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  #51  
Old 09-15-2017, 08:10 AM
HopDavid HopDavid is offline
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I am pretty sure that you could use a space elevator to leave the Solar System if you aimed the payload just right- using a flyby manoeuvre at Jupiter, for instance, like Voyager.

It is a remarkable thing that once you get past geostationary orbit all that acceleration would come from the rotation of the Earth. But launching payloads towards the outer solar system would have an effect - there ain't no such thing as a free lunch. It would cause a drag on the Earth, and slow it down, That's why you need to attach the elevator firmly at the bottom - otherwise the bottom end of elevator would career off through the atmosphere, and extract angular momentum by friction.
Yes, it is the upper part of elevators that I find most interesting. They can act as a sling hurling payloads in any direction in the orbital plane of the elevator.

As you say, an assist from a gas giant could send an elevator flung payload out of the solar system. Or just make the elevator a little longer and the elevator could provide all the velocity. The acceleration gradient grows much shallower as you move outward up the slopes of earth's gravity well. So it would be possible to extend the elevator without adding a horrendous quantity of upward newtons.

Not that I regard an conventional Clarke style beanstalk as plausible, mind you. At least not on earth.
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  #52  
Old 09-15-2017, 08:23 AM
Darren Garrison Darren Garrison is offline
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If I'm doing my math right, if you extend the trailing portion of the space elevator out to a little over 2.5 billion miles, you can exceed the speed of light. So let's get started on building that sucker to launch starships!
  #53  
Old 09-15-2017, 08:43 AM
Marvin the Martian Marvin the Martian is offline
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Originally Posted by eburacum45 View Post
I am pretty sure that you could use a space elevator to leave the Solar System if you aimed the payload just right- using a flyby manoeuvre at Jupiter, for instance, like Voyager.

It is a remarkable thing that once you get past geostationary orbit all that acceleration would come from the rotation of the Earth. But launching payloads towards the outer solar system would have an effect - there ain't no such thing as a free lunch. It would cause a drag on the Earth, and slow it down, That's why you need to attach the elevator firmly at the bottom - otherwise the bottom end of elevator would career off through the atmosphere, and extract angular momentum by friction.


That raises an interesting point. Any materials discussion regarding a space elevator that I have seen concentrates on the tensile strength of the structure, but if you launch a payload from the top of the elevator that would produce a large torque about the center of mass and a shearing force at the anchorage point. How strong are carbon nanotubes to shear force?


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  #54  
Old 09-15-2017, 10:46 AM
Chronos Chronos is offline
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You'll only get a shearing force until the cable bends and turns it back into a tensile force. And for a cable that long, the amount of bending needed to do that is minuscule.
  #55  
Old 09-15-2017, 05:25 PM
RedSwinglineOne RedSwinglineOne is offline
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So my intuition tells me that that as weight is pulled up the cable, the weight "tries" to fall behind the cable because as the weight goes up, it is being pulled up to a height for which it does not yet have sufficient speed. This would cause the weight to act as drag on the cable. The cable would need to not only raise the weight, but pull it forward to increase the weight's speed. Does the cable have a way to compensate for the drag or do we assume the mass of the weight being lifted is insignificant when compared to the weight of the elevator?
  #56  
Old 09-15-2017, 06:48 PM
carnivorousplant carnivorousplant is offline
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Does the cable have a way to compensate for the drag or do we assume the mass of the weight being lifted is insignificant when compared to the weight of the elevator?
I am an engineer, and of instrumentation, not a physicist, but I should think that a prerequisite would be that the mass of the cable car and it's load would be insignificant compared to the mass at geosynchronous orbit.
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  #57  
Old 09-15-2017, 07:49 PM
scr4 scr4 is online now
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If the elevator has two tracks, with cars going up one track and down the other at the same time, the lateral forces should cancel out. Assuming the separation between cars is reasonably short, and that the amount of cargo coming down is the same as going up.

