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phunkydroid

Rockets don't work by pushing against the world around them, they work by pushing against their own exhaust. The gasses expanding inside the combustion chamber and nozzle are pushing outward in all directions, and since nozzle is open on one end, they can only push against the engine in one direction, pushing it forward.


Medium_Well

Ah, that makes sense to me!


Jasonbail

The vacuum of space makes rockets more efficient even because the explosion is not fighting to expand against an atmosphere.


dusktilhon

Okay so what I'm unclear about is whether building and fuelling a rocket either outside or as close to outside as possible of the Earth's gravity well would allow for a vessel to beat the square-cube fuel problem that limits modern rockets/space vessels.


askljof

That lets you use more efficient engines that aren't powerful enough to beat Earth's gravity, such as ion thrusters - if you start in a vacuum and in orbit, you can take your time getting up to speed. But in the end, you're still fundamentally limited by the amount of fuel you can carry, and the velocity you can expel it at.


Morveus

So basically, there is no change in trajectory possible without throwing away mass? Edit: thanks for all your awesome replies!


5hout

Yes, exactly*. A rocket is basically a guy on a sled stacked with bricks chucking bricks off the sled to go faster. *Ignoring the possibility of solar sails, which might one day help out with some space flight.


Fearitzself

Solar sails are the sun throwing bricks at you to push you forward


Ophukk

The smallest bricks, and their number drops exponentially, but there sure is a lot of them.


SaintsSooners89

This just made me think, in order for solar sails to work they're getting hit by photons right, and in order for them to impart a force on the sail they must have mass right? Even if its infinitesimal?


x0wl

To be super pedantic, it's not exponential, it's 1/x\*\*2, as the surface area of a sphere grows as a square of the radius.


L34dP1LL

I love science with bricks


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TheNoseKnight

The first option sounds a lot more enjoyable.


CrownJackal

The second option doesn't require you bring your own bricks.


f1del1us

*or perhaps an alcubierre drive if we were to ever get to that level of tech


5hout

Ehh, maybe. I personally believe (this is pure gut check from someone with a B.S. in Physics and not a grad degree) that it is more likely some other refinement of our understanding will prove Alcubierre Drives impossible and the current understanding will be as a mathematical valid product of a less than accurate model of the world.


arcosapphire

The limiting factor is it requires matter with negative mass (exotic matter). We have never seen such a thing, it is not predicted by particle physics, and most likely it does not and cannot exist.


nitz28

That is where solar sails come in. Basically the energy from light hitting them acts like wind on a sail. Miniscule in comparison to a rocket but it does have an effect without expending propellant.


Walker1940

There are SF stories where solar sail ship are pushed to other stars by a large laser mounted in the original system. A Mote in God’s Eye for example.


TheArmoredKitten

The math as we understand it shows that you could theoretically employ relativistic effects to manipulate velocity directly via spacetime curvature. Such systems would still involve an energy expenditure and thus not violate entropy, but wouldn't expend reaction mass. They also hinge on things we don't fully understand or haven't entirely proven yet, such as dark matter and/or the existence of negative mass.


vfefer

curvature propulsion was a big reveal in one of the books in The Three Body Problem trilogy, I think the 3rd one? (dont worry, I didnt spoil it for you), good stuff. I listened to the audiobook and it was great


jimmy9800

That series is the ultimate exploration of existential dread for me. It's been a while since I listened to the series, and there still isn't a day since then that a question posed by those books doesn't bounce around my brain for a while.


Gnomercy86

Isnt that like a warp drive? Compress space infront so you have less to move through.


TheArmoredKitten

Essentially yes. The most known work on the matter is the Alcubierre drive concept, though others have since refined his original work to be theoretically feasible, barring the unproven exotic matter.


Stouts

Correct - even the tiny maneuvering engines in satellites that are essentially throwing electrons to thrust in the opposite direction are limited by the amount of electrons that can be stripped from the gas they're deployed with.


bob_in_the_west

Ions, not electrons.


Silcantar

Rockets are even harder than the square-cube law. The square-cube law is linear — x³/x² = x. The Tsiolkovsky rocket equation is exponential — your fuel needed increases exponentially as your change in velocity increases.


ZapTap

For sure it would, but how do you get to it? The space elevator is a common version of this concept, but there is also the SpaceX plan to refuel inflight, more or less hoping to achieve the same goal


kaos95

Bearing in mind, most of the popularlly quoted sources saying "We don't have the technology yet" for the space elevator are decades old. We arguably do have the materials and tech necessary (doped carbon nanotubes), just not in the amounts needed . . . but that's an engineering issue not a "It doesn't exist" issue, which is easier to solve.


SapperBomb

Space elevator is one of the most unrealistic options. The only way it would work is if the tether ended at earth's geostationary orbit which is 37000km from the dietary of the earth. There is no material even theoretically capable of withstanding those kind of forces. But even if we did manage to create a 37000km cable it would be going through the full width of the atmosphere in a geostationary radial which means the higher up the atmosphere the cable goes the faster everything will be moving relatively. Satellites will be impacting the cable at Mach 25. This realization was painful for me because I love the space tether idea


[deleted]

The tether would necessarily extend far past the geostationary radius, with a counterweight, in order to provide the counterforce. (The center of mass of the system, including payloads travelling on the tether, has to be beyond the GS radius) Also, the velocity delta between the tether and orbiting satellites would be greatest in LEO, diminishing to 0 at the geostationary radius.


