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Thread: Mars Direct/indirect

  1. #1
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    Mars Direct/indirect

    hey folks,
    I been watching a lot of stuff lately on the interweb about Zubrins Mars direct, and its various watered down versions.
    Well, hes got me convinced- But I am not so naive as to realise this is just one side of the argument.
    So what I am wondering, are there any critiques of the plan available, ones which deal with the practicalities and technical challenges rather than politics?
    I am particularly interested in the concept of centrifugal artificial gravity, tethering a spent engine and rotating around center of mass. This just looks a bit too good to be true. What are the issues with it? Why isnt it being done, does it solve physiological problems by inducing new ones?
    cheers muchly

  2. #2
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    I can't direct you to any specific discussion, but it has
    all been discussed in considerable detail many times,
    and will continue to be discussed in even more detail.

    Why does a tether centrifuge look "too good to be true"?
    It's an idea I've known about for decades. It hasn't been
    done yet mainly because there has been no pressing
    need to do it. It will most likely first be done as a test
    of a manned Mars system, the test being done in Earth
    orbit, and not lasting as long as an actual Mars trip.

    The only physiological problem a centrifuge would be
    likely to induce is nausea from strong coriolis forces
    on the head. That is most easily avoided by making
    the turning radius very large. Which makes structural
    strength of the system more challenging, which is a
    problem in that it makes the system heavier.

    I favor using multiple launches of moderately large
    boosters to get the parts of the system into LEO, and
    assembling them similar to ISS construction. I don't
    believe any EVAs should be required. Just connect
    the modules together, test the connections, and go.
    NASA launched two Gemini spacecraft a few days
    apart in the mid-1960's. We should be able to launch
    two or three or four modules over a period of a week
    or two in order to have all the desired equipment and
    supplies without requiring a super-sized booster.

    -- Jeff, in Minneapolis
    Last edited by Jeff Root; 2012-Apr-03 at 04:59 PM. Reason: where'd that typo come from?
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

  3. #3
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    hi jeff, thanks for reply.
    i say too good to be true because Zubrin suggests manned exploration is being held up on the promise of faster propulsion systems which are necessary to prevent the problems of zero gravity.
    there does also seem to be a focus more on centrifugal chambers within within craft subsequently.
    excuse my ignorance, ive not really looked that closely at proposed missions before

  4. #4
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    The Coriolis effect means that when you drop an object it will hit the floor in a place you don't expect. It won't fall straight down since your feet (and body) have moved since the object were released.
    Imagine peas falling off your fork. They won't hit your plate. They may not even hit the table.
    Also imagine looking out the window.
    We don't know what the long term effects of the Coriolis effect will have on the psyche of astronauts.

  5. #5
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    Plus, along with the structural mass that Jeff talked about to make the rotation as long as possible, there's shielding to add to that mass, unless there's further progress on generating a field that will steer most radiation away.

    I think we can get to Mars okay. I just don't think we're ready to stay just yet, and that's a long trip to just collect a few rocks and head home.

  6. #6
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    Quote Originally Posted by samkent View Post
    We don't know what the long term effects of the Coriolis effect will have on the psyche of astronauts.
    It's making me crazy just thinking about it.

    Have any experiments been done to study how much we'd be able to feel the abnormalities of gravity due to rotation? How big would the spacecraft need to be to avoid noticeable effects?

  7. #7
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    isnt the answer to making you feel dizzy when looking out the window....dont look out the window?
    i read that the coriolis effect is pretty minimal at lower rotation speeds, so its really an engineering problem for how long you can make the tether right?
    from wiki (sorry) i got the figure that to create 1g at 2rpm would require radius of rotation to be 224meters
    The point raised about radiation- well thats another issue i wanted to understand if Zubrin is under playing the problem.
    As i am sure you all know, his plan was to use supplies as a partial shield, and his case being that his plan would expose the crew to double the radiation that has been recieved by long stay space station crew(i dont recall the figures, sorry) and that it equates to a 1% increase in developing a cancer at some time in the future.

  8. #8
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    Lets see if we assume that .75 g is good enough.
    And if the peanut misses your mouth and falls 4.5 feet to the floor.
    I would accept a 2-3 inch radius of expected impact zone.
    Would that be 3 inches of rotation in .5 seconds? Off the cuff numbers.
    That would be a BIG radius.

