Capitalism Doesn't Work... So What Would?

Discussion in 'Business & Economics' started by matthew809, Sep 22, 2008.

  1. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    To Keith1

    Attacks directed to me do not change astronomical facts* I cited, so there is no need to reply more to you until you answer my now four times asked question:

    What do you think one can bring back from an asteroid to even just a near Earth orbit that is not now available on Earth for less than 1% of the cost?**
    -----
    * Nothing man can do will ever change these facts but you are welcome to ignore them and dream instead. (More details in last paragraph.)

    ** Or less as advance in technology do reduce the cost of production on Earth. As an interesting example of this, is the top of the Washington Monument in DC. It is a tiny pyramid of not very pure aluminium, chemically produced before the Hall processed was invented. - It was the most precious metal then, far more valuable than gold, but now that metal is thrown in trash cans when you finish drinking your soft drink.

    However no future technology advances will cause the Earth and moon to not move around the sun at ~107,000 km/h or asteroids to cease their very different directions and speeds in orbit about the sun. Landing on one requires huge energy to change from Earth's velocity (speed and direction) to their's and then after taking on the cargo, huge energy is required to again re-match the Earth's velocity, just to co-orbit with it around the sun in near Earth orbit. Technology will not change the kinetic energy changes required. Going to the moon and back was very easy by comparison as both go around the sun with the same speed and direction.
     
    Last edited by a moderator: Jun 20, 2011
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  3. ElectricFetus Sanity going, going, gone Valued Senior Member

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    Jesus Billy, why do you exaggerate to such egregious extremes? The ship going there may or may not be solar sailed powered, the ships return, cargo ships manufacture at the asteroid from asteroid materials would be, lets say each one weighs 100 tons, and need to do a delta v of 0.8-2 km/s to return from a near earth asteroid to earth.[1] At earth's distance from the sun a 1 km^2 solar sail exerts 9 newtons of thrust[2], thus it would take only 0.28-0.7 years (15-37 weeks) to get 100 ton automated cargo ships up to speed, not "10,000 years", so once again stop talking out of your ass!

    [1]: http://echo.jpl.nasa.gov/~lance/delta_v/delta_v.rendezvous.html
    [2]: http://www.ugcs.caltech.edu/~diedrich/solarsails/intro/intro.html
    [3]: http://www.moonminer.com/Solar_sails.html
     
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  5. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    [1]:http://echo.jpl.nasa.gov/~lance/delta_v/delta_v.rendezvous.html
    [2]:http://www.ugcs.caltech.edu/~diedrich/solarsails/intro/intro.html
    [3]:http://www.moonminer.com/Solar_sails.html

    To answer your first question - I was just guessing but now want to do some checking. So here is the data from first link about the first choice (closest asteroid) with some non-essential parts omitted and rearranged to make it print better:

    ASTEROID..................... Earth.... MOON ... MARS
    a (AU)... e ..... i (deg)
    1.033 .. .024 ... 0.6 ....... 3.820 ... 0.637 ... 0.606 (Delta Vs in KM/s)

    The meaning of "a" is surely the semi-major axis, "e" the eccentricity and “I” the inclination of the asteroid's orbital plain wrt earth’s. Unfortunately, the text does not make clear if these Delta-Vs are just to get to the asteroid's location or to arrive there with velocity of its surface (i.e. to land on it). Can you clarify? I am not concerned with the gravitational attraction of what I think is a very tiny asteroid, perhaps not even having the 100 tons of mass you want to build a solar sail powered space out of and surely not likely to have any useful metal content.

    This asteroid is just outside the earth orbit and has 5.00% longer year,{sq root (1.033^3 )= years} so 10 years would be required for it to come to its annual closest approach to Earth from the other side of the sun and a far great number of years before that was also with it approximately in the Earth’s orbital plain. I.e. the distance between it and Earth ranges from slightly more than 2 AU down to little more than 0.033 AU.

    I mention this to point out that the distance traveled between by a solar sail returning can be increasing faster than the solar sail velocity can be accelerating it to catch up to Earth for many years, I think, as a launch from the asteroid it would initially have the asteroid’s slightly lower orbital speed than the earth has if it was launched when the asteroid was near its closest approach to Earth. Thus, the launch for return would needs to be when the Earth, with it higher orbital speed, was reducing the angular separation to the asteroid and these opportunities would come only about every 13 or 14 months. The return launch speed need to be accelerated to Earth’s speed almost exactly and timed so both asteroid and Earth arrive at the same point in space simultaneously (obviously a point on Earth’s trajectory).

