Instead of a space elevator

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crossovermaniac

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Why not make a composite structure for launch vehicles from carbon nanotubes? Think about it, for every pound shaved off a launch vehicles is about a pound more that can go into orbit. Or compositive cars. Think about the gas savings. What do the rest of you think?
 
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mrmorris

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<font color="yellow">"Why not make a composite structure for launch vehicles from carbon nanotubes?"</font><br /><br />Well -- because they don't exist yet in a usable form would be one reason. If CNT-based composites become available that would serve this purpose -- then I'm confident that they will be used.<br /><br /><font color="yellow">"Think about it, for every pound shaved off a launch vehicles is about a pound more that can go into orbit. "</font><br /><br />Not true. Every pound shaved off the <b>upper stage</b> of a launch vehicle means another pound of payload. I don't know the figure for all launch vehicles -- but for the Falcon 1 -- there was a note in their updates which indicated each pound shaved off the first stage meant about 1/7th of a pound increase in payload.<br /><br />Companies are already working with composites to reduce the weight of the launch vehicles. SpaceX uses a carbon-fiber composite interstage which brought them a significant weight reduction. Northrup Grumman recently developed a composite tank capable of handling liquid Hydrogen. NASA recently awarded a contract to XCOR for development of a cryogenic composite tank for LOX.<br /><br />Until CNT tech develops to the point where they can be manufactured in sufficient size and quantities to test making composites using them -- there's no telling how useful they may or may not be. Composites, by definition, are a combination of two or more materials -- generally a fiber (like fiberglass or carbon fiber) and an epoxy to glue the fibers together. Much of the properties of the composite depend on the material used as the epoxy. CNT-based composites might not have much benefit over carbon-fiber based composites for this reason. <br /><br />Or they may... who knows? At any rate -- it's too soon to start addin
 
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najab

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><i>Not true. Every pound shaved off the upper stage of a launch vehicle means another pound of payload.</i><p>I guess it's splitting hairs here, but it might be more accurate to say that a pound less structure put on orbit means a pound more payload on orbit. I know you were talking about expendables, but think about the shuttle Orbiter or a SSTO vehicle (a what? Aren't they fictional?). Reduce the mass of the (non-propulsive, therefore not technically upper-stage) vehicle structure, and you increase payload. Or the Shuttle ET - who's mass budget do you charge that to: upper stage, or vehicle?<p>Anyway, with that hair sucessfully split, I'm outta here. <img src="/images/icons/laugh.gif" /></p></p>
 
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crossovermaniac

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<font color="yellow">Not true. Every pound shaved off the upper stage of a launch vehicle means another pound of payload. I don't know the figure for all launch vehicles -- but for the Falcon 1 -- there was a note in their updates which indicated each pound shaved off the first stage meant about 1/7th of a pound increase in payload. </font><br /><br />Oops! Sorry about that. Should have known better. To explain myself. True, the technology has to mature some before use, but in case the space elevator concept isn't practical for any reason like space debris, political, etc, nanotubes would make great composites. And the reason why nanotubes could bring down the cost of space travel is because every pound shaved off the upper stage means an extra pound in orbit. Launch vehicles prices are per launch, not per pound. Launching a 10 ton payload from a $190 million dollar Delta IV heavy will cost the same as launching 25 ton payload from a $190 million dollar Delta IV heavy. Using a compositive lets you launch a few extra tons per payload for the same cost if a nano-compositive upper stage is the same cost as the aluminum one. Although, after looking at the Delta-IV as a base example, if you did cut the weight by half for both stages and using the assumption that 1 pound shaved off the first stage=1/7 pound of payload and 1 pound shaved off the upper stage=1 pound of extra payload, all you get is 3 extra tons. Then again, the extra three tons could be used to make the space craft reusable (parachutes, wings, added heat shields) and maybe even a SSTO.
 
