Moonwards

kim holder

02:50

oh dear, disqus seems to have eaten that long reply i wrote...

kim holder

03:02

1. tether mass, like the calculator on the learn page

2. i saw a post on chris wolfe's blog that he likes the idea of using Spectra/Dyneema. maybe we could include that as a material option in analyses too. I should check into it more.

3. something about the engines needed to have sufficient station-keeping ability, and the fuel needs of them. that will require a healthy dose of estimation, i guess.

that doesn't need to be precise, of course. it just needs to be a reasonable estimate, probably better it be somewhat on the conservative side.

The big thing about that, is calculating the dip in COG when a shuttle grapples the foot. And now that i'm getting warmed up, it occurs to me you probably need a bunch of information from me about sizing of things, and then that needs to be refined if you see problems with it, and really a bunch of things would need thinking through in order to put in figures.

I think the foot platform needs to be rather elaborate, and heavy. I thought that would help some with stability, to have a weight on the end basically, but am i wrong?

But it needs to be able to transfer stuff, have engines to dampen oscillations, have a grappling area that is as large as is reasonable, maybe it would be a good idea to make it able to telescope, maybe there are some other things it ought to do.

It ought to have big cameras constantly taking images of the surface, for instance.

Yeah, this is going to take some consultation... are we going to say there are power lines running through the cable?

Should we put two cars on the cable from the outset, so the COG can be kept in the same place if desired as cars go up and down?

The hoisting idea doesn't seem good to me for a cable this length, and i'm not sure it is a good idea in the system Eagle proposes, but i'm open to being educated on that.

kim holder

03:30

actually i'd like to know your opinion of that proposal. Graphite fibers in a thermoplastic matrix? That doesn't sound ideal, it sounds heavy and stiff. He talks about using hot platens to patch the cable when damage is found, by pressing on new material. That also makes it sound like the cable is stiff and heavy. I don't know, look at it if it is interesting to you...

Hohmannfan

08:49

From the kittens game code :)

1. Tether mass: Yes, in the process of implementing it.

2. Collecting data

...done

3. COG. Yes, scaling. I need numbers.

Tether stress on shuttle docking is purely vertical. Climbing on the other hand...

Power lines? I hope not.

Remember we are talking about hundreds of kilometres here.

kim holder

14:55

somewhere you made some reference to having something like that in the cable. i can't find it again now. it must have been referring to something different.

i stumbled across it when i was looking for where we started discussing vertical tethers in order to respond to Eagle

so, would having a mass on the end aggravate oscillations caused by the climbing car?

i thought it would keep the cable more taut, which would dampen oscillations. but also you say it is easy to compensate, so maybe it doesn't need to be thought about.

let's see, what is a reasonable size for a shuttle, once the tether is fully developed...

10 ton payload?

kim holder

16:00

yeah, maybe that is good. it needs to deliver food for up to a few hundred people, at the peak of its duty.

the cargo ship is sized as having a maximum payload of 40 tons.

anyhow, it is round numbers, and so much needs analysis still, in order to shape the whole colony.

so, from your forum thread on nuclear shuttles...

Hohmannfan

I have concluded tether mass can not be found analytically. You have to solve an integral of the form e^(a+b/x+c*x^2), and WolframAlpha says that is not doable.

I need to numerically iterate over tether lengths to find an exact mass instead.

But for now I am going to use the following approximation:

kim holder

yeah, i think you ran into that the last time it came up and did something like that

Hohmannfan

(I assume taper ratio to be linear over the tether)

This approximation will always give a slightly higher mass, but is very close for low taper ratios.

Ok, now the first mass estimates are pushed to the repo.

Be a little careful with the Zylon numbers, I used ultimate tensile strength instead of yield strength.

kim holder

where do i find that?

