Friday, May 27, 2011

Rebirth Of The Spaceship

Over the last year the space advocate community has splintered into two major groups in answering the question "where should we go next?" Moon First or Mars First. This division was present in the Review of Human Spaceflight (aka Augustine) committee's final report in late 2009, with the surprising conclusion that there isn't the funds for either, suggesting a number of intermediate destinations first - including asteroids. However, as few people consider asteroids to be truly interesting destinations for the human utilization of space (except me!), the debate rages on.

Many Moon First advocates are "Mars Next" advocates while most Mars First advocates are "Moon Again?" detractors. The former claim that Mars exploration will benefit from lunar exploration, particularly in experience and risk reduction, and perhaps the procurement of propellant. The latter claim that lunar exploration is just a distraction and want to avoid the risk of being bogged down by another expensive obligation (read: another ISS).

NASA, and the Congress, is hedging their bets.. declining to select a camp and insisting that the so-called flexible path will have "off-ramps" for either lunar or Mars exploration.

Perhaps as a result of the deferment, an old camp has resurfaced with a strong central tenant: the true "spaceship". Defined loosely as a vehicle which is assembled in orbit and is never intended to land on a planetary body - although it may do aerobraking maneuvers in a planetary atmosphere. Spaceship advocates talk about lander vehicles rarely and, although the Moon is recognized as a nice buoy to fly around in a shakedown cruise, the intended destination is clearly Mars.

For many years, this camp has been silenced by a powerful force: The Mars Society and its charismatic leader. With a desire to cut out all distractions, Bob Zubrin has rallied against "Battlestar Galactica scale plans" for getting to Mars, advocating trips of endurance of small crews in tightly packed modules - small enough to fit on the top of a single heavy lift launch vehicle and launched directly from the surface of the Earth to the surface of Mars. What happened?

It seems that the last 20 years of advocating for the simple, elegant, and dangerous Mars Direct plan has been easily swept aside with just a single picture:



This sharp looking spaceship, with its command deck off to the side like the Millennium Falcon, and its inflatable artificial gravity ring promising Bigelow budget sweetness, has inflamed a deep longing for the sci-fi universe we were all promised - humans exploring space for years at a time with large crews.

The problem is propulsion. The tiny Firefly-like cluster on the rear of the ship is woefully inadequate for even the most advanced nuclear thermal propulsion system. The solar panel array is football fields too small for a solar-electric propulsion (or SEP) system. A chemical rocket stage to throw this vehicle to Mars and back would be so much bigger than the vehicle that we'd have trouble seeing it without zooming in... or so I've heard. How true is this objection?

The first problem comes when the objector talks about assembling the spaceship in low Earth orbit. This is an understandable assumption given that all on-orbit assembly to-date has been done in LEO, namely the international space station. However, for some time now the Earth-Moon Lagrangian points have been identified as the perfect location for staging for deep-space missions. This is not to say that no on-orbit assembly would be done in LEO, but once completed the resulting module could be ferried by a SEP tug up to L1. Whereas crew transfer vehicles would take the faster, more energetic path.


Often platforms at the Lagrangian points have been called "gateway stations" and for good reason. It takes less than 1 km/s of delta-v to go from L2 to a Mars transfer orbit. Transiting a large structure from L1 to L2 requires about 100 m/s of delta-v if you need to do it fast, but can be done with just 10 m/s or less if you take your time.

The Moon is so close and lunar water is so abundant that can be cracked into cryogenic propellants or used for radiation shielding, drinking, grow crops, etc. A purely chemical propulsion system quickly becomes feasible, but some other techniques such as solar sailing appear to be very interesting to me.

Although that could be because I just saw the latest Johnny Depp pirate movie. Arrgghh.

44 comments:

  1. Sigh! here we go again
    600 million cubic metres. Doubled (For the South Pole. Not a given.) = 1.2 x 10^9 metre^3 A little over a Gigatonne!
    Sounds a lot doesn't it?
    But, for conveniences sake, let's convert that into cubic km =1.2 km^3

    Lake Superior (Now that is a lot of water!) 12,000 km^3 Whoops too big! Perhaps a little smaller and human made:
    Lake Mead 31.92 km^3 (Currently running out of water...) Getting closer. O.K. let's look for something smaller
    Loch Ness Scotland 7.4 km^3 Smaller still

    (and so on...) until

    Lake Windermere 0.98 km^3 (being generous) the largest natural fresh water lake in England.
    Again being generous it supports a rural foodstuff growing population of ~16,000.
    Doesn't seen so "abundant" now does it?
    Even with perfect recycling and perfect life support systems and perfect seals ...and a policy of making sure that any transients lose weight during their Lunar Vacation; any "Lunar Authority" allowing this precious hydrogen to be exported as fuel or in the form of foodstuffs would be overthrown by violent revolution. Perhaps someone should write a book...