    Quite a similar launch timing problem exists for the rocket powered trip to this nearest of all asteroids, but is less critical as the rocket has (while still with fuel) much greater acceleration ability. I.e. the quasi optimum launch opportunity to the asteroid also only reoccurs about every 13 months. Note this does not very directly relate to the fact the solar sail has extremely low acceleration capacity – just launch and return opportunities.
    Also note for more distant asteroids which might have some useful metal content, this minimum delay between the two launch is more than ~26 moths and the distance the solar sail is must travel (with less intense flux) is greater.

    I don’t have time now (1 AM for me) to continue but hope to later. In the mean time can you make clear what speed you are speaking of with the part of your text I have made bold. Is it how long (at 1AU) the huge sail you assume requires to get up to Delta-V? If so, are you then neglecting the time for traveling the constantly changing distance between asteroid and Earth when estimating the trip duration?

    Also is my memory correct that all of the 3 or so attempts to deploy much smaller solar sail than the 1 sq Km one you assume have failed to get the sail spread out as hopped? (I think it essentially impossible to deploy the large sail you are assuming.)
     
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  7. ElectricFetus Sanity going, going, gone Valued Senior Member

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    I'm pretty sure that is the delta-v to match velocity. But as you imply or exaggerate from, this is not landing on the moon or mars or any planet with significant gravity that must be faught against. Once velocity is match its an inconsequential delta-v to land on even a asteroid of a few kilometers in size. The escape velocity of Eros for example is only 10 m/s (23 mph) and its roughly 34.4×11.2×11.2 km and weighs 66.9 trillion metric tons, even if say 0.1% of that was nickel iron it would be worth up to $300 trillion.

    Stop being silly, you know that none of this adds up to 10,000 years of travel! Just because solar sails provide very little acceleration and thus must provide continues thrust for months to years does make timing and launch opportunities any more difficult, if it did than mission like Deep Space 1 and DAWN with ion propulsion engines providing only millinewtons of thrust, would never have been launch.

    Lets take a real good look at what the DAWN space probe is now doing. It weighs at most 1250 kg including 425 kg of propellant. Its ion thrusters put out at most 90 mN each, but it can only operate one at a time. This comes to 72-109 micronewtons of thrust per kilogram of space craft. I'm proposing a solar sail ship that weighs 100,000 kg and has a most 9 newtons of thrust, or 90 micronetwons per kilogram. So my hypothetical solar sail ship is in the same class as this real ion propelled ship, thrust wise. Now Dawn is on a mission, launched in 2007 its traveling to the giant Asteroid Vesta, accelerating there via ion propulsion and a gravity assist past mars, it then will decelerate to enter orbit about Vesta in 2012, hang there for 1 year, then accelerate leaving orbit and travel to the dwarf planet Ceres, decelerating and entering orbit about Ceres in 2015. Its doing all that in 8 years with the same amount of thrust as I'm proposing for my solar sailed cargo ship which is only traveling from an asteroid to earth, no decelerating on its way either simply rolling up its sail and crashing into earth or aero-capturing into earth orbit.


    Its not 10,000 years is it?
    But now lets do the math, lets assume we need to do 5 km/s of delta-v, not only to match the speed of the earth but also to catch up and slow down, so more then double the minimum, lets also assume we start out beyond mars space with an average thrust from the sail at only 4.5 newtons during the trip, it would take a 100 ton, 1 km^2 solar sailed ship 3.52 years to accumulate all the delta-v, not 10,000!

    Sail deployment from earth is a much different problem then sail manufacture in the weightless vacuum of space. I'm pretty sure that if we could produce a strong AI that smart enough to mine an asteroid and manufacture cargo ships, than the problems of deploying solar sails are trivial! Now until the former is achieve I'm not proposing we mine asteroids.

    Now I think I laid out the evidence pretty well, if you don't admit that 10,000 years was grossly wrong that that even less then 10 years is possible via solar sail to travel from an asteroid to earth, then it would be useless for me to argue with you any further on this thread.
     
  8. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    Sure that was a wide guess, based on memory and really referring to the whole go and return time laps (plus all the time lost in failed attempts, for an average). You have now made clear (or I have just understood) than you plan to go out by conventional rocket and build 1sq Km sail after arrival on asteroid I will reduce that wild guess to 5000 year.

