Space station artificial gravity - how to spin up to speed?

[sophiecentaur]

Let me rephrase it, too.
As long as your path is tangential and your speed is within fairly lax limits, the actual time of your arrival doesn't matter. I still get the feeling that you have not understood what I am describing. So many of your questions are based on colliding with the centre of the wheel. The centre of the wheel ( and the docking bay of the ISS) is a 'solid' object with little resilience. What I am discussing is nothing like that; it doesn't need to be because contact is 'outside' the station body. It can be flexible and allow for much more variation in position and speed. If you still have a problem, try drawing out the situation and do the same sums I have done, to work out the relative velocities and positions. The mechanical requirements are very much on the same lines as for aircraft landing gear. You do not seem to take that into account.
If you can tell me of a mechanism that can stop the ship from traveling in a straight line whilst drifting towards the station and how that line cannot be set up at a significant distance away then I could take on board what you are claiming.
I understand that the tolerances (lateral, alignment and speed) are very stringent for a normal docking. But the situation is really not the same for what I am describing. Why do you keep insisting that it is?

You'll be off by 15 meter in some random direction if your velocity vector is wrong in arbitray directions.

Why should it be, if all you are doing is aiming to keep to a line that's a tangent to the wheel? Do you not see how your time of arrival is of minor importance?

 

[A.T.]

all you are doing is aiming to keep to a line that's a tangent to the wheel

Which requires corrections for which you have limited time, because you have to approach the point of tangency at a fixed speed. At the hub you can approach as slowly as you wish.

 

[Baluncore]

Radiation pressure will work very, very slowly, to start with.

Yes. But I wanted to reapply riverboat paddle-wheel technology to the future.

I was also considering delivery of construction material by attachment while passing on the fly to a spinning loop, a polygon, or a star of carbon fibre cable with a great radius. Then using solar power to operate winches that symmetrically pull the construction modules towards the centre with the radius reduction and inherent angular acceleration.

It might not even have spokes, it might have low-g upper floors.

I think a docking hub with two spokes would be essential for access and construction. Note that transport of stores from the hub to the periphery would decelerate the 'station'.

The access hub of a rotating station is another possible application for the anti-twister mechanism.
https://en.wikipedia.org/wiki/Anti-twister_mechanism

 

[sophiecentaur]

Which requires corrections for which you have limited time, because you have to approach the point of tangency at a fixed speed. At the hub you can approach as slowly as you wish.

I have asked, in vain for some numbers here. How is it that the best artillery can achieve such high accuracy in placing shots under all the perturbations you get on Earth but you guys are claiming that course corrections are constantly necessary in the space docking scenario? What is it that can disturb the relative velocity of the ship so that a tangent to the wheel will be waving about? I realize that, over a longish period, there will be gravitational effects but will there really affect the motion, once the ship is a few km from the station? The objections I read have been a bit too arm waving and not quoting calculations or results of experiment. I am totally prepared to be convinced but the least you guys can do is to give me some numbers. PF threads don't work well on 'assurances' alone.
You can tell me as often as you like that a near zero velocity approach to the hub has its advantages. I accept that.

 

[Baluncore]

I don't think it is necessary to dock a delivery vessel to the rotating station to unload materials.

Delivery of most of the stores, material and crew to the rotating station should be by attachment of a modular delivery capsule to a long station tether while on a synchronous tangential flyby. The delivery capsule is pulled in, unloaded, then dismantled for reuse. The capsule would be assembled from a regular set of parts that are reused to fabricate more station modules.

Passengers in transit or crew on exchange could arrive and depart from the hub so as to save materials and conserve rotational energy. Exports should depart from the central hub. With time the station can grow in size while maintaining the same virtual gravity.

 

[A.T.]

I have asked, in vain for some numbers here.

https://en.wikipedia.org/wiki/Space_rendezvous#Rendezvous_phases

They currently have 45-90min for corrections over the last 100m. With your method they would have only a few seconds.

