How does a rocket work in space

sophiecentaur

Science Advisor
Gold Member
22,903
3,596
Z8
Fair enough but when we stop using terms like momentum it's a thin line between that and saying things like "the force carried the cricket ball out of the ground" as if the force was there all the time.
The only time a force is involved in our problem is when 'momentum is conserved'. You can either say this happens during molecular collisions or on a macroscopic level, after all the collisions have taken place and we're just dealing with the mass of gas on the way out of the back. Momentum ideas have the beauty of working at all levels.

I must say, I am always a bit twitchy about insisting on using particles to explain virtually everything. I have heard a poor Science Teacher try to explain Convection in terms of particles to a group of 14 year olds because he thought he should be able to. It was a nightmare for him and they went away shaking their heads.We do use appropriate 'levels' of complexity for a very good reason.
 
803
8
OP specifically said that he understands Newton's laws.

The reason he got confused is because of these "microscopic effects" that got needlessly thrown in there.

If OP intrinsically understands why a rocket works, but is led to believe that in fact he doesn't because of his lack of understanding for the string theory and super symmetry, then there is a problem.
 
Last edited:
361
2
Z8
Fair enough but when we stop using terms like momentum it's a thin line between that and saying things like "the force carried the cricket ball out of the ground" as if the force was there all the time.
The only time a force is involved in our problem is when 'momentum is conserved'. You can either say this happens during molecular collisions or on a macroscopic level, after all the collisions have taken place and we're just dealing with the mass of gas on the way out of the back. Momentum ideas have the beauty of working at all levels.

I must say, I am always a bit twitchy about insisting on using particles to explain virtually everything. I have heard a poor Science Teacher try to explain Convection in terms of particles to a group of 14 year olds because he thought he should be able to. It was a nightmare for him and they went away shaking their heads.We do use appropriate 'levels' of complexity for a very good reason.
I really don't get your point. What different does it make if you once heard some guy give a poor explanation about something completely different? It's not like what I said earlier is difficult to understand. Or is it? Oh well, I'll clear off back to the Quantum Physics forum.
 

D H

Staff Emeritus
Science Advisor
Insights Author
15,326
680
Conservation of momentum is not a poor explanation. It is a very good explanation. Explaining how real rockets work at the molecular level is a very, very difficult undertaking. The best we can do is computational fluid dynamics models, and those are well beyond someone with a (somewhat shaky) freshman physics level of understanding.
 
361
2
The reason he got confused it because of these "microscopic effects" that got needlessly thrown in there.

If OP intrinsically understands why a rocket works, but is led to believe that in fact he doesn't because of his lack of understanding for the string theory and super symmetry, then there is a problem.
But before I do, I mean what? What a gross mischaracterization of this discussion.

The OP says "..but surely there is nothing for the reaction force to push against?". I say "molecules bang on the upper side of the reaction chamber" and now you're saying "Ooh, lah di dah, no need to talk about string theory and supersymetry". Come on.. this is silly.

The guy understands the conservation of momentum well - but for that to operate there needs to be some interaction between two objects. He's just asking "what objects?".
 

DaveC426913

Gold Member
17,884
1,537
It's a perfectly reasonable question with a perfectly reasonable answer, OK?

- the rocket motor is a hollow sphere with very thick walls. There's a hole in one side of the sphere opening into space.
The difficulty with the above explanation is that it is specious. There are many, many ways of getting propulsion without using exploding, expanding fuel and a nozzle. You can get propulsion by throwing a book, no expanding gases involved.

The general principle is indeed conservation of momentum as explained. Ejected mass goes left, rocket goes right. How you get that ejected mass to move (and how much and how fast) is entirely up to your imagination, and your engineers.


Also,
To all the sarcastic people who are having a go at the OP and just quoting laws at him...
So, despite what has been said by the mickey-takers...
Ooh, lah di dah...
The only one here being sarcastic is you. Please, let's keep the emotional bickering out of it.
 
Last edited:
361
2
Conservation of momentum is not a poor explanation. It is a very good explanation. Explaining how real rockets work at the molecular level is a very, very difficult undertaking. The best we can do is computational fluid dynamics models, and those are well beyond someone with a (somewhat shaky) freshman physics level of understanding.
No - producing a numerical simulation of a real rocket engine so that the computer model behaves exactly the same as the real one is a difficult computational fluid dynamics problem. Understanding how it works in general is not.
 

