Space elevator update

[vt_hokie]

I always thought there would be too many practical issues, even if we had the unobtanium with which to build the thing. How will it stand up to weather? Is there a counterweight on the end of this thing? If so, it would be subject to perturbations just like anything else and develop eccentricity and inclination over time.

 

[nexium]

Hokie may be correct: One of the practical issues may be a show stopper for either The space elevator or the rotating tether/ribbon. I think the eccentricity and inclination are easilly remedied with small amounts of down time. Only the first space elevator needs a counter weight. A counter weight is optional for subsequent space elevators. Neil

 

[mlorrey]

Well, it should be obvious that anything made of pure carbon cannot travel for more than a microsecond in the dense lower atmosphere at 12,000 mph without burning up. Furthermore, if the tether is rotating, its rotation would lose energy to drag if its end dipped into the atmosphere at all. Thus, the tip should drop down to no less than 400k feet altitude. NASA says that orbits of less than 185 km are not stable, which is 607,000 ft.<br /><br />If the tips have platforms that operate like the flight deck of an aircraft carrier or helicopter pad, then an autopilot flown vehicle that gps guides its way to rendezvous should be viable. Getting to 12k mph at 607k ft is a lot easier than getting to orbit.<br /><br />However, even 16k mph is easier than orbital velocity and allows for conventional airframe designs with mass fractions that are actually possible with current technology and allow for significant payloads.<br /><br />Keep in mind there are two things slowing a stationary 1000 km tether down at the bottom end: firstly orbital velocity is less at the higher altitude. If your bottom end is 185 km (about 607k ft), your CG, if both end stations are of equal mass, is at 685 km. The higher your CG is, the slower the velocity of the entire system.<br /><br />The second thing slowing down your bottom end station is the fact that it is already spinning end for end once per orbit. The orbit circumference is smaller at 185 km than at 685 km. Ergo with an Earth radius of <br />6378.137 km our lower end is at 6563 km and the CG is at 7563 km from the center of the earth, and orbital circumferences equal 41236.5 km and orbiting at the orbital velocity of a 685 km altitude the bottom end will be going 15,614 mph, which is 6.98 km/sec, without having to spin end for end.<br /><br />An elevator doesn't need to put energy into lifting a whole launch vehicle, and shouldn't have to. All it needs is to have cargo dropped off at the bottom end platform by a launch vehicle. Given that most lau

 

[torino10]

I kind of like the design as depicted in figure 4 of the previously posted PDF on the nasa site.<br /><br />The only real improvements on that design I might suggest would be to have the lower end at an altitude of about 300km so as to further reduce drag on the lower end. <br /><br />It would also be nice to have a drag line that could be lowered and help reduce the amount of velocity required to mate with the lower end. <br /><br />The sub orbital launch vehicles could also serve a dual purpose in that when they hook up to the lower end of the cable the drag line could give them a boost during cargo transfer , making the suborbitals also capable of high speed intercontinental transport.<br /><br />I prefer a more mobile flying elavotor system mostly for the fact that if it is oriented along the solar planetary plane it will offer more launch windows from the far end to the moon and various planetary bodies in the solar system without expensive plane change maneuvers and could also have a resonant orbit so that it passes within launch range of many international airports on a fixed schedule.<br /><br />One of the main benefits that NASA seems to fail to mention in there article is the fact that the first organisation to create a space based launch infrastructure will have great influence and control over space development in general, the space age equivalent of owning the Panama canal.<br /><br />Oh yeah I almost forgot, the shorter flying bridge can be built using currently existing materials and then strengthened when better materials begin to become more feasable. I was wondering if anyone new if microgravity had been considered as a way of making longer CNT?

 

[mystex]

Why couldn't the space elevators orbital counter balance make use of the atmosphere's electrical charge to keep it repelled and in orbit? I remeber reading somewhere about somethign along these lines but being used the exact opposite to help degrade sataliite oribts faster.

 

[mlorrey]

<blockquote><font class="small">In reply to:</font><hr /><p>Why couldn't the space elevators orbital counter balance make use of the atmosphere's electrical charge to keep it repelled and in orbit? I remeber reading somewhere about somethign along these lines but being used the exact opposite to help degrade sataliite oribts faster. <p><hr /></p></p></blockquote><br /><br />You are trying to get something for nothing. If you use the electrical charge of the atmosphere, you create electrical drag on the system, thus degrading its orbit. Putting that energy back into keeping it in orbit is counter productive, because any such system will have efficiency losses, so you wind up losing orbital energy. To use the tether to stay in orbit when the low end is encountering drag, you need to pump electricity into the tether that is supplied by solar power or nuclear power, etc.