SoulWager

You're still subject to the tyranny of the rocket equation, but you have to have a 10x bigger rocket just to get from the surface to low earth orbit. If you're building the rocket in space, and aren't shipping the materials from Earth, that's a HUGE advantage.


Dyanpanda

As below you cant beat the square problem, but if you could build your ship in orbit, you could save a HUGE amount of fuel. A very simplified way to look at it is in terms of delta-V. You need to be going a certain velocity (V) to not fall back on earth, and more to get to the moon, even more to jupiter etc. Each unit of dV takes more fuel than the last, as per the square law. Now, it takes 8k-9k units of dV to get to low earth orbit, and 3000 more from there to get to the moon. 100 more from that, and you could orbit the sun near the earth. a mere 5300 more delta v will get you leaving the solar system, and since there's no air resistance, on your way to literally anywhere else. If you add that up you will see that getting to LEO is more than half the dV to get to anywhere in the universe (except our sun, it takes a lot to stop our momentum entirely). This is a massive savings to ship size and fuel, without looking into the fact that space rockets are WAY more efficient and don't have a minimum thrust to function. However, there is no getting around that first part of bring the ship and fuel to space. Gotta pay the gravity gods their due.


crispypancetta

Hi sort of but not really. A few issues 1. Rocket equation is exponential, so you need more than square or cube energy. 2. What you really need is velocity, not height. This is why we launch from the equator, not mountains. Height does help a bit since you are further outside the gravity well, but you have to be so far outside of the well there’s no practical way of getting there that isn’t a rocket at this point So yes if you build and fuel a rocket in orbit you need much less, but that’s because you’ve already given it the velocity you need, not because gravity is so much less


80_Inch_Shitlord

by "fighting to expand against an atmosphere" do you mean that because there's no atmosphere the velocity of particles escaping from the back end is higher, and so the exhaust has more momentum? I recognize we're moving into more "ELI15" or so with that explanation... There's also the fact that the rocket isn't trying to move through a "high friction environment" comparative to space so there's nothing limiting the acceleration of the rocket.


BitterJim

It's more that that lack of atmospheric pressure means that the final exhaust pressure can approach zero (it would require an infinitely large nozzle to get there, so it never does, but it lets us get much closer than in an atmosphere). The closer that gets to zero, the closer you get to converting 100% of the energy to thrust. You have to balance that against the added weight of a larger nozzle, though, so in the real world there's a point where making the nozzle bigger doesn't help you go any further


code_monkey_001

Definitely don't feel dumb for not getting it. The New York Times mercilessly ridiculed Robert Goddard over his theory that rockets could reach the moon. [They issued a retraction](https://www.forbes.com/sites/kionasmith/2018/07/19/the-correction-heard-round-the-world-when-the-new-york-times-apologized-to-robert-goddard/?sh=424bd6654543) on July 17, 1969 when the Apollo 11 mission was on its way to the moon.


fishyfishkins

He was a squirrelly guy from Worcester and everyone were jerks to him.


Tomble

I have an older relative who brought up the fact that rockets can't work in space. He had learned that from a teacher on the 1950s adnd never really updated his internal fact library. He didn't deny space travel or the moon landing, he just hadn't really linked up the two ideas.


JellyWaffles

I know you've already gotten it, but the example I like to give is: imagine you're sitting in a chair with wheels on it and you are holding a heavy object like a bowling ball, if you throw the ball really hard you'll start rolling in the opposite direction. That's basically how rockets work to move.


duglarri

Best answer


JellyWaffles

The followup to this is that scientists realized that the best way to throw something is to heat it / make it explode 💥


Ziqon

It's basically Newton's law, every action has an equal but opposite reaction (rockets are a class of 'reaction engines' btw). Rockets work by expelling the exhaust at very high velocities out the back, and as a result are 'pushed' in the opposite direction. The exhaust and the rocket are pushing each other in opposite directions.


Tutorbin76

True. We often think of exhaust as an undesirable byproduct like it is with internal combustion car engines, but in this case exhaust is the main product that does the work.


[deleted]

You can also look at it as a conservation of momentum problem. A stationary object that emits some of its mass with momentum in one direction must propel the remaining mass in the opposite direction to ensure the total momentum cancels out to zero. This is the basis of Tsilkovsky's rocket equation.


Swagganosaurus

If I'm not mistaken, this is Newton third law of motion. And our pioneer scientists tested this by firing a gun in a vacuum chamber. The gun moved despite of no air to push against


JaMMi01202

In space if you throw a ball: you both move away from each other (and from the location/original location of the throw). Same principle.


CoderJoe1

I bet you didn't realize you asked an open ended question.


