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Author Topic:   Quick Questions, Short Answers - No Debate
hooah212002
Member (Idle past 802 days)
Posts: 3193
Joined: 08-12-2009


Message 46 of 341 (615662)
05-15-2011 2:29 PM


Propulsion in the vacuum of space
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.

Replies to this message:
 Message 47 by Dr Adequate, posted 05-15-2011 2:31 PM hooah212002 has replied
 Message 49 by fearandloathing, posted 05-15-2011 2:46 PM hooah212002 has seen this message but not replied

Dr Adequate
Member (Idle past 285 days)
Posts: 16113
Joined: 07-20-2006


Message 47 of 341 (615663)
05-15-2011 2:31 PM
Reply to: Message 46 by hooah212002
05-15-2011 2:29 PM


Re: Propulsion in the vacuum of space
The rocket is pushing against its own exhaust.

This message is a reply to:
 Message 46 by hooah212002, posted 05-15-2011 2:29 PM hooah212002 has replied

Replies to this message:
 Message 48 by hooah212002, posted 05-15-2011 2:35 PM Dr Adequate has not replied
 Message 51 by dwise1, posted 05-15-2011 7:45 PM Dr Adequate has replied
 Message 57 by Percy, posted 05-16-2011 10:24 AM Dr Adequate has not replied

hooah212002
Member (Idle past 802 days)
Posts: 3193
Joined: 08-12-2009


Message 48 of 341 (615664)
05-15-2011 2:35 PM
Reply to: Message 47 by Dr Adequate
05-15-2011 2:31 PM


Re: Propulsion in the vacuum of space
Makes sense. Thanks.

"What can be asserted without proof, can be dismissed without proof."-Hitch.

This message is a reply to:
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fearandloathing
Member (Idle past 4145 days)
Posts: 990
From: Burlington, NC, USA
Joined: 02-24-2011


Message 49 of 341 (615666)
05-15-2011 2:46 PM
Reply to: Message 46 by hooah212002
05-15-2011 2:29 PM


Re: Propulsion in the vacuum of space
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 285 days)
Posts: 16113
Joined: 07-20-2006


Message 50 of 341 (615694)
05-15-2011 7:20 PM


Fish Identification
(1) Is it real?
(2) What is it?
(3) How can I be sure of never encountering one?

Replies to this message:
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dwise1
Member
Posts: 5930
Joined: 05-02-2006
Member Rating: 5.8


Message 51 of 341 (615697)
05-15-2011 7:45 PM
Reply to: Message 47 by Dr Adequate
05-15-2011 2:31 PM


Re: Propulsion in the vacuum of space; the true story
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:
As a side effect, these pressures on the rocket also act on the exhaust in the opposite direction and accelerate this to very high speeds (according to Newton's Third Law).[1] From the principle of conservation of momentum the speed of the exhaust of a rocket determines how much momentum increase is created for a given amount of propellant. This is called the rocket's specific impulse.[1] Because a rocket, propellant and exhaust in flight, without any external perturbations, may be considered as a closed system, the total momentum is always constant. Therefore, the faster the net speed of the exhaust in one direction, the greater the speed of the rocket can achieve in the opposite direction. This is especially true since the rocket body's mass is typically far lower than the final total exhaust mass.
As the remaining propellant decreases, rocket vehicles become lighter and their acceleration tends to increase until the propellant is exhausted. This means that much of the speed change occurs towards the end of the burn when the vehicle is much lighter.
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.
PS
On 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

This message is a reply to:
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Replies to this message:
 Message 52 by Dr Adequate, posted 05-16-2011 12:33 AM dwise1 has replied

Dr Adequate
Member (Idle past 285 days)
Posts: 16113
Joined: 07-20-2006


Message 52 of 341 (615715)
05-16-2011 12:33 AM
Reply to: Message 51 by dwise1
05-15-2011 7:45 PM


Re: Propulsion in the vacuum of space; the true story
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.

This message is a reply to:
 Message 51 by dwise1, posted 05-15-2011 7:45 PM dwise1 has replied

Replies to this message:
 Message 53 by dwise1, posted 05-16-2011 1:16 AM Dr Adequate has replied

dwise1
Member
Posts: 5930
Joined: 05-02-2006
Member Rating: 5.8


Message 53 of 341 (615721)
05-16-2011 1:16 AM
Reply to: Message 52 by Dr Adequate
05-16-2011 12:33 AM


Re: Propulsion in the vacuum of space; the true story
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.

This message is a reply to:
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Dr Adequate
Member (Idle past 285 days)
Posts: 16113
Joined: 07-20-2006


Message 54 of 341 (615728)
05-16-2011 3:04 AM
Reply to: Message 53 by dwise1
05-16-2011 1:16 AM


Re: Propulsion in the vacuum of space; the true story
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 4306 days)
Posts: 442
From: Fremont, CA, USA
Joined: 06-18-2004


Message 55 of 341 (615733)
05-16-2011 5:27 AM
Reply to: Message 44 by Dr Adequate
05-13-2011 12:45 AM


I found this tidbit from your link interesting:
quote:
A better word to translate deus might have been P.Gmc. *ansuz, but this was used only of the highest deities in the Germanic religion, and not of foreign gods, and it was never used of the Christian God.

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Coyote
Member (Idle past 2106 days)
Posts: 6117
Joined: 01-12-2008


Message 56 of 341 (615744)
05-16-2011 9:23 AM
Reply to: Message 50 by Dr Adequate
05-15-2011 7:20 PM


Re: Fish Identification
(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?

This message is a reply to:
 Message 50 by Dr Adequate, posted 05-15-2011 7:20 PM Dr Adequate has replied

Replies to this message:
 Message 61 by Dr Adequate, posted 05-16-2011 1:11 PM Coyote has replied

Percy
Member
Posts: 22392
From: New Hampshire
Joined: 12-23-2000
Member Rating: 5.3


Message 57 of 341 (615745)
05-16-2011 10:24 AM
Reply to: Message 47 by Dr Adequate
05-15-2011 2:31 PM


Re: Propulsion in the vacuum of space
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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Replies to this message:
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dwise1
Member
Posts: 5930
Joined: 05-02-2006
Member Rating: 5.8


Message 58 of 341 (615749)
05-16-2011 11:30 AM
Reply to: Message 57 by Percy
05-16-2011 10:24 AM


Re: Propulsion in the vacuum of space
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 in
L = 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.
PS
I 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 1479 days)
Posts: 2161
From: Cambridgeshire, UK.
Joined: 02-05-2002


Message 59 of 341 (615752)
05-16-2011 11:41 AM
Reply to: Message 14 by Tram law
05-02-2011 1:40 PM


Re: *Bump for Tram Law*
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 4145 days)
Posts: 990
From: Burlington, NC, USA
Joined: 02-24-2011


Message 60 of 341 (615754)
05-16-2011 12:25 PM
Reply to: Message 59 by Peter
05-16-2011 11:41 AM


Re: *Bump for Tram Law*
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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