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Question inspired by Neil deGrasse Tyson's tweet:

In @StarWars #TheForceAwakens, BB-8, a smooth rolling metal spherical ball, would have skidded uncontrollably on sand.

Is this ever explained in-universe?

How was BB-8 able to travel through the sand without skidding?

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85  
Given that it was an actual prop, rather than CGI the answer is "very well thank you". – Valorum Jan 5 at 16:12
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It uses the Force! – DVK-in-exile Jan 5 at 16:17
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Wouldn't the better question be "How did he go up the stairs at Maz's?" – Kevin Jan 5 at 19:33
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@DVK That's not how the Force works! – Fatalize Jan 6 at 8:46
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@Chloe: That's not how the real robot, the one they actually built and operated on sand, works. The real question is how does Neil deGrasse Tyson fail so hard here? – slebetman Jan 6 at 11:44
up vote 4 down vote accepted

TL;DR: He was able to move in sand without skidding because there was much more to some versions of BB-8 than we see onscreen.

One version of BB-8 has a trolley attached to give him traction, and moved across a hard surface covered with a thin layer of sand, rather than across loose sand; another version of BB-8 is pushed from behind by a puppeteer.

Note: All photos are screencaps from the behind-the-scenes featurette Secrets of The Force Awakens: A Cinematic Journey


Different versions of BB-8:

We can finally answer this question, thanks to the bonus materials on the newly released home video edition of The Force Awakens.

There are several versions of BB-8. At least one of these is a functional remote controlled device, but it seems that the functional version was not used as-is when BB-8 has to move over sand.

We came up with seven versions for the film — three main ones and some variations.
- Josh Lee


"Trike" version:

enter image description here Trike version of BB-8, with the trolley mounted behind his body

This model of BB-8 has a wheeled grey cart or trolley attached, either in front of his body or behind it; Josh Lee calls this BB-8 the "trike" - short for "tricycle". The trolley is designed to give him some more traction.

We built the trikes because we needed a stable driving version. We motorized the ball and had the head move around on top by means of a curved track system. Motorized castors on the back allowed us to steer it. That version could go over pretty much any terrain. The only thing that defeated it was deep, very fine sand.
- Josh Lee

In the behind-the-scenes featurette Secrets of The Force Awakens: A Cinematic Journey, we can see that in at least one scene, he isn't moving over a dune of loose sand - he is actually rolling over a thin layer of sand spread over a hard surface, perhaps a board.

enter image description here Trike version of BB-8, with the trolley mounted in front of his body - note the sand-covered board he is rolling across


"Rod Puppet" version:

enter image description here Rod puppet version of BB-8 in the studio

One of the puppeteers who worked with BB-8 described this version as a "rod puppet", because it is manipulated via two articulated poles wrapped in green screen material and attached to his back. This version is the one used in the scene where we see BB-8 speeding across the desert (the footage of which was used in the early trailers).

And then there was the puppet, which had an axle going through the ball, rods coming out, and a track system for the head. A puppeteer in a blue or green suit would hold the rods, and have very fine control over the head and ball.
- Josh Lee

In short, this version of BB-8 can move across the sand without skidding because there is a puppeteer (also wrapped in green screen material) pushing him from behind.

These photos show the filming of the scene in question:

enter image description here enter image description here

And these photos show the filming of the scene in which Rey first meets BB-8:

enter image description here enter image description here enter image description here enter image description here

Filming BB-8's town scenes:

enter image description here enter image description here

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That BB-8 droid actually works, as it was an actual prop that was used for actual filming, on actual sand. Even though Neil deGrasse Tyson says it will skid uncontrollably, we know that it does not.

The proof is in the pudding, and comes straight from a horse's mouth (if Mark Hamill were a horse):

That said, we also have NASA pointing out in this article that it is a poor design:

"Looking at the BB-8 droid I would have to say the physics, it doesn't follow particularly well," Kennedy said in a video for Wired. "Trying to roll up and over anything is extremely difficult."

It is also worth noting that the Sphero BB-8 toy does not work on sand:

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14  
I think the real BB-8 is pretty heavy, and thus it gets traction on surfaces where the much lighter toy does not. (Also, I'd like to see a test with the toy where it starts out on a hard surface and then rolls onto sand.) – Martha Jan 5 at 16:47
    
@Martha We don't see how it gets to the sand (and I had it muted) but it seems like it can get around a bit if it starts elsewhere, but this one got stuck in a hole: youtube.com/watch?v=WaYe0RZs0oI – Dave Johnson Jan 5 at 16:49
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FWIW, we don't really know the nature of the actual sand BB is traversing in the movies. It's possible that the ground is hardened in the scenes we see him being highly mobile in and so traction isn't much of a problem. – joshbirk Jan 5 at 18:50
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Actually - it looks like the toy is getting too much traction on sand. Both the toy and presumably the prop work by over balancing. Just like a hamster ball. – Wayne Werner Jan 5 at 19:48
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Even if it is a real prop, that doesn't mean it does not skid 9 times out of 10. (I know for experimental bipedal walking robots, its common to film them making dozens of attempts to get one good proof of concept video). It also doesn't mean that that there is not another explination, E.g. it may have been rolling on carpet which was then CGI'ed out. Of course neither of those are Inuniverse concerns. – Oxinabox Jan 5 at 23:19

Neil's question seems to assume two things which aren't really knowable:

  1. BB-8 is a truly smooth sphere

  2. The sand on Jakku lacks a layer dense enough to provide friction

The first is really really up for grabs - while BB-8 looks pretty round and smooth, remember he's really made up with different plates which have ports, crevices, etc. It also assumes that a robot equipped with grappling hooks to steady itself in flight doesn't also have some kind of tread or cleats for traction that we might not even notice due to BB's speed in the scene.

