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The first docking of two objects in space was Gemini 8 docking to the Agena Target Vehicle.

Naturally, for two vehicles to dock, they need compatible docking connectors. They must be designed to attach to each other perfectly.

Careful manufacturing to standards can be enough to gain confidence that the systems will connect properly - today, every vehicle going to the ISS is designed to match its ports, despite never having a chance to do a physical fit test prior to launch.

For this first docking, were the physical flight units of the docking interfaces of the two vehicles ever fit-checked with each other on the ground, or did they rely on the "design, measure, and trust it" approach that is necessitated today?

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    $\begingroup$ Despite having answered, I don't think the premise of your question as I understand it is sound. We invented interchangeable parts and precision manufacturing long before we got into docking spacecraft. We don't need or want to match-fit docking adapters, we want them to meet the manufactured tolerances so they'll fit together, but we absolutely do have physical test rigs on the ground that are representative. Some (many?) are on motion rigs. $\endgroup$ Commented 1 hour ago
  • $\begingroup$ I'm a bit confused - what is unsound about the question? I build space hardware and even though we have CAD drawings and can cross reference dimensions, we still do fit-checks when possible, for a multitude of reasons. And it seems like the answer is "yes, they did do a test docking before launch", indicating that they indeed wanted to double check it. Therefore this seems to vindicate the premise and suggest that the premise is sound. Am I missing part of your analysis? Not aiming to be combative here, just curious. $\endgroup$ Commented 16 secs ago

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The docking adapters for Gemini and Agena were apparently tested for Gemini VI; there's at least one photo, S65-52015, of the ground test. Gemini VI's Agena "failed to go into orbit" and the Gemini VI-A mission rendezvoused with Gemini VII instead.

GEMINI-TITAN (GT)-6 - AGENA TESTS - DOCKING EXERCISE - BORESIGHT RANGE - CAPE S65-52015 (1965) --- The Gemini-6 spacecraft (right) and the Agena Target Vehicle (left) on the Boresite Range Tower for the Plan-X docking exercise. Photo credit: NASA or National Aeronautics and Space Administration

For Gemini 8 specifically, NASA Press Kit for Friday AM March 11, 1966, Release No 66-52, Gemini 8 Launch Set for March 15 tells us

The Agena and its docking adapter were shipped to the Kennedy Center Jan. 21. These two components and the Gemini spacecraft were mounted atop a 50-foot "Timber Tower" at KSC'S Radio Frequency Test Systems site Jan. 27-28. The prime Gemini 8 crew and their backups boarded the spacecraft on the tower to conduct a series of Radio Frequency Capability Tests between Gemini 8 and the Agena target. Docking compatibility checks also were made between the two vehicles.

Emphasis mine. It isn't clear to me if the "Boresight Range Tower" of the Gemini VI photo is the same as the "Timber Tower" referred to in the press release.

On Astronautix such tests appear to be referred to as "Combined Interface Tests", starting with Gemini 6. The same term is used in the link to SP-4002 that Controlgroup shared.

All found by Googling "gemini agena interface test."

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  • $\begingroup$ It's interesting that the Agena clearly has adjustment for axial and rotational movement. The photo appears to also show provision for yawing the Gemini about some point, it's unclear whether there's any provision to move it laterally or provide pitch, but I speculate that one of the reasons for the relatively high tower was to provide space for the capsule positioning actuators below what we can see. $\endgroup$ Commented 3 hours ago

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