Space Elevator/Sky Hook

[Laudnum]

Question:
Listening to Dr. Kaku's show on Coast to Coast. . .He mentioned a Space Elevator or Sky Hook.
I am a graphic designer. . .not a physicist, but would there not be a lot of centrifugal force at the outer end of such a cable when it is hundreds of miles long? I realize the concept is to drop the 'cable' from the space station in orbit, but once it is anchored to the earth's surface would it not be like swinging a ball on a string over your head? I appologize for my lack of understanding, but I feel if anyone can help me. . .It would one of you. Thanx

Tom Hanson

[enigma]

Yes, that's actually the point.

It's the centrifugal force which keeps the line taut by countering the weight trying to pull it down to earth.

[Eddie French]

I agree, that is the whole point.
We are looking at two opposing forces. At the optimal orbital distance the effect of gravity on the lower (Half) of the Cable/Ribbon exactly equals the CF force on the higher half!
Imagine this cable being lowered to a point on the Earth's surface during construction - It would descend at a lower and lower rate as the (End) aproached the surface.
It would actually be possible to leave the Cable/Ribbon suspended at a particular distance above the ground (Without actually 'Anchoring' it) while allowing operations to commence.
Hope this is clear.

[Ubu]

Useing Geo syncronise orbit positioned directly over the equator for the anchor is the general concept to ensure the line is completly vertical with no slant. The cintrifical force strained by the anchor im pretty sure would require a material with 6 carbon atoms perfectly alligned, this does not happen naturally in nature, and to assemble this atom by atom would cost millions per pound of this material

[notal33t]

Low earth orbit is all fine and good but, I wonder about concept of a magneto-hydrodynamic MHD propulsion system using a torodial superconducting magnetic field for gathering and emitting ambient charged solar particles as a method for both internal power and propulsion. A charged solar particle system would provide a large delta V from the solar environment with no need for a "nuclear" option (fusion, fission, or antimatter). I wonder if any of these NASA rocket jocks has ever stopped to think of the raw energy that solar activity pours daily into earths polar magnetosphere region just waiting to be tapped. Dont forget about the space elevator option. but once were in low earth orbit lets use a MHD system, and we can explore the solar system with ease.
PS! I seem to recall a recent report that a coronal mass ejection front was reported recently having been detected by the voyager probes at the fringe of the solar system!
HMMMMMMMMMM!

[drag]

Greetings !

It's probably rude of me, but - Dr. Kaku should stick to physics
and stay out of other things, he just talks too much.
I couldn't even finish Hyperspace because of all that waste
of time art history and stuff.

Anyway, in this case too - the idea of a "space-elevator" is
very unreal for now. As it was mentioned here, the base station
needs to be in GEO which is a LONG (36K km) distance, you need
a very massive station there to keep it in place and compensate
for what you bring up, and there are cable strenght and weight
problems and more.

An MHD propulsion system uses on board propellant and ionises
and ejects it. Collecting space plasma, even in LEO, is very ineffective
because there's too little of it. It also, by no means, rids you of
the need for energy to accelerate these particles. And instead
of an efficient heavy noble gas like mercury or Xenon or even the new
heavy C60 molecules, you'll have to mess with helium and hydrogen.

A prop system that collects the momentum of space plasma -
solar particles does exist in theory and maybe in some limmited
experiments. The newest and best idea there is the M2P2 system
which disperses a gas in space and using its magnetic dipole
and a week magnetic field contains it in a huge bubble many miles
across. Solar particles exchange momentum with this gas
bubble and propel the spacecraft.

Live long and prosper.

[Adam]

Unreal, eh? Please read: http://flightprojects.msfc.nasa.gov/fd02_elev.html

[drag]

Unreal, eh? Please read: http://flightprojects.msfc.nasa.gov/fd02_elev.html
So ? (You do know it's not a photo, do you ? :wink:)

[LURCH]

He was probably referring to the text, not the image.

In response to the original post; yes, the tension between the opposing forces (centrifugal and gravitational) would be immense. No currently-existing material could withstand these forces. However, progress in materials development in the field of graphite nanotubes may provide the construction materials needed.

[drag]

He was probably referring to the text, not the image.
I know... :wink:

[Tau_Muon_PlanetEater]

The sky elevator would have no centrifical force if the attached space station were in a geosynchronous orbit (stationary) at an altitude of more than 36km, since there is no gravitational force from the Earth there. Well actually there is a minute gravitational force at 36km, but at over 50km its basically zero. There is a mathematical tradeoff to consider as well... The longer the cable, the faster the space station would orbit, but the longer the cable, the further away from Earth's gravitational field would be. Therefore, although the space station may orbit at several thousand km./sec., there is no gravity in this region, and there for no G-forces (centrifical forces).

However, the cable will undergo significant stress near its mid point, since gravity will still be appreciable at this altitude, and conversely the cable speed will be high. The difficulty in building the cable will be dealing with the range of stresses put upon it at various altitudes.... Just above the Earth's surface will be very little stress, at the midpoint above the Earth will be the most, and at the apex of the cable where the space station will be, will have no stress.

As for harnessing the energy of the cosmic winds, its very possible to collect gamma ray energy and lower energy photons by harnessing the photoelectric effect. This is no problem at all.

[Adam]

Crap. Due to the sudden disappearance of gravity, half our satellites just tumbled off to the deeps...

[Tau_Muon_PlanetEater]

Adam why be a jerk?

[notal33t]

Please examine the diagram at the following sites.

http://www.ae.utexas.edu/design/phoenix/vasimr.html

http://www.artsci.washington.edu/newsletter/Autumn99/Winglee.htm

My proposal substitutes all the messy, 'stored' hydrogen/helium fuel and 'implied' reactor plumbing. As for the low level of plasma in LEO, this is of course dependent on the superconducting field intensity and the cross sectional collection area, not to mention the effect of increasing delta v, on collection efficiency. Even a minor foray into the Van Allen radiation belts would bring about a MAJOR increase in particle density, The need for acceleration energy is obvious, but remember were already in the center of a highly charged ambient solar medium. To paraphrase Marshal McLuen, the medium is the energy!
I see no need for gases, such as Mercury or Xenon or even C60 molecules, as electrically charged hydrogen has a large specific impulse at MHD ejection velocities.

Lets take a 'none dismissive' look at use of solar plasma, for vehicle power & propulsion. :rolleyes:

As to M2P2, 'solar sailing', although I realize that as quoted.
"There is enough power in the solar wind to move a 300-pound spacecraft at speeds of up to 4.3 million miles a day." This comment alone hints at the forces available for my proposal!
I think it's reasonable that this proposed MHD solar plasma collection/emission drive have at least a shot at research funding.

Greetings !


An MHD propulsion system uses on board propellant and ionises
and ejects it. Collecting space plasma, even in LEO, is very ineffective
because there's too little of it. It also, by no means, rids you of
the need for energy to accelerate these particles. And instead
of an efficient heavy noble gas like mercury or Xenon or even the new
heavy C60 molecules, you'll have to mess with helium and hydrogen.

A prop system that collects the momentum of space plasma -
solar particles does exist in theory and maybe in some limmited
experiments. The newest and best idea there is the M2P2 system
which disperses a gas in space and using its magnetic dipole
and a week magnetic field contains it in a huge bubble many miles
across. Solar particles exchange momentum with this gas
bubble and propel the spacecraft.

Live long and prosper.

[drag]

Adam why be a jerk?
Why talk of things you do not understand, apparently ? :wink:

[drag]

Greetings !

My proposal substitutes all the messy, 'stored' hydrogen/helium fuel and 'implied' reactor plumbing. As for the low level of plasma in LEO, this is of course dependent on the superconducting field intensity and the cross sectional collection area, not to mention the effect of increasing delta v, on collection efficiency. Even a minor foray into the Van Allen radiation belts would bring about a MAJOR increase in particle density, The need for acceleration energy is obvious, but remember were already in the center of a highly charged ambient solar medium. To paraphrase Marshal McLuen, the medium is the energy!
I see no need for gases, such as Mercury or Xenon or even C60 molecules, as electrically charged hydrogen has a large specific impulse at MHD ejection velocities.