The second is questionable from scene to scene. In the scene where Rey finds BB it looks like dune like sand which can be tricky even for bipeds. In the scene around the trading post, it appears well packed and would be easy to traverse for a sphere (which, as a physical prop, BB-08 proves himself).

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^ The sand near Abu Dhabi. It's not only a physical prop, it was filmed at a physical on-Earth location. – Slipp D. Thompson Jan 6 at 7:06
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There's a difference between 'physical prop' and real system. For example, just because there's a real speeder bike the actor sits on, it doesn't mean that Disney has created antigravity bikes which fly. The independent rolling BB-8 didn't turn up until after the shooting, and IIRC the fast sand scenes had BB-8 on a stick which was edited out in post-production (there are now too many clips on youtube to find the one I saw). – Pete Kirkham Jan 6 at 12:43
  1. How do you know how smooth it is? Because it looks smooth? So what?

  2. We know that anti-gravity technology exists and is commonplace to the point that Luke's and Rey's vehicles happily hover above the sand even when they appear to be powered down. It's not a big leap to conclude that a similar technology can be used to exert a force on the ground not through the friction between the droid's surface and the sand in which it's in direct contact.

enter image description here

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While that may be true, is there any reason to assume that BB-8 is fitted with an anti-gravity unit of some description? – Valorum Feb 22 at 17:58

This Popular Science article has some discussion of how they implemented BB-8.

http://www.popsci.com/qa-how-to-create-robot-icon

"So we came up with seven versions for the film—three main ones and some variations. ... One was the wiggler. It didn’t roll around, but it could wiggle its head and body on the spot. We would bolt that to the set or bury a baseboard in the sand. Then we built the trikes because we needed a stable driving version. We motorized the ball and had the head move around on top by means of a curved track system. Motorized castors on the back allowed us to steer it. That version could go over pretty much any terrain. The only thing that defeated it was deep, very fine sand. And then there was the puppet, which had an axle going through the ball, rods coming out, and a track system for the head. A puppeteer in a blue or green suit would hold the rods, and have very fine control over the head and ball. That’s how we achieved some of the more-subtle acting shots."

So Tyson is probably correct that a robot that was constructed, in reality, just as BB-8 is depicted onscreen would have trouble.

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People tend to think of metal as perfectly smooth and not subject to deformation, but even if you take a perfectly smooth piece of stainless steel and put it on a sand hill, it rolls down the hill, it doesn't just slide. There is still significant friction between the irregular sand grains and the smooth metal surface.

This is enhance by the texture of the sphere, and the paint. For instance, you could use a smooth enamel or even teflon paint and reduce the friction, or you can use a softer rubberized or plastic paint with a minutely rough texture to increase friction on a variety of surfaces.

Without a microscopic analysis, though, it would be hard to determine the friction between the robot and sand. The sand grains are small and don't show up individually on screen, and thus the resolution of the camera isn't good enough to tell us in the surface of the robot interlocks, or slides, against those grains.

If the robot were instead on a planet of 1" (2.5cm) diameter ball bearings, then we could draw such conclusions. In fact I'd go so far as to say that as it appears on screen, the robot would have a very difficult time in such an environment.

But without further detail, we cannot discern the amount of friction between the surface and the robot.

Notably, though, we do have footage of it moving. I don't have the time to sit down and analyze it, but one sign that the robot has difficulty would be if we see it accelerating and decelerating on the sand onscreen. If it has difficulty, it would do so slowly. It wouldn't change directions very quickly. If there are jump cuts when it's about to change direction, or accelerate/decelerate quickly, that's a sign the robot is having difficulty.

Do note that while there exists a live prop, and perhaps most of the shots are without CGI, there's no guarantee that all the shots are with the prop.

Lastly, there were several purpose-built R2-D2 props used in the early films, and I fully expect several BB8 props exist for different scenes. They will no doubt have differing abilities on the sand.

But most of the friction will be defined by the coating and texture of the material that covers the sphere.

Do keep in mind, however, that the head has to slide and maneuver around with little friction on that same surface, so there's a balance that must be maintained between higher friction on driving surfaces, and lower friction on the head, with the sphere maintaining the same surface coating and texture for both.

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Assuming the head does not use repulsors or something similar to maintain a tiny gap between head and body. – JAB Jan 6 at 14:06
    
@JAB And given a few scenes where the forces on BB8 were large enough to rip just about any magnetically held head off the body, I'm certain they were using technology we don't currently have. If they are using it there, then they're likely using it elsewhere on the robot to enhance its usefulness. My post is essentially proving the point using technology, knowledge, and materials we have, with the implication that if we can handle it now and here, then it could certainly be managed a long time ago in a galaxy far, far away. – Adam Davis Jan 6 at 14:10

It could be possible that his mass created enough friction between him and the sand. Just a thought. He would have to be pretty heavy though. But in all actuality, I couldn't see him climbing a steep dune with his design, along with going up stairs, with any type of ease. He would have to roll at a high speed for both of the obstacles.

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They did show him going down stairs fairly competently. As for going back up? I imagine those grappling hooks might come in handy. To be fair though, I feel like he'd have a much easier time on a sand dune than, say, R2-D2. Even with all 3 wheels out, I don't think he'd be able to climb a dune without tipping over... – Darrel Hoffman Jan 5 at 21:33

In case OP wanted a physics-oriented answer: to not skid around during a turn, the centripetal force must not exceed the frictional force. (Both are independent of mass.) Centripetal force and frictional force can be changed by BB-8 leaning into corners, just like how you (out of universe) ride a bike by leaning in.

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