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  1. Standard member sonhouse
    Fast and Curious
    03 Nov '15 18:13
    http://www.ibtimes.co.uk/nasa-says-emdrive-does-work-it-may-have-also-created-star-trek-warp-drive-1499098

    Forget the warp drive, this EM drive seems to actually work, why nobody really knows.

    It shoots electromagnetic energy out the rocket nozzle and some thrust is produced, looks like about 250 pounds per megawatt of energy in.

    I have two observations.

    One is, wouldn't this be equivalent to using light the same way and wouldn't there be more thrust, watt for watt, than microwaves?

    The second thing I thought about and I don't think anyone else has, suppose you have one of these rockets in space producing its thrust, whatever that is. The microwave energy leaves the bell of the rocket to be lost in space. But suppose behind that is a giant microwave dish capable of concentrating that energy on a second rocket. You are basically doubling your efficiency right there and not using any more energy to do it.
    Then you have a large dish behind the second one, collecting ITS output of microwave energy for yet a third rocket. Obviously the energy is not all collected, some is lost after each rocket uses the energy but you can see a train of these things hanging on behind, maybe each one having 10% less thrust, 10% less payload and so forth but still you are reusing the energy that would otherwise be lost heating up interplanetary gas and such.
  2. Standard member DeepThought
    Losing the Thread
    03 Nov '15 18:58
    Originally posted by sonhouse
    http://www.ibtimes.co.uk/nasa-says-emdrive-does-work-it-may-have-also-created-star-trek-warp-drive-1499098

    Forget the warp drive, this EM drive seems to actually work, why nobody really knows.

    It shoots electromagnetic energy out the rocket nozzle and some thrust is produced, looks like about 250 pounds per megawatt of energy in.

    I have two observa ...[text shortened]... you are reusing the energy that would otherwise be lost heating up interplanetary gas and such.
    No, the engine "works" as a microwave cavity, no microwaves leave the cavity. The point with EMDrive is that it is a reactionless engine. It seems to violate the law of conservation of momentum. So I don't believe a word of it. The inventor has a theory about pushing against vacuum fluctuations, but I don't think that gets him round the problem of it not conserving linear momentum.
  3. Standard member sonhouse
    Fast and Curious
    03 Nov '15 19:05 / 1 edit
    Originally posted by DeepThought
    No, the engine "works" as a microwave cavity, no microwaves leave the cavity. The point with EMDrive is that it is a reactionless engine. It seems to violate the law of conservation of momentum. So I don't believe a word of it. The inventor has a theory about pushing against vacuum fluctuations, but I don't think that gets him round the problem of it not conserving linear momentum.
    There is no way energy can be trapped in the engine, it has microwaves in an open bell. there is no difference between that and a cone antenna where microwave energy from a waveguide enters the bottom of the cone and energy shoots out the end of the trumpet. If you have a way to store the EM inside the rocket, the temperature would rise in a hurry because you are feeding a couple kw of energy in. RF has to be leaving the system. That energy goes somewhere, and for sure it is not into the 5th dimension.
  4. 03 Nov '15 19:38
    http://www.projectrho.com/public_html/rocket/reactionlessdrive.php#id--EmDrive

    http://blogs.discovermagazine.com/outthere/2014/08/06/nasa-validate-imposible-space-drive-word/

    https://medium.com/starts-with-a-bang/how-to-fool-the-world-with-bad-science-7a9318dd1ae6

    So what are the claims of the EmDrive?

    A team in China, an inventor in Britain and a third person, Guido Fetta, have built three separate versions of the EmDrive. All three versions supposedly give different results varying by a factor of ~500 from one another.
    The “test” performed at NASA was sensitive to a minimum thrust threshold of about 10-to-15 microNewtons, and the “positive result” claimed detection of somewhere between 30-to-50 microNewtons of thrust.
    And finally (and most damning), there was a “true” version and a “null” version of the EmDrive that were both tested at this facility, with the anticipation that the true version would produce this thrust and that the null version wouldn’t. But both versions produced the same thrust.

