Ep. 803: Mission Tech Series: Engines & Stages

Welcome and show introduction

0:00Welcome to the show.

0:30Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. I'm Fraser Cain. I'm

1:01the publisher of Universe Today. With me, as always, is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of CosmoQuest. Hey, Pamela, how are you? I am as well as I can be. There has been chaos today. I will be posting pictures of the unholy dongle chaos I have created in order to record this show. I can't wait to see a picture of the... Yeah. You have fashioned together a dongle Frankenstein, the likes of which we have never seen before. So busy wondering if you could, never stopped

1:34to ask if you should, and yet you should, because that allows the audio that you were speaking to make its way into this computer through some circuitous route. It's glorious. Yeah. It is truly, truly glorious. Very impressive. So before we get into this week's episode, I just want to give a shout out to our good friend, Dylan O'Donnell, who has posted on Facebook that he is cancer-free. And for those of you who weren't aware, he had cancer of the esophagus, which is like

2:07a death sentence. It's a really terrible kind of cancer. Get your HPV vaccine, people. Get your HPV vaccine. And it's so ironic because, of course, in every video that he does, he says life is meaningless and everybody dies, but not this soon, not Dylan O'Donnell. And he posted that after a lot of treatments and almost dying, he has come out the other side and is feeling a lot better. And scans say that he's cancer-free.

2:39It's going to take years for it to go through the full loop. But I'm so glad. Man, the world is made better with Dylan O'Donnell on it. And if you are not familiar with Dylan, he does an incredible YouTube channel, astronomy. He's one of the best astrophotographers out there and just a wonderful human being. And just, I read that post, brought a tear to my eye of the happy variety to see that he's doing okay and we get to spend more time with

How rocket physics work

3:11Dylan. But today, our series continues with our deep dive into rockets, their engines and stages. We fall under the tyranny of the rocket equation and discover what it takes to escape Earth's gravity and head off into deep space. All right, Pamela. I didn't have to say we're going to stop for a break because we don't do a break in the world because we are funded by our patrons. Advertisers get lost. And we are getting close to 1,200 patrons. I haven't looked today to see how close we are, but we could cross 1,200 this weekend. That would be amazing.

3:44As a reminder, we need to get to 1,500 patrons to make sure we never have to add. I mean, we're not going to. To make sure that we can pay everybody and stay ad-free, we need to get to 1,500. So the ads aren't coming back, but my sorrow and my ability to pay people might come back. And if we hit the 1,500, you just will never hear from us again about this. It's true. So this is kind of like an ad right now, I know, but only to put us into this safe place. Yeah, please do that.

4:14Okay. So first, let's just talk about rockets and how they work, because I think this is going to be really, really important, which is that on Earth, you are able to push against the great fluid that is the atmosphere. Or if you want to be in the water, you can push against that fluid, or you can just use friction to drive your car down the road. Once you're out in space, there is nothing to push against. And that's fine.

4:44And this befuddles the, you know, some of the science deniers. But how do rockets work? I had no idea this befuddled anybody. I have learned something, and it made me very, very sad. You started the episode by making me sad. Really? Okay, yeah. There are people out there who use as their gotcha, there's nowhere in space. If there's nowhere in space, how do rockets work? Momentum. It's called conservation of momentum, people. Yeah, yeah. Newton's, one of Newton's laws for reaction equal ups and reactions.

5:17It started with Galileo, but yes. Okay. Yeah. How do they work? So the thrust equation that governs rockets has two parts to it. Part one is you have the mass of the stuff exiting the, whatever place you have put the thruster or the engine on the rocket. So you have the mass of the stuff you're flinging away, and then you have the velocity of the stuff that you're flinging away. And the mass and the velocity is able to fling you. It's just conservation

5:54and momentum. So if you think about it, you can be in ice skates with a bag of books that weighs half of what you do, which sometimes felt like what was happening in college. And if you start flinging the books, you'll actually cause yourself to start moving backwards. Right. And it's just because your mass is changing, mass is going that way at a velocity, and the mass velocity