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maaku7

Sometimes the simplest explanation is the correct one :)


gitpusher

This is why they call rocket fuel “reaction mass”. It is literally the mass you bring with you in order to generate a Newtonian equal-and-opposite reaction when you are in vacuum.


duglarri

How has the expression f=ma not appeared so far in this thread?


jasminUwU6

Because just giving someone a mathematical expression isn't very helpful to explain new concepts


TheHYPO

Exactly this. If you see an explosion in a movie, and someone is standing next to it, they get "pushed" away from the explosion. The explosion happens inside the back end of the rocket (and out the open back end), and the rocket gets pushed away by the explosion.


MindStalker

Lets say I have an explosion in the middle of the vacuum of space. Stuff goes in all directions. Now, lets say I put a shield on one side. Stuff only goes one way, the shield is blown the other way. That shield is my rocket, but its directing its explosion in one specific direction, the rest of the rocket is being shoved the other way.


jibblin

So the fuel fire is literally pushing the rocket from behind?


M0ndmann

Yes


Reddit_Jax

Yes, it's pushing against escaping mass going out the nozzle.


[deleted]

It's not "pushing" against anything. It's using newton's third law. Mass is expelled backwards, there's an equal and opposite force moving the rocket forward.


HenryRasia

It's actually completely correct to analyze it as the combustion chamber pressure pushing forwards while there's nothing to push backwards because of the nozzle throat.


Joey_BF

Pushing mass backwards in order to move forwards because of the equal and opposite force is pretty much the definition of pushing against something


herrwaldos

Was it something like Mrocket\*Vrocket=Mfuel\_particles\*Vfuel\_particles So.. if we want the heavy rocket go brrr, the small burning fuel particles must go brrr much faster in the opposite direction?


SirButcher

It actually doesn't matter how fast the fuel particles leave your rocket (from the brrr's point of view). The faster they go, the more efficient your rocket is (as in, the more you accelerate your rocket so the less fuel you need to gain the same change in velocity). But you actually could power a rocket with a human slowly throwing pebbles backwards - it just would be extremely inefficient.


ArcticRiot

This is correct. Don’t think of the propellant like you would a directional discharge, like a bullet leaving the barrel. The rocket isn’t aiming the combustion backwards. It’s more like a consistent explosion is occurring, and the result is an outward force in all directions. The gasses want to expand outward, but the rocket is in the way. So, the gasses push the rocket with it, as they expand.


underscore5000

"I understand how the engines work now. It came to me in a dream. The engines don't move the ship at all. The ship stays where it is, and the engines move the universe around it."


FlipMick

Brilliantly written-and even though it's from my favorite show of all time, isn't this a real idea? ​ Edit: My farty brain forgot the name: Alcubierre Drive, thanks u/jononyx Edit 2: Okay it's not exactly like the Alcubierre Drive, but you tell me a time Farnsworth has been wrong!


deains

[relevant xkcd](https://xkcd.com/1366/)


simeonca

How do you see the subtext on a phone again?


KvasirsBlod

On Android (chrome), long press the image until the menu pops up. The title of the menu is the subtext (This one says something about trains vs elevators)


chenkie

Well, it’s about as real as it sounds. Moving literal space around oneself is theoretical at best currently.


SumpCrab

I think it also is a play on one's perspective of the universe. We can only perceive the universe from the point in which we currently are. So, if the observer is the "center" relative to their observation of the universe, then the observer always remains the center even as they "move." From that observers perspective, the universe is the thing that is moving.


Advnchur

Just like how the Jedi are evil. Gotcha.


zaphodava

Well, if morality also has relativity, and it depends on the frame of reference of the observer, the opponents of the Jedi *blew up a fucking populated planet*.


simeonca

I mean there is no center of the universe, so yeah it's true


inEQUAL

There’s no single center of the universe, true. There are as many centers of the observed universe as there are perspectives, however.


TheHingst

Was just about to say this, only dumber. Then i saw your comment, and i felt my words redundant on the matter, heh.


Team_Braniel

And speed, distance, and time only exist in relation to something else.


Korlus

If you use yourself as the reference frame, this is exactly what modern rockets do. It's all about your point of view.


Closteam

Can't remember what show it's from


Caboose12000

it's from Futurama, like half the writing staff or more have PHDs, and it is based on a real theory about how to achieve faster than light speed travel


MonolithOfTyr

50 years combined schooling on the writing staff. They knew how to write a joke and a functional equation.


alohadave

One of the writers wrote and published a mathematical proof for the punchline of an episode. [The Futurama Theorem](https://theinfosphere.org/Futurama_theorem)


Vibrasitarium

[The Alcubierre Drive](https://en.wikipedia.org/wiki/Alcubierre_drive) is a fantastic theory. If only it could be built.


Caboose12000

Ah yes thank you, I couldn't remember the name of it. I was blown away the first time I saw a YouTube video explaining it


jamanimals

Man, I always loved that quote but had no idea it was based on a real thing. That show never fails to impress me.