  9. #9
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    I would expect the artificial gravity on a manned Mars
    spacecraft to be about equal to Mars surface gravity,
    which is 0.38 Earth surface gravity. That would be its
    maximum value, in the spacecraft's lowest habitable
    level.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

  10. #10
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    Quote Originally Posted by samkent View Post
    Lets see if we assume that .75 g is good enough.
    And if the peanut misses your mouth and falls 4.5 feet to the floor.
    I would accept a 2-3 inch radius of expected impact zone.
    Would that be 3 inches of rotation in .5 seconds? Off the cuff numbers.
    That would be a BIG radius.
    I think that it will be one of those things that as long as it's not way off what's normal, you'd learn to adapt to the differences. From this 1997 paper (http://www.spacefuture.com/archive/a...habitats.shtml) I found this graphic, and at number 1 for 1 g seems a reasonable difference. Of course that's a big habitat, so the first travelers will have to adjust, but that's always true.



    Falling objects are the least of the problems. Rapid movement up would be very disorienting, and movement spinward and antispinward would have to be carefully done as well.

    Dropping the g value (and thus the rotation speed for x radius) reduces the problems, so likely we wouldn't do 1 g until we build very large structures, and maybe not even then. Why settle for earth normal if less (like .75 or so) works okay?

  11. #11
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    thanks everyone for their contributions.
    The reason i was so taken by what Zubrin has to say is that what he proposed was not a flag and footprint exercise.
    I would hate for the eventual plan to be that. going to plant a flag would be such a waste of resources when there is still so much science that needs to be done. Two or three scientists on the surface of Mars for 18 months.... that is what made me sit up and listen.

  12. #12
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    Quote Originally Posted by Glom View Post
    It's making me crazy just thinking about it.

    Have any experiments been done to study how much we'd be able to feel the abnormalities of gravity due to rotation? How big would the spacecraft need to be to avoid noticeable effects?
    While early studies suggested a slow, and therefore large wheel would be required, some more recent studies suggest that, with time to acclimatize, higher revolutions per minute may be possible.

  13. #13
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    I haven't found any singular critique essays of Zubrin's Mars Direct, but small criticisms here and there. I think he tends to gloss over some of the potential engineering difficulties and "unknowns", such as

    1. Landing several 40+ ton vehicles on Mars. A review of the "Mars Underground" Film about Zubrin said that one of the NASA guys brought this up.
    2. The effects of low-gravity on human beings for extended periods of time (such as the 0.38 gravity his astronauts will be in for more than a year).

  14. #14
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    As a side question.
    If one weighs 150lbs with the Earth spinning. What would we weigh if the Earth stopped spinning?

  15. #15
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    Using a=v^2/r for Earth's values, the force from the spin at the equator, ignoring anything else, is .0358 m/s^2. So you would feel a bit heavier, a few ounces more, but probably not notice. Everything crashing around you because the Earth stopped would be a lot more distracting.

  16. #16
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    Quote Originally Posted by samkent View Post
    As a side question.
    If one weighs 150lbs with the Earth spinning. What would we weigh if the Earth stopped spinning?
    The centrifugal effect is a little less than half a percent. So in your example, the difference would be less than a pound.
    Conserve energy. Commute with the Hamiltonian.

  17. #17
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    So all I have to do to lose a pound is go to the North Pole? Cool!
    Cum catapultae proscriptae erunt tum soli proscript catapultas habebunt.

  18. #18
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    Quote Originally Posted by Trebuchet View Post
    So all I have to do to lose a pound is go to the North Pole? Cool!
    No. You go from the pole to the equator.

    But that's only force we're talking about. It doesn't change your waistline and the lighter weight probably means the muscles won't get as much of a work out shifted your fat butt around and so your figure will get even worse.

  19. #19
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    This is a total tangent, but on one of the "Wonders of the..." shows Brian Cox in a centrifuge to show the effects on the human body.

    The funny bit is the segment starts with Cox in the centrifuge with the man who was going to be controlling the machine. Obviously, they just wanted to show the exciting part. Then they filmed this man strapping Brian Cox in, latch and lock the door on the centrifuge, leave the room and lock the door, travel up several flights of stairs and enter a locked room where the controls are located.

    The man then asks "How are you doing?"

    I was expecting Brian Cox to respond "Why did you run so far away?!?"
    Solfe

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