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space elevator
3

space elevator

space elevator

(OP)
Did anyone see references to the above mentioned project and is there a consensus? Is it a hoax, a bunch of dreamers or a legitimate new technology?

Dave

RE: space elevator

I met the people involved with the space elevator at a forum fo high-tech investors.  Believe me they are quite serious, and include a number of very sharp people,  This is not a hoax, but they will be dependent on technology breakthroughs in nanotubes to be successful.

RE: space elevator

I'm afraid that the "serious investors" are involved with "serious scam artists" (my quotes).  This orbital elevator, I assume, is the scheme I saw in OMNI magazine (that paragon of engineering accuracy and knowledge) a decade or so ago.  It involved a cable loop, 44,000 miles in circumference, a geostationary satellite with a pulley and a similar pulley on the ground; a payload on the ground is hooked onto the cable and a motor on the ground turns the crank and the payload is hoisted into orbit.  This violates a number of laws of physics, conservation of angular momentum comes immediately to mind.  A slight twist to this scheme is to have an equal mass aboard the satellite, hooked onto the cable as it is cranked.  This allows the net angular momentum of the system to not change; however, the angular momenta, potential and kinetic energies of the "up" mass and that of the "down" mass must be changed by the force in the cable.  While a geostationary satellite appears to be hovering over a spot on the Earth, it is really travelling at roughly five plus miles a second; this velocity would have to be imparted to it by a cable that is running perpendicular to this direction of travel.  

If any of the investors are interested, I have a few concepts of my own they might be interested in.

RE: space elevator

nhughes, I'm afraid but you are partly right and partly wrong.
What you describe is laughable as elevator, true. But that it's not the current concept.
AFAIK, orbital elevators are fixed space Structures, more like "Space stairs", ON which loads are transported, usually by means of induction/magnetic motors

They are veryadvanced concept and, in my opinion, still very SF-like.
They work like this: the base of the elevator is connected somewhere on Earth's equator, the "tip" is somewhere up there in space. The length of the elevator is not important but the whole structure MUST have a property: its COG must be at the geostationary orbit level to be orbitally stable. Some schemes require a big equilibrating mass (a captured asteroid usually) on top, to achieve the COG requirement.
The main drawbacks to this scheme are two:1) the asteroid and 2) the enormous tensional forces.
POint 2 is the reason why joe922 was speaking of technological breakthroughs: at the moment there is no material able to resist the tension generated by its own weight under these orbital conditions.
A further problem would be the electrical induced currents (make a google search on tethered satellites) but I think this post is long enough now.

RE: space elevator

(OP)
Wow long time between responses. As a follow up, since my original post I looked further into this topic. While it is still pretty SF, it is close to realization, or the proponents are very convincing. I say this becuase NASA has pumped several million dollars into a group to explore the possibility. The key technology is carbon nanotubes. The strength of this material would make it possible if it could be produced in large, continuous quantities. I beleive the group doing the research is on the website www.ISR.us. While I am skeptical, I assume NASA has given the project some consideration as to feasibility before providing research funds.

RE: space elevator

Are you kidding me??????   A rigid structure tall enough so that its center of gravity is at geosynchronous altitude????  Come on.   As an engineer I am ashamed of having absurd concepts such as this attributed to my profession.  Millions of dollars spent????   Billions on single stage to orbit????  I am appalled.

RE: space elevator

It does seem incredible. One suggestion for how it would be built is to build a factory at geostationary orbit, then have it start building cables in opposite directions, directly toward the Earth and directly away from it. The COG would stay at geostationary orbit, and tidal forces would stretch the cables taut. Eventually the lower end would reach close enough to the earth to be reachable somehow, perhaps even anchored to the ground. Another way is to build a little sky elevator one to hoist other cables up in a kind of pyramid scheme.

When it is built, you ride an elevator up to the far end. That far end would be whipping around at much higher than Earth escape velocity, so to launch that coummuter bus to the Moon, you just let go of it and it sails away, like a rock from David's (the Goliath-killer) sling. The entire structure will have lost some kinetic energy and its orbit will shrink a little, so it would need some way to correct its orbit (maybe by running electrical current through it to push against the Earth's magnetic field.)

It would seem crazy, but once it is built you can have relatively cheap interplanetary space flight, with no limit on payload. Also, it might not be practical on Earth, but it might be practical on the Moon or even Mars, perhaps even using existing materials like steel. People have suggested it as a way to return mined materials from fast-spinning asteroids.

