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How does a rocket work in space

  1. Jan 17, 2010 #1
    when there is nothing for the reaction force to push on?

    My textbook says, ironically rockets work better in space because in atmosphere it has to do work against the pressure thingy. But it doesn't explain why, all it says is "at the microscopic level its complicated"

    I remember asking my teacher once and he said the fueling pushes against the spent fuel, but thats not really "complicated"
     
  2. jcsd
  3. Jan 17, 2010 #2

    Doc Al

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    Rockets work by ejecting material (from burning fuel) at high speed. The rocket pushes the exhaust and the exhaust pushes back on the rocket--Newton's 3rd law. Rockets don't need anything else to push on.
     
  4. Jan 17, 2010 #3

    DaveC426913

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    Rockets are the purest application of Newton's Third Law:

    For every action there is an equal and opposite reaction.

    Fuel goes backward, rocket goes forward.

    Think about sitting on a rolling chair and throwing heavy books away from you. You will move in the opposite direction. It has nothing to do with what the book "pushes on" once you throw it.

    In fact, it works better in a vacuum.
     
  5. Jan 17, 2010 #4
    Yes I understand how the third law works, but I'm REALLY asking is, how it works in space, where there is nothing to push against

    There was sort of a scandle thing before the apollo mission in the 1960s where Goddard was ridiculed by the New York Times for suggesting a rocket moving in space, they argued that the fuel isn't pushing against anything, but in fact it worked better.


    If a bird keeps flapping its wings in space its not gonna go underwhere. If I "kick" the empty space in space, i'm not going to move anywhere. So how does the rocket move?
     
  6. Jan 17, 2010 #5

    Vanadium 50

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    Apparently not.

    Where is the "push against" in the 3rd law?
     
  7. Jan 17, 2010 #6

    DaveC426913

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    These two statements are contradictory. Not to put too fine a point on it but, if you are asking what it pushes against, you don't understand the 3rd law.


    Correct. The bird's wings stay attached to it. Your leg stays attached to you.

    By expelling a portion of itself.

    Fuel goes left, rocket goes right.

    Think about the centre of mass of the entire system in all the examples.

    1] Bird beats its wings: centre of mass stays stationary, so does bird.
    2] PF member kicks his legs, centre of mass stays stationary, so does PF member.

    as opposed to:

    3] PFer throws book. Centre of mass of [book/PFer system] stays stationary. But book goes left, PFer goes right.
    4]Rocket expels exhaust plume. Centre of mass of [rocket/exhaust-plume system] stays stationary. But exhaust plume goes left, rocket goes right.
     
    Last edited: Jan 17, 2010
  8. Jan 17, 2010 #7
    Or think of it as conservation of momentum.

    Momentum of expelled fuel = Momentum of rocket
     
  9. Jan 17, 2010 #8
    perhaps I've been confusing y'all with my dumb person talk. To make sure that there are no mistakes being made on my side, I will now quote from the book "College Physics 8th edition from Serway"
    From chapter 6. In the section "Rocket Propulsion" on page 178 - 179
    Starting now. (dont worry its not a long read)
    "In a now infamous article in the New York Times, rocket pioneer Robert Goddard was ridiculed for thinking that rockets would work in space, where according to the Times, there was nothing to push against. The Times retracted rather belatedly, during the first Apollo moon landing mission in 1969. The hot gases are not pushing against anything external, but against the rocket itself-ironically the rockets actually work better in a vacuum. In an atmosphere the gases have to do work against the outside air pressure to escape the combustion chamber, slowing the exhaust velocity and reducing the reaction force.

    At the microscopic level, this process is complicated, but it can be simplified by applying the laws of conservation of momentum to the rocket and its ejected fuel"
    *end*
    What I wanted to ask about, is the process in the microscopic level thats complicated
     
  10. Jan 18, 2010 #9
    Yeah, sure, if you want to get into the "microscopic" level, of how forces are actually transmitted according to quantum mechanics, or what the ejected "mass" really is, and where it all came from, and WHY?

    Yeah, you can go as deep as you want into it. Deeper than even the top scientists understand.

