4 futuristic space technologies — and when they might happen

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Of these 4 technology concepts, the nuclear reactor on the Moon seems like a sure bet, but the others seem more fanciful than realistically evaluated.

Regarding the "space elevator" using a cable that is terminated on Earth and reaches past geosynchronous orbit, that just does not seem to have been thought through the dynamics properly. Even if we can eventually make a cable that can stand the load of tying the far end satellite to the Earth's surface, and assuming that a space "elevator" could "climb" that cable instead of being hauled up by another cable. there is still the issue of where the energy ultimately comes from to get the capsule to geosynchronous orbit.

Think about it this way: Assuming attachment of the bottom end at Earth's equator, it has a rotational speed to the east of 24,901 miles/24 hours = 1037.5 miles per hour. But, geosynchronous orbit is 6,876 mph. So, the capsule must be accelerated sideways by 5.838.5 mph.

How is that sideways acceleration going to be provided? If it is provided by a vertical cable pushing the capsule to the side as it climbs the cable, that will tend to move the cable to the west, which will lower the upper end of the fixed length cable as it takes on a more spiral path from Earth's surface to the upper end satellite. But, that will destabilize the geosynchronicity of the orbit, causing the upper end of the cable to eventually fall out of orbit.

So, maybe that could be fixed by putting rocket motors on the capsule that fire sideways to keep the side pressure on the cable to zero.

But, we also need to consider the effect on the upper satellite caused by hauling the capsule upwards. That will also haul the upper satellite downwards, again taking it out of geosynchronous orbit.

So, again, we could put rocket engines on the capsule to push it upwards, instead of "climbing" the cable by pulling itself up mechanically (and simultaneously pulling the satellite downward).

So, to have a vertical cable, we would need rocket motors that pushed the capsule sideways and upward in the right amounts to reach geosynchronous orbit. But, wait a minute, isn't that like what we already do without the cable? Are we really saving rocket fuel that way? Nope.

So, how would we design a cable stretching past geosynchronous orbit that could be used to haul mass to geosynchronous orbit without using any rocket fuel?

We would need to have that cable spiral in a path that is effectively tugging on the Earth's rotational energy directly along the axis of the cable at all points between the Earth's surface and the upper satellite. I am not sure that such a path even exists, but think that at least most of the energy could come from slowing the Earth's rotation a tiny amount but probably still requiring some rocket thrust on the upper satellite end of the cable to keep things stable, there.

And, besides the effects of hauling mass to geosynchronous orbit, there are also gravitational effects from the Sun and the Moon that need to be worked out. Just like the surface of Earth's oceans get pulled towards the Moon and the Sun, the satellite(s) in geosynchronous orbit and the cable will get pulled on a daily basis by the Sun and Moon with the Moon's pull changing into and out of phase with the Sun's pull on monthly period.

So, to show how this would need to be designed to really work, somebody needs to figure out the Lagrangian equation for the cable spiral, solve it, and determine how long that cable would really need to be. And, they need to determine what amount of rocket fuel would need to be hauled up that cable to keep it stably in geosynchronous orbit while people hauled masses up the cable and the Sun and the Moon affect it.

My guess is that it would need to be a much longer cable than the proponents of this concept envision for a vertical cable. And, I suspect that even thousand-mile-long nanotubes are not going to have the strength to weight ratio needed. That is the issue for a cable from Earth to orbit.

I have not thought about the idea of one from the Moon to lunar orbit. For one thing, it could be pointed directly at Earth, and use that for stability. The cable would be tidally locked, just like the Moon. And, it would have far less distance and far lower forces to contend with. Still, the dynamic effects of hauling loads up and down with just the cable tension needs more thought than I have time to give it. Pulling a mass to the satellite in lunar synchronous orbit is still going to be pulling that satellite closer to the Moon. Even the Lagrange point between the Earth and the Moon is not stable for a free satellite. So, pulling on a satellite in that location to haul loads off the lunar surface is not going to leave the upper end satellite in-place for continued service without some sort of compensating force.

Maybe somebody can show me how the energy to lift the loads off the lunar surface can be "harvested" from the energies of lunar orbit or lunar rotation or Earth's rotation, but I am not envisioning a process for that, at the moment.