Hohmannfan

/moonwards.com/tools/teth.html

After you build it, of course.

kim holder

16:14

ah.... the building... :P

Hohmannfan

Actually, be a little careful with all the numbers. It is not like anything is tested :P

kim holder

should there be a fudge factor to account for that?

Hohmannfan

Try to input the parameters of some reasonable tethers. If the numbers are not making sense there are errors.

Comparing with the spreadsheet should also help to find bugs.

kim holder

yeah, it looks good

Hohmannfan

It needs more stuff. What do you want?

kim holder

16:23

i sort of wanted to put this stuff in the spreadsheet, so it is easier for me and others to look at the formulas

but then, that could be made the title of the fields on the page, or something like that. or in a details tag.

Hohmannfan

The thing is, spreadsheet formulas are unreadable. Bugs will happen and be impossible to find.

kim holder

i hadn't expected you to create a whole finished page. it of course needs to go on the site today :]

Hohmannfan

No design done yet though, it is just a list.

I am creating a bunch of unfinished stuff :D

kim holder

the design part isn't a big deal. it already looks fine, it just needs to be formatted with the site's standard design.

so, is the next logical step to think about how much the orbit lowers when a shuttle is grappled?

Hohmannfan

Yes.

kim holder

16:27

in that case, the mass of the anchor, and the shuttle, and the platform, all need defining

i maxed out what i thought a committed program could reasonably fetch and place in lunar orbit as something massing 7500 tons

Hohmannfan

But even setting the centre altitude as an input parameter is a bit of a stretch. That assumes the central station is significantly more massive than the tether if you are not guessing the right altitude.

7500 tons?? That is like... 2 1/2 Saturn V's

kim holder

the Keck report figured they could go get something up to 1500 tons

Hohmannfan

Is this 'something' an asteroid?

kim holder

it's an asteroid, all you need to send out is a ship that can slowly nudge something into orbit

Hohmannfan

Ok then. I thought you was talking about supplies from Earth.

kim holder

16:31

yes. dear me, hoh, you do all this work on this, and you haven't actually read the website?

oh - a misunderstanding

Hohmannfan

I read the website occasionally, mostly in source form.

kim holder

so, that is enough to make the center of mass pretty close to the asteroid, right, meaning the error introduced by estimating isn't that much?

Hohmannfan

Yes. You just say: "I want the centre of mass to be there" and it works out.

One more thing

It is not really the centre of mass.

It is what part of the tether has orbital velocity.

kim holder

ok, i kind of understood that, but that is clearer

the payload specified in the timeline right now for the shuttle is 10 tons. what delta v should it have for a round trip to the tether foot...

Hohmannfan

What was the tether specifications again?

kim holder

16:37

(i don't know how that can be explained and referred to on the site in a way that is clear - i should work that out.)

right now, it is sort of a cycle - i am guessing 50 km will be sufficient for a safety margin, which means a delta V of 820 m/s if 20% of the fuel is for maneuvering

Hohmannfan

Sounds about right.

kim holder

that is from the spreadsheet, it should be right

Hohmannfan

It is the 50-5000 thing, right?

kim holder

yeah

Hohmannfan

=sqrt((M6^2)+(2*C6/A6)-(2*C6/(A6+(K6*1000)))) can be simplified to =sqrt((M6^2)+2*C6/A6-2*C6/K8)

kim holder

16:50

oh - that's why you made that field...

Hohmannfan

Spreadsheet formulas are unreadable. You have to look up every single one of those C6, A6, M6...

kim holder

hm, the result changes when i do that...

Hohmannfan

Because K7 contains nonsense: =K6^2+E6*2^2 what??

It should be =K6*1000

kim holder

i thought that was something you put in... it was finding the center point of the tether, and i thought you figured that would be where the structure would be going at orbital speed

that's why i put in the note

so, change that then?

changed...