Last edited by scr4; 09-15-2017 at 07:50 PM.
  #58  
Old 09-15-2017, 08:03 PM
eburacum45 eburacum45 is offline
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I've seem some detailed analyses that suggest that the forces on ascending and descending cars are going to be complex and unusual, and it won't be a simple matter of just cancelling the forces out. To reduce the forces to manageable levels you could raise and lower the cars slowly, but that means a transit time measured in days or weeks. You weren't in a hurry, were you?
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  #59  
Old 09-15-2017, 08:13 PM
HopDavid HopDavid is offline
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So my intuition tells me that that as weight is pulled up the cable, the weight "tries" to fall behind the cable because as the weight goes up, it is being pulled up to a height for which it does not yet have sufficient speed. This would cause the weight to act as drag on the cable. The cable would need to not only raise the weight, but pull it forward to increase the weight's speed. Does the cable have a way to compensate for the drag or do we assume the mass of the weight being lifted is insignificant when compared to the weight of the elevator?
What you referring to is called Coriolis force. An ascending payload would exert a Coriolis force pushing west which would induce an oscillation. When the oscillation reaches earth, it would be absorbed and give earth a tiny shove that would have negligible effect on earth.

An payload descending the elevator would exert an eastward Coriolis force which would also induce oscillations.

However oscillations in the elevator aren't desirable. But oscillations can be damped by timing when elevator cars would ascend or descend.

______

Earth anchored Clarke Towers aren't remotely plausible. Not only are we unable to make Bucky tubes of sufficient length, there is also an excessive amount of debris and satellites in low earth orbit. Impacts would cut the elevator.

Also with an elevator tens of thousands kilometers in length, throughput would be excruciating. Isaac Kuo has correctly called it drinking a very thick milkshake with a very long, thin straw. It is doubtful the throughput would even be adequate to maintain the elevator.

But smaller elevators are plausible. Orbital vertical tethers would be stabilized by a gravity gradient. If each tether handled just 1 km/s delta V, stress would be manageable. It could be made from conventional materials like Zylon. Orbital tethers could also be placed in regions that are free of debris and satellites thus avoiding likelihood of impact.

I talk about the possibility of such elevators at Trans Cislunar Railroad.

______

A Mars elevator could be built from Zylon if you use a safety factor of 1. However no sane entity would risk human lives or valuable payloads on such a razor thin safety margin. The slightest scrape or nick would cause the tether to break. Given a more sensible safety factor of 3, Zylon Mars elevators are impractical.

However Zylon Mars' orbital tethers anchored at Deimos and Phobos are doable.

Last edited by HopDavid; 09-15-2017 at 08:14 PM.
  #60  
Old 09-15-2017, 09:35 PM
LSLGuy LSLGuy is offline
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Originally Posted by Darren Garrison View Post
If I'm doing my math right, if you extend the trailing portion of the space elevator out to a little over 2.5 billion miles, you can exceed the speed of light. So let's get started on building that sucker to launch starships!
We could probably make the project easier (or at least cheaper) by instead expending the effort to spin the Earth up to having just a 12 hour rotational period instead of the traditional 24.

Then we could save 1.25 billion miles of tether material.

I'm pretty sure that's a money saver. Even considering the cost of all the rest of our infrastructure we'd need to update. All new clocks, new school books, 4 hour workdays, etc. Still cheaper than 1.25 billion miles of tether & the power systems to traverse it.


Last edited by LSLGuy; 09-15-2017 at 09:35 PM.
  #61  
Old 09-16-2017, 08:57 PM
si_blakely si_blakely is offline
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It's been a long time since I read The Fountains of Paradise, and I don't have a copy, but I'm pretty sure that Clarke has an astronaut die during construction of one of the intermediate waystations below geosynchronous orbit - betrayed by years of experience in orbital construction work, he stepped off the platform untethered and fell. It could have been at the counterweight and he was flung.
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  #62  
Old 09-16-2017, 09:14 PM
carnivorousplant carnivorousplant is offline
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It's been a long time since I read The Fountains of Paradise, and I don't have a copy, but I'm pretty sure that Clarke has an astronaut die during construction of one of the intermediate waystations below geosynchronous orbit - betrayed by years of experience in orbital construction work, he stepped off the platform untethered and fell. It could have been at the counterweight and he was flung.
The only death I recall is the protagonist while rescuing people stranded on the cable.
  #63  
Old 09-16-2017, 09:53 PM
SamuelA SamuelA is offline
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So with landable, reusable rockets, the only savings you get from an elevator are :

1. You don't burn rocket fuel. Which can be liquid methane, which is about $1.52 a gallon.

2. You don't burn rocket components. Eventually, the reusable rockets will wear out and need to be overhauled or recycled.

I've mentioned this in many other threads, but classic science fiction is out-dated. The new thing is that since machine learning allows for making robots that are more flexible and can learn to perform well defined tasks on their own (and learn how to clear faults on their own), manufacturing is about to get a whole lot cheaper. I'm predicting a 'manufacturing singularity', actually. Once robotic factories can make the components used in robotic factories, the cost of robotics would plummet to just barely over the cost of raw materials and energy. And you can then have those cheap robots build mining robots, lowering the raw material costs. And you can have the cheap robots manufacture solar panels and batteries for cheap, easy to deploy production of energy.

