People Are Still Trying to Build a Space Elevator | Hacker News Reader

People Are Still Trying to Build a Space Elevator

smithsonianmag.com

33 pointsdnetesn54 comments

The article says, "His conclusion: The space elevator could be built with existing technology—minus the super-lightweight tether necessary to make the whole thing work." In other words, it cannot be built with existing technology.

NASA uses the idea of "Technology Readiness Level (TRL)" [1] to help assess speculative technology, the (obvious) idea being that each stage of deployment of a technology relies on passing tests at previous stages.

On this scale, it's not clear to me that space elevators are at TRL1 ("basic principles observed and reported") yet. Space elevators depend critically on having a material that's strong enough to build the cable. Feasible designs for climbers, debris avoidance systems, power transmission and so on, can't make up for the lack of this critical component.

[1] http://www.hq.nasa.gov/office/codeq/trl/trl.pdf

The article could have mentioned Skyhook [1], which can be built with existing materials.

[1] https://en.wikipedia.org/wiki/Skyhook_%28structure%29

Holy scrap, I had no idea a skyhook could be built from off-the-shelf materials. Thanks for pointing this out!

The way these are described in Seveneves is awesome. I didn't realize there's so much theory behind it!

Carbon nanotubes are seductive. They are very strong, diamond like, in that circularized plane. But when you extend the tube to macro dimensions, you have problems with phonons and the 'rogue waves' that add up over such long distances (relative to atoms). These phonon effects can and do add up to break the nanotube as there is so much energy in the phonon that it overcomes the intermolecular bonds. You can help a bit with an onion layering of nested nanotubes, each a little bit bigger. But that doesn't get you much further. Doping the tubes with other atoms reduces the strength by a lot, but may increase the length. Isotopes of carbon, like C10 or C14, do help a fair bit. However, they tend to be too radioactive to be stable for a project like this. You'd have to continually replace the rope as the dopant isotopes would decay away and the nanotubes would self destruct again. In the end, we need new ideas to help with this. Perhaps some strange electron-phonon interaction? I don't know.

Yes, but so what if you need to continuously replace parts of the cable? As long as you can identify the damage and cheaply perform the repair, it doesn't seem that bad.

The idea is that the cable would be one long crystal, not sections. You'd have to continuously extrude the crystal from orbit to the ground or something. You can't bolt in new pieces. Whatever the solution will be, cheap is not one of the answers. Just the maintenance on it would be a world-wide effort. And for what? Cheap orbital travel? The real hard part is to find a reason to leave Earth that the oceans and Antarctica don't already fulfill. The only one I can think of is orbital foundries or other processes that need 0g. Living space we have a lot of, though cold, Antarctica has air, something Mars and Venus do not.

> pls don't ask me about space elevators until someone at least builds a carbon nanotube structure longer than a footbridge

https://twitter.com/elonmusk/status/559557786514632704

Yes, people are still working on it. It's not going to get built if no one tries.

Some things aren't worth trying because they are batshit insane.

It might be more cost effective to build a really long, narrow elevator into space vs. paying $450 million per launch of a rocket/large spacecraft.[1] And keep in mind that launching a large spacecraft usually carries less than 12 individuals.

[1] http://www.nasa.gov/centers/kennedy/about/information/shuttl...

Not really this is like saying that It would be more cost effective to build jumbo jets instead of sail boats in the 15th century. Even if an alien descended from the sky and gave you the materials to build it we don't have a single analogy as far as the actual engineering and construction process goes to even begin drawing blue prints.

So yeah in the "long run" it might be more cost effective but it's irrelevant you can say the same thing about any hypothetical technology but you can't stop spending money in the mean time if nothing else we need to be able to be really good at launching extremely heavy payloads and building stuff in micro-gravity before we can even begin to start dreaming about how to build a space elevator. We also need rockets that can go out quite far and come back with a huge payload like a small asteroid for the anchor and even most likely have to master asteroid mining for the raw materials for any potential space elevator. My bet is that it may become feasible around 2250-2500 (yes I know this is a 250 years window :)) if it's possible at all (because when all said and done even carbon nano-tubes aren't good enough they are a good candidate and most likely will be used but we need much stronger composites than what carbon-nano tubes alone can offer).

See also: horseless carriages, flight, splitting atoms.

fusion power?

What I never seem to see addressed, is the following: As the elevator ascends the cable, it's "ground speed" must continually increase. What is the source of the force that provides that acceleration (parallel to the ground)?

A traditional rocket expends a great deal of energy firing its rockets with a component parallel to the surface of the earth to gain acceleration in that direction. Where is the equivalent coming from w/ the space elevator?

That is not at all how it works. The elevator does not go into low-Earth orbit(LEO). It is part of the elevator system whose center of gravity is at Geostationary Orbit. So the craft itself, and the ribbon, is always over the same spot on the Equator.

