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Questions about thrust in pressurized environment

  1. Sep 15, 2015 #1
    is the thrust equation F=MA applicable only in atmospheric condition? can we use this equation in vacuum or pressurized space, thanks!
     
  2. jcsd
  3. Sep 15, 2015 #2

    andrewkirk

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    It applies everywhere, including in vacuums. Apollo spaceships used it while in a vacuum to land on the moon and to set their trajectory for re-entering the Earth's atmosphere. In the recent film Gravity, you see it being constantly used by the astronauts' jetpacks.
     
  4. Sep 15, 2015 #3
    if that is correct, then it means that the amount of thrust generated by a jet in vacuum is also the same if the same jet is put in a pressurized space. because if we analyze it, it seems to me that that the amount of thrust generated in vacuum is bigger than in pressurized space because the resistance against the exhausting gas in vaccum is lesser than in pressurized space.


    im not sure, just my analysis, correct me if im wrong
     
  5. Sep 15, 2015 #4

    andrewkirk

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    The thrust is the same, but in an atmosphere it will produce less acceleration on an already moving vehicle, because it is partly offset by drag. It's a gross vs net thing.
     
  6. Sep 15, 2015 #5
    so youre saying that the thrust produced by exhausting gas in both environment were the same, they only differ in speed of the vehicle because of friction(the speed of the vehicle in vacuum is larger than in pressurized)?
     
  7. Sep 15, 2015 #6

    Drakkith

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    Hold on. I think rickitek is asking specifically about the thrust generated from the engine, not about the thrust vs drag stuff.
     
  8. Sep 16, 2015 #7

    andrewkirk

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    Replace 'speed' by 'acceleration' in that sentence and it will be correct.
     
  9. Sep 16, 2015 #8
    ok, if acceleration is the correct term, no problem, sorry for my incorrect use of term, im not a physics expert. but the idea is there and i think thats is the most important.

    one more thing, if i put a rigid steel plate perpendicular to the exhausting gas, will the thrust produce by the exhausting gas increases? or it will remain the same with or without the steel plate.

    and sorry for my english/grammar, im from asia...
     
  10. Sep 16, 2015 #9
    Have you thought about what would happen if the external environmental pressure was equal to the pressure in the chamber. would there then be any thrust?
     
  11. Sep 16, 2015 #10
    of course there would be no thrust because outside and inside environment will be in equilibrium, the gas wouldnt flows out. what im asking is if the thrust would change if the outside environment specifically pressure changes. or is the thrust also is dependent on the outside pressure,

    thanks for your reply,
     
  12. Sep 16, 2015 #11

    andrewkirk

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    What is the steel plate attached to?

    If it's rigidly attached to the rocket, it'll reduce the thrust because the high-speed gas hitting the plate will provide a reverse thrust.

    If it's not attached to the rocket, and a long way away, it'll make not much difference. If it's close, maybe some gas will rebound off the plate then strike the rocket and provide some extra thrust, but I imagine that would be pretty minor compared to the thrust of expelling the gases.

    It doesn't matter what happens to the gas once it has left the rocket engine, unless it then strikes the rocket again, or something connected to the rocket.

    By the way, in post 3 you referred to using a jet in a vacuum. Jets don't work in a vacuum, because they rely on taking in air at the front and expelling it at the back When there's no air (and no substitute gas) they can't do that. That's one of the reasons spaceships have to use rockets rather than jets.
     
  13. Sep 16, 2015 #12
    So, ( forest for the trees ) doesn't that show you that the outside pressure can alter the thrust of a fluid exiting a container.

    You initial inquiry says nothing about how the thrust is being accomplished, or specifically to what type of flow, compressible or not, choked flow, sonic, subsonic, what type of nozzle - convergent or convergent-divergent, upon which an answer can depend.

    Specifically, as from andrewkirf, Fnet = ma applies everywhere. Determining Fnet is the interesting part.