    From the abstract itself:

    “Thrust was observed on both test articles, even though one of the test articles was designed with the expectation that it would not produce thrust. Specifically, one test article contained internal physical modifications that were designed to produce thrust, while the other did not (with the latter being referred to as the “null” test article).”



    Also it should be noted, that if this were a photon drive that used light as it's 'reaction mass' then it would have a thrust
    of 1 Newton per ~300Mw

    http://www.projectrho.com/public_html/rocket/enginelist.php#id--Other--Photon
  5. Standard member sonhouse
    Fast and Curious
    06 Nov '15 12:00 / 1 edit
    Originally posted by googlefudge
    http://www.projectrho.com/public_html/rocket/reactionlessdrive.php#id--EmDrive

    http://blogs.discovermagazine.com/outthere/2014/08/06/nasa-validate-imposible-space-drive-word/

    https://medium.com/starts-with-a-bang/how-to-fool-the-world-with-bad-science-7a9318dd1ae6

    [quote]So what are the claims of the EmDrive?

    A team in China, an inventor ...[text shortened]... ton per ~300Mw

    http://www.projectrho.com/public_html/rocket/enginelist.php#id--Other--Photon
    One newton per 300 megawatts. Wow, not much bang for the buck. 3.6 ounces. I can see why there is not much work done on photon drives There still is work being done on solar sails though.

    Is the thrust of a solar sail the same? 300 mw=3 ounces of thrust?

    If so you would need about 50 acres of sails to even get that much thrust at 1 AU.
  6. 06 Nov '15 13:50 / 1 edit
    Originally posted by sonhouse
    One newton per 300 megawatts. Wow, not much bang for the buck. 3.6 ounces. I can see why there is not much work done on photon drives There still is work being done on solar sails though.

    Is the thrust of a solar sail the same? 300 mw=3 ounces of thrust?

    If so you would need about 50 acres of sails to even get that much thrust at 1 AU.
    Not quite the same, because you have to factor in angle/shape/reflectance/ect.

    https://en.wikipedia.org/wiki/Solar_sail#Physical_principles

    Solar radiation exerts a pressure on the sail due to reflection and a small fraction that is absorbed. The absorbed energy heats the sail, which re-radiates that energy from the front and rear surfaces.

    The momentum of a photon or an entire flux is given by p = E/c,[11][12] where E is the photon or flux energy, p is the momentum, and c is the speed of light. Solar radiation pressure is calculated on an irradiance (solar constant) value of 1361 W/m2 at 1 AU (Earth-Sun distance), as revised in 2011:[13]

    perfect absorbance: F = 4.54 μN per square metre (4.54 μPa)

    perfect reflectance: F = 9.08 μN per square metre (9.08 μPa) (normal to surface)

    A perfect sail is flat and has 100% specular reflection. An actual sail will have an overall efficiency of about 90%, about 8.17 μN/m2,[14] due to curvature (billow), wrinkles, absorbance, re-radiation from front and back, non-specular effects, and other factors.

    The force on a sail and the actual acceleration of the craft vary by the inverse square of distance from the Sun (unless close to the Sun[15]), and by the square of the cosine of the angle between the sail force vector and the radial from the Sun, so

    F = F0 cos2 θ / R2 (ideal sail)

    where R is distance from the Sun in AU. An actual square sail can be modeled as:

    F = F0 (0.349 + 0.662 cos 2θ − 0.011 cos 4θ ) / R2

    Note that the force and acceleration approach zero generally around θ = 60° rather than 90° as one might expect with an ideal sail.[16]

    Solar wind, the flux of charged particles blown out from the Sun, exerts a nominal dynamic pressure of about 3 to 4 nPa, three orders of magnitude less than solar radiation pressure on a reflective sail.


    You can get increased thrust by using ground/orbital based lasers focused on the sail, and/or by using capillary action to
    spread coolant [water?] over the surface that then evaporates off on the sunward side to produce extra thrust.
    The downside of that being that you now have to carry reaction mass which kinda removes the point of a solar sail.
    However it does still mean that your energy source [fuel] is external and doesn't have to be carried on the ship.