The important points to remember are:
1. the energy gained by the hoisted capsule must be taken from somewhere; and
2. it cannot come from the satellite orbit on the upper end of the cable without reducing the energy of that satellite's orbit, so that it goes lower to the body that it is orbiting.
 
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Of these 4 technology concepts, the nuclear reactor on the Moon seems like a sure bet, but the others seem more fanciful than realistically evaluated.

Regarding the "space elevator" using a cable that is terminated on Earth and reaches past geosynchronous orbit, that just does not seem to have been thought through the dynamics properly. Even if we can eventually make a cable that can stand the load of tying the far end satellite to the Earth's surface, and assuming that a space "elevator" could "climb" that cable instead of being hauled up by another cable. there is still the issue of where the energy ultimately comes from to get the capsule to geosynchronous orbit.

Think about it this way: Assuming attachment of the bottom end at Earth's equator, it has a rotational speed to the east of 24,901 miles/24 hours = 1037.5 miles per hour. But, geosynchronous orbit is 6,876 mph. So, the capsule must be accelerated sideways by 5.838.5 mph.

How is that sideways acceleration going to be provided? If it is provided by a vertical cable pushing the capsule to the side as it climbs the cable, that will tend to move the cable to the west, which will lower the upper end of the fixed length cable as it takes on a more spiral path from Earth's surface to the upper end satellite. But, that will destabilize the geosynchronicity of the orbit, causing the upper end of the cable to eventually fall out of orbit.

So, maybe that could be fixed by putting rocket motors on the capsule that fire sideways to keep the side pressure on the cable to zero.

But, we also need to consider the effect on the upper satellite caused by hauling the capsule upwards. That will also haul the upper satellite downwards, again taking it out of geosynchronous orbit.

So, again, we could put rocket engines on the capsule to push it upwards, instead of "climbing" the cable by pulling itself up mechanically (and simultaneously pulling the satellite downward).

So, to have a vertical cable, we would need rocket motors that pushed the capsule sideways and upward in the right amounts to reach geosynchronous orbit. But, wait a minute, isn't that like what we already do without the cable? Are we really saving rocket fuel that way? Nope.

So, how would we design a cable stretching past geosynchronous orbit that could be used to haul mass to geosynchronous orbit without using any rocket fuel?

We would need to have that cable spiral in a path that is effectively tugging on the Earth's rotational energy directly along the axis of the cable at all points between the Earth's surface and the upper satellite. I am not sure that such a path even exists, but think that at least most of the energy could come from slowing the Earth's rotation a tiny amount but probably still requiring some rocket thrust on the upper satellite end of the cable to keep things stable, there.

And, besides the effects of hauling mass to geosynchronous orbit, there are also gravitational effects from the Sun and the Moon that need to be worked out. Just like the surface of Earth's oceans get pulled towards the Moon and the Sun, the satellite(s) in geosynchronous orbit and the cable will get pulled on a daily basis by the Sun and Moon with the Moon's pull changing into and out of phase with the Sun's pull on monthly period.

So, to show how this would need to be designed to really work, somebody needs to figure out the Lagrangian equation for the cable spiral, solve it, and determine how long that cable would really need to be. And, they need to determine what amount of rocket fuel would need to be hauled up that cable to keep it stably in geosynchronous orbit while people hauled masses up the cable and the Sun and the Moon affect it.

My guess is that it would need to be a much longer cable than the proponents of this concept envision for a vertical cable. And, I suspect that even thousand-mile-long nanotubes are not going to have the strength to weight ratio needed. That is the issue for a cable from Earth to orbit.

I have not thought about the idea of one from the Moon to lunar orbit. For one thing, it could be pointed directly at Earth, and use that for stability. The cable would be tidally locked, just like the Moon. And, it would have far less distance and far lower forces to contend with. Still, the dynamic effects of hauling loads up and down with just the cable tension needs more thought than I have time to give it. Pulling a mass to the satellite in lunar synchronous orbit is still going to be pulling that satellite closer to the Moon. Even the Lagrange point between the Earth and the Moon is not stable for a free satellite. So, pulling on a satellite in that location to haul loads off the lunar surface is not going to leave the upper end satellite in-place for continued service without some sort of compensating force.

