1. ace.1991
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    ace.1991 Member

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    EM-Drive space ship to Mars

    Discussion in 'Research' started by ace.1991, Apr 22, 2016.

    Anyway, I have a few questions relating my current WIP.
    Basically a story about a mission to Mars with a space ship that uses EM-Drive technology as it's propulsion. I have tried my best to figure this out myself but I can't seem to make sense of it.


    My questions are (with the assumption that the EM-Drive actually works in large-scale and that its power-to-thrust ratio holds to current theories):

    With a vessel approximately 1,500,000Kg in weight, with a 4.7 Megawatt power supply...

    What would be the maximum velocity achievable for the vessel?
    and how long would it take to travel to mars and back under its own power?

    Thanks

    Links for more information:
    http://emdrive.com/firstgenapplications.html
    https://en.wikipedia.org/wiki/RF_resonant_cavity_thruster
     
  2. SethLoki
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    SethLoki Unemployed Autodidact Contributor

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    I'm figuring you might have to dig more with Google for the numbers. Unless there are any resident physicists/top drawer space engineers here? It's a very nascent technology—so much so the power to weight ratios are terrible. I've read an emdrive couldn't even lift one fifth the weight of its own battery!

    FWIW I tried the maths... quite amateurishly.

    I ran a=F/m and got 2.76m/s sq (nearly a 3rd of a G) - so with Mars being 55 million kms away I came up with 3 and a bit days.
     
  3. doggiedude
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    doggiedude Contributing Member

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  4. newjerseyrunner
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    newjerseyrunner Contributing Member

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    Mars really isn't that hard to get to, why do you need EM technology for this? A regular ion engine would give you a little extra speed, but you'll still need a chemical rocket to get you into orbit and do a descent, and EM drive, even considering the faulty math wouldn't be able to provide that sort of force.

    The problem is that it doesn't. The EM Drive as described in pop-science journals violates known physical laws.

    Roger Shawyer came up with the idea and did the original mathematics using special relativity, but the math fell apart when trying to replicate the result using General Relativity. An understandable mistake, but something that should not have been published without the full description. The Chinese math was just plain fraudulent, they used classical electrodynamic laws to attain positive thrust, it again falls apart when trying to do the same math with quantum electrodynamics.

    It made it's way into the news because a NASA team at Eagleworks started to study the device, which in the public's mind legitimized it. That particular research group is designated specifically for fringe theories. They've already conclusively shown that it didn't work in either of the ways described by Shawyer or Yung. The news focused on a handful of positive thrust results, but science demands repeatability and most of the time, nothing was measured.

    Hi, I'm new, but I'm more bottom drawer.
     
  5. Kiem Hoang
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    Kiem Hoang New Member

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    If you are writing sci/fi and EM tech does not exist today, why don't you just make it up. If it is only a theory right now, don't you as the author have all the answers in this case? I think if you don't dwell on it, people will go with what you have if it makes some sense.
     
  6. doggiedude
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    doggiedude Contributing Member

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    Yup, Not many sci-fi readers bitch about FTL or wormhole travel. You make the universe. If you say it's the way things happen then that's the way it happens.
    (Throws fireball into the air and watches the pretty lights.)
     
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  7. ace.1991
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    ace.1991 Member

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    Okay, let me rephrase what i'm asking:

    I know that the em-drive is still theoretical and may not even work. I know this.

    In my story, it does work. Or at least a variation of it does.

    What i'm having trouble with are some plausable numbers like 'transit time to mars', for example.
     
  8. doggiedude
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    doggiedude Contributing Member

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    If I was reading it as a sci-fi piece I would be less concerned with how long the trip took and more concerned with how you presented the voyage.
    If you tell me it took a year, then I want explanations as to how people survived the trip.
    Were they in cryostasis? Explain it a bit.
    Were they awake? How did they eat, breathe, recycle water? Were they at each others throats because of boredom?

    If you tell me it took a week. Fine. Tell me a bit about their experience on the way.

    I think most readers of sci-fi are just used to accepting certain things.
     
