Image credit: NASA
What does momentum have to do with rockets? Why are nozzles so dang important? What is a “potent mixture”, anyway? I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
Let's make one thing abundantly clear. I am not a rocket surgeon. I know basically nothing about the intricate details of the insides of a modern rocket engine. I'm a physicist, so I'm gonna look at this problem of designing a rocket ship through the lens of physics because that's basically all I know how to do because that's what I was trained to do. So the approach used in physics, and physics is not alone in using this kind of approach, it's kind of a good approach for a variety of problems that you might encounter in your life.
The physics approach is that when you're faced with a complex problem, like, I don't know, how does a rocket work? First, you find the simplest description of that problem that hits at the fundamental properties that grabs at the essence that that the most important pieces, the simplest model regarding those that incorporates those important pieces. They use that to gain understanding and insight and intuition, and then you go from there. Then you actually go out and solve the more complex problem, and you do it in layers. So I'm not gonna get into the intricate details of powered space flight because it gets really complicated really fast.
And because of that, I have a heaps, tons, loads of admiration and respect for the scientists and engineers who do work on these absolutely fascinating systems that we call rockets. So with that to the side, and I hope, you know, if there's any actual rocket scientists in the audience, I hope what I'm about to say doesn't offend you. I'm intentionally taking an incredibly simplistic approach so we can use rockets to explore some very cool physics, which is the point of this show. And I am going to keep it simple. Also, there's going to be a bunch of cheesy metaphors, but you probably figured that going in when you downloaded it.
So I'm gonna take a step backward. To keep it really simple, I'm gonna take a step backwards by taking a step forward. And I one of my favorite things to do is to break down normal everyday experiences and examine the very important essential physics happening in those everyday experiences because physics isn't something that happens around a black hole or in a galaxy or the evolution of the universe. Physics is happening something right in front of you, in front of your eyes. It's happening underneath your feet.
It's it's happening in the world around you. Everything, all the physical laws that we understand and we divine from nature apply right here in the real world right now in the situation you're in. And the physics I want us to explore together is if you're, I want you to take off your shoes. That's the first step. The first step of this physics experiment is to take off your shoes because I really want you to feel this with your bare feet.
I don't care if you're in a city sidewalk. I don't care if you're sitting down. I want you to take off your shoes and I want you to start walking. And I want you to think of the sensation of walking as as your foot places on the ground. Feel where your force, your weight moves.
Feel how it gets distributed. And if people give you weird looks, just tell them you're doing something for science and they'll leave you alone. Trust me. It works all the time. When you walk, when you have your foot on the ground, you're about ready to take your next step forward, what do you do?
Feel how your foot moves, how your muscles and your tendons and your bones all react to each other against the ground. You press you're pressing down on the ground, of course, that's your weight, But you can also feel yourself. You push a little bit back like you you roll onto the balls of your feet and then your balls of feet push back ever so slightly on the ground and you're relying on the fact that there's a lot of friction between your foot and the ground. All those microscopic molecular nooks and crannies hooking into each other, resisting each other so that in order to go forward, you have to push back. And this works because of conservation of momentum.
You're pushing on the earth, the earth is pushing on you. The earth is a little bit larger than you, I hope, and so the earth doesn't move hardly at all. But through conservation of momentum, you have only a tiny bit of mass compared to the Earth, so you move a lot. The momentum is the same. And it's only because in order to move forward, you must must push the Earth back.
This is conservation of momentum, and conservation of momentum is the most important physical principle when it comes to rockets. It's all based on momentum. It's all based on pushing backwards in order to move forwards. And this works in the air too. It's not just for walking on the ground.
If you think of what a propeller does or a jet does, it takes air, it sucks in air, now it's got big chunk of air right in front of its moving bits and then it pushes on the air. It pushes the air behind it, and that makes the airplane go forward. If you look at a bird wing, the way it flaps, it pushes the air behind it, and that's what makes it go forward. Obviously, that's a little bit more difficult than walking on the ground or swimming in the water because air is such low density, it doesn't give you a lot of pushback, which is why it took a while to, you know, build airplanes. We had that after, you know, we had ground vehicles for a very, very long time because it you know, you get the point.
You have to push something back if you wanna go forwards. What about the vacuum of space? What if you're out there 100% vacuum? There's no ground. There's no water.
There's no air. How do you push against something? How do you push something backwards in order to go forwards in space? Well, here's a surprisingly related scenario. You know, we talked about, you know, walking on the ground, relying on that friction between your feet and the ground in order to push against the Earth and the Earth pushes against you and you go forwards.
