
Space Tethers: Stringing Up the Solar System - sohkamyung
https://toughsf.blogspot.com/2020/07/tethers-all-way.html
======
hinkley
Tethers shift the economics of landing ships or materials on earth, since they
can trade delta v with things outbound.

But it seems like you have to trade power for accuracy; if you can hit a tiny
window moving in three dimensions inside of a 3 body problem, then you don’t
need to be able to manage as much delta V (increasing to leave, or decreasing
to land). Smaller rockets, less need for ablative materials.

But if you miss, boy are you fucked.

One solution for this I’ve heard is you take enough fuel to get where you’re
going with plenty to spare. If everything goes right, you offload your
emergency supplies (reaction mass, fuel) and get a credit.

I’m not sure how much handwaving goes along with moving liquids and gasses in
vacuum, at zero g... burst a line you need a lot of reaction mass to reorient
yourself.

Maybe there’s an emergency venting design where all of the forces cancel
out...

~~~
hirundo
> But if you miss, boy are you fucked.

Imagine closing in on Mars knowing that if you fuck up a super-orbital speed,
split second maneuver by more than a couple of meters you will drift in space
until asphyxiated. It makes a carrier landing seem bland.

If you carry enough fuel to decelerate without the tether it defeats the
purpose of the tether.

~~~
FlyMoreRockets
The comparison between tethers and an aircraft carrier catch cable seems
particularly apt. Thanks you. And yes, most times, you can initiate a go
around if you fail a carrier landing. Not really an option with an orbital
tether. Some sort of cable pair with a targeted coupler would probably work,
each side with a heat emitter an heat seeker RCS system. Obviously, there
would need to be slack in the system, (nearly local) winches a ways up-cable
in the system, may be an answer.

~~~
mLuby
The relative motion drops to zero though, at the moment of capture. So it's
more like two cars on the highway that pull alongside each other just as they
match speed. Might still be too precise for a human, but automated systems
should have little trouble.

And it might be easier to store the emergency retrieval system on the tether's
hub. That way it doesn't have to be hauled around to be used (hopefully)
infrequently.

~~~
hinkley
You have to get the distance and velocities to line up, and it’s challenging
to change only one of those in a three body situation.

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karmicthreat
Its always felt like a better near term way to boost around is just use
massive (literally) platforms that have large installations of solar,
batteries and electric thrusters. Have enough ballast that its orbit is only
going to change a small amount when throwing something. Then you just keep
thrusting until you are back where you were.

Electric thrusters are getting huge specific impulse improvements and the
future seems to be pretty bright for them. We can trade time for efficiency
essentially.

For interplanetary transit, just keep the platform running a circuit between
the planet and earth. It's going to be way easier to hit your entry without
getting into an unrecoverable situation.

~~~
matterbeam
That is similar to the concept of the Aldrin Cycler.

~~~
karmicthreat
Ah perfect, I knew there was a similar concept but couldn't remember the name.

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unchocked
For more on space tethers, check out Hop David's excellent blog (cited as such
in the article), wherein a laypersons understanding of orbital dynamics and
momentum exchange is developed in posts spanning several years.

[http://hopsblog-hop.blogspot.com](http://hopsblog-hop.blogspot.com)

Best read in chronological order.

~~~
baq
IME the best way to quickly get a very practical grasp of orbital mechanics is
to play Kerbal Space Program. I felt like a 6 year old kid who just got his
favorite candy when I docked one spacecraft to another the first time.

~~~
Loughla
I am, honestly, astounded at some of the videos online of KSP. I can barely
get something into orbit (if I can even do that). Let alone set up entire
space stations and explore other planets.

KSP is one of my favorite, infuriating things to exist, and I'm not sure why.

------
jp57
It seems like this system is crucially dependent on the balance of departing
and returning vessels to conserve momentum. While that balance might be
maintained over long timescales, it seems unlikely over short ones, especially
the first years of operation at earth, which are likely to have many
departures and few returns. And anyone who's seen a UHaul lot in a college
town on the first week of school knows that departures and returns are often
quite out of balance for short periods.

It also seems like there is a conservation of energy issue here that I don't
understand. In a state of balanced departures and returns, the energy used to
launch departing vessels would be captured from returning vessels, but in the
Earth-Mars loop, the returning vessels were themselves launched from the
surface of Mars and accelerated by tethers there. It feels like a perpetual
motion machine.

~~~
ec109685
They discuss this in the article:

“ And of course, none of these deltaV savings are for ‘free’. Accelerating
payloads means the tether will slow down. If it slows down too much, it will
de-orbit itself. The momentum lost with each catch-and-release operation must
be recovered either by absorbing momentum from payloads being slowed down, or
by using its own propulsion system.

A major advantage of an orbital tether is that you do not have to immediately
recover that momentum - it gives time for slower but more efficient propulsion
systems like a solar-electric thruster to gradually accelerate the tether. A
chemical propulsion system limited to 450s of Isp is not needed as the
acceleration can be done over time with something that has thousands of
seconds of Isp. The propellant needed to run the tether’s engines is greatly
reduced. Even more interesting is the possibility of propellantless
propulsion, such as electrodynamic tethers that push off the magnetic fields
around a planet.”

------
gkfasdfasdf
FYI This concept is employed extensively in the excellent novel Seveneves. The
author Neal Stephenson worked at Blue Origin for a time.

------
mcdoh
> It is ironic that the person who first described how hard spaceflight by
> rocket is, due to the exponential nature of the deltaV equation, is also the
> person who described the best way to side-step that problem with non-rocket
> launch.

That's not ironic at all!!

------
aaron695
It annoys me that Makani (tethered kites) closed this week but released
everything from patents to a documentary and a 1000+ page ebook and their
modelling software for free [6]

But hardly anyone cares.

But several orders of magnitude harder, maybe impossible, it's exciting?

If we can't tether in our atmosphere what chance is there in space.

[6]
[https://www.x.company/projects/makani/](https://www.x.company/projects/makani/)

