Designing, manufacturing and testing a set of lunar wheels for the Canadian Space Agency (CSA) was well within the longstanding areas of expertise of Canadensys Aerospace’s highly-skilled Stratford-based robotics team. But as Peter Visscher, General Manager of the Stratford Facility revealed in an interview with SpaceQ, there were still a lot of genuine surprises along the way to the successful conclusion of the project.
The SWARM Contract
The CSA has a long history of contributing high-quality robotics to international space missions. A Canadian company Héroux (now called Héroux-Devtek) created the legs for the lunar landing gear for the Apollo space missions, for example, and everyone knows about the Canadarm.
More recently, Canada created the Juno rovers, which were used for lunar simulations in NASA-led field tests. The CSA knew that wheels based on traditional rubber tires would not be suitable for long-duration lunar rover operations, and also knew that they would likely need a more sustainable solution than the “chain mail” design of the Juno wheels.
So, in line with their current Lunar Exploration Accelerator Program (LEAP), the CSA put out an RFP for SWARM, the “Scaleable Wheels and Advanced Rover Motion” project. It was intended to create a public-private partnership that would help Canadian robotics experts to design, create and test wheels that would work for a variety of sizes of lunar rovers.
Canadensys Aerospace’s Stratford team has long-standing experience with lunar robotics going back to their time as the Special Projects and Space Exploration team at Ontario Drive & Gear, before OD&G divested itself of its space robotics business and they moved to Canadensys. That expertise got them the contract, and with the aid of the CSA, they set to work.
Regolith and Rubber
To say it’s challenging work is an understatement. Visccher went over many of the different issues that make it difficult to design and build lunar wheels.
First and foremost is regolith itself. As Visscher explained, “the soil on the Moon is created by micrometeor and meteor impacts, so it’s microscopically like shattered glass. Very fine, very jagged, and very hard.” That jagged lunar dust attaches itself to absolutely everything, and can be incredibly abrasive—Visscher said that “even titanium parts wear down a lot quicker than you might expect.” You definitely don’t want to breathe it, but it can also easily damage porous substances like rubber. Tires might be able to survive it, but not forever, and it creates big problems for rubber used in seals or gaskets or the like.
The bigger problem, though, is temperature. Without either oceans or an atmosphere, the Moon’s temperatures swing wildly between 130°C during the lunar day to -200°C at lunar night. Rubber would be hard pressed to survive either to begin with. Constant shifting between the two, like on the Moon, would cause tremendous damage. Visscher said that regular rubber tires would “shatter like glass.” Combined with outgassing from the vacuum, even well-designed solid rubber tires would “look like an old car tire after about a week.” Canadensys can’t use air in their tire designs, either; air condenses to a liquid in the frigid lunar night, letting vacuum in and rendering the wheels useless.
Designing a Rubberless Wheel
Canadensys’ solution: The SWARM prototype doesn’t use rubber or tires. Instead, it consists of “20 ground-contacting traction plates, each of which is located by a pair of leaf springs and a pair of tension cables.” These are assembled and joined in a circle, creating a rotating “wheel” with springy contact surfaces but no rubber tire. Thanks to their unique design, they conform to the shape of the ground underneath them, and have almost no issue handling small holes and rocks.
(This smaller size is for the Canadian Precursor to Human and Scientific Rover (PHASR). The larger Pressurized Rover (LPR) SWARM wheels use 30-35 of these traction plates.)
This would require special materials. Visscher said that Canadensys dedicated time, money, and specialized personnel to the task of developing the proprietary alloys that were used for the SWARM leaf springs and cables. While he couldn’t provide details on the alloy compositions, he said that they were indeed able to sort out metals that could handle the 200 degree temperature swings, vacuum, and regolith dust. They were also able to develop materials that would serve in place of rubber to, as Visscher put it, “keep the lubricant in and keep the dust out” for critical components like the rover axles.
They also designed it to ”fail gracefully.” On the Moon, it can be difficult-to-impossible to repair or replace a rover’s tires out on the regolith if something breaks down. Even when manned missions to the Moon begin, the wheels must be able to take damage and still work long enough to get the rover back to base for repair, as human lives may be on the line.
Testing the SWARM
Once the design and manufacturing was done, Canadensys was required to test the SWARM. As the CSA SWARM site said, “extensive and rigorous durability testing was a critical component of the SWARM project.” The wheels needed to be efficient, dependable, and able to allow the rover to move freely and easily.
After some initial thermal vacuum testing, they moved to the Dust Utilizing Space Terrain Environment (DUST-E) lab, a “special room that reproduces the dusty conditions of the lunar surface using a lunar regolith (soil) simulant.”
After initial testing, Canadensys began extended testing on (from the CSA) “a unique five-metre diameter turntable wheel test bed/apparatus to place the wheels on,” which began the longest testing period ever conducted on this kind of wheel: 600 km on the small PHASR wheel and 2000 km on the larger LPR wheel. They received some “Chenobi” simulant from the CSA (made by Deltion Innovations) then got to work.
Immediately, they found that lunar dust gets everywhere. Visscher said that “it tracks, so if you get it on your shoes and start walking around, you get footprints everywhere.” They used special equipment, much like a hazmat suit, and had an “airlock” set up, but they were still concerned about the simulant showing up outside of the testing area. Even aside from the potential health risks, it was a hazard to equipment, and a constant reminder of the harshness of lunar conditions.
(The turntable was also very, very loud.)
Nevertheless, the testing continued. The wheels were occasionally turned during testing, to ensure that they weren’t traveling over the same patches of simulated regolith, and small rocks were added to test the wheels’ ability to handle uneven terrain. But they mostly just went over that turntable, day after day, kilometre after kilometre. The PHASR wheel completed its 600 km testing without incident, and the LPR wheel went for 1997 km before a breakdown happened.
The “breakdown,” however, wasn’t in the wheel. The wheel itself was fine. The problem was that the lunar dust had done so much damage to the turntable that it broke down beyond repair. Visscher said that “it essentially fell apart” beyond hope of repair. So, to finish the tests, the team attached the wheel to an industrial robot and took it outside for the final 3km run. They took the opportunity of using “repeated rock strikes” to make sure that the wheel was tough enough to handle rough lunar terrain.
Will it make it to the Moon?
With the job done and the SWARM project successfully completed, the question becomes what next? With the SWARM wheel make it to the Moon?
Even if SWARM stays on Earth, Canadensys is already working hard to be a part of Canada’s own efforts on the moon. They’re one of two teams competing for the Lunar Rover Mission contract, and their Phase A contract was just augmented for an additional $270,000 by the CSA. Canadensys told SpaceQ earlier that they are “aggressively moving forward with the LRM activities.”
CSA Robotics Engineer David Gingras said that they were impressed by Canadensys’ work on the wheel. While the SWARM wheel that was tested may or may not be appropriate for the upcoming LRM rover, depending on the results of the competition, he said he believed that the use of the SWARM wheel is “very feasible” in other contexts. He said that he’s convinced that they are “true experts in their domain,” and that he’s “very confident we’ll see Canadensys on the Moon.”
Visscher, for his part, says that Canadensys is currently focused on the CSA’s lunar rover project, and is confident that the SWARM will touch regolith. He said that “they wouldn’t have spent so much time on this if we didn’t think it would be used,” and that Canada “has always had a role providing critical robotic elements to international projects.” And while the wheel is unlikely to see terrestrial use —rubber is still quite effective here on Earth — they believe that the expertise they gained with the SWARM will definitely help them with future terrestrial work.