What if you're on, like, super slippery ice, the slipperiest ice you've ever encountered in your entire life? Well, if you do the normal walking thing that you're accustomed to, you're gonna fall right on your face. Right? Because you're gonna push against the ice, but the ice isn't gonna push back on you because it's so slippery, and your feet go out from underneath you, and you just fall. But what if I carried something heavy with me?
What if I had, I don't know, a shoe? And I took my shoe and I held it in my hands and I threw it as hard as I could? Well, conservational momentum still holds, doesn't it? Of course it does because conservation momentum is like a universal law. It's always gonna hold.
If you throw something forward, if I take that shoe and throw it forward, I'm gonna move back. I'm gonna scoot back on the ice. Now I may not scoot back very, very far because the shoe isn't very heavy. And so conservation momentum, I I don't get a lot of velocity, but the velocity, but the shoe will get a lot of velocity. That's fine, but maybe I maybe I brought a box of shoes, a whole bunch of shoes with me out onto the ice so I can just keep throwing them one after the other, and I can make my way to the edge of the lake and back onto the ground in safety.
So if I'm in space with nothing to push against, I can only push against myself just like on the slippery ice. But in order to do that, I have to carry something with me and then throw it away. And the act of throwing it away will push me around from conservational momentum. So a good rocket if you wanna make a rocket that goes around space, you need two things. One, you need a propellant.
You need something to shove out, to throw away so that you can move, like a cargo hold full of shoes. I don't know. We'll get to what the propellants actually are. But it doesn't matter. It doesn't matter what it's made of.
It just needs to be a thing that you take with you that you can just throw out the window so you can move around. And two, you need a source of energy. You need to do work on the propellant on your box of shoes in order to expel them away from you and so that you can move. That costs energy, and so you need to bring that energy with you or acquire that energy from the environment. In the case of me on the ice, I let's say I ate a croissant for breakfast.
It was a really buttery one. It was great. I took that chemical potential energy. I stored it in a different way as chemical potential energy in my body. That was a store of energy that could carry around with me for such situations as ending up in the middle of a frozen lake.
And then I can expend some of that energy to throw the shoe and then get my way to shore. And then by the time I'm at the shore, I might be hungry because I used up all my energy, and I also used up all my shoes. So, yeah, to make a rocket that goes into space, it's the exact same principle, but we're probably gonna have to move past the power of croissants and a cargo hold full of shoes. We need we need something more energetic, more propellant. And remember, especially with rockets here, the challenge of escaping the Earth isn't the distance.
It's just 60 miles. It's just a hundred kilometers if you wanna get to this edge of space. That's like nothing. You have but the speed is what kills you. You have to get fast enough to go into orbit or to escape the Earth.
You need 11.2 kilometers per second. That's per second. Every second. Eleven point two kilometers. 11.2 I can't even say it in a second.
That that's how fast it is. Or, 25,000 miles per hour, which is also a large number and also very fast. So you need to pack a lot of energy if you wanna do anything remotely interesting in space. You need a lot of energy, and you need to throw things out that back window really fast. And if you're wondering why after a hundred years of developing rockets we still have the same basic plan that we did a century ago, It's because that same basic plan really, really, really works.
Number one, you need that tons of energy, which means you need a very potent mixture. And and I'm gonna leave that to the side for now. I'll get to what those potent mixtures are in just a little bit. So just hang on to that. You have a source of energy, a potent mixture.
This potent mixture isn't just a source of energy, though. It's also going to be your propellant. This is the basic plan behind what's called a chemical rocket, where you store a potent mixture on your rocket, and it's going to do something to release a lot of energy, and you're gonna use the same mixture to squirt out the back to make your rocket go. It's like shoes but made of bombs or something. It's two birds, two space birds, one space stone kind of deal where the same fluid, the same thing is going to be your propellant and your energy source.
That's kind of handy. So here's how to make a rocket. You start with your potent mixture. You put it in a box. Box seems a little bit ungainly, so why don't we make it a tube instead?
And then you poke a hole in the bottom of the tube. Then you ignite your potent mixture, then you ignite your potent mixture, and it explodes and squirts out the bottom. Conservation momentum means your tube will move forwards. Ta da. That's it.
You put something like a satellite, a brave astronaut, something you don't like very much, whatever, depending on how well you've tested your your tube and potent mixture combination. You put that at the top of the tube, and you've got a rocket. The earliest rockets in history, these things were developed centuries, millennia ago, by the Chinese. It's basically gunpowder in a tube. Not powerful enough.