Maybe somebody can show me how the energy to lift the loads off the lunar surface can be "harvested" from the energies of lunar orbit or lunar rotation or Earth's rotation, but I am not envisioning a process for that, at the moment.

The important points to remember are:
1. the energy gained by the hoisted capsule must be taken from somewhere; and
2. it cannot come from the satellite orbit on the upper end of the cable without reducing the energy of that satellite's orbit, so that it goes lower to the body that it is orbiting.
The issue 'unclear engineer' appears to be raising is what to do about the Coriolis force acting on the cable by an ascending climber. I published a paper in 2008 in Acta Astronautical that explains how it causes the cable to sway, but the system is stable, and oscillates about a vertical equilibrium. The paper proposes several solutions to minimize and even undo such oscillations.

Regarding energy required to lift the climber to GEO, the majority is provided by the climber's motor, but some is taken from Earth's rotation (the Earth will slow its rotation... Negligibly, of course). Ascent beyond GEO requires no input energy, as it 'falls' upward.
 
OK, I found it. The first place I found it seemed to be some sort of condensation that stopped in the middle of sentences and went on to the next section. But, I did find a PDF that is 127 pages, so it is going to take me a while to read through it all.

However, the first thing I saw is that the calculations were "simplified" by assuming that the cable is "rigid". That tends to make me discredit the rest of the study right there. A rigid structure that needs to be well over 22,000 miles long seems like a non-starter.

So, how about some simplified explanation here. Are you assuming that this "tether" goes in the vertical direction except for the sinusoidal oscillations in the horizontal direction? Or, do you let it assume some sort of spiral shape with altitude?
 
This seems to be the crux of the issue, from the paper:

"Therefore, the work per unit mass done by the motor to move the climber from the
surface of the Earth to the geosynchronous altitude is about 48.5 Ml/kg. An apparent
anomaly occurs when examining the change in the energy per unit mass of the climber
before and after transit, EH2' given by
(2.33) [equations did not copy]
or
(2.34)
The increase in the total energy per unit of mass of the climber in moving from the
surface of the Earth to the geosynchronous altitude is about 57.7 Ml/kg. This result begs
the questions, "How did the climber gain more energy than the work the motor put into
it?" and "Where did this free energy come from?" Thefree energy came from the spin of
the Earth. For a transit to the altitude dj. the energy per unit mass extracted from the
Earth is given by .Q2 [( df + R)
2
- R2
] ; for this particular case, it is .Q2 (R~ - R2 ). The
slowing of the spin of the Earth due to this energy extraction is negligible."

So, basically, the side force on the tether needed to impart 57.7 - 48.5 = 9.2 MI/kg to what is sent up the tether to the geosynchronous level. That is what challenges the assumption of effective rigidity.

Also, I note that trying to launch something off the tether at any other altitude will require rockets to match the vehicle velocity to the local orbital velocity. Otherwise, the ones launched from the tether below the geosynchronous orbit would not be going fast enough (for a circular orbit) and the ones launched from above geosynchronous altitude would be going too fast (for a circular orbit).

Getting back to that side force, that is what I want to see explained, beyond the assumption that the tether is "rigid". Side force cannot be exerted with a non-rigid tether unless the tether is deflected to the side to some degree, so that the spin of the Earth is pulling on the climber to accelerate it. And, the assumption that a tens of thousands miles long tether could actually be rigid is not something that I expect an engineer to be able to design, not with carbon nanotubes, or any other "unobtainium".
 
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Indeed this paper uses a simplified model. A rigid model indicates the basic behavior. My thesis published in 2006 also looked at an elastic cable model. Many papers since have done so as well. An ascending climber excites an elastic model primarily with the rigid body mode but it also induces sinusoidal lateral elastic mode shape participation. The most concerning mode would still be the rigid body one, because it is effectively undamped.
 
When you say others used an "elastic" model, was that only in the longitudinal direction of the cable, or could it deflect sideways in the two other dimensions.

And, what is the assumption about the upper end mass? Is it assumed to be immobile, or can it move up and down and east and west and north and south?

Not only will a climber have to pull on that upper end, the moon going by is going to pull on it too. Just assuming that it is immobile is a fatal flaw in the analysis. It needs to be SHOWN to not move significantly BY the analysis.
 