Hohmannfan

That is defined in E6 as user input.

kim holder

17:00

yes... i don't know how that field came to be then...

i do find spreadsheets helpful in the sense that even though i have to check around what each connected thing is, it helps me get a sense of the set of connections that make a system

moonwards.com/forum/index.php?mode=thread&id=93

so, that approach of using water for launch and landing?

Hohmannfan

Yup, still good.

That is optimizations part 2

kim holder

ok

Hohmannfan

Not yet written but important.

Though the current lower delta-v budget means that water-all-the-way is perhaps the best option, assuming propellant is mined on the Moon.

But just maybe.

kim holder

hm, interestng

i'd looked at the LANTR concept of using LOX afterburned during launch and landing, but that adds complexity and so i guess is no advantage

Hohmannfan

I can find some numbers.

kim holder

17:15

the shuttle doesn't currently need a lox system, one would have to be added

and it could break, which would be quite hard to repair remotely i expect.

considering that this version is for the first tether, when people aren't there yet.

i had just found it interesting. i thought i should mention it, but i don't know that it is worth you having to look all that up at this point.

water-all-the-way also means no splitting of water into hydrogen and oxygen, and no storage of cryogenic hydrogen.

that sounds like a big advantage for a remotely operated system

Hohmannfan

Numbers! :

Water mass ratio: 1.44

Hydrogen mass ratio 1.18

Water equivalent launch mass: 1.78

(That is, a shuttle with dry mass 1 using hydrogen could have a dry mass of 1.78 using water.)

kim holder

hm...

Hohmannfan

Equivalent hydrogen usage: 0.27

So we are actually saving fuel using water and can have a larger payload on the same rocket.

kim holder

wow!

we're geniuses!:D

i mean... i guess sort of you are the genius...

Hohmannfan

And as you said, we can avoid cryogenic storage and water splitting

kim holder

17:30

i really am starting to love this system. We can start with the ARM system that goes and gets the asteroid, which has already been documented in detail and there is no argument it would work.

Hohmannfan

I guess a nuclear/LH2/LOX system could get a little better performance than nuclear/water, but it would be 10x more complicated.

kim holder

do i understand correctly as well, that unlike chemical engines, a nuclear engine can be restarted many times with no trouble?

Hohmannfan

Well, they have the same troubles.

Except nuclear engines do not care about ignition and such.

I can also see problems with balancing neutron poisons. Perhaps you will have a situation like "No, we can not launch the shuttle in a week after use".

kim holder

oh? that's interesting, i didn't know about that.

can the water fuel be used to reduce that?

Hohmannfan

Nope, that is only dependent on the reactor.

kim holder

17:37

well, it will be quite a while before the shuttle needs to launch more than once a week. as long as it isn't much longer than that

Hohmannfan

"A week" was just an example. I have zero information about how you manage a high power reactor operating in short power spikes.

kim holder

i'll put in on the wackyTech list of things to look into?

Hohmannfan

It is a potential complication.

Perhaps I have an answer after fifteen years and a phd in nuclear engineering.

kim holder

the thing would be to look for information, it must be out there

but for now i suppose we could say something like the engines can be swapped out if needed for another one

Hohmannfan

My guess is that many of the sequential tests in project Rover/Nerve was to explore things like this.

I can ask a question on SE later.

kim holder

17:45

oh, good point.

ok, so for a shuttle with a payload of 10 tons... and you figure the engine would be about 2500 kg?

that was for that forum post, but perhaps in this situation it is now oversized

although, for a few missions it will have to go right to orbit, because the tether doesn't exist yet

Hohmannfan

2500 kg is about the minimal mass of a nuclear engine. You could cut back the thrust a lot but that will just reduce the mass by a few hundred kg. Making the engine a little larger will give a lot more thrust though.

kim holder

well, that is probably a good idea.

Hohmannfan

10000kg payload is about the first stage of the lunar lander. That is smallish.