Lets take a 'none dismissive' look at use of solar plasma, for vehicle power & propulsion. :rolleyes:

As to M2P2, 'solar sailing', although I realize that as quoted.
"There is enough power in the solar wind to move a 300-pound spacecraft at speeds of up to 4.3 million miles a day." This comment alone hints at the forces available for my proposal!
I think it's reasonable that this proposed MHD solar plasma collection/emission drive have at least a shot at research funding.
I assume you know, but your first link about the VASIMR engine is
not related to MPD propuslion. The VASIMR uses microwaves to
heat a propellant. Its variable nozzle geomtery can allow it
to change the thrust and exhaust velocity (specific impulse) of the
gasous propellant.

Of course the M2P2 engine does have its disadvantages, but I believe
that it could be developed and improved. One important disadvantage
is that it can not be operated near a planet with a magnetic field,
thus ussualy only in deep space. One important advantage on the
other hand is that it requires very little power to operate, so if you
could have a VERY big cloud, solar wind particles travelling at an
average of about 600 km/sec can make a difference, aspecialy for smaller
spacecraft.

Your idea can work, but it isvery inefficient. First, you need new
technology which will take power and mass to collect enough
fuel which will take a lot of time and slow down the spacecraft.
All, this extra technology with extra power and mass is instead
of just bringing up a fuel tank, and since electric propulsion systems
operate at high Isp they don't need relatively that much fuel.
Also you'll have different gases - helium/hydrogen and its always
easier to adapt and make eficient a thruster that operates with one
type of propellant.

Second, the reasons that we'd want to increase the mass of the
propellant atoms is due to gains in power efficiency - there are less
atoms to ionise for the same propellant mass and the ionisation
energy is lower, and they're also easier to handle. For a VASIMR engine,
however, light gases are better because the tempratures are lower.

If you're intreseted in space propulsion concepts, this's a good link:
http://www.islandone.org/APC/index.html

Live long and prosper.

[Tau_Muon_PlanetEater]

Thanks for the insult. I know I am ignorant that's why I am here to learn (jeez what's with you cocky wannabe scientists anyways?)


Apparently I don't understand the concept of the sky hook, that's fine. But you my friend don't understand the concept of proper English. You said, 'travelling at an
average of about 600 km/sec can make a difference, aspecialy for smaller
spacecraft.'

Funny, I thought aspeacialy wasn't a word, perhaps you were looking for especially. The 'a' key is too far from the 'e' key to chalk this mistake up to a simple typo, so I suggest you go back to school. I feel sorry for your editor if you have one.

Insults aside... (I feel we are now even)

What do you think about the idea of a propulsion system based on the photoelectric effect? Since a photon striking a metal surface knocks out an electron in the opposite direction minus the 'work function' of the metal, couldn't we harness this 'work funcition energy'? Or is it too neglibable to be considered?

Thanks.

[drag]

Greetings !

Tau Muon PlanetEater, I don't quite understand why you took that
as an insult. It's simply not a good idea, ussualy, to speak of something
as if you're an expert when you're not, aspecialy :wink: when it's the main subject of discussion. Doing so is, ussualy, an insult to yourself and others.

btw, English is my third language and I ain't :wink: trying to be
100% "proper" on a web forum.

What do you think about the idea of a propulsion system based on the photoelectric effect? Since a photon striking a metal surface knocks out an electron in the opposite direction minus the 'work function' of the metal, couldn't we harness this 'work funcition energy'? Or is it too neglibable to be considered?
A photon has momentum, when it is absorbed the absorbing material
"receives" that momentum. That's how a solar sail is supposed to work.
The thing with the photoelectric effect ussualy discussed is that
an anode (positive electrode) is located relatively near the surface,
that's exposed to light, and it attracts the electrons. Without it, however,
the electrons won't (maybe in some special poliraziation modes of
the material though, don't know...) have any preferable direction
of movement.

Peace and long life.

[notal33t]

Greetings !

I assume you know, but your first link about the VASIMR engine is
not related to MPD propuslion. The VASIMR uses microwaves to
heat a propellant. Its variable nozzle geomtery can allow it
to change the thrust and exhaust velocity (specific impulse) of the
gasous propellant.

Of course the M2P2 engine does have its disadvantages, but I believe
that it could be developed and improved. One important disadvantage
is that it can not be operated near a planet with a magnetic field,
thus ussualy only in deep space. One important advantage on the
other hand is that it requires very little power to operate, so if you
could have a VERY big cloud, solar wind particles travelling at an
average of about 600 km/sec can make a difference, aspecialy for smaller
spacecraft.

Your idea can work, but it isvery inefficient. First, you need new
technology which will take power and mass to collect enough
fuel which will take a lot of time and slow down the spacecraft.
All, this extra technology with extra power and mass is instead
of just bringing up a fuel tank, and since electric propulsion systems
operate at high Isp they don't need relatively that much fuel.
Also you'll have different gases - helium/hydrogen and its always
easier to adapt and make eficient a thruster that operates with one
type of propellant.

Second, the reasons that we'd want to increase the mass of the
propellant atoms is due to gains in power efficiency - there are less
atoms to ionise for the same propellant mass and the ionisation
energy is lower, and they're also easier to handle. For a VASIMR engine,
however, light gases are better because the tempratures are lower.

If you're intreseted in space propulsion concepts, this's a good link:
http://www.islandone.org/APC/index.html

Live long and prosper.
--
Read your critique regarding the VASIMIR. You assumed correctly that I knew the VASIMIR wasn't a MPD system. I inserted it to show the earliest stages of my direction of thought on the system I was proposing. After all I wasn't attempting to design the drive on this forum. :wink:

Using collected solar emitted particles for a propulsion system IMHO, is the future. Recent advances in the area of high intensity toroidal plasma magnetic field generation makes a coupled Magneto Hydro-Helium Dynamic(MHD) accumulator/generator, and a MPD Magneto Plasma Dynamic(MPD), accelerator system leap to the front of the line. I see a very high Isp drive on the drawing boards in the immediate future. As for the "low levels of particles", I again refer to toroidial field intensity & delta v as the answer to that aspect of the equation.
Let me make this as plain as possible...
The system I'm proposing is a plasma dynamic vampire drive. It sucks in solar plasma via the plasma torus field coil, and channels the high energy plasma through a MHD generator for power generation and the particles are then ejected into the MPD section where it is accelerated and emitted.
As I see it the vampiric use of a high energy hydro-helium plasma in no way alters the principles of conservation of energy in a power and propulsion system. Again the medium provides the energy, not some reactor.
As for a variable gas nozzle problem, a magnetic field is the nozzle, problem solved...

[drag]

btw, more important than efficiency loss are space-charge limmitations which abviously make massive particles favourable for ion thrusters, for example.

notal33t, could you elaborate a bit on that power generation part, please.
I don't understand what you plan to do once you contain the particles
with a magnetic field.

Live long and prosper.

[Deca-of-CD]

Okay... Callll me stupid, but... Could someone explain how in the hell the sky hook would be put up and ..stay up in geosync? (preferably someone knowledgable on the subject, not just someone guessing..)

[notal33t]

btw, more important than efficiency loss are space-charge limitations which obviously make massive particles favourable for ion thrusters, for example.

notal33t, could you elaborate a bit on that power generation part, please.
I don't understand what you plan to do once you contain the particles
with a magnetic field.

Live long and prosper.
:approve:
There are two primary performance parameters of concern:

Power density and the specific energy.

1. The power density is the ratio of the electrical energy output to the internal volume of the generator channel.
The MHD process is a volumetric process and the power density is therefore a direct measure of the compactness of the system. As such, it controls the size and weight of a MHD power generating system for a given power output. The greater the power density, the smaller and lighter the channel, magnet flow train needs to be.
2. The second parameter, the specific energy, is the ratio of the electrical energy output to consumable solar plasma energy used for its production, and electrical and gas dynamical parameters on the length of a linear MHD duct. The ionized solar plasma would be injected into an magnetic contraction nozzle preceding the MHD duct. If power densities in the order of magnitude 100 MW/m3 are desired, high magnetic fields and Mach numbers in the supersonic range are needed. There also needs to be investigation into the influence of the variation of the channel length to determine power generation.
Hope this answers your question.