    As, by memory, all of the attempts to deploy sails in NEO only ~1% as large as you have assumed all failed to deploy, I would assume you need to think in those smaller terms (especially as you are assuming light reflecting sail can even be made on the asteroid from asteroid mass) which cuts my wild guess down to ~50 years. (Also I would guess at best you have a 1 chance in 10 of get sail to deploy – like an increase in cost by factor of 10)

    Then one must consider how many years on the asteroid it would take to build the return ship (I’m not sure how much of rocket ship you plan to use.) without any of the resources used on earth to build space ships. Probably several trips / orbits about the sun - say 20 years of construction before launch of the solar sailer.

    I also think that the exact thrust the sail produces will have more than1% variation from plan so instead of arriving at Earth orbit planed intercept point with exactly Earth’s velocity (speed and direction) the return ship will miss Earth by more than 1000 miles and /or have "crash only speed" if it were lucky enough to get to same point in space that Earth is. As you only have, even with the assumed 1km^2 sail only 2 pounds of thrust to maneuver your 100 ton ship just getting back to earth I think is highly unlikely. A lot would depend upon how much you can tilt the sail or fold parts up* etc. to modify velocity months before planned arrival back to Earth. You will have accurate radar data on what the velocity and locations is for perhaps last half of return trip but it may already be too late to save when you start getting it. (At launch, Earth may even be on the other side of the sun.) You know asteroids out gas as they come back nearer to sun - that is more thrusting than you will have, so the initial velocity at launch and shortly afterwards may not be what was expected. This is MORE than "rocket science."

    I sort of summarized these and other unstated thoughts in my 10,000 year wild guess to quickly show my opinion /guess of the extremely low IMHO economic prospects and the technical prospects even in a few centuries from now.

    I think we should start a new thread to discuss more accurately solar sailing’s role in asteroid mining, if you are inclined to do more. As this is in B&E forum, I almost sure I and move current off thread posts there.

    *Folding up parts so sail is not symmetric about center of mass and makes a torque and may be most practical way to tilt the sail. Some means of "thrust control" is absolutely essential.

    PS If all goes much better than I expect, I think Earthlings will get to see for several hours after sun set a "man-made" moving "star" come close to earth before it swing by towards the sun. Last phase of the attempt to return to earth has space ship near where the earth recently was and sort of moving along the Earth's trajectory (must match directions of travel). I.e. chasing Earth but not yet fast enough to catch Earth - losing ground intitialy until Earth's gravity helps it but not so much that it crashes - damed critical to get that just right (Perhaps impossible to do without use of conventional rockets?) Perhaps I should jack my "over all index of success" back up to 1000 years? - i.e. could get a success once every 1000 years, if you are stupid enough to try again immediately after each failure.
     
    Last edited by a moderator: Jun 22, 2011
  9. ElectricFetus Sanity going, going, gone Valued Senior Member

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    First off stop speaking from memory because that clearly flawed! Second, your an asshole.

    First off solar sail deployment has in fact been achieved, and its probability of achievement is technology based, so once mastered it should work most of the time. Of course that is a useless skill for this task and it has nothing to do with manufacturing a solar sail in space.

    Second and again the big show stopping step is not solar sail deployment but strong AI, we need the automated mining ship to be smart enough to mine the asteroid, and build cargo ships. I'm sure that is at least a few decades away and as such provides plenty of time to master the other necessary technologies.

    That all dependent on how much material can be processes per unit of time by our mining ship. If it can convert nickel-iron asteroid feedstoke into purified metals at say 250 kg a day then it could manufacture a cargo ship every 4 months. Perhaps if it could also manufacture on site more mining and purifying equipment or perhaps even replicate its self than its manufacturing abilities would increase exponentially.

    Again the same problem occurs with ion propelled ships like DAWN and that mission is so far going as planned. Deep Space 1 proved that with something called "computers" it was possible to calculate the trajectory and adjust for targets ahead of time even while using a pitifully weak propulsion system, and to top it off they even demonstrated auto-navigation from the spacecraft its self!

    If strong AI is in fact several centuries away I would agree, if not then IMHO its economical viable far sooner. But the technology it self is not what you questioned, you originally stated:

    This statement raised nothing about the technology being problematic, it merely supposed that a solar sail would take that fucking long to just move about! So sure change the argument if you so want, but be warned that it appears that you simply will make up excuses to validate your belief in its economic inviability, that your not in any way open to changing your mind and as such there is no reason arguing with you. This is the same reason I've stop replying to you on many other threads: you bring up arguments, I show why they are wrong then you either nitpick about irrelevant things as if they somehow change the whole argument, and/or worse you simply resort back to your original arguments after a number of posts.

