Topic: Improvement of the wheel design of lunar mobility
The development of mobility is essential for efficient lunar exploration and sustainable lunar infrastructure development. For example, the total distance walked by the astronauts on Apollo 12 was only 2.3 km*1. In contrast, the Lunar Roving Vehicle (LRV) used by the astronauts on Apollo 17 totally traveled 35km*2. This fact clearly shows that it would be difficult to efficiently conduct scientific research and resource exploration missions on the Moon without mobility.
There are many challenges in developing mobility for lunar exploration. One of these challenges is the development of wheels. Since the lunar surface is covered with very fine regolith, wheels can easily get buried in the regolith and become immobile. In fact, the LRV of Apollo 15 got stuck on the lunar soil, and the astronauts had to move the LRV manually. In addition, the terrain around craters, which is suitable for establishing facilities such as lunar bases, is often steep, which will be a hurdle for the vehicles of lunar exploration. In order to overcome these problems, it is very important to develop the design of wheels.
Many different types of wheels have been invented so far. For example, the LRV used in the Apollo program had wheels with the mesh of zinc-coated piano-wire and titanium treads*4. These wheels actually traveled on the lunar surface and completed missions of tens of kilometers, but as I mentioned above, they got stuck during Apollo 15 mission. In addition, wheels with a waterwheel-like structure have also been developed. The lunar exploration robot*5 created by the Japanese team (HAKUTO) that participated in the unmanned lunar exploration race which was organized by the Google Lunar XPRIZE has wheels with a waterwheel-like structure. The strength of this type of wheel is its simple structure, but its high ground pressure may cause it to sink on the soft surface of the moon.
In order to overcome these problems, some approaches are being considered. For example, a crawler-type wheel*6 can be one of the options. The large grounding area of this crawler-type wheel can mitigate the ground pressure, so this method will be promising as wheels for lunar vehicles. In addition, Bridgestone, a Japanese tire manufacturer, is developing the bio-mimic tire*7 with Japanese JAXA. This tire has a metal frame and tread, and its double tire structure increases the ground contact area. Furthermore, inspired by the feet of camels walking in the desert, the tread surface is covered with soft metal like steel wool in order to further increase the ground contact area. This tire is currently being installed on a test vehicle and is being tested on sand and slopes. This tire expected to be used in a four-passenger lunar vehicle*8 being developed by JAXA and Toyota, and its launch to the moon is scheduled in 2029.
As the LRVs of the Apollo mission showed, some wheels for lunar vehicles have already been in practical use, so the technology readiness levels (TRL) are at level 9, if only in terms of running on the Moon. However, in order to survey the entire surface of the Moon in the future, it will be necessary to be able to travel several tens of thousands of kilometers, so the TRL of the wheels to meet this requirement is still at level 4~6. In order to improve the TRL, it is necessary to improve durability and to study and verify tire structures for all types of lunar’s terrain and geology.
*1: Apollo 12 The sixth mission: The second lunar landing, NASA
*2: A brief history of the lunar roving vehicle, NASA (https://www.hq.nasa.gov/alsj/MSFC-LRV.pdf)
*3: Lunar Mobility Review, David A. Kring
*4: The development of wheels for the lunar roving vehicle, NASA
*5: HP of HAKUTO
*6: Design and mobility evaluation of a crawler-type lunar vehicle, Sachiko Wakabayashi et al. (2006)
*7: To drive on the moon, we’ll need tough-as-hell tires, Popular mechanics
*8: JAXA and Toyota Reach Agreement on Consideration Toward International Space Exploration (https://global.toyota/en/newsroom/corporate/27059582.html)