# Thread: How is thurst in space possible? (hopefully correct place)

1. i know that newtons third law is one object pushing against another object in the oppsite direction but i just am a little confused with the fact that theres no air pressure and no gravity (or very little that is) that the propulsion would have nothing to push on. like how a bird flaps its wings and the air resistance keeps them in the air (mythbusters birds in a truck episode 77) some areo-spiecialist something arather tried to explain it as:

This is sort of a common misconception - that thrust is created by pushing against the air surrounding the engine. I like to explain it in terms of a simple 4 wheeled cart standing on a flat surface.

If you stand on the cart while it is just sitting there, and then throw a bowling ball off the back of the cart, the cart will move forward. The bowling ball is not pushing against the air. But when you toss it one way, the cart will move the other way. If you continue to toss bowling balls, the cart will continue to accelerate. A rocket engine works in a similar manner. Think of all those hot gas molecules coming out the exhaust as little bowling balls. They aren't very heavy, but there are a lot of them and they are moving really fast. They are not pushing against anything, except the vehicle that they are leaving.

but with no gravity there would be no shift in g-forces between the cart and the mass inside it and throwing a weightless bowling ball would do nothing right?? then this leads me to ask can you even create g-forces without gravity? i read somewhere they invented a gravity simulator for space travel that spins the astronaut in circles to create g-forces but if the astronaut is weightless wouldnt this just make him nausious and dizzy lol? anyways could just be a pipe theory but i dunno couldnt grasp the concept. this leads me to think that until we discover that gravity is a form of energy like all matter space travel is impossible...

i could see two objects pushing off of eachother in space like two space crafts having the rockets face each other and go in opisite directions but then what the pressure in the engines would have nothing else to push off, you can blow all the air in your lungs and space and not go anywhere and you could flail and kick in all directions but it would do you any good right?

im not saying space travel is impossible with current technology im even willing to admit this is wrong im just open to the possibility its not lol inertia and acelleration could get you into space but (with my theory) it could only propel you into space and then youd continue in one direction which would then say the soviet space dog laika would of just went in a straight line and died of bordom lmao i could see orbiting earth using its gravity and having sattelites and i could see that you could possibly go to the moon by lasoing its gravity if you calculated it perfectly you could even land on the moon and calculate how to get back to earth but i cant seem to understand if its possible to just go into the middle of space and do a couple loop-da-loops and donuts lol then just fix your position and come back to earth without using a gravity grapple laser or something lol thanks for any responses from anybody smarter than me lol (also sorry for the mispellings 3 am here and too tired to give a rat's maze about it lmao)

2.

3. throwing a weightless bowling ball would do nothing right??
This is you basic misconception. When there is no gravity the bowling ball still has mass so there is still a reaction and Newton is still right.

4. okay so its actually an exchange of mass through thrust? hmm interesting i kinda of get it though settled my mind a little bit lol

5. but im just wondering if using gas compression as thrust in space wouldnt the vacuum of space play a factor as its own force of sucking the expansion? wouldnt it just be an exchange of compression into filling the void i could see the exchange of mass as propulsion but wouldnt that be very slow? (sorry if this sounds retarded lol)

6. The propulsion of a rocket comes from its propulsion engine, that launches a lot of gas in one direction and drives the rocket in the other. To launch the gas in one direction, the engine must exert a force on the gas, which is done by its combustion in a suitable system. By exeting a force on the gas, Newtons third law tells us the gas will exert a force of equal strength on the engine, thus pushing the rocket in the other direction.

The vacuum of space will increase this effect, since a vacuum outside the engine compared to say atmospheric pressure, will mean there's a greater net force on the gas. However it is not crucial to the phenomenon.

7. Originally Posted by Maverick Earhart

If you stand on the cart while it is just sitting there, and then throw a bowling ball off the back of the cart, the cart will move forward. The bowling ball is not pushing against the air. But when you toss it one way, the cart will move the other way. If you continue to toss bowling balls, the cart will continue to accelerate. A rocket engine works in a similar manner. Think of all those hot gas molecules coming out the exhaust as little bowling balls. They aren't very heavy, but there are a lot of them and they are moving really fast. They are not pushing against anything, except the vehicle that they are leaving.
You have to understand the concept of inertia. The more massive an object is, the more it resists being accelerated. The way it resists is by pushing back.

