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...
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.
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.
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.
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.
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.
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.
“ 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.
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.
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.
> 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.
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.
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)
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].
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.
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.
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.
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.
[+] [-] hinkley|5 years ago|reply
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|5 years ago|reply
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.
[+] [-] elihu|5 years ago|reply
[+] [-] restalis|5 years ago|reply
[+] [-] sandworm101|5 years ago|reply
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.
[+] [-] unknown|5 years ago|reply
[deleted]
[+] [-] karmicthreat|5 years ago|reply
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|5 years ago|reply
[+] [-] unchocked|5 years ago|reply
http://hopsblog-hop.blogspot.com
Best read in chronological order.
[+] [-] baq|5 years ago|reply
[+] [-] jp57|5 years ago|reply
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|5 years ago|reply
“ 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.”
[+] [-] mabbo|5 years ago|reply
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.
[+] [-] TeMPOraL|5 years ago|reply
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.
[+] [-] hinkley|5 years ago|reply
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.
[+] [-] gkfasdfasdf|5 years ago|reply
[+] [-] mcdoh|5 years ago|reply
That's not ironic at all!!
[+] [-] aaron695|5 years ago|reply
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/
[+] [-] jpm_sd|5 years ago|reply
https://despair.com/products/mistakes?variant=2457302467
[+] [-] sohkamyung|5 years ago|reply
[1] https://spectrum.ieee.org/energywise/energy/renewables/exclu...
[+] [-] sleepy_keita|5 years ago|reply
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)
[+] [-] jessaustin|5 years ago|reply
[+] [-] kaonwarb|5 years ago|reply
> Let’s imagine a modestly-sized tether orbiting at a high altitude above the Earth. It is 1,000 km long...
[+] [-] matterbeam|5 years ago|reply
[+] [-] swader999|5 years ago|reply
[+] [-] kristianp|5 years ago|reply
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/
[+] [-] ncmncm|5 years ago|reply
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|5 years ago|reply
[+] [-] _Microft|5 years ago|reply
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
[+] [-] MayeulC|5 years ago|reply
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.
[+] [-] remarkEon|5 years ago|reply
[+] [-] minitoar|5 years ago|reply
[+] [-] rini17|5 years ago|reply
[+] [-] unknown|5 years ago|reply
[deleted]
[+] [-] jcims|5 years ago|reply
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.
[+] [-] aaron695|5 years ago|reply
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
[+] [-] ncmncm|5 years ago|reply