Gunpowder doesn't pack enough energy to get you up into space, but they're great for fireworks celebrations and or shooting at people, so very successful over the centuries. The big development came in the nineteen tens that instead of using a tube with just a hole in the bottom, you're gonna replace a hole at the bottom with a nozzle. Yes. A nozzle. And not just any nozzle, a special nozzle invented by Gustave de Laval in 1888.
And Robert Goddard, if you've ever wondered why NASA has its headquarters named after Robert Goddard, this is why. He took a nozzle and he attached it to a rocket. These were the good old days when a simple combo, cemented your place in the history books. We wish we could do that again where you could just take two random things and put them together and get a Nobel Prize or something. But, anyway, Robert Goddard put a DeLaval nozzle on the back of the tube and called it a rocket.
And here's how the DeLaval nozzle works and why it's so special. It actually, like, performs a magic trick on your potent mixture as it's ignited, as it's squirting out the bottom of your rocket. If you put it through this nozzle, something really special happens. So the nozzle starts out wide, and then it narrows down to a place called the throat. And then it passes through the throat, a very narrow part, and then it starts to widen again, but the way it widens is different than the way it funnels into the throat.
It's, much more it it tapered. It looks like a bell. It's more fluted. It's very long. It's like an it's it's like two different bells attached to each other on their pointy ends, and then you make a tube in the middle.
Something like that. This is my best way of describing a Delaval nozzle using just words. Here's what happens. You have a bunch of gas. It's hot.
It's ready to go. It wants to squirt out the other end of the nozzle. So it crams down. It funnels into that very narrow constriction called the throat, but this fluid, like most fluids, is incompressible. You can't really squeeze it very well and make it take up less volume.
Like if you take water and just squeezing on it, the water's gonna squeeze back. It's gonna have pressure on you. It's gonna say, I'm not squeezing any smaller. I'm water. Quit bothering me.
Water is incompressible. So and a lot of fluids are. So this fluid is incompressible, which means all the mass that goes into this tube has to come out at the exact same rate. So whatever your mass rate going in, if it's like 10 globs per second coming in, has to be 10 globs per second going out. And if it's 10 globs per second on a very wide opening, that can be rather slow, like all 10 globs go at once.
Then through the throat, in order to maintain that same mass rate, they have to line them past each other. It's glob number one coming out, then glob number two, then glob number three, glob number four. But to maintain that rate to get all 10 globs out in the in one second, they have to go really fast. They only get a tenth of a second each. So it's boom boom boom boom boom.
One, two, three, four, five, six, on and on and on. So the point is the fluid speeds up. Now if you so arrange it that the fluid speeds up so much that at the narrowest part of the throat, you can make the fluid become sonic. The fluid can start traveling at its own sound speed. And when that happens, it completely flips.
The fluid starts doing way weird stuff that you don't expect. Like, Now that this sonic or supersonic fluid is exiting the throat and coming back into the wide section, normally at the back end as it winds up, you'd expect it to slow down. But now it's supersonic, and supersonic fluids actually as they expand, they actually go faster. So you're converting a hot but slow fluid into a fast and sonic fluid into a very fast and supersonic fluid on the way out. You've been able to convert through the Delaval nozzle.
You're able to convert a lot of that raw heat energy in the fluid. You've been able to harness that and turn it into kinetic energy, into motion. It's basically that. It's a machine for turning heat energy into kinetic energy. And that special fluted bell like structure, it's tapered in just a way so that as this gas, as this fluid is expanding supersonically and speeding up, it gives a little bit of push on the sides of the rocket of the or sorry, of the funnel as it leaves and also a little bit forward.
So you get some bonus kick from the fluid as it's exiting your fancy nozzle. The end result is this little tiny little detail of switching out a hole for a nozzle dramatically improves the efficiency of your rocket. You get so much higher thrust, so much higher velocities, and you're so much more efficient for the same amount of fluid. You get a lot more work done on your rocket than you would normally expect. Okay.
That's nice. A nozzle. But what about the potent mixture? Yo, it can't just be gunpowder, but what can it be? Perhaps the best potent mixture is Patreon.
That's right. Your contributions keep this show going. Visit patreon.com/pmsutter to see how you can contribute. Really, I'm incredibly grateful. It really is a potent mixture.
It is your small donations, a dollar, 5 dollars, 10 dollars, whatever amount you wanna contribute that keep this show going, that amplify, that allow me to do all sorts of other cool education outreach initiatives. There is no better potent mixture except maybe liquid oxygen. But I'll I'll take I'll settle for the Patreon contributions. Patreon.com/pmcr. Also, check out astrotours.co.