    I don't know, make me believe its not futile and I might reply to such a thread.
     
  10. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    To ElectricFetus

    Part of the problem in discussing solar sail return cargo ships is I see so many ways project can fail that I can only lump disasters together to talk about them. Thus let’s first just focus, in some detail, on the last week(s ?) of the return to earth, by solar sail. Say starting with craft having exactly the desired velocity at a point where separation from Earth is such that solar sail and gravity accelerations have same magnitude. I somewhat suspect that rocket thrust is required to both catch earth and yet not burn up in the atmosphere.

    My gut expectation / concern is that if the starting point is a meter closer to Earth than “perfect” ship burns up in atmosphere and if it is a meter more distant than prefect, Earth runs away too fast and ship orbits the sun instead of returning to earth.

    BTW, I have never raised any AI concerns as I am willing to grant that solar-powered AI robots will be MORE capable and MORE intelligent than man when ready to try to mine an asteroid to return some iron/ nickel (or just pure nickel, if that is more economical) metal to earth.

    I will start new thread called “Asteroid Mining with Solar Sail Return to Earth – Economically Feasible? In the “General Science & Technology” forum. Then clean up the mess we have made in B&E, mainly by deletion – I.e. start over, hopefully mainly solving problems working backwards from mission termination as first paragraph suggest.

    PS you said solar sail has been successfully deployed (i.e thrust nearly as expected) in near earth orbit. Can you give a reference?
     
  11. ElectricFetus Sanity going, going, gone Valued Senior Member

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    I disagree, and again space craft like Deep Space 1 and DAWN verify that with even today's computer automation precision travel through the solar system can be achieved even with a micronewtons per kilogram of thrust. I don't see how you can keep denying this fact, with the already flight tested proof!

    Now if your assuming there will be no propellant at all I don't see that is a requirement, certainly it could be possible to have some conventional propulsion system for attitude control and minor flight correction, say 50 detla-v worth.

    Space probes have at best had to hit points of several kilometers, not meters. MER for example, and it did so with a tiny amount of onboard propellant after enter mars transfer orbit in order to make corrections.

    http://www.planetary.org/news/2004/0810_Japanese_Researchers_Successfully_Test.html
    http://en.wikipedia.org/wiki/IKAROS

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    Again try to use the internet to learn.
     
    Last edited: Jun 22, 2011
  12. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    To Electric Fetus:From your wiki link:

    "The IKAROS probe is the world's first spacecraft to use solar sailing as the main propulsion." It was only launched 13 moths ago, and I did not know about it. - Thanks. I only knew about some earlier ones that were not able to deploy their sail.

    The IKAROS sail is a square 20m on an edge. You are suggesting one that is 1000m on an edge 50^2 = 2,500 times more area to deploy. Thus this first successful deployment, after failures, does not persuade me a 1km square sail, made on the asteroid from asteroid materials, can be deployed on or near the asteroid, especially if the asteroid is out gassing as all do when they come back near the sun after long time in cold space. (The out gassed mass moves with / near the asteroid, until solar pressure stretches it behind like a comet's tail.) So I'll stick with my estimate that there will be 9 failures for each successful deployment until there is some evidence to the contrary. I.e. that alone would make the average cost a magnitude greater than cost of a successful mission.

    For half a day, I am not able to start new thread. I get: "HTTP error 500 (Internal Server Error): An unexpected condition was encountered while the server was attempting to fulfill the request." msg. Other functions do work (like posting this).

    You are missing my point about the last, near-Earth, stage of the return. I think/ guess even 1km square solar sail's tiny acceleration of your 100ton ship will provide much too little reduction in Earth's gravitational acceleration. I.e. the returning craft will fall to Earth (if it was lucky enough to even get near Earth with ~1% variation from designed solar sail thrust magnitude and ~1 degree errors in orientation) and burn up in atmosphere. That is you will need real rockets to break the fall to Earth like currently used, so for return to Earth, the solar sail is not worth the problems. I think out bound only solar sails are a good idea.

    Thus in the thread I wanted to post, we start with just this "final problem." Its solution will determine the sail area, A, required for a mass "m" returning. I have admitted my prior comment were just guesses*, but now want to start calculating in a new, not B&E forum. Determining the least A/m ratio, required not to burn up, seems to me like a good starting point as does not depend up which asteroid is chosen.