6:24going that direction is going to get compensated by the mass velocity going that direction. Now, the reason I said the book bag needs to weigh like half of what you do is you're not going to be able to fling things very quickly. But if the mass is high enough, you can fling things slowly and have an effect. But if you fling things fast enough, the mass can be very low. This is why the recoil on a shotgun is so bad, is those bullets are going at a really high velocity. There's also

6:55explosive in there. I understand. But conservation of momentum plays part of why your shoulder just got hurt. Right. Paint gun balls is a fun way to do conservation of momentum. So you just need to get something going at a high velocity and conservation of momentum takes over. So the other half of the thrust equation is you have the pressure of the exhaust, of the stuff coming out. So it is at a

7:30certain pressure that is governed by that whole how many molecules are there, what temperature are they at? That pervener equation from college or chemistry, whenever you learned it, minus the atmospheric pressure. So you can actually wreck yourself if you don't have the balance between the exhaust pressure and the atmospheric pressure correct. And then it's multiplied by the area of the exhaust. So the bigger the area of the exhaust, the more force you're going to have.

8:04Now, it turns out that kind of ideally, you just want the two pressures to equal and that term goes away and you live and die by conservation and momentum. That's a good place to be. Mm-hmm. So the gist being you're essentially hurling mass at velocity and you multiply those numbers together, that gives you a thrust, you get that in the opposite direction. And all rockets are just that either. And so then you don't have, you don't, you don't end up with a lot of levers that you can

8:36really control. You can control the amount of mass that you're throwing off. You can control the speed that you're throwing that mass. And then you can control your own mass that's getting the kick in the opposite direction. You, so where it gets trickstery is you also need to control the pressure to as large a degree as you can, because the atmospheric pressure at sea level and the atmospheric pressure, uh, in space where there is negligible atmosphere, uh, those are not the same.

9:11No, but that's just trying to get, I mean, then you're just trying to make those, that math efficient. Yeah. You're trying to optimize. You're trying to optimize that math. Right. But at the end of it, if you could, in a perfect world, turn your entire spacecraft into small particles of mass, and then hurl those particles in the opposite direction at high velocity. You'd have an ion drive. Yes. You'd have an ion drive. That is a rocket, right? And, and as long as you have stuff that you can throw at the back, you can keep moving in whatever direction you choose. As soon as you run

9:44out of stuff to throw at the back, you are now stuck in moving in that direction. And, and everything else, like, it's just like everything else that we're going to talk about today. And there's a lot just ties back to, but how can you do it better? How can you do it fancier? How can you fire, how can you not have to use some of the energy instead of bringing it in from an outside thing? How can you, as you say, with, you know, manage the pressure so that you're firing the, the particles directly out the back as opposed to firing them off the side. And I think,

10:14you know, sorry, let me just, one thing is just, you know, a lot of the times you talk about this idea of fuel versus propellant. And I think we've really kind of shifted. I know over time, the conversation has really shifted to, we talk about it as propellant because then it can be laser beams. It can be, um, it can be ion, you know, ions that you're firing out the back, or it could be, you know, nuclear bombs that you're dropping out the back of your spacecraft, whatever, right? These all fall into this idea of it being a propellant. You are propelling

10:45yourself forward by throwing this thing back. Yeah. So, so then let's get fancy. Okay. Um,

Nozzles and exhaust velocity

10:53and so is it more important for the mass or the velocity or does it not matter? Velocity. Okay. So you, you have two different problems that you have to take care of with the velocity. One is you have to make sure that it's in the correct direction. Right. Uh, so one of the things a lot of effort has gone into is figuring out the optimal shape of both the pressure chamber above the

11:24nozzle so that it directs the velocity downwards from the chaos that occurs where, where the acceleration is taking place, where you're building up pressure. We should talk about this, right? Because you're essentially igniting fuels in this chamber. Oxidizer and the not oxidizer. Yeah. And then you are directing the exhaust gases in the direction of your choosing. Yes. And so you have this nozzle and the speed of sound varies depending on, uh, material pressure, all sorts of other things. And