[notal33t]

Okay... Call me stupid, but... Could someone explain how in the hell the sky hook would be put up and ..stay up in geosync? (preferably someone knowledgable on the subject, not just someone guessing..)
====
:surprise:
As the system now stands the "fullerine line" for the space elevator consists of a payload delivered to geo-syncronous orbit at the equator. It's 'paid out' to earth and since both sites are 'motionless' relative to each other the fullerine line is attached to the earth station. As I understand the process further lines are fed up the initial line until the network of fullerine tube lines is sufficient to begin operation with a payloaded elevator system. As of this moment the estimates on the weight of the initial fullerine line is within operational limits for LEO delivery systems. I suggest that you might study the physics involved with centripetal forces acting on geostationary orbital objects. The system is really quite elegant.

[selfAdjoint]

The top end stays up because it's in orbit, just like any other satellite. Because it's in geosynchronous orbit, its period of rotation is 24 hours. And because it's over the equator and orbiting at the same speed the earth turns, it stays over the same point on the equator; that's what "geosynchronous" means. So if you establish a cord between that point and the satellite, at least the ends will stay fixed relative to each other, while rotating with the earth. There are other forces acting, and all sorts of technical problems, even if you get a material strong enough to bear its own weight for 23,000 miles. But that's the basic idea.

[Deca-of-CD]

What kind of material would be used? That.. carbon.. nanocentigrade??? shhhtuff? And wouldn't it be in extreme danger of being hit with airplanes, other satellites, weather, ..uhh...whatever else is floating through our atmosphere and whatnot?

O_O What does centripetal :confused: force have to do with a 'line' from the Earth station and the sat, or whatever the hell the end is being anchored to..and if it is even being anchored at all..?

[LURCH]

What kind of material would be used? That.. carbon.. nanocentigrade??? shhhtuff? And wouldn't it be in extreme danger of being hit with airplanes, other satellites, weather, ..uhh...whatever else is floating through our atmosphere and whatnot?

O_O What does centripetal :confused: force have to do with a 'line' from the Earth station and the sat, or whatever the hell the end is being anchored to..and if it is even being anchored at all..?

As I see it, other than the material problem, the idea of being hit by satellites is the main obstacle. The cable strong enough to hold its own weight at those altitudes should be able to withstand any weather that might come its way, and air traffic could be routed around it. However, no matter where the cable is placed, any satellites that are in orbits lower than geosynchronous orbit must eventually passed through the space where the cable stands.

There have been some proposals to make a cable that can "sway" to avoid satellites when they pass through.

[MonstersFromTheId]

:eek:
I should apologize for inadvertently starting a separate thread on Space Elevators over in the Celestial Mechanics section. I didn't realize this thread existed until today. From now on I'll confine my posts to this thread. Sorry guys. :redface:

This just occurred to me.
Wouldn't it be dammed near IMPOSSIBLE to dock a ship with a Space Elevator at ANY point along its length that lies an appreciable distance from the c.g. of a Space Elevator?
In order for a Space Elevator to remain geosynchronous its c.g. would have to lie exactly at the g.s.o. point wouldn't it? Additionally that would be the ONLY point at which a ship's orbital speed could be matched to the orbital speed of the Space Elevator wouldn't it?
If you try to dock at any point lower than g.s.o. then your ship is going to be moving too fast to dock, and if you try to dock at any point above g.s.o. then your ship would be moving too slow wouldn't it?
An extreme example:
In my dismally placed previous thread I brought up the idea of locating essentially a "railroad station" or "space port" at a point just 300 miles above the Earth's surface so that passengers, unaccustomed to a low gee environment, wouldn't have to deal with things like becoming nauseous en mass (picture a chain reaction of 400 people aboard a maglev car suddenly feeling the need to fill their "space sickness bags" all at once! :bugeye: :yuck: ) or having difficulty handling carry-on baggage or small children or a tendency to run into walls and/or each other because they're not accustomed to moving around in a low or micro gee environment.
Restricting untrained crowds of "civilians" to lower levels of a Space Elevator would eliminate those problems, and preclude the need of trying to move large numbers of people from sections of the structure that spin, through sections that don't, and onto other sections that spin (possibly an engineering and crowd control nightmare), but it wouldn't work would it?
I mean there's just no way in HELL you could dock a ship with a Space Elevator just 300 miles above its base is there?

[enigma]

Not without a ~7.5km/sec delta V burn, no.

It would certainly be possible to save on launch costs, though.

Lift a package to a higher than required orbit. Release it. It will fall into the field and increase its velocity. When you're at the desired altitude, do a delta V burn to circularize the orbit. It wouldn't be free, but it would certainly be cheaper.

The real advantage would be in launching interplanetary probes. Simply have them climb up past GEO. There is a point (I haven't calculated it) where you could let go from the elevator and be on an escape trajectory.

[MonstersFromTheId]

Considering that a "7.5km/sec delta V burn" would, I imagine, have somewhat of a tendency to "rearrange" the plumbing of yer passengers, I suppose docking a ship that far down would be a less than practical idea. ;-) Especially if you've got the slightest interest in your passengers becoming regular customers as opposed to corpses.
Being an aerospace engineer you might actually be able to make a decent guess at this...
If you're talking about *passengers*, untrained, poorly conditioned, chubby, colesterol and booze ridden passengers, not to mention small children, how much of an accelleration can they reasonably be expected to take?
If you take THAT limit as the limit to how many gees a ship can be allowed to employ in a docking manuver, how far from the g.s.o. point could you place docking facilities on a Space Elevators for use by passengers?

[enigma]

That's 7.5km/sec, not 7.5km/sec^2.

Velocity not acceleration. The acceleration can be as slow as you need it to be.

Last semester, I participated in a senior year design competition for a space tourism project. We designed for a peak re-entry 'g' loading of 4g. That was with eyes away from the direction you were travelling - loading through the chair.

We assumed that we could get away with it being that high (the shuttle has 2.5-3g during re-entry, IIRC) because our market research showed that people wanted to 'feel' that they were in space.

If we're talking a space-train type application where we couldn't guarantee the optimum health of each passenger, I wouldn't go any higher than 2g through the seat.

All of this is moot if you're talking about docking procedures. If you need to do a big delta-V burn, you just start it earlier.

[notal33t]

What kind of material would be used? That.. carbon.. nanocentigrade??? shhhtuff? And wouldn't it be in extreme danger of being hit with airplanes, other satellites, weather, ..uhh...whatever else is floating through our atmosphere and whatnot?

O_O What does centripetal :confused: force have to do with a 'line' from the Earth station and the sat, or whatever the hell the end is being anchored to..and if it is even being anchored at all..?
==
Here's an excellent site with an animation of the system.

http://www.gizmonicsinc.com/

Suprising news on this front just hit the WWW, seems there's a contemplated Space Elevator 'X prize' in the works with a "proof of concept" date of 5-10 years.

[The Binary Monster]

So we might soon see commericial companies going for a Space Elevator? Fantastic! It'll be great if it works as well as the space flight "X Prize" has done.

[Deca-of-CD]

O.o Thanks for the site, Notal33t.. just skimmed it..looks informative.. Gonna read it later tonight..

[MonstersFromTheId]

I was thinking you'd have to dump 7.5km/sec in just one sec.
See I really didn't have a grip on this yet honestly.
In fact I still don't. Let me strain the hell out of your patience here for a minute by making clear where I *thought* the problem lies (which is evidently wrong as all hell).
K,..
The "a little info is a dangerous thing" version... (**please** excuse the baby steps review here, this is going to be a bit like a kid repeating what he picked up watching Barney sing about it, but,.. lets face it, that's about the level of understanding I'm workin with.)

If you draw a straight line from the c.g. of any satellite (be it a ship, Space Elevator, whatever), to the c.g. of a planet, that line is always going to sweep through a "pie slice" of equal area regardless of how close or far from the planet it is.
The only way a satellite can do that is by going around the planet faster when it's closer to the planet, and slower when it's father from the planet.

The Space Elevator's c.g. is waaaaay the hell out at around 22,000 miles, a very long leg to sweep out the area of the pie slice with, so it goes around the planet relatively slowly.
A ship just 300 miles up has a much shorter leg with which to sweep through that same area per second, (around 21,700 miles shorter), so in order to sweep through that same area per second the ship has to go around the planet a LOT faster (7.5km/sec faster?).