    But...if you just want to know how a rocket works, it's pretty much the simplest concept in physics. It "pushes" against its fuel. I'm not sure why the article claims that it's so "complicated". Everything can be more complicated than anybody understands on some fundamental level.

    Maybe the editor just had some recent vision or something....
     
  11. Jan 18, 2010 #10

    Char. Limit

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    The editor doesn't quite sound like he entirely knows what he's talking about...

    Why are you going into the microscopic level when the macroscopic one works just fine?

    "Rocket goes up, exhaust goes down." No need for microscopics at all.
     
  12. Jan 18, 2010 #11
    It sounds like editor just recently saw some small glimpse of slightly deeper physics, and has somehow been enlightened to the existence of a deeper reality. He wants to make sure that the world knows that HIS knowledge goes beyond the average.

    But yeah...to every action is an equal and opposite reaction. You don't need to split the atom and unfold the fabric of space-time to understand this.
     
  13. Jan 18, 2010 #12
  14. Jan 18, 2010 #13

    sophiecentaur

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    Why should you expect a newspaper to get something like Newtonian Physics right? They don't seem do a very good job with anything that you happen to know a lot about yourself. 'Even' Science Journalists (who really should know better) make some amazing howlers.

    Char.limit
    "No need for microscopes"
    A man after my own heart!!
     
  15. Jan 18, 2010 #14
    To all the sarcastic people who are having a go at the OP and just quoting laws at him. It's a perfectly reasonable question with a perfectly reasonable answer, OK?

    Let's simplify the design:

    - the rocket motor is a hollow sphere with very thick walls. There's a hole in one side of the sphere opening into space.

    - continuously inject flammable gas into the sphere through tiny pipes of negligible cross-section. Set fire to the gas. The gas molecules acquire huge kinetic energy/momentum and fly about every which way, and either end up hitting the inside of the sphere, or flying out the hole into space.

    - at every point on the sphere the average force is radially outwards. Add up all these vector forces around the whole interior surface of the sphere and find the resultant.

    - If there were no hole in the sphere, the resultant is zero and the thing doesn't move at all. As there is in fact a hole on the sphere, the forces on that bit don't exist and the resultant is a very big arrow pointing radially outwards on the side of the sphere opposite to the hole.

    - Result : the rocket flies off very fast in that direction.

    So, despite what has been said by the mickey-takers, the stuff that just gets ejected straight into space (i.e. flies out the back of its own volition) has no effect whatsoever on the movement of the rocket. The molecules have to bounce off the interior of the forward hemisphere inside the engine before anything gets to move. So all the '[Slaps forehead] For God's sake, the rocket goes right, the fuel goes left, you idiot' explanations the OP has been getting are actually seriously misleading.
     
  16. Jan 18, 2010 #15

    sophiecentaur

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    You could say that your spherical rocket loses only those molecules that happen to be moving in a certain direction (with a certain direction of momentum vectors). Total momentum is conserved so blah blah.
    But is that any more useful or less confusing than talking about the reaction force due to material of the fuel being ejected and giving a simple example of throwing a book from an office chair?
    The nonsense being pointed out was surely in the notion that you might need to 'push against" anything like an atmosphere.
    Personally, the only micky taking I was doing was against newspaper editors - who should be capable of receiving as well as dishing it out.
     
  17. Jan 18, 2010 #16
    That's what I said, yes.
    Don't quote laws. He's asking for an explanation of why the rocket moves forward.
    Sophie - read what I wrote. There is no reaction force if something just flies out the back - the analogy with throwing a book is not correct.

    There is a force on the rocket moving forward because there is an unopposed pressure on one side of the combustion chamber. Here is a nice picture:
     

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  18. Jan 18, 2010 #17

    Dale

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    Hi zenith8, I know you understand this stuff, but this is an incorrect statement imo. Let's say that we are using a fuel where one molecule of fuel gets split into two molecules of exhaust, and let's say further that a given molecule of fuel burns in the combustion chamber such that one molecule of exhaust goes straight out the back and the other goes straight forward. The one going straight out the back has a momentum of -p and the one going forward has a momentum of p, when the forward one collides with the ship and rebounds its final momentum is -p and the change in momentum of the rocket is 2p. From this interaction the total momentum of the two exhaust molecules is -2p and the total change in momentum of the rocket is 2p. So the molecule of exhaust which is ejected straight back does in fact have an effect on the movement of the rocket because it has an effect on the movement of the fuel which strikes the rocket. This is why it is sufficient to simply consider the total change in momentum from a macroscopic level.
     