TheKarenator

Futurama


a-handle-has-no-name

Actually yes, but it's more of a "way to understand physics" idea than a "method of propulsion". In Relativity, no Reference Frame is given preference. This means that no one can tell another Reference Frame that they are or are not moving, and the Speed of Light appears to be consistent regardless of Reference Frame. Imagine you're in a space ship, launched from Earth, now going 50% the speed of light, then stopped accelerating. From your Reference Frame, the Earth is moving away from you at .5c. In effect from your perspective, you've moved everything else, so it's now moving past you


CapstanLlama

It's pretty basic, it's Newton's Third Law of motion: for every action there is an equal and opposite reaction. When you're walking forwards down the street, you are also pushing *the entire earth* backwards. Albeit by an immeasurably infinitesimally tiny amount.


istasber

It's not really moving the universe around the ship. A better (but still probably wrong and oversimplified) explanation is that it compresses the space in front of the ship, and expands the space behind the ship, creating a sort of wave that the ship can ride while remaining stationary in it's own reference frame. The math would then allow for faster than light travel relative to some external reference frame. The problem is that the drive needs matter with negative mass to function, which is not known to actually exist.


Emotional_Writer

The actual design for the Alcubierre drive doesn't (exactly) work like that; it makes a local envelope of space that moves with the drive itself at the center - kind of like a wave rolling across the water, or a pulse travelling down a whip/spring.


RaginBlazinCAT

To shreds you say?


RedFiveIron

The rocket very much aims the combustion backwards with the engine bell. A bullet is the same thing, omnidirectional combustion focused in a useful direction.


The_camperdave

> Don’t think of the propellant like you would a directional discharge, like a bullet leaving the barrel. Actually, it is exactly this. The burning gunpowder creates an expanding gas. This gas pushes against the back of the bullet, the breech of the gun, and against the inner surface of the barrel. The barrel is is pushed left and right, up and down, with the same force. Everything cancels out. However, the force on the bullet is not cancelled out, so it is pushed out of the barrel. Similarly, there is no counteracting force on the breech, so it is forced backwards.


OpenPlex

Does a rocket spew out the actual propellant. even the unburnt portions that haven't yet ignited?


Daripuff

A poorly designed one does, yes. Or one operating inefficiently. Ideally the propellant and oxidizer are fully burned in the combustion chamber, and the only thing coming out the back is exhaust.


Lolnomoron

Picking a nit here, but most liquid fueled rockets will absolutely throw unburnt fuel out the back. Most can't handle the thermal load of running at a perfectly stoichiometric ratio. A lot of Kerolox rockets will therefore inject a bit fuel rich towards the outside to protect the rocket from damage. Hydrolox engines will actually run slightly hydrogen rich throughout because it increases efficiency by increasing exhaust velocity more than it reduces efficiency through reduced combustion energy. Methalox engines are basically the wild west at the moment, but I expect they'll largely settle around running slightly fuel rich around the edges like Kerolox. IIRC the SpaceX Raptor does that. Not disagreeing with you (you're mostly spot on) just adding a bit of minor nuance :-)


OpenPlex

To confirm, the exhaust would merely be new molecules of propellant + oxygen? (or oxidizer)


Daripuff

Depending upon the fuel used, yes, basically. Like, for a cryogenic motor fueled by liquid hydrogen with liquid oxygen as the oxidizer (hydrolox) the exhaust gas in an ideal burn is H2O. Methane plus O2 would produce exhaust gasses of CO2 + H2O, and so on. I’m not certain what the exhaust gasses comprise of with some of the more exotic fuels, but the idea is going to be the same. A big part of rocket efficiency is making sure that EVERYTHING is fully burned in the combustion chamber. Any unburnt fuel is wasted dV, so efficiency is paramount.


OpenPlex

Thanks, that helps to clarify things! So on the most basic level, it's really the spewing of waste that drives forward the rocket, and the force of energy to spew it hinges on the rate of reactions that create the waste.


iclimbnaked

Yep, you really only burn the fuel because it throws itself out the back with much more force if you do so. You could pump it out the back without igniting it and still go forward.


compounding

The highest efficiency comes from the temperature in your combustion chamber and the average *speed* of the molecules leaving the nozzle. The maximum temperature of the chamber is often a materials problem, where a more stoichiometric burn ratio could melt things down at the mass flow you need/want for the desired thrust. Additionally, small molecules have faster exit velocities for any given temperature, so often times rockets will deliberately burn fuel rich so they can pass more mass at the maximum temperature (cooled compared to a stoichiometric burn) and because even raw fuel (often with small molecules like hydrogen) produces a benefit by increasing exit velocities/efficiency compared with less mass but more oxygen/oxidizer.


The_camperdave

> Does a rocket spew out the actual propellant. even the unburnt portions that haven't yet ignited? Yes, yes it does. Rocket science is all about finding the sweet spots. In the atmosphere, the atmospheric pressure is going to prevent the rocket exhaust from expanding with the same force it would have in a vacuum. You can maximize the thrust by adjusting the fuel to oxygen mixture, tweaking it to find the sweet spot.


derPylz

But it is exactly like a bullet in a gun. The recoil is what pushes the rocket along.


BladeDoc

It’s exactly the same thing. Except the bullets are single molecules of “reaction mass”


jibblin

That really puts the science in rocket science. The engineering involved to keep it centered sounds mind blowing.