My opinion: larger payload.

kim holder

i sized the cargo ship to have a payload of 40 tons

the shuttle was going to need to empty it in a few trips

Hohmannfan

The question: Why not just one?

kim holder

17:55

yeah, i just thought that was too much to ask

40 tons of robotic equipment and supplies just seems like a wonderland. that's what the early missions have to haul.

Hohmannfan

So, a shuttle carrying 40 tons, and perhaps a much smaller LH2/LOX craft for transportation of crew?

kim holder

here is what the timeline says right now, it is probably the fastest way to discuss this

wait, let me update the live site

moonwards.com/machines2.html#PodShip

actually, it seems to be there

erm...

Hohmannfan

"Uses an oxygen 'afterburner' to increase thrust during launch and landing" is in the timeline though.

kim holder

yeah, i haven't taken it out yet. i just noticed that.

it was all just updated, but it doesn't have that change yet

Hohmannfan

Replace it with something about water.

kim holder

18:02

yes

there, how is that?

Hohmannfan

Yup.

All time, you have 1 in 7 posts in The Pod Bay. But counting this room as well you have 1 in 5

kim holder

:P

Hohmannfan

The pod bay tags are strange, but still so oddly relevant canada cheese hand-drawn-circle-snobs procrastination space unobtuse-alcoholic-ducks

kim holder

alright, we have an awesome shuttle that hauls a 40 ton payload. its engines mass let's say 3 tons and the rest of it could be 7 tons, for a total mass loaded of 50 tons?

TW began that approach, i've always liked it

Hohmannfan

18:17

Yeah, and multiply that with 1.44 to get the gross launch mass, about 75 tons.

kim holder

75 tons, that sounds good. Look, a harmony with the anchor mass - it is 0.1% its mass. :)

Hohmannfan

room topic changed to Moonwards: about moonwards.com and the Virtual Moon Colony project [basalt] [colony] [tethers] [the-moon]

kim holder

did you search for the most common words?

Hohmannfan

No, but perhaps tags makes it easier to find when browsing the rooms.

kim holder

yeah, i just wondered about the particular choices. you are right that it needed to be done.

Hohmannfan

18:22

Nothing particular in mind behind the choice.

kim holder

it should probably have space settlement or something

Hohmannfan

room topic changed to Moonwards: about moonwards.com and the Virtual Moon Colony project [colony] [nuclear-engines] [settlement] [space] [tethers] [the-moon]

kim holder

that looks good

Hohmannfan

What happens if I add items to the schedule? This looks a bit scary:

"Because you are a moderator there, an event that you schedule in this room may be advertised on space.stackexchange.com and tweeted by the site's Twitter account."

Free but questionable advertisement.

kim holder

based on what happened when i scheduled things for the pod bay, it is no big deal

Hohmannfan

18:26

What does "advertised on space.stackexchange.com" mean?

kim holder

the site's twitter account is sort of a black box. it apparently selects things according to an algorithm that is very hard to figure out

@Hohmannfan it posts an entry in the sidebar at the top

very similar to the 'featured on meta' box

so, is there any point to making a foot platform that is more massive than necessary, or is that not an advantage?

Hohmannfan

No advantage of pure mass. But you can fit a lot of useful equipment there. The acceleration is even less than at the surface.

So if we have a release version of the website in the future we can get some traffic and a ban? Sweet :P

kim holder

oh, if SX graduates, we can apply to have an ad :]

13

A: Community Promotion Ads - 2017

Hohmannfan

I saw it linked from that other room earlier.

kim holder

whenever i pull it all together for it to be time to promote, i'll raise my profile on SX in order, partly, to generate traffic.