And the earliest source of the idea I am aware of is the source of so many other visionary ideas, Arthur C. Clarke, in his 1979 novel, Fountains of Paradise. He imagined using diamond cables, but he later suggested buckminsterfullerene, aka carbon nanotubes.

This wikipedia article has a nice rundown of the design issues, costs, etc.
http://en.wikipedia.org/wiki/Space_elevator

RE: space elevator

This is an interesting twist on the space elevator hoax.  Let's look at the basic concept of conservation of momentum.  (not to mention the energy needed to transport the geostationary factory and its raw materials to geo orbit)  Orbital (or any other object's) angular momentum is equal to w**2*r where w is rotational rate and r is the radius.  Thus an object attached to an orbiting vehicle that is closer to Earth than the orbital altitude is forced to move forward relative to the vehicle; an object at a higher altitude falls behind.  Thus the cables streamed out from the geo factory would be forced into a line leading and trailing the vehicle.  This would dominate the dynamics of the cable if it were only a few miles long, let alone 22000 miles.  This concept, as well as all the others, violates basic laws of physics.
If there truly is significant money available to study the space elevator, please let me know.  I will become an advocate and request financial support for my studies.  This takes advantage of a more basic law than any of physics, the  one about one being born every minute first (???) promulgated by P.T. Barnum.

RE: space elevator

(OP)
Mr. Hughes,

I assume you were not serious as I already stated NASA has provided funding for this particular "hoax". Perhaps you should instead get a job with them, as you are obviously much smarter, being able to dismiss this based on basic laws of physics, where the obviously uneducated types at NASA gave them money. Or perhaps you could put your doubts on hold for a moment and look at the link I posted. Perhaps you, not NASA's engineers who do this for a living, are missing something.

Nothing wrong with being critical, assuming you are the only one who is being critical is a little egocentric, don't you think?

RE: space elevator

Mr. Worf,
I am, and have been, a spacecraft dynamics and attitude control engineer for pushing thirty years.  I have a Bachelor's degree in Aerospace Engineering and an MSME.  I have worked for three of the largest Space/Defense contractors as well as the Federal Government during my career in this field.  In this time I have known engineers and scientists (a small number) whose scruples are  such that they would not hesitate to convince naive decision makers, who control the money, of whatever it takes to receive said money.  I have seen many millions of dollars wasted on projects on which the technically knowledgeable personnel have no expectation of success.  The fact that NASA is funding this concept may confer apparent legitimacy to it, but it does not mean it is based in reality.

By the way, I do "do this for a living" and have for alomst three decades.
As far as being smarter than the NASA scientists, with my training and experience, many might agree that I might be.

RE: space elevator

(OP)
Mr. Hughes,

I understand that you feel your experience gives you insight into this. While I am happy to grant you that, I, by the same token grant that credit to NASA's enginers. While you feel you may exceed their abilities, I find it hard to believe you can dismiss their approval of funding in hundreds of millions out of hand. While I agree mistakes are made, it seems to me just a likely that you are making one, as they are. And though I am an engineer and DAR (Candaian equivalent to DER) I have not carried out the analysis of this to convince myself one way or the other. Just an interested party, not wanting to hold onto a belief without reference to changes in technology or understanding. I have no doubt that many experts dismissed new technologies as impossible in the past. That may or may not be the case here, I am just willing to keep an open mind. Einstien seemed pretty far out there in 1914. By 1920 what he said was accepted without question.

As always the proof is in the pudding. Since I have no money to invest, I will continue to observe with interest. Thanks for your input, I value educated opinions. I see you mention angular momentun and the difficulties it would entail in the design of such as system. Difficulties do not equal violating basic laws of physics and can be overcome with new technologies (let me define the ideal material I can make you vertually anything). What law of physics do you suggest is being violated?

David Pasquill, P.Eng, DAR

RE: space elevator

Let's do a thought experiment.  Let's say that we have invented a material, unobtanium (a commonly used engineering material) infinitely strong and zero density.  We have made a structure/tether to geosynchronous orbit.  Remeber that strength does not equate to stiffness, so a structure made out of unobtanium 22000 miles long would act like a string (I think you'll grant me that).  So now we want to raise a payload up the structure/tether, using a crawling mechanism.  Since as soon as it leaves the ground it is supported by tension in the structure/tether, the weight of the payload is transfered to the geosynchronous satellite.  This will pull it into a lower orbit; the satellite will no longer be geosynchronous - it will move in the prograde direction from its station.  The longer this condition persists, the farther it will move.  