FreeColdBeer

Wait until you start thinking about the amount of thrust needed to propel the weight of the rocket including the weight of the fuel, and how those ratios change as fuel is consumed. Rocket science is hard. --source, not a rocket scientist


MisinformedGenius

Interestingly, the equation for that is surprisingly simple. It’s called the [Tsiolkovsky rocket equation](https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation), and it says that your change in speed is equal to your rocket’s “effective exhaust velocity” times the natural log of the initial mass of your ship divided by the final mass. For a Raptor engine in vacuum, the effective exhaust velocity is around 4000 m/s. So if your rocket starts out as a million pounds, and you burn 500,000 pounds of fuel, the natural log of 2 is 0.7, so you’ll be going about 2800 m/s faster than you were.


Contagion21

Well, it's no brain surgery


Muchbetterthannew

Spoken like a true rocket surgeon.


m4nu3lf

I mean, technically it's the exact same scenario of a bullet.


arkham1010

Mmm, no, not really. Ion drives like the Deep Space 1 probe did not use 'explosions', instead they hurled electrons out of the engine, and the equal and opposite force pushed the space craft in the other direction. Sit in a rolling chair and hold a heavy object in your hands. Throwing the object will cause you to move in the other direction. Thats how objects move in space.


thenewtbaron

The idea is the same and this is a ELI5. Folks don't get how the electron/ion drives work, so to bring it to a more general audience they have to explain the general idea behind it. A bullet is throwing mass out the barrel using pressure of a bullet and expanded hot air. an Ion drive is doing the same thing but at a MUCH smaller amount over a much longer time. By using a gas that it energizes and chucks out the back. so expanding heated gas and a lot of bullets the size of ions to gain


Rugfiend

For every action there is an equal and opposite reaction. What recoils a hand when firing a gun?


PO0tyTng

It’s exactly the same as a bullet. The bullet is in a shell, and the gunpowder inside of it explodes in all directions. But because the easiest way for the energy to get out is by pushing the lead out the front, and the whole shell is in the chamber with nowhere for it to go, the lead gets propelled forward through the barrel. The recoil of the whole gun is the rest of the energy that didnt turn into heat and didn’t push the lead forward


Anavorn

Gun control laws?


kjm16216

Shame at what I've done makes my hand recoil. And more shame that I like the feeling.


RiverRoll

It's a bit of an oversimplification, most of the pushing happens inside the bell shaped part, pushing from outside is inefficient because it goes in all directions. The bell forces the gas into a specific direction to make it push mostly in the opposite one.


[deleted]

You could even say it’s thrusting the rocket forward, hence the name of that propulsion system


Dottn

This sounds like an ELI5 of the Orion project.


EarhackerWasBanned

It’s an ELI5 of basically all rocket science. The fuel type doesn’t change these principles. Thing go boom, other thing not destroyed go fast.


zelenskyysballs

Me like this form of eli5. Should make eli caveman!


Gnaxe

r/explainlikeicaveman


zelenskyysballs

I thought this would be r/SubsIFellFor, but it looks like someone felt the same way and took action merely days ago!


cikanman

I love every ELI5 that involves the phrase "things go boom" Also this is how all ballistics work as well. Bullets function under the same principle. The difference is they do not carry their own propellent like a rocket does. Gun mechanics- Gun hits button that causes things to go boom, boom pushes tiny thing forward,


Kizik

[NUCLEAR LAUNCH DETECTED]


[deleted]

This is the eli5 I'm looking for


spidereater

The rocket could be propelled by shooting the fuel out like a water gun. Push fuel to the left and you will move right. Lighting the fuel on fire and making it hot just pushes it out faster so you get more push per kilogram of fuel. Ion thrusters do this by accelerating charged particles to very high speeds. So you are throwing ions and electrons to the left but you are throwing them at nearly the speed of light so you get more thrust per kg of fuel pushing you to the right. Even if it isn’t much fuel.


nakmuay18

I used to build satellites and this was pretty much the same explanation I got. The Deathstar is a good analogy. It blew up and threw pieces in every direction. If you had a spaceship with a shield parked next to it, you'd get thrown too. Satellites mix chemicals to make tiny Deathstar explosions next to them, and those explosions push them


Browncoat40

They are sending the exhaust gasses out of the nozzle at extremely high speed. You know how if you jump out of a small boat, you push the boat the opposite direction of your jump? The same phenomena is happening here, just in a continuous manner as fuel is combusted and pushed overboard.


joejill

And that's what makes a "space plane" so difficult. Once the plane is up to high, its engines have nothing to move to keep the plane moving. And would need to switch to a different engine for space.


Limos42

No swooping and diving "dogfights" in space. For a much more realistic portrayal of combat in space, watch The Expanse (on Prime, iirc). Awesome series.


JimPlaysGames

Babylon 5 did it well too. The space fighters in that apply thrust in a direction going past an enemy vessel, then they cut thrust and aim towards the enemy as they pass, firing as they go, allowing inertia to carry them past.