We have trips to Iceland, Costa Rica, and Ireland active right now, taking bookings. Sign up. It's tons of fun. Trust me. Now back to the actual potent mixture.
It's actually pretty simple. It's pretty crazy how simple the mixtures that go into a rocket are. You just need a fuel, like say kerosene, and an oxidizer like say, you know, oxygen. And you just bring them together and ignite it, and then the chemical reactions between the fuel and the oxygen heat up the surrounding area, which keep the chain reaction going. So as long as you can keep pumping in fuel and oxygen, you can keep this mixture going at very, very high temperatures.
Then you squirt it out the nozzle and you can get a lot of thrust and you can start moving your rocket around. It's it's basically a controlled fire. I mean, fire is a chemical reaction between a fuel like wood and oxygen, where the chemical reactions themselves release heat. You just need a little bit of spark to get yourself going, and then the part once the party starts, it's self motivating as long as the music keeps playing. And that's it.
Like, yeah. Yeah. Of course, there's tons of other kinds of mixtures. Sometimes you just need one fluid that if you put it under the right conditions, it will ignite with itself. It doesn't need any other partygoers to to have fun.
Sometimes it's a solid where these are easier to store, but a little more volatile, harder to control, You know, etcetera, etcetera, etcetera. There's there's a bunch of different combinations, but this it's it's always the same thing, the fuel and the oxidizer in some sort of mixture, in some sort of state. And, let me just toss in a few more etceteras just for completeness because I know I'm barely scratching the surface, etcetera, etcetera, etcetera, etcetera, and etcetera. So we have a tube, we have a fancy nozzle, and we have a potent mixture. To reach the incredible speeds you need for Orbit, you need a lot of power.
You need just the right amount of potent mixture in nozzle design, in tube manufacturer, etcetera. And it turns out that a good chunk, like 80 to 90% of your rocket is just gonna be fuel. Straight up. If you wanna go to Orbe, if you want that 11.2 kilometers per second, twenty five thousand miles per hour, hour, you need a lot of fuel, and most of your rocket is just going to be fuel that is gonna be used up in your goal of reaching those incredible speeds. On top of all that fuel, you're you have the actual engine and the cylinder that's keeping everything together.
That's a relatively small fraction. And then at the teensy at the top of the you have this teensy tiny payload, the actual thing you want to go into space like the satellite or an X Lover, you know, whatever is gonna go at the top. And you might be tempted. Well, hey, if I just wanna go further, maybe I just don't wanna go to the edge of space. I wanna go really far into space.
Maybe I want something heavier, maybe two ex lovers at the same time. You might be tempted to say, oh, just load up more fuel, man. Just just keep piling it on, add some more fuel, and then we can get those suckers into orbit, but you run into a problem. If you wanna heft more stuff, if you wanna go further, you need to add more fuel, but more fuel is more weight, and more weight makes it harder to get off the ground. It get it makes it harder to push around from conservation of momentum.
So there's this lovely, lovely phrase called the tyranny of the rocket equation, where the rocket equation itself is this very simple relationship between the energy you need to do what you want, the energy that's available in your fuel, and then how much of your rocket what percentage of your rocket's mass is is in the fuel. And so you can plug in. So if you decide, okay. I wanna go here, and my fuel is capable of delivering this much energy, boom, this is how much of my rocket needs to be fuel, and you can't get away from that. There's nothing you can do to change that.
So increasing the mass, if you wanna increase what you want to put into orbit, that means more energy, which drives up how much of your rocket is fuel, meaning you don't get to launch what you wanted to launch. And that that kinda sucks, but that is the tyranny of the rocket equation. Like, you can't just put on some extra fuel in order to launch a heavier thing because the fuel itself has weight, and that's gonna cost you even more energy. And so you're not instead of having two x levers, it's only gonna be, like, one and a quarter x levers for twice the amount of fuel, for example. This is why staging is so important in rocket design where you instead of just one big giant rocket, you actually have a bunch of smaller rockets put on top of each other.
And then the first rocket at the bottom goes, and when it runs out of fuel, you decouple it, you detach it, and it goes screaming off into the atmosphere to be buried in the ocean or retrieved or, you know, whatever. And that way you're not carrying around that dead weight for the rest of the mission, this big shell and engine and all that kind of stuff. You can just toss it. Okay. I'm done.
Move on. Staging is absolutely critical for getting to orbit because the energy of the chemicals that we use for our rockets, while it's impressive, you know, it's not like the super most energetic thing in the universe, and there are limits. There are limits to what chemical rockets can do, and staging is a way to get around it. So that's chemical rockets. Is there any alternative?