Okay, let's say we have thrusters on board to maintain orbital position.  As the payload leaves the ground, the thrusters will have to be producing thrust equal to the weight of the payload (this ignores other effects, like the necessity to accelerate the payload tangentially).  As the payload climbs the structure/tether, the differential between weight and the Mw**2r centripital force will decrease, reaching zero as the payload pulls into the hangar on the geosynchronous station.  I've seen estimates of days to weeks to get a payload up; so we have this thruster firing for that amount of time.  

The most efficient rockets we have today - hydrogen/oxygen - have a specif impulse of around 400 seconds.  That means you can ge 400 seconds of one pound of thrust for every pound of propellant.   A more exotic system - ion engines - I believe have specific impulse of around 1000 seconds, although at present they only produce fractions of a pound of thrust and require large amounts of electrical power.  Okay, let's say we have ion engines that produce enough thrust to provide for a useful payload to climb the structure/tether.  And let's say that the typical transit to geo orbit takes a week - 604800 seconds.  The average thrust over that week will be half the weight of the payload (assuming that it climbs at a constant rate).  So for every pound of payload we lift we will need to expend 604800/(2*1000) pounds of fuel or roughly 300 pounds of propellant to move it vertically to geo orbit altitude.  

Also, the payload will have to be accelerated tangentially from equatorial velocity, about 1000 mi/hr, to geosynchronous orbit velocity, about 6800 mi/hr.  This tangential velocity must be imparted by a tension only device which can provide tangential velocity only when deformed, much like a bowstring imparts velocity to an arrow.  Watch a bow closely as you draw an arrow and you can observe that the tension in the string is far larger than the force required to pull it back - it goes, I believe, with 1/sin(angle between the string at its undeformed position and the position at any time).  This sounds like very large tension force, which, of course, is also transfered to the orbiting geosynchronous station.  We could make some assumptions and estimate this force but my head is starting to hurt - I think it would be very large - probably orders of magnitude larger than the thruster we calculated above.

Okay, this does not, strictly speaking, "violate a law of physics" as I suggested before.  It's more that we cannot violate the laws of physics, and therein lies the problem.

RE: space elevator

I am not the smartest guy in the world, but I can see some flaws in nhughes1's argument.  First of all, I would put the tethered satellite in an orbit that is bigger than a geosynchronous orbit.  The tether will now have to be in tension and any load which weighs less than the tension can climb it and have no effect on the tension that the satellite feels.  The tension in the portion of the tether that is below the climbing load will be the original tension minus the climbing load.  In reguard to the tangential speed, doesn't the base of the tether have the same angular velocity as the satellite at the start.  After all, we are on the surface of a spinning ball.

RE: space elevator

Orbital mechanics is far beyond my knowledge but the governing equations is this application is Newton law of gravitation.
F= G* m1*m2/r^2
This equation is integrated over the mass volume of the objects in question
This will give the following result:
Given a string is orbiting a sphere, Where the mass of the string is significantly small than the mass of the sphere.  
The string will line up pointing towards the sphere, the longer the string, the stronger the forces that will point it towards the sphere.
The prime example of this phenomenon is the moon orbiting the earth, the rotation of the moon about its own axis has degraded so the same face is always pointing toward the earth.  The earth is trying to do the same thing toward the moon and the sun as evidenced by the tides of the seas.
The basic orbital mechanics that you have been describing only applies to bodies taken about its mass centroid as whole bodies assumed to be concentrated at the centroid, applicable only to the limits of the assumptions.  The space elevator equations require the full volumetric integration of both the elevator and the earth to understand the process.

