laboratory program.

[5] Another nuisance problem that occurred during the Gemini program was code-named "Green Man" and involved momentary pitch oscillations of the Titan second stage following engine cutoff.

As with the Mercury-Atlas launch vehicles, a high degree of workmanship was stressed as well as more thorough testing of components and improved handling procedures compared with Titans designed for uncrewed flights.