    For a rocket engine, and similar operating engines, the nozlle is designed so that vast majority of the thrust is due to the momentum change of the fluid, with only a slight change in thrust as a result of a difference of exit pressure from the nozzle with that of the environment.

    http://www.grc.nasa.gov/WWW/k-12/airplane/thrsteq.html
    You can try reading some of that stuff that Nasa has put out to the public.
     
  14. Sep 16, 2015 #13

    CWatters

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    The key is that the F in F=ma is the net force. eg It's the total force acting on the rocket after taking into account all other forces such as air resistance or any extra thrust developed by the engine for whatever reason.

    As 256bits said... Working out the net force is the interesting part.
     
  15. Sep 16, 2015 #14
    Yes. As a rocket rises through the atmosphere, its thrust increases slightly. When you read the thrust ratings of various rockets, you have to consider whether they are at atmospheric pressure (boosters) or vacuum (upper stages).
     
  16. Sep 16, 2015 #15
    the steel plate is not attached to the rocket, so the steel plate has no significant effect to the thrust because the effect is so small or negligible.

    thank you so much!
     
  17. Sep 16, 2015 #16

    Drakkith

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    That's what I was thinking. Do you have any references elaborating on this that I can read?
    Thanks.
     
  18. Sep 16, 2015 #17
    ok guyz, thank you for all your responses and unfortunately i can not reply to all of you one by one because there so many,

    for now i still havent get yet what i am looking for and its my fault, my questions is incomplete which causes confusions and more questions than answers.

    to make it more clear and comprehensive, below are the things that i know and things that i dont know(or not sure)

    things that i know:

    1. the thrust in low pressure environment is greater than in high pressure environment because the resistance or drag is less in low pressure environment than in high pressure environment.(assuming the jet/rocket carry its own oxygen, it doesnt suck air from the outside)

    2. as the rocket goes up the thrust increases because of the ff:
    2.1 the weight of the rocket decreases as fuel burns out
    2.2 effect of acceleration
    2.3 the resistance or drag is lesser in high altitude than in low altitude. because the pressure in high altitude is less than in low altitude

    3. the resistance(drag) in outer space is lesser than on earth. because outer space pressure is negative(vacuum) and earth has a positive pressure.

    things i dont know or not sure and need answers:

    1. when the exhaust gas flows out the rocket nozzle. the only resistance to the exiting gases that i can think of is the pressure of the outside environment. with this in mind i conclude that the outside pressure indeed affects the thrust, am i right? (note: in answering this question please neglect for a while other factors such as the drag on the vehicle, just focus on the exhaust gas and the resistance of the outside pressure(if there is any) on the exhaust gas.)

    2. if the outside pressure indeed affects the thrust, how it affects relative to the outside pressure? is the thrust increases if the outside pressure decreases or vice versa?

    3. how big is the effect? is it big enough that you need to consider it in your design. or it is just very small and negligible that you can safely eliminate it in your design.


    thank you in advance and i hope i made everything clear...
     
  19. Sep 16, 2015 #18

    Drakkith

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    The acceleration of the vehicle from any given amount of thrust is lower when the vehicle is in a high pressure environment, thanks to drag, but the thrust, the force from the engines, is not affected by drag effects. This was what I was trying to get at in my earlier post.
     
  20. Sep 17, 2015 #19

    CWatters

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    Yes.

    http://www.nasa.gov/returntoflight/system/system_SSME_prt.htm

    Brief explanation/basic maths here....

    https://exploration.grc.nasa.gov/education/rocket/rockth.html
     
  21. Sep 17, 2015 #20

    CWatters

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    In the above you confuse "thrust" with the "net force on the rocket".

    The net force on the rocket in low pressure environment is greater than in high pressure environment for two reasons:

    * The thrust from the rocket is higher see post #19 and
    * The resistance due to drag is less.

    The acceleration in low pressure environment can be greater than in high pressure environment for several reasons:

    * The net force on the rocket is greater (see two reasons above).
    * The mass of the rocket is lower (if some fuel has been burnt getting into the low pressure environment of space).
     
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