As a result, if you were sittin in the cockpit of the ship orbiting 300 miles up, it'd look to you like you're goin waaay too fast as you approach the dock.
Exactly the WRONG thing to do would be to "put on the brakes" by firing yer "retro-rockets" to slow down, because it wouldn't slow you down, at least not in relation to how fast you're coming up on that dock.
If you did that, what would happen is as you "put on the brakes" you'd start to drop from your orbit 300 miles up, to a lower orbit (and lower orbits are faster orbits), so you'd wind up not only missing the dock completely (by hitting the Space Elevator someplace below the dock), but you'd also wind up hitting the Space Elevator even harder than if you'd done nothing at all, and just allowed yourself to ram the dammed dock at the speed you were originally going.
Paradoxically enough, from what little I understand, to slow your approach to the dock you'd have to "hit the gas" (to get yourself into a higher/slower orbit), instead of "hitting the brakes" (which would put you into a lower/faster orbit).
The problem with THAT idea is that by the time you're in a high enough orbit to approach the Space Elevator at a speed that's not going to get you creamed, you're going to be all the way up at a height of 22,000 miles, 21,700 miles above the dock.
Hmmm.
Now that I'm writing this I'm beginning to suspect I see where the flaw in my own thinking lies. All this is based on the idea that you can only fire your "retro-rockets" in one of two directions. Directly forward to slow down, or directly backwards to speed up.
K,..
So suppose this is like approaching a dock with a hell of a cross current.
Your turn your ship so that the resultant vector can be split up into two component vectors, one comp vec points out, away from the Earth, that keeps your orbit from droppin, the other comp vec points against yer direction of travel to slow your approach to the dock?
Unless I'm just gettin even more lost in me undies here I think I'm beginning to see what yer sayin.
If you point your ship in the right direction how quickly you match the motion of the dock when you fire your rockets is a function of acceleration. You can take a week to complete the docking maneuver with a low acceleration, or a half a minute to complete the docking maneuver if you don't mind having to pluck yer eyeballs back out of their sockets with a suction cup when you arrive?
But there WOULD be a *minimum* acceleration you'd have to undergo to dock with a port 300 miles up on the Space Elevator wouldn't there?
In fact that minimum would be anything greater than the acceleration someone feels standing on the Space Elevator's dock 300 miles up, something a little over 0.8 g?
(From Jenab's helpful explanation in the old thread)
[GM[earth]/(R[earth] + altitude)^2] - [(R[earth] + altitude) * (angular vel @ gso)^2]
In order to dock you'd also have to keep yer foot on the gas right up until the moment the dock latches grabbed yer ship too wouldn't you?
Cut the engines any sooner and the ship would instantly start to fall away from the dock into a lower faster orbit?

BOTTOM LINE
You COULD put a "railroad station" or "space port" just 300 miles up from the base of a Space Tower.
That WOULD preclude all the problems of having to deal with crowds of untrained passengers having to deal with low gee environments.
It would also spare you from trying to figure out how to get large numbers of people (not to mention heat, power, air, baggage, data, sewage, etc) into and out of spinning sections.
The only real downside would be fuel costs?

Long ago, when I worked as a design devel engineer in the packaging industry, I learned that anything big and complex that has to spin, is inherently expensive, and dangerous.
There are always "pinch points" that have to be guarded with **literally** fool-proof ways of keeping errant body parts from being crushed. (The kinds of insanely idiotic things people will do to circumvent safety systems is something that never ceased to amaze me.) And no matter how you try to do it, getting anything like power, data, fluids, or anything else, through some type of rotary union is always expensive, and always leaves you with maintenance headaches above and beyond those you're going to have anyway without things that spin involved in the design.

Higher ticket prices to pay for higher docking fuel costs might not be all that bad a trade off to make.

-------------------
2 g max through the seat for passengers huh? That's actually a helluva handy number to know.
---------------------
How much of what I've said above makes sense, and how much have I got badly wrong?

[enigma]

I was thinking you'd have to dump 7.5km/sec in just one sec.


Nope. You just have to drop your velocity by ~7.5km/sec.

Orbital maneuvers are usually approximated as instantaneous because a) they are usually small duration w.r.t. the period of the orbit and (more importantly) b) if they are long, then you can't solve the problem analytically and have to resort to numerical methods. Believe me... They're a pain in the butt. A teammate and I tried to code a launch simulation/optimizer for the project I mentioned earlier and we gave up after about 4 days (and several trips to several professors... the problem we were having was coding the control system).


If you draw a straight line from the c.g. of any satellite (be it a ship, Space Elevator, whatever), to the c.g. of a planet, that line is always going to sweep through a "pie slice" of equal area regardless of how close or far from the planet it is.
The only way a satellite can do that is by going around the planet faster when it's closer to the planet, and slower when it's father from the planet.


Don't worry yourself about Kepler at this stage. It's correct, but it is really only used to get to other results. Let's cut right to the meat.

Vis-Viva Equation

V=\sqrt{\mu \times (\frac{2}{r}-\frac{1}{a})}

This is one of the most (if not the most) important equations in orbital dynamics. It relates velocity, orbital energy, and position.

r is the distance from the center of the planet (6378+alt) {km}
V is the velocity of the satellite. {km/sec}
a is the semimajor axis of the orbit. All bounded orbits are ellipses. The semimajor axis is 1/2 the distance between periapsis (the closest point) and apoapsis (the farthest point) {km}
\mu is the Gravitational parameter, and is equal to the gravitational constant times the mass of the body. It is unique to each body (the Earth has a different value than the Moon or the Sun). For the Earth it is 398,600.4 {km3/s2}

With this equation, you can find all sorts of things.

For one, you can find the orbital velocity of a satellite in a circular orbit, 300km up. (Set a = r for a circular orbit. Don't forget to add the radius of the Earth in)

Another: you can find the escape velocity to leave the Earth system from any altitude. (Set a = infinity for a parabolic escape orbit)

The top level thing to take away is that if you have a set semi-major axis (and unless you do a maneuver or take atmospheric or other disturbances, you will have one) as your radius increases, your velocity will decrease.

Now, a space elevator would have slightly different rules. It isn't 'orbiting' per se.

The elevator has its center of mass just higher than GEO. That makes the entire thing in tension, and allows you to climb it without pulling it back down to Earth. Some sort of thruster system would have to keep it stable due to it orbiting just slower than 23hr56min/orbit... just one of the thousands of engineering details needed... (someone said proof of concept 5-10 years for an X-prize-like contest?!? I sure as heck don't want to be on THAT design team... I value my sanity). Let's just assume everything works for this thought experiment and it rotates perfectly in phase with the Earth.

Any point on the elevator will have an orbital speed of:

\omega \times r

Where \omega is the rotational speed of the Earth, or ~7.29e-5 rad/sec.

r, again, has units of {km}

On every point on the elevator (except for GEO) you'll actually have to hold on, otherwise you'll fall away.

******************
To do a straight dock, you'd have to slow yourself down from your orbital speed to the elevator dock's speed. (Do these calculations... :biggrin: only way to learn it)

For the other docking method I described, you'd do what is called a Hohmann transfer. This (http://liftoff.msfc.nasa.gov/academy/rocket_sci/satellites/hohmann.html) page has one of the better descriptions I've seen only it doesn't cover the calculations.

You start on orbit A, do a burn increasing your velocity and semi-major axis putting you on orbit B. You can calculate the semimajor axis of orbit B. Perigee is orbit A's altitude. Apogee is orbit C's altitude. The semi-major axis is half of the sum of the two. (draw a picture of the three orbits if this doesn't make sense)

Now, you'll travel on orbit B until you get to the intersection with C (the dock's altitude). You then will have to do a much smaller burn to get to the dock's speed than you would have if both were at 300km.

For practice with this assume you're starting in a circular orbit at 300km, and you want to get to a hypothetical base at 20,000km altitude. Just use the Vis-Viva to find required velocities before and after each burn and find the difference needed. Remember that the 'r' is the same both before and after the burn. The only thing which changes is the semi-major axis.

More homework: Find out how much delta-V you'd need to get into a circular orbit at 20,000km and how much it would take to get to the elevator dock's speed. Both situations start in a circular 300km orbit.

IIRC, there is a demonstration of procedure in one of the very helpful "Hillbilly Tutorials" in the Celestial Mechanics forum. EDIT: Here it is! (http://www.physicsforums.com/showthread.php?t=29524)

There is a dock location where you could get away doing a one-sided Hohmann transfer. Basically, you'd be going slow enough at the B-C intersection where you would be at the elevator's speed and could just "step in".