The earth's rotation. Much like how your rotational velocity increases when you climb stairs. Also, the cable is not vertical, you want a large mass past geostationary orbit which pulls the cable fairly thought though there is some bend.

PS: Try and calculate just how much energy is in earth rotation it's a rather large number.

> climb stars

I love this typo.

That's caused by the Coriolis force. Rising elevator cars would leach the Earth's angular momentum, whilst lowering cars would replenish it.

> As the elevator ascends the cable, it's "ground speed" must continually increase

That is backwards. The higher you are the less speed you need. This is true in all orbits. Contrast the orbital velocity of Mercury vs Neptune.

The rocket is aiming for low orbit. I.E. you go fast enough that you fall around the earth rather than into it. The only reason a rocket's ground speed increases as it's altitude increases because it's hard to add a lot of speed while still in the denser sections of the atmosphere. Altitude is needed merely to clear the drag imparted by the atmosphere.

We go fast and low because it's the easiest way to get something to stay in orbit using rockets. But mechanically as we gain altitude we go slower relative to the ground. Eventually when you go high enough you are standing still relative to the ground (geosynchronous orbit) or even go backwards relative to the earths rotation (high orbit).

A space elevator is aiming to go so high that there is no parallel acceleration necessary. The rotation of the earth provides all the acceleration needed. A space elevator that ends in geosynchronous orbit requires no parallel acceleration relative to the ground. A space elevator in high orbit could get something of a free ride out of earth's gravity well powered by the rotation of the earth.

A space elevator only provides access to geostationary orbit. One can climb up to LEO altitudes along an elevator, but that doesn't mean much. You wouldn't be in orbit, but sitting stationary as if atop a tall building. So we would still need to launch rockets for any space use in LEO (GPS and imaging sats for example). And any such orbiting objects would of course have to be navigated around this obstruction. Many useful orbit types would have to be outlawed to protect the elevator.

I wonder, is easier to launch into a LEO from the ground or from a fixed point at say 300km altitude? If you climbed a space elevator you would still need 8km/s of speed laterally. So you jump off and fire your rocket. You still fall towards the ground, requiring some thrust to keep out of the atmosphere. Without the arcing trajectory of a ground launch you would have to accelerate roughly twice as quickly, requiring larger engines. Is that really any better than starting from the ground as we do today?

Or you could climb to a near-geostationary position, burn retrograde until you touch the atmosphere, then aerobrake down to LEO. That's still a heck of a lot of effort.

You wouldn't have to contend with gravity loss (energy expended by performing work against gravity) or atmospheric drag, so all else being equal it would be a net energy win. Definitely not a free ride though.

I believe you have a fundamental misunderstanding of the speed of the top of a space elevator. The counterweight at the top of a space elevator would be approximately at geostationary orbit. If you were to be up there and "drop" a baseball, say, it would appear to hang there motionless, as it would also be (approximately) at geostationary orbit.

I believe you have misread my post. I see no issue with geostationary orbits. I'm talking about the low earth orbits on which we so depend.

Neither. For LEO the object is released at well above the desired orbit and as it drops it acquires the needed velocity. Some fuel is still needed but not nearly so much.

So it drops, but how does one translate that it into orbital, lateral, velocity?

One doesn't. Its already in orbit, but not travelling fast enough. As it trends downward, it gains velocity until it reaches stable orbit. If the maths were done correctly, that will be the desired orbit. If the maths are in error, an opportunity for experience is gained.

It doesn't drop vertically back down the elevator cable! It performs a Hohmann transfer from GSO to LEO.

The delta-v to transfer from GSO to an LEO at 400km is 4km/s, whilst to go from ground level is about 7km/s.

That's a significant fuel reduction, _and_ that fuel doesn't have to go up all at once: you can keep fuel tankers at the GSO station and fuel up a rocket from there.

https://en.wikipedia.org/wiki/Hohmann_transfer_orbit

4k/s is still a huge number. This will still involve a rocket 1/4 to 1/8th the power of a ground launch. AND, pieces or not, that thing has to be hauled up the elevator.

So my point still stands. The space elevator will only provide ready access to geostationary orbit. Journeys to more useful low orbits will benefit marginally at best.

LEO is useful for current satellites. But not that useful for moving human influence beyond Earth's surface.

Only if you release it from GSO, but that's not ideal. Release it from a higher altitude than LEO, but far less than GSO and it takes vastly less fuel.

Ah yes.

There's a lower point on the cable where you're already in an elliptical orbit with the perigee at the altitude you want.

For a target orbit of 400km, that happens at about 23.8Mm, 57% of the way up the cable. At that point you just have to let go of the cable and you'll only need a circularizing burn when you reach 400km: Δv of ~2,137m/s.

This is not taking into account the further savings from aerobraking as I don't know how to calculate that.

I believe you are overestimating the cost of a transfer from GEO to LEO and underestimating the costs from ground to LEO. Specifically you haven't accounted for the earth's atmosphere.

