Original at: Part-Time Scientists Blog

OK, we know a lot about footsteps on the moon, but how much do we know about wheel tracks on the moon?

We were facing the exact same problem when we first started working on our rover prototype “Asimov Jr.” Designing the wheels of a lunar rover may seem like quite an easy and intuitive thing to do: they're supposed to be round, as light as possible and they gotta have good traction. Well, things actually weren't t h a t easy ...

Let's start the game by piling up some sheets of paper and call them “mission statement”. Sounds boring? It certainly is. But without such a statement you're most probably going to sift through various wheel designs for, like, two years, without ever getting to a result. What's important is to figure out the conditions of where your wheels are supposed to operate. The rules of the Google Lunar X-Prize state that you have to rove at least 500 meters over the lunar surface and, ideally, survive a lunar night. Key facts for us are: we aim for a landing on a lunar day, which means it's going to be real hot with temperatures of up to 125 degrees C (that's 257 degrees F for you imperials and 398.15 degrees K for you trekkies). Second, we plan on landing close to the equator, for compared to the poles the equator is as smooth as a freshly built parking lot and far easier to overcome than the cluttered landscape of the lunar poles. If Asimov Jr. is to survive a lunar night, the wheels need to withstand extreme temperature shifts in the range of +125 to -125 degrees Celsius(-193 degrees F and 148.15 degrees K).

Now, let's discuss our options a little bit. Air-filled rubber wheels are probably best known, but their fate on the moon – and on space flights – is pretty much doomed. Due to the vacuum, the wheels would explode and burst into pieces. Even without the exploding part, the rubber would soon “vaporize into thin vacuum” due to its outgassing properties. “Outgassing” basically means that all materials contain a certain amount of air, and in space this enclosed air will do anything to break free, destroying the rubber in the process. The amount of enclosed air particles and their behavior defines the material's distinct outgassing properties. What about non-air filled rubber wheels then? They perform somewhat better when it comes to outgassing and, of course, they wouldn't explode, but the extreme temperature shifts would pose a big problem. It's like pulling a piece of rubber out of the freezer and putting it right into a pre-heated oven: the result isn't gonna make up a wheel anymore. Now, let's skip materials like wood, plastic, steel or iron, and let's take a look at aluminum. Aluminum is light as a feather, plus, it is robust and tolerant towards temperature extremes ... congratulations! You found yourself a suitable material!

OK, so we handled the temperature. The only thing left is the roving around the lunar equator. The lunar equator is quite a special place in our solar system; the complete lack of an atmosphere means there's a complete lack of wind, too. The best word to describe the lunar surface is probably: debris. After 4.6 billion years of constant galactic bombardment, the lunar surface is covered in layers of dust which are between 5 to 10 meters – or 16 to 32 feet – thick. Since there are no currents and winds, moon dust has different properties than ordinary earth sand. While a footstep on the beach is gone within minutes, the bootprint of Neil Armstrong was kind of made for eternity. This is due to the characteristics of the particles. Sand grains are round and electrically neutral. You put some on your hand and they will rain down through your fingers. Regolith grains are more like spikes meshed together and they're electrically charged. You put some on your hand and you just won't get rid of them easily. But what does this mean for our wheels? Travelling on wheels is always about good surface traction. The goal is to design the wheel pattern in a way that the wheels have the least weight and the best traction. And in this environment: don't get clogged up by regolith. There is no distinct answer on how to find the perfect wheel pattern. The only thing for sure is that using the wrong wheel pattern can leave your rover digging itself deep into the regolith without moving an inch.

We did a lot of regolith and traction surface simulations using SolidWorks and ended up with almost 50 suitable designs. We picked out the most appropriate one from an engineer's point of view and had the wheels for our prototypes manufactured accordingly. Towards the end of the year, we will be conducting extensive tests with synthetic regolith in different testing facilities. Among the best-known experts in the field are the folks over at the California Space Authority. They have a large and realistic testing site designed for the Lunar Excavation Challenge. If you're interested in synthetic regolith and cool video footage, just visit this site: (link), or drop them (link to matts official page) an email.

Umm … I got a stupid question! I mean, if the environment is so complicated ... why doesn't everyone use the exact same wheels?

Well, as with every problem, there are always multiple solutions.

For example, let's take a closer look at the wheels of the LRV – the Lunar Roving Vehicle, or simply: the Apollo rover.

As you can see, its wheels look as if they were made of rubber, but in fact, they do not contain any plastic at all. They consist of a composite mix of materials, starting with a basic frame of iron and ending up with a mesh of woven piano strings to provide the needed traction. They behave like normal rubber wheels but have the distinct advantage of being more lightweight than filled wheels. As the landing module had already reached its weight limit, those wheels were deemed the best option. Mixing multiple materials to composite materials can lead to a much stronger structure. However, most composite materials need time, intensive development and testing to survive severe temperature shifts. So, why has it worked for them? As the Apollo missions only took play during the period of a lunar day, the wheels didn't have to survive a lunar night. It is most likely that by now the the wheels of the LRV would have dismantled completely.

In other missions with longer lifetimes, NASA/JPL and Russia tried out different wheel designs based on groups of meshes supplying the needed traction. The main reasons for this design change were again weight savings. Cut down costs and spare weight for scientific instruments, while reaching for an extended mission lifetime. The only drawback is that their wheels behave pretty much like tracked wheels, meaning they got a lot of junctions and thus single points-of-failures. All these potential points-of-failures need to be checked and certified to withstand the stress of launch and descent.

As for our wheels and their design, you can we keep up on that by following us on Facebook, Twitter or by reading our blog. The first test results and footage will be available for you by the end of August!

To learn more about wheels on the moon take a look at the following sites:

Science - HowStuffWorks.com
Wikipedia - Apollo Lunar Rover
Wikipedia - Lunokhord Rover

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