 

[sophiecentaur]

https://en.wikipedia.org/wiki/Space_rendezvous#Rendezvous_phases

They currently have 45-90min for corrections over the last 100m. With your method they would have only a few seconds.

Can you explain that reasoning? That argument would suggest that landing an aircraft would be too risky to undertake.
The fact that present docking uses a long period of time is no reason to suggest that the same limits apply to the alternative method. Position, velocity and time are all less critical. You do not need to have zero relative speed and the lateral tolerance could be a matter of metres. Please tell me what perturbations can be present to cause a problem. Does the 30m/s speed difference produce an incalculable uncertainty in the rendezvous point?

 

[A.T.]

That argument would suggest that landing an aircraft would be too risky to undertake.

An aircraft cannot hover / land slowly without using lots of fuel and be specially build for it. A spaceship can do this naturally.

 

[sophiecentaur]

An aircraft cannot hover / land slowly without using lots of fuel and be specially build for it. A spaceship can do this naturally.

What is the relevance to that remark? Aircraft land perfectly well and, but for the vagaries of weather and the military, they would have an even better record of making contact with Earth at high speed. If an aircraft could hover for free, are you sure it would wait for 45-90 minutes to land, as a spacecraft takes to dock? You are not putting logical arguments together here. I asked for some (quantitative) reason to suggest that the accuracy of manoeuvring would be as bad as you claim. If it works for aircraft the why not for spacecraft ? (Same potential cost of human life.)

 

[mfb]

Aircraft don't land on ceilings where they have to get attached within a fraction of a second. They are also quite sturdy - their mass is not as important as it is for spacecraft , so they can handle larger relative velocities and accelerations. Compare to spacecraft s hey have a very large region to land, even on aircraft carriers.

Aircraft that can land vertically do that slowly as well. They don't take an hour for that, but they also don't do a hoverslam where you reach zero velocity exactly on contact.

 

[sophiecentaur]

We may as well finish this conversation if you can't give me a serious reason that the errors would be as high as your intuitions tells you.

 

[Baluncore]

With today's navigation technology the docking process would be automatic and use minimum fuel.

But I doubt a visiting ship would ever dock physically to the periphery of a rotating station without causing huge stresses within the station structure as the centre of the rotating mass was relocated. The angular momentum of the station would immediately fall as the visiting ship attached and changed from linear to circular motion. With the arrival of the visiting ship's linear momentum, the rotating station would move onto a new trajectory.

Departure would have a similar set of disadvantageous changes to angular and linear momentum.

 

[sophiecentaur]

With today's navigation technology the docking process would be automatic and use minimum fuel.

But I doubt a visiting ship would ever dock physically to the periphery of a rotating station without causing huge stresses within the station structure as the centre of the rotating mass was relocated. The angular momentum of the station would immediately fall as the visiting ship attached and changed from linear to circular motion. With the arrival of the visiting ship's linear momentum, the rotating station would move onto a new trajectory.

Departure would have a similar set of disadvantageous changes to angular and linear momentum.

I agree in principle. I must say, I have been thinking in terms of a massive space station and a modest sized ship. But the impulses could be spread over time by suitable use of resilient mountings for the rail and capture 'arm'. It would not be practicable to have to warn the crew "Incoming!"

 

[A.T.]

If an aircraft could hover for free, are you sure it would wait for 45-90 minutes to land, as a spacecraft takes to dock?

If an aircraft could hover for free, then it would use this ability to land. And no sane person would suggest it should instead land by approaching a rotating platform tangentially.

 

[sophiecentaur]

If an aircraft could hover for free, then it would use this ability to land. And no sane person would suggest it should instead land by approaching a rotating platform tangentially.