12:02it's kind of the speed you want to go at to make sure you get the exact right maximum flow. In atmosphere. You have the speed of sound anytime you have a material. So, so the thrust coming out needs to be going at the speed of sound for it and whatever it's encountering. So through that nozzle, you want your material exiting at the speed of sound for that particular situation. Right. And then I

12:35think what you were getting at earlier is, is just the shape of this nozzle. And I think, you know, you imagine that you're firing these particles out and they want to come out in all kinds of directions and you don't want them to come out in all kinds of directions. You want them to come out in the direction that is exactly opposite the direction that you want to go. Well, and, and they're expanding as they cool. So you need to shape that expanding cloud of material coming out of the nozzle. Right. And so the whole shape of the nozzle that you see at the back of rockets is designed to direct

13:07the flow of the hot gap, the mass at high velocity that you are driving out of your, of your spacecraft and try to get as optimal push in the opposite direction as you can. Yeah. And have you ever seen with like a hose is the place I've seen this most. If you get the flow just right, all the water is like nice, beautiful stream, you get it wrong. And the water is going kind of all over the place with turbulent flow. And you can have the exact same problem with thrust

13:43coming out of a rocket engine. If you get the shape wrong and, and so you need to keep that flow nice and smooth laminar is a fancy word, because if you don't, it's not going to come out smooth and propel you in a smooth way. And one of the problems that they have is they are now having to model bigger and bigger rockets. So you have a different flow over the rocket assumptions that we can make

14:13small rockets, don't work with big rockets. Then you're also running into, okay, so we have more nozzles. So their flow is interfering with each other. We have to model all of that. So you have this combination of, you need to account for the flow off of the rocket that is affecting that, that I don't think it's called a backdraft. I have to admit, I just realized, I don't know the vocabulary word for the air behind the butt of a rocket as it takes off. Right. I don't know either.

14:47Yeah. So you have to take into account the flow of air caused by the aerodynamics of the rocket that affect the thrust coming out the back of the rocket. You have to account for the flow from all the rocket nozzles around you and make sure none of this becomes turbulent and destroys you because that's a bad day. That would be very bad, yeah. But this is why we see different shapes of rockets depending on when and where they're going to be used and what kind of medium or lack thereof.

15:19So the, I think Starship is a great example of this, that you look at the engines that are at the bottom of the super heavy and they are kind of small. They're very pointy and they're designed to operate in the atmosphere. Yeah, there's a ton of them. And so, yeah, you're not only considering just the performance of each one of these, but you're also considering the performance of the, just of them together as a unit. But then you look at on Starship, the part that's designed to fly up in space, and it's got three of the littler ones designed to operate in the

15:54atmosphere and then three of these much bigger bells designed to operate in vacuum. We actually saw that on the space shuttle as well. So you've, you've got this, this different shape and it's really tuned for what is your atmospheric environment. And there, there's a great idea that NASA was working on. It never, and, and other companies were working on as well, this idea of the aerospace engine. Did you ever sort of learn about these? I remember it, but right now I don't remember it. So like I can see the name on a page.

16:24Right, right. So this is an engine that was being designed for the X-34, X-33. It was the X-33, right? The Venture Star. And this was going to be this single staged orbit spacecraft that, that NASA was working on with Lockheed Martin. And that, that this used an, an engine that could shift from the atmospheric version to the vacuum engine and back. In other words, you wouldn't have to have separate nozzles, some that could operate in the atmosphere,

16:54some that could operate in the vacuum. This one engine could do both of them. And then that's a weight savings. Instead of you having two separate bells, two separate rocket engines, you just have one. And, and the engines, you know, and, and development is still continuing on for these, these kinds of engines, but it just shows you how important it is to get the, the, the shape of the nozzle to, because you're just, it's what makes it efficient in firing those gases in the opposite direction. And, and one of the things that I think has really helped with both the weight and the

17:29diversity is stages, which is the next topic in the talk, because.