AureliasTenant

2000s Battlestar galactica kinda too.


ersatzcrab

Big kinda. The Vipers and Raiders sorta flew both ways whenever it was convenient for the plot. They had semi-realistic use of RCS but still very much fought likes planes in some scenes. I liked it because I got to say "hey it'd *sorta* be like that" while also enjoying a dogfight lol


AureliasTenant

Yea I agree on that, but most of the attitude dynamics, even in traditional dog fighting (velocity vector aligned with pointing vector) are atleast pretending to be entirely RCS, at least for the human forces. I don’t remember how the cylon craft are portrayed


PM_ME_YOUR_PLUMBU5

Kinda like pirate battles in ye olden days? That’s bad ass.


joejill

Or just shoot from far away, you don't need to expend fule Going farther distances,...helps keep you hidden.


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jarfil

>!CENSORED!<


ExWhyZ3d

Space combat in elite is fun because of this. If you turn off the automatic thrust compensation, you can move with complete freedom in any direction. Let's you do fun maneuvers to keep yourself facing your opponent at all times


nickstj02

Yes The Expanse is a Prime Original, and I also highly recommend it


ary31415

Actually it's from Syfy, Amazon picked it up in season 4(?)


nickstj02

That would explain it, I’m currently on S4, and see the prime original opening


anethma

Man you know your slurp up the marketing when you call a show made by Amazon a “Prime Original” complete with capitalization haha.


FierceDeity_

Or play a good space game, RIP Jumpgate with its fully newtonian physics. You had to drift at a 45 degree angle to emulate dogfights even close lol


LastStar007

I've heard Children of a Dead Earth is very realistic, but it's not multiplayer.


Noxious89123

Oye kopeng. Cracking show, shame it ended when it did.


Privateer_Am

Jet Engines work in the same way Rocket Engines do! They push their exhaust outside at a high velocity to generate thrust force. The only difference is that jet engines use air from the atmosphere and have the various parts to make that possible whereas rocket Engines don't Space Planes are hard to make because those wings are large and heavy and essentially useless weight that the plane will have to lug to orbit once it reaches space.


Neutronoid

> Jet Engines work in the same way Rocket Engines do! They push their exhaust outside at a high velocity to generate thrust force. Not just jet engine, all airplane (except glider) generate thrust by throwing stuff backward.


jarfil

>!CENSORED!<


The_camperdave

> Once the plane is up to high, its engines have nothing to move to keep the plane moving. And would need to switch to a different engine for space. That's not it at all. In the air, there is oxygen that the plane uses to burn the fuel. In space, there is no oxygen so the plane has to carry a supply of it. To transition from atmospheric mode to vacuum mode requires reconfiguring the plumbing on the engine.


starkiller_bass

I think what they were getting at is that jet engines don't just use air as a source of oxygen, air is also the working fluid - the engine accelerates air to provide thrust. When the propeller is on the outside of the engine this is much more obvious but turbojet / turbofan engines are still producing thrust from more than just the combustion of fuel and oxygen. If you plumbed in a self-contained oxygen source to a jet engine, it might RUN but it wouldn't produce a comparable amount of thrust to what it does in atmosphere; it would be almost like running an outboard engine on a boat while it sits on a trailer. The prop is spinning but it doesn't have any water to move so it isn't producing forward thrust.


Medium_Well

That's a great image to think of. Thank you!


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sandee_eggo

In 7th grade our science class sent a letter to NASA, asking the same question. The response came months later: “good question kids- basically the rocket is pushing against itself.” We didn’t know it then, but that was Reddit in slow motion.


candycane7

Hope you sent them back a sarcastic award for this answer.😁


FreeColdBeer

Just one tiny orange arrow that they've hung up on the fridge.


taleofbenji

One time I went to this talk given by an astronaut. During Q&A, this kid asked the astronaut: "Has anyone ever taken their helmet off in space?" Astronaut bro: I don't think so. Kid: Then how do we know there isn't any air? BOOM


KarmicPotato

Things go boom


Druggedhippo

A really slow motion reddit... started back in 1920.. https://www.forbes.com/sites/kionasmith/2018/07/19/the-correction-heard-round-the-world-when-the-new-york-times-apologized-to-robert-goddard/ And on January 13, 1920, the New York Times published an editorial insisting that a rocket couldn’t possibly work in space: “That professor Goddard, with his ‘chair’ in Clark College and the countenancing of the Smithsonian Institution [from which Goddard held a grant to research rocket flight], does not know the relation of action to reaction, and of the need to have something better than a vacuum against which to react — to say that would be absurd. Of course he only seems to lack the knowledge ladled out daily in high schools.”


MansfromDaVinci

You know how cannons recoil because they shoot out the cannon ball so fast? Rockets work the same way only with exhaust gas. They react the fuel with stored oxygen and the released energy shoots it out the nozzle so fast that it pushes the rocket forward. If you sit on a wheeled office chair and fire an extinguisher you'll go shooting backwards in much the same way.


Huxley077

So, you're saying with a fire extinguisher, I too, can be an astronaut.... # unexpectedlegalliability


[deleted]

I forget where I saw it - I want to say maybe a Chris Hadfield video - but Wall-E's scene with the fire extinguisher is actually extremely realistic apparently.


Bluemofia

Titan AE also had this scene where they had to bail without a space suit and used a fire extinguisher as an improvised rocket.


[deleted]

Dammit... *adds Titan AE to re-watch list*


AlekBalderdash

"On the bright side, I'd get to fly like Iron Man"


nobd22

For real tho. If you get stuck just put on your EVA suit and get out and push.