As a result, if you were sittin in the cockpit of the ship orbiting 300 miles up, it'd look to you like you're goin waaay too fast as you approach the dock.
Exactly the WRONG thing to do would be to "put on the brakes" by firing yer "retro-rockets" to slow down, because it wouldn't slow you down, at least not in relation to how fast you're coming up on that dock.

{snip}


Real-life docking isn't quite as simple as these thought experiments present.

One common way is to line yourself up on the nadir (bottom) side of the craft at a very similar velocity. You then do a small forward burn which brings you up a hare. When you start to come back down again, you give it another burn. The process can take hours.

Smarter people than I would have to come up with the least fuel-intensive method to get into an object at a non-orbital speed.


-------------------
2 g max through the seat for passengers huh? That's actually a helluva handy number to know.
---------------------


There's nothing magical about that number. It's an off-the cuff guess. It's just slightly less than the max-loading you'd get from a rollercoaster (although in a rollercoaster it's the rapidly changing acceleration which makes you sick).

I also realized that when I said "through the seat" it wasn't clear... I've been in school so long I forget what isn't common knowledge. By through the seat, I mean from the back. If you are sitting in a rocket on the launchpad, your seat would be facing up. You'd be getting your 'g' loading from the back.

If you get a 'through the seat' 'g' loading sitting so you're looking out of the side of a rocket, all the blood would rush from your brain into your legs, making your legs hurt a lot right before you passed out.

[MonstersFromTheId]

This is a lot to absorb, and I've got a plane to catch, but I didn't want to go before sayin...
THANKS! No kiddin.
:approve:
Guess who's going to be doin a few "homework exercises" on the trip.
Beats the snot out of paging through catalogues full of programable tie racks, digitally monogrammed jelly beans, and solar powered golf caps.

[MonstersFromTheId]

I was just listening (once more) to Dr. Kaku's broadcast where he's talking to Dr. Edwards about I.S.R.'s version of the Space elevator. In both the broadcast and on I.S.R.'s site (http://www.isr.us/default.asp?s=1) mention is made of the initial Space Elevator having a lift capacity of 20,000 kg/payload, with thought toward an eventual 2nd generation Space Elevator with a lift capacity of around 100,000 kg/payload.

First question: Do I have those numbers right? (I'm hoping I misheard, or just have that figure screwed up).

The primary argument for a Space Elevator seems to be cost savings in that current methods of getting things, even just into low orbit, stands at around $10,000 per pound, where the S.E. might get that figure down to around a couple of hundred dollars per pound.
The thing is, to a neophyte like me, 20,000 to 100,000 kg/payload sounds like ridiculously small payload capacity for any kind of cargo transport system regardless of how cheap it is.
My initial impression is that a payload capacity THAT low would limit the usefulness of the system to use for essentially tiny science projects, but never reach the level of offering "bridge into space" that could be used for large scale commerce. I mean consider how many trips it would take to match the carrying capacity of something the size of a container vessel or single train of railroad cars.
Before getting to the point where you could really call a concept like this "a bridge", in the commercial sense of the word, you'd have to get to the point of having many thousands of payloads running up and down them per day.
What I'm getting at here is that we're not talking about a "bridge into space" as in something that could match the delivery capacity of something like the Golden Gate Bridge with hundreds of thousands of tons of cargo and a million or so people running over it per day.
What we're really talking about here is more like a large construction crane that can lift 20,000 kg to orbit, and later, at some point, a second generation crane that could lift as much as 100,000 kg to orbit, or in other words a very big block and tackle.

Have I got that right?

Second question: Has ANYTHING, anything at all, ever been envisioned in the way of a transport system for getting things into and out of orbit at the kinds of raw tonnage per day required to sustain commerce between Earth and low orbit at the kind of levels you see now between say Pennsylvania and New Jersey running over route 80, or container vessels running between Newark and Europe?
Or is THAT kind of transport capacity REALLY waaay over the rainbow kind of thinking for at least the next two to five hundred years, let alone the next fifty to a hundred years?

[enigma]

First question: Do I have those numbers right? (I'm hoping I misheard, or just have that figure screwed up).


It seems a little smaller than I would have thought, but a 20,000kg payload is HUGE to space. Most satellites weigh in much less than 5,000kg. The lunar lander weighed just under 15,000kg.


My initial impression is that a payload capacity THAT low would limit the usefulness of the system to use for essentially tiny science projects, but never reach the level of offering "bridge into space" that could be used for large scale commerce.


One thing you need to consider is what demand is there to go to space? You've got science missions. You've got communications and spy satellites. If the price were much lower, you'd have a very little bit of tourism. There is a huge demand for things to travel interstate. The demand isn't really there to go to space en masse.


What we're really talking about here is more like a large construction crane that can lift 20,000 kg to orbit, and later, at some point, a second generation crane that could lift as much as 100,000 kg to orbit, or in other words a very big block and tackle.

Have I got that right?


Yeah, more or less.


Second question: Has ANYTHING, anything at all, ever been envisioned in the way of a transport system for getting things into and out of orbit at the kinds of raw tonnage per day required to sustain commerce between Earth and low orbit at the kind of levels you see now between say Pennsylvania and New Jersey running over route 80, or container vessels running between Newark and Europe?

Not to my knowledge.


Or is THAT kind of transport capacity REALLY waaay over the rainbow kind of thinking for at least the next two to five hundred years, let alone the next fifty to a hundred years?

If we ever get to the point where large amounts of stuff needs to be put into space, I'm sure we'd quickly develop the capability to manufacture the stuff up there to begin with. No use lifting it out of the gravity well at $10 to $1000/kg when you can build/grow/mine/etc. it there.

[MonstersFromTheId]

I'm probably looking at this all bass ackwards but, it seems to me that the demand, the need, on a very wide spread commercial basis already exists. The problem isn’t "demand" so much as the prospect of fulfilling that demand is currently just to costly to ;-) "supply".

Take the example of the other thread I put up about space based h2o separation.
Suppose:
20,000,000 gallons of water a day is run up a S.E. in tank cars little different than the tank cars currently used on railroads.
Once there, the water is separated out into hydrogen and oxygen by way of solar powered electrolysis in very large orbital "refineries" (picture a slew of orbital facilities the size of Earth based petroleum refineries).
Once separation is complete, the resulting hydrogen and oxygen gasses are cooled into liquid form (for more compact transport), a fairly easy thing to do in any shadowed area in orbit.
The resulting 13.3 million gallons of liquid oxygen and 6.6 million gallons of liquid hydrogen a day is run back down the S.E. where it is then sent to power generation and distribution facilities.

If, and from what I've been able to gather so far, only if, you could move that kind of mass into and out of orbit, then space based power generation becomes one HELL of a lot more feasible, not just as some kind of energy supplement, but as an energy staple.
You no longer have to try to generate 3.602 trillion kWh of electricity *in real time*. That energy is now in a storable form (liquid h & o2) for use in generating electricity, powering ground transport systems, or whatever, as required, when required.
You no longer have to find incredibly HUGE open areas for rectennas or solar collectors on Earth that have to expand as demand expands. Which also eliminates (what in my mind at least is) another very real concern, the idea of putting vast energy conglomerates like BP Exxon and Royal Dutch Shell (or their eventual equivalents) in competition with vegetation for available sunlight (who do you honestly THINK would win that competition?).
Mind you, in this conversation we're not talking *just* "starting points" here, or supplements to existing power generation systems. We're talking in terms of meeting long term energy demands that have to be met for markets of hundreds of millions of people, and quite possibly much more than that, preferably sooner than wars start over dwindling supplies, as opposed to later, where you're forced not only to dedicate the resources required to win those wars, but to develop substitute energy resources at the same time.

One of the more fatal problems I see with all the talk about "alternative energy sources" is that by far the bulk of it is restricted to tinkering around the edges of one very BIG problem. "Starting points" that revolve around providing a few hundred kWh of energy are, imo, pretty worthless wastes of time if they aren't accompanied by at least some idea of how they can be expanded over time to provide the multi-trillions of kWh of power we all know is eventually going to be needed.

Again, imo, the "demand" for getting things into and out of orbit in vast commercial quantities cheaply *already* exists. Exxon wouldn't be interested in buying a new fleet of hideously expensive supertankers, or expanding oil drilling and refineries into wilderness areas or politically unstable areas, if they had a viable alternative. They'd LOVE to sell off all their overseas assets, and use that money to position themselves at the forefront of a new, more stabile, and more sustainable emerging energy production and consumption model.
The problem they've got is that, at least for now, THERE ARE NO ALTERNATIVES - PERIOD, at least not when you're talking about alternatives that can be expanded to deal with the STUPTIFYING levels of energy consumption the world not only demands, but requires.