Anyways, this is immediate term. Better robots are being deployed now and really, really amazing robots are about 4 years away (autonomous cars).

So if we wanted to leave the planet in mass, the way it would be done is that the rocket fuel would be made with solar power to convert air to methane and liquid oxygen. Maybe buy the Sahara desert and cover the whole thing with photovoltaics, and the electricity runs the gas compressors and Sabatier plants when the sun is out, with everything shutting down at night.

Then there would be thousands upon thousands of reusable rockets, all based on common components, all made without a human hand and inspected by machines to a far greater level of precision than current manufacturing tolerances. Most of these rockets would be flying cargo missions every few hours until the end of their service lives (which might be just a few weeks or months before the rockets get recycled and rebuilt in automated factories back to brand new). Over thousands of launches a month, you can get the statistics needed to really verify a launcher's reliability for crewed payloads.

This is something we can basically start on today. This technology is real. Orbital tethers aren't possible or feasible.
  #64  
Old 09-16-2017, 10:09 PM
scr4 scr4 is online now
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Originally Posted by si_blakely View Post
...I'm pretty sure that Clarke has an astronaut die during construction of one of the intermediate waystations below geosynchronous orbit - betrayed by years of experience in orbital construction work, he stepped off the platform untethered and fell. It could have been at the counterweight and he was flung.
Both, actually. One fell "more than 15,000 meters" and burnt up upon reentry. Another was flicked off from the counterweight and could not be rescued before her air supply ran out. Here's a Google Books link to that section of the book.
  #65  
Old 09-16-2017, 10:19 PM
Chronos Chronos is offline
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Liquid methane might be a buck fifty a gallon, but it takes an awful lot of gallons of liquid methane (and of liquid oxygen; don't forget that part) to lift useful payloads to orbit. And the time might come when we can make methane really cheaply, but if we ever get to that point, we'll be able to make everything else really cheaply, too, and space travel will still be one of the most expensive things humans do.

And, of course, if you can use your miracle machines to make methane, can you also use them to make nanofiber? Because that really could make space travel cheap, even in comparison to other cheap things.
  #66  
Old 09-16-2017, 10:32 PM
SamuelA SamuelA is offline
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Liquid methane might be a buck fifty a gallon, but it takes an awful lot of gallons of liquid methane (and of liquid oxygen; don't forget that part) to lift useful payloads to orbit. And the time might come when we can make methane really cheaply, but if we ever get to that point, we'll be able to make everything else really cheaply, too, and space travel will still be one of the most expensive things humans do.

And, of course, if you can use your miracle machines to make methane, can you also use them to make nanofiber? Because that really could make space travel cheap, even in comparison to other cheap things.
We're not talking about 'miracle machines'. We're talking about robots doing the same tasks humans are needed to do now. The same robots available now, just optimized for easy of automated assembly and connected to large datacenters to provide their edge case intelligence.

Liquid methane has only a little less energy per gallon than kerosene. Currently, it costs about a million bucks worth of fuel and LOX to put a F9 into orbit. So in this future world of mass production, the cost might be a little lower, and the cost of the rocket itself would be immensely lower, but yeah, it would probably still be hundreds of thousands of dollars per launch.

This type of robots solves none of the problems with space tethers. It doesn't make it possible to produce the flawless chains of CNT you'd need, nor does it solve the inherent problem of having a cable that is 70,000 kilometers long that if it fails at any point, will negate your entire investment.

This is the reason space tethers won't work. With thousands of separate rockets made in separate copies of the same automated plant, fueled by distributed arrays of solar panels and gas processing plants, there's no single point of failure. Any failure only destroys a tiny part of the infrastructure.

Last edited by SamuelA; 09-16-2017 at 10:34 PM.
  #67  
Old 09-16-2017, 11:58 PM
si_blakely si_blakely is offline
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Both, actually. One fell "more than 15,000 meters" and burnt up upon reentry. Another was flicked off from the counterweight and could not be rescued before her air supply ran out. Here's a Google Books link to that section of the book.
Thanks or the confirmation, scr4.
My knowledge of space elevators and orbital mechanics mostly stems from early 80's hard sci-fi. Glad to know I can still remember the details.