So, if you and a bowling ball that weighs 5 kilograms are both stationary, and you decide to push on that bowling ball enough to accelerate it up to 10 kilometers per second (easier to use km per second rather than miles per hour. ) Rather than just give in and freely accept your accelerations, the ball will stubbornly resist, and the way it resists is to push back on you with the same force. So if your mass is 100 kilograms, the bowling ball will push back on you with a force sufficient to accelerate you to 0.5 kilometers per second.

All objects do this.

but with no gravity there would be no shift in g-forces between the cart and the mass inside it and throwing a weightless bowling ball would do nothing right?? then this leads me to ask can you even create g-forces without gravity?
The word g-force sometimes gets used to describe things that have no relationship at all to actual gravity. It's because they are similar to gravity, even though gravity is not causing them.

i read somewhere they invented a gravity simulator for space travel that spins the astronaut in circles to create g-forces but if the astronaut is weightless wouldnt this just make him nausious and dizzy lol? anyways could just be a pipe theory but i dunno couldnt grasp the concept. this leads me to think that until we discover that gravity is a form of energy like all matter space travel is impossible..
It doesn't just spin him around in circles. Have you ever been on the carnival ride, where they strap you against the inner walls of a huge cylinder shaped drum, then start the drum spinning and slowly tilt it sideways? The fun of the ride is that when you're looking down toward the ground, you actually feel no gravity pulling you toward the ground because the centripetal force is pushing you more strongly toward the wall than the ground.

.... I don't know if I'm describing that very well, but most carnivals and fairs have a ride like that one, so if you should just go find one and locate the ride and try it out. The effect that ride uses is the same effect that would be used to create artificial gravity in space. If you really want to understand it, the best way would be to experience it first hand.

8. Why not use a ferro/magnetic fluid electrically excited to rotate in a ring around the space craft and use it's inertia for thrust ? Producing electricity has to be more efficient than dumping matter into space ?

9. Originally Posted by Max Time Taken
use it's inertia for thrust
How exactly? What you describe is an electric motor with internal power generation. You will have a rapidly rotating ring of fluid (contained presumably so that it doesn't fly off into space) and a spaceship rapidly rotating in the opposite sense. How do you convert this into motion along the axis of rotation?

10. Is there a physicist in the house ???

Its simple conservation of momentum.
A rocket and its fuel, at rest ( no thrust ) has zero momentum. When it starts thrusting, it accelerates hot gases rearwards with a given momentum, say X ( vector quantity ). For the system as a whole to stay at zero momentum, the rocket must aquire a momentum of equal but opposite direction, ie -X ( again vector quantity ).

11. Kojax, you have centripetal force confused. It is actually the force that the frame of the ride exerts on your back and pushes you down, otherwise you would continue undisturbed in your ballistic trajectory.
What is commonly known as centrifugial force, which pushes you up, is a fictional force ( doesn't exist ).
If you're studying physics, you should have learned this in second year classical mechanics.

12. Originally Posted by MigL
Is there a physicist in the house ???

Its simple conservation of momentum.
A rocket and its fuel, at rest ( no thrust ) has zero momentum. When it starts thrusting, it accelerates hot gases rearwards with a given momentum, say X ( vector quantity ). For the system as a whole to stay at zero momentum, the rocket must aquire a momentum of equal but opposite direction, ie -X ( again vector quantity ).
Whose post are you responding to?

13. Holy shit- google - howstuff works and STFU

14. Originally Posted by Maverick Earhart
i know that newtons third law is one object pushing against another object in the oppsite direction but i just am a little confused with the fact that theres no air pressure and no gravity (or very little that is) that the propulsion would have nothing to push on. like how a bird flaps its wings and the air resistance keeps them in the air (mythbusters birds in a truck episode 77) some areo-spiecialist something arather tried to explain it as:
This is very good.