Hydrae

RE: space elevator

To Timelord:
You're correct - the angular velocity, w, of the mass "climbing" the "tether" remains constant, but the angular momentum is equal to mwr^2.  Angular momentum of the mass increases as the square of the radius from the center of rotation.  This increase in angular mometum must come from somewhere - Newton and God say angular momentum is always conserved.  Either the angular momentum of the orbiting station, irrespective of its original momentum (an interesting dynamics problem - a mass at higher than geo altitude at geosynchronous rotational rate attached to a tether - the big fallacy many people make is confusing what they see when they swing a rock on a string with the interaction between gravity and centripetal force) will decrease proportionately or an external force (thrust?) must be applied to make up the difference, either to the orbiting station or to the climbing mass.  The same priciples I outlined in my earlier missive apply - I made assumptions about what the system would look like to get some numbers.  This increase in angular momentum is another way of viewing the required increase in velocity.  Trust me, constant angular velocity does not translate into no need to accelerate a body going to geosynchronous (or any other) orbit.  Geosynchronous orbit is the same as any other orbit (it requires the correct angular momentum, velocity, energy, altitude, etc.) with the minor proviso that its angular velocity happens to match that of the Earth.

To Hydrae:
You are referring to the gravity gradient effect.  This is, indeed, the reason that the Moon always shows one face to the Earth and Mercury always shows one face to the Sun.  Gravity gradient torque is proportional to (r X Ir)/R^3 where  r is the radial unit vector from the central body, R is the distance from the center of the central body and I is the inertia matrix of the body in orbit (X is the vector cross product operator).  Gravity gradient is used to stabilize some vehicles, dump internal momentum for others.  Notice that it is proportional to 1/R^3 - it decreases very rapidly with altitude. It will, I think, be orders of magnitude smaller than the forces exerted by conservation of orbital momentum, etc. By the way, it took billions of years for the Moon and Mercury to stabilize with their rotational periods equal to their revolutionary periods.

I stand by my assertion that a space elevator is - the appropriate word eludes me.  I hesitate to insult by using the words that immediately come to mind, but I refuse to dignify the concept with any word that suggests that it merely suffers from difficult complexity or a lack of sufficiently advanced technology.

I hope you all had a nice Mother's Day.

RE: space elevator

>>>>Angular momentum of the mass increases as the square of the radius from the center of rotation.  This increase in angular mometum must come from somewhere - Newton and God say angular momentum is always conserved.

As I see it, yes, angular momentum is conserved. The rotation of the earth slows when you hoist your payload. This is because the center of gravity of the earth asteroid system shifts twards the asteroid. But no fears, the angular momentum of the earth is close to infinatly bigger than any object we will ever be hoisting...I hope.

Lucas

RE: space elevator

Grayocean,
All you say is true.  The problem is how to transfer angular momentum, in this case from the Earth, to the payload climbing the tether.  Any structure that is 22,000 mile long will NOT support any bending moment which is what would be required for the structure to provide the angular momentum to the payload.  Such a structure, if it could be built, would support ONLY tension; any compressive load would cause immediate buckling as would any bending moment.  Think of a high (1000' ??) radio tower with its guy wires removed - not very stable.  Now think of a structure roughly 120,000 times as high!!!!!

I stand by my position that a space elevator is impossible.

RE: space elevator

Shorten the Rope!

RE: space elevator

Not shorten the rope, lengthen the unidirectional unobtainium crystalline structured cord to 40,000+ miles. Make gravity negligible. The Centrifugal force that keeps the asteroid in orbit is not the gravity of the earth, but the tension of the cord. mv*v/r + GmM/r*r = Tension in unobtainium cord.

Example one, a bowling ball and golf ball tied together and spinning in free space. Now there is no reason why an ant with his own Niel Armstrong space suit could not walk from the surface of the bowling ball to the surface of the golf ball. In this case where gravity is essentialy zero he better not let go, but it helps having six legs.

No torque needs to be applied to move the ant or make the system spin slower.. Angular momentum will conserve itself. Example two, a spinning figure skater, by pushing her arms out, her spinning will slow, but angular momentum will remain constant because her angular inertia will have increased.

Hope this helps clear it up.
Lucas

RE: space elevator

Grayocean,
Thanks for your comments.  Some of your facts are correct, that rotational momentum is conserved, that the moment of inertia of an ice skater decreases with arms pulled in so rotational rate increases and vice versa.  
 
The c.g. of the bowling ball-golf ball system will be very close to the center of the bowling ball.  The contribution to the moment of inertia of any mass is proportional to the square of its distance from the center of gravity.  Thus, the ant's mass, moving from the surface of the bowling ball to the golf ball increases its distance from the c.g. of the system and thus increases the moment of inertia of the system which would slow the rotational rate of the system very slightly.  