I may need to check out some books from the library. The Descent of Anansi (Barnes/Niven) has nanowires and orbital tether physics, as well.
  #68  
Old 09-17-2017, 07:12 AM
HopDavid HopDavid is offline
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So with landable, reusable rockets, the only savings you get from an elevator are :

1. You don't burn rocket fuel. Which can be liquid methane, which is about $1.52 a gallon.
It's not about the cost of fuel. As you say, propellent is dirt cheap.

It's about breaking the delta V budget into smaller chunks and reducing velocity change that needs to be accomplished via reaction mass. The exponent in the rocket equation scales with the delta V budget.

Presently it takes about 9 km/s to get to LEO (Low Earth Orbit) and around 13 km/s for GEO (Geosynchronous Earth Orbit) or TMI (Trans Mars Insertion).

Given a 9 km/s delta V budget and chemical propellent you have a dry mass fraction of around 4%. This 4% includes the payload, structure, rocket engines, power source and propellent tanks.

This tiny mass fraction means upper stages are about as sturdy as a hollowed out egg shell. With this slim mass fraction it hasn't been doable to give upper stages the structure and thermal protection they need to survive the extreme conditions of an 8 km/s re-entry.

Elon Musk and Jeff Bezos seem well on their way to making reusable boosters. But reusable upper stages are another story.
  #69  
Old 09-17-2017, 07:21 AM
Chronos Chronos is offline
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There's no reason why a failure at a single point should cause an entire cable to fail. As with anything else humans make, you build in a safety factor. If you make the cable 10 times stronger than it needs to be, then you can survive a cut 90% of the way across the cable. And then you send up a robot to fix the cut, using the same technology you used to make it in the first place.

And of course there's no reason to only have one space elevator. The marginal cost for making more would be orders of magnitude less than the cost for the first one, because you're re-using all of the R&D, and you're using the first one to launch all the rest of them. So even if you do lose one cable entirely, you just use one of the other nine you have to cheaply launch a replacement.
  #70  
Old 09-17-2017, 11:04 AM
SamuelA SamuelA is offline
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There's no reason why a failure at a single point should cause an entire cable to fail. As with anything else humans make, you build in a safety factor. If you make the cable 10 times stronger than it needs to be, then you can survive a cut 90% of the way across the cable. And then you send up a robot to fix the cut, using the same technology you used to make it in the first place.

And of course there's no reason to only have one space elevator. The marginal cost for making more would be orders of magnitude less than the cost for the first one, because you're re-using all of the R&D, and you're using the first one to launch all the rest of them. So even if you do lose one cable entirely, you just use one of the other nine you have to cheaply launch a replacement.
The problem with adding a significant safety factor is the same as that joke about making an airplane out of the same stuff they make black boxes out of. (the highways aren't wide enough)

It's likely not physically possible to do that. Adding a safety factor means making the cable thicker, which makes it heavier, which increases the stress, so it doesn't work.

Sure, if you can build hundreds of orbital tethers, in the far, far future, maybe you'd do it.
  #71  
Old 09-17-2017, 11:09 AM
SamuelA SamuelA is offline
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This tiny mass fraction means upper stages are about as sturdy as a hollowed out egg shell. With this slim mass fraction it hasn't been doable to give upper stages the structure and thermal protection they need to survive the extreme conditions of an 8 km/s re-entry.
Well, then you throw them away. Ultimately, the cost of rockets is tied to the cost of manufacturing. The raw materials and fuel are cheap, as you say.
  #72  
Old 09-17-2017, 05:06 PM
Weisshund Weisshund is offline
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I think I'd lose more than just 3 lbs if I climbed a 400km tower.
Does it count as being lost if it is still in your shorts?
  #73  
Old 09-18-2017, 04:06 PM
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Well, then you throw them away.
That's what we do. Which is why spaceflight is so expensive. Imagine what a transcontinental flight would cost if we threw away a 747 each trip.

As I said, full blown Clarke Towers aren't plausible. But their smaller cousins orbital tethers are. And they can lend a hand reducing delta v budgets which could make ships reusable.
  #74  
Old 09-18-2017, 04:56 PM
SamuelA SamuelA is offline
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That's what we do. Which is why spaceflight is so expensive. Imagine what a transcontinental flight would cost if we threw away a 747 each trip.
Sigh. I am tired of repeating myself. Mass production and automation works. Complex goods are available today for less than the price of much simpler goods in the past. The problem is that mass production requires mass order volumes. What makes it cheaper is that there are a lot of fixed development costs in producing anything, from programming the automation to coming up with the optimal steps for each part of the process to developing the fixtures and custom tooling.

