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Author | Topic: Quick Questions, Short Answers - No Debate | |||||||||||||||||||||||||||
hooah212002 Member (Idle past 822 days) Posts: 3193 Joined: |
This sounds like a dumb question (and it probably makes me sound stupid): how does rocket propulsion work in the vacuum of space? It seems to me as though the engines would have nothing to propel the rocket/ship/whatever off of; nothing to "push" off of, so to speak. Is my understanding of space, or air in general, skewed?
"What can be asserted without proof, can be dismissed without proof."-Hitch.
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Dr Adequate Member (Idle past 305 days) Posts: 16113 Joined: |
The rocket is pushing against its own exhaust.
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hooah212002 Member (Idle past 822 days) Posts: 3193 Joined: |
Makes sense. Thanks.
"What can be asserted without proof, can be dismissed without proof."-Hitch.
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fearandloathing Member (Idle past 4166 days) Posts: 990 From: Burlington, NC, USA Joined: |
Also remember every action has an equal and opposite reaction, I have seen a few sci-fi movies ignore this when characters fire guns, bullet type, in space/ zero-G.
"I hate to advocate the use of drugs, alcohol, violence, or insanity to anyone, but they always worked for me." - Hunter S. Thompson Ad astra per aspera
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Dr Adequate Member (Idle past 305 days) Posts: 16113 Joined: |
(1) Is it real?(2) What is it? (3) How can I be sure of never encountering one?
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dwise1 Member Posts: 5948 Joined: Member Rating: 5.5 |
Negatory, good buddy! Conservation of linear momentum.
Linear momentum is mass times velocity. Expell part of that mass (AKA reaction mass, which is why they're called "reaction drives") at high velocity in one direction and that will result in a change of velocity in the other direction. Nothing pushes against anything else. A common example from a related principle, conservation of angular momentum, might help. There, angular momentum is the product of angular velocity (how many degrees/radians/grads per second the body is rotating) and the moment of inertia, which is analogous to mass. Actually, the moment of inertia, I, is a good source of integral calculus problems, if you're thinking of writing a math textbook. It is the infinitessimal summation of the products of infinitessimal points of mass times their distance from the axis of rotation. The farther out mass is distributed, the greater is I, and the closer in it's held, the lesser is I. Skaters and dancers use this principle all the time. In your office swivel chair, extend your arms and legs out (high I) and have somebody start you to slowly spin. Angular momentum is that initial rate times your high I. Now pull your limbs in tight, suddenly lowering I. To keep angular momentum constant, angular velocity increases and your spinning speeds up. At any point that you want to stop, simply increase I by extending your limbs out again. Works every time!
http://en.wikipedia.org/wiki/Rocket#Physics (though it also graphically plays on the idea of forces pushing on all walls of the combustion chamber except for the exhaust hole):
quote: It is also covered in Chapter 8, "Conservation of Linear Momentum", of my physics textbook, Fundamentals of Phsics, by Halliday and Resnick, Revised Printing, 1974, John Wiley and Sons, $13.50 in 1977 (present-day students, eat your hearts out), as Example 7 starting on page 145. PSOn the subject of conservation of angular momentum, I just have to share this one dance move. It was taught to me by a West Coast Swing teacher. Fan moves are common. That's when you're turning supported on a bent leg with the other leg straight describing a circle on the floor. OK, you go into a fan, but then you draw that extended straight leg in, which greatly speeds up your turn, after which you extend that leg again to slow your spin down. And maybe get one more speed-up in if you're lucky (keep in mind the frictional losses through the contact between your supporting foot and the floor). Looks phenomenal when you see it. Edited by dwise1, : PS
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Dr Adequate Member (Idle past 305 days) Posts: 16113 Joined: |
I don't see that your longer version actually contradicts my shorter version.
From the WP article:
In a closed chamber, the pressures are equal in each direction and no acceleration occurs. If an opening is provided in the bottom of the chamber then the pressure is no longer acting on the missing section. This opening permits the exhaust to escape. The remaining pressures give a resultant thrust on the side opposite the opening, and these pressures are what push the rocket along. The rocket is pushing against its exhaust; equivalently, the exhaust is pushing against the rocket.
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dwise1 Member Posts: 5948 Joined: Member Rating: 5.5 |
I do not see your explanation at all describing the action of the conservation of linear momentum.
Consider ion drives, which are also reaction drives. Electric grids accelerate ions to form the exhaust. Where is the pushing there? Acceleration of the vehicle is by conservation of linear momentum, not by pushing against the sides of the chamber.
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Dr Adequate Member (Idle past 305 days) Posts: 16113 Joined: |
I do not see your explanation at all describing the action of the conservation of linear momentum. I didn't particularly see the need to state that law explicitly any more than if I was describing how a rowing boat or a punt works.
