6 ms·

Doesn't it seem like a slowdown caused by infrared light emitting from one part of Pioneer and reflecting off of another part of Pioneer is kind of like powerin

by jarin 16y ago

Doesn't it seem like a slowdown caused by infrared light emitting from one part of Pioneer and reflecting off of another part of Pioneer is kind of like powering a sailboat with a giant fan attached to the back of the sailboat? I'm not a physicist by any means, but doesn't conservation of momentum apply to photon emission and absorption/reflection as well?

deleted 16y ago
[deleted]
dexen 16y ago
The original direction of the (infrared) light is irrelevant, only the end result matters -- the `net force', with reflections and losses factored in. And factoring in the reflections is exactly what the article is about. So it's not like a sailboat with a fan, more like a jet airplane with thrust reversers [1] engaged. While the original thrust is ordinarily pointed rearwards, it's redirected when the reversers are engaged and the `net force' (original thrust sans losses) is pointing frontwards. ---- [1] http://en.wikipedia.org/wiki/Thrust_reverser http://en.wikipedia.org/wiki/Thrust_reverser
- jarin 16y ago
  Ah, that totally makes sense. Apologies if that was a dumb question, I am a high school dropout after all. :)
- ars 16y ago
  The reason it's like a jet engine is that the engine "creates" wind - by burning fuel, the hot exhaust is the new wind. If a jet engine only moved wind (like a turbo prop), then a thrust reverser would not work.
  - dexen 16y ago
    EDIT: wait, what? I'm convinced that thrust reverser would (in principle at least, construction details be damned) work with a propeller engine. In my understanding, even if the air stream was accelerated rearwards at first, and only subsequently redirected frontward, it'd still create net force pointing frontward (decelerating one), without need for increased temperature nor extra mass from burned fuel. But I don't have solid physic background; somebody please correct me. It may be easier to visualize it with a ducted fan. As used on some RC aircraft models, modeled after jet-engined crafts. I believe a thrust reverser would work in such a setup, minus friction and turbulent losses.
    - falcolas 16y ago
      With a propeller engine, the "wind" is a side effect caused by to the primary effect of the props getting "lift" in the forward direction, similar to how the wings themselves work. The prop itself is a wing [1], which twists to create even thrust across the prop despite the speed differences between the root and tips. Turbofans (and ducted fans) are different, as they really do produce thrust due to the particles they are throwing backwards (direct thrust).[2] Turboprops can get reverse thrust by inverting the propellers, but adding a thrust reverser behind them would not be enough to negate the forward lift (though they could cause problems with the airflow over the props that would negate the forward lift - but it wouldn't be due to the redirected airflow). [1] https://secure.wikimedia.org/wikipedia/en/wiki/Propeller_%28aircraft%29 https://secure.wikimedia.org/wikipedia/en/wiki/Propeller_%28... [2] https://secure.wikimedia.org/wikipedia/en/wiki/Turbofan https://secure.wikimedia.org/wikipedia/en/wiki/Turbofan
    - ars 16y ago
      Imagine a bent tube in a 'u' shape with a fan in the middle. Both openings of the tube point toward the front of the plane. That's basically what a propeller engine with a thrust reverser is. And I think you can visualize how it will do nothing if you turn on the fan. The force from the air exiting and entering the tube will cancel out.
      - eru 16y ago
        Do you think so? The outward stream would probably be more directed than the inward stream, which comes in more diffuse. (Though I don't know if that matters.) An actual experiment would be nice.
        ars 16y ago
        Maybe. After writing the previous post that I realized it sounds a lot like a Feynman sprinkler. So now I'm less sure.
        dexen 16y ago
ars 16y ago
You can power a sailboat with a giant fan. What's the problem? Don't point it at the sail though - just point it away from the boat. :) But anyway the difference here is that the "wind" is not being "pulled" from external sources (like real wind), it's created by Pioneer. And that makes a huge difference. With wind, the fan pulls the wind (which creates force), and also pushes the wind (which also creates force). Next you impact the wind into the sail, which subtracts from our force (since the wind is pushing in the wrong direction). And finally the wind "bounces" off the sail, also subtracting, and leaving you with no net force. i.e. +2 then -2. With pioneer one step is missing - the force from pulling the wind (since the photons are created on the fly). So you have +1 then -2, leaving you with a net force of -1. PS. If you don't understand why pulling and pushing the wind counts as two forces, think of a compressor which sucks in air and stores it. It still causes motion by pulling the air, but it doesn't push it.
omellet 16y ago
Yes. But the equipment on the back is radiating heat (roughly speaking) in every direction, as I understand it, so absent a reflector, the equipment would have a net zero effect on thrust. But because some of the heat is being reflected, it's contributing a non-zero amount of thrust.

