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Old 04-20-2012, 12:04 PM   #16
jedidia
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 So if you place the heat-generator on the BACKside of the vehicle, it will accelerate the vehicle continuously?
It's called a photon drive. It takes about 300 megawatt to produce one newton of thrust...
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Old 04-24-2012, 04:12 PM   #17
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 It's called a photon drive. It takes about 300 megawatt to produce one newton of thrust...
Thanks

Can we say each nuclear powerplant, when running, accelerates down on the ground in the neighborhood of an extra 1 or 2 newton then (beyond their mere weight)?
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Old 04-24-2012, 05:23 PM   #18
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Can we say each nuclear powerplant, when running, accelerates down on the ground in the neighborhood of an extra 1 or 2 newton then (beyond their mere weight)?
Not unless they try really hard to focus their luminosity into the same point of the sky...

Power Plants usually get rid of their heat by thermal conduction, not by radiation.
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Old 04-24-2012, 10:08 PM   #19
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Isn't it when the photons hit the back of the antenna they create a push-effect on the space-craft? The antenna was on the 'back' side (pointing to earth) and the heat-source on the 'front' side (in the direction of travel). That's how I understood it at least. Somewhat similar to flying with a tiny amount of head-wind.

So I was figuring in the nuclear power plant scenario it would be similar in reaction to whatever the radiation hit that was beneath it. That that radiation-pressure would then push down on the earth ever so slightly. Not that a few newtons make a huge difference anyway though, but I was just wondering if such an effect was present
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Old 04-24-2012, 10:12 PM   #20
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 Isn't it when the photons hit the back of the antenna they create a push-effect on the space-craft? The antenna was on the 'back' side (pointing to earth) and the heat-source on the 'front' side (in the direction of travel). That's how I understood it at least. Somewhat similar to flying with a tiny amount of head-wind.

So I was figuring in the nuclear power plant scenario it would be similar in reaction to whatever the radiation hit that was beneath it. That that radiation-pressure would then push down on the earth ever so slightly. Not that a few newtons make a huge difference anyway though, but I was just wondering if such an effect was present
Anyone know how effective Earth might be as a solar sail?
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Old 04-25-2012, 03:53 AM   #21
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 Anyone know how effective Earth might be as a solar sail?
A value above zero?
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Old 04-25-2012, 06:25 AM   #22
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Isn't it when the photons hit the back of the antenna they create a push-effect on the space-craft?
Not exactly. If I understood the article correctly, then the dish is the main reason why the radiation is anisotropic (that is, not regular to all sides). Photons don't have mass, but they have momentum, so whenever a photon is radiated away, or reflected, they impart an equal and opposite reaction on whatever they have been radiated or reflected off of.

If that radiaton is regular to all sides, the effects cancel each other out, but due to the dish deflecting a lot of the radiation into a specific direction the effect became measurable (over long periods of time).

As said before, a powerplant does get rid of its heat by thermal conduction. Of course there's quite a bit of IR radiation, but that is usually not directed to one side. Also, the pressure of the sun burning down on it would probably be larger than the pressure by its own radiation, but not even that is really noticable...
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Old 04-25-2012, 06:50 AM   #23
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 Also, the pressure of the sun burning down on it would probably be larger than the pressure by its own radiation, but not even that is really noticable...
That's what I meant with my last post but I think I made it far from clear.
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Old 04-25-2012, 04:54 PM   #24
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@jedidia; thanks
Would you say, that if we made a vehicle with the shape of a large dish-antenna, or a bowl, with a strong radiating device placed underneath it, that it might theoretically be possible to make this vehicle levitate by the radiation-pressure coming from it's radiative 'pod' at the bottom? Theoretically at least
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Old 04-25-2012, 05:02 PM   #25
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Theoretically, sure, if you can build a power source and a dish with almost zero mass. Although the dish size only has a secondary influence. The more focused the light is, the more thrust you generate (but the 300 MW/N stated above are a (approximation of a) theoretical maximum, based on the plank constant).
So what you need is a laser, and a larger dish generally would mean a better focused laser, but there's a point when increasing dish size will lead to dropping thrust to mass ratio and it will hit pretty quickly I guess.

The hole you burn in the ground when levitating the contraption would likely be the bigger problem, though. TBH, a photon drive with a thrust to mass ratio bigger than one (needed to overcome earths gravity) is about as impossible a thing as I've ever heard although the general concept is researched for laser assisted launch by some people, where you'd have the whole powersource and laser dish on the ground and push the vehicle from there (successfully done with lightweight objects to an altitude of something like 40 meters, if memory serves me right, but I also think there was ablation involved to produce additional thrust).

Last edited by jedidia; 04-25-2012 at 05:11 PM.
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Old 04-25-2012, 06:32 PM   #26
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 photon drive
...as a search-term on google gave me this:
http://en.wikipedia.org/wiki/Nuclear_photonic_rocket http://en.wikipedia.org/wiki/Nuclear_photonic_rocket


Fascinating stuff, even if still mostly theoretical
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Old 04-25-2012, 10:17 PM   #27
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For something to impart a reaction (during an impact) It needs to have mass. A massless sub-atomic particle isn't going to affect anything (in the newtonian sense of push and push back) regardless of the momentum.
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Old 04-25-2012, 11:03 PM   #28
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 For something to impart a reaction (during an impact) It needs to have mass. A massless sub-atomic particle isn't going to affect anything (in the newtonian sense of push and push back) regardless of the momentum.
Then how do solar sails work?
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Old 04-25-2012, 11:15 PM   #29
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 For something to impart a reaction (during an impact) It needs to have mass. A massless sub-atomic particle isn't going to affect anything (in the newtonian sense of push and push back) regardless of the momentum.
Not exactly. Einstein postulated and was later proven correct that photons have do have momentum equivalent to p = E/c.

Edit: Whoops. Never mind I misread your post.

Last edited by Mattyv; 04-25-2012 at 11:42 PM.
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Old 04-26-2012, 12:41 AM   #30
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 (successfully done with lightweight objects to an altitude of something like 40 meters, if memory serves me right, but I also think there was ablation involved to produce additional thrust).
I think I know what you're referring to. I once saw video of a laser hitting a ...disk-ish rocket thingy... but as far as I remember, the laser served to heat the air, which expanded and produced thrust that way...
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