Looking at this thought experiment using a free body analysis of the ant, his tangential velocity increases significantly - it starts at W(RBB), where RBB is the radius of the bowling ball, and W is the rotational rate of the system, and ends up at W(LGG), where LGG is the length of the tether holding the golf ball (W doesn't change much).  The force accelerating the ant is supplied by the tether in the bowstring mode, which I described in an earlier missive, so the tension force in the tether is large, depending on how rapidly the ant climbs, and is transfered to the golf ball whose distance from the c.g. of the system decreases and position relative to the original radial vector from the bowling ball also changes.  The ant chooses his rate of climb which determines whether a large force is applied to the golf ball (space station) for a short time or a small force for a long time; in either case, the angular momentum of the golf ball (space station) decreases the same amount - roughly equal to the change in angular momentum of the ant.

Also, gravity is not zero, it decreases as 1/R^2, where R is the distance to the mass center of the system, so it decrease from 1 g to (4/26)^2g as a mass moves from the surface of the Earth to Geo altitude.  (Gravity is not zero in orbit, it just appears to be so because all bodies are in free fall).

The ant (or the space elevator climber) would be climbing subjected both to gravity and to the force that is required to increase its tangential velocity (accelerate it).

Any way, whether the system is bowling and golf balls with an ant or the Earth, a geosynchronous space station and a climber, Isaac Newton and God agree, you don't get something for nothing.

An asteroid in orbit????  The effort and energy expenditure to modify an asteroid's Sun central orbit to one around the Earth would buy an awful lot of Titan IVs (whoops, sorry, no more Titan IVs.  They would be Delta 4s or Atlas 5s)

RE: space elevator

(OP)
Now at last, I think I can see some of the issues. I think Mr.Hughes arguement rests on an energy balance question, first law of thermodynamics and all that nice stuff. The crux of the issue making it an impossible feat is the fact that the energy required by the system (both to overcome gravity and to modify the angular momentum of the system) is so high that the added energy to carryt the propellant with the elevator (hence the comment about the weight of propellant) make the thing impossible. Other issues are clearly technological, the use of unobtanium for instance.

Is this a reasonably clear statement of your concerns Mr, Hughes? My impression that is the getting something for nothing comment in the last post.

Dave

RE: space elevator

This space elevator concept is a thought experiment, as such let your mind wander and wonder.

As for the amount of energy required to lift a payload using the ‘elevator’ it would be many orders of magnitude less that today’s methods.    Yes the orbit of the elevator will be modified by the lifting of payloads, but the reverse is true of any payload dropped to earth also through the elevator.   I imagine the operator of such a system would keep a detailed tally of loads in both directions.  
Orbital corrections in the elevator could and would use the more efficient methods of thrust that would be available when unobtanium is invented or discovered such as ion drive (current today) or a proton beam, (accelerating protons to near the speed of light as reaction mass.)  It would not require the very very high energy consumption of a chemical rocket engine needed just to lift its own mass with its resulting expended reaction mass to lift off the ground. I am not saying it its free but the energy will be from where we can make it cheaply.
The power for either payload lift or orbital corrections could be from photovoltaic solar power stations in orbit or power plants on earth again based upon technology available at that time, or who knows what source of energy then.

Leonardo Da Vinci did a drawing of a helicopter when steam power had very limited uses, but that did not stop him from dreaming an impossible then but now everyday reality.  A lot has and can happen in four centuries.

Hydrae

RE: space elevator

(OP)
As I was pondering this an interesting point came up. How about a system with 2 elevators, one going up[ and one going down. Very hard to implement perhaps, but there went the angular momentum problem out the proverbial window. I believe the people currently working on this are looking at a microwave beam type power source so no propellant is actually part of the lifted weight.

Thx all for the participation btw, this has been very enlightening so far.

Dave

RE: space elevator

You've convinced me!!!  Please tell NASA to send me 1% of the "Hundreds of millions of dollars" in research money.

I downloaded the NASA report "Space Elevators  An Advanced Earth-Space Infrastructure for the New Millennium",  "based on the findings from the Advanced Space Infrastructure Workshop on Geostationary Orbiting Tether Space Elevator Concepts ...".

I had a few questions that I need to have answered so I can advocate Space Elevators most effectively.  For instance, on page 6 they state that the energy required to move payload to geo orbit is 14.8 kWh/kg (I'm paraphrasing).  My calculations show that the change in kinetic and potential energy of a 1 kg mass going to geo orbit is about 8512273721.64 kWh.  I would like someone to explain where the other 8512273706.84 kWh will come from or where it goes.