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No study I am aware of has assumed the counterweight to be immobile. My studies and others have allowed longitudinal and transverse mode shapes that respect all boundary conditions.
 
Here is a more succinct explanation of the space elevator concept: https://en.wikipedia.org/wiki/Space_elevator

It includes the following:
"As a payload is lifted up a space elevator, it would gain not only altitude, but horizontal speed (angular momentum) as well. The angular momentum is taken from the Earth's rotation. As the climber ascends, it is initially moving slower than each successive part of cable it is moving on to. This is the Coriolis force: the climber "drags" (westward) on the cable, as it climbs, and slightly decreases the Earth's rotation speed. The opposite process would occur for descending payloads: the cable is tilted eastward, thus slightly increasing Earth's rotation speed.

"The overall effect of the centrifugal force acting on the cable would cause it to constantly try to return to the energetically favorable vertical orientation, so after an object has been lifted on the cable, the counterweight would swing back toward the vertical, a bit like a pendulum.[61] Space elevators and their loads would be designed so that the center of mass is always well-enough above the level of geostationary orbit[64] to hold up the whole system. Lift and descent operations would need to be carefully planned so as to keep the pendulum-like motion of the counterweight around the tether point under control.[65]

"Climber speed would be limited by the Coriolis force, available power, and by the need to ensure the climber's accelerating force does not break the cable."

So, my issue is addressed by the cable deflecting to the west while the climber goes up. Yes, it would deflect to the east as a climber comes down. But, the net effect would depend on the net transfer of mass up or down. Launching spacecraft would get at tiny fraction of Earth's rotational energy, while importing asteroid ores would add a tiny bit to the Earth's rotational energy.

The actual energy/momentum transfer between the Earth and the climbers is accomplished both by the sideways deflection of the tether + climber mass and the sideways return of the tether mass to the vertical orientation once the climber has stopped moving. There has been more analysis of that than I was aware of when I made my first post, here.

So, the issue becomes the necessary strength of the materials needed, and it requires consideration of these side forces as well as the force along the cable between the Earth and the top mass of the elevator.

So far, despite earlier projections that are presented in the Wiki link, we do not have demonstrated material properties that come close to the necessary parameters for both strength and fabricated length. The Liftport company that, in 2005, projected accomplishing that with a factory and launching a space elevator by 2010, actually never even built the factory. Actually being able to create an operational tether still seems to be the major hang-up.

But, I am also wondering about other issues that I do not see even being mentioned. For example, there is some discussion of avoiding lightning strikes by placing the surface end on a barge in the middle of the equatorial Pacific Ocean. And there is some discussion about the electrical conductivity of the tether material being able to provide electric power from Earth to move the climbers on the cable. But, has anybody thought about what electrical effects will occur when we put a conducting cable through both of the radiation belts that surround Earth that are presently directing incoming solar plasma toward the poles? Would the tether make a "lightning rod" ground for those radiation belts down to the Earth's surface? And cables extending from orbiting satellites have been proposed as propulsion devices for maneuvering the satellites, so it also seems prudent to ask what forces the magnetic field of the Earth and coronal mass ejections from the Sun would have on an electrically conductive tether that is tens of thousands of miles long.

I am not trying to argue that a space elevator is impossible. But, my opinion that one is not close to deployment isn't getting changed by what I am learning as I look into it more. It doesn't seem to be as far into the fantasy thinking realm as "interstellar travel", but does seem to share the over-optimistic thinking with "space based solar power". At least we think that the physics may be possible, even if the engineering eludes us for the present, and the economics are highly dependent on the engineering "solutions" eventually developed.
 
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I've been studying the mechanics of space elevators for 20 years on and off. I find it enjoyable on a personal level, and can state that we have yet to find a true show-stopper from a physics point of view. The economic viability is another question, and no one can say when that becomes a win. As you state, it depends on many factors, and one critical factor is humanity's will to explore the solar system.

I stand by my statement from this article... We will need a proper infrastructure if we intend to pursue big space efforts, like Mars habitation. We are not ready to deploy a space elevator today, but if our space pursuits are to grow, we would need it, or something like it, to replace chemical propulsion as our principal means of exiting Earth's gravity well. I have yet to hear of a more promising method to do this. That is why it remains an exciting project for research.
 