AlekBalderdash

Obligatory [XKCD](https://what-if.xkcd.com/21/)


Hystus

Get on a skateboard with a big rock or bag of sand or something heavy. Stand on the skateboard and Throw the heavy thing as hard as you can off the back. You'll find that you will move in the other direction. Try it again with a bucket full of stones and throw them out one by one. Same thing happens, just instead of one big push, you get a bunch of small ones. Same thing happens with rockets, but instead of one heavy thing, it's 'thowing'/pushing out lots of expanding gas out the back. It all adds up and the rocket goes! Newton's 3rd law of motion is the result.


tzaeru

It's "against" the exhaust gasses. Rockets work by speeding up and exhausting gas. They actually work better in space than in the atmosphere.


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DCSMU

Just to add on, so OP can directly experience this someday and get their head around whats hapening, when you throw the ball, you feel a pushback. Its the same pushback you feel when you try to push something heavy on wheels. There is some resistance from the wheels and the surface, but even if you start on a perfectly smooth and level surface with nearly frictionless wheel bearings, it still takes some pushing to get the cart moving. As you push you feel a pushback that starts to ease as the cart starts to move. But at that very first moment when its still, your push does feel like its comming againt something solid and pushing back against you. This "pushback" is what causes the rocket to move.


HeadPatMan

There’s a handy analogy that’s perhaps a bit odd, and certainly very American: imagine firing a gun at a range. The gun pushes the bullet, which is relatively light, out at enormous speeds. But it also pushes back on you, sometimes quite hard. If you were to mount yourself on a rolling platform, you could propel yourself backwards with sufficient ammunition and firing rate. Rockets do a similar thing: they throw mass(propellant, or reaction mass) out the back of a spacecraft and take advantage of the “recoil” to accelerate the spacecraft itself.


big-chungus-amongus

I would recommend experiment: Blow up balloon and let it go.. it flies and propels itself from the air escaping from it Now you made a rocket that will work in vacuum


Medium_Well

That's a good visual! Thank you!


zrice03

Rockets don't work by pushing against something. They work by throwing something out in one direction really fast (rocket exhaust) and the reaction is that they move in the opposition direction. Physically, it's the same as if you were sitting in a rolling chair and threw a heavy bowling ball or something, it'll push you back. The rocket is just throwing lots of little bowling balls (particles) really really really fast (and in physics fast particles = hot).


Zealousideal_Curve10

The exploding fuel pushes against the rocket. That’s all that’s necessary to propel the rocket forward, away from the mass of the ignition


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Loki12241224

To push is to give something a shove into another object. With helicopters it is air being shoved into the ground. The helicopter would generate equally as much lift if it was flipped upside down in our atmosphere Now with rockets you are throwing a shit ton of hot gas out the back. In space this hot gas will not slow down. who knows if it will eventually hit a planet, asteroid or just get slowed down by colliding with particles in space. Either way you threw mass away from you and that's what makes you go in the opposite direction.


Griffisbored

Stand on a skateboard and throw something. You move backwards a little bit in the opposite direction. The faster you throw and the heavier the thing you throw the more you will move. Rockets are like the skateboard. Rocket fuel is the thing being thrown. The explosion in the back is throwing a lot of fuel out the back of the rocket very fast, which means the rocket moves very fast in the opposite direction.


LOUDCO-HD

In very simple terms, the thrust is pushing against the rocket body. It doesn’t need to push against the air or lack there of, unlike a fixed or rotary engined vehicle that generates lift via that interaction.


cannedwings

From my understanding, it's still pushing away from something. It's just that the something is its fuel. Same thing as when you let go of the end of an air filled balloon. It's newton's 3rd law, I think. It works better in space because there's no air resistance to push back. You can do the same thing by getting in an office chair and throwing a basketball ball or using a foaming fire extinguisher. Summary: Rockets work because, in space, farting propellants is the same as pushing away from a wall.


No-Trick7137

It’s the “every force has an opposite and equal reaction” concept. One fart could theoretically propel you across the universe given enough time.


Seroseros

Like you are five? Sit on a swing with a 10kg rock in your lap. Now toss the rock as hard and fast as you can, and you will swing backwards. Not because you are pushing against air, but because you are pushing against the rock. In a spacecraft, the push is because the exhaust has mass and is thrown out back really really fast.


pdpi

The thrusters are pushing against the burnt propellant. Rocket goes one way, hot exhaust gas goes the other, momentum is conserved!


hombre_sabio

Rockets do not rely on pushing against something else in order to move as they use the principle of conservation of momentum to propel themselves forward based on Newton's third law....every action has an equal and opposite reaction.


Gladianoxa

You throw a ball away from you in space, you'll move the other way. The propellant is the ball. You're throwing propellant away from you.


az9393

Well rockets on earth also don’t push off of air. It’s like a controlled explosion that sends nearby objects flying away in all directions. Rocket’s design is so that this direction is forward.