    ---------
    * You are just making guesses too (without admitting it) as extrapolation of first solar sail success by 2500 fold does not prove anything. E.g. it is only your guess that 1km^2 is OK for 100ton ship. The A/m ratio must be calculated. Also you are postulating the ability to even fabricate the sail on the asteroid with nothing available when the out bound ship arrives there. Making a 1Km^2 sail that can be deployed in space even with all the construction resources here on Earth, in view of the tiny sail failures, I doubt can be done successfully on the first try.

    Having some rim mass and slow spinning it probably will work to keep sail deployed approximately flat once it is nearly spread out, but the surface tension (force /unit area) near the hub will be much greater than near the rim so the thickness of the sail probably need to decrease by something like a guessed factor of 100 or more as you go away from the hub - that ain't going to be easy to make on the asteroid, not even on Earth for that matter and surely has never been done.

    Also, do you plan to steer the sail's thrust direction by tilting the sail orientation? To me that would seem achievable by covering parts near the rim briefly to make a torque. (Center of solar force no longer passes thur center of mass.) Doing thrust vector control this way must cope with the gyroscopic force due to the spinning rim mass. - This will at the very least make the sail non-plainer when done, and probably induce oscillation waves in it as rim masses "overshoot" when trying to return the sail to the flat form. (There ain't no air up there for damping.) These problems and many others are best deferred, IMHO, until we know the minimum A/m ratio that is possible.
     
    Last edited by a moderator: Jun 22, 2011
  13. ElectricFetus Sanity going, going, gone Valued Senior Member

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    First of all not all asteroids out-gas, in fact few do, your thinking comets! Second off your estimate is based off of nothing, you can't make such an estimate with just one deployment. We don't know if the IKAROS design would work 95% of the time or only 1% of the time and thus its successful deployment so far is a fluke, we don't know if the design can be scaled up or not, we know very little. From that it impossible to just assume that it won't work most of the time. That like it being 1870's and saying heavier than air manned flight won't work 9 out of 10 times thus it will never be economical or safe. In short this is pure biased bull shit on your part.

    And again and again I repeat such accuracy has been achieved with existing spacecraft propelled with propulsion systems of the same thrust as a solar sail. The DAWN space craft managed a perfect gravity assist maneuver around mars, achieving far less then 1% variance in accuracy! It flew by mars precisely passing to within 550 km of Mars, did it crash, no, did it miss its target, no, it did that with ion propulsion of the same thrust range as a solar sail. So in short your continued use of this argument in light of the facts is pure denialism!

    There is no such value. You need to ask how much thrust you want and how much percentage mass you want dedicated to the sails and sail structure. As PROVEN BY EXISTING SPACECRAFT ~90 micro-newtons per kilogram of mass should be adequate for enough precision for re-entry or aerocapture. Assuming 3 g per meter sails, which is achievable with today's technology and should easily be achieved with a space produced sail, a 1 km^2 sail would weigh just 3 tons, lets quadruple that to account say for a support structure allowing for a static sail (a centrifugal one would probably weigh less) and that still only 12% of the 100 tons mass, so the rest can be purified metals and heat shield for entry or aerocapture. Now the sail might be designed to fold up or not, it might be better to have it "drop" the cargo and navigate to fly by earth while the cargo hits earth or is aerocaptured, then the sail can travel back to the asteroid. Without its cargo mass its going to have ~750 micro newtons per kg of thrust available, ~8 times improvement so its going to accelerate home ~8 times faster then it left.

    IKAROS is a centrifugal design, and does attitude control via LCD panels along the out perimeter of the sail. I would assume that for a kilometer or larger sail assembled in space a static design would be better. A static square sail would have smaller attitude controlling sails that could be turned to face the sun and induce torque.
    http://physics111.files.wordpress.com/2010/12/solarsail_msfc.jpg
     
  14. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    I'll only reply briefly to part here, more later in new thread.
    You are still missing three of my points. (1) The gravitational attraction acts on the total mass, m, as craft nears earth, not just the sail, but I admit it will distort the surface of the 1km^2 sail, changing the net force and thrust direction. (vector F) when nearing Earth on the return.

    (2) Thus, if the sail area, A, is too small, then the A/m ratio will be too small to avoid burn up as the gravity act on m but the sun force only on A. I.e. too small A/m will experience nearly all of the gravitational pull on the craft, fall towards Earth and burn up in the atmosphere. The effect of gravity's acceleration on m must be very significantly reduced / off set / by another acceleration acting on m, as it always is with rockets. If the A is too small for the m, the craft burns up. - That is why there is a minimum A/m ratio.

