O'Neill cylinders assume you assemble in orbit — an economic nightmare. But what if we built a 1-g rotating habitat on Earth's surface, tested it with actual humans for a decade, then dismantled and launched the qualified sections? The engineering problem becomes: can a spin habitat survive its own gravity while being built in Earth's gravity?

Sizing the ring. For 1 g at a comfortable 2 rpm (below vestibular disturbance threshold), radius r = g/ω². With ω = 2 rpm = 0.209 rad/s:

r = 9.81 / (0.209)² = 224 m

So a ring 448 m in diameter — think two Eiffel Towers laid tip-to-tip, spun on edge. Rim speed: v = ωr = 47 m/s (170 km/h). A modest habitat width of 20 m and length of 100 m gives ~600,000 m³ of pressurized volume for maybe 2,000 residents.

The killer: building it on Earth. On a rotating hab in space, "down" is outward. On the ground, gravity pulls down, meaning half the ring is upside-down relative to its intended orientation. You cannot furnish it. You cannot pressure-test the floors. You'd need to build it on a horizontal axis inside a giant cradle, then tilt it vertical for spin tests, then tilt it back for habitation trials.

A 448 m diameter ring with 20 m × 100 m cross-section, built as an aluminum-lithium pressure vessel at ~4 mm skin plus internal decks, masses around 15,000 tonnes. That's roughly 1.5 Eiffel Towers. Supporting it during ground-based spin — the rim carries its own mv²/r hoop stress plus Earth-gravity bending — pushes the hull to yield. Hoop stress alone at spin: σ = ρv² = 2700 × 47² ≈ 6 MPa. Fine. But add gravitational sag of a 224-m cantilever half-ring and local stresses spike above 200 MPa — right at Al-Li yield.

The launch problem is worse. Even split into 40 sections of ~375 tonnes each, that's beyond Starship's ~150 t to LEO. You'd need Starship-class vehicles delivering triple-payload configurations, or you cut into ~100 sections of 150 t. At $50/kg optimistic future launch cost, 15,000 tonnes = $750 million just in launch. The ground build itself — think aircraft-carrier construction with rotating cradle — is probably $10–20 billion.

Reassembly in orbit. Here's where it collapses. Each section was qualified in 1-g bending, but the joints between sections were never load-tested as a complete ring under spin. You'd be doing final integration test with lives inside. Worse: launched sections experience 3-4 g axial acceleration during ascent, deforming precision-machined mating flanges by millimeters — enough to leak air at pressure seals.

What actually works from this idea: Build a partial arc — say 60° of the ring, 75 m tall — on the ground as a habitability testbed. Never spin it. Use it to shake out life support, radiation shielding, agriculture. Then build the orbital ring from scratch using validated subsystems, not validated structure. Every rotating structure ever built (centrifuges, tire test rigs) has taught the same lesson: scale and rotation don't superpose. You test full-size or you don't test.

The romance of "assemble it in your backyard first" dies on the hoop-stress equation the moment you tilt the thing.