Astronomy Cast Ep. 805: Mission Tech Series - Gyroscopes & Servos

Welcome and introduction

0:00It's the 365 Days of Astronomy podcast, coming in 3, 2, 1.

0:30We'll be right back.

1:00We'll be right back.

1:30Hello, Pamela. Hello, Fraser. How does it feel to be in your own bed again? It's nice. Yeah. Well, so I was in my bed for two nights, and then I was on the road again. I have absolutely—I guess we haven't switched places, because normally I'm the one who's at home, and you're the one who's on the road. But we are also both on the road quite a bit, and we're going to be on the road again shortly, except we're going to hopefully be in the same city at the same time, which is going to be cool.

2:01Yeah. Yeah, we're going to be meeting up in Tokyo to record an episode, so that's going to be cool. Our journey through space exploration technology continues. Today, we look at the things that spin. And gyros, reaction wheels, servos, Newton's laws are large and in charge today, as spinning has a very useful benefit when you're in weightlessness. And we don't pause for breaks anymore because we don't do ads, and so we just keep rolling. But, like, thank you, everybody who's already joined our Patreon cause.

2:33I'm not sure what the total count is, but things seem to be moving in the right direction, and we are well on our way to being a fully sustained, a fully operational, bulletproof, science-teaching machine. I just, like, made an ad. Science? Please do not shoot us with bullets. We are going to be advertising revenue proof. Yeah, advertising revenue proof. Search engine optimization proof. AI slop proof. It's glorious. Until the robots come for us with their bullets.

3:05All right. With the driving and the skulls and the wheels. All right. Pamela, this is a sore spot for me. Well, not a sore spot, but this is, like, this is a meme on our podcast right from almost the very beginning of us doing astronomy cast, which is that we have identified what is the weak spot of all space technology, and that is the lowly reaction wheel. Well, the spinny gyro. Today, we are going to understand the nemesis of space exploration missions everywhere, get to the bottom of how these things work, why they fail, and, you know, maybe what's being done about it, or maybe not.

Newton's laws and inertia

3:48But, so, Pamela, let's sort of talk about, before we do that, though, you know, I mentioned we'd be talking about Newton's laws. Why does spinning matter, and what is the sort of, like, why does this whole thing work in space? All right. So, the first thing you need to wrap your head around is the idea of inertia. And we're used to thinking about this strictly in a straight line. So, if you roll a ball in a straight line, or it's actually way better in the way Galileo originally did this experiment was to slide something in a straight line.

4:25It has its motion forward, and then it has friction between itself and the thing that it's on that is slowing it down. Friction is a force. Now, the less and less friction there is, so think ice skater, you give an ice skater a good push, tell them not to do anything other than hold themselves in position, and they will just keep going. Now, there's still going to be a little bit of friction. They're still probably going to stop, or they're going to hit the railing of the ice rink, which is unfortunate. Right. And that an object in motion stays in motion unless acted upon by another force comes initially from Galileo's work looking at things on inclined planes, sliding blocks down super slippery inclined planes while polished.

5:12Now, what we don't usually think about is how inertia works in circular settings, and we mistake inertia as a force. So, we've all been there at some point in your life. You're sitting in the backseat of a vehicle. The vehicle turns a corner, and everyone goes squish against whoever's against the door. And it feels like there is a force pushing you towards the outside of the car as the car turns the corner.

5:46We even call this centrifugal force. It is not a force. What is actually happening is your body is trying to keep going in a straight line. Your body really wants to keep going in a straight line. The only force acting on you is the seat belt and any friction that might exist between you and the seat. And so, as you attempt to go in a straight line, the car is moving into you. Right. And that's why they call it a pseudo-force. Or a fictional force. I like the idea of fictional force.

