Okay, I don't think this requires handwaving, because even in the current day we have space craft that are changing course and moving around in space, but... can someone explain to me how that works? Like, my understanding is that propulsion comes because the force from the space craft is pushing against something. Is this not accurate? I think it must not be, because space is a vacuum so there's nothing to push against, so when jets fire or whatever, they're effective because... why? This is probably more a physics question than a space question, but when we're not in space it makes sense to me because the jets could be pushing against the atmosphere or the Earth itself or... something. But in a vacuum, how do we create propulsion? (It's entirely possible I don't understand propulsion on Earth, either....)
I don't quite know all the nuances, but propulsion relies on the release and acceleration of fuel of some sort as it source. You're right that in a vacuum, there would be no molecules in space to push from, but the spacecraft's fuel is used as the propellant: as fuel is accelerated in one direction, so the object (the spacecraft) moves in the opposite direction. There'll be some physicist or science fiction enthusiast who will be able to explain this more eloquent than me, I'm sure!
Technically dark matter makes up most of what we understand is 'space', and there is technically a micro gravity in it. Just that it is so insignificant, that it is almost negligible. So in a way you are pushing against 'something' when you fire a rocket (or whatever type of propulsion system your using). Think of it kinda like traversing water, if it happens to be mostly friction-less. At the atomic level the particles will bounce off each other, creating a means of movement in 3 dimensions if you have an RCS system to help orient your vessel in the direction you want to go, outside of just going forward from your main propulsion system. The more thrust you can create, the faster in that direction you will go, just like how a plane works in atmosphere. Though the drag you will have in space is like the micro-grav, and won't slow you down too much, and you might not even notice it. Unless you observe it over a long period of time after you have stopped accelerating and are just coasting. So basically the more exhaust you can produce, and the faster you can produce it will determine how fast you will accelerate through space. You can use large bodies such as gas giants (and even stars) gravity wells to sling shot you up to a good speed to save on relying strictly upon your fuel reserves. Though you could push yourself along with a flashlight, but it will take an eternity before you make much progress.
Space isn't quite empty. A theoretical method of space travel is utilizing photons via solar-sail. Rocket propulsion is above my pay grade to explain, but I can try. The best I can do is: the exhaust isn't necessarily pushing against the vacuum of space. The force of the exhaust is pushing against the spaceship, and in the effort of doing so is sent out; like you shoving a brickwall and proceeding to fall down flat on your back, except here you're also creating enough force to move "the brickwall" (spaceship). Or, to try and put it another way... Think this: --->(spaceship) Not this: |<---(spaceship) Or pushing a child on a zipline / swing. You are acting as "the propellant", the child is "the spaceship". You don't need to simultaneously brace or push yourself off of a wall or something in order to give the child a push on the zipline / swing. I suppose you might be able to make an argument that latter particles are pushing against the initial particles of the exhaust, but I don't think this is the primary source of the propulsion. Anyway, there's a reason why I'm not an astrophysicist.
The following may be true. I think it is. The "push against something" concept would apply to an old-fashioned plane with a propeller. The propeller blades shove air around, producing a difference in air pressure that pulls the plane forward. But that's not what's going on with space flight. I believe that the most useful analogy might be that of a gun. When you fire a gun, the bullet goes flying in one direction. There's an equal and opposite reaction that causes the gun to kick back. When a spacecraft explodes "stuff" in one direction, there's an equal and opposite reaction that shoves the spacecraft in the opposite direction. It doesn't matter that the stuff has nothing to push against--the important thing is that you shoved something that has a nonzero mass.
This is what happens. Spacecraft engines don't provide continuous thrust like atmospheric craft do, they fire in bursts and the effect is as ChickenFreak described. Essentially it's recoil, the engines don't push against something else, they push against themselves.
I went out of my way to make a very advanced and accurate drawing. Basically, the force of the engines firing pushes on the ship, but the ship is also pushing back on the ejected material. The ejected material doesn't have to push on anything else in order for the ship to move. However, the material will push on the earth, making a crater, and the earth will push back on the ejected material, kicking it into the air. This reaction isn't needed for the ship to fly. Any ship flying away from the earth will have to obtain an escape velocity to overcome the earth's gravity, which is why vehicles shot into the air will tap out at a certain altitude if it can't accelerate fast enough. So, you could add two more backward facing arrows. Force of Air Friction on Ship, and Force of Gravity on Ship, so the thrust arrow will have to be as big as those two together to hover, and more to fly. Once in space (always really), the force of rockets firing on the ship always add, so to stop, the jets have to fire in the opposite direction. Firing down jets wont slow forward momentum, so you will get some kind of diagonal motion.