Consider ion drives, which are also reaction drives. Electric grids accelerate ions to form the exhaust. Where is the pushing there? Well apparently the ions are being pushed ("electric grids accelerate ions"). And, by Newton's Third Law, so is the spaceship.
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Trae Member (Idle past 4327 days) Posts: 442 From: Fremont, CA, USA Joined: |
I found this tidbit from your link interesting:
quote:
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Coyote Member (Idle past 2127 days) Posts: 6117 Joined: |
(1) Is it real?
No.
(2) What is it?
A clever photoshop.
(3) How can I be sure of never encountering one?
Avoid the internet?
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Percy Member Posts: 22480 From: New Hampshire Joined: Member Rating: 4.8 |
The Dwise1 explanation left me confused, so maybe I'll just be explaining the same thing in a different way.
The controlled explosion of fuel in the combustion chamber causes gas to push outward in all directions. In the forward direction it encounters the wall of the combustion chamber, and it pushes against this wall, which being part of the rocket ship imparts a forward force to the whole vehicle. The forces of all the gas that pushes against the sides of the combustion chamber cancels out. There is no force from the gas that exits to the rear because the rear of the combustion chamber is open. The net of all this is a forward force. Jets work on the same principle. The difference between rockets and jets is that jets get their oxygen from the atmosphere while rockets have to supply their own. --Percy
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dwise1 Member Posts: 5948 Joined: Member Rating: 5.5 |
To understand the role of the conservation of linear momentum in rocket propulsion, calculus is required.
Thrust is based on the rate of mass lost per second. The mass lost per second and the velocity at which it leaves the rocket gives that lost mass, which is the exhaust, a linear momentum directed rearwards which is balanced an acceleration forwards of the rocket body which is losing the mass of the exhaust. More simply put, linear momentum L is equal to mass m times velocity v (a vector, so it has both direction and magnitude). So inL = mv, for L to remain constant while m is decreasing, v must decrease. dv/dt = acceleration. And if that loss of mass is due to separation (eg, rocket exhaust), then the subtraction of that mass' linear momentum (subtraction since it's in the negative direction; remember, v has direction) has to be balanced by the addition of momentum to the rocket body in order to keep the overall momentum constant. It's basic physics. http://en.wikipedia.org/...m#Conservation_of_linear_momentum For a more complete, rigorous, and hopefully more enlightening explanation, I would recommend consulting physics books and introductory books on the physics of rocketry, but only those that employ calculus. Or to chat with a physicist about it. Otherwise, we will be violating the title of this topic. PSI had already stated my reason for adding angular momentum to the discussion. That was explicitly because it is much easier to change the "mass" term and to observe the effects of that change on the "velocity" term. To keep the product of two terms constant, if you reduce one term you must increase the other term, and vice versa. In playing with angular momentum, you can directly experience that that idea is far from abstract. Edited by dwise1, : PS
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Peter Member (Idle past 1500 days) Posts: 2161 From: Cambridgeshire, UK. Joined: |
Tram law writes: Oh wow, thank you. I appreciate it. I'll repeat the question below: My friend says the primordial soup, the soup that from where all life came from, is nothing more than myth. He says it's a myth because the odds of such a primordial soup are astronomical to make it completely impossible for life to form from it, and therefore, the primordial Soup is a myth. Is he correct? Edit: often I do want to discuss thing, but i simply lack the capacity to. No. Not if his sole argument is based around probabilities. Apart from the afore mentioned lack of knowledge of what would have had to be there, there is also the simple fact that anything with a finite probability, no matter how small, can happen (I'm a Safety Engineer -- I know ) Combine that with the vast size, and extreme age of the universe and you'll see that the typical 'astronomical' probablities become rapidly reduced.
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fearandloathing Member (Idle past 4166 days) Posts: 990 From: Burlington, NC, USA Joined: |
Peter writes: Tram law writes: Oh wow, thank you. I appreciate it. I'll repeat the question below: My friend says the primordial soup, the soup that from where all life came from, is nothing more than myth. He says it's a myth because the odds of such a primordial soup are astronomical to make it completely impossible for life to form from it, and therefore, the primordial Soup is a myth. Is he correct? Edit: often I do want to discuss thing, but i simply lack the capacity to. No. Not if his sole argument is based around probabilities. Apart from the afore mentioned lack of knowledge of what would have had to be there, there is also the simple fact that anything with a finite probability, no matter how small, can happen (I'm a Safety Engineer -- I know ) Combine that with the vast size, and extreme age of the universe and you'll see that the typical 'astronomical' probablities become rapidly reduced. This makes me think of the Drake equation "I hate to advocate the use of drugs, alcohol, violence, or insanity to anyone, but they always worked for me." - Hunter S. Thompson Ad astra per aspera
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