6:17Yeah. And this is like every pedantic's favorite argument to have. Well, actually. But the point being that you will experience force. You will experience a pressure on you that something is happening. Yeah. Yes. And so, when you have an entire wheel of stuff that's rotating, that's a whole lot of stuff that wants to keep going in a straight line. Yeah. And in the case of a wheel is being forced by like the physical atomic and molecular connections of the wheel holding it together is being forced to turn.

6:52Right. And so, that stuff wants to just keep turning, keep turning, keep turning. And this is why bicycles stay upright for people who have terrible balance. Now, there are people out there who are capable of getting onto a bike. And you've all seen them at intersections if you've driven in California. And they're doing this weird thing where they're not putting their feet down. And they're somehow staying balanced at the stoplight. Those people are crazy. But your typical person, me, can only keep their bike upright when they're moving forward at a certain rate.

7:30And it's because of the centrifugal, not actually a force, the inertia of the tires going round and round and round. That's what's keeping the bike upright. And if you try and push over the bike, it's going to resist because all of the parts of the bike want to keep going in the same orientation round and round and round and round. And there is a beautiful science experiment that you can do in your school.

8:02Most science museums. Yeah. But they always do them at science museums. Yeah. Which is that they have you sit in a seat, like an office chair. They give you a wheel, like a bicycle wheel, that you have handles on either side. They spin up the wheel so it's going very quickly. And then you just start to turn the wheel. You just rotate it on the axis. You turn it in one direction and you will spin in the opposite direction on your chair. You turn it the other direction and now you're spinning in the opposite direction from that. That you can literally change your orientation, your rotation, your direction, and fine tune it.

8:34Yes. You can just perfectly spin yourself around and then stop just by how you orient this wheel.

How spacecraft use flywheels

8:42And this is the key. And in terms of when we start transferring these ideas to spacecraft, your arms, which are turning the spinny thingy, are playing the role of the servo-mechanical actuators. The servos. Yeah. The wheel is playing the role of the flywheel. And so on a spacecraft, in order for them to ever so carefully reorient themselves, they do the same thing you do on that bicycle and they go snert.

9:17And the whole spacecraft goes snert and it has now moved a very precise amount without having to have step motors. Right. Podcast listeners, Pamela just made a lot of really interesting hand gestures. We're sorry. When she said snert, but I think you kind of imagine, right? You turn the thing a little bit, it starts to turn the spacecraft, you turn it the other way, you can come to a stop again. And I mean, what's great about this is that it's all electricity, that you're not requiring any propulsion, any maneuvering thrusters.

9:49You're not throwing anything overboard like we talked about in that previous episode. Now, you are just spinning this wheel and you can effectively change your orientation forever. Yeah. Relative to the wheel. Right. You turn the wheel one way, the spacecraft wants to turn in the opposite direction. You stop, you turn the wheel, turn it back and forth. You can make yourself reorient. As long as you have three axes of rotation, essentially three different wheels, you can move in all of the three axes that you might want to turn your spacecraft at. You have solar, you have the sun's energy coming at you, you're collecting it with your solar panels.

10:25You now can change your spacecraft in any direction forever, theoretically. And this is where it gets trickstery because it is not easy on you when you're doing the science museum experiment to move that wheel. Right. It takes force and it takes leverage. Now, in the case of the servomechanical actuator, it is bolted on one side to the spacecraft.

10:58So that's its leverage. And then it's trying to move that flywheel. Give us a sense of the actual physical, like how big are these things? It entirely depends on what they're trying to move. So what's really cool is they're now getting to the point that they can make itty bitty little tiny ones for cansats and stuff. Yep. And so they vary from these itty bitty little tiny things that are basically something that looks like you could have bought it in Radio Shack in the 80s to...

11:29Right, like a little gyroscope that you could... Yeah. To tabletop-sized units designed for moving much more massive flywheels. Yeah. I mean, they can be... Like on a spacecraft like James Webb or Hubble, you're looking at things that are hundreds of kilograms. Yeah. They're big. And they're spinning fast. How fast are these things spinning? I don't know how many rotations per second, but what gets me is that the servos have to respond at the 20 millisecond level.