Earth based solar power generation is a joke. There's just no way in hell you can set aside the collection area required to supply even current energy requirements. Even if you could, the result would be an environmental catastrophe even more devastating than the one we're currently stuck with. Solar collectors of any kind at all are really "heaters" that produce electricity one way or another as essentially a by product. You think we've got problems with "global warming" now? Ha! Run a few calculations. Take THE most efficient solar power system you can come up with. Figure out how much area is required by a system like that to produce just the 3 TRILLION + kWh of electricity that the U.S., all by itself, sucks up. Add to that what it would take to also run the number of cars and trucks currently on the highways, not to mention things like farm equipment. Figure out how much waste heat would be thrown off by a system that massive. Picture what a world like that would look like. That's not a solution it's a nightmare.

Wind power can't produce the levels of power we need, not as a *primary energy staple* it can't.
Geothermal won't do it.
Hydroelectric won't do it.
I hope I don't have to delineate the problems involved with nuclear fission as a primary energy staple.
Fusion power is another HIDDIOUS idea *as a primary energy staple*. Nuclear fusion, unlike ANY other process that takes place on Earth, doesn't just *use* water, IT DESTROYS WATER - PERMANENTLY. It turns hydrogen into helium, and once that's done, so far as I know, there IS no practical way to turn that helium back into hydrogen so that it can be recombined with oxygen to replace the water that was used. Turn to nuclear fusion *as a primary energy staple* and you'll wind up dehydrating the planet! Or... be forced, FORCED mind you, to acquire replacement water from off-planet sources, which brings you right back to having to have the capacity to get things like water into and out of orbit in the kinds of quantities that supply the kinds of power levels you need to run a planet full of people.

And all that is just looking at energy production.

As Twain said "Buy real estate. God ain't makin no more of it."

Rather than get as long winded with that one as I did with the previous idea I'll sum it up this way...

There is, I would argue, right now today, this very minute, as you read this, ALREADY an absolutely HUGE demand for as much more of three things as we could possibly get our hands on:
1) Energy
2) Raw materials
3) Arable land.
If we haven't already reached the point where the law of diminishing returns is going to strangle us to death as a species in trying to get more of those three things here on Earth, we're going to be there all too dammed soon.

The problem in my mind isn't "demand". The problem is the bottleneck between ourselves and the abundant supplies of those three things, that we could tap into, if that bottleneck were opened wide enough to where the cost of getting at those three things wasn't so prohibitively high that it put the prospect of tapping into them well beyond our current reach.

[enigma]

How much electrical energy can you get out of fusing 1kg of water from Hydrogen and Oxygen?

How much energy does an object which is in GEO have? (1/2mV^2 + mgh, V calculated from the above equations)?

You'll have to spend that energy twice. Once to bring it up, once to bring it down. How much will that cost per kWh? Compare with current energy prices for solar/nuclear/wind energy.

Back up your case with numbers, and you may have a reasonable proposal. My hunch is that nuclear or solar will win out, but I haven't run any numbers either. :smile:

[MonstersFromTheId]

... *would* you have to pay for that energy twice?
(once on the way up, once on the way down)
Or would it be that you only pay the piper on the way up?

See the problem here is I'm in the position of a guy with a paper one ya know? I can do the simple arithmetic, but I'm never all that sure I'm applying the formulas properly.
For example:
It seems to me (not knowing a whole heck of a lot better), that you *wouldn't* have to expend energy to get tanks of water back down a S.E., or more accurately, you wouldn't have to spend all that much energy. The impression I get (which could be TOTALLY wrong), is that if a tank full of liquid h or o2 were at G.S.O. on the S.E., all you'd have to do is give it a nudge toward Earth and gravity would do the rest. Once pushed say a couple of miles toward Earth the tank would no longer be weightless (because the tank would only be weightless when it's actually *at* G.S.O.). Any closer to Earth at all, and the tank would have a little weight pulling it toward Earth, that weight would not only keep it going, but start it accelerating in what is essentially a slow fall. As it kept getting closer to Earth, Earth's gravitational attraction would steadily increase, thereby increasing the speed of its fall.

Additionally I'm not at all sure that you'd have to pay for the initial push to get the tank back down the S.E.
If what I'm saying above is true, then as the tank drops and begins to pick up speed I'd imagine you're going to absolutely positively have to have, well, for lack of a better term, brakes. I mean by the time that puppy's around a mile above the Earth's surface it's going to be going like a bat out of hell isn't it? So suppose that instead of just burning off that energy as heat with something like a brake drum, you use the wheels or whatever that pinch the ribbon of the S.E. to drive a set of generators the way "hybrid" cars do? At least some of that energy could be stored in a battery or beamed back up the S.E. with a laser couldn't it? And again, I'm not really all that sure how you'd calculate the amount of energy you'd have to burn off to keep the tank from creating a video spectacle for the six 'o clock news to lead with, but it'd be a fairly substantial amount of energy wouldn't it?
That energy could power things like a refrigeration system to keep the liquid h & O2 cooled and in liquid form on the trip down, or to run the "tank car's" internal control systems, or as I said, beamed back up the S.E. with a laser to power the initial shove to get the next Earth bound tank headed on its way down, or, well, whatever the hell you can use that energy to do constructively instead of just burning it off as waste heat.

As for the trip up the S.E., although it's true that a HELL of an energy bill is going to have to be paid, there's nothing says that the payment for that bill has to be picked up by the water you're shipping up the line is there?
Couldn't the energy bill for dragging that water out of Earth's gravity well be paid by collectors on the upper end of S.E.?
At times that the collectors on the S.E. are exposed to sunlight, some of the power they generate could be used to separate water into h & o2, while the rest is used to power lasers beaming power to cars on the way up. At times the S.E. is in Earth's shadow the h & o2 that had been produced while the S.E.'s collectors were exposed to sunlight could be burned to drive the lasers powering the cars coming up, until the S.E. again emerged from Earth's shadow, and the cycle would start again.
Or is that getting more than a bit iffy in approaching one of those perpetual motion machine ideas?

As for running the numbers, essh, I'll TRY, (I really will), but there are a few problems with that idea.

1) I have absolutely NO idea how to figure out how much energy you can get out of a gallon of water by separating it into h & o2, and then burning the resulting gasses. I have no idea where to even start when it comes to something like that.

2) I have no idea how to figure out how much energy it takes to separate water into h & o2 by way of electrolysis either. I.e. what kind of collection area do you need to separate "20,000,000 gallons of water a day" into h & o2? How much more do you need to drag water tanks up into orbit? How much h & o2 do you have to sep out and store for use when the S.E. is in Earth's shadow?

To at least get SOME numbers to play with in those two areas I suppose what I'll do is Google the hell out of phrases with the term electrolysis in it to try and find some numbers that have already been run by somebody who knows what the hell they're doing.

3) None of those calculations are likely to be worth all that much without answers to a whole SLEW of other questions I have no idea how to answer, such as:

Do you even WANT to move that water all the way out to G.S.O.? Or would it be smarter to just haul it say 300 to 500 miles up the S.E., load it aboard shuttle tankers, distribute it to dozens of separation facilities in the lower faster orbits you get 300 to 500 miles up, separate it, and shuttle the liquid h & o2 back to the S.E. for the trip back down (where, in those lower positions on the S.E., the tanks now require NO initial "push" to get 'em headed back to Earth, ya just let 'em drop)?
"Shuttling" the water from the S.E. to separation facilities, and then shuttling the liquid h & o2 back to the S.E. for the trip back down ain't gonna happen for free. So where's the trade off? Where's the best spot to do all this?

Oh well. Gotta start someplace right? I'm off to Google electrolysis for a while I guess. At least it'll give me SOME place to start.

[selfAdjoint]

You wouldn't have much of a tank of water or anything else if you just let it fall from orbit. Remember Columbia? Things in orbit are going around 5 miles a second, or 30-something thousand MPH, they have to shed all that speed by friction with the air, and it gets plenty hot. And after all that they get to fall from a hundred or more miles up.