    Sorry Trent but apart from the few American exceptionalists who seem to believe that 20 July 1969 somehow gives America and its corporatist vision: pre-eminent domain over such a (very finite) lunar resource is not living in the real world.
    However if we consider Ceres. Now that is (potentially) a lot of water. Even more than Lake Superior!
    http://brobof.wordpress.com/2009/11/27/moon-pie/

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  2. If you don't think 600 million tons is a lot then you're not going to think anything is a lot. To send a 200 ton spacecraft from L2 on its way to Mars you need about 53 tons of LH2/LOX.. you can send that up from Earth if you prefer.

    Oh, and who said anything about NASA.. it's not like they're capable of doing anything anymore.

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  3. Victor Moraes10:44 PM

    The moon is cheaper to go, but more expensive to stay. Mars is more expensive to go, but cheaper to stay. The moon has no protective atmosphere, zero atmospheric pressure, extreme variation in temperature, with extremely hot (and this is hell), and water on the moon, although it is abundant, is very mixed with lunar soil and to this semparar water quality, it will consume much energy (and if you think of solar panels, you must remember that the moon is a long time without the sunlight in some of their faces, it is in opposition). A base on the moon is expensive and complicated.

    On Mars au think it's simpler. NASA is already know. Both know that they changed the mission of Opportunity, which sought to water, to seek life. Mars because water is simple. Simply break down the methane. The pyrolysis of methane. You add oxygen to methane and burns down, releasing carbon monoxide and water. Hot water. So we have fire and water on Mars. And the oxygen on Mars, NASA already knows how to get from the CO2. Mars also has a large variation in temperature, but is cheaper and easier to protect you from the cold (Mars is cold) than the heat (the moon is hot). And the air to breathe easy, getting oxygen from carbon dioxide. Nitrogen is abundant in the soil of Mars. So it is easier to colonize Mars. And cheap, despite the distance.

    I say this because NASA has carried out studies of new engines of internal combustion is not my fault deduction. A new propulsion system, a revolutionary. The displacement of the ship will be done by physical impulses, not by displacement of hot material and expansive as is done today. Certainly better and simpler than the ion engine.

    It is true that NASA is with good plans, will take the opportunity to build something new. MPCV is just a name to attend the Congress. The Orion design their systems can be reused, but will certainly not be taken advantage of its design, its form. I believe MPCV be a completely different ship Orion, although many use the Orion systems. Bolden said that both MPCV will be 360 ​​cubic meters of living area. It is big thing. And I bet MPCV, or as will be called, will be self propelled. Perhaps a kind of shuttle that does not need an external tank and rocket fire, and soar like an airplane or helicopter.

    I think people are underestimating the capacity at the core of NASA, and they will also share going to dominate the interplanetary travel.

    This is what I think

    Victor.

    (Trent, please forgive any errors of translation or typing - this time I paid more attention before speaking)

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  4. As pretty as Nautilus-X is (solar panel "wings", that lovely ring, a pointy bit out front like a true ship), there has to be something sized in between a tiny capsule and a decade-long construction effort.

    Isn't a BA-330 supposed to provide life support to six people for some number of months? Would that + a Dragon Mark II + a (suitably refueled) FH upper stage be enough to make up a decent-sized exploration craft?

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  5. Hey Chris, the problem is zero-g exposure. We know it destroys bone mass and that exercise has no beneficial effect - not withstanding claims that have been floating around for years that exercise helps, which have never been published. We know virtually nothing about partial gravity. Before anyone is going on any long space journeys (more than 6 months) we need to crack this problem. Be it via drugs, exercise, skinsuits, vibration, diamagnetic fields, rotating rings or tethers.. we need to get started on this right away.