12:04So these servos are stopping and starting with an error of 21 thousandths of a second. Yeah. So there, I just looked it up, a few hundred to up to 6,500 RPM. Yeah. So this is fast computing, fast reacting. Yeah. And figuring out how to do this was super key because stepper motors, which are the other way that they can do things like this, where they know exactly where they are oriented in space,

12:37the way a stepper motor works is it looks and says, okay, where is tooth zero, now advance to tooth 100, advance to tooth 300, rotate a few times, go around four times, stop. And when you turn a stepper motor on and off, it has to re-zero itself, which is not ideal for your spacecraft, which can't necessarily re-zero itself. And so servo motors don't need to do that. They have a lot of built-in systems that allow them, in part because of the flywheels and everything else, to know exactly how they're oriented all the time.

13:11And ideally, as you said, you want to have three of these things, one for each spinny orientation you can possibly have. So any given object in space, because we live in three-dimensional space, you need to have three points of contact with the universe around you to be stable. Yeah. And they don't have to be symmetrically distributed, but you have to have three points in space to orient yourself. And when you're a spacecraft not really touching anything, you fake it with gyroscopes, with magnetic fields.

13:48And Hubble was originally designed, as were spacecraft in general, to function using three primary sets of gyroscopes, flywheels, servo motors. And we've had to get good over time. And they always had backup systems, so you launch these with like six. We have had to get good over time, though, at operating with only two of these working, which means a whole lot of keeping track of where you are, using stars to keep track of where you are, and using magnetic fields wherever possible to also exert a force on the spacecraft.

14:27Right, right.

Reaction wheel failures

14:29Well, so you're starting to kind of move into the next piece of technology. But before we talk about gyros, you know, like the issue that we had, say, with the Kepler mission, and there have been many other missions that have identified this, the reaction wheels have failed. And so now your super, essentially, forever free system no longer functions. You can no longer reorient the spacecraft. You don't have enough fuel beyond just a couple more maneuvers, and then you're done. Why do these things break so often?

15:00Or why did they break so often? So a lot of it has to do with just the friction, and you get the slightest instability in there, and the sucker is just not going to rotate freely. So with the reaction wheel, you want to be able to spin that thing up while it's not moving the spacecraft. So you spin it up, and then you connect to it, and you use it to either add rotation to the spacecraft or take rotation away from the spacecraft.

15:30And that is jarring. That's going to wear things down over time. You're going to just – things don't like staying spinning forever. Right. We've seen the first thing to break in a nuclear power plant is going to be the spinny bits. The first thing to break in your car is probably not going to be the spinny bits. It's going to be a gasket of some sort. But spinny bits just like to break because any imbalance, and they start wobbling incontrollably, and then they're dead.

16:03But this is going to be – sorry, everyone, this is the podcast where I'm making many hand gestures and weird noises trying to explain things that really deserve an animation instead of words. So if you imagine a tire on a car that starts wobbling while rotating, it's eventually going to pop up. Yeah. So the thing to keep in mind is generally these reaction wheels, they are big, they're heavy, and they are bolted into their orientation. And so the reaction wheels start spinning in whichever direction you want.

16:38Your spacecraft gets a counter spin in that axis, and you can – by a combination of the three axes of rotation, you can reorient your spacecraft into any new position. Yeah. And it's that – in a fixed position is the thing you need to really keep in mind.

Gyroscopes versus reaction wheels

16:57So let's talk about the gyros then because they are – we have gyros on the Hubble Space Telescope. We have gyros on the International Space Station. So what is the sort of distinguishing feature between reaction wheels and gyros? So reaction wheels, you want to change the orientation of to change the orientation of your spacecraft.

17:21Gyroscopes, especially the ones on spacecraft, are often spinning within essentially a pocket of air. So there's pressurized air around them that is being used to reduce the friction on the system. So they're spinning round and round and round. The system can keep track of how they're oriented. And then as the spacecraft moves around the gyroscope, the gyroscope maintains its orientation in that compressed air pocket.

