Space Tethers: Stringing Up the Solar System | Hacker News Reader

Space Tethers: Stringing Up the Solar System

toughsf.blogspot.com

144 pointssohkamyung55 comments

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...

> 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.

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.

Maybe don't open the tether until you have tugboat sufficient to retrieve a bolter.

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.

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.

Because if you arrive with a full tank, it's useless and you vent it?

Maybe there could be uses for the fuel, if you succeed in getting it there.

You keep it for the next tether or you sell it at destination.

How is that different from aerocapture, which is what Starship is going to do?

You are coming in at hyperbolic velocity and have to use the thin and very variable atmosphere to break into an orbit.

At least the tether is completely predictable.

Hard to miss an atmosphere. If you're off to the side a bit, no big deal.

Actually it's completely plausible to skip on the surface of it. Bit easier to hit sideways, yes.

Bringing a few maneuvering thrusters to adjust the trajectory would still helpful and doesn't require tons of fuel.

If it works in KSP, it will probably work in reality.

I think the idea is brilliant, since you don't need rocket fuel to to generate the force you would transfer to the craft.

I’ve seen some proposals where there’s a low-mass extension to the cable that can change delta-v quickly and then reel in the real tether. Moral equivalent of bridging a ravine by tying a thread to an arrow, pulling a string across with the thread, pulling a rope across with the string, pulling a cable or two across with the rope, and pulling or building a simple bridge across using the cable.

But with the bridge, it’s just distance you’re crossing. With a tether you’re also dealing with velocity. And changing velocity changes distance...

In theory I suppose the tether could be equipped with a large-ish rocket that's capable of going out and grabbing anything that misses and putting it back on a correct course. If you had to use it all the time, it wouldn't be worth constantly refueling it -- you might as well not use the tether at all and just use bigger rockets. But if it only gets used occasionally, it's probably a good insurance policy to have on hand and would just be a cost that gets amortized across all the successful launches.

There will be a lot of factors constantly perturbing the trajectories, so lacking the ability to take corrective actions should be out of the question. Unless something happens too close to the point of contact, I expect the situations to be calculated and compensated for. Also, that tiny window that you talk about doesn't actually have to be that tiny. There can be engineered a string of potential contacts with multiple tethers on slightly different orbital altitudes to lower the risk, and for fungible cargo like fuel or other kind of supplies, that may fall be within the acceptable risk margin. Of course, people and other high value cargo can be serviced by other means if necessary.

>> you take enough fuel to get where you’re going with plenty to spare.

Then what is the point? 90+% of spacelaunch is lifting fuel, and the fuel you need to lift that fuel, and and and... all fuel. The whole thing is an exercise in moving reaction mass around using other reaction mass. Hauling around an extra 2km/s of deltaV would require a rocket on the pad double or triple the size of not bringing it. Spacelaunch equipment has to work every time or else it isn't worth the investment.

Even if we treat the tether as a potential 'bonus' rather than a necessity, think of all the fuel spent on the intercept with the tether. Your launch time would have to coincide with the tether, meaning you launch at a non-ideal time. And you would only every launch into the tether's orbital inclination/plane, radically reducing the number of potential destinations.

Tethers are about not treating launches as isolated events. So why are you still thinking about the rocket equation as a single interval?

You could imagine one company having a more reliable but expensive launch platform and another having a cheaper one that occasionally explodes, or has a smaller launch window. You'd fly up fuel on the cheap ones, and your deep space probe on the other. And any fuel left from previous missions can be resold or spent on good will.

Essentially instead of 3 stages to high orbit, or 4 stages to another planet, you have 7 smaller stages (2 + 2 + 3) to another planet.

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.

That is similar to the concept of the Aldrin Cycler.

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

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

Best read in chronological order.

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.

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.

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.

The momentum transfer is never perfect, so every tether station will need to have a propulsion system to boost it back to the base orbit. But since the tether is already in orbit, its station keeping system doesn't have to be chemical rockets. On top of momentum exchange, it can use various form of electric propulsion that are slower but much more efficient.

RE balance of incoming/outgoing boosts, that's why the article mentions the option of having the ship carry some extra fuel as payload to refuel the tether - because even with that, tethers still get you energetically and economically ahead over having the rocket fly itself all the way to destination, due to exponential nature of the rocket equation.

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.”

One option available: add momentum very efficiently on the tether.

Rockets have to balance their specific impulse (efficiency) against their need for massive amounts of thrust immediately. You can't take off from the ground using an ion thruster, despite what Kerbal Space Program tells us.

But a tether system in orbit could be using a very efficient engine to gain momentum over a very long period of time (months, years, etc). Give it large solar arrays or a small nuclear plant on-board. It's getting a much better exchange rate on that delta-V than the rocket, which needs to fight earth's gravity and air resistance.

You can trade time for delta-V.

There are 7 million seconds in three months. That's a lot of time for a Hall Effect thruster running off of solar power at 1 AU to boost the momentum of a tether.

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

> 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!!

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/

IEEE Spectrum did write an article about its closure [1]

[1] https://spectrum.ieee.org/energywise/energy/renewables/exclu...

I had the same thought. Makani had an interesting idea but the reliability wasn't there -- even if they had all of the problems worked out, the system having only one tether and such a fragile glider would have been inherently unreliable.

That said, it might be easier in space because there's less atmosphere to worry about (not nothing though, debris and micrometeoroids would probably pose a problem in the long run)

"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

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.

The tethers in Seveneves were a clear sign that things had changed.

My favorite part:

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

I think I would barf.

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

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

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

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.

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?

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

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

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

Well, you need a rotor and a stator. Alternators work by transferring energy to the stator, which is extracted as an electrical current thanks to it being held in place.

You cannot really "hold" something in place (barring an external propulsion system that costs energy), so while you could extract some energy at first due to inertia, you will slow the orbiter down and barely collect more (if any) than you needed to put it there in the first place.

An alternative but close system was described in Stephen Baxter's Sapce (Manifold series), with (IIRC) superconducting loops around Io, to collect energy from Jupiter's magnetic field, technically extracting energy from Io's momentum.

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

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

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

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.

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.

Watch the movie or doco on Dr Gerald Bull

It probably doesn't take anything special like a 50km railgun. We probably could have one by now if we had of just not stopped Gerald Bull's Canadian funding rather than assassinating him.

https://www.imdb.com/title/tt0109650/ https://www.youtube.com/watch?v=UvL9qcJKqOw

Very familiar and totally agree. I had plans to make a basic light gas gun a while back but never followed through...probably a good thing lol

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

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

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.

No loss, I’m already prepared to die before seeing square dancing spacecraft.

Reply on news.ycombinator.com