Spin Population Architecture — Moon Base Alpha

Executive summary

A modular system of independently rotating concentric rings constructed inside a pre-existing lunar crater can provide sustained Earth-normal gravity (1 g) for long-term biological sustainability. The innermost ring operates at a maximum of 1 RPM and measures approximately 1.76 km in diameter. Successive outer rings spin more slowly while maintaining 1 g, with each ring’s radial width limited so that its vertical rise never exceeds one-fifth of its own diameter.

Motivation

Prolonged exposure to lunar gravity (≈1/6 g) produces well-documented adverse effects on human physiology, including bone density loss, muscle atrophy, and cardiovascular deconditioning. A practical method of generating sustained 1 g environments on the lunar surface is a foundational requirement for permanent settlement and closed-loop biological systems.

Core concept

Discrete annular rings rotate about a common vertical axis inside a large lunar crater, each driven independently. Lunar gravity acts downward; centrifugal acceleration acts radially outward. The resultant effective gravity vector has magnitude 1 g and is oriented nearly horizontally, requiring the local floor of each ring to be banked at approximately 80.5° from horizontal. Because the rings are independent, spin rate can be matched to radius so every long-duration habitat zone experiences the same 1 g target — outer rings turn more slowly, and the innermost ring is limited to 1 RPM for comfort and mechanical practicality.

Key design parameters

Target effective gravity

1.0 g9.81 m/s², all primary zones

Required centrifugal acceleration

≈ 9.67 m/s²

Banking angle

≈ 80.5°from horizontal

Maximum rotational speed

1 RPMinnermost ring

Vertical-height rule

≤ 1/5 of diameterradial width ≈ 6.7% of radius

Innermost ring and outer rings

At the 1 RPM limit the radius required for 1 g is ≈880 m, giving a diameter of ≈1.76 km, a maximum radial width of ≈59 m, and a maximum vertical rise of ≈352 m. Larger rings require lower rotational speeds to maintain the same 1 g. All rings share the same banking angle — the height rule simply permits proportionally wider living bands at larger radii.

Innermost ring

1 RPMØ 1.76 km · rise ≈ 352 m

10 km ring

≈ 0.42 RPMrise ≈ 336 m

20 km ring

≈ 0.30 RPMrise ≈ 672 m

40 km ring

≈ 0.21 RPMrise ≈ 1.34 km

60 km ring

≈ 0.17 RPMrise ≈ 2.0 km

Example capacity

Inside a Copernicus- or Tycho-class crater (≈85–95 km diameter) the radial distance from the 1.76 km core outward allows 12–20 distinct 1 g rings while respecting the height constraint and leaving space for structural buffers and access. A Clavius-scale crater (≈231 km) could accommodate substantially more.

Construction approach

1. Select a suitable large crater with a workable floor.
2. Prepare a fixed, paved foundation system from regolith-derived materials.
3. Install and commission the central 1.76 km / 1 RPM ring first.
4. Add successive outer rings outward, each on its own concentric track and driven at the speed required for 1 g.
5. Provide velocity-matching transfer systems — elevators, ramps, or vehicles — between rings.

Advantages and challenges

Advantages

• Sustained 1 g without continuous high-speed rotation of an entire massive structure.
• Incremental construction and expansion.
• Specialized zones can sit on different rings.
• Existing crater topography supplies structural support and radiation shielding.

Challenges

• Steep banking angles demand careful design of floors and circulation.
• Independent drive systems and power distribution for multiple rings.
• More noticeable Coriolis effects on the faster inner rings.
• Significant civil-engineering effort to manage dust and thermal extremes.

Status and next steps

The kinematic and geometric framework is now defined: 1 g baseline, 1 RPM inner limit, fixed banking angle, and a clear height-to-diameter rule that sizes every ring. Immediate follow-on work includes selecting a specific host crater and detailed ring schedule, first-order structural mass and power estimates, conceptual design of the ring-to-foundation interface and inter-ring transfer systems, and human-factors evaluation of daily living on an 80.5° banked surface under 1 g.

The concept is internally consistent and scalable. It offers a pathway to Earth-normal gravity on the lunar surface using discrete, purpose-built rotating rings rather than a single monolithic rotating structure.