Neither your comments nor mine on that particular topic are relevant to the question I have been asking about the quantitative practicalities involved. Sanity is not a parameter that affects the risks (apart from a maniac behind the wheel, I suppose) The fact is that aircraft land in thousands, every day, with pretty basic automation and their approach speeds are very much higher than I am suggesting and conditions are orders of magnitude more variable. So there is no basic principle that says it can't be done. There is no point in more comments against the idea unless they involve actual numbers or new insights - such as . . . .

With today's navigation technology the docking process would be automatic and use minimum fuel.

This has been my opinion about the idea. I looked at a Wiki page about docking in space and it actually showed a picture of some guy using a rangefinder, through the spacecraft window, whilst docking with the ISS. I would agree that, if that's the sort of navigation you have available then creeping up to the docking bay as slowly as possible would have to be the way. We would have to assume next generation (or the one after that) systems would be involved.

With the arrival of the visiting ship's linear momentum, the rotating station would move onto a new trajectory.

Now that's a new and relevant idea, not based on intuition. The only question there would be 'by how much?' and that would depend on relative sizes. A change of orbit would not have to be a disaster if the station were in its own band of operation and there would be momentum changes in the other direction when the ship leaves.
Something that struck me is that the station could be used as a construction platform for building big ships for big voyages. They would certainly need to be non-rotating and, I guess, so would the materials, throughout their passage through the station.

 

[A.T.]

So there is no basic principle that says it can't be done.

Nobody argued otherwise. But the mere physical possibly doesn't make it a good idea for a standard operating procedure, if there are safer methods.

 

[sophiecentaur]

Nobody argued otherwise. But the mere physical possibly doesn't make it a good idea for a standard operating procedure, if there are safer methods.

Hmm. That's not how I have been interpreting your posts.
Safety in such matters is based on numbers and actual risk and that's what I'm after at the moment. Your comments have been along the same lines as what people say who have a phobia of flying. Percieved risk etc.
Apart from telling me that it's hard to aim at the periphery of a spinning wheel in space, you have actually added very little. I assume that most of your objection is based on what wiki can tell us and, from what I have read there, the majority of the stuff on Wiki describes historical or present systems. Not relevant.
The word "safer" is an advertiser's term. You are safer standing ten metres from the edge of a cliff than standing five metres away but how relevant and what is the actual risk involved? The risk of some medical conditions according to lifestyle are often discussed without talking of absolute risk. That's no use if you want to know the relevance to your life. How does the risk of a bad tangential docking compare to the risk of failed takeoff or re entry? Anyone who wants to be an astronaut will be putting themselves at more risk of dying at work than your average office clerk I reckon they all must be batty.

 

[A.T.]

Safety in such matters is based on numbers and actual risk and that's what I'm after at the moment.

Fixed time for last X meters versus as long you need, just in case something takes longer. It's obvious which is more fail save.

 

[sophiecentaur]

Fixed time for last X meters versus as long you need, just in case something takes longer. It's obvious which is more fail save.

So I take it, you have no answer and that you did not read my last post.

 

[mfb]

A faster approach is so much more risky that no one ever did it to save an hour.
Going to the ISS currently takes several hours. Which is a huge improvement over the previous standard procedure which needed 2 days.

No one has specific numbers for impact and other failure probabilities for such a system. That would need development of such a system. I'm not aware of anyone even considering that at the moment. Which means the experts think it is so risky that they don't even bother evaluating the risk in more detail.

 

[sophiecentaur]

Non risky space travel is an oxymoron.

 

[mfb]

Everything has a risk. 100 years of R&D brought the risk to die in an airplane flight down to 1 accident in several million flights - so low that you can safely neglect it as customer. Let's see what decades of commercial manned spaceflight can do, once they start.