If space is empty, what are you pushing against?

This is not logically decidable under current mainstream physics.

15. I am responding to anyone who thinks a rocket's exhaust has to push against something such as air, the ground or any other surface for it to make thrust, Bunbury. I don't think you're in that group judging by your bowling ball comment.

16. Originally Posted by MigL
Is there a physicist in the house ???

Its simple conservation of momentum.
A rocket and its fuel, at rest ( no thrust ) has zero momentum. When it starts thrusting, it accelerates hot gases rearwards with a given momentum, say X ( vector quantity ). For the system as a whole to stay at zero momentum, the rocket must aquire a momentum of equal but opposite direction, ie -X ( again vector quantity ).
You can look at the problem in either of two (equivalent) ways.

Thrust is the net result of integration of pressure over the surface to which it is applied.

Thrust also results from conservation of momentum. It is fairly straightforward at the moment of initial acceleration. Later on it gets more subtle. Once the speed of the rocket (relative to the initial "rest" inertial reference frame) equals and then exceeds the speed of the exhaust (relative to the rocket nozzle) the momentum of the rocket actually decreases, even as the speed increases. Remember that a rocket is a variable mass system. Also remember that, relative to the initial rest frame, the exhaust gasses eventually are going in the same direction as the rocket, only slower.

The final velocity of the rocket, in a vacuum and away from gravitational fields, is

where is the velocity of the exhaust gasses at the nozzle exit plane, relative to the rocket..

17. Originally Posted by chinglu

This is very good.

If space is empty, what are you pushing against?

This is not logically decidable under current mainstream physics.
Please read MigL's post on 5/28 @ 10:27 am:
...Its simple conservation of momentum.
A rocket and its fuel, at rest ( no thrust ) has zero momentum. When it starts thrusting, it accelerates hot gases rearwards with a given momentum, say X ( vector quantity ). For the system as a whole to stay at zero momentum, the rocket must aquire a momentum of equal but opposite direction, ie -X ( again vector quantity ).
Thrust from rocket burns in space are not only "...decidable under current mainstream physics...", but the exact length of thruster burns for desired changes in spacecraft velocities have been routinely pre-calculated and successfully executed for the many spacecraft missions that have been conducted over the past 50 years based on the very principle that MigL describes.

NOTE: "A rocket and its fuel at rest..." in MigL's post describes the inertial frame of reference of the rocket in space - whether it has any motion relative to the Earth or not.

Chris

18. Originally Posted by CSMYTH3025
Originally Posted by chinglu

This is very good.

If space is empty, what are you pushing against?

This is not logically decidable under current mainstream physics.
Please read MigL's post on 5/28 @ 10:27 am:
...Its simple conservation of momentum.
A rocket and its fuel, at rest ( no thrust ) has zero momentum. When it starts thrusting, it accelerates hot gases rearwards with a given momentum, say X ( vector quantity ). For the system as a whole to stay at zero momentum, the rocket must aquire a momentum of equal but opposite direction, ie -X ( again vector quantity ).
Thrust from rocket burns in space are not only "...decidable under current mainstream physics...", but the exact length of thruster burns for desired changes in spacecraft velocities have been routinely pre-calculated and successfully executed for the many spacecraft missions that have been conducted over the past 50 years based on the very principle that MigL describes.

NOTE: "A rocket and its fuel at rest..." in MigL's post describes the inertial frame of reference of the rocket in space - whether it has any motion relative to the Earth or not.

Chris
Right. We understand the law very well.

Specifically why is the law true.

19. Originally Posted by fizzlooney
Holy shit- google - howstuff works and STFU
I see you are maintaining your reputation for polite, informative, relvant posts. Well done! Take the rest of the year off.

20. Originally Posted by chinglu
Originally Posted by CSMYTH3025
Thrust from rocket burns in space are not only "...decidable under current mainstream physics...", but the exact length of thruster burns for desired changes in spacecraft velocities have been routinely pre-calculated and successfully executed for the many spacecraft missions that have been conducted over the past 50 years based on the very principle that MigL describes.