Better automation should lead to robotic systems where you can get the same cost benefits you would get with ordering a million units with ordering only 10. That, in turn, would mean that you in fact could scrap the 747 after each transcontinental flight, and the tickets would only be 10 times as expensive.
  #75  
Old 09-18-2017, 05:43 PM
Chronos Chronos is offline
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Better automation should lead to robotic systems where you can get the same cost benefits you would get with ordering a million units with ordering only 10.
Not necessarily. You mass-produce things by making a mold or die, and then using the mold or die to make many parts. The original mold or die can't be made in the same way, and has to be made in some more expensive way, but that's OK, because its cost is spread out over many final products.
  #76  
Old 09-18-2017, 06:05 PM
SamuelA SamuelA is offline
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Not necessarily. You mass-produce things by making a mold or die, and then using the mold or die to make many parts. The original mold or die can't be made in the same way, and has to be made in some more expensive way, but that's OK, because its cost is spread out over many final products.
Correct. But a robot that can flexibly make a lot of things, and any molds needed can just be 3d printed, would be like having a mold or die already made for any order.
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Old 09-18-2017, 07:49 PM
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Being able to 3D print a mold or die is not the same thing as having a mold or die already made. I'll be the first to say that 3D printing is cool, but it's a lot more expensive and time-consuming than mass manufacturing techniques, especially in materials suitable for use as dies or molds.
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  #78  
Old 09-18-2017, 08:52 PM
SamuelA SamuelA is offline
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Being able to 3D print a mold or die is not the same thing as having a mold or die already made. I'll be the first to say that 3D printing is cool, but it's a lot more expensive and time-consuming than mass manufacturing techniques, especially in materials suitable for use as dies or molds.
That might have been true a few years ago. Near instant 3d printers are now available for plastic, and there are extremely high speed metal printers as well. Again, you would print the mold, not the final product.
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Old 09-18-2017, 09:00 PM
carnivorousplant carnivorousplant is offline
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That's what we do. Which is why spaceflight is so expensive. Imagine what a transcontinental flight would cost if we threw away a 747 each trip.
Maybe if they didn't show movies and didn't give out those goldfish crackers?
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Old 09-18-2017, 09:24 PM
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The "near instant" 3D printers you're referring to are still several orders of magnitude slower than mass-production methods.
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Old 09-18-2017, 10:49 PM
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So they print a negative mold, you hot pour a positive mold, then use that to mass produce. Point is, if the process were completely reliable and automated, this additional step could just be queued up in the automation system and completed within a few minutes. It's the reliability. Current automation isn't this good, and things will go wrong, requiring manual intervention.

Last edited by SamuelA; 09-18-2017 at 10:51 PM.
  #82  
Old 09-19-2017, 06:45 AM
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Also don't forget, by the way, the cost of design, which also must be amortized over an entire production line. CAD software has made that more efficient, too, but you still need highly-skilled humans spending a lot of time on that, even when everything goes right.
  #83  
Old 09-19-2017, 03:10 PM
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Sigh. I am tired of repeating myself. Mass production and automation works. Complex goods are available today for less than the price of much simpler goods in the past.
So now I can buy a Tesla Model S?!

(Checking….) Ummmm…. Nope. Not yet.
  #84  
Old 09-19-2017, 03:25 PM
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So now I can buy a Tesla Model S?!

(Checking….) Ummmm…. Nope. Not yet.
No, but the S is cheaper than the Roader which is cheaper than experimental handmade lithium battery EVs from the 1990s.
  #85  
Old 09-21-2017, 12:18 AM
jharvey963 jharvey963 is offline
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Let's say the prefect the Space Elevator. You get on it and ride it up to the top, 400km above the surface of the Earth. Will you be "weightless" in the sense that astronauts aboard the ISS are?
I haven't read EVERY reply, but this fact wasn't mentioned in the ones I did read.

At 250 miles up, you still feel .88 G. If the car is stopped, you won't feel too much different than 1 G at sea level. If you let go of the car, you will fall (pretty much) straight back to earth.

At the height of geosynchronous orbit, 22,236 miles above sea level, you still feel .023 G, but your orbital velocity is exactly enough so that you will remain at the same relative position to the space elevator. Technically, you aren't weightless, you are in free fall.

J.

p.s., this is an amateur's view. Some of the rocket scientists on the board will be able to give a much more accurate / correct interpretation of this.
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