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

[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.

 

[mfb]

Retro failure in the present system involves a major impact.

It does not, because the approach speed is slow. The spacecraft never approach each other head on at significant speeds. At points where the relative velocity is larger (still small compared to 30m/s), they don't have to fly directly in the direction of the station.

 

[sophiecentaur]

So they have to fly a course that will not collide, until they fire the retros and then they have to do all manoeuvring at safe, low speed (= a long time). You can't have it both ways.
I have already acknowledged that the time factor is not that important these days.

 

[Al_]

Would it be better to have the pressure skin of the station remain static, and just spin the internal structure?
I'm imagining a station that does not have spokes - it's shape is just a wide disc or even a sphere.
This would mean windage losses between the hull and the spinning structure, but it would have one big advantage -
crossing between parts rotating at different speeds or in different directions would be much easier. You would cross at the hub.
There could even be a zero-g section there for experiments, processes, manufacturing etc.

 

[Al_]

Non risky space travel is an oxymoron.

Oooh, I don't know about that. The ISS has done maybe 2.7 billion miles already without a fatality on board.

 

[mfb]

This would mean windage losses between the hull and the spinning structure

Orders of magnitude higher than the overall power consumption of the space station, probably.

crossing between parts rotating at different speeds or in different directions would be much easier.

A vacuum-tight connector for different rotation speeds is certainly not the easiest component, but I would expect it to be possible.

Putting a zero-g section into a separate rotating part within the space station would be easier than the other option: much lower wind speeds.

You can't have it both ways.

Right, you cannot have the same safety as in a slow approach if you approach the station quickly, no matter where you dock. No one ever questioned that. No one suggested a hoverslam approach to dock at the hub.

Oooh, I don't know about that. The ISS has done maybe 2.7 billion miles already without a fatality on board.

~3% fatality rate for astronauts, not from in-orbit operation but from launches and landings.

 

[Al_]

~3% fatality rate for astronauts, not from in-orbit operation but from launches and landings.

What is the rate so far for docking maneovers? I can't think of one fatality.

 

[mfb]

0%.

1967: Parachute failure during landing: One Soyuz cosmonaut dies.
1971: Decompression while preparing for landing (but still in space): All 3 in a Soyuz capsule die, still the only deaths in space so far.
1986: Challenger explodes shortly after launch: 7 die
2003: Columbia disintegrates on re-entry: 7 die

Some more people died in atmospheric test flights or during ground operations

 

[A.T.]

I have already acknowledged that the time factor is not that important these days.

When will it ever be? When space travel becomes like air travel today? Here approach, landing, parking and docking to gate also takes tens of minutes. You are obsessing about a non-issue.

 

[csmyth3025]

AL: You mention a disc shaped space station. This concept has been described in detail by Al Globus, et al ( http://space.alglobus.net/papers/Easy.pdf )

I'm not sure why your concerned with a reaction wheel. A spinning space station will continue it's initial spin with very little need for adjustments to its angular velocity. The movement of supplies, people air and water within the structure presents a negligible change in the distribution of mass compared to the overall mass of the station. Moreover, cabled counterweights (similar to those used for elevators) can easily be reeled in and out to compensate for any minor wobble caused by changes in mass distribution.

The notion that a spinning space station will be spinning while it's under construction is no more sensible than expecting that a passenger airliner will be flown while it's under construction.

Docking at a central docking port is much safer because the closing velocities can be kept much lower. All that needs to be done is to impart a spin to the docking spacecraft that matches the spin of the station. This assumes, of course, that docking spacecraft are purpose designed with a docking port aligned with one of its axes of rotation.

A docking craft that matches a tangential velocity of 30 m/sec necessarily approaches the station at a closing velocity of 30 m/sec (67 miles/hr). Any navigational error or malfunction that results in an impact will potentially result in the same catastrophic damage as driving a truck into a brick wall at 67 mph. .

 

[jbriggs444]

Any navigational error or malfunction that results in an impact will potentially result in the same catastrophic damage as driving a truck into a brick wall at 67 mph. .

On the other hand, a navigational error or malfunction risks the same impact on a two lane highway every day of the week.

 

[sophiecentaur]

~3% fatality rate for astronauts, not from in-orbit operation but from launches and landings.

3% is an amazingly high rate. Who (in their right mind - certainly not a PF member) would ever volunteer for any other activity with such a risk of death? The total actual number is small because there are so few participants. It's more risky than pretty much any other activity I can think of - apart from Russian Roulette. Base jumping, by comparison, is a stroll in the park.
The perception of risk is so dependent on subjective factors and the way the statistics are stated. The number of millions of miles is not a meaningful measure for any activity. People travel no miles at all when they are struck by falling objects on building sites and that is a very common form of accident.

You are obsessing about a non-issue.

I am not "obsessing" at all. I have already said that I accept the proper objections to the system. What I am objecting to is the skewed arguments against it. The perceived risk of a serious collision is so overblown and you are not comparing like with like. If you want to discuss retro failure then you have to consider it for both cases. How far away does your visiting ship need to be from a Massive space station before you can be sure of avoiding a collision due to thruster failure? How many minutes / hours away from docking does that represent? On the grounds of collision cross section area alone, a station that's ten times the cross section of the ISS would need ten times the docking time. Otoh, a tangential approach would reduce the consequences of a slight mis-registration because the closing speed could be less; the approaching ship would only need to deflect by a few metres to avoid a collision, compared with needing to veer off by the total radius of the station. All you have done is to apply existing 'rules' to the two possible future options. Is that reasonable? (Or not obsessional)

 

[A.T.]

On the grounds of collision cross section