~~~
jpm_sd
"Nobody cares" because they were solving the wrong problem. Existing wind
turbine designs work just fine and they are cheaper and far more reliable.

[https://despair.com/products/mistakes?variant=2457302467](https://despair.com/products/mistakes?variant=2457302467)

~~~
aaron695
Gas works fine to create energy, they mention gas in the doco. Wind turbines
also solve the wrong problem.

The nobody cares bit is around HN not caring all their info was made open
source (Try algolia). There's no reason you couldn't try and pivot. It's
something you can contribute code to. Tethers can also supply electricity
upwards. You could make permanent platforms, perhaps.

Space tethers are probably impossible, certainly nothing can be started for
decades. It's fun to LARP but it'd also be nice to get equally keen about
things that are possible and actionable.

------
jessaustin
The tethers in _Seveneves_ were a clear sign that things had _changed_.

------
kaonwarb
My favorite part:

> Let’s imagine a modestly-sized tether orbiting at a high altitude above the
> Earth. It is 1,000 km long...

~~~
matterbeam
It's modest when you compare it to the 36,000km+ geostationary tether or the
200,000km+ lunar elevators we see proposed!

------
kristianp
Surprising that no one here has used the term "skyhook" in the comments here.
Adding it to increase discoverability of this thread. Could SpaceX's starship
reach the tether without the superheavy 1st stage?

The video in the article is here: [1] and the sources to the video [2].

[1]
[https://www.youtube.com/watch?v=dqwpQarrDwk](https://www.youtube.com/watch?v=dqwpQarrDwk)

[2] [https://sites.google.com/view/sources-
skyhooks/](https://sites.google.com/view/sources-skyhooks/)

------
ncmncm
I prefer the idea of a rapidly spinning hoop. It means there are a lot more
chances to latch on.

The usual way they talk about keeping these things aloft is to set up an
electromagnetic accelerator track on the moon and aim buckets of ore, or
industrial product, at the upper rim. They latch on at the top and get dropped
off the bottom.

Timing is critical. It takes computers.

All around the circumference you have radial tethers with scoops at the end.
You just have to dive into the scoop when it dips down in front of you. Then
it's a wild ride up and out.

------
fareesh
My physics knowledge is quite poor, but I've always wondered if it was
possible to use the rotation of the earth as some kind of alternator in
conjunction with an orbital station. Is that idea fundamentally ignorant? Or
is it something that has been toyed with before?

~~~
_Microft
It's not an answer to your question but there are indeed ways to utilize
interaction of Earth's magnetic field with spacecrafts.

Power production (or thrust if reversing the process by feeding a current into
the wire) is described in [0].

Use for stabilization and attitude control is described in [1]. These might be
combined with [2], which have more immediate effect but from time to time need
to be 'desaturated' which is where [1] comes in again.

[0]
[https://pwg.gsfc.nasa.gov/Education/wtether.html](https://pwg.gsfc.nasa.gov/Education/wtether.html)

[1]
[https://en.wikipedia.org/wiki/Magnetorquer](https://en.wikipedia.org/wiki/Magnetorquer)

[2]
[https://en.wikipedia.org/wiki/Reaction_wheel#Implementation](https://en.wikipedia.org/wiki/Reaction_wheel#Implementation)

------
remarkEon
Has anyone built something like this in KSP? Is it possible (with mods of
course)?

~~~
minitoar
Not possible as I believe maximum physics range is 2.5km -- not long enough
for these sorts of devices.

~~~
uj8efdkjfdshf
There are mods that let you extend the physics range, but they don't address
the underlying issue of the KSP physics engine bring numerically unstable over
long distances

------
rini17
I am afraid we'd have to clean up orbital debris first. Even after that, the
system to negotiate orbital "corridors" is going to be maddeningly complex.

------
jcims
These things always seem like they would be so hopelessly fragile and finicky
that nobody would commit to them to do real work.

By the time we can build these we should be up to our eyeballs in fusion
energy. Seems like you could just build giant 50km railguns to blast raw
materials and a kick stage into orbit and leave rockets for the squishy
payloads.

~~~
ncmncm
If you wait for fusion to work before you do something, be prepared to die
first.

~~~
adrianN
We are a lot further along with fusion than with alternatives to massive
rockets.

~~~
ncmncm
I expect to die long before we get useful magnetic-confinement D-D fusion.

There are a lot of alternatives being worked on, with venture money, that
might yield before I die. But the Tokamaks have always been a jobs program to
maintain a good population of high-neutron-flux techs ready to draw upon for
weapons programs. It is why p-B fusion has never attracted any gov money: no
neutrons.

