Note: The SketchUp 3D model (Image 11) illustrates the updated harness system.

Background: Why the Exoskeleton Was Abandoned

Early designs of NIMBUS included a rigid exoskeleton—a network of structural tubes surrounding the envelope. While functional, the exoskeleton presented serious drawbacks: significant added weight, manufacturing complexity, and structural bulk that worked against the project's core philosophy of simplicity.

The Tropical Ring Update eliminates the exoskeleton entirely, replacing it with a lighter, more ingenious tendon-driven solution.

The New System: Two Belts and a Ring

The core idea is to harness the sphere's own geometry to do the structural work.

Two continuous belts wrap around the upper hemisphere of the envelope, crossing at the North Pole at 90° to each other. They run downward through a rigid Tropical Ring before anchoring to the four corners of the gondola.

Why "Tropical"?

The ring does not sit at the equator. Instead, it sits approximately 20–25° above the widest point (similar to the Tropic of Cancer on a globe).

This positioning solves a major structural problem:

• Self-Retaining Geometry: Because the ring is smaller than the sphere's maximum diameter, the curvature prevents it from sliding downward.
• Zero Bonding: The wider lower hemisphere holds the ring in place naturally—like a ring held on a finger by a knuckle. No brackets or adhesives are required.

The current material candidate is Aluminum 6061, with a planned upgrade to Carbon Fiber (CFRP) to further reduce mass.

The Rotating Cylinder Mechanism: Selective Friction

The belts feature a series of small, integrated cylinders mounted on axles oriented parallel to the belt's length. This creates a differential friction system that enables envelope rotation without internal motors.

1. The Driving Axis: When a belt is tensioned to rotate the envelope, its cylinders are loaded perpendicularly to their axles. They cannot rotate, causing them to "grip" the ETFE skin via friction.
2. The Yielding Axis: Simultaneously, the cylinders on the other belt are loaded along their axles. They spin freely, allowing the envelope to glide underneath them with near-zero resistance.

This replaces complex internal machinery with a simple, external "mechanical logic" system, significantly reducing points of failure.

Weight and Structural Implications

Preliminary estimates suggest the harness system (ring + belts) weighs between 6–11 kg, a fraction of the weight of a rigid exoskeleton.

While the lower hemisphere has no direct structural contact with the harness, the internal pressure of the ETFE envelope maintains the aerodynamic shape. This "tension-only" architecture is common in high-performance aerospace design and will be the primary focus of our upcoming stress tests.

Status

✅ Concept Finalised — Pending prototype fabrication.

Contributions regarding anisotropic friction simulations, CFRP ring fabrication, or belt tension dynamics are welcome. Open an Issue or contact the author directly.

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