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  6. That seems like a digression, though. A giant ship without a tested zero-g solution exposure is no different from a tiny capsule without a tested zero-g solution, as far as this is concerned.

    If we don't know what the solution is yet, what stops us from testing a few options while taking our little Dragon+BA runabout for a spin around the Moon?

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  7. Anonymous12:00 PM

    "but some other techniques such as solar sailing appear to be very interesting to me."

    What?! You want to build a sail with an area of thousands of m^2 and then you're gonna waste all that wonderful solar energy (10MW/ha at one AU from the Sun) by just bouncing it away??

    Solar thermal, solar thermal, solar thermal.
    Isp using LH2 of 900s.

    Oh, and that Nautilus thing needs a aerobraking shield.

    Andrew W

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  8. It only needs a shield if you're doing aerocapture.

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  9. Anonymous1:46 PM

    If you're going to Mars, or returning to Earth, it's hard to justify not using aerocapture, you'd need a very low weight power supply and a high Isp for the weight of an aeroshield not to make more sense.

    Andrew W

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  10. Anonymous1:51 PM

    The Nautilus-X is entirely modular. The pretty picture shows the mammoth - all options in place version.
    A much smaller, 1 Sundancer, modified Centaur upper stage is a great starting point for a small BEO habitat.
    As time goes on and budgets allow you can add all the bells and whistles. More habitats, for more people and storage on longer missions and so on.
    The major show stoppers at the moment are micro gravity health effects and radiation.
    Using the current NASA safety levels for radiation exposure The maximum possible mission length is 150-200 days.
    That's not Mars.
    How to solve these problems is the research that needs to be done now, before we start trying to do too much BEO

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  11. "you can send that up from Earth if you prefer."
    Yes please. It's cheaper!
    Hand waving aside please Trent. Consider the robotic infrastructure required to extract a mixture of: cyanogen; ammonia; acetylene; complex organics and all the other cometary ices *that are not water* from stratified deposits spread over thousands of square kilometers of rugged terrain... That is BTW in permanent shade and as close to absolute zero as to make any difference. Vs a big dumb booster: Falcon Heavy or a multitude of cheap and cheerful expendables. Or more apposite: because of the real time frame we are talking about: SKYLON. Or probably a launch loop!
    Add humans and the costs skyrocket!
    "you're not going to think anything is a lot. "
    I did. I said: "Lake Superior (Now that is a lot of water!)" And implied that Ceres has a "lot" more.. But I return to my main contention:
    "Abundant"
    Well bearing in mind "peak oil" or indeed "peak water" is fast approaching us...
    Remember that your Lunar GigaTonne OF ICE. Not water ice but ice with a hydrogen component that may be water, has to last forever... well until you can import some water from an in space source like the cometary NEOs. Or Phobos & Deimos. Or Ceres.
    But hang on a minute! If it is cheaper in terms of deltaV and MONEY to go to the asteroids in the first place. Using something like NAUTILUS-X: "FlexiStyle." Why not extract their water FIRST. With the added bonus that 2045 AB6678 - a yet to be discovered 'celestial body' - may be exempt of the Treaty on Outer Space and thus capable of being owned!
    It is my firm belief that this is the current thinking ahead. Hence Hayabusa; DAWN; Rosetta/Philae or, more topically, OSIRIS-REx. And I might add NAUTILUS-X and the SEV reborn.
    Without the water resources of the rest of the Solar System: the Moon is a Dead End as well as a Harsh Mistress.
    I never mentioned NASA. I implied Paul and Jack! (There are others...)

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  12. Oh and I forgot to mention: The biggest and deepest deposits that characterise the cometary bombardment record will have a SCIENTIFIC value far too great to be grubbed out for mere rocket fuel. Sorry.

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  13. Anonymous2:45 PM

    I don't think getting lunar water to (for instance)EML1 and EML2 are as much of a challenge as you suggest brobof, the deposits are quite thick and built up in small areas of permanent shade (not spread out over many thousands of km^2 as you suggest), thawing them out should be easy enough with reflected sunlight, these areas of permanent shadow are after all, in close proximity to area of near permanent sunshine. And distilling out various volatiles, a challenge? Hardly. The only challenging bit, compared to going all the way to Ceres, would be lifting them off the Lunar surface. Are you familiar with the idea of using tethers for transport to and from the Lunar surface?