Hohmannfan

18:38

I have to test the event system. I am going to add the next Soyuz launch to the pod bay. It is a little unusual after all.

kim holder

sure. i added a timezone app to my phone because i made so many mistakes in the time

but that's me...

they usually garner little attention, those posts. When i was first on the site, i found them useful in that they made me aware of other things in the space exploration scene online.

community ads i believe are one of the cunning ways SE makes it worth the time of talented people to make big contributions to the sites here, as it can lead to goodwill in the community, that leads to votes that allow you to post a very well-targeted ad on a popular website.

okay, the first event will start displaying in the sidebar tomorrow - it does that starting a week before the event by default.

there is another event scheduling tool available only to moderators that controls that, i couldn't tell you any more how to find it

chat room events are automatically hooked up to it, but you can override it there, to determine if it is going to show on the site home page

as room owner, i can also schedule events in the pod bay, or here.

if you want to explain what the events actually are, and how to watch them, you have to add a post in the chat room that is just about that

the event system was not actually designed to be a way to bring the community together to watch something, TW just used it that way, and so it takes some extra steps to make it work.

you can't put a link to the site with the stream in the schedule, you have to post it in the chat room.

Hohmannfan

18:56

Yes.

kim holder

i gotta keep my post count up so my domination doesn't slide :D

ok, so, the thing is you are saying oscillations aren't going to be big and can be dampened with thrusters?

Hohmannfan

Dampened with small weights, I believe.

kim holder

ah - slide them in the opposite direction to motion?

right

Hohmannfan

A little like that.

But keep in mind that west means up, up means east, east means down and down means west.

kim holder

move mass up to create counter-motion to the east, et cetera?

no, wait, i don't know what you are saying there.

Hohmannfan

19:11

More like that to compensate for an attached shuttle you move a weight down, forcing the tether to have a higher velocity to conserve momentum, raising the periapsis again.

Low centre of mass? Move stuff down! That sounds counter-intuitive :)

kim holder

oh - i wasn't thinking about orbital motion here, i was thinking about the tether beginning to swing

Hohmannfan

Move stuff up below the centre of mass: swing east.

Move stuff down below the centre of mass: swing west.

kim holder

oooh...

why does that happen, is it a coriolis thing?

Hohmannfan

exactly.

kim holder

but you say that in that 200 km tether idea doesn't need to worry about oscillations. Can the motions be balanced out somehow over the course of an orbit or something?

Hohmannfan

19:18

Oscillations can be killed, yes.

kim holder

because in that system there is only the end stations, there is no counter-weight system, or a second car.

Hohmannfan

Meh, add smaller cars to taste. But with an asteroid that big it does not mean much.

kim holder

yeah, i'm just anticipating the reply to what i posted yesterday, about that Earth-orbiting system. i messed up some stuff, and i expect he will reply.

his system doesn't have those kinds of options, so i don't know what his plan is about oscillations

Hohmannfan

5.75 Gm²s⁻¹ for the centre altitude, so about 4*10^16m²s⁻¹kg as a momentum bank using the asteroid.

kim holder

the lunar system i'd think can almost ignore swinging, until you want to berth a shuttle to the foot platform, in which case you want it to keep still

please, continue...

Hohmannfan

19:28

400 Mm²s⁻¹ at the foot, so hoisting 40 tons is

2,14×10^14 m²s⁻¹kg of that.

If you do it in one go.

kim holder

about 1/200th

and if you don't use a car moving in the opposite direction to keep the COG in the same place

Hohmannfan

Yes. That is a orbital slowdown of about 4m/s without a counter-car. I am not sure how that translates to a periapsis drop, but it is not a lot.

kim holder

Metre squared per second

Metre squared per second or square meter per second is the SI derived unit of kinematic viscosity and of specific relative angular momentum. The unit is written in symbols as m2/s or m2·s−1 or m2s−1. == Specific relative angular momentum == Specific relative angular momentum is defined as the cross product of the relative position and relative velocity. The unit may be better understood when phrased as "metre per second times metre", i.e. the velocity of an object with respect to a position....

that is a funky unit

Hohmannfan

Specific relative angular momentum

See also: Classical central-force problem In celestial mechanics the specific relative angular momentum h → {\displaystyle {\vec {h}}} plays a pivotal role in the analysis of the two-body problem. One can show that it is a constant vector for a given orbit under ideal conditions. This essentially proves Kepler's second law. It's called specific angular momentum because it's not the actual angular momentum ...