by jarin 16y ago

deleted 16y ago

[deleted]

dexen 16y ago

The original direction of the (infrared) light is irrelevant, only the end result matters -- the `net force', with reflections and losses factored in. And factoring in the reflections is exactly what the article is about. So it's not like a sailboat with a fan, more like a jet airplane with thrust reversers [1] engaged. While the original thrust is ordinarily pointed rearwards, it's redirected when the reversers are engaged and the `net force' (original thrust sans losses) is pointing frontwards. ---- [1] http://en.wikipedia.org/wiki/Thrust_reverser http://en.wikipedia.org/wiki/Thrust_reverser

jarin 16y ago
Ah, that totally makes sense. Apologies if that was a dumb question, I am a high school dropout after all. :)
ars 16y ago
The reason it's like a jet engine is that the engine "creates" wind - by burning fuel, the hot exhaust is the new wind. If a jet engine only moved wind (like a turbo prop), then a thrust reverser would not work.
- dexen 16y ago
  EDIT: wait, what? I'm convinced that thrust reverser would (in principle at least, construction details be damned) work with a propeller engine. In my understanding, even if the air stream was accelerated rearwards at first, and only subsequently redirected frontward, it'd still create net force pointing frontward (decelerating one), without need for increased temperature nor extra mass from burned fuel. But I don't have solid physic background; somebody please correct me. It may be easier to visualize it with a ducted fan. As used on some RC aircraft models, modeled after jet-engined crafts. I believe a thrust reverser would work in such a setup, minus friction and turbulent losses.
  - falcolas 16y ago
    With a propeller engine, the "wind" is a side effect caused by to the primary effect of the props getting "lift" in the forward direction, similar to how the wings themselves work. The prop itself is a wing [1], which twists to create even thrust across the prop despite the speed differences between the root and tips. Turbofans (and ducted fans) are different, as they really do produce thrust due to the particles they are throwing backwards (direct thrust).[2] Turboprops can get reverse thrust by inverting the propellers, but adding a thrust reverser behind them would not be enough to negate the forward lift (though they could cause problems with the airflow over the props that would negate the forward lift - but it wouldn't be due to the redirected airflow). [1] https://secure.wikimedia.org/wikipedia/en/wiki/Propeller_%28aircraft%29 https://secure.wikimedia.org/wikipedia/en/wiki/Propeller_%28... [2] https://secure.wikimedia.org/wikipedia/en/wiki/Turbofan https://secure.wikimedia.org/wikipedia/en/wiki/Turbofan
  - ars 16y ago
    Imagine a bent tube in a 'u' shape with a fan in the middle. Both openings of the tube point toward the front of the plane. That's basically what a propeller engine with a thrust reverser is. And I think you can visualize how it will do nothing if you turn on the fan. The force from the air exiting and entering the tube will cancel out.
    - eru 16y ago
      Do you think so? The outward stream would probably be more directed than the inward stream, which comes in more diffuse. (Though I don't know if that matters.) An actual experiment would be nice.
      - ars 16y ago
        Maybe. After writing the previous post that I realized it sounds a lot like a Feynman sprinkler. So now I'm less sure.
        dexen 16y ago

ars 16y ago

You can power a sailboat with a giant fan. What's the problem? Don't point it at the sail though - just point it away from the boat. :) But anyway the difference here is that the "wind" is not being "pulled" from external sources (like real wind), it's created by Pioneer. And that makes a huge difference. With wind, the fan pulls the wind (which creates force), and also pushes the wind (which also creates force). Next you impact the wind into the sail, which subtracts from our force (since the wind is pushing in the wrong direction). And finally the wind "bounces" off the sail, also subtracting, and leaving you with no net force. i.e. +2 then -2. With pioneer one step is missing - the force from pulling the wind (since the photons are created on the fly). So you have +1 then -2, leaving you with a net force of -1. PS. If you don't understand why pulling and pushing the wind counts as two forces, think of a compressor which sucks in air and stores it. It still causes motion by pulling the air, but it doesn't push it.

omellet 16y ago

Yes. But the equipment on the back is radiating heat (roughly speaking) in every direction, as I understand it, so absent a reflector, the equipment would have a net zero effect on thrust. But because some of the heat is being reflected, it's contributing a non-zero amount of thrust.