In late 1964 McDonnell proposed use of the Gemini as a circumlunar spacecraft to scout the Sea of Tranquility prime Apollo landing site and prove navigation and communication systems prior to the first Apollo missions.
The manned portion was a modified earth-orbit Gemini. The aft modules would be retained, but with the retrorockets removed. The retro module space would be used to install Apollo-type lunar distance communications, navigation, and test equipment. Deployable DSIF omni-directional and parabolic antennae would deploy from the aft modules to support lunar-distance communications. To handle re-entry from lunar distances, several modifications were necessary. The capsule's heat shield would be beefed up, and the Rene 41 corrugated shingles of Gemini's skin would be replaced with ablative shingles. The load of attitude control propellant for the capsule's reaction control system was substantially increased. Additional strap-down gyros and solar sensor packages would be added to provide navigation system redundancy. The ejection seats would be deleted and a Mercury-style launch escape tower added. The then-planned Rogallo wing recovery system would be used to glide to the Gemini to a runway landing on US territory after return from the moon. To handle the scientific payload, a camera compartment was added to the nose below the parachute/Rogallo wing housing. The Gemini spacecraft modified in this way had on on-orbit mass of 3955 kg as compared to the 3207 kg of the earth-orbit version.
The circumlunar version used the Saturn IB launch vehicle. This would put the Gemini mated to a partially-fuelled Centaur upper stage in low earth parking orbit. The Centaur would fire, sending the Gemini on a loop around the moon.
The scientific equipment would consist of a modest camera array installed in the nose of the spacecraft. This consisted of a long focal-length telescope, to which were attached a telephoto camera, a 40 degree field-of-view mapping camera, a 180 degree field-of-view panoramic camera, and two 16 mm film cameras. The tube of the telescope would be jettisoned with the nose compartment during parachute deployment, but the telescope base, mirror, and film cassettes were retained. The cameras would be used to film the moon during the relatively brief flyby period. This began 96 minutes before closest approach to the lunar surface, with the spacecraft 9700 km from the lunar surface. For 92 minutes the spacecraft could film in detail the Apollo primary landing site on the Sea of Tranquility and the those portions of the unseen back side of the moon that were in daylight (the flight would be timed for the sun angle to be 45 degrees at the Sea of Tranquility). Closest approach to the moon would be 160 km. Beginning 36 minutes after closest approach, at a distance of 4500 km from the surface, the sunlit portion of the moon would again appear, and remaining film could be used for reconnaissance of the area around Copernicus crater.
Crew Size: 2. Habitable Volume: 2.55 m3.
Gross mass: 3,955 kg (8,719 lb).