NIMBUS - Solar Balloon for Human Flight

Botanical Biomimicry: The Solar Physics of a Buttercup

The glossy, concave petals of Ranunculus (the common buttercup) function as a natural parabolic mirror. They are physically shaped to concentrate sunlight directly onto the center of the flower, raising its internal temperature to attract pollinators.

The NIMBUS concept relies on the exact same thermodynamic mechanism, just scaled up for aerospace. Instead of relying on a passive black envelope that loses heat to external wind, we are using a transparent envelope with an internal concave mirror to focus the alpine sun onto a central receiver.

Nature uses concentrated solar power for thermal regulation at ground level; we are applying it to generate volumetric lift.

Note: This update replaces the belt-and-cylinder mechanism described in the Tropical Ring Update. This solution offers superior mechanical logic and omnidirectional efficiency.

The Problem with the Belts

The previous belt-and-cylinder design had a fundamental geometric weakness: in the equatorial zones, the contact angle was often ambiguous. It could not always guarantee a 100% grip when tensioned or a 100% release when idling. We needed a cleaner mechanical "binary"—either rolling or gripping.

The Solution: The "Pearl Necklace"

Imagine a high-tensile wire (Dyneema or Steel) strung with small, independent spheres—like a pearl necklace. Each sphere is free to rotate in any direction and makes direct contact with the ETFE envelope.

How it Works:

1. At Rest (Rolling): When the necklace is not under drive-tension, the pearls act as omnidirectional bearings. Any movement of the envelope beneath them causes the pearls to roll freely, allowing the balloon to shift or expand with minimal friction.
2. Under Tension (Gripping): When the operator pulls the necklace, the increased downward tension presses the pearls firmly against the ETFE. Because the pearls cannot "slide" along the wire itself, they lock and transmit the pulling force directly to the envelope, driving the rotation.

Material Specification: To optimize this, the "pearls" will feature a soft silicone or nitrile rubber coating. This ensures high static friction for the drive phase while maintaining smooth rolling during the idle phase.

The North Pole: Crossed PTFE Tubes

To handle the crossing of the two necklaces at the top of the sphere, we have implemented a Crossed-Tube Guide:

• Independent Movement: Two slightly curved tubes, oriented at 90°, act as a bridge.
• Ultra-Low Friction: The tubes are lined with PTFE (Polytetrafluoroethylene). This ensures that even when one necklace is under high tension, it can slide through the crossing point without interfering with the other axis.

The Tropical Ring: Passive Horizontal Stabilization

A common question regarding this harness is how the Tropical Ring stays level without motors. The answer is Symmetric Tension:

1. Gravity Alignment: The weight of the gondola pulls downward on the four lines anchored to the ring. This constant, symmetric tension naturally forces the ring into a horizontal plane.
2. Surface Stabilization: Because the ring is in continuous contact with the pressurized sphere, the envelope itself acts as a massive stabilizing jig. The ring cannot tilt significantly because it would have to "deform" the pressurized sphere to do so.

This creates a self-leveling architecture that requires zero electronics, zero sensors, and zero power to maintain orientation during flight.

Status

✅ Concept Finalised — Transitioning to prototype fabrication for friction-coefficient testing.

Call for Contributions:

We are currently seeking data or simulations regarding:

• Pearl Spacing: Optimal density to prevent ETFE "tenting."
• Wire Selection: Evaluating Dyneema vs. Stainless Steel for long-term creep resistance.
• Coating Durability: UV-resistant rubbers for high-altitude exposure.

"The crisis requires sharing, not secrecy."

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.

"The crisis requires sharing, not secrecy."

Note: images referenced below are planned. Prompts for generating them with AI image tools are included at the end of each section. Contributions are welcome.

Concept

Once the core thermal lift system of Phase 1 is validated, the natural next step is to add onboard electrical generation. This transforms NIMBUS from a passive solar aerostat into a partially steerable hybrid airship: NIMBUS-E.

1. Spectrum Splitting at the Receiver

Sunlight concentrated onto the ceramic receiver contains three components:

Component	Share	Role
Near-infrared	~50%	Heats internal air → lift
Visible light	~45%	Intercepted by PV layer → electricity
Ultraviolet	~5%	Absorbed as additional heat

A thin photovoltaic layer is added to the front face of the ceramic receiver. Visible light is converted to electricity before the remaining radiation heats the SiC honeycomb behind it. This is called spectrum splitting and is an active area of research in concentrated solar power (CSP) systems.