  19. Jan 18, 2010 #18

    sophiecentaur

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    So if you don't want to "quote laws" how are you going to describe what has happened to the molecules of the gas inside and what relevance their particular direction of travel has? How does a gas exert pressure?
    That's a nice picture and it gives another (useful) view of the situation.
    But what causes the pressure and what has the fact that it is "unopposed" is certain directions got to do with the movement of the rocket, IF you're not going to "quote laws" when you want to relate how the particles are involved in the process? Why should the pressure on the inner surface only cause the rocket to go in that direction? How intuitive are we allowed to be and which bits of "laws" are we supposed to / allowed to use?
    If you really want to 'get down to it' then you can't be picky. The fact is that there is no limit to how deep you can go into a topic. Dunno why you seem to be getting so upset.
     
  20. Jan 18, 2010 #19

    vela

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    You guys do realize that that excerpt is from a textbook, not a newspaper article, don't you?
     
  21. Jan 18, 2010 #20

    sophiecentaur

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    A textbook that uses the word "thingy" sounds a bit suspect?
     
  22. Jan 18, 2010 #21

    vela

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    I don't see the word "thingy" in there.
     
  23. Jan 18, 2010 #22

    Doc Al

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    But the quoted excerpt does refer to a famous New York Times editorial that appeared in 1920:
    http://astronauticsnow.com/blazingthetrail/gruntman_btt_pages/gruntman_blazingthetrail_p_117.pdf" [Broken]

    Another reference for that NYT editorial: http://www.clarku.edu/research/archives/goddard/faqs.cfm#question8" [Broken]

    Note that the Times printed a "correction" to that 1920 editorial in 1969, just three days before we walked on the Moon.
     
    Last edited by a moderator: May 4, 2017
  24. Jan 18, 2010 #23

    sophiecentaur

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    I read "thingy" in the OP, actually.
    In any case, there are occasions when a textbook can be wrong. I have had lots of trouble with irate students. They get more upset than when they find out "there ain't no sanity clause".

    Actually, it doesn't sound so hard to accept that atmospheric effect. The average exit velocity could also be affected by the presence of slower moving air molecules absorbing net KE of the jet. Some air molecules could find themselves in the end of the nozzle, due to turbulence, etc. The energy imparted to these molecules would be in the form of more random motion - KE being given to molecules moving from side to side - not backwards. This represents work done and would reduce the useful work done on the rocket nozzle. Without the presence of air, the molecules leaving the nozzle would be undisturbed.
    Perhaps you could also look on it as a heat engine losing some thermal energy to its surroundings which can't then be used for useful work.
    I suppose the simple fact would have also been noted that there is no friction from passing through an atmosphere.
     
  25. Jan 18, 2010 #24
    Hi Dale,

    Everything you say is, of course, correct. All the OP and others should take from my post is that what is important for propelling the rocket forward is (1) the impact of molecules on the forward half of the combustion chamber, and (2) the absence of impacts on the rear half of the combustion chamber, because there isn't a rear half. This is merely intended to answer the question about 'what is there to push against in space?'. I'm not trying to derive the conservation of momentum from first principles.
     
  26. Jan 18, 2010 #25
    I'm not upset, dear - you're being overly literal. "Not quoting laws" doesn't mean that one has to rederive the whole of physics from first principles. It's just that sometimes if you say "the conservation of momentum means that the rocket goes up and the exhaust goes down" or "sugar cubes glow blue if you crack them with a pair of pliers because of triboluminescence" then that isn't enough to answer a question.

    Here it's a given that we know that force (pressure) is caused by the electromagnetic repulsion of atoms brought into close proximity, that forces are bigger if things bang into each other very fast, and that we understand Newton's third law and the conservation of momentum. And yet here, the OP still doesn't understand why the rocket goes forward. Deeper explanation required. Hence my post.
     
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