Crookes Radiometer Thrust and Vane Geometry Study

ISEF Category: Engineering Technology: Statics and Dynamics

Subcategory: Aerospace and Aeronautical Engineering  ·  Difficulty: Advanced  ·  Setup: University Lab  ·  Time: Full Year

The Hook

A tiny spinning vane can teach you a lot about thin air. In the right pressure range, molecules start acting less like a smooth fluid and more like a crowd of ping-pong balls. That makes this project a strong fit for high-altitude flight ideas, where the air is too thin for normal lift. You can turn that odd behavior into real data.

What Is It?

A Crookes radiometer looks like a little windmill in a glass bulb, but light alone does not simply push the vanes. The key action happens when one side warms more than the other. Gas molecules near the warmer side bounce off differently than molecules near the cooler side, so the vane feels a net force.

Your project studies that force as a function of vane shape, surface area, and chamber pressure. The pressure range matters because the effect changes in the Knudsen regime, where the gas is so thin that molecules travel far between collisions. Think of it like this, in normal air, the molecules jostle each other constantly. In very thin air, each molecule behaves more like an individual messenger, and your vane geometry starts to matter in new ways.

Why This Is a Good Topic

This topic works well because you can vary one design choice at a time and measure a real physical response. You can compare vane shapes, surface areas, coatings, or angles, then connect those results to a problem that matters for thin-air flight and passive thermal propulsion. A strong version teaches you vacuum basics, force measurement, calibration, and data analysis. It also gives you room to ask a real engineering question instead of just building a cool demo.

Research Questions

• How does vane surface area change thrust at a fixed chamber pressure?
• How does vane aspect ratio change the force curve across pressure levels?
• Does a blackened vane generate more thrust than a reflective vane at the same geometry?
• To what extent does vane angle affect the onset of measurable motion in low pressure conditions?
• Which vane geometry gives the highest thrust-to-area ratio in the Knudsen regime?
• What is the effect of pressure on the peak force produced by a given vane design?

Basic Materials

• 3D printer or access to a printer with design files
• Assorted lightweight filament for vane prototypes
• Mason jar or small vacuum chamber rated for low-pressure use
• Vacuum pump or chamber access through a school or university lab
• Pressure gauge or vacuum sensor compatible with the chamber
• Digital scale or force sensor with fine resolution
• Temperature sensor or infrared thermometer
• Small heat source or light source for thermal loading
• Rigid mounting hardware for holding the vane assembly
• Ruler or calipers for measuring vane dimensions
• Matte black paint or heat-absorbing coating samples
• Reflective foil or mirror film for comparison vanes

Advanced Materials

• Vacuum chamber with viewports and controlled pumping
• Calibrated microforce sensor or torsion balance
• High-resolution pressure transducer
• Thermal camera for surface temperature mapping
• Laser displacement sensor or optical lever setup
• Surface profilometer or microscope for finish checks
• Arduino or data logger for synchronized pressure and force capture
• Controlled illumination system for repeatable heating
• Precision balance for component mass measurements
• Test coupons with known emissivity coatings
• Finite element or molecular flow modeling inputs

Software & Tools

• ImageJ: Measures vane dimensions, tracks motion, and helps compare prototype geometry from photos or microscope images.
• Python: Organizes pressure and force data, fits curves, and plots thrust-to-area trends.
• Excel: Stores trial data, checks calculations, and makes quick graphs for early analysis.
• GeoGebra: Helps you sketch geometry changes and compare vane aspect ratios before printing.
• Fusion 360: Lets you model and revise vane shapes before fabrication.

Experiment Steps

1. Define the force signal you will measure, then choose a sensor or balance that can resolve it above noise.
2. Select one geometry variable to change first, such as area, angle, or aspect ratio, and keep the rest fixed.
3. Plan a pressure sweep that covers the transition from normal flow behavior to rarefied-gas behavior.
4. Build a calibration strategy that converts sensor output into thrust or force units.
5. Design control trials that separate thermal effects, mounting effects, and chamber drift from true vane performance.
6. Decide how you will compare results, such as thrust-to-area ratio, normalized force, or curve shape across pressures.

Common Pitfalls

• Using a chamber that leaks slightly, which makes the pressure reading drift while the force signal changes.
• Measuring force before the setup reaches thermal steady state, which mixes heating transients with real vane effects.
• Changing vane mass along with vane shape, which hides whether geometry or inertia caused the result.
• Letting the mounting arm flex, which adds fake motion that looks like thrust.
• Comparing trials with different surface finishes or coatings without tracking emissivity, which confounds geometry with heating behavior.

What Makes This Competitive

A stronger project goes beyond showing that the vanes move. You need clean calibration, a clear pressure curve, and a fair way to compare shapes by normalizing force to area or mass. If you pair measurements with a simple flow model or a rarefied-gas analysis, your project becomes much deeper. A novel geometry comparison, like curved vanes, perforated vanes, or asymmetric coatings, can also make the study stand out.

Project Variations

• Test how vane curvature changes force compared with flat vanes at the same area.
• Compare matte, reflective, and mixed-surface vanes to see how coating affects thrust.
• Use different gases or humidity levels, if your lab setup allows it, to see how molecular properties shift the force curve.

Learn More

• NASA Glenn Research Center: Search for educational pages on rarefied gas flow, vacuum physics, and thermal propulsion concepts.
• MIT OpenCourseWare: Search for propulsion, fluid mechanics, and heat transfer lecture materials that explain flow regimes.
• PubMed: Search review articles on thermophoresis, gas-surface interactions, and micro-scale force generation.
• NOAA National Centers for Environmental Information: Use atmospheric data to connect thin-air behavior with high-altitude conditions.
• USGS Publications Warehouse: Search for vacuum, materials, and measurement methods that support experimental design.