On a workbench, gravity is a silent inspector. Parts sit flat, brackets hang true, and a slightly warped panel announces itself by rocking on the surface plate. In orbit, none of that exists. The only thing holding your geometry together is the drawing — which is why geometric dimensioning and tolerancing stops being paperwork and becomes the mission.
A position callout of ⌖ Ø0.05 A B C looks like bureaucratic
fine print until you picture a star tracker bolted to a panel that was machined
in a 1-g shop, shipped through 8 g of vibration, and is now floating in vacuum
at a 90-degree thermal swing. The feature control frame is the only sentence
in the entire assembly that survives all three environments unchanged.
The Datum Is the Spacecraft
Every GD&T scheme begins with a datum reference frame, and spacecraft are unusually honest about theirs. Datum A is almost always the primary structural deck — the interface plate that everything else bolts to. B and C are typically machined edge or a pair of tooling holes that were reamed before the first bracket was ever modeled.
Three Habits That Survive Orbit
- Dimension to function, not to convenience. If two parts must align in flight, tolerance them to each other — not to an arbitrary sheet edge that felt easy to click.
- Never let the CAD default pick your datum. The origin of your model is a modeling convenience; the datum is a physical, touchable surface. Know which is which at all times.
- Profile of a surface is your thermal friend. Panels breathe under thermal swing. A profile callout constrains the shape while letting the part grow — a flatness callout alone will fight physics and lose.
Gravity is the cheapest fixture ever made. Orbit charges interest on every assumption it let you skip.
The practical takeaway for modelers: build your CAD assemblies the way the drawing will be inspected. Mate to datum planes that correspond to real surfaces, suppress nothing that inspection would measure, and let the feature control frame — not the mate folder — be the source of truth.