Perhaps you have guessed that there is a touch of sarcasm in my above remarks.  

The document is rife with similar unsupported statements and calculations, fanciful drawings and paintings, bits and pieces of engineering reality woven into an absurd concoction.

I am appalled at what I found in that report.  It makes me ashamed to admit that I am in the same profession as the individuals who wrote the report.

However, if someone will send me the sum that I mentioned in the first paragraph (I'd settle for something a little smaller - chump change by these standards) I might be persuaded to change my point of view.

RE: space elevator

Well, I get about 16 kWh/kg, 14.7 kWh/kg change in potential energy and 1.3 kWh/kg change in kinetic energy.

Integrating GEMEdR/R2 from RE to RE+22300mi for potential energy

escape velocity at 7mi/s results in 17.6 kWh/kg, geosynchronous orbital speed is only 1.9 mi/s, so kinetic energy is only about 1/14th of escape velocity KE

TTFN

RE: space elevator

I hope the viability of the theory has now been solidified. Ok, time for me to switch sides. On science friday they discussed the what if's of an airliner crashing into the ribbon, the 1400 Tons of satellite would hurl off into space. Gone. But my biggest concerns is space junk/meteor showers problem. There is over 40 million square meters of ribbon up there, the probability that it would be hit in the 20 year time scale that it would take to construct the thing would probably make even the ribbon construction impossible. Sooner or later the ribbon would be reduced to Swiss cheese, and then part. Can anyone comment on the probabilities on say a grain of sand sized meteor passing though a specific one square meter at various altitudes?

Lucas

RE: space elevator

Thank you for the discussion.  I am a high school teacher, not an engineer.
Years ago, one of my Chemistry students informed my class and me of this Space Elevator.  We had a great discussion.  At the time I confess that I sided with nhughes1.  
But now as a teacher of young dreamers, I want to keep it alive.  Could there be any chance that returning to fundamental high school principals we might find the obtainium and the possibility that the Space Elevator could work?
Let me review the things that I understand (I think), but from more of the high school linear point of view.
First of all there must be a heavy mass tethered at a distance well beyond 22000 miles, the free fall geosynchronous orbit altitude, with the anchor at the equator. I think I have read the proposed radius is 62000 miles.  The heavy mass may be referred to as a geosynchronous satellite.
At the earth surface the linear speed of the elevator is 25000 miles per day = 1042 mph = 0.289 miles per second.
At a radius of 62000 miles the distance for the satellite to make one revolution is C = 2pir = 389557.5 miles =  389557.5 miles per day = 16232 miles per hour, approximately a linear velocity of 4.5 miles per second.  That is less than, but close to free fall Low Earth Orbit orbital velocity.
As the elevator moves outward, its linear tangential velocity must increase from about 1042 mph to about 16232 mph to maintain a geosynchronous orbit.  I believed it was assumed that the force to provide this acceleration would come from the satellite located at 62000 miles that keeps the tension in the tether.  This force would slow the satellite, and it may have been assumed to have a negligible effect.
Did I forget anything?
I have a couple of questions.  Can the anchor be offset from the equator?   Could the cable go from the N.pole to the S.pole?

RE: space elevator

dEARQ, you might have something there.  Just modify the cable to go from the Equator to the N.Pole?   Is there anyway that could work?

RE: space elevator

First of all, similar to the author of the previous msg., I am a HS teacher myself.  I hope it is ok to join the discussion.
Ok, from the HS point of view, how about this argument for trying to keep the project alive?
The satellite (massive captured asteroid or similar) tends to travel in a straight line (inertia).  As a planetary satellite, its direction of motion is constantly and uniformly being changed.  The force responsible for this directive component of the satellite's acceleration, which would normally come from only from the nanotube (obtainium) cable will be shared by the force that is driving the elevator's acceleration in the tangential direction.
Finally, here are the results of an experiment I performed at home.
I attached a hacky-sack to about one meter of fishing line attached to my ceiling fan.  I also attached a heavy, symmetrical mass with a set screw (from my kid's erector set) adjusted to barely slide along the fishing line when under centrifugal force.  I turned on the fan.  The mass slowly slipped outward.  The fan slowed, but the two masses seemed to constantly and continually swing around in a nice, neat uniform motion.

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