
The Spaceline: a space elevator alternative achievable with current technology - bookofjoe
https://arxiv.org/abs/1908.09339
======
mytailorisrich
> _By extending a line, anchored on the moon, to deep within Earth 's gravity
> well, we can construct a stable, traversable cable allowing free movement
> from the vicinity of Earth to the Moon's surface. With current materials, it
> is feasible to build a cable extending to close to the height of
> geostationary orbit, allowing easy traversal and construction between the
> Earth and the Moon._

If we still need rockets to reach the bottom of the cable 36,000 km above
Earth's surface, doesn't that negate the point of a "space elevator"?

~~~
opwieurposiu
Appendix B calculates a fuel savings of ~25% vs using only rockets to get to
the moon.

The saturn 5 rocket weighed 3,000 tons and was 96% fuel. Even if we only save
10% fuel, that's an extra 300 tons we can get to the moon.

~~~
mytailorisrich
25% fuel saving seems like a small saving compared to the cost of building
such cable. And of course, the saving up to geostationary orbit would be
zero...

I don't think you have your maths correct, either.

~~~
jefftk
It's not about paying for the fuel, it's about not having to lift the fuel.

Going from a rocket that is 2880T fuel and 120T for everything else to a
rocket that uses 25% less fuel gives you 2160T fuel and 840T everything else.
Decreasing the fuel by 25% increased the rest of your budget by 7x!

~~~
mytailorisrich
That's not how I interpret " _using the spaceline can save around 25% of the
cost per launch in fuel_ " (c.f. paper).

If the starting point is 2,880t fuel for 120t payload, so 24t of fuel per ton
of payload, then 25% less fuel gives 18t of fuel per ton of payload and thus
your 3,000t rocket can now carry 158t of payload, or 32% more.

Even " _we can see that the spaceline cuts the fuel cost of such a mission by
approximately two thirds_ " is not a game changer compared to the cost of
building the cable.

Space elevators are touted as game changers because although building one
would be extraordinarily expensive it would slash the cost of sending payload
to space by orders of magnitude, making it worthwhile.

~~~
pavel_lishin
But amusingly enough, a spaceline costs orders of magnitude less than a space
elevator. It might make sense as an intermediate step, where we launch a
Spaceline using ol' fashioned rockets, and then use it to haul up the
materials we're going to build an actual space elevator from.

~~~
TheSpiceIsLife
A space elevator needs to be anchored in geostationary orbit at about 36,000k
altitude, which isn't possible with available materials.

This article talks about a cable from geostationary orbit to the moon.

That still leave the ground-to-geostationary part for conventional lifters.

~~~
pavel_lishin
I must have misunderstood - I thought the Spaceline was going to actually
reach into the atmosphere?

Dang, I totally misunderstood, I thought this thing was going to go much
lower.

------
andyjpb
My favourite space elevator alternative is the space fountain (
[https://en.wikipedia.org/wiki/Space_fountain](https://en.wikipedia.org/wiki/Space_fountain)
).

It's an active structure made of small parts that are held together with a
stream of magnets.

If the fountain fails then each part can have its own parachute so that it can
safely return to Earth. i.e. without destroying entire cities as would be the
case for a traditional elevator design.

~~~
db48x
A space elevator wouldn't destroy cities if it fell. Most of it would be flung
away from earth, the part that fell would mostly burn up in the atmosphere.
The rest would flutter like leaves as it came down.

------
gene-h
TL;DR we can stretch a really long piece of fishing line from the Moon's
surface to geostationary orbit. And we can probably send payloads to the moon
by docking with said cable in geostationary orbit too. The funny thing is I
literally mean fishing line here, they call for using a constant cross section
cable made of zylon or dyneema with a diameter of about 0.35 mm. At least
initially. Both dyneema and zylon with this about this diameter are sold as
fishing line. Initial payload to/from moon seems a bit low as the line at the
Moon end can only support 100 kg. The dynamics of such a thin, flexible cable
which passes through an unstable(!!!) lagrange point should be interesting. We
know that very long space tethers can behave in weird ways[0]. This idea is
crazy and attractive enough that it warrants further investigation to at the
least find out what the problems are so other people don't make the same
mistake.
[0][https://pdfs.semanticscholar.org/6aff/70794d94b8979d0c5a67ff...](https://pdfs.semanticscholar.org/6aff/70794d94b8979d0c5a67fff7c2500cda082d.pdf)

~~~
pavel_lishin
> _Initial payload to /from moon seems a bit low as the line at the Moon end
> can only support 100 kg._

But on the Earth-side, it can support up to 2000kg! And launching from the
Moon is cheaper, anyway, since it's a less massive object than the Earth, and
there's no atmosphere to deal with.

~~~
hanniabu
That's interesting, I wonder if they meant 100 Earth kilograms or 100 moon
kilograms.