 

[torino10]

Also there is an altitude limit to the use of electrical tether boost sytems.<br />The lower end of the elevator may be able to use them but the upper end will need a reaction mass thrust system to maintain orbital stability.<br /><br />On a side note I much prefer the term Space Bridge to space elevator, While elevator may be more accurate in describing how it works , the word bridge is more descriptive of what it does.<br /><br />Bridges are monuments and are often dedicated to heroes or grand ideals. elevators are cramped alternatives to using the stairs.<br /><br />

 

[josh_simonson]

>2 Tip rockets are likely essentual, to fine tune the rondevous, reduce the stress on the ribbon, and the g loading of the payload.<br /><br />Perhaps not, instead of moving the tip 'station' one could hang a loop of cable off the end. On the SSTT aircraft, a small rocket could be fired attached by a tether to the SSTT (like the silkworm missile). The rocket does the high-G work of intercepting the hanging loop and deploys a grapple to catch the loop. If the intercept fails, the SSTT cuts the tether to the rocket and aborts. Placing a spool with a brake on the tether would allow it to play out line immediately after intercept to reduce G forces on the cargo.<br /><br />Alternately, the rocket-tether could attach to a payload in a top mounted payload bay (similar to STS) and on intercept the payload is ejected by a pnumatic pusher. That way the tether doesn't have to raise the entire SSTT, and the unloaded SSTT could return to base immediately to prepare for the next intercept. This would double the capability of the SSTT fleet.<br /><br />ABM elements have already done much of the work on in-space intercept of fast moving objects, so threading a large loop with a rocket aught to be a slam-dunk with current technology.

 

[mlorrey]

What elevator are you talking about? The cartwheeling one? So long as the earth end is 185 km or higher, it won't deal with significant loss issues. Keep it above 220 km and it'll be safe as houses. The cartwheeling one would save more dv at first at the expense of greater risk of collision with orbiting objects, and taking more work to get spinning.<br /><br />The ideal cartwheeling tether is one where the ends essentially drop down motionlessly to pick up objects on pop-up vertical trajectories like SS1 (in actuality, they'd have a velocity of about 1000 mph at the low end to match earths rotational velocity). This would suffer no low end erosion of the orbit.<br /><br />I also suggest, steve, that you actually look at the math, like I've done, to see that starting with a 1000 km stationary tether actually saves significant dv, enough to make SSTO RLV feasible without special technology as soon as that first tether is put in orbit.

 

[torino10]

I tend to think that the first space tether, or other orbit based launch infrastructure, should not be considered as a purely economical method of reducing launch costs.<br /><br />In all likelyhood it will not be able to pay for itself in the short term, ie under two or three decades. Instead it would be much more politically benificial to those who build it for the simple reason that it will give it's operators great influence over the development of space based technologies.<br /><br />The ability to affect comerce in an already demonstrated highly lucrative environment has many rewards besides just the imediate income one might realise from cheaper launches.<br /><br />

 

[mystex]

Wouldn't the country who built it also reap the benefits of faster turnaround on lifting items to space? Plus if they have a stable platform at the other end they could for all intensive purposes use it as a dry dock to build ships to go to the moon or other places. I mean if you could build a ship in orbit to travel to the moon you would have all kinds of systems you wouldn't nesicarily have to implement. No need to install a system to land on earth. Etc.

 

[nexium]

~These are likely authoritative answers from www.liftport.com~<br />Our goal is 200km/h. This makes for a trip time of 7days 10:55:54.8 to GEO (considered the "top" for most cargo)<br /><br />2. There's still quite a few assumptions there that haven't been determined yet. We are shooting for a higher capacity than the NIAC report.<br /><br />3. Our design goal is an ocean based liftport for a few reasons.<br />The area of the pacific that we are planning for has been under study for quite some time do to it's lack of adverse weather.<br />The ribbon will need to be moved occasionally to avoid lower orbit debris. 1000 miles out into the pacific we'll have a lot of room to move about.<br />Name an equatorial country that's stable?<br /><br />4 will have to wait until there's a proven track record. No insurance is going to cover human cargo right away.<br /><br />Your angular momentum questions are answered in the December technical report: http://www.liftport.com/papers/2005Dec_SEquestions.pdf<br />

 

[mlorrey]

Brazil is pretty stable.<br /><br />However, stability is overrated. A small, poor, and unstable isolated country on the equator would lend itself to becoming a 'company town' to whatever syndicate chose to invest in building a base for a tether there. Particularly if the country also had attractive topography for rail/tube launch site to put tether building materials into orbit. A divided electorate means that whatever side you put your support behind in elections would owe you their support.<br /><br />A launch assist system providing subsonic airspeed of 1000 km/hr, when on the equator also provides the full 1660 km/hr earth rotational velocity boost as well. Starting off with 2660 km/hr before lighting off is a significant savings to one's mass fraction.<br /><br />There are a few locations like this. One is Sao Tome, which has one island on the equator AND has a 2,000 meter peak, AND has a useful port and some airfields. The national population of 184,000 is small enough for a tether launch project to drastically improve the standard of living of every islander. It also helps that the island is 70% catholic: getting the Vatican behind the project would ensure its success.

