A ski braking system for the lunar rover

Taiyo Hozack, John Jin, Julian Larsen, Christian Pikor, Hooman Pirouz and Kai Shang

Community Partner: Canadian Space Agency
Degree:
- Bachelor of Applied Science
Program:
- Mechanical Engineering
Campus: Vancouver

Our project

We worked with the Canadian Space Agency to design, manufacture and test a ski-based braking system for the CRATER lunar rover. The agency is working with capstone teams from universities across Canada to build a scaled-down prototype of a lunar cargo transport rover, and they wanted us to investigate the feasibility of using skis to brake and control the speed of the rover when descending slopes on the moon.

This is relatively uncharted territory as ski-based braking has only been used on polar and Arctic vehicles and not in a space environment.

Conventional braking systems rely on wheels or tracks, which can slip on loose lunar soil and in the moon’s low gravity environment. Our scope was to design, manufacture and test the mechanism for positioning and tilting the skis to control braking force, and to integrate it with the rover for testing.

Canadian Space Agency

Our design solution

The design we developed allows for two independent motions: the skis can rotate inwards and they can tilt. In a standard braking configuration, both motions happen simultaneously to create a classic snow plow effect. In our design, these motions are uncoupled and each ski can be controlled independently. This gives the Canadian Space Agency the option of exploring using the skis to steer in future iterations.

The tilt mechanism uses a NEMA 17 motor with a 5:1 gearbox driving an ACME 1/4” lead screw. This moves a linkage linearly, which transmits force through ball joints to tilt the ski. The rotation mechanism uses a separate NEMA 17 motor with a 4:1 gearbox driving a worm gear, which rotates the main strut and ski.

Tilt position is controlled with closed-loop feedback using a linear potentiometer, and rotation position uses a rotary encoder. A Raspberry Pi Pico serves as the primary microcontroller, with DRV8825 motor drivers and a 12V lead-acid battery stepped down to 5V via a buck converter.

CRATER Lunar Rover Project Capstone Poster.

The technical challenges we faced

Designing a braking system for a lunar environment meant addressing constraints that don’t apply to projects here on earth. One of the most significant was lunar dust. Regolith is extremely fine, and dust can easily seize up mechanisms. We had to design our system with dust sealing in mind, allowing future incorporation of flexible materials like bellows to protect moving components.

At the beginning of the project, there was considerable uncertainty as we explored different design options. We basically needed to commit to a certain path and have trust and confidence that we would be able to figure out the inevitable challenges that would emerge later on.

We came away with a more holistic view of the workflow associated with complex and complicated projects!

Another challenge was our limited budget. We built large parts of this project using materials sourced in the scrap room in Rusty Hut and trying to engineer around the budget we were given. We manufactured our components on the lathe and mill, and some of our components were not as accurately dimensioned as they could have been. Small errors caused misalignments that we needed to troubleshoot or rework.

Rusty Hut

How we validated our solution

We had modelled the behaviour of the skis as a hydrostatic system, treating the regolith as a fluid under pressure. That assumption and hypothesis were confirmed by our experimental data.

To test our system, we built a large sandbox test bed and a pulley system constructed from scrapped parts found across campus.

We attached weights to the cable to simulate the gravitational loads that the rover would experience on a slope. We pulled the rover across the sand bed to test power draw, tilt mechanism performance, engagement time, pull force at different tilt and rotation angles, recovery (to see if the ski could be recovered if it is buried below sand) and active braking.

The results confirmed that our system could achieve full braking within the required time and showed controlled descent, although there were some oscillations in velocity control.

The measured plowing forces matched our theoretical model closely, providing solid experimental validation of the concept.

What’s next for our project

We will be sending our ski braking system to the Canadian Space Agency, along with complete documentation so future teams can build on our work. Given more time, we would have liked to explore different ski geometries, manufacture and integrate flexible bellows for dust sealing, test our system on a slope to analyze soil dynamics and address torque loss in the tilt mechanism.

What we’re most proud of

Our project scope was to design and test the ski braking system. But to test it properly, we ended up building a fully functional rover prototype, complete with a 10”-diameter wheel setup, a test bed and a movable centre-of-gravity weight system. Many capstone teams came by our corner of the lab and were surprised to find we’d built a rover platform for testing, not just the braking mechanism!

We’re also proud of our work on the firmware and software side of the project. We managed to get wireless telemetry working so we could stream live sensor data back from the rover in real time and send commands remotely.

Perhaps most of all we are proud of our work as integrators. Integration is often overlooked when it comes to real-life builds, particularly in situations like this where it has not been done before and you don’t have documentation.

We are very proud of having successfully integrated the components of our system to create a functional working prototype – within a constrained budget and a compressed timeline.