The earths atmosphere adds to the cost of ground launch (drag) and subtracts from the cost of GEO->LEO transfer (aerobrake). Instead of 4km/s transfer vs 7km/s direct, it should be 2km/s vs 9km/s.

Any height is an advantage in achieving orbit. It's an exponential problem - you need fuel to go up, and you need more fuel to lift the fuel you need to go up, etc.

Also, air resistance is much more at the surface than higher up.

So by starting your ascent at a high altitude you need much less fuel both because you need less fuel, and because there is less air resistance (friction).

Space elevators still need to use the same energy to lift something, but they use an external power source so do not need to lift their own fuel, and they lift more slowly and greatly reduce the effect of air resistance.

So no, a space elevator won't get you to "outer space", but it creates a stepping stone which greatly reduces the cost of getting there.

Air resistance is a very minor issue for rockets. Long/thin things are very slippery and once at 10-20-30,0000 meters the air is basically gone.

Jumping off the elevator from LEO altitudes would still require one to fight gravity, to avoid dipping back into the atmosphere before getting to orbital velocities. So there are still gravity-related losses to account.

This isn't correct. Air resistance increases at the rate of velocity squared. At the speed those rockets achieve, is is a big factor. Elon Musk himself likened the atmosphere to a "thick soup" when it comes to launching rockets to space.

My understanding is that atmospheric drag on a standard launch vehicle to leo is on the order of 300-400m/s, a small fraction of the total budget.

> A space elevator only provides access to geostationary orbit.

No, it provides access well above geostationary orbit.

https://en.wikipedia.org/wiki/Space_elevator#/media/File:Spa...

Time things right and you can basically fling off the end like the tip of a whip and head anywhere in the system.

Anywhere up, I cannot see how it is to fling things down to low earth orbits.

Delta-V here's a good map http://i.imgur.com/SqdzxzF.png

The Delta-V required to get from the surface of the earth to leo is 9-10, meaning it requires less energy to launch a payload from the moon to earth's LEO than from the surface of the earth to LEO. This can even be more optimized using aerobraking, anything that will be launched (or flung) from the launch platform of the space elevator will have higher orbital velocities than an object in LEO so what you really need to do is to slow down. Because there is still a pool of gravity the object will be in free fall so it only requires relatively small delta-v to bring it down to a lower orbit and you can use the atmosphere to both slow it down further (which can cut down on fuel) and if you design your delivery vehicle correctly can even get into very interesting orbits cheaply by skipping on top of the atmosphere and using the earth for gravity assist.

You are right jumping off at LEO altitude would be silly. Which is why you would go to GEO then transfer to LEO. GEO is something like 35,000 km high. You burn your rockets to fall towards low orbit, then pick up all the velocity you need falling towards LEO. Burn again once at perigee (or aerobrake) to steer your orbital path into circular rather than elliptical.

A transfer from GEO to LEO is significantly cheaper than reaching LEO from the ground. Additionally transfer from GEO to anywhere else in the solar system is really cheap. It is hard to overstate just how much of an advantage a space elevator would be in reaching any location in the solar system including LEO.

The only disadvange is travel time. Rockets to 300km get there quickly. Elevators to 35000km get there slowly.

A space elevator on Earth is very hard to build and would require exotic materials which we can't yet synthesize in bulk. But you can make a decent lunar space elevator with Kevlar.

http://hopsblog-hop.blogspot.com/2012/09/beanstalks-elevator...

I certainly believe an underthought of this article, which is that improved capabilities with materials such as carbon nanotubes might lead to a paradigm shift in the possibilities of what can be done with materials.

"But you’ve had civilizations rise and fall by a change in material strength by a decimal point, and now you’re talking about orders of magnitude."

Does anyone know what he's referring to in this comment?

I would assume Bronze -> Iron -> Steel would be the changes he's alluding to.

My understanding of the biggest challenge is that each point in the cable must be strong enough to carry the weight of all the cable beneath it, which by the time you reach the upper atmosphere would be tremendous even using carbon nanotubes. The cable would also be perpetually struck and damaged by high velocity micrometeoroids.

Can't you have multiple stages with multiple cables? I guess acceleration and deacceleration between stages could be costly, though.

What do you connect your stages to other than the next stage?

I would imagine the cabin could stop halfway, were it would attach to some magnetic or mechanic lock system. There it would detach from it's cable and be attached to another one that would pull it out the rest of the way. Actually, the first cable could be the lock system; you just have to keep it attached until you setup the second one.

Could one use a counter-weight, as in Kim Stanley Robinson’s Red Mars?

Space elevator shadow wouldn't look like that: https://en.wikipedia.org/wiki/Umbra,_penumbra_and_antumbra

I remember back in the early 2000's when predictions for when it would be built was 2015-2020. 4 years to go.....

To be fair, the advances in space tech by the private sector have been nothing short of amazing as of late. It's really an exciting time.

I fear we're heading into Year of Linux Desktop memes with the private sector. I'd love to be proved wrong.

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