I am not trying to discourage further thinking and analyses of the space elevator concept.

But, I think it is well past time for proponents to realistically evaluate the motions of the upper mass created by the climbers in order to best understand what those effects create in the way of limitations on the amount of mass per climber and the speed of the climbers.

That is no simple task, because the mass of the tether is also a factor, and the mass per unit length of the tether varies along its length by a large amount to try to keep the weight down by keeping the tension force per unit cross sectional area close to constant along its length. And, the side force on the tether increases with altitude, having its greatest effect near the top, where it also is most effective in moving the top mass. These are not trivial forces, and conceptualizing them as being delivered by a rigid lever or a completely inelastic rope are not sufficient. It is going to require some complex finite element dynamic simulations to get realistic results, and those are dependent on having a pretty well detailed design to evaluate.

Because this concept has already been associated with commercial startups that promised more than they even tried to deliver, such as a working version by 2010, this concept is often viewed by the general public as mostly a wild idea with some disingenuous advocates mixed-in, rather than as an engineering concept with serious analytical efforts being applied to short-term development efforts for actual prototype designs.
 
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The earliest one is by Dave Lang in the early 2000s using a GTOSS software he created. A more recent advanced study (May of this year) is called " high Fidelity flexible multi-body model considering torsional deformation for non-equatorial space elevator" published in Acta Astronautica.
I think I would prefer to end this conversation here. I prefer to have research discussions in another setting, like ResearchGate.
 
From https://answers.library.american.edu/faq/405403

"ResearchGate is a business that hosts open access research. It is neither a publisher nor a journal. It is a popular hub on the web for sharing academic publications. There is no editorial review board, nor does ResearchGate require that articles be peer reviewed, although they may be. Since it is an academic social network and there is no process for vetting the articles, evaluate each source carefully. If you choose to use an article found on ResearchGate, cite it using the citation information provided by the authors. No mention of ResearchGate is necessary."

Not a very convincing departure from the conversation here. I was at least hoping for a link to a paper that you think is a proper evaluation of the limitations of a rational space elevator design, considering the dynamic effects of climbers on the structure. Going off into the effects of locating the Earth end off the equator is not what I want to hear about, first. The first concern is how useful it would be if located in the optimal position, which is on the equator.
 
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The paper I recommended is peer reviewed and analyzes the motion you claim to be interested in. It includes equatorial results (as others have), but then goes further.

Your response is off-putting. I took the time to find the exact paper that would most benefit you based on your query. You read the title and then sent me a definition of ResearchGate.

ResearchGate is indeed a place where researchers can share work and have fruitful discourse. Your last reply indicates you are neither a researcher nor someone wishing to have fruitful discourse. Bye.
 
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Stephen Cohen, you are apparently not trying to actually discuss the issues, and are sending me off to find things that you say exist, but are not willing to provide me with links, much less extracts, to support your point.

FYI, I have gone to ActaAstronautica and searched for and found the title you suggested. It is here https://www.sciencedirect.com/science/article/pii/S0094576524002522#bib58 and it does provide a lot of information. So, why didn't you provide that link, here, when I asked?

As I started reading it, I saw a reference to "Space Elevator: Stability" (see https://www.sciencedirect.com/science/article/abs/pii/S0094576508000507 ), and started to read that. But, it comes up to me as a truncated summary that stops in the middle of sentences in one heading and goes to the beginning of the next heading. To get the full document, I need to "access through your organization" or "purchase PDF". So, I am expected to pay for access to see the things that you allege support your position that space elevators are well understood and feasible.

I simply don't have the time to do the work for the posters here who are not willing to make any effort to support their own positions.

If you want to make a positive impression about the feasibility of space elevators, you are going to need to post an available link and provide a quote of the results portion that addresses the issue being discussed. That is what I do, and that is what I expect others to do if they want credibility.

So, your excuse for leaving the discussion here is neither accepted nor appreciated.