Edit: On 2nd thought they are probably using kg in terms of mass, not weight.

~~~
db48x
Grams are always mass, never weight.

------
papito
Even if we built a space elevator, imagine that the payload travels up at
100km/h - _really_ fast, basically a car on a highway. Even in that ludicrous
mode, it would take 15 days for the payload to reach 36,000km. Probably
unrealistic for human cargo.

~~~
aruggirello
You've got it... upside down. The line is dangling from the Moon, down to
~36000Km above us. That car would have to reach geostationary orbit, jump on
that tiny cable, _then_ travel... oops, the remaining ~350,000Km to reach the
Moon. To do that trip in 15 days, it'd have to run at mach 1.0.

Edit: it gets even weirder, if you consider the Moon's distance from Earth
goes from 363,300Km at perigee to 405,500Km at apogee... just how long should
the line be? If it's 327,300Km, we might only be able to operate it near
perigee, sure if it's a bit longer it'd be more usable, but it cannot extend
past 363,300Km - or it would touch down here :) - that means there's no way we
might use it near apogee, since it would always be too far away.

~~~
Tepix
> That car would have to reach geostationary orbit

No, it would not have to reach orbit, it would have to reach the altitude but
not the horizontal speed. Huge difference.

------
logfromblammo
This is not a launch alternative. This is _maybe_ a get-to-geostationary
alternative. This is really an L1-base station-keeping alternative. The Moon-
side cable anchors the base in the radial direction, and the Earth-side cable
keeps the Moon-side cable in tension, and lowers the fuel cost of reaching
orbits that the cable crosses.

The math suggests that putting a rendezvous base 41% of the way from the Earth
to the Moon would be better than extending more cable down into Earth's
gravity well. Meeting the cable closer to Earth would always cost more fuel.
You could save fuel to GEO by meeting the cable further out, and then climbing
back towards Earth. But every bit of extra cable you add on the Earth side
increases the tension at the max-tension point at L1.

So just stop at the lowest-fuel rendezvous height, and add enough mass there
to keep the whole cable in tension, but not enough to break the cable at L1.

(The L1 point is at about 84% the distance from Earth to Moon.)

The same type of material would likely be usable to anchor a halo relay
satellite slightly beyond Earth-Moon L2, just by bolting it down to the
darkest antipodal point on the dark side of the Moon.

~~~
dredmorbius
If you can source materials _in situ_ , this _might_ be a useful way to create
GEO platforms at lower cost than Earth launch.

~~~
logfromblammo
If the theory is correct, you could build satellites on the Moon and slowly
walk them to GEO at zero fuel cost with solar power.

Any circular orbit with height between 5% and 41% of the Earth-Moon distance
could be placed more cheaply by flying out to rendezvous at 41% with fuel,
then walking it back in along the cable with solar power.

------
marshray
Well if we can get a cable to the Moon, we could attach it to a crank at the
North or South poles to generate electric power.

~~~
k_sze
Wouldn’t that slow down the speed of rotation of the earth and the revolution
of the moon around earth? That would be catastrophic.

~~~
Doxin
I'd imagine that the amount of energy stored in orbital momentum is a lot
higher than anything we could use up in any reasonable amount of time.

------
mirimir
This is an interesting idea, for sure.

But relative to the feasibility of this, how far are we from building
automated systems to mine raw materials, refine and process them, and build
stuff? Initially, of course, to build more such automated systems.

------
dann0
Forgive my ignorance, but if the moon orbits around the earth (ie not
geostationary), how can the end of a line attached to the moon stay in one
place above the earth?

~~~
Leo_Verto
The moon is tidally locked to earth, meaning it rotates at such a speed that
the same side is always facing earth. (This is also why the moon can have a
"dark side")

It usually takes millions of years of tidal forces from a planetary body
acting upon its satellite for the latter to be tidally locked to the former.

~~~
dann0
Thank you. I understand that part, but is the line supposed to be at the same
point above the earth?

~~~
Tepix
No. It wanders around the earth every 28 days.

------
sgt101
ohh - is there anyone here who can answer a question for me. I have been
writing a science fiction story which features large rotating space stations
like the one in 2001 and I wondered...

What is the force on a rotating structure in space. Is it possible to build a
space station that rotates with 1G force on people at the rim, and if so what
scale of structure could be built with current, or plausible technology? Where
does a structure go from "imagined future" to "magic"?

~~~
zaroth
There’s discussion on /r/spacex that two Starships could be teathered nose to
nose and set in a rotation during their ballistic flight to Mars, in order to
provide at least Martian (~.3g) level artificial gravity along the way.

This is important because even things like convection in the air stop working
well at 0g, so for example, you can suffocate on your own CO2 if you fall
asleep and there isn’t a fan running.

Funny enough the hardest part about this doesn’t even seem to be the tethering
or rotation, but two things; 1) you have to make slight course corrections
along the way because the initial burn is never precisely accurate enough to
target something as far away as a planet, and 2) occasionally you have to put
as much mass as possible between you and the sun due to solar storms spewing
radiation, so you have to stop the rotation occasionally to point the engines
sunward to use your fuel as a radiation shield, or to course correct.