What about a car with a specific relative angular momentum moving up a tether at a constant speed? That means the specific load on the tether is measured in ... meters. erhm. what?

kim holder

:}

Hohmannfan

19:38

The measure of acceleration potential / tether material strength is funky in itselft, m²/s²

kim holder

50 km is 1/100th of 5000 km, so if the whole thing is proportional, it is more than enough to cause the foot to hit the ground, if nothing is done

and it would only take a few hours

Hohmannfan

why?

Equation coming in ~10mins

kim holder

i'm just casting about. i don't feel able to sit down and work through the math to learn how it works right now. but when i work out where the altitude is that has an orbital speed that is 4 m/s faster on the spreadsheet, it is about 65 km lower

and i just thought, that would be where the periapsis would move to, which would be located on the opposite side of the moon from where the shuttle grappled the tether foot

Hohmannfan

19:59

Your method for finding the new altitude is indeed correct.

So, counter car needed for large payloads like 40 tons.

kim holder

disq.us/p/1h6kbro

i think i'd say the reply to my questioning of the 200 km vertical tether did not go well

long, though. i wish it had numbers.

so, i wonder would it be worth it to do something like send mass up the outer tether to compensate for the arrival of a payload

i guess there is no need. the car that moves down can slowly move back up again if there is nothing you actually want to send to the ground

Hohmannfan

Burning fuel at different altitudes has a different effect on the total momentum. That is kinda funny.

kim holder

that's Oberth, right?

Hohmannfan

So, moving water up the tether from the Moon as a propellant should be hoisted some distance, but not too far :)

No, not Oberth.

Something something conservation of angular momentum.

The momentum gain is proportional with the radius.

So higher up is more effective

But hoisting has a quadratic momentum cost...

... so somewhere on the tether the linear and the quadratic functions are equal.

That depends on the exhaust velocity of the propellant used.

kim holder

20:16

Hop is going to adore this

Hohmannfan

So ion engines should be situated pretty high

But, I expect the downmass of the tether to be larger than the upmass, so you have to slow it down.

kim holder

but they can't be above the asteroid, can they? wouldn't it be a problem that they are on a flexible cable?

Hohmannfan

They can be above the asteroid.

The flexible caple will move west... causing the tether to go up.

kim holder

that isn't lost in bending?

Hohmannfan

Nope.

The bending is the whole point.

kim holder

20:18

oh... i think i can imagine that...

it is pulling up on the whole thing, while also pulling west

Hohmannfan

It is like burning retrograde in an orbit is the most effective way to get back to the planet, instead of pointing straight down.

kim holder

yeah, but spaceships can be treated as a point

Hohmannfan

Tethers can be treated as confusing momentum

By the way, the momentum difference centre-top is twice that of foot-centre.

kim holder

i would have thought a retrograde burn at the top of an outer tether would create slack in the cable

Hohmannfan

Yes.

If you do it instantly, you get a lot of nested elliptical trajectories.

Think of it as doing it gradually, like an ion spiral

In fact, exactly like an ion spiral

kim holder

20:23

ahhh, alright, that is sort of approachable

Hohmannfan

"Fast" burns will change the eccentricity, just like in a Hohmann transfer.

The same thing with "fast" hoisting

Observation: The best way to slow down the tether with propellant produced on the Moon to compensate for downmass would be to hoist it all the way to the top and burn it.

kim holder

no need for ion engines?

Hohmannfan

Ion engines are perfect!

kim holder

but they don't burn water

Hohmannfan

They would be at the very top, using propellant from Earth.

(assuming downmass > upmass)

In the case downmass < upmass, they would be a little further down, I think.