Like everybody else has said; it's just recoil. If you sit in a chair and use your feet to push yourself away from a wall, you end up moving. This is the same thing that happens in space but there's no gravity or air resistance to slow you down. I'm going to recommend two or three games for you to play in order to understand how space propulsion, orbiting and physics in general works. Space Engineers - Amazing for building ships of your own creation and traveling through space. It's physics is incredible, it'll help you get a grasp on how every action has an equal and opposite reaction. You can also build some monstrous ships in that game, which might help your writing. The physics engine provides tons of fun but also interesting experiments that can be performed by colliding ships with other ships to see how objects would travel and react to each other. Kerbal Space Program - I think it's the closest thing to actual rocket science that has been simplified enough for the average person to understand. Definitely try this one out, you learn how achieving orbit works, how propulsion works and how planning out space missions actually works. Although it can be quite overwhelming and complex at first, it gets easier. Space Engine - This isn't exactly a game, more of a simulator and it doesn't really have anything valuable to teach physics wise but it does provide you with an enormous understanding on the scale of our universe. It is literally "The Universe" crammed into a single game, you can explore an entire universe, all its galaxies, stars, planets, moons, asteroids, black holes, nebulae and other celestial objects. The distances are realistic in this game, meaning that if you traveled at the speed of light from the Milky Way to the Andromeda galaxy which is roughly 2 million light years away, it would literally take you 2 million years in real life to reach it. You can go up to a maximum speed of 325 million ly/s (light years per second) and it will take you roughly 10 minutes to reach one end of the universe to the other at that speed. This game is free by the way and can be downloaded on the internet.
They have mods to make it easier to say, make a basic orbit with the navball, but it is still a learning curve. Though you kinda have to understand just how many kilo-newtons of thrust you will need to get your ship up into space. Me, I just trial and error it. Granted I have a Warp Engine (100,000 Kn) from a mod, and it can move around 500-600 tons pretty good. But yes it is still quite complicated to learn and master all the stuff in KSP, but I have seen one guy, Scott Manley on youtube who plays it close to reality.
My concern is that in space, there's no wall. So it feels like you'd be kicking your feet at the air (actually, at something even less than air), which isn't going to push you anywhere. So this analogy isn't working for me. I'm struggling with the gun analogy, too, because there's nothing for the spaceship to kick back against in space. That is, there's no gun for the the opposite force to act upon... but I guess I need to think of the spacecraft as being the gun, rather than the spacecraft being the bullet?
Okay, so you're in space and there's a solid metal box in front of you which is significantly heavier than you. You and that box's weight is now mass and when you use your legs to push yourself off this box, that's the force in action using your own mass and kinetic energy with your muscles, similar to firing a gun. This provides you with acceleration as you move away from this box. The amount of energy it took to set that mass into motion had both an equal and opposite reaction, when you pushed that box with your legs, not only did you move but so did the box. That's how moving in space works, you literally use kinetic energy to get around. The same principle is applied when you fire a gun in space, when you use propellants on a spacecraft to move around and gyroscopes to turn in any direction. Even a pin has mass and if you threw a pin at a 10 mile asteroid, you wouldn't notice any immediate effects. But give or take a few million years? The trajectory will have changed significantly from its original path. Anything with mass and speed will produce force and acceleration. Every action will have an equal and opposite reaction, everything in physics follows this law to our current knowledge with the exception of a few odd things like black holes.
It's more like standing on an icy lake. You can't walk because it's too slippery (basically kicking the air) so you grab a brick out of your bag and throw it as hard as you can. It goes hurtling in one direction and you the other. The harder you throw the brick, the farther and faster you both move. Rockets are basically a way to use a controlled explosion to throw scads of tiny bricks (or exhaust, if you prefer) really, really hard in (or at, if you prefer) One Direction.
