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Thread: sailboat paradox and em sail propulsion

  1. #1 sailboat paradox and em sail propulsion 
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    So I currently have a hypothesis on solar propulsion via an onboard engine that would utilize cherenkov radiation and quite possibly freely propogating terahertz beams. I really don't know much about the subject but am quite obsessive and feel as though I have learned a bit since I started on this subject yesterday as a hobby. I feel as though people are too quick to dismiss self propelled systems such as with the popular sailboat discussion where a fan is mounted on a sailboat and blows air into a sail. I understand this is cartoonish and silly, but I believe that with some work a method could be developed to release electromagnetic energy in opposing directions generating rear-facing propulsion similar to rocket propulsion while at the same time 'catching' the forward traveling photons from the terahertz beams in a massive solar sail. With the little to no drag in space I don't see why this wouldn't create the possibility of travel at near-light speeds. The traditional argument I hear that opposes the sailboat paradox is that the 'fan' would be pushing backwards (as it is mounted to the boat) at the same rate as it would be pushing the sail forwards. If energy was being released in opposing directions would this not be counteracted? My idea would actually be to have an engine cylindrical in shape releasing terahertz beams outwards in all directions but only being caught by a sail in the front of the craft. The use of parabolic mirrors has also occurred to me but I don't want to get ahead of myself quite yet and would like some more input and feedback on whether or not this is even possible.

    ..I apologize if my recent obsession seems naive, I don't know why astrophysics has piqued my curiosity. My focus really is quite partial and I can be extremely intelligent but ONLY in fields that are interesting to me. Hard to explain... I've never seen a professional about it. On the plus side it is great to find a place where other minds gather to talk about these things! Please if I could have some input I am really enjoying working on this theory but am afraid I lack the background to accomplish much more by myself. Thanks!


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    Welcome khagelund

    The traditional argument I hear that opposes the sailboat paradox is that the 'fan' would be pushing backwards (as it is mounted to the boat) at the same rate as it would be pushing the sail forwards. If energy was being released in opposing directions would this not be counteracted?
    That is the traditional argument, yes, but it is based on simple physics. Can you explain a bit more about how you are proposing to get around this problem?

    What you have is a simple force equation. 1 + (-1) = 0. No way around that.


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    You need to learn about conservation of momentum and energy, and how to calculate the energy required to accelerate something to near-light speed. Then you will understand why people are "quick to dismiss" this kind of idea.
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    I guess it is a bit hard to explain, I do understand Newton's law where every action has an equal and opposite reaction. Maybe if I explain myself one step at a time you could pick out the step where I am missing something. If I had a vertical cylindrical engine with both its top and bottom attached to a craft pulse energy outwards along the x and z axis, the engine itself would tend to stay put due to the simple force equation you just mentioned. However after this point the 'equal reaction' would be the photons moving outward in all directions. The 'opposing reaction' would be somewhat of an implosive force on the engine itself thereby not creating any propulsion. Stray photons at this point would eventually exert their 'equal reaction' against something via electromagnetic pressure whether it happened soon or after a very long period of time. However a sail mounted at the front of the craft would catch the forward traveling photons thus causing propulsion.
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    Quote Originally Posted by Harold14370 View Post
    You need to learn about conservation of momentum and energy, and how to calculate the energy required to accelerate something to near-light speed. Then you will understand why people are "quick to dismiss" this kind of idea.
    I completely understand that light pressure is an extremely negligible amount of energy and accept the more-than-likely possibility that traveling at very fast speeds wouldn't happen this way. I am more interested for now if it can even be used to achieve momentum especially considering the extremely low drag in space.
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    Quote Originally Posted by khagelund View Post
    I guess it is a bit hard to explain, I do understand Newton's law where every action has an equal and opposite reaction. Maybe if I explain myself one step at a time you could pick out the step where I am missing something. If I had a vertical cylindrical engine with both its top and bottom attached to a craft pulse energy outwards along the x and z axis, the engine itself would tend to stay put due to the simple force equation you just mentioned. However after this point the 'equal reaction' would be the photons moving outward in all directions. The 'opposing reaction' would be somewhat of an implosive force on the engine itself thereby not creating any propulsion. Stray photons at this point would eventually exert their 'equal reaction' against something via electromagnetic pressure whether it happened soon or after a very long period of time. However a sail mounted at the front of the craft would catch the forward traveling photons thus causing propulsion.
    I am afraid this doesn't make any sense. Please describe this "vertical cylindrical engine". By "pulse energy", do you mean fire out photons?