    (3) Yes an ion engine also has low acceleration capacity, but its thrust can be quickly and precisely adjusted. To do that significantly with a large sail is complex and adds more weight. For example, a 90 degree rotation is the best way from a weight an cost POV, I think, to turn off the accelerating force. (You may need to turn off the force periodically if the surface reflectivity turned out to be 79% instead of the expected 75%, etc.) That is quite more complex than flip of a switch with an ion engine, especially if sail is spinning to keep extended and flat. The gyroscopic forces when it is turned will bend it, set up oscillations in its surface with no air to damp the "overshoots" etc.

    With a solar sail, the net direction and magnitude of the vector force are not independently adjustable by slight tilting - both change together. Thus this also makes control of the vector force of one big sail a lot more complex and slower than with several small ion thrusters. Thus, saying I am ignoring the fact precise navigation has already been done with weak thrust of ion engines is not an acceptable reply to may concerns. This has NOT been done with a large sail.
     
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  15. ElectricFetus Sanity going, going, gone Valued Senior Member

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    I'm sure tidal forces can be accounted for ahead of time!

    No i'm not understanding what the fuck your saying! Are you saying gravity does not act equally on all mass, that somehow the cargo would be pulled on more then the sail? Are you saying that anything that gets near a planet must ultimately be sucked in and crash with the planet, that a fly-by is not possible? Are you saying tidal forces would ripe it apart?

    Why? How does changing the tack of the sail add weight? If you want to talk about precision how about the MESSENGER probe with microNewtons of thrust proved by its solar panels were used to making flight corrections and INCREASED PRECISION of its course.

    Again this can be predicted and compensated for, this thing isn't going to be flown by monkeys but bey very precises computers! And a static sail is not going to have significant gyroscopic forces. Turning and changing tack could either be done with reaction wheels or with secondary sail flaps or both, the former has been used in space crafts for decades, and weigh very little.

    So? This does not explain how you believe a solar sail could not make a fly by or not undergo precision courses, your argument has no viable premise, your merely saying "oh it has not been done exactly like this, therefor it can't!".
     
  16. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    Then let me make it very simple for you. If sail area A is reduced to zero, you just fall in Earth's gravity, which act on m, and burn up in the air. You break the fall by sun acting on A. You need a certain size A for any total m to prevent this burn up. Thus there is a minimum A/m ratio. (Which I hope will be the first calculation if the new thread can ever get set up.) You can not just guess the OK A/m ratio as you have done. Return to earth is very different problem than travel out to asteroid X, where more time for the trip can let a smaller sail work.

    I try more, by analogy, to explain to you why even a meter shift in location from the "perfect one" for the ship's velocity is important. It is sort of like a rain drop falling near the top of a mountain. - One meter shift can result in it running down the other side.

    The solar sail ship must approach the Earth from behind (where Earth recently was) with slightly more than Earth's orbital speed and essentially the same direction of travel to catch up to Earth, but but it cannot keep the sail set to accelerate ship towards Earth as it was when "catching up." At some quite critical distance from earth, the sail must turn NOT ONLY to begin to reduce the ship's speed to Earth's orbital speed, BUT ALSO to cancel out most of Earth's gravitational acceleration to make the net acceleration of the ship towards earth small enough to survive entry into the atmosphere.

    Like the raindrop analogy, turning sail too soon will let the earth escape / run away from ship / and ship goes into orbit about the sun. Turning the sail to late and ship burns up it the atmosphere. Is this problem clear now?
     
    Last edited by a moderator: Jun 22, 2011
  17. ElectricFetus Sanity going, going, gone Valued Senior Member

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    Bullshit, then spacecraft as is would do this, they don't, they can fly by, make gravities assists, etc, they don't need to spend any fuel either, they merely rely on their existing velocity.

    Double bullshit, once imparted with appropriate velocity the solar sail can coast right past the earth if desired, our past precise altitudes for re-entry or aerocature, just like any other spaceship. So there is no area to mass ratio requirement.

    No this is complete bullshit.

    If this was true then all existing spacecraft would not be able to do what they do: their course precisions is in kilometers not meters.