See, to bring it SAFELY back, you need just as much fuss and feathers as it took to get it up there, and that is going to cost you just about as much.

[enigma]

1) I have absolutely NO idea how to figure out how much energy you can get out of a gallon of water by separating it into h & o2, and then burning the resulting gasses. I have no idea where to even start when it comes to something like that.


You probably don't have to go into too much calculations for this. I'm sure manufacturers of fuel cells have specific energy densities listed (W*hr/kg) in their product specs. That should be good enough for back of the envelope numbers.

You may be able to search some chemistry websites (or put a post in the Chemistry section) asking how to get rough approximations for power draw to electrolyse water.

Returning something to Earth by 'dropping' it out of orbit will cause it to burn up without expensive re-entry devices. My guess is slowly descending it on the already-presupposed elevator would be less expensive than an ablative (like Mercury, Gemini, or Apollo) or composite (like the Shuttle) re-entry vehicle capable of de-orbiting several tens of thousands of payload.

[MonstersFromTheId]

You wouldn't have much of a tank of water or anything else if you just let it fall from orbit. Remember Columbia? Things in orbit are going around 5 miles a second, or 30-something thousand MPH, they have to shed all that speed by friction with the air, and it gets plenty hot. And after all that they get to fall from a hundred or more miles up.

See, to bring it SAFELY back, you need just as much fuss and feathers as it took to get it up there, and that is going to cost you just about as much.

I do appreciate that S.A. Things in orbit make bullets look like snails :surprised , but we're talking here about something on a Space Elevator (in this case a tank full of liquid h & o2) that's making its way down a ribbon or rail. I'd offer that under those circumstances, as it drops and begins to pick up speed it'll likely get going pretty dammed fast left to its own devices, but not re-entry speed kind of fast. Something with wheels that pinch a ribbon is (I'd HAVE to believe) going to have a comparatively miniscule "terminal velocity" as it comes down, at least with comparison to something aero-braking its way in the way the shuttle does. I'd have to expect that even if the pinch wheels are allowed to "free wheel" completely out of control (which I don't think any competent design engineer would ever allow), there's *still* going to be a VERY real limit as to how fast those wheels can reasonably be expected to turn.
In other words, I don't think the tank would burn up on reentry unless the pinch wheels burned up, the tank jumped the tracks of the S.E., and the thing just dropped into the ocean.
On the other hand...
Without a breaking system for the pinch wheels I COULD very easily see were a tank car weighing thousands of pounds would probably arrive at the base of the S.E. running at speeds that'd curl the hair of the craziest NASCAR driver, and wind up providing the six 'o clock news with some pretty dramatic video of the ensuing results. :bugeye:
My point however is this: I DON'T think you'd be dealing with anywhere NEAR the kinds of "fuss and feathers" (I like that term), that you'd have to deal with in trying to get a tank car of several thousand pounds to de-orbit safely by traditional aero-braking. In this case the tank is on tracks. Its speed could be controlled with considerably more precision all the way down. Hell, if you wanted to, you could govern the speed of decent to the point were you never allow the thing to get going faster than a comfortable walking pace.

As I learn more about this it seems to me that where a Space Elevator is really saving you money in the first place is by eliminating the problems of having to deal with the level of :smile: "fuss and feathers" you inevitably get into when forced to deal with getting things to work across the exceptionally steep energy gradients of methods like building one big assed BOMB with a slow leak built into one end, that's powerful enough to blow your payload clear into orbit (a device often referred to as a "rocket"), and then getting it home by shooting the payload through a solid wall of air inside a shell casing that keeps it from getting cooked or torn to shreds in the process.
By comparison, dragging payloads, however slowly, up and down a very long rope, is a heck of a lot cheaper and easier to do - IF - you can make a strong enough rope.
Or so it would seem to a neophyte like me.

[MonstersFromTheId]

Ya read my mind Enig.
That's exactly what I'm doin now, tx!

[Myclicheisbetter]

Wouldn't the docking of a shuttle of any sort throw off the pull on the station? Granted the pull wouldn't be severe enough to physically notice it at the station, wouldn't the slight movement be compounded through the distance of the chord and create a shockwave? Also, would the movement of the elevator up and down the chord also disrupt the chord, i.e. The chord itself might be able to disapate the force of the wind, but would the elevator cause enough of a draft as it moves up through the atmosphere to completely disrupt the stations overall postition? This might make quiet an issue with any sort of stability control programs used in the station itself. Also the weight of the elevator would also change as it progressed up the chord as it collected moisture from the lower levels of the atmosphere?

[enigma]

Wouldn't the docking of a shuttle of any sort throw off the pull on the station? Granted the pull wouldn't be severe enough to physically notice it at the station, wouldn't the slight movement be compounded through the distance of the chord and create a shockwave? Also, would the movement of the elevator up and down the chord also disrupt the chord, i.e. The chord itself might be able to disapate the force of the wind, but would the elevator cause enough of a draft as it moves up through the atmosphere to completely disrupt the stations overall postition? This might make quiet an issue with any sort of stability control programs used in the station itself. Also the weight of the elevator would also change as it progressed up the chord as it collected moisture from the lower levels of the atmosphere?

Yes to every point you made.

There are serious engineering hurdles to be overcome before a space elevator could be built. It's much, much more than just: "Get the nanotubes long enough!"

[Garth]

I suppose you have all read Arthur C Clarke's "The Fountain's of Paradise" (first published 1979)? When the thing goes commercial I vote he gets a cut!
Most of the theory and technical details are covered pretty well in that SF novel.
(Apart from actually describing how to build a rope strong and long enough of course.)

One problem though, if the thing is built won't satellites lower than GEO bump into the elevator?

Just a thought,
Garth

[MonstersFromTheId]

Nice to see an article like this in the mainstream press. :smile:

http://www.msnbc.msn.com/id/6227455/

[pervect]

I

One problem though, if the thing is built won't satellites lower than GEO bump into the elevator?

Just a thought,
Garth

Yes - current plans are to have the bottom terminal of the space elevator be a mobile sea platform, so that it can dodge satellites and space debris.

Re: Myclicheisbetter's comments:

As far as the dynamics issues go, at a gross level the space elevator is very much like a very long pendulum. The stable, lowest-energy configuration will be when the cable points straight up. Transients will indeed occur as objects move up and down the cable, displacing the pendulum from it's low-energy position. Some damping of the oscillation will happen naturally. More damping of the oscillation can be achieved by the proper timing of the loads - if the pendulum is oscillating due to previous activity, you bring the load up at a time when you will decrease, not increase, the oscillation.

The dynamic loads will increase the structual loading somewhat.

I haven't read everything available in detail, but my impression is that a lot of the dynamic analysis as far as the magnitude of the transients has already been done and has been found to be acceptable.

For instance, there's the amazon.com book


http://www.amazon.com/exec/obidos/ASIN/0877033706/spacerefcom/103-4772094-8997446

Online, there are the phase 1 and phase 2 studies

http://www.spaceelevator.com/docs/521Edwards.pdf http://www.spaceelevator.com/docs/472Edwards.pdf

One big concern is that the natural frequency of the cable must not be close to a intergal multiple or sub-multiple of 24 hours.

[Myclicheisbetter]

[QUOTE=pervect]Yes - current plans are to have the bottom terminal of the space elevator be a mobile sea platform, so that it can dodge satellites and space debris.

Re: Myclicheisbetter's comments:

As far as the dynamics issues go, at a gross level the space elevator is very much like a very long pendulum. The stable, lowest-energy configuration will be when the cable points straight up. Transients will indeed occur as objects move up and down the cable, displacing the pendulum from it's low-energy position. Some damping of the oscillation will happen naturally. More damping of the oscillation can be achieved by the proper timing of the loads - if the pendulum is oscillating due to previous activity, you bring the load up at a time when you will decrease, not increase, the oscillation.

The dynamic loads will increase the structual loading somewhat.

I haven't read everything available in detail, but my impression is that a lot of the dynamic analysis as far as the magnitude of the transients has already been done and has been found to be acceptable.


Ah thank you very much. That's something I've been curious about for a while, I'd still like to see (if this is completed) what safety precautions are used to dampen vibrations and tremors with in the cable. Hopefully I'll live long enough to atleast see this started. Granted, we will most likely find an even more cost effective method to enter low orbit that constructing a fishing pole from the stars. :)

[docbill]

I'm afraid the people who are proposing this space elevator as a viable idea, just don't understand the difference between a stable system v.s. an unstable system.