NOTE: "A rocket and its fuel at rest..." in MigL's post describes the inertial frame of reference of the rocket in space - whether it has any motion relative to the Earth or not.

Chris
Right. We understand the law very well.

Specifically why is the law true.
If you're looking for a scientific "why" then I'd say that it comes down to Newton's Second and Third Laws of Motion:
Second law: A body of mass m subject to a net force F undergoes an acceleration a that has the same direction as the force and a magnitude that is directly proportional to the force and inversely proportional to the mass, i.e., F = ma. Alternatively, the total force applied on a body is equal to the time derivative of linear momentum of the body.

Third law: The mutual forces of action and reaction between two bodies are equal, opposite and collinear. This means that whenever a first body exerts a force F on a second body, the second body exerts a force −F on the first body. F and −F are equal in magnitude and opposite in direction. This law is sometimes referred to as the action-reaction law, with F called the "action" and −F the "reaction". The action and the reaction are simultaneous.
(ref. http://en.wikipedia.org/wiki/Newton%27s_laws_of_motion )

Basically, the combustion of fuel in a rocket chamber causes rapid heating and expansion of the combustion products which produces pressure (force) against the walls of the combusion chamber which is directed through the rocket nozzle and which causes the comustion gases to undergo an acceleration in a direction dictated by the orientation of the rocket nozzle opening (First Law).

The force exerted on the expanding and accelerating exhaust gases also exerts an equal and opposite force on the rocket nozzle - which is attached to the spaceship. (Second Law).

If you're looking for a philosophical "why" then I can only offer you the rather overused (but still profound) street-wise phrase uttered by many of the younger folks today: "It is what it is".

Chris

21. Originally Posted by CSMYTH3025
Originally Posted by chinglu
Originally Posted by CSMYTH3025
Thrust from rocket burns in space are not only "...decidable under current mainstream physics...", but the exact length of thruster burns for desired changes in spacecraft velocities have been routinely pre-calculated and successfully executed for the many spacecraft missions that have been conducted over the past 50 years based on the very principle that MigL describes.

NOTE: "A rocket and its fuel at rest..." in MigL's post describes the inertial frame of reference of the rocket in space - whether it has any motion relative to the Earth or not.

Chris
Right. We understand the law very well.

Specifically why is the law true.
If you're looking for a scientific "why" then I'd say that it comes down to Newton's Second and Third Laws of Motion:
Second law: A body of mass m subject to a net force F undergoes an acceleration a that has the same direction as the force and a magnitude that is directly proportional to the force and inversely proportional to the mass, i.e., F = ma. Alternatively, the total force applied on a body is equal to the time derivative of linear momentum of the body.

Third law: The mutual forces of action and reaction between two bodies are equal, opposite and collinear. This means that whenever a first body exerts a force F on a second body, the second body exerts a force −F on the first body. F and −F are equal in magnitude and opposite in direction. This law is sometimes referred to as the action-reaction law, with F called the "action" and −F the "reaction". The action and the reaction are simultaneous.
(ref. http://en.wikipedia.org/wiki/Newton%27s_laws_of_motion )

Basically, the combustion of fuel in a rocket chamber causes rapid heating and expansion of the combustion products which produces pressure (force) against the walls of the combusion chamber which is directed through the rocket nozzle and which causes the comustion gases to undergo an acceleration in a direction dictated by the orientation of the rocket nozzle opening (First Law).

The force exerted on the expanding and accelerating exhaust gases also exerts an equal and opposite force on the rocket nozzle - which is attached to the spaceship. (Second Law).

If you're looking for a philosophical "why" then I can only offer you the rather overused (but still profound) street-wise phrase uttered by many of the younger folks today: "It is what it is".

Chris

If you're looking for a philosophical "why" then I can only offer you the rather overused (but still profound) street-wise phrase uttered by many of the younger folks today: "It is what it is".