    Bullshit, existing spacecraft don't need to make any such maneuvers to fly by or enter. They don't need to expend any fuel fighting gravity, decelerating anything like that for flying by, only to enter orbit or land do they need to decelerate and a heat shield has been proven time again to make the possible.

    No, because its bullshit. There is no need for continues thrusting, there is no need for the maneuvers you describe and there is no need for the precision you describe.
     
    Last edited: Jun 22, 2011
  18. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    NO question! Yes you can fly by, etc. I though you wanted to land the cargo. Even to go into captured low Earth orbit you will need thrust (or air drag).

    It is well known, for example, that the moon could not been captured by the Earth (without a third body helping). Your returning no-thrust cargo ship is also impossible to capture. Both moon creation and a burn up crash of no-thrust capacity cargo ship are possible with high speed impact.

    Understand this this way: Cargo ship came from far away to near earth (less than a radus away) thus it will gain essentially the earth's escape velocity as it falls in Earth's gravity field, and that is what it will do (escape), not be captured. Perhaps it will help you to think you have made a movie - it can be played backwards - same as forward beginning and end for the movie have cargo ship far from Earth. Only way to avoid this is have the "third body" be some air - skim thru without burning up, but the density of the high atmosphere varies a lot - why NASA never tried to reduce the retro rocket fuel requirements by dip thru the high atmosphere - very critical and unpredictable trick to do.
     
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  19. ElectricFetus Sanity going, going, gone Valued Senior Member

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    First off to land/re-enter requires no thrust, take Star Dust for example, the capsule had no propulsion of its own yet returned to earth precisely. Once the trajectory is set, the cargo can be dropped from the solar sail or the sail is folded up and it will slam into the earth atmosphere, slow down and land via the help of a heat shield and maybe a parachute (considering its cargo is hard metal it could just crash land).

    Second, look up aerocapture. In which the ship would bounce off the atmosphere after losing enough velocity to the air remain in orbit. I admit some fuel may need to be used immediately after aerocapture to raise the perigee, but after that the sails could be unfurled and the orbit modified how ever desired. Total fuel used would only be for a ~0.1 km/s delta-v.

    I prefer the fly-by approach better now as the sail can be reused and doesn't need to be folded allowing for extremely light sails of less than 2 g/m^2.
     
  20. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

    Messages:
    23,198
    To ElectricFetus:
    First note that the stardust mission confirms what I said. – I.e. that the re-entry speed, without retro-rockets or huge A/m that can do the same breaking of fall in Earth’s gravity job, will be like the Earth’s escape velocity (11.2 km/sec.) or a little higher if one does not want to spend more years in the trip coming back from the asteroid.
    “... the Sample Return Capsule successfully separated from Stardust and re-entered the Earth's atmosphere at a velocity of 12.9 km/s, the fastest re-entry speed into Earth's atmosphere ever achieved by a man-made object. ...” From your link: Star Dust
    Yes, I also said it is impossible to capture / land on earth/ from far away without either (1) a direct hit which vaporize most if not all of the returning mass Or (2) conventional "retro-rockets," which you have assumed are replaced by a solar sail, Or (3) some “third body” assisting the capture. I mentioned the air can be that “third body” and energy dissipation in the air was used for the stardust return capsule, SRC, shown below open as it was in space to capture "space dust" in a foam gel array.

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    Here you can see two men putting the ablative heat shield on the SRC and some of the capsule’s controls:

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    I could not find the mass of the heat shield so will just guess from this photo it was more than 20 pounds and less than 100, so let’s say 25kg. The mission is well named (“stardust”). It collected particles too small to be seen with the unaided eye. Let’s guess 10 micrograms total. I.e. the heat shield mass was on the order of 2.5 billion times the payload mass.

    But that is a very unfair comparison if you were returning more than say 100 kg of iron/nickel (worth less than $1,000, I think, as mixed, not pure) as the re-entry protected controls could be essential the same weight as used in the star dust mission. In the limit, let’s assume the control mass can be neglected – I.e. only the pay load and heat shield masses need to be considered.

    One needs to know the energy per gram required to vaporize the heat shield material. I would guess that is on the order of 1000 calories /gm as for water it is 540cal./gm if memory serves me correctly, say 4000 Joules/ gram but let’s be generous and say, heat shield dissipated 5E6 J / kg. Now a kg traveling slower than stardust was at re-entry, let’s say at say only 10km/s (0.5mV^2 = 5E7 joules) assuming the sail did do some slowing below escape velocity during the fall to earth.