The mechanics of multi-stage rockets

17:34Well, before we get to stages, actually, I want to talk about, I want to talk about essentially, um, the exhaust velocity because, because, you know, back to that idea of the, of the velocity, different kinds of fuel, different kinds of propellant give you different velocity. They give you this specific impulse. And so we see hydrogen, oxygen, we see kerosene, we see solid rockets, you know, different kinds of chemical rockets have slightly different performance

18:04characteristics, methane rockets, you know, they have other advantages and disadvantages, but they are firing these exhaust gases out. But, you know, you mentioned this briefly before, you know, there are other ways to generate the particles that you were then firing at the back of this rocket. One is a nuclear engine. So you are heating up. We haven't gotten one of those working yet. I just want to make that clear. Well, no, they, they've totally been working. They just aren't in space. Oh, are you talking about the, the thermal powered ones or are you talking?

18:39Yeah. Okay. Okay. Sorry. I thought you were going somewhere else there and I was about to freak out slightly. No, no, no. Like a nuclear, no, no, no. So like a nuclear thermal rocket, like, and I forget the exact numbers for what is the. Thermal and good with. Yeah. Yeah. So, so you take this fission reactor and you use that to heat up, say some kind of propellant like hydrogen and you fire that out the back of your, of your rocket. And you've got, instead of a controlled explosion, you're heating it in a different way from a fission reactor. It all works. So, so Pervenert is, is kind of king

19:10here. That pressure equation from college where we're looking at what is the pressure? What is the temperature? What is the number of molecules you have to fit in there? And each different thing that you're burning with an oxidizer heats with a different efficiency. And so the question is how efficiently can you raise the temperature on your fuel? Well, if you're raising the temperature with a nuclear reactor, it, it's all the same physics, but you're looking at a completely different way of

19:42getting at the efficiency. So instead of a chemical burn, you're just heating that up with nuclear fission. Yeah. And it gives you a much higher exhaust velocity. Because you have a higher temperature. Because you have a higher temperature and that allows your, you to essentially use your hydrogen fuel more efficiently than if you use your hydrogen, burn it with oxygen, the way they did in say the space shuttle. And then the last one, and you were mentioning this earlier, is that you can go even more efficient and you can fire ions out the back of your spacecraft using an electric field.

20:14And we have an entire episode on that later. Yeah. Yeah. But we'll, so we'll give you the short version. And so you are using electricity, essentially solar panels. So you don't have to carry the thing that is, you have to carry the oxidizer essentially. And you're able to fire the ion engine, the ions out the back of your spacecraft. Magnetic fields. Yeah. With the magnetic fields. You use both. At hundreds of kilometers a second. And, and so you don't have very much mass, you know, essentially individual ions, but you have enormous velocity and these are incredibly

20:49efficient. And the other cool thing about them is they don't require, because it's not, you're not expanding something. So they don't require the nozzles the same way. They don't require the thrust chamber. They don't require a whole lot of the stuff is just not there. Yeah. You essentially have to have a rail gun for particles and that has its own weight associated with it, but it's, it's like a rail gun for particles. Yeah. Oh, so, so Zephan is saying it's not higher temperature, but the

21:20hydrogen is lighter than water. And so the molecules are lighter and faster and they get a bigger kick in the comments. So this gets back to how people argue about what temperature is, because in my head, how fast the particle is moving is the definition of temperature. So we're going to wave our hands at that one and say it's a philosophical discussion for when we eventually get to meet in person. Okay. So now I'll let you talk about stages. I will allow it. Um, but, but I guess the point is like, you know, earlier on we talked about how half of this conversation is about how fast you kick the

21:53stuff out the side. And the other half is how much mass you are that you're trying to move. Yeah. As you're a skater is running out of books, each book that they throw makes them faster. Right. Now imagine that you have all of your books in a 10 pound camping backpack that you made the mistake of buying in the eighties. Sure. Now it's finally empty of books, but you still are carrying around this 10 pounds that had books in it. And if you're a child who weighs 80 pounds, sorry to be

