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I have read many times that the thrust is produced when the ions leave the satellite, not while they are accelerating.

I have asked the same question twice in openAI ChatGPT and it gave me two different answers and so I am confused.

Does thrust occur on the satellite when the ions being accelerated are still inside the satellite?

These are the answers from ChatGPT:

first answer

second answer

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    $\begingroup$ Why would you assume a CHAT system would know anything about physics? It will tend to proritise grammar/sentence structure from the learning set over the factual details. I can see the structure of a lot of text books on both those chunks, and they are just that, mashups of texts - note for example talking about fluids in a vacuum thruster. $\endgroup$
    – GremlinWranger
    Commented Jan 3, 2023 at 7:57
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    $\begingroup$ -1 Shall we at least temporarily ban chatbot-generated content (e.g. ChatGTP) until it becomes clearer what it means for Stack Exchange? This computer-generated gobbledygook does not qualify as prior research. $\endgroup$
    – uhoh
    Commented Jan 3, 2023 at 9:35
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    $\begingroup$ @DavidHammen the tooltip guideline for the down vote is "This question does not show any research effort; it is unclear or not useful" and I think that characterizes this and likely many "chatterbot says"-based questions. To me, asking a chatterbot is not research effort, and the OP has not included any evidence that they "...have read many times that the thrust is produced when the ions leave the satellite, not while they are accelerating." If they cite a source or three ("many times") that something beyond chatterbot noise is saying this, I can easily reverse the down vote. $\endgroup$
    – uhoh
    Commented Jan 3, 2023 at 11:16
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    $\begingroup$ @DavidHammen: I believe there is a relevant difference between ChatGPT and a badly-written textbook. A textbook is generally expected to be correct, precise, accurate, and instructional. There is no expectation that ChatGPT is any one of those things – it says so right on its own homepage, where the authors talk about "plausible-sounding but incorrect or nonsensical answers". ChatGPT essentially generates random strings of words with no expectation whatsoever that the result makes sense. That is not true for textbooks. $\endgroup$
    – Jörg W Mittag
    Commented Jan 3, 2023 at 15:21
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    $\begingroup$ While I agree that chatbots are not reliable sources of information I do think the downvotes are unfair. The poster has put effort into posing a question and is asking for clarity, and there are good answers. $\endgroup$
    – GdD
    Commented Jan 4, 2023 at 9:06

4 Answers 4

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First off, ChatGPT doesn't provide answers. It strings words into plausible looking sentences based on the prompts you provide. It knows nothing and understands less. If you rely on a chatbot for information, you will not learn anything real.

In all reaction drives, the acceleration imparted to the ship occurs at the same time the reaction mass is accelerated.

If you sit in a boat floating on water and try to throw a big, heavy rock off the stern, you and the boat will experience acceleration with every motion of the rock. You accelerate the rock, and the reaction accelerates you and the boat.

This video illustrates action and reaction on a hovercraft. It works just like the written example with the boat. The hovercraft accelerates as the ball is thrown. It also accelerates when the ball is caught. Everytime the ball is accelerated (sped up or slowed down or just changing direction,) the hovercraft speeds up or slows down or changes direction. It is the same in a rocket engine. As long as the engine is accelerating the exhaust, the exhaust is accelerating the engine.

enter image description here Image from YouTube video.

Ion thrusters and rockets are reaction drives. You accelerate the exhaust and the reaction accelerates the ship. The acceleration of the ship and the exhaust takes place at the same time.

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I have asked the same question twice in openAI chatgpt and it gave me two different answers and so I am confused.

The answer to that is obvious; it is garbage.

I have read many times that the thrust is produced when the ions leave the satellite, not while they are accelerating.

I doubt that you have read this many times since it conflicts with basic high school physics; thrust is absolutely produced while ions are accelerating. Depending on the type of thruster, that location might be physically inside the thruster our outside it, wherever the electric field is accelerating ions, that's where thrust is being produce.

Basically the electric field allows the ions and the spacecraft to push on each other. Stick with good ole' Newton and the three laws, especially #3:

  1. An object at rest remains at rest, and an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force.
  2. The acceleration of an object depends on the mass of the object and the amount of force applied.
  3. Whenever one object exerts a force on another object, the second object exerts an equal and opposite on the first.

Caveat that "thrust" and "accelerating" require a reference frame.

Assuming you were reading a reasonable source and not Chatterbot noise, that frame would have been defined.

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Newton’s 3rd Law is pretty clear about “equal and opposite”. There is no “and then a while later…”.

ChatGPT does not converse. It confabulates. https://en.wikipedia.org/wiki/Confabulation This is a psychiatric term used to describe abnormal speech as a result of brain damage. It is seen in Wernicke’s Encephalopathy (better known as “wet brain”) in alcoholics. Confabulation is superficially plausible nonsense.

We have all met confabulators. They sit on a barstool, going on and on with great authority but little accuracy. If some one points out they are self-contradicting, they just keep mouthing off.

Confabulation is not lying. A liar tries to convince you of something they know to be false. A confabulator doesn’t care if they are caught in a lie. A certain politician comes to mind.

“... ChatGPT gave me two different answers and so I am confused.” Is equivalent to “Two different chronic alcoholics with Wernicke’s gave me different answers so I am confused.”

I don’t think SE should post input generated by confabulating computer programs. It muddies the waters of evidence-based discussion.

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  • $\begingroup$ Being able to confabulate is a big step toward being able to pass the Turing test. I'm quite impressed with what CatGPT comes up with. I'd categorize it as "close but no cigar". That said, a good number of drunkards sitting at a bar most likely would also fail the Turing test. Once again, close but no cigar. $\endgroup$
    – David Hammen
    Commented Jan 4, 2023 at 13:25
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By way of analogy, where does the thrust from a chemical rocket with a de Laval nozzle occur? The answer is that occurs at the points where the molecules in the exhaust last contacted the nozzle. From a modeling perspective, that is the last thing a modeler who is creating a model of the entire spacecraft wants to hear. We don't want to do computational fluid dynamics to model the thrust from a thruster. We want to model the thrust as if it occurs at a specific point. Computational fluid dynamics experts will grudgingly supply a fictional point for modeling purposes that represents where on average the thrust can be modeled as being applied to the vehicle.

To get back to the question, on a microscopic scale, acceleration of the vehicle with an ion thruster happens at all of the points where the to-be-ejected particles are accelerated. On a macroscopic scale, the thrust can be modeled as occurring at one point, which sometimes is modeled as the center of the exhaust plane.

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