On the other hand, other posters here that are interested in the space elevator concept should look at the link that I posted above, It is primarily intended to address the torsional loads and motions of the tether if it is attached to Earth at some latitude other than the equator, and ends with

"In this study, we elucidated the mechanism of the tether torsion due to the gravitational perturbations. The gravitational perturbations excite the torsional moment arm in the tether. The trend becomes more significant as the anchor latitudes are higher. However, the gravitational perturbation did not significantly affect the tether compared to other dominant deformation or motion. However, a torsional rotation of the tether can easily increase due to its low torsional stiffness. A twisted tether can affect the attitudes of other components on the tether. In future, a flexible multibody space elevator model that considers the coupling of the attitudes of the rigid-body component and torsion is required. In addition, other effects such as air drag, and lunisolar gravity, which were not considered in this study, should be incorporated in future studies." [emphasis added]

But, it does include a lot of information about the physical attributes assumed for the tether and climber. Quoting from the linked paper:

"Table 1. Analysis parameters.

Material density[kg/m3]ρ1.30 × 10^3
Young's modulus[GPa]E1.00 × 10^3
Shear modulus [63][GPa]G4.00 × 10^2
Cross-sectional shape[−]Circular
Anchor cross-sectional area at rigid bar model [26][m2]1.00 × 10^−6
Overall length of tether[m]1.00 × 10^8
Anchor latitude[°]20
Anchor longitude[°]0
Counterweight mass at anchor latitude 0°[kg]mcw2.539 × 10^5
Counterweight mass at anchor latitude 10°[kg]mcw2.543 × 10^5
Counterweight mass at anchor latitude 20°[kg]mcw2.590 × 10^5
Climber mass[kg]mcl1.00 × 10^3
Geocentric constant of Earth[m3/s2]μ3.987348 × 10^14
Mean equatorial radius of Earth[m]RE6378.137
× 10^3
J20 coefficient of Earth[−]J201.082627 × 10^−3
C22 coefficient of Earth[−]C221.574422 × 10^−6
S22 coefficient of Earth[−]S22−9.037666
× 10^−7
Angular velocity of rotation of Earth[rad/s]ωE7.336149 × 10^−5
Number of iterations in Newton-Raphson method[−]2.00 × 10^1
Convergence threshold in Newton-Raphson method[N]1.00 × 10^−10
Time step

[TD]–[/TD]
[TD]1.00 × 10^0[/TD]


{I had to modify the copied version to insert "^" to indicate exponents of 10, because the superscripts copied as full sized characters]

So, at least there is something for others to discuss, even if Cohen has "left the room".

In particular, note that the climber mass is only one metric ton, 2,200 pounds. And note that the upper mass, which needs to be well above geosynchronous orbit, is 254 metric tons, for the tether located on the Earth's equator. It would take a SpaceX SuperHeavy type lifter multiple launches to get this material into position.

Strangely, the mass of the tether itself is not in the table and I did not find it in the text. I wonder how the FAA would license a mission that puts something in space that would more than wrap completely around the planet if it comes down in an uncontrolled manner. And, assuming it does not have an infinite lifetime, getting it down is another issue. If there is worry about StarLink satellites burning up the upper atmosphere, how much of this tether would burn-up and how much would be impacting the surface if it fails?

And, do the economics work with only one-ton climbers with something less than one ton payloads climbing in speed and numbers limited by the tether dynamics. That is why I would like to see the results of the tether dynamics presented better. There are some results in the link, but they seem to be basically examples for a rather small climber, rather than exploration of the limits for climber mass, speed and numbers on the tether.
 
To extend this discussion a bit more relative to the state of understanding of the real issues for a space elevator:

1. Note that the table I copied above says that the tether at Earth's surface is assumed to be only 1 square millimeter in cross sectional area. It will taper to much larger diameter up to geosynchronous orbit level, then back to smaller diameter at the upper mass. So, what is the mass of the whole tether? Somebody should be able to answer that, if they have done analyses for such a tether.

2. I note that the climber is only 1 metric ton, which seems to be about 15% of the tensile strength of a 1 square millimeter cross section multiwall nanotube. And, the speed of the climber is limited, so the trip up was stated to be about 8 days in one of the references. So, unless there are ways to use multiple climbers on the same tether at the same time, that seems to limit the payload to orbit for the whole system to 365/8 x1 tonne = 46 tonnes per year. And, that assumes that the climbers can come down while another goes up. If not, then maybe only 23 tonnes per year? It seems to me that multiple climbers are going to be needed just for financial competitiveness, so there needs to be analyses that address multiple climbers simultaneously using the tether.