  9. newjerseyrunner
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    newjerseyrunner Contributing Member

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    Do you want to calculate the transit time to Mars, or do you want the transit time to Mars to be something specific and you're trying to calculate the force required? Wattage is not the correct unit to use, as it just describes the rate of energy conversion over time, you could do it that way, but it'd be hard.

    The force required to get to Mars fairly quickly is actually fairly low. You've given me a weight, if you tell me how long you'd like your trip to do, I can do the differential and give you the force required.
     
  10. SethLoki
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    SethLoki Unemployed Autodidact Contributor

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    Given the mass and the power you stipulated. I'll stick with 3.5 days till someone corrects me. There are probably lots of other plausible nos if you muck about with the variables: weight of the ship, power of the engines and where Mars is in its relational orbit to earth. Like @doggiedude said though, readers accept.
     
  11. newjerseyrunner
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    newjerseyrunner Contributing Member

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    Assuming that the ship will be accelerating at it's maximum acceleration the entire time, you'd get a bell curve for the velocity, which would have to peak at the halfway point, so that the force can be turned around so you arrive at Mars at low enough speed to get into orbit or land.

    The closest distance to Mars is 50e6 km, so you want to know how to relate force and mass to distance. Force and mass are acceleration so you need to take the first integral of the velocity formula and come up with d = v(i)t + (1/2)a(t^2).
    v(i) would be zero so you can drop it and simplify the constants: 50e6 km = a(t^2)
    Relate it to force and mass by replacing acceleration: 50e6 km = (F / m) * (t^2)
    Replace your mass constant and normalize the km out: 50e9 m = (F / 15e6 kg) * (t ^ 2).
    Rearrange

    To figure out how much force you need to reach Mars in t seconds: F = 7.5e17 / (t ^ 2)
    To figure out how long it'll take to reach Mars based on F newtons: t = sqrt(7.5e17 / F)

    If you want it to take 3 days, you need 11 million Newtons of force, the most powerful machine humans have ever built was the Saturn V rocket, which put out about 35 million Newtons. But because of the square in the time variable, the longer you make the trip, the exponentially easier it gets. A 1 week trip will only take 2 million Newtons. 1 month: 100,000N: the force of an airbag.
     
    Last edited: Apr 22, 2016
  12. Guttersnipe
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    Guttersnipe Member

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    As has been said, decide what you want to happen in your story, then write it that way. The full range of possibilities are available, from 8 months in The Martian with an ion engine and very low thrust, all the way to a few days with constant acceleration. At 35 million miles (closest approach), with 1 G acceleration and deceleration, You get there in less than 2 days. So pick your requirement and write the tech accordingly.
     
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  13. ace.1991
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    ace.1991 Member

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    That is what I plan to do.

    I'm fully aware that the EM-Drive is very theoretical, but it's what I've chosen to use for my story.

    I've read various time-scales of using the technology ranging from 6 months to 10 weeks to get to Mars.

    All I'm asking for is some help coming up with a plausible transit time so I have a 'ballpark' to work with. Part of my story opens while in transit to Mars, so I kind of need that number, if nothing else.
     
  14. Guttersnipe
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    Guttersnipe Member

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    What people have said is that almost any number you care to use is plausible, as long as you match the technology to the transit time.

    Here's a suggestion: Write the story, using whatever timeline is needed for the plot to unfold. Then go back, look at the transit time you ended up with, and decide on a technology.

    You really are overthinking this.
     
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  15. tonguetied
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    tonguetied Contributing Member Contributor

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    "NASA's twin robot geologists, the Mars Exploration Rovers, launched toward Mars on June 10 and July 7, 2003, in search of answers about the history of water on Mars. They landed on Mars January 3 and January 24 PST, 2004 (January 4 and January 25 UTC, 2004). "

    As copied from http://mars.nasa.gov/mer/overview/

    So figure NASA took nearly 7 months to get there, of course their rocket technology was not what you are proposing. I think another recent post mentioned that it took 3 days to get to the Moon for the Apollo missions, I think that was a shadowfax contribution. You can obviously shorten it. You might find "Saturn Run" by John Sandford handy for a reference, however I will add to that the entire section about the ion propulsion system got boring very quickly and I doubt that very many people bothered to check the math.
     

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