Let’s talk about “Vacuum Thrust”. Simply put, thrust that is exerted in a vacuum. Space is a vaccum correct? So therefore there is no matter is space, it is just empty space. Thrust is measure in pounds or kgs. So basically an engine that is producing a thrust pushing force of 200 pounds means that it is exerting an opposite force of the same weight.

Now I’m no Einstein but “every action has an opposite and equal reaction”. Once a space shuttle, or rocket is propelled into space let’s say it docks with the ISS for supply transfer or fuel. (I only added the stop so there are no arguments that the said object already had forward momentum and that is how it is propelled forward). Fuel docking or equipment transfer complete! Great! Now we head back home or move on to next destination. How? For thrust to work you need an opposite reaction pushing against the object to achieve movement. But space is a vaccum? Empty space with no matter? If you accidentally went spiraling through space in a space suit and started flailing your arms and legs around hoping to stop yourself, you’d better just hope a little more.

So how is it possible for the propulsion of “rockets” “spacecraft” through space if there is no matter to push against? I guess thrust is a picky choosy thing and wants to defy Newton’s laws lol.

P. S. Again, the point of my questions aren’t to get other theories. The point of my question is to point out that all we can go by are “theories” and “educated guesses”. The speed of sound at sea level is 700+ because it has been measured and proven to be at that speed. And if I am wrong in any of my question in question lol I am a young mind and would love to be corrected!

  • $\begingroup$ Remember that thrusters expel lots of fast-moving vapor. This is what is being pushed against. $\endgroup$ – probably_someone Nov 23 '18 at 19:03
  • $\begingroup$ @probably_someone Okay makes sense. But I must be thinking deeper than that. So you’re saying that the propelling forward is being caused by the ejected vapors due to the burning of fuel? Not saying you’re wrong but stick with me here. What are the expelled vapors pushing against in order for the thrust to push against? Idk if I’m asking it correctly or not explaining it right. Basically I’m saying if the vapors are acting like a wall to push against, what is supporting that wall? $\endgroup$ – Begons18 Nov 23 '18 at 19:15
  • $\begingroup$ @BrandonGonzales They're not "pushing" against anything. The expelled "vapors" (not a great term - "gas" is more accurate) just go straight on out to infinity at a constant velocity. (Unless something else intervenes - but even if it does, that's irrelevant for the rocket propulsion.) The "wall" moves, and that's OK. $\endgroup$ – Emilio Pisanty Nov 23 '18 at 19:21
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    $\begingroup$ The vapours push against the rocket, that's why it moves. $\endgroup$ – PhysicsDave Nov 23 '18 at 19:31
  • $\begingroup$ @Emilio Pisanty Yes sir. That sounds pretty reasonable. But that my friend would be a contradiction. If what is said about the “gas” (sorry that one was hard to say and spell out) is true in the comments above then they are in fact “pushing” (I guess I need a smarter word for that one) on something. And that something would be each other. The “gas” thrust “pushing” on the object in question. $\endgroup$ – Begons18 Nov 23 '18 at 19:32

Rockets operating outside the atmosphere don't "push" on the vacuum - there is no such thing. Instead, they operate on the very principle that you mention,

every action has an opposite and equal reaction,

by expelling propellant at high speed out of the engine nozzle. And as a consequence of Newton's Third Law, the action of pushing the propellant out through the engine nozzle implies an equal and opposite reaction, which is to confer thrust to the spacecraft.

Nevertheless, since the system (spacecraft + expelled propellant) is an isolated system and subject to Newton's Third Law, its combined centre of mass remains (i) at zero velocity (if we choose a frame of reference in which it started that way), and (ii) at the position it started with (in that frame of reference). This is possible because the spacecraft's motion is offset by an even faster motion by the (smaller) mass of the propellant.

If you do want to analyze things in terms of 'walls', consider a $100\:\rm kg$ astronaut floating in space holding on to a $100\:\rm kg$ section of brick wall, and suppose the astronaut pushes on the brick wall with all their might (say, some $100\:\rm N$ for $1\:\rm s$), so that the force of the push accelerates her away from the wall, at $1\:\rm m/s$.

But, as you said, the wall has nothing to support it, and it's just been pushed with $100\:\rm N$ of thrust, so the wall section also accelerates at $1\:\rm m/s^2$ for $1\:\rm s$, and it also ends up going away from its initial position at $1\:\rm m/s$.

It's the same deal with rocket engines, though the "wall" is lighter (so it moves much faster) and the "push" is simply the action of the gas pressure on the engine bell. But the fundamentals are the same.

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  • $\begingroup$ Well, looks like “lol” is out of your vocabulary for sure. So I guess I’ll teach you the meaning of that one, “lol” is used when a joke is implied my good sir. “Lol” isn’t something used to state that somebodies theory is incorrect. That being said, you just hit me with some “facts” (I’m guessing) from your choosing of implied tone. So, for us apparent undereducated and non understanding people. I’d like to challenge you to explain further in much simpler terms. I wanna learn something today! $\endgroup$ – Begons18 Nov 23 '18 at 19:24
  • $\begingroup$ Think of yourself floating in space but not moving and you have a baseball in hand. Throw the baseball and you will move opposite to the ball. $\endgroup$ – PhysicsDave Nov 23 '18 at 19:34
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    $\begingroup$ @BrandonGonzales Perhaps you missed the point on that one - the tone of the language you choose matters. The tone implied by your word choice there doesn't convey the impression of someone willing to learn - so consider whether that's the impression you want to present. $\endgroup$ – Emilio Pisanty Nov 23 '18 at 19:38
  • $\begingroup$ @PhysicDave Okay. I guess I’m just thinking too deep into it. But that’s how questions get answered correct? We ask them! Thank you sir. $\endgroup$ – Begons18 Nov 23 '18 at 19:38
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    $\begingroup$ @BrandonGonzales Attacking the folks trying to help you isn't great, either. That comment was meant as a friendly pointer that your tone (not your grammar) is not appropriate for this site; please consider that carefully - but there's really no need for further discussion of that here. $\endgroup$ – Emilio Pisanty Nov 23 '18 at 20:01

Rocket engines do not require an atmosphere to push against. Here is how they operate:

The rocket engine contains within it a source of very high-pressure gas that the rocket scientist can turn on and off. This gas can be stored in a tank with a valve on it or furnished by the exhaust products of a very energetic chemical reaction.

That gas presses hard against the walls of the rocket motor, generating high pressure inside it, but the motor has a nozzle at one end of it through which the gases can escape.

That high pressure accelerates the gases to great velocity in the nozzle. Since the gases have mass, we write one of Newton's Laws as m x a = F which means the pressurized gas inside the motor is exerting a force on the gas leaving the nozzle.

Newton also tells us that forces occur in action-reaction pairs, which means that the action force performing work on the gases flying out the nozzle is accompanied by a reaction force in the opposite direction which is exerted on the motor. We call that reaction force thrust.

Thus the motor is applying thrust to the rocket to which it is attached, which causes the rocket to accelerate.

The atmosphere surrounding the rocket plays no role in the thrust generating mechanism.

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