 

[sophiecentaur]

I agree that time will tell. Incidentally, the R&D times for space flight and terrestrial flight are really not that different. (60 yrs vs 100yrs). I think it's more a matter of money spent than time taken.
But are you suggesting that commercial space flight will always involve docking between craft which uses the same techniques that they use these days?
Perhaps we are talking at cross purposes and our timescales are just very different.
At the moment, though, space travel is very risky and I can understand that you look upon every manoeuvre as a potential disaster. The survival statistics are really pretty dodgy, up till now but the whole business is so attractive that people choose to forget that. A climate in which there are people lining up to take a one way trip to Mars is perhaps one in which the actual sums should be made more public.

 

[mfb]

I don't make predictions about "always". They would be as pointless as people in 1800 discussing how advanced mechanical calculators might be in 2000.
Who knows how spaceflight will look like in the distant future. Maybe we don't dock at all because the concept of spacecraft flying around is as outdated as mechanical calculators are today.

The only timescale where we can hope to get predictions right is the not so distant future. Rotating space stations are possible with today's technology (and dedicated R&D of course). Within the foreseeable future, docking will look similar to today. The details change, but the main concept does not: Spacecraft approach each other, connect to each other, establish a solid mechanical contact, and make it airtight and combine their pressurized volumes if docking is done for manned spacecraft .

 

[sophiecentaur]

Who knows how spaceflight will look like in the distant future. Maybe we don't dock at all because the concept of spacecraft flying around is as outdated as mechanical calculators are today.
The only timescale where we can hope to get predictions right is the not so distant future. Rotating space stations are possible with today's technology (and dedicated R&D of course). Within the foreseeable future, docking will look similar to today. The details change, but the main concept does not: Spacecraft approach each other, connect to each other, establish a solid mechanical contact, and make it airtight and combine their pressurized volumes if docking is done for manned spacecraft .

Did we ever disagree about that?

as pointless as people in 1800 discussing how advanced mechanical calculators might be in 2000.

In my experience, the majority of threads about space flight on PF are doing the equivalent of just that. You are being very pessimistic if you think that landing on the periphery of a rotating wheel couldn't be considered in the conceivable future. We wouldn't get far if we only did things 'the way we've always done them'. Your objections have been very backward looking from the beginning of this thread.

 

[mfb]

I'm sure someone studied that. No follow-up studies happened, at least none I would be aware of. Which usually means the approach was discarded as impractical.

It might become more realistic in the distant future. But then you still have disadvantages without advantages. I don't see the point.

 

[sophiecentaur]

I'm sure someone studied that.

Very likely but a citation would be useful - as for most PF topics. Their studies would, no doubt, have involved more calculations than intuitions. And that's all I am after.

without advantages.

Not so sure about that. Unless the central docking uses a lot of retro thrust at the last moment, there is a not inconsiderable delay for all arrivals. (In which case, your 'take as long as you like' argument doesn't apply and a failure would produce a serious crash*) In a tangential approach, you do not need to 'slow down' your linear approach speed and can more or less step off the ship onto the 'platform'. The deceleration is over in a couple of seconds. Mass transit type of process.
Of course, most of these ideas take us further and further into the future.
* The hub could be open and the ship could fly into the 'hole' and into an arrestor net; there's a possibly solution for every problem.

 

[A.T.]

In a tangential approach, you do not need to 'slow down' your linear approach speed

Why would this final delta v matter, given orbital speeds are several orders of magnitude higher? And all that linear impulse the tangential docking transfers to the station has to be corrected by the stations thrusters, so you don't save any fuel in total.

 

[sophiecentaur]

The effect on station orbital speed would depend on relative masses. It could be negligible. Also, when the ship leaves, the linear momentum would be back to what it was.

 

[sophiecentaur]

I read this again and I think I get the message this time. My point is that a constant approach velocity actually gets you there and at the right speed. The conventional method involves a gradual change in speed towards the final docking. (Zeno' Paradox at work ;-). This takes more time than what happens when you latch on to the rotating wheel. The approach involves no changes, once the right velocity is achieved. It's actually 'more' fail safe, involving, at worst, a miss or a glancing blow at low speed. Retro failure in the present system involves a major impact.