22:33working in pounds, rest of the world. Um, that, that 10 pound bag is actually significant. So you hurl it away and that gives you a little bit of extra momentum, but it makes it easier for you to figure out how to keep going. Now with rockets, you have double advantage here. So as you're taking off, you have that initial stage that has to carry its own fuel that has all of its own electronics that has all of the nozzles designed for atmospheric travel. And at a certain point you've exhausted that

23:11fuel. If it's a multi-stage rocket, or you've exhausted all of the fuel that you're putting into launching the stuff above it in the stack and you separate it from everything above it and you either destroy it or you bring it back depending on which technology, but nonetheless, all of the weight associated with it now goes somewhere else. You're not lugging it around with you and you've switched to using nozzles that are optimized for wherever you are next. And so you have this double gain in efficiency

23:47where you're now using better engines for where you are and you're no longer carrying all that extra weight. It is really amazing to think about how much of a rocket is just the fuel. Yeah, it really is. That when you look at say Starship on top of super heavy, you're essentially looking at a big tube of fuel, right? From bottom to top, that there is this little bit at the top where you could put your cargo. But really, it's just a ludicrous amount of fuel from the bottom to the top.

24:20Yeah. And then you need all of the stuff associated with that fuel. You need the pumps, you need the tanks, and you need the engines that you can use to fire to carry this stuff out to use it as propellant. But to give a sense of scale, if you've ever drank, as I am doing currently, out of a double walled steel container, that double wall area that you can't put, in my case, coffee into, that's where all the electronics and stuff goes. And then it's fuel. And that's not a

24:55perfect example, but it's really close. Yeah. Yeah. I forget the numbers, but these things are 95% fuel and the associated stuff. Yeah. And that as your rocket launches in the beginning, it is trying to not only carry all of the weight of the astronauts and all of their electronics and all of that stuff, but you're carrying the fuel and all of the tanks and stuff to hold the fuel. And so as soon as you have emptied out a fuel tank, get rid of the fuel tank. It is no longer needed. And so this

25:27is why we have multi-stage rockets and you really feel it when you play a game like Kerbal Space Program and you design your rockets with multiple stages. You realize the benefits of a two-stage rocket, a three-stage rocket. That's what the Saturn V was, a three-stage rocket, which is really impressive. Most things are in their own way more than two stages. So the thing that you have to always think about is like, so stage one we're used to, especially with the Falcon series where

25:59you see them come back and it's glorious. Now, if they're launching something particularly far, particularly high, they're going to use up all the fuel and not bring that first stage back home because they wanted the extra fuel. So first stage be gone, keeps going. Then it gets to a point where it releases the pharynx. That's a different episode. And whatever's inside comes out and often will have its own engines associated with it. Um, when I was a small child, I was deeply disturbed

26:35because it was the payload assistance module with the abbreviation Pam, which is one of the reasons I like Pamela better. I am not a payload assistance module. Um, until you work on a mission and assist with the payload, you will be. Yeah. I mean, it's a goal. It's not going to happen at this point, but we put engines everywhere we need and then we banish them as soon as we can. So Bethel Colombo two weeks ago, three weeks ago, dropped its ion engines away because they were no longer needed.

27:08And that was weight it didn't need. So it went through the multi-stage launch, traveled through space for forever. As far as I'm concerned, finally is nearing Mercury. And it's like, be gone ion engines, just shed what you don't need. I think that's a good moral for all of us. So stages are so incredible, um, that they really make modern rocketry possible. And people are always trying to come up with this idea of like, could there, could we have a single stage to orbit