17:51And what's kind of amazing, right, is you can imagine you've got this thing that's spinning. And you've probably played with gyroscopes, right, as a toy. Anyone who has an image-stabilized camera has played with one of these things. Or a chicken. That's a good point, too. Yeah. A chicken, right. So you've got this thing spinning, which is – it's the same effect, but it's going to both resist being turned. And you will feel the resistance of it being turned if the spacecraft starts to turn.

18:22But the other thing that you can do is you can actually then change the orientation of the gyroscope inside your spacecraft. And you will get a reaction in the same way that you get with a reaction wheel. So there's a ton of kind of overlap between the gyros and the reaction wheels. But just think of it as reaction wheels are generally just – they're locked in place. They give that three-axis of rotation. The gyroscopes, you are turning the gyroscope or feeling the effect of the turned gyroscope. And this has all kinds of benefits. And which we'll talk about.

18:54So let's talk about it for kind of like maneuvering and reorientation. So with maneuvering and reorientation, this is literally where you have your spinny bit. You start it spinning. You walk into it. And then it spins in one way. And you spin in the opposite direction due to conservation of angular momentum. So we're all used to the idea of conservation of linear momentum where one thing hits another.

19:27And based on the difference in their velocity and mass, you get something that moves. Pool is 100% about conservation of momentum. If you've ever seen someone kick a ball, that is – your foot has momentum. The momentum, as well as additional forces and stuff like that, is imparted into the soccer ball. It goes flying. So anytime you have one thing in motion hitting another thing in motion, the quintessential science museum example of this is where you have the sets of pendulums.

20:08And you pull one ball up on one end, drop it, its momentum transfers through however many middle balls are hanging down. And then the one on the far side swings out because the momentum has gotten transferred from ball to ball to ball until it hits one that can fly freely. And this just goes back and forth. So that transfer of momentum can occur linearly or it can occur rotationally.

20:40And it also depends on the distribution of mass. This is why ice skaters can pull their arms in and out and change their velocity. We are ignoring that spacecraft. They're not expanding, contracting their arms. They do do it with their solar panels but once and then they stay put. So we're good. We're good. So all we're worried about is you have the mass of your flywheel that is rotating. It's going in one direction. You have your mass of your spacecraft and when you lock in, it will rotate in the opposite direction.

21:15Right. And the thing that's different, like we talked about the reaction wheels, right? They're spinning. You spin one up, spin one down. You get a kick in the opposite direction along that axis. The gyroscopes, they spin and they never turn them off. So like the gyroscopes on the International Space Station were spun up, say, in 2001 and have in some cases, you know, and they're running 6,500 RPM and have been running continuously. They just run them and run them and run them. And normally they want to just keep the International Space Station in their preferred attitude, right?

21:50Whatever direction they want to have it. And then the gyroscopes, by spinning this quickly, are putting in that almost like… Stabilizing. Yeah. I mean, stabilizing. But it's that, again, it's like if you do that experiment and you feel the wheel fighting you as you try to turn it or as you, you know, that when you're holding that wheel in your hands at that science center and you're spinning it up and you can feel the force. If you try to turn the wheel away from what it wants to be, it's going to try and fight you. And it's this stabilization that they're looking for with the gyroscopes.

22:25But the other thing that's really important, we talk about the ones on the Hubble Space Telescope, they are looking to point the Hubble Space Telescope at some star, some target. And then they want to do the longest exposure that they can with no movement of the spacecraft. And so they're spinning these gyros up and that is causing the entire telescope to be incredibly stable and wants to fight moving away from the thing that has been targeted. Yeah. And even if it does, they can feel the movement because the telescope is kicking, is pushing back against what the gyroscopes want to do.