A.T.

Science Advisor
9,368
1,382
Conservation of momentum is not a poor explanation. It is a very good explanation. Explaining how real rockets work at the molecular level is a very, very difficult undertaking.
You can use Conservation of momentum, but make it more detailed than just saying "If fuel goes back, rocket has to go forward". The forces on the walls of the burning chamber are too just a result of Conservation of momentum. But for laymen they are easier to accept as a cause for the rocket's acceleration, than some bookkeeping rule like Conservation of momentum.
 

DaveC426913

Gold Member
17,884
1,537
The forces on the walls of the burning chamber are too just a result of Conservation of momentum. But for laymen they are easier to accept as a cause for the rocket's acceleration,
The trouble is, it doesn't grok the problem. Expanding gases are NOT necessary for propulsion. Throwing a book will work just fine.

It will be much more beneficial for the layperson to understand the overarching principle that applies in ALL cases, rather than having them get caught up in details of a specific example.



The principle of momentum, which you call "bookkeeping" is, in fact, the fundamental principle. Everything else is mere bookkeeping.
 
803
8
But before I do, I mean what? What a gross mischaracterization of this discussion.

The OP says "..but surely there is nothing for the reaction force to push against?". I say "molecules bang on the upper side of the reaction chamber" and now you're saying "Ooh, lah di dah, no need to talk about string theory and supersymetry". Come on.. this is silly.

The guy understands the conservation of momentum well - but for that to operate there needs to be some interaction between two objects. He's just asking "what objects?".

He went back and forth between "I understand" to seeming like he doesn't. With the final explanation, he led me to believe that he DOES, just that the "microscopic" stuff is throwing him off. So, I tell him not to worry about that.

You believe that he doesn't understand, so you gave a deeper explanation which perhaps will make him understand. I applaud you. You interpreted the problem differently than I did, and you're probably right.

If you are right, I still don't think we should be held accountable for giving OP the wrong or simplistic answer. I believe the problem rests in how OP went about asking for his answer. Specifically, this example "Yes I understand how the third law works, but I'm REALLY asking is..."

Basically he had an overinflated (almost cocky) assumption of what he understands, and he dismissed attempts at trying to give him a proper answer. Can't blame us for that.
 
27,487
4,007
The principle of momentum, which you call "bookkeeping" is, in fact, the fundamental principle. Everything else is mere bookkeeping.
I agree. Whatever the nature of the interaction between the exhaust and the rocket conservation of momentum works. The exhaust can be a book, a hot gas, an accelerated particle, or a photon, the same principle applies. That is what a fundamental principle does, it ties together seemingly disparate things into one overall concept.
 
Last edited:
Ok this is getting more "English 101" than anyone reading this thread initially would expect. I mean I wake up the next morning to check this thread and this thread just got 10x bigger full of possible character analysis of the editor and me.

That microscopic and complicated part is what I'm asking, (bravo if you figured that out) but that seems to be utterly unnecessary, and from some of your perspective, the editor putting something in there just for the fun of it.
 

Char. Limit

Gold Member
1,198
12
Ok this is getting more "English 101" than anyone reading this thread initially would expect. I mean I wake up the next morning to check this thread and this thread just got 10x bigger full of possible character analysis of the editor and me.

That microscopic and complicated part is what I'm asking, (bravo if you figured that out) but that seems to be utterly unnecessary, and from some of your perspective, the editor putting something in there just for the fun of it.
Er... yeah, that last paragraph pretty much sums it up. The microscopic level is unnecessary.
 

DaveC426913

Gold Member
17,884
1,537
That microscopic and complicated part is what I'm asking,
If I may be so presumptuous, I think the thread has served a greater good, since you have come out of it minus a misconception you had been carrying about how rockets work in the first place. Your newfound knowledge of the micro/macro mechanics of rocket exhaust would not be much good if the foundation upon which it was built was wrong. :wink:
 

sophiecentaur

Science Advisor
Gold Member
22,903
3,596
A.T.
You really undervalue the concept of momentum conservation as a mere "book keeping rule". There are few aspects of Science more fundamental than Momentum Conservation; it applies to the macroscopic and the microscopic. Any 'lay' person who wants explanations which don't hang on that principle is likely to be getting the wrong end of a lot of sticks. It just isn't fair to suggest to anyone that they should try to get to know Physics without it..
 

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving
Top