27:39vehicle? Yes, but you shouldn't. Right. Yeah. So you could theoretically do that. And this is what the X-33 that I mentioned earlier on, the Venture Star, um, there's the Skylon and in theory, Starship is a single stage to rocket, single stage to orbit rocket. And people have talked about various ideas of like hybrid airplanes and things like that. And that the problem is, is that even if you could make it work, the amount of payload that you could actually carry to space is tiny compared to what you could do with a multi-stage rocket. And so I think the, the dream of us having a single stage

28:16to orbit rocket is just overwhelmed by a fully reusable multi-stage rocket. If you can nail it so that you can recover all the parts of your rocket, it's the same, you know, you, you, you stack it up together. The parts come off as needed. Then they all come back together and get reassembled on the, you know, for the next launch. That's the dream. And, and there's no one way to do it. Like one of my favorite ideas that I heard talked about at some point, I forget exactly which conference I was at

28:46is imagine a future where you have some kind of small space plane. I think dream chasers, the small space plane Sierra Nevada is working on. Yep. Yep. And, and that carries the humans because those are more comfortable to return home in. And, and then you launch all of your cargo in like standard capsule format. And, and so you essentially have comfort vehicles for humans and you have semi trucks for cargo and, and there's going to be multiple ways to come up with something

29:22that is both most efficient and most comfortable and maximizes reusability. Um, and there's some interesting ideas of, of like mixing and matching. So, you know, you talk about BepiColombo, you've got this, it launched on traditional rocket, then it used its ion engine. So that's kind of like the upper stage, right? Like it had a single stage, I forget what it was probably an area on, but anyway, launched on its first stage, use second stage to get to orbit, kicked on its ion engine to carry it to its destination, ditched the ion engine, switched

29:55to a chemical rocket again to do the sort of high thrust. Orbital insert. Yeah. That's a four stage rocket when you think about it. Right. Um, and so always you just plan around these, these different stages as necessary. And one of the things that's really interesting is, you know, people always think about planets with higher gravity. Like what would it be like to live on a planet with, with much higher gravity? You're trapped better. Yeah. You are trapped a little better, but, but the, the more like stages get you there. And so, uh, there was this paper that I reported on

30:26where even if you're on a planet with like twice the force of gravity, twice the escape velocity required to get away from that planet, you could still do it with like a 10 stage rocket. And you maybe couldn't, couldn't launch more than a few hundred kilograms into an escape velocity from your planet. And yet it would be crushing gravity, but the rocket could still get you there. The, the inefficiency comes in with, you're now carrying like 12 sets of engines, because like

30:57you, you can imagine every small nerdling may have at one point or another tried to essentially build the centipede rocket. That's just like segment, segment, segment, segment, segment, segment. Yeah. And you can do that with your Etsy rockets. And I have done that with my Etsy rockets, but the reality is you end up with inefficiencies because you have to carry all the stuff that goes to allow. And the danger. Yeah, exactly. There's more, there's more single points of

31:29failure. Yeah. But it, but, but that, that is the, that is the way forward that if, if you wanted to be able to launch off of one of these high gravity worlds, you would have to rely on stages and engines. And you, you know, I forget what the number was. It was like a hundred engines. Oh, good Lord. On 10 stages. Yeah. Yeah. Just crazy. But, but it would work. It would get you there. And then there was another interview that I did that was great, um, about using multiple stages for an interstellar flight, that there are different

32:02kinds of, of propulsion systems that, that work at different, in different regimes. And so you might be using a nuclear rocket for one part of fusion rocket for another part of light sail for another part of this. And you're, you're sort of stacking up the stages and ditching the light sail when it is no longer necessary. Right. And it's the same idea, which is just use the tool while you can, and then ditch it when you don't need it anymore.

Patron acknowledgments and credits

32:27Bepi Colombo. Bepi Colombo. All right. Thanks Pamela. Thank you. Our series will continue with our next episode. And thank you to all of the patrons out there who are allowing us to go ad free. We are so close to 1200 patrons. I would love to get there this weekend if it's at all possible. Um, so please help us be able to make up the difference between what we are making in with ads and what Patreon is supporting us with. And, uh, here are your names read out pre-recorded

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