    Thus this first cut at the problem shows that the heat shield, required to keep the pay load from vaporizing in the atmosphere will be about 10 times more massive than the pay load.* Now as a “second cut” at the problem we realize that the re-entry mass is 10 times larger than the payload mass if returning from an asteroid without much breaking of the fall in earth’s gravity. I.e. the return capsule delivering 100Kg of iron/nickel weights at least (I don’t bother to do the “third cut at the problem”) 1100Kg. This 1000 kg of heat shield material costs per gram more than iron (almost anything but water does) but let’s just say it is only twice as expensive per installed kg.

    Thus already, just consideration of the re-entry problem, shows that the cost is twenty times greater than the value returned cargo!**

    ---------
    * Don’t tell me that the heat shield used on the Apollo return capsule was not a significant cost as I know that. Yes my estimated heat shield to pay load mass ratio (~10) is a much higher ratio than the Apollo return capsule needed, but it was returning to atmosphere at much slower speed than escape velocity when entering the atmosphere. The KE to be dissipated goes as the square of the velocity. If you can find the percentage of Apollo return capsule that was heat shield mass and increase it by the square of the velocities ratios at the moment of atmosphere re-entry that would avoid needing to know (or guess, as I did) the vaporization energy per kg of the heat shield.

    I suspect that the use by Apollo of retro-rocket breaking during fall to Earth made Apollo capsule’s speed at atmosphere re-enrty lower by more than a factor of 100 (with square of >10,000). But will accept a correctly done calculation based on Apollo heat shield facts, if you can find the needed data. (I.e. speed ratios at atmospheric re-entry, percent of Apollo capsule mass that was heat shield was heat shield & cost of installed heat shield per kg)

    ** Thus as the cost of the heat shield is a negligible part of the mission cost, I will stick with my earlier estimate that mining metals on Earth (or recycling them from trash) is ~10,000 times cheaper than bringing them back from an asteroid. Also, I will again admit that in early posts, without any cost analysis, I quickly, and erroneously, summarized the 10,000 factor as 10,000 years. - One can not estimate the duration of the mission until the specific asteroid is chosen and many other mission design factors are known but for a "one liner" the "10,000 year" gives a good idea of how silly asteroid mining is.
     
    Last edited by a moderator: Jun 24, 2011
  21. billvon Valued Senior Member

    Messages:
    14,518
    Let's see:

    The one successful solar sail we have seen is on the Ikaros; it's 20mx20m and weighs 15kG. Scaling that up, your sail is going to weigh 3750 tons. (It would likely weigh more than that to handle the additional thrust, but let's go with this for now.)

    If you go with your assumption of .7 years to accelerate 100 tons, then accelerating 3850 tons to the same speed would take 27 years.

    Even if you cut that weight by a factor of 2 (difficult, but let's say you can do it) it's going to be well over 10 years before you see that speed change. And of course that's just the start of its journey.

    Skitt's Law would certainly seem to apply here.
     
  22. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

    Messages:
    23,198
    Yes it does, and it would also help if she attacked my facts or analysis instead of me. I am growing tired of being told I am posting BS or talking out of my ass with nothing but irrelevant links (like her most recent to the Stardust fly-by mission which actually confirms what I said) to back that up.
     
    Last edited by a moderator: Jun 23, 2011
  23. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

    Messages:
    23,198
    More than five years ago, mynameisDan said: " America is the greatest nation on the face of the earth in all of history in terms of freedom for the most amount of people. Warts and all, nothing else compares, nothing." I replied in post here: http://www.sciforums.com/showthread...o-What-Would&p=2045013&viewfull=1#post2045013 most of which follows
    ** This "I am my brother's keeper" POV instilled by their early education system is why Norwegian's would never think of spending their oil wealth on the current generation. It belongs to Norwegians who will be born 1000 years from now too.*** It is why "We are a mutually responsible for the advance of all group" they gladly pay high tax rates so all can have high quality health care, essentially for zero out of pocket cost. Why education thru College is free, but teachers often do have a year of two obligation to teach in rural areas needing them. (Why they need to be fluent in all 3 versions of Norwegian) It is why the streets are clean and why my less than 12 year old (and very pretty) daughter was safe traveling alone on a long and complex journey in Norway. (She was not allowed to even go alone to a local movie in the USA.)

    *** But they are smart enough to know that if it just remains in the ground, their is never any benefit, so the oil wealth is being transformed in a diversifired asset for the benefit of all Norwegians, even Norwegians born in 3001.
     

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