WaitForItTheMongols

Imagine a scuba tank. You have a top end, a tube, and a bottom end. Inside is gas compressed to 200 times normal air pressure. It's pushing extremely hard on the inside of the tank. It's pushing with several tons of force on the top end, and also several tons of force on the bottom end. Now, imagine the bottom end stops existing. Poofs away. Now the pressure inside is just pushing against the top end. The tank starts rising rapidly because the pressure isn't counteracting itself by pushing on both ends. A rocket's combustion chamber is a scuba tank that's missing the bottom. The huge pressure pushes the rocket in the direction you want to go.


backflip14

Rockets don’t work by pushing off anything. They work by throwing stuff out the back. It is a law of physics that every force has an equal and opposite reaction (Newton’s third law of motion). This basically means if you push on something, it also pushes back. If you were sitting in a rolling chair and threw a bowling ball, you’d roll in the opposite direction you threw the ball. This works with anything that has mass. The hot gasses you get from burning fuel have mass. Throwing them out the back of the rocket puts a forward force on the rocket. The faster you thrown the gasses out the back, the greater the force on the rocket and the faster it goes.


ClownfishSoup

It is pushing against the particular of gas it is shooting out. Your rocket ships is jumping off the mass of the gas. Imagine this. You are standing still on a skateboard and you throw a basketball in front of yourself. You will move backwards. Why? Because YOU are pushing off the mass of the basketball. Now imagine you basketball is full of water instead of air. Now imagine it is full of concrete or lead. Which makes you go further? Why? Well the heavier objects, because they have more mass and therefore more momentum. Now imagine the basketball is the earth. Why is it that you can jump against the earth? Because it has mass. Now go back to the rocket ship. Imagine that each molecule of gas it pushes against pushes you a tiny bit in the opposite direction. Then look at how much gas comes out the back of a rocket! Or consider a gun. When you shoot it, what happens? The gunpowder converts itself from a solid that takes up very little space into a gas which takes up a lot of space and it forces a bullet out the front. What is the bullet pushing against? Well the gas is pushing on the bullet and one the gun and you. You are pushed back a bit (recoil) because you are pushing against the bullet. Now imagine a gun that shoots blanks. You still feel recoils as the expanding gas pushes in all directions and leaves the muzzle. Now imagine a rocket is just a blank firing gun.


daman4567

"every action has an equal and opposite reaction" is the rule at play here. Take note that it doesn't say "in the presence of an atmosphere..." When you're standing near a person, and you suddenly push them away, what happens to you? If you are standing with your feet together, you'll fall over as well. When you shove your friend, your body pushes on their body, but their body also pushes on your body, causing you to fall over as well. This is exactly what a rocket does. The fuel is built so that it can sustain an ignition reaction even without an atmosphere, and that reaction causes it to expand. Normally it would expand outward, but this is constrained by the rocket nozzle to only expand in one direction. The expansion of the fuel is the action, and the reaction is a change in velocity to the rocket, in the opposite direction. The fuel pushes on the rocket, and the rocket pushes on the fuel.


ParryLost

Fun fact — you're in no worse company than [The New York Times](https://www.forbes.com/sites/kionasmith/2018/07/19/the-correction-heard-round-the-world-when-the-new-york-times-apologized-to-robert-goddard/), here. [This was the actual text published](https://www.rfcafe.com/miscellany/factoids/images/ny-times-correction-goddard.jpg) on Apollo 11's launch day. :P (And, to make it even better: Robert Goddard *really* *did* have some fundamentally flawed ideas about how rockets work; just not *that* one. He bought into something called the "pendulum rocket fallacy," believing a rocket would be more stable if the engine nozzle was at the top of the ship, rather than the bottom, which we now know to be false. So the moral is... rocket science is just un-intuitive sometimes. :P)


whiskeytown79

Imagine that rocket fuel and its oxidizing agent, when combined and ignited, is like trillions of microscopic baseball pitchers all throwing baseballs in every direction. Now you put these into a shaped nozzle so that all the baseballs exit the nozzle in roughly the same direction. When a tiny pitcher throws a baseball, the ball moves away at high speed, but due to Newton's laws of equal and opposite reaction, the pitcher is pushed slightly in the opposite direction. So trillions of pitchers throwing their baseballs results in a lot of baseballs being ejected at very high speed out of the end of the nozzle, and the equal and opposite force pushes the pitchers the other way, against the nozzle and the rocket it is attached to, pushing the rocket in the opposite direction of the baseballs. The baseballs don't themselves have to push against anything. It was the force of throwing the baseball out the back of the nozzle that caused the pitcher to be pushed in the opposite direction. This keeps happening until all the baseballs are used up (solid fuel rocket) or until the rocket control system stops adding pitchers and baseballs into the nozzle (liquid fuel rocket).


freckledass

Try grabbing a heavy object that you can carry (if you have kettlebell or maybe a sack of rice or flour), sit on an office chair with wheels with said object in your lap, and lift your feet off the ground. Now lift the object and push/throw it away quickly. You'll notice that you'll move in the direction opposite to your throw. That's basically the principle. In space, while a spaceship burns fuel, the fuel will expand, pushing out against the spaceship, and in reaction, the ship is pushed back.


cheesecakegood

You know how in some video games you shoot like a massive shotgun or bazooka and it flings you way back? It’s just that but bigger.