    Andrew W

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  14. brobof, when you find yourself writing really long comments like that on my blog, please consider taking it elsewhere. We've all heard your opinion, three times now, and I for one respectfully disagree. Yes it is "hard", so what? We're not talking about Apollo era stunts here. We're talking about becoming a real spacefaring people.

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  15. My profound apologies Trent.
    My final words shall be brief.
    @Andrew W the diameter of MiniSAR NP image is a BOTE 1200km with 30 odd widly spread craters of interest. Is 2 metres quite thick?
    Wrt Tethers: Yes. But a nanoparticulate Mg/LOX monopropellant tug would be my suggestion.
    Still like the NAUTILUS-X tho :)

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  16. Warren Platts5:03 AM

    The 2-meters is the minimum thickness required to send back same-sense polarized radar. In reality, it's probably much thicker. Also, water ice crystals were directly observed by the LCROSS experiment. Also, the site of base will be on peaks of permanent sunshine where the temperature is a constant -50 degrees plus or minus 10. This is nicer than Mars. Moreover, some of these spots are right next to permanently shaded craters with the high CPR on the inside (e.g., esp. Whipple Crater). An operation the size of a modest gravel pit on Earth (excavating maybe 20,000 to 30,000 cubic meters per year--the volume of a small farm pond) could likely produce 10,000 tons of propellant per year. This is plenty for fueling up a big MTV with enough leftover that aerocapture wouldn't be needed at all. The total cost and complexity of such a Lunar ice pit wouldn't differ qualitatively from ISS. Also, it's not the case that getting water from asteroids would be "easier".

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  17. I think mining the asteroids will be easier once you have a spaceship.. it'll be one of the things you do with your spaceship.

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  18. LFTR and VASIMR are both solutions to the propulsion problem.
    I could easily see something like the NAUTILUS-X being produced by technologies currently being developed by private companies.
    Propulsion - Ad Astra
    Power - Flibe Energy
    Inflatables - Bigelow
    ECLSS - Paragon
    All could be done soon and for much less than NASA themselves ever could.
    What we need is an X-prize for every major contract, rather than the traditional RFP.

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  19. Anonymous11:09 AM

    "LFTR and VASIMR are both solutions to the propulsion problem".

    Would you like to try to give some weight and performance figures for what you think is achievable, especially acceleration rates?

    There was a 39 days to Mars proposal a while back using a vapor core reactor to get weights down, but even doing it that way you run into other problems eg. the rate of the consumption of the nuclear fuel become problematic for ships doing more than a handful of trips.If you want to do deep space on a commercial basis in the forseeable future (next 50 years?), it's not going to be using nuclear powered propulsion. Solar sails are a nice idea for a gentle cruise, but not an efficient use of the suns free energy.

    Andrew W

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  20. Anonymous6:37 PM

    Anyone come across this idea?

    Take 5000m of 15mm carbon nanotube tether, wind it up on a large spool on a turntable on the lunar surface, attach a 50 tonne payload to the tethers free end and attach a cam arm to the turntable, now, making sure the payload swings clear of the lunar surface, start to spin it slowly while winding out the tether and speeding up the turntable. When the payload is zipping around the turntable 5km out and at about 6rpm (and experiencing 5g centripetal acceleration), release it, it's above lunar escape velocity, and if you've put your turntable in the right place, and released the payload at the right moment, it might just end up where you want it. The tether weighs about a tonne, and as long as it's anchored well, the mechanical side of the turntable wouldn't need to be all that heavy either. A lunar launch system that uses no propellant, doesn't weigh hundreds of tonnes with complex electrical systems and engineering, and doesn't require thousands of km of tether.

    Andrew W

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  21. Anonymous6:50 PM

    Oops, dammit,I think acceleration is actually 192g, might have to double the length of the tether an accept a higher acceleration.

    Andrew W

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  22. Warren Platts1:09 AM

    I think mining the asteroids will be easier once you have a spaceship.. it'll be one of the things you do with your spaceship

    Operationally, I don't see how to do it. On the Moon, there's enough gravity so that the mining operation would be recognizably similar to a gravel pit on Earth; you could have regular excavators, hauler trucks, etc. Plumbing's also easier to do in gravity. Dealing with a rubble-pile in near-zero gravity is going to require a brand new approach.