Well, yes. Of course. You can still have the principles of a rocket, and in fact, these principles will always hold, but your design to meet those principles might change. So just like you can push your propellant out the back using, you know, a controlled explosion and a fancy nozzle, you can use electric and magnetic fields. Electric and magnetic fields are perfectly capable of shoving stuff around like a compass needle.
That's that's a force. That works. That's legit. And the ion engine is the best example of this. You take something neutral, like a random neutral gas like xenon, and you zap it with a bunch of electricity.
Some of the electrons get ripped off their atoms. Congratulations. You've made a plasma. Then you pump those ions, the positively charged particles that are left over. You pump them through.
You slam them with a big, strong electric field, and that electric field shoves them out the back, and then conservation momentum takes over and your spacecraft moves. It's the same fundamental deal. You need a propellant, in this case xenon. You need a source of energy. In this case, energy can come from, say, solar panels to power the electric field, so you're getting it from the sun.
The good news of something like an ion drive is you're not blowing anything up, which is kind of safe and stable. It's easier to manage. The bad news is it's not the most powerful thing in the world. Like, you're never gonna use, I shouldn't say never. You're almost certainly never gonna use an ion drive to actually launch into orbit.
It can't even overcome atmospheric drag. You'd just be sitting there like, come on. Come on. Come on. Come on.
Come on. And then, like, air air is able to say, no. Hold on. You're not going to orbit. So its best use is in space over really, really long periods of time.
I mean, you're just squirting xenon out the bag. You're not getting a lot of thrust that way. And so it's it's but the fact that it can just keep operating and operating and operating, you don't need a lot of xenon at a time. It just takes, like, 10 kilograms or something to get you to some decent speeds, but it's gonna take you a while. So if you have some applications where you don't care how long it takes, you just care you just wanna do it as cheaply and energetically efficient as possible, ion engines are your best bet.
There's other somewhat related technologies, you know, gridded iron, hall effect, colloid iron. The list keeps going, and we'll put in a four few more etceteras for you, etcetera, etcetera, etcetera. You can also just heat stuff up and shove it out the back, which I hope you recognize by now as a common component of rockets. Like, just just thermalize, just take a gas and heat it up and let some of it go out the other end. What's good at heating stuff up?
All nuclear power is pretty good. If you add, you know, a reactor sitting there that's generating a lot of heat doing its thing, just take your propellant, whatever it is, run it around near the reactor so it gets really hot, and then voila, squirts out the back and you got yourself a rocket. That's different than nuclear propulsion, by the way, which in a simplified view is if you, you know, drop nuclear bombs off behind you and let those bombs push you, that's the nuclear equivalent of taking a shoebox with you to the middle of the frozen lake and throwing the shoes around. But there's nuclear propulsion and also using nuclear power to make a spacecraft or rocket ship go. Those are both just hypothetical theoretical constructs.
We haven't we don't have, like, actual working rockets doing those things because turns out, kinda tricky to put a nuclear reactor into space. Who knew? What about the sun? You know, that giant ball of raw fusion power in the center of our solar system. You can use concentrated light like mirrors to heat up your propellant.
And then once you've got hot, you square it out a hole and you've got a rocket. You could also ride the light. Light sails, which are completely different beasts, but still rely on conservation momentum. The point here is that there's, like, a million different rocket designs out there, but it's always based on the same principles, always about conservation of momentum, always about the need for a propellant in a source of energy, which is why physics is so awesome. I'm not saying it's better than engineering.
I'll let you be the judge of that. But but using this simplified view that physics provides us, we can get at the essential I I hope you came out of this episode realizing what the essential parts of doing rocketry is like, and that's why in rocketry, we make the decisions that we make because they're governed by very basic physical laws that we can't get around, and so we have to build to that. We have to optimize within those parameters. We're always gonna have conservation momentum. We're always gonna have the rocket equation.
We always need an energy source. Those are the physics. And and, yeah, yay, physics. Now go out there and build some rockets. Thanks to at mark reep, Peter, Dan h, and sean p via email for the questions that led to this episode.
And, of course, thanks to your my Patreon contributors, patreon.com/pm. Sorry. You are what make these shows possible. I'd like to especially thank my top Patreon contributors of this month, Robert r, Justin g, Kevin o, Justin r, Chris c, Helga b, and Andrew p. Go to patreon.com/pm.
Sorry. Have I said that enough? No. I haven't. Patreon.com/pm.
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