But that depends on the ISP

Anyway, are we sure downmass > upmass?

kim holder

20:27

well, the idea is for it to be the other way around, with time

Hohmannfan

But then we are talking 50 to 60 years, right?

kim holder

yes, that is probably a good estimate

it is hard to see how upmass could be more than downmass until the construction industry is large.

but hell, you can just send up mass and tack it on to the anchor mass

Hohmannfan

^

You get 9x more momentum change by hoisting water to the top and burning it than to hoist it to the centre though.

kim holder

what do you mean by burning it though? with what, a shuttle?

Hohmannfan

NTR at the top is the number I used.

kim holder

20:34

i don't understand why you'd want ion drives and that too

Hohmannfan

Just an example of what you can do with the mass instead of tacking it on the anchor.

kim holder

oh.

Hohmannfan

But the benefit would be larger for extending the tether beyond the 9500km required for an Earth transfer.

Hmm. A little faster transfer to Earth should not increase the atmospheric entry velocity that much too... I need to investigate that as well.

kim holder

there is no need to let go of the tether at the very end

Hohmannfan

Further out than the minimum gives a faster transfer.

And letting go before the minimum should not be very expensive in terms of delta-v

kim holder

20:38

i think of the upper tether as being a ferry that carries ships that were berthed at the anchor up the tether and lets go of them wherever is most convenient

a faster transfer would be good for people, that would seem to be the only case

Hohmannfan

A very rough estimation suggests hat he outer tether is at least 10x less massive than the lower one.

kim holder

so, if we want an efficient way to get the foot of the tether up before it hits the ground, would it be better to have a little nuclear thermal engine at the tether tip fire engines?

because i was sort of hoping to put the foot even closer than 50 km

Hohmannfan

Or a car moving from the tip to the centre (or foot)

kim holder

tip to center is more efficient, is that also true?

because you have a few hours to do this, and i don't know how fast the cars can move.

Hohmannfan

The more massive the car, the slower it has to move.

But the tether only slows down when the payload starts to climb

Worst case is that the payload has to climb very slowly to the centre.

kim holder

20:43

oh - the shuttle's mass at the foot makes no difference?

Hohmannfan

A little, but not nearly as much.

kim holder

because it might be there for a while, as payload is switched and such

hm, i misunderstood then.

the 4 m/s penalty is what happens when the 40 tons has moved the entire distance - right?

Hohmannfan

Yes.

kim holder

ah, well, that's easy...

Hohmannfan

But the force of the shuttle hanging is down, meaning we get some eccentricity.

My head hurts when trying to think about how eccentricity even works in conencted elliptical orbits.

The best I can think of is to have the tether initially in a slightly eccentric orbit before the docking.

kim holder

20:49

actually, in order to send the shuttle back to base, it has to be docked for almost a full orbit.

that makes a ton of sense, make the orbit apoapsis on the opposite side from the base, and when the shuttle docks, it becomes circular.

Hohmannfan

90 degrees actually

Because this does not make sense at all :)

kim holder

one day.... :]

Hohmannfan

Wait, somewhere between 90 and 180 degrees, because of the connected orbits.

kim holder

when fuel supplies arrive for the ion engines, the ship will dock at the anchor, and the xenon or whatever will be taken up the cable to the engines. which is a net gain in momentum...

right...?

Hohmannfan

My brain did that wrong. The docking is not instantaneous force, it is continuous afterwards. It is almost 180 degrees, as you said.

kim holder

20:54

not very much, it just occurred to me

@Hohmannfan well, that was not because i logically thought it through :P

Hohmannfan

But it is an actual intuitive mechanic regarding momentum! That is incredible...

@kimholder Or perhaps move a weight up the tether before the docking, compensating with xenon, and then move the weight down when the payload starts to climp

It gets better, you can move the weight in a "fast" fashion multiple times at the same place in the orbit to create the initial eccentricity.

kim holder

how fast are we talking about?

Transcript for