~~~
dredmorbius
Water supply as rad shield might be better, presuming you have a large water
store. Designing a chamber within the water supply would put mass between you
and radiation.

------
mmazing
Another interesting angle - is it possible that there is a continuous void
between the earth and moon that _never_ intersects _any_ space debris already
in orbit?

~~~
tejtm
Considering geosynchronous orbits and that the moon progresses around that
band of highest distribution of stuff to bump into, I would not bet on any
window lasting very long without serious maintaince

------
java-man
I want to see an analysis of the different failure modes (catastrophic
failure, hit by a satellite) and the consequences of the failure.

I believe this idea is just incompatible with reality.

~~~
corodra
Let's ignore the physical catastrophe of a cable at 380,000+ km long
dislodging and becoming either locked in earth orbit or crashing into earth.

Let's ignore that.

What if the cable attached to the moon and then locking the other end to earth
orbit alters the month's orbit? There was some stuff I read that the moon
doesn't neccesarily orbit the earth. It's the earth and moon orbit each other.
Essentially our orbit is significantly affected by our moon compared to other
planets and their moons. We have a ton of wildlife that depends on the current
orbit pattern. Wouldn't this cable greatly alter said orbit?

Plus, 380,000km worth of material? Along with they just hand wave that all
failure will happen on the moon side and never affect earth... um okay?
Because of testing?

This seems more like a joke paper in all honesty. Kind of like the paper based
on Silicon valley where they dev an algorithm to find out how long it would
take to give an audience handjobs. d2f ratio

~~~
pavel_lishin
> _they just hand wave that all failure will happen on the moon side and never
> affect earth... um okay? Because of testing?_

They don't _handwave_ it, they _calculate_ it. It's significantly less
handwavy than your back-of-napkin objections.

~~~
wbhart
Can you point to the specific section of the paper where this is calculated. I
admittedly only skim read, but I did go through the whole thing, and I'm
convinced I missed this kind of analysis somewhere.

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tauntz
Relevant and entertaining take on this: [https://what-
if.xkcd.com/157/](https://what-if.xkcd.com/157/) (A direct moon - earth
elevator, not really what's proposed here though)

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JoeAltmaier
The earth-moon cable is just a construction fixture. The space elevator
extends to geosynchronous orbit, as usual.

------
tpmx
Ok, I didn't read the paper...

But everyone knows the moon orbits the earth with a cycle sort of similar to
what we call a month.

How is an earth-moon cable feasible again?

~~~
chkaloon
Suggestion - read the paper.

~~~
tpmx
"The Spaceline. By extending a line, anchored on the moon, to deep within
Earth’s gravity well"

That's the concept. They could have fit that within in the title of the post.

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orthecreedence
I'm excited for a space elevator almost exclusively just so we can get rid of
nuclear waste. Nuclear power is the only current viable alternative to fossil
fuels at scale, from my understanding. However, people are squeamish about the
spent fuel...if we could take it up to space and launch it into the sun
(without having to deal with the perils of escaping gravity) it would be a lot
easier to get rid of.

~~~
jandrese
That sounds way more dangerous than just burying it in the ground. An accident
in space spreads radioactive materials over some part of the Earth almost at
random. An accident in a buried site stays mostly contained, with the biggest
danger being contaminated groundwater.

~~~
orthecreedence
OH yeah?!

------
wbhart
So they found a material which won't degrade in 24 hours due to radiation from
the sun, or a way to shield it? And what about the mismatch between the
rotation of the moon around the earth and geostationary orbit? And they
figured out how to go around the van Allen belts to prevent frying sensitive
electronics in the payload? And when the payload gets close to the moon, they
have a way to stop it plunging to the surface of the moon for unscheduled
disassembly?

Of course the cheapest part of getting to the moon is the bit between the edge
of space and moon orbit. Approximate current cost: $0. Approximate cost after
placing 40,000kg fishing line in the way....

I'm also pretty sure crawling along a fishing line will be prohibitively slow
compared to letting a rocket coast with its engines off.

~~~
jimrandomh
> Of course the cheapest part of getting to the moon is the bit between the
> edge of space and moon orbit. Approximate current cost: $0.

No, it costs 4.1km/s of delta-v.
[https://en.wikipedia.org/wiki/Delta-v](https://en.wikipedia.org/wiki/Delta-v)

~~~
wbhart
I meant to compare getting to GEO (as proposed in the paper) as opposed to
Lunar orbit.

Of course the way I worded this is a bit lose, and it's not completely free,
but according to the figure in that Wikipedia article, the difference between
getting into geosynchronous orbit and getting to lunar orbit _from earth_
seems to be only about 0.1km/s, which is negligible compared to the total
cost. So my basic point still stands.

It's actually really expensive to get to GEO.