Important design note: Standard silicon PV cells fail above ~80°C and cannot survive near the ceramic receiver. NIMBUS-E will use either GaAs (Gallium Arsenide) cells — which tolerate high irradiance and elevated temperatures — or position the PV layer slightly offset from the ceramic core, in the cooler airstream, so that infrared passes through to the honeycomb while visible light is captured. Both approaches are being evaluated, as well as the possibility to use a Dichroic Mirror or a Beam Splitter.

Estimated electrical output: modest but sufficient for tracking motors, sensors, communications, and partial propulsion.

Image 7 — Receiver cross-section diagram AI image prompt: "Technical cross-section diagram of a solar receiver: a Silicon Carbide honeycomb block behind a thin photovoltaic layer, labelled arrows showing near-infrared passing through to the ceramic, visible light absorbed by the PV layer, white background, clean engineering style."

2. Thermal Battery — Flying After Sunset

Because the receiver operates at high temperature, it naturally acts as a thermal mass. Phase 2 will explore integrating Phase Change Materials (PCM) — substances that store and release large amounts of heat during melting and solidification — inside or around the ceramic receiver.

This would allow NIMBUS-E to remain buoyant for 1–2 hours after the sun goes down, using stored heat rather than active solar input. This is a significant safety and operational advantage over conventional aerostats, which lose lift immediately when solar input drops.

Image 8 — PCM thermal storage diagram AI image prompt: "Schematic diagram of a thermal battery integrated into a ceramic receiver block, showing Phase Change Material layer surrounding a SiC honeycomb, arrows indicating heat storage and release cycle, clean technical illustration, white background."

3. Onboard Electrical Systems Enabled

The electricity generated opens four new capabilities:

Autonomous Sun Tracking The two-axis cable mechanism of Phase 1 is motorised, replacing manual control with a sensor-driven automatic tracking system. This improves optical precision and reduces pilot workload.

Image 9 — Tracking motor integration diagram AI image prompt: "Diagram of a two-axis cable-driven solar tracking mechanism inside a transparent sphere, with small electric motors at the cable anchor points, labelled, clean engineering drawing style, white background."

Altitude Control The electricity produced can be used for a Ballonet: it's an internal bag you inflate with outside air to make the balloon heavier and descend.

Steering and Station-Keeping Small electric thrusters or vectored fans on the gondola provide limited directional control — enough to compensate for wind drift and hold a position. Full horizontal flight is beyond the scope of this system, but meaningful station-keeping transforms NIMBUS-E from a balloon at the mercy of the wind into a genuinely steerable platform.

Image 10 — Propulsion pod concept AI image prompt: "Concept illustration of a small gondola beneath a large transparent spherical balloon, with two small vectored electric thrusters mounted on the sides, clean technical concept art style, blue sky background."

Onboard Instrumentation Sensors, cameras, communication systems and data loggers powered indefinitely during daylight hours, making NIMBUS-E a capable aerial observation and research platform.

4. Impact on Envelope Size

Recovering an estimated 8–10% of total solar input as electricity reduces the thermal load required for lift. This allows a modest reduction in envelope diameter — approximately 5–10% compared to Phase 1 — while maintaining the same payload capacity. Supporting calculations will be published in the /calculations folder.

5. Commercial Applications

Station-keeping capability fundamentally changes the commercial proposition of NIMBUS. A steerable, fuel-free solar airship opens applications in:

• Aerial photography and surveying
• Environmental monitoring
• Telecommunications relay
• Tourism and recreational flight

These are markets with established demand — making NIMBUS-E a significantly more attractive proposition for the crowdfunding campaign planned for Phase 3.

Status

⏳ Planned — pending successful validation of Phase 1 thermal lift system.

Contributions and simulations related to spectrum splitting, high-temperature PV cells, PV-thermal hybrid receivers, Phase Change Materials, and electric propulsion at aerostat scale are welcome. Open an Issue or contact the author directly.

"The crisis requires sharing, not secrecy."