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Comment by jarin | Hacker News Reader

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?

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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.

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[1] http://en.wikipedia.org/wiki/Thrust_reverser

Ah, that totally makes sense. Apologies if that was a dumb question, I am a high school dropout after all. :)

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.

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.

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_%28... [2] https://secure.wikimedia.org/wikipedia/en/wiki/Turbofan

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.

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.

Maybe.

After writing the previous post that I realized it sounds a lot like a Feynman sprinkler. So now I'm less sure.

A difference in kinetic energy -- E(outflow)-E(inflow) -- of air would result in net force pointing forward (a decelerating force, if we consider a plane). Basically, we'd need the air to get accelerated forward more than rearward, producing net force.

Kinetic energy is E = (mV^2)/2

Obviously the mass of inflow is equal to the mas of the outflow. We can achieve a difference in speeds if the effective cross-section of the (forward-pointing) outlet was smaller than cross-section of the inlet.

That's very off-topic, anyway :^)

Don't you think that a momentum based approach would be more suitable than energy based ones?

That doesn't sound correct.

Consider it as a mass flow problem. You're ingesting a certain mass of air m at a velocity v and releasing it in the opposite direction. Assuming no losses in the U tube, the only thing that changes is the direction of v. What matters is the total momentum, mv.

If the tube were straight, mv would point out the back and the airplane would go forward. With a U shaped tube, you have instead -mv which is the same momentum, but a different direction, and the airplane moves backwards.

Now that I think of it, it might be easier to visualize as a conservation of energy problem.

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.

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.

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