    In any case, an asteroid will never be able to keep up with a couple of DC-X-style, SSTO fuel tenders in terms of mass flux to an L2 depot because your turn-around time is measured in days, rather than years. Asteroid mining will be useful for getting back from the asteroids....

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  23. Anonymous8:19 AM

    Doing the maths again I get 18.5g at 0.75rpm with radius of 30km giving payload throw velocity of 2380m/s. The load on the pivot for a 50 tonne payload would be around 1100 tonnes, the tether would weigh about 14 tonnes if made from carbon nanotubes of at least 50 GPa capacity.

    Though other combinations of radius and rotation speed would no doubt make more sense.

    Hunting around the only other reference to this I can find is in a comment by Robert at Selenian Boondocks a year ago where he calls it a "hammer-throw type surface based spinning tether lunar launcher".

    http://selenianboondocks.com/2010/05/lunar-land-grants-discussion/#comment-8434

    A system of this size could throw hundreds of thousands of tonnes a year to lunar escape velocity at high energy efficiency, the limit would probably be power supply.

    Andrew W

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  24. Anonymous8:53 AM

    A 200km radius of rotation gets 50 tonne manned ship to escape velocity at an angular acceleration of 28m/s (2.8g), rotation speed of 6.8 rpH, tether weighs about the same (longer but thinner).

    To decrease the time between launches, it would probably be better to have the payloads travel down a permanently extended tether. When you get to this radius of rotation, the possibility exists of the tether catching payloads from space, making low g rocket free transport available in both directions.

    Andrew W

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  25. Andrew, whenever people talk about throwing stuff off the surface of the Moon they never seem to get around to talking about how they're going to *catch* that material.

    For once I'd like to see someone work out the catching side of the equation first.

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  26. Anonymous1:42 PM

    There's no reason a rotating tether couldn't be used at the EML1/2 points to capture payloads. Slide 19 shows a mechanism that'll increase the capture window on an Earth rotovator to 12 seconds, a rotating tether at EML2 would have a much bigger window to play with.

    http://www.niac.usra.edu/files/library/meetings/annual/jun01/391Grant.pdf

    O'Neil's group calculated the payloads from their lunar mass driver would arrive at EML2 at 600 MPH, which isn't that quick when the end of the tether is moving at the same speed.

    http://www.nss.org/settlement/ColoniesInSpace/colonies_chap06.html

    Andrew W

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  27. Andrew, they also proposed catching them in a giant catcher's mitt.

    I'd like to see someone put the effort into designing both ends of a lunar mass launch system to a Lagrange point.. and actually show their work when it comes to the math. I'd like to see them take criticism and improve their design. I'd like to see numbers for how much it will cost to start.

    I hope I won't have to do it myself.

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  28. Anonymous2:30 PM

    "they also proposed catching them in a giant catcher's mitt."

    In using the mass driver they faced a major problem in power storage and rate of discharge, the solution they came up with was to launch small payloads; using the "hammer-throw type surface based spinning tether lunar launcher" the power storage issue disappears, the tethers rate of rotation can be increased as slowly as power supply allows, and the high tether/payload mass ratio allows large individual payloads.

    I never liked their mass catcher idea as it would have meant having to continually replace and repair the damage done by the impacting payloads.

    I'm afraid that when it comes to calculating orbits and the like, don't look at me, I couldn't even get the centripetal acceleration right the first time, fingers crossed my later attempts are an improvement...

    Andrew W

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  29. Anonymous2:55 PM

    "the high tether/payload mass ratio"

    Probably should be low tether/payload mass ratio.

    Rule of thumb for costings; use system using as little mass as possible , use system using cheapest energy available.

    I've been getting a bit hacked off with space cadets on other forums lately, you offer a cheaper and simpler way of doing something, maximizing existing technology, requiring far less mass, and nobody likes it because it's not cool enough, or else they're getting excited about something as mundane as a fuel depot, expecting that if it exists it'll suddenly make going to Mars easier to sell to the politicians.

    Andrew W

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  30. Anonymous3:57 PM

    A tether with a radius of 4km at EML2 could catch payloads arriving at 270m/s subjecting them to a centripetal acceleration of 18ms^2, tether rotation is at 3.86 degrees/sec. A carbon nanotube tether for a 50 tonne payload/manned ship need only weigh about 200kg(!) at 50GPa strength, so would need to rotate around something substantial, EML2, as you know, is unstable so the momentum of the arriving ship/ment could probably be used to maintain the receiver at EML2 if positioning at capture was done with that in mind.