It sure is. But, that is not a why, which was my valid point in the first place you thought you claimed you refuted. Now, that you understand you were wrong, hopefully you will learn.

22. Say we had a spaceship and on the nose of this spaceship we had a long pressurized tube protruding.

If we had a small "gate" at the front of this pressurized tube with which we could open and allow the pressurized gas to escape into space, would this cause the spaceship to be tugged forward too?

Or maybe even backwards... I'm just curious.

Rich.

23. The "why' for the conservation of momentum follows from simple symmetry, chinglu, not even a gauge symmetry but a simple geometric symmetry. Look it up, before you dispute or question laws which have been valid and actually used for rockets for a couple of hundred years

24. Originally Posted by CSMYTH3025
NOTE: "A rocket and its fuel at rest..." in MigL's post describes the inertial frame of reference of the rocket in space - whether it has any motion relative to the Earth or not.

Chris
A reference frame attached to a rocket is not inertial, unless the rocket is a failure. The rocket had better be accelerating. Neither does F=ma apply to a rocket, since the rocket had better not be a constant mass system.

I posted the correct "delta v" rocket equation earlier. It is based on conservation of momentum, with proper attention to a real inertial reference frame and the changing mass of arocket.

25. Originally Posted by MigL
The "why' for the conservation of momentum follows from simple symmetry, chinglu, not even a gauge symmetry but a simple geometric symmetry. Look it up, before you dispute or question laws which have been valid and actually used for rockets for a couple of hundred years
Conservation of momentum for central forces follows from Newton's laws. It also follows from translation invariance of the equations of motion using Noether's theorem.

Arguing with chinglu is like arguing with a house plant.

Correction: a house plant doesn't say stupid stuff.

26. Wow, you can be harsh Doc. Hope I don't get on your bad side.

27. Originally Posted by DrRocket
Arguing with chinglu is like arguing with a house plant.
LMFAO! My mother used to do this all the time!

28. Originally Posted by MigL
The "why' for the conservation of momentum follows from simple symmetry, chinglu, not even a gauge symmetry but a simple geometric symmetry. Look it up, before you dispute or question laws which have been valid and actually used for rockets for a couple of hundred years
The symmetry is based only on the laws of Newton, look it up.
That is not a why.

29. Originally Posted by DrRocket
Originally Posted by MigL
The "why' for the conservation of momentum follows from simple symmetry, chinglu, not even a gauge symmetry but a simple geometric symmetry. Look it up, before you dispute or question laws which have been valid and actually used for rockets for a couple of hundred years
Conservation of momentum for central forces follows from Newton's laws. It also follows from translation invariance of the equations of motion using Noether's theorem.

Arguing with chinglu is like arguing with a house plant.

Correction: a house plant doesn't say stupid stuff.
I have a thread still sitting out there quite intact that you visited and ran from in which I proved Sr cannot supply the vectors necessary over an interval strictly by using LT.

It shows clearly vectors are missing. Now, if I am wrong, you could supply said missing 4-D vectors and would have but failed and ran.

This is a circle argument, you know, the circle you can't see without a mirror.

30. Originally Posted by chinglu
I have a thread still sitting out there quite intact that you visited and ran from in which I proved Sr cannot supply the vectors necessary over an interval strictly by using LT.

It shows clearly vectors are missing. Now, if I am wrong, you could supply said missing 4-D vectors and would have but failed and ran.

This is a circle argument, you know, the circle you can't see without a mirror.
You were shown the error in your argument several times, by several people.

You are just too damn stupid to understand it.

31. Originally Posted by DrRocket
Originally Posted by chinglu
I have a thread still sitting out there quite intact that you visited and ran from in which I proved Sr cannot supply the vectors necessary over an interval strictly by using LT.

It shows clearly vectors are missing. Now, if I am wrong, you could supply said missing 4-D vectors and would have but failed and ran.

This is a circle argument, you know, the circle you can't see without a mirror.
You were shown the error in your argument several times, by several people.

You are just too damn stupid to understand it.
Well, maybe you are correct.

Let's see you write the why without using the laws of Newton.

Everyone will be watching.

