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Trunnion pins are devices which anchor the payload to the launcher, in this case the Space Shuttle.
The trunnion pin is 3" in diameter and protrudes 9.5" past the scuff plate. The scuff plate is 28" in maximum diameter, with a flat section 22" in diameter, and a flange mounted at close to a 45-degree angle. Neither the flat part nor the outer flange is a complete circle; see the drawing.
The protruding part mounts in a payload release latch assembly on the Shuttle Orbiter payload bay sill. The scuff plate serves as an alignment guide as a payload is mounted in the Orbiter and protects the payload from getting scuffed by the payload bay sill (hence, its name).
The trunnion pin is usually bright polished metal and the scuff plate is anodized a bright orangish yellow.
When sizing a spacecraft, the trunnion pin and scuff plate assembly shown is outside the 15-foot-diameter dynamic envelope allowed for the payload. That is, your payload can be a full 15 feet in diameter; then you add the mounting hardware to bolt it into the Orbiter.
Shuttle payloads also usually have a KEEL PIN. This is a similar pin, but no scuff plate, that goes into a hole in the Shuttle Orbiter keel. The keel is the main structural element that runs along the bottom center of the payload bay.
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