    Andrew W

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  31. Anonymous5:01 PM

    Energy costs at 10c/KWhr are about $75/tonne assuming negligable losses, perhaps $100/tonne including losses.

    Andrew W

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  32. Warren Platts10:19 PM

    The way to "catch" a payload launched from a mass driver is to launch the entire lander catapult-style. An advantage of this approach is that you don't have do the whole 2.5 km/sec delta v at first; the idea is to simply take the edge off the total delta-v requirement. Even a 0.5 km/sec boost would bump up the payload mass fraction quite a bit.

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  33. Warren Platts10:27 PM

    Hey Trent, I see you cited Zegler and Kutter (2010): What do you think of the MTV they briefly discuss at the end? I kind of like that one better than Nautilus. No centrefuge, but it's got an 11 km/sec delta v and can carry 16 people. All the propellant/sun shades do the radiation mitigation. Plus if you could refuel at Mars (or Ceres), you could burn it all on the 1-way shot; I haven't run any simulation, but I bet it could cut transit times by at least 2/3 compared to Hohman transfers.

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  34. Hello Trent,

    Intersting idea.
    The main problem with the launcher though is that the 50 Gpa nanotube cable doesn't exist yet...
    Zylon cable at 5,9 MPa is, unfortunately, just a bit too weak. Still it's a much simpler solution than the magnetic launcher, doesn't require large energy accumuation and I like the fact that it is both very light and scalable: the more power you have, the faster you can accelerate and increase the system's output. A 1 km, 30 Gpa cable launching 1000 kg payloads would weigh only 300 kg! The tower would need to resist a lateral force of 600 tons, which does't seem unreasonable at first glance.

    Does anybody know how strong real life nanotube cable is?

    Regards,

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  35. Hello QuantumG and Trent,

    I think an moon orbiting catcher would be a simpler solution than the L2 located target envisioned by O’Neil. It would show up every 90 minutes or so, and the launch could be timed so the packets arrived at a very low relative velocity. For 1 ton loads, 16 t per day, 4500 tons per year. A harpoon could be used to catch the packet, and a reel to bring it in, since it would basically be ‘hanging’ beside the catcher.
    The catcher should be in a relatively high orbit, so that the deltaV to other targets would be small.

    Once the catcher reached a certain mass, (500 T?) it would use some of the packets as reaction mass (fill some packets with water) and move to L5 or wherever it’s contents might be needed. Another catcher could then take its place.

    This is also a very scalable system, that could reach huge capacities eventually, but that could start very small.

    Regards,

    Michel Lamontagne

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  36. Why bother with a catcher? a lunar space elevator could be made out of steel.

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  37. Ed, you mean to EML1 or EML2? At about 62,000 km from the Moon how much is the basic cable going to weigh, there has to be a minimum size of cable for the climber to hang on to? How are you going to power the climber and what does that power system weigh? I was looking for a low weight system that could be set up easily with materials from Earth.

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  38. Well, Andrew, L1 is a lot closer to the moon than 62000 km, more like 39000km from the center of the moon. In lunar gravity, the tensile strain is low enough that conventional materials like steel and Kevlar are strong enough to support their own mass, IIRC without taper.

    For the specifics of masses, I'd have to look those numbers up in the archives of the Space Elevator yahoo group and get back to you. Power would be by microwave or laser beaming. For the Space Elevator challenges every year, they've been using very bright spotlights to beam power, though some teams have started using lasers.

    There are a couple of fundamental problems with the rotovator idea you presented above.

    First, although the tensile strength of your cable or ribbon or whatever may be adequate, the pull on the hub is equal, opposite, and rotating. This means that the hub has to handle shear, torque, and strain equal in magnitude and opposite in direction to the tensile strength of the cable. The hub and bearings need to be made of unobtainium.

    Second, how do you keep the tether from colliding with the moon? Whether it's 5km or 30km or whatever, it is still spinning with an axis perpendicular to the surface. Even in an ideal location (smooth surface all the way to the horizon in all directions) the cable and payload will still be attracted by the moon's gravity for its entire length. You won't get a perfectly horizontal cable short of an infinite speed at the bottom of a gravity well. Best case would be to match the curvature of the moon, which means a smooth surface well beyond the horizon is required in all directions, or an equally-strong tower on which to mount your spindle. Again, unobtainium.