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vogon13

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I've brought this up before, but I really like this idea.<br /><br />How about a space tether that only descends to the maximum altitude of Spaceship One?<br /><br />Launch SS1 and dock with the low end of the tether. Climb tether. Explore space from there. <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>
 
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mrmorris

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<font color="yellow">"Launch SS1 and dock with the low end of the tether."</font><br /><br />The idea has been brought up before and dismissed before. In general -- the tether concepts which perform services like this are 'momentum exchange tethers'. These are multi-km long tethers in orbit that are spun up over time using electricty working with (against) the earth's magnetic field (i.e. propellantless). A suborbital ship is sent up to meet the lower end of the tether as it's swinging around. They connect, and the tether expends some of its rotational energy in 'flinging' the ship to a higher orbit. The problem with using this and SS1 is that the ship must match speeds with the end of the tether at the time they meet (multiple mach ~7 IIFC). Since SS1 is essentially motionless at 100km (it's simply at its apogee and about to start falling back to Earth), if it were to meet up with an ME tether at 100km -- the result would be the tether smacking the crud out of SS1.<br /><br />However -- since you specifically state:<br /><br /><font color="yellow">"...dock with the low end of the tether. Climb tether."</font><br /><br />It sounds like you're thinking of a motionless tether (i.e. as opposed to a rotational one). If so -- I have to wonder exactly what this tether is anchored to? Ordinarily, 'climbing' space tethers are anchored at one end to the Earth's surface, and a counterweight at the far end anchors the other and keeps the tether taut. There's no 'cleat' at 100km that you can tie a bowline off to. So how is it going to stay there?
 
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vogon13

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Center of mass (for lack of a better term) is at geosynchronus altitude. Expect there to be some deflection of low end of tether, but SS1 (or a follow on vehicle) would have manuevering capability. <br /><br />Think of a suspended plumb bob. <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>
 
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mrmorris

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<font color="yellow">"Center of mass...is at geosynchronus altitude."</font><br /><br />No -- won't fly. Orbital mechanics doesn't work that way.<br /><br />However -- let's <b>pretend</b> for a moment that it would be 100% stable and examine this concept. Geosynchronous orbit is 35,786 kilometers above the surface of the Earth. You want to make a tether that is 35,<b>6</b>86 km long instead of the full 35,<b>7</b>86. That's a savings of almost 0.28 % of the total tether length. The downside of that savings is that you can not simply bring materials to the base of the tether via surface transportation, but instead must launch it via a suborbital rocket at significant expense (albeit less than conventional rockets). <br /><br />In addition -- the standard 'plan' for building a ground-to-GEO tether is that a booster will be launched to GEO with a spool of less-than-hair-thin tether (at this point the speculation is carbon nanotubes). One end of the spool will be lowered to the ground, and then construction will continue with a succession of small climbers sent up with additional threads. This process will repeat until the cable is thick enough to sent up cargos of the derired mass. With a cable ending 100km in the air -- construction of the tether is going to be just a wee bit more difficult.
 
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the_unknown

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Someone has got to subscribe to Popular Science magazine, right... If I recall correctly, a company designed a tube that would connect New York with London. That tube is flexable, went under water and could even produce a vacume on the inside of the tube, which is where a train would travel faster than the speed of sound in a <b>frictionless environment</b>. We have the biggest vacume on our side if we build that same tube vertical toward the heavens (i mean space).What I am saying is why build the tube from New York to London? That is more than enough distance (not to get too technical but that would be more than 10 times the distance?!? -im guessing-) to build one from lets say, Florida to near weightlessness. Or better yet, from California go eastward and have the tube follow up mountains for a little support so that the tube wouldn't be just 'floating'.And I would almost put money on it that if you equalized the pressure in the tube to match that of space, the tube wouldn't require NEARLY as much support as you might would think.Hmmm... what do you think?!?
 
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the_unknown

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In order to keep the tether vertical you would have to have it be hollow, like a tube, so that you could suck all the air out of the tube and have the pressure equal to that of space... I think that should keep the tether 'bow-tied to space'.
 
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jurgens

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Because a tube going upwards into the sky has to support its own weight and everything above it, a tube going from new york to london doens't have to support nearly as much weight.
 
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nexium

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I agree with JurgenS: Even with optimistic projections for CNT the vertical tube supports more weight. In both cases the boyancy is reduced by the weight of air removed from the tube, but that is not enough, unless we get thousands of times stronger than Kever composits. Neil
 
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nexium

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As mrmorris suggests rotating tethers don't help much especialy those only a few kilometers long, but we should build one anyway, to better understand how tethers work. We can launch a kevar thread from Low Earth Orbit with a climber similar to the one designed by Dr. Edwards for the space elevator. The climber can add a strand to the thread, then change direction repeatedly to understand how to control transients in a tether. It might be practical to use carefully timed transcients = snap the whip to slow the tether end considerably below mach 7 at the moment of attachment. An other transcient can flip the the pay load, in theory, giving the pay load a bigger boost. The main difficulty is a one second timing error means the pay load gets smacked distructively, and/or the tether end and the pay load miss each other, resulting in the payload falling back to Earth. Neil
 
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