    Could you provide a diagram of some sort?

    It sounds almost as if you are describing a similar setup to if you had two fans attached to a boat with a sail in the front to catch the wind from the forward facing fan. Something like that? Why not just mount a single rear-facing fan?
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

    "Gullibility kills" - Carl Sagan
    "All people know the same truth. Our lives consist of how we chose to distort it." - Harry Block
    "It is the mark of an educated mind to be able to entertain a thought without accepting it." - Aristotle
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    chenkovpropulsion.jpg

    yes, by vertical i meant that it would be upright along the y axis perpendicular to the desired direction of travel. i only used the word 'pulse' as a verb to describe the engine firing photons outward in all directions repeatedly as necessary for the desired momentum. The sail would catch all photons firing in the same desired direction of travel and photons would be fired in all other directions just as a means to counteract the opposing force generated by the photons traveling forward.
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    Ok, but then it looks like I was thinking about the right thing. So I'll ask the same question: why not simply have a light shining backwards? You'll get the exact same propulsion, but use a quarter of the energy.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

    "Gullibility kills" - Carl Sagan
    "All people know the same truth. Our lives consist of how we chose to distort it." - Harry Block
    "It is the mark of an educated mind to be able to entertain a thought without accepting it." - Aristotle
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    The photons going forward and hitting the sail, only to be reflected backwards, is a wasted step. The ones going sideways are useless. Just send them all backwards in the first place. This will give you some propulsion, but very little considering the small momentum of the photons.
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    The conventional mind design is to mount the laser on the Earth, or even better in orbit, thus greatly reducing the required mass of the space craft. However the material in the interstellar medium, apart from eroding the sail, will limit its speed to well below c.
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    Quote Originally Posted by Harold14370 View Post
    The photons going forward and hitting the sail, only to be reflected backwards, is a wasted step. The ones going sideways are useless. Just send them all backwards in the first place. This will give you some propulsion, but very little considering the small momentum of the photons.
    I was hoping to eventually design a spherical engine at the centerpoint of a sphere with a parabolic mirror along the edge of the sphere containing the engine. It would be a semispherical parabolic mirror that's centerpoint would indicate the intended direction of travel. The mirror (solar sail) would be capable of rotating along every axis in order to act as a brake and perhaps even re-absorb the energy perhaps through photoelectricity while the rear facing photons would propel the ship forward with thrust. The latest part of this changed because I was viewing the idea from a different perspective calling the rear thrust and forward thrust the negating forces and the sail the forward propulsion. It is easier to view it as the sail and the rear thrust negating each other and the forward thrust propelling the ship. The location of the solar sail in relation to the spherical engine would determine the direction of thrust. Unfortunately I am having trouble coming up with information on the actual energy output caused by a photon shockwave found in cherenkov radiation, but I'm thinking it is not very energy efficitent. Also because of this penetrating type of radiation I'm sure it would be harmful to human life and could only be used on unmanned craft, but hey that's fine with me.