    Third, look at the failure modes. For a rocket or a mass driver or a space elevator, it is possible to have minor failure without catastrophic failure. With a rotovator the margin for error is so much lower than the aforementioned systems that even minor errors (i.e. a faulty bearing in the hub) will almost certainly cascade into catastrophic failure of the system (i.e. sudden fractional change in speed of hub sends a wave down the cable, causing the cable to impact the surface halfway to the payload).

    Bottom line: if you have the materials available to build a rotovator, then you already have the materials required for simpler systems (with much higher throughput rates) like mass drivers or a space elevator.

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  39. Ed, L1 is 58,021 ± 3183 km from the center of the Moon toward the Earth.
    http://www.niac.usra.edu/files/studies/final_report/1032Pearson.pdf

    Lateral loads of my 50 tonne ship at 2.8g are around 150 tonnes plus a few tonnes for the tether, not a high loading, rotation rate is 6.8 rph, not a fast rotation rate. No unobtainium required.

    "Even in an ideal location (smooth surface all the way to the horizon in all directions)"

    Are you being foolish or dishonest with that?

    The ideal location is on the top of a mountain. With a 3g load the tether will drop 1km for every 18 km of length, the point at which the length of the tether is closest to the moon is at 100km from the pivot,the pivot needs to be 2.8km (plus a safety margin) higher than any obstacle at that distance. Closer than this the high location of the mount will keep the tether above the moons curvature, further than this from the mount and the moons curvature falls away faster than the 18:1 slope of the tether.

    Are there such locations on the lunar surface? yes, not in ideal locations to fire loads to L2, but still possible with in flight course corrections, and certainly good enough to get stuff into lunar orbit.

    "Third, look at the failure modes..."
    This is just silly, in the event of loss of power the inertia of the system would keep it going for hours with no significant loss of speed. If bearings fail in aircraft and spacecraft engines the failure can indeed be catastrophic, that's why they're engineered not to fail. With this system, because a maximum weight of the bearings isn't important to successful operation, the bearing can be over engineered with no loss of performance.

    "For a rocket or a mass driver or a space elevator, it is possible to have minor failure without catastrophic failure."

    You are kidding right? Rocket failure during ascent = failure to reach orbit = crash to surface. Mass driver failure during launch = sub orbital speed = crash to surface. Break in space elevator cable at maximum load = crash to surface.

    And you haven't addressed my points about the mass of an elevator (not to mention its counter weight) and yes, you probably would have to resort to using lasers to power the climbers. incidentally, how fast do you expect the climbers to climb? 60,000 km is a long way at 100kph, wouldn't it be a bugger if a climber broke down half way up?

    (Having said that, I think a lunar tether is a good long term solution, but they still have problems, mass drivers are crap)

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  40. Microwave wouldn't do for power because beam spread is too big.
    I struggle to believe a mass of less than 10gm/meter is practical, and that works out at 600 tonnes just for the tether. I can't imagine you wouldn't be into the thousands of tonnes for the whole system.

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  41. Well I've finally found time to properly read my own link and am a bit embarrassed to see the system I've been talking about covered at some depth near the end of it.

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  42. Thanks for that link, Andrew, I hadn't seen it before. I agree that microwave receivers could be pretty big, and I think laser transmission of energy is more likely.

    I'm not kidding about minor failures being survivable in rockets. That's why Saturn V had engine-out capability, as does the shuttle for the SSMEs and Falcon for the Merlins.

    I haven't looked up the exact numbers for mass and so on for a lunar space elevator, but I expect I'll find that information in the link you provided. I admit I just took 10% of the Earth-Moon distance for L1, which was kinda lazy.

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  43. ken anthony6:03 PM

    We need to be designing mars missions based on storable fuels of 300s rather than hydrogen 450s engines. During transit you don't want any clouds of hydrogen popping up in bad places.

    You send two or three ships tethered together for gravity and redundancy. We should be testing various radiation mitigations. Including those English fellows that came up with a low power shield solution. I hear styrofoam is a pretty good radiation insulator. It shouldn't be too difficult to send a test package into hard radiation space and see what works best.

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