3. And, what mechanism is thought to be capable of gripping a 1.1 mm diameter circular cross section tether with enough force to lift a tonne? Without doing any wear damage to the tether material at all? How could that mechanism adjust to the changing diameter of the tether? For that matter, how could climbers pass if there are more than one on the tether at the same time and some are coming down while others are going up? (Yes, I can think of "passing devices", but those would have additional mass that would need to be deducted from the useable payload mass of the already light climbers.)

4. What electrical current will be induced along the highly conductive tether as it passes through the Van Allen radiation belts? Will that heat the tether sufficiently to cause problems with its properties? Will it cause problems at the ground termination point? (On the plus side, could we use the power constructively? )

5. What types of accidents have been considered? If an orbiting booster like a Chinese Long March rocket hits the tether at 17,000 mph, what happens? Does it break? Does it fall? Will the upper mass sail off into solar orbit - perhaps with people on board that would not be able to be rescued?

These are the types of issues that actual designers of a prototype system will need to address before anything is going to be started to be built. So far, what I am seeing is what I would call "research papers" that academically explore one or a few aspects of the basic physics in ever increasing detail, but use arbitrary parameter values for things like climber mass and number. I am not seeing the types of analyses that indicates anybody is seriously planning to commit to constructing even a prototype of a tether attached to the Earth's surface..

Regarding the other topics that are the subjects of this article:

The solar energy from orbit topic seems much further advanced in its engineering phase, with demonstrations being conducted. Still, it might not turn out to be financially viable, or safe enough. But at lease we do know how to design it so that it could actually function.

The "travel to the stars" topic is something that we don't have any idea how to do, except perhaps for gram sized "probes" propelled by super lasers from Earth (or the Moon), which is not going to "carry crew". We have no clue how to actually make a warp drive or a subluminal interstellar space craft that could keep humans alive all the way to even the nearest star.
 
Why i don't think the 'space elevator' makes physics sense:

(Any competent physicist feel free to correct any erroneous ideas of my objections).

To lift something to orbit takes a given amount of energy. It consumes the same energy as a rocket would.

That energy must be stored in the angular momentum of the orbiting body and held in tension (a lot of tension) by the ribbon/cable.

Just moving stuff/mass up into orbit increases the mass of the orbiting body but doesn't add a proportional vector/angular momentum to the orbit of all that mass,
which will, i believe, slow the orbit speed [lose stored energy].
A trailing orbital body means the a constant length ribbon will draw the orbiting body closer to the Earth's surface, which may encounter atmosphere friction at some point.

A counterweight doesn't work because a weight starting at the top of has much less weight [even given equivalent mass] than the stuff to be lifted at the bottom of the ribbon/cable.

The only way it could make sense is if one had a cable to the Moon, which is already moving outward and is super massive.
One would need a cable base that could slide around [near] the Earth's surface once daily.

That along with a quarter million mile unbreakable cable makes that impracticable.

Even with the Moon eventually its escape energy would eventually be consumed.
 
I won't have time to make extensive comments, today.

At this point, I just want to say that it does not necessarily take the same amount of energy to use a space elevator as it does to use rockets. There are several aspects to consider, but the main one is that a rocket needs to lift its fuel as it goes. A space elevator would be taking energy from the Earth's rotation in a more efficient way for getting a specific amount of payload to geosynchronous orbit (or above).

And, the cable does not need to fall down if mass is added at the top end, It only needs to bend back a bit, so that the Earth's rotation is adding to its velocity by pulling on the tether a bit in the circumferential direction. When mass is no longer being added (climber stops), then the upper mass should return to its undisturbed position - if the mass addition was small enough and slow enough.

However, adding too much mass too fast could definitely cause the tether to fall, and somewhat less than that would probably cause some pretty wild position oscillations of the upper mass, plus a lot of strain on the tether.

So, my thinking is that the whole concept needs a realistic look at how much mass can be moved up the tether per day, so that it can be compared to using rockets. The numbers I have seen in the papers I have looked at just don't look like they would be at all competitive, much less an improvement in heavy lift capacity.
 
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