What force would keep the cable pointed vertically outwards from Earth? What will keep it from either falling to the Earth's surface or flying off into space?

Granted if you place it initially at just the right velocity, there will be an unstable equalibrium. But keep in mind the solar system is a many body system. The moon, other planets, and even the sun would constantly try to move the space elevator from it's orbit, and spin it about it's center. Even the Earths magnetic field would probably produce too much of a force for such a large object.

Arthur Clarks, idea was viable only because he proposed anchoring the sky hook on the Earth. So provided you could produce a substance strong enough, you just make the cable long enough for the spin of the Earth to hold the cable in place. Presumably you could then make the cable as long as you wanted (to the limit of the materials available), so you could release objects as speeds considerably faster than just escape velocity.

While there is no known cable that would have the strength needed, a few years ago a carbon tube structure was discovered with the strength great enough, provided we learn how to make the carbon tubes into long cables. It might be possible with nano technology sometime this century...

Bill

[docbill]

You don't want to think of a floating sky hook as having centripital force from it's rotation about Earth. You'll never properly understand the orbital dynamics that way. Instead think of it as an object orbiting at a speed determined by it's center of mass about the Earth. This object is tumbling, or turning in space at a rate that keeps the same end always pointed towards the surface of the Earth, just like the moon does. Is this possible? Certainly if the only two bodies where the Earth and the floating sky hook.

Now consider that every body in the solar system can have two effects on this sky hook. 1. Each item can pull the sky hook into a non-circular orbit. 2. Each item can change the rotation of the object so the same side does not remain pointing at the Earth.

Are these forces significant? Just look at the tidal effects on the Ocean's of the Earth. For such a large object the energy cost for correcting for these effects would be too great.

Bill

[docbill]

The next idea to dismiss is the idea that somehow you gain from having a floating sky hook. Now that you have the correct image of a sky hook in your mind, an object in a free fall orbit rotating about an axis, it should be clear to you how Newton's third law applies. For every action, there is an opposite reaction. Consiquently, 100% of the energy a spaceship gains from using the space elevator will be lost from the space elevator. This not as suggested cause the space elevator to sink closer and closer to Earth. Rather it will make the orbit more and more elliptical.

At first a more elliptical might seem like a good thing, since a more eliptical orbit means that at some point the elevator will extend even lower and it will then move even farther away from Earth. But eventually this effect will cause the edge of the sky hook to hit the atmosphere. Consiquently, 100% of the energy taken from the sky hook will have to be returned to restore it to the proper orbit. There is no free ride here...

You might say, well yes we have to return the energy, but we can take our time with the most efficient means possible. Again no. Remember our sky hook was rotating at just the right rate to have the same side always pointed towards the Earth. As soon as its orbit changes, that is nolonger true. So the longer it takes to correct the orbit, the more of a change in orientation that will also need correcting.

You might then argue, that you still gain, because you could afford to use a more efficient engine that you could on each ship using the sky hook. That argument might be valid, but remember there is a constant energy cost to correct the sky hook change in positions due to tidal forces.

Now keep in mind my arguments only apply to orbiting sky hooks. A sky hook anchored to the Earth follows a completely different set of rules, because it can spin at a rate faster than orbital velocity for its center of mass.

Bill

[docbill]

... *would* you have to pay for that energy twice?
(once on the way up, once on the way down)
Or would it be that you only pay the piper on the way up?
...

As for the trip up the S.E., although it's true that a HELL of an energy bill is going to have to be paid, there's nothing says that the payment for that bill has to be picked up by the water you're shipping up the line is there?


You miss the point, if the sky hook is anchored to the Earth, there is no energy bill going up and down. When going in both directions, you could actually generate energy.

Picture it this way. Lets say you take a bucket of water and you spin it around in the air? Does the water fall out? No, because of the apparent force caused by the spin causes the direction of the spin to be "down". Now picture your bucket at the end of the sky hook. At the far end, "up" will actually feel like "down" to objects connected to the cable. So your bucket of water would be swing upside down at the end of cable and nothing spilling out, the same as if you where swinging it around in circles with your arm.
Now the longer you make your cable, the stronger the force is as the other end. Initially, you have to work very hard sending your object up. But once you get half way, it starts falling away from the Earth on its own at an accelerating rate, as if it was falling.

Now if you wanted to run your elevator for zero energy, you would make the line just the right length so you could have multiple elevators running at once with a looped cable, so the elevators going "down" pull the other ones up.

But wait you ask, if you are launching things into space where do you get the counter weight? One answer would be a hose. Just like emptying gas from your gas tank, you could empty water from the ground into a container at the other end of the cable. However, there are other solutions to this problem as well. The main point is each time you send something up the cable you have a net energy gain, provided your cable is long enough. This can pay for your transportation costs. Each time you send an item down the cable, you can also have a net energy gain provided you can match velocity with an appropriate part of the cable.


Now you might ask how you can actually gain energy going up? The answer is you make the cable longer than what you need. At the break even point, you connect your ship directly to a non-looped cable instead of the loop one and then let your ship fall to outer-space. After you reach your necessary velocity, you turn on the fly wheel breaks and send the extra energy back to Earth.

Now you ask what is the cost of all of this? Well ultimately you are going to end up with more junk in space, so you might as well fill the elevators with garbage to serve a useful purpose.

The energy is actually coming from the spin of the Earth. So the amount of free energy is available is finite but huge.

Bill

[MonstersFromTheId]

A new face! Welcome docbill! (Just an aside- I grew up about two miles from Bell Labs Holmdel. Knew quite a few neighbors that had to put up with all the confusion caused by the Bell Labs/Lucent split. EEESH!)
---------------------------------------------------------------------

You might want to read the Phase I NIAC (NASA Institute for Advanced Concepts) paper on this subject (which covers most of the problems you've cited, you'd probably love reading this). It's available here:

http://www.isr.us/Downloads/niac_pdf/contents.html

"Subsection 10.8: Induced Oscillations" covers most of the things yer noodlin through, and it's cool as hell. Trust me, you'll love this.

[notal33t]

Question:
Listening to Dr. Kaku's show on Coast to Coast. . .He mentioned a Space Elevator or Sky Hook.
I am a graphic designer. . .not a physicist, but would there not be a lot of centrifugal force at the outer end of such a cable when it is hundreds of miles long? I realize the concept is to drop the 'cable' from the space station in orbit, but once it is anchored to the earth's surface would it not be like swinging a ball on a string over your head? I appologize for my lack of understanding, but I feel if anyone can help me. . .It would one of you. Thanx

Tom Hanson
----
Can do Tom...

Check out these sites!

http://www.cnanotech.com/

http://www.liftport.com/index.php

http://www.star-tech-inc.com/spaceelevator.html

http://www.tetherapplications.com/

http://www.tethers.com/

http://groups.yahoo.com/group/nsecc/

I realize the concept of a Space Elevator is difficult for any none space cadet to wrap his head around quickly, but trust an old rocket man. This system WORKS, or it will when fullerine nanofibers are made in sufficient quantities! AND YES, gigapascal level FNF'S have been made. It's just a matter of time (and a short time) before they're ready to make the S.E. an operationally feasible system.

[RoboSapien]

I got the idea that Centrifugal force due to rotation of earth will be used but once the force is being exerted how will it be counter balanced. I mean will it not throw the space station out of orbit.

I think i misunderstood something here.

[addman]

using a simple model of the space elevator to be a single cable, of length 72000 km, and mass 20 000 kg,
the centre of mass would be at geo-stat orbit.
The forces acting on the cable as a whole are
(GMm)/(r^2)

and (mv^2)/r

i work out (GMm)/r^2 to be 4489.8 N
and (mv^2)/r to be 4400

the fact that these two are fairly close and acting in opposite directions suggests that they are in equilibrium.

However, adding a climber onto the bottom half of the elevator would unbalance this equilibrium and cause gravity to pull the elevator back to the earth.

To prevent this, the centre of mass needs to be further out. When i calculate, at centre of mass at 72000km, i get (GMm)/r^2 = 1288.5N and (mv^2)/ r = 1303N. The forces acting on the cable are less, and they still seem to be in equilibrium. I was hoping that the (mv^2)/r would be much greater than gravity, can anyone tell me what im doing wrong?