23:02And that is a measurement that they can sense and they can account for that and detect that in those movements. So just the, I guess the simple thing to remember is the reaction wheel, spin up, spin down, the gyroscopes just go and go and go. And then you turn the gyroscopes to get that push in the opposite direction. And one of the things I learned early on with the Hubble Space Telescope is they could move the Hubble quickly. And if they did, it was really, really bad for the system. And so when they first launched it, it had a whole lot of problems.

23:34And so they knew they needed to do a servicing mission to fix the optics. And they realized as that servicing mission was approaching that they were also having problems with the servos. Because if they tried to move too quickly, they'd get a coarse wobble in the system. And that wobble was just, they're trying to move the spacecraft. The spacecraft is trying to say, no, I am stable here.

24:05Yeah. And that was significantly shortening the life expectancy of these servos. And so what they had to do was literally slow their roll and move the spacecraft more slowly so that they wouldn't get this forcing it to the limits that they were experiencing. And they were able to extend the lifetime of the servos substantially. Right. And, you know, we've talked about how the Hubble Space Telescope has gone through so many gyros in its life.

24:36Yeah. Because, again, these are things spinning at thousands of RPM. Heavy mass in space. All of the electronics surrounding it, everything that's keeping this thing spinning, wears down over time. You've got bearings and bushings and all that kind of stuff. And these things wear down and require maintenance. Anything that spins is a thing that's going to have to be maintained more often than the things that are just solid state that just sit there and do their job. Right. You would sort of set other kind of servo-related spinny things.

25:07Was there some other stuff we wanted to add to this?

Managing multiple rotating systems

25:09Well, so I think it's important to understand that they don't use servos only to move the spacecraft as a whole. They're also using them to move anything that might need to rotate and change orientation. So the solar panels are going to have servo-electric actuators on them. The antenna of the spacecraft are something else that they have to keep extremely precise alignment on. So you're worried about your antenna being able to track that ground station that you're sending your message to.

25:43And so the antenna need to be able to steer in their own way. So spacecraft have a lot of different points of failure. The thing is that if your solar panels stop moving, you just move the entire spacecraft, which if it's a space telescope means it's dead. But, hey, it had a good life, hopefully. If it's your antenna, you move the entire spacecraft. And when you're sending and receiving, you're stuck in that given orientation. But when everything is working, you have all these different systems that are tracking orientation in three dimensions while the spacecraft is moving in three dimensions.

26:24So you have solar panels staying oriented facing the sun. You have antennae staying oriented tracking ground stations. You have spacecraft staying oriented relative to the stars or whatever else it is that they're staying oriented to. And it's really kind of amazing to me how much effort, computational power, mechanical effort goes in to just keeping track of everything in rotating systems.

26:55Yes. It's wild and awesome. And a reason we need to learn trigonometry that isn't linear. But think about that, that even to just change the orientation of your antenna, you're going to be firing this servo. It's going to be turning in multiple axes to move the antenna so that it's facing exactly at your downlink and tracking against the downlink as the spacecraft is in orbit around it. The whole spacecraft is going to be getting a kick in the opposite direction from the directions that you're turning this stuff and has to compensate for that.

27:29That there is never, nothing is ever just a completely independent system. We're so familiar here on earth, you know, you turn the wheel on your car to move in a new direction, but you don't have to think about the fact that the entire car was given a kick in the opposite direction. Friction makes that all go away. You turn your head, but your body is being turned by you turning your head. You just don't feel it because, you know, the only time you really get to experience it is when you're in a low friction environment and socks and linoleum mostly count and ice skates, roller skates, all of those things are great.

28:10So my challenge to all of you is to channel your inner four-year-old pretending to ice skate while wearing socks and linoleum or hardwood floors also work and stand still and jerk your body around and the top of your body will move in one direction and the bottom of your body will actually rotate relative to the floor due to this conservation of angular momentum. Yes, so you can mimic that feeling. And then that's all spacecraft are doing. They're just standing there on the giant linoleum floor of the vacuum of space, twisting their arms and experiencing what happens, the reaction from that.