    Thanks to the information you have provided me with so far, I have been able to see a few flaws in my thinking so far but I'm not going to let it slow me down! I appreciate the responses very much especially because I'm admittedly not very experienced in this field, you must all find me naive haha.
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    The problem with building this machine is essentially the same problem as trying to pick yourself up off the ground.
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    Very confused... your diagram does not depict a sphere at the center of a cylinder within a semisphereical parabolic mirror which is the sail attached to a [rocket?]. Huh? Presumably, if you were to capture photons via parabolic mirrors (at the edge of a sphere) and direct them to propulse your engine, what would be the sail, what would be the engine? Electromagentic energy in opposing fields? Wouldn't this create equalibrium? How would you direct the particles using a parabolic mirror and harness energy? Perhaps I should read more about what you propose but questions are questions; please elaborate.
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  15. #14  
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    Quote Originally Posted by khagelund View Post
    Quote Originally Posted by Harold14370 View Post
    The photons going forward and hitting the sail, only to be reflected backwards, is a wasted step. The ones going sideways are useless. Just send them all backwards in the first place. This will give you some propulsion, but very little considering the small momentum of the photons.
    I was hoping to eventually design a spherical engine at the centerpoint of a sphere with a parabolic mirror along the edge of the sphere containing the engine. It would be a semispherical parabolic mirror that's centerpoint would indicate the intended direction of travel. The mirror (solar sail) would be capable of rotating along every axis in order to act as a brake and perhaps even re-absorb the energy perhaps through photoelectricity while the rear facing photons would propel the ship forward with thrust. The latest part of this changed because I was viewing the idea from a different perspective calling the rear thrust and forward thrust the negating forces and the sail the forward propulsion. It is easier to view it as the sail and the rear thrust negating each other and the forward thrust propelling the ship. The location of the solar sail in relation to the spherical engine would determine the direction of thrust. Unfortunately I am having trouble coming up with information on the actual energy output caused by a photon shockwave found in cherenkov radiation, but I'm thinking it is not very energy efficitent. Also because of this penetrating type of radiation I'm sure it would be harmful to human life and could only be used on unmanned craft, but hey that's fine with me.


    Thanks to the information you have provided me with so far, I have been able to see a few flaws in my thinking so far but I'm not going to let it slow me down! I appreciate the responses very much especially because I'm admittedly not very experienced in this field, you must all find me naive haha.

    I think you are making this way more complicated than you need to.

    The basic principle here is that of a photon rocket.

    Simply place a photon source at the focus of a parabolic reflector. the photons striking the mirror will be reflected backwards along a line parallel to the direction of motion providing forward thrust, like thus:



    The reflector does not have to be of solar sail size, just large enough to contain the photon source at its focus, so you make the mirror as compact as possible. (The reason you make Solar sails as large as possible is so that they can capture as much light from the exterior source as they can. ) As such, the whole engine is solidly mounted at the rear of the ship. Steering and braking are easily accomplished by turn the whole ship with attitude jets. (Much easier and less trouble prone than mounting the engine on gimbals.)

    One reason that this works compared to the fan and sail idea is that the air hitting a sail is deflected sideways and not backwards, otherwise it would have to fight against the air coming from the fan. Light does not interact with itself in this way, so the photons reflected off the mirror have no trouble traveling back against the flow of the photons coming from the source.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
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    Quote Originally Posted by khagelund View Post
    I guess it is a bit hard to explain, I do understand Newton's law where every action has an equal and opposite reaction. Maybe if I explain myself one step at a time you could pick out the step where I am missing something. If I had a vertical cylindrical engine with both its top and bottom attached to a craft pulse energy outwards along the x and z axis, the engine itself would tend to stay put due to the simple force equation you just mentioned. However after this point the 'equal reaction' would be the photons moving outward in all directions. The 'opposing reaction' would be somewhat of an implosive force on the engine itself thereby not creating any propulsion. Stray photons at this point would eventually exert their 'equal reaction' against something via electromagnetic pressure whether it happened soon or after a very long period of time. However a sail mounted at the front of the craft would catch the forward traveling photons thus causing propulsion.
    very well explained thanks it works in my mind

    so if you blow in all directions the blowing in the sail works

    thats a very creative idea thanks for sharing

    edit besides that doesnt contradict newton principle taking the universe as a whole since the wind-electrons which doesnt hit the sail hit eventually something making net momentum of the universe zero
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