 

[mystex]

Any equitorial country the elevator would be build in would definetly see a sharp rise in their standard of living. The only question would become protection of the facitlity. <br /><br />I just can't see any one country or company being allowed to soley control it. It seems to me that some how or another the UN would try to step in. If nothing else all countries would either want to safeguard to take it out depending on how they stood to benefit. Plus every terrorist around the world would probably see it as a prime target.

 

[chriscdc]

I was just thinking about the effects of thermal expansion on the cable. Do you know if thy have put this into their calculations or plan on shielding it some way?

 

[mlorrey]

carbon nanotube is a good thermal conductor, and most of it will be in constant sunlight, given the equator is only in line with the ecliptic twice a year, so once it acclimates to space and is long enough to reach above about 10k miles, it shouldn't exhibit any further expansion or contraction. A full length GEO cable should be stable and of constant near-uniform temperature

 

[nexium]

If conciderable epoxy or other binder is used, thermal conductivity may not be good enough, to avoid CNT degrading due to rapid temperature changes. Squeezed between the high speed rollars, CNT temperature may increase several degrees c per nanosecond. The ribbon will rotate, slowly, perhaps rarely several RPM, So the sun lit surface will change places with the unlit side. Part of the laser beam, will sometimes heat the ribbon on one side.<br />100,000 kilometers increasing to 101,000 kilometers will likely occur briefly, but I think more because the counter weight moved outward and/or there are more climbers on the ribbon. I think it will be important to monitor the tension on every kilometer of the ribbon. Neil

 

[rocketman5000]

You are forgeting that for there to be stress from thermal expansion there has to be something for the heated body to push against. ie. concrete cracks under thermal expansion because it is pushing against other slabs of concrete. Internal stress is created as the concrete trys to expand but can't.<br /><br />In the ribbon of the space elevator there won't be much to push against. It will simply expand letting the counterweight raise to a slightly higher altitude. As long as this centripetal force of the counterweight was accounted for in the design of the ribbon I do not see and issue. <br /><br />As for the wheels causing localized expansion that could be solved by only allowing a constant pressure of the wheels onto the ribbon using a spring or some other variable mechanism<br /><br />All this being said it would still be a smart thing to monitor the tension of all segments of the elevator cable.

 

[chriscdc]

I realise that a 30K km cable would mostly be in direct sunlight. In fact being a good thermal conductor might work against it as the lower end would lose heat as the atmosphere cools at night.<br /><br />Of course though, your sig says it all.

 

[chriscdc]

Yes but centre of mass is very important for these things.<br />Also if the fibres expand at different rates then it could damage the bonds between them (this is important unless we work out how to make 30,000 long tubes).<br /><br />Alot depends upon how quickly the tube could contract. Depending on how quickly it transmits force, your GEO station (counterweight etc) could experience a significant acceleration. You might need to add heating elements to avoid this. I'd suggest mirrors as electricity would interfere with the magnetic field, or you could use this at the same time as orbital boosting.

 

[pmn1]

There is a report out today about metals running out (I've put it in the Environment section) now what would having a nice metal rich lump of rock at the end of your elevator do for its economics? <div class="Discussion_UserSignature"> </div>

 

[chriscdc]

Well they do say that 1 average C-type asteroid would be worth 12trillion dollars. <br /><br />Of course you'd have to get it into the correct orbit. Then it would take decades to move it first. You might not have a civilisation to use its resources!<br /><br />You could get plenty of resources out of an asteroid without an elevator, however you could get energy out of it using an elevator.

 

[mlorrey]

rocketman has it. You need to think of the CNT ribbon like the cable on a ski lift. The cable is cold at night and contracts so there is less sag between towers and the tension also picks the counterweights at the peak up further. After the sun rises the cable starts to warm, so that by afternoon the counterweight has sagged and the cable sags more between towers. <br /><br />Also note: CNT handles stress like a spring: unlike metal it doesn't have a stress-strain threshold in tension, it simply stretches out like a telephone headset cord. You have to nearly stretch to double the length of a CNT to get it to snap from stress-strain in tension.

 

[nexium]

I had been thinking the ribbon would stretch 1%, perhaps 2 percent. Almost double means huge stress transients. Keeping track of their movement and predicting their effect on the climbers will likely be a major project. Won't such vast stretching degrade the epoxy binder rapidly? Neil

 

[mlorrey]

This stretching happens in the region of what would be considered in metals to be overstressing: where metal would permanently deform, CNT would simply get springy. So long as your cable operates below this limit, you should see minimal stretching.<br /><br />However, qualified climber engineers should design their climbers to be as safe as ski lift chairs: since light travels faster than vibrations in the cable, a radio controlled system should allow climbers to know when transients are headed their way, and with proper spacing, and given the climbers would tend to act like fingers on a guitar string, they should be able to harmonically cancel out transients.