The physic principle of rocket movement is Momentum Conservation: https://en.m.wikipedia.org/wiki/Tsiolkovsky_rocket_equation After derivating the ecuation you can let alone the Speed of the ship as a logaritmic function.
Fun things to think about. Fuel has a mass, just like a bullet. If your ship weighs 1 million kilos, fully fueled, it will weigh less as it flies, and less fuel will be required to accelerate the ship as it gets lighter. So when you get to planet X, you will weigh only a quarter of a million kilos, let's say. It will be easier to stop. So, these considerations dictate how far you go and how big your ship can be, yet it has to carry that initial weight. Bang for buck is a big deal in space.
Okay, related question - once you're moving in space, you'd keep moving almost indefinitely, right? So you'd use the fuel to get out of atmosphere, you'd use some to get started in the right direction (and then I guess some more for whatever directional adjustments you need to make) and of course some to keep the passengers alive. But you wouldn't actually be using all that much fuel to keep going, right?
Correct. This is why something like an ion drive is feasible. It creates only a small amount of thrust but is able to create that thrust over a very long period of time, unlike conventional combustibles that "blow their wad" very quickly and violently. As the acceleration slowly builds up, the ion drive keeps pushing and that speed continues to increase. The problem is slowing back down again. Most modern Sci-Fi takes this into account, but older Sci-Fi often doesn't. You either need an engine on the nose or you need to flip that ship around and start thrust in the opposite direction when you approach your destination.
My biggest peeve when it comes to this question is Star Wars sized battle cruisers measured in kilometers quickly picking up speed as if they were small ships. Those things would be a monstrous amount of mass to accelerate, and you're reaching into the land of magic to come up with internal structural integrity that allows that kind of acceleration without the engines pushing clean through the body of the ship and out the nose.
Just as a curiosity. There's a Aeronautical "Law" (based in the Earthling chemical combustion energy limit) which dictates you have to left some mass behind to leave the planet surface. It's impossible to leave the surface of a planet with a escape velocity of more than 8000 km/second with a sigle-phased rocket (again, only if you use the chemical combustion that's been used nowadays to boost your rocket). You need to leave some mass behind. That's why the NASA rockets usually have three phases in order to get to the orbit. Of course, sci-fi ships don't use chemical combustion (some real one at least), they couldn't land at the spaceport and take off so easylly as we see in the movies.
BTW, and just as an aside... If you've not given The Expanse a go, I would consider it a good starting point for how many of the things in this question and in your last question concerning effects of microgravity on the human body could/would/should (you pick) best be handled. They do a fair job of paying attention to the laws of motion. They do go into the effects of microgravity as well, but only a bit since the actual thing would tend to create a story that's about "How to survive microgravity" rather than a story about humanity's first expansion (get it, get it??) and what we find just past our own astronomical front door. You also get to meet some kickass characters like GySgt. Roberta Draper. Is there a word for what I feel for her? Worship? I don't know. It's not a groin kind of thing. It's a plant my sword (or plasma rifle) in the ground and pledge my undying fealty kind of thing.
Have you tried books and TV show? One better than the other? ETA: I may have to go with books... doesn't look like there's any good way for me to actually watch the TV show.
Yes, but remember you also have to slow down and stop at some point, which in the vacuum of space is no easy matter. Whatever fuel you expend to get going, you'll need it again to stop. There are some tricky ways to accelerate and decelerate in space without using up valuable resources. If you want to get your feet wet in the subgenre of Hard Science Fiction you may want to read one of Ben Bova's Grand Tour books. Ben is a tad polemic in his views, but still very readable.
Yes, I've done both, all the books, the glorious books! Safe to say, the books trump the show. The books are much better in regards to the effects of microgravity. It costs nothing extra to write those details. It's a whopping amount of CGI to show those details in a TV show or film that's about people who were born and live their whole lives in microgravity, how this would alter their body-plan, growth, shape, etc. Also, one of the central characters, Chrisjen Avasarala - played by the stunning Shohreh Aghdashloo in the show - who is pretty much President of Earth, is a much, much, much better character in the books. In the show they had to tame her mouth a bit. In the books, this is Chrisjen being polite and demure.
I watched the four first episodes and I can't say why I didn't like it. The plot was a little mess, I think. I got bored but I'll try the books.