28:51Very cool. Yeah, man, I think a lot about reaction wheels and gyroscopes. It's been a lot of time reporting. And as soon as – it's funny because, like, whenever I see an idea for a new mission, I will specifically go through the technical paper and go, like, what do I got for gyroscopes? What do I got for reaction wheels? How do they plan to change their orientation? How many spares do they have? Do they have enough? Yeah, it is a thing that I spend too much time thinking about.

Patron acknowledgments

29:16And this brings us to thrusters, which is the topic of our very next episode, which we're going to record right now live. But those of you listening to the podcast, you get to wait another week. All right. Thanks, Pamela. Thank you, Fraser. And thank you to all of our patrons out there. As Fraser talked about at the beginning of the show, we are now ad-free, and we are relying on you to make this mathematically work out for us. We are very slowly approaching 1,200 patrons.

29:47We need to get to an estimated 1,500 for this to work long term. Um, so if you're listening and you haven't joined yet and you have a little bit of extra, please consider joining us. And if you join at a high enough tier level, I will endeavor to say your name correctly and probably fail. And here is where I say your name and probably mispronounce it, and I'm so very sorry.

30:12AstronomyCast wouldn't be here without the amazing support we get on Patreon. This month, we'd like to thank the following $20 a month and up patrons. Adam Anise Brown, Alan Glass, Alexis, Amy Zhang, Andrew Palestra, Arno DeGroote, Astro Sutz, Boogie Nett, Boldrinette, Burry Gowen, Buzz Parsec, Conrad Haling, Daniel, David, David Rossietter, Diane Philippon, Dylan Gee, Eric Lee, Felix Gutt, Frodo Tannenbaugh, Gerhard Schweitzer, Gordon Dewis, Jack Mudge,

30:50Jarvis Earle, Jason Kwong, Jean Baptiste, Lou Martineau, Jeanette Wink, Joe Holstein, Joe McTee, Josh Huggins, Caleb Axon, Katie B, Keith Murray, Kenneth Ryan, Kevin Lyle, Kim Barron, Laura Kettleson, Mark Thompson, Matthew, shut up Wesley, Kennard, Michael Atkinson, Michael Compatera, Michael Purcell,

31:20Michael Witchman, Mike Hoosie, Nick Boyd, Olga, Paul D. Disney, Rakesh Chitauri, Red Bar is watching, Richard Drum. There's two pages of you. Must load the second page. Robert Hundle, Roger Isaac, Sergei Manilov, Zegi Kemmler, Space Today, The Lonely Sandperson, Tiffany Rogers, Tim Goresh, Tom Grant, Travis C. Porco, Vitaly, Wanderer M101, William Andrews,

31:55Zachus Maximus, Zero Chill. And thank you all so much for supporting us and being okay with my bad pronunciation. Thank you. Thank you, too. Thanks, Pamela. And we appreciate you hilariously trying to pronounce everybody's names. And we'll see you all next week. Bye-bye, everyone. You are listening to the 365 Days of Astronomy Podcast.

32:29Cool.

32:36The 365 Days of Astronomy Podcast is produced by the Planetary Science Institute. Audio post-production is by me, Richard Drum. Project management is by Aviva Yamani, and hosting is donated by LibSyn.com. This content is released under a Creative Commons attribution, non-commercial 4.0 international license. Please share what you love, but don't sell what's free. This show is made possible thanks to the generous donations of people like you.

33:08Please consider supporting our show on patreon.com forward slash CosmoQuestX and get access to bonus content. Without your passion and contribution, we won't be able to share the stories and inspire the worlds. We invite you to join our community of storytellers and share your voice with listeners worldwide. As we wrap up today's episode, we're looking forward to unraveling more stories from the universe. With every new discovery from ground-based and space-based observatories and each milestone in space exploration,

33:43we come closer to understanding the cosmos and our place within it. Until next time, let the stars guide your curiosity. We'll see you next time.