32. Originally Posted by chinglu
Originally Posted by DrRocket
Originally Posted by chinglu
I have a thread still sitting out there quite intact that you visited and ran from in which I proved Sr cannot supply the vectors necessary over an interval strictly by using LT.

It shows clearly vectors are missing. Now, if I am wrong, you could supply said missing 4-D vectors and would have but failed and ran.

This is a circle argument, you know, the circle you can't see without a mirror.
You were shown the error in your argument several times, by several people.

You are just too damn stupid to understand it.
Well, maybe you are correct.

Let's see you write the why without using the laws of Newton.

Everyone will be watching.
You have been told numerous times that special relativity and the Lorentz transforms follow from the geometry of 4-dimensional spacetime with the inner product of signature (+,-,-,-). The Lorentz group preserves that binner product, hence the norm assocuated with it, which is known in physics as the spacetime interval.

See -- no "Newton's laws" anywhere. Just Minkowski spacetime.

Yep, I'm correct. You won't understand this either (or again).

Now, after this very short interlude, let's stick to the topic of the thread.

33. Originally Posted by DrRocket
Originally Posted by chinglu
Originally Posted by DrRocket
Originally Posted by chinglu
I have a thread still sitting out there quite intact that you visited and ran from in which I proved Sr cannot supply the vectors necessary over an interval strictly by using LT.

It shows clearly vectors are missing. Now, if I am wrong, you could supply said missing 4-D vectors and would have but failed and ran.

This is a circle argument, you know, the circle you can't see without a mirror.
You were shown the error in your argument several times, by several people.

You are just too damn stupid to understand it.
Well, maybe you are correct.

Let's see you write the why without using the laws of Newton.

Everyone will be watching.
You have been told numerous times that special relativity and the Lorentz transforms follow from the geometry of 4-dimensional spacetime with the inner product of signature (+,-,-,-). The Lorentz group preserves that binner product, hence the norm assocuated with it, which is known in physics as the spacetime interval.

See -- no "Newton's laws" anywhere. Just Minkowski spacetime.

Yep, I'm correct. You won't understand this either (or again).

Now, after this very short interlude, let's stick to the topic of the thread.
Now, what does SR have to do with the topic of the thread? I would hate to be corrected so often.

34. Err, you're the one that brought SR up...

35. Originally Posted by MagiMaster
Err, you're the one that brought SR up...
Err, I brought it up only in the context of another thread and not this one as a specific off topic reaction to a post from Rocket.

Rocket brought up SR in the context of this thread which is not applicable.

Now do you understand?

36. Originally Posted by chinglu

I have a thread still sitting out there quite intact that you visited and ran from in which I proved Sr cannot supply the vectors necessary over an interval strictly by using LT.

It shows clearly vectors are missing. Now, if I am wrong, you could supply said missing 4-D vectors and would have but failed and ran.

This is a circle argument, you know, the circle you can't see without a mirror.
Why don't you post a link to your thread, so people don't have to try and figure out which one you are talking about? It would be helpful.

Originally Posted by MigL
Kojax, you have centripetal force confused. It is actually the force that the frame of the ride exerts on your back and pushes you down, otherwise you would continue undisturbed in your ballistic trajectory.
What is commonly known as centrifugial force, which pushes you up, is a fictional force ( doesn't exist ).
If you're studying physics, you should have learned this in second year classical mechanics.
Yeah. I know it's not a real force as such. I should have mentioned that so as to avoid the OP becoming confused later on if he decides to study physics. It's handy to think of it as one if you want to keep things simple, but technically it's as you said. It's an artifact of a number of other forces interacting.

The N force between your body and the carnival ride is similar to what you would experience if you were being acted upon by gravity, but you're not. You're just being continually accelerated around in a circle by being pushed on from a gradually changing direction. It's quite a similar effect to what you would experience if you were in a rocket that is being continually accelerated in one direction, a situation that is, according to the equivalence principle, indistinguishable from gravitational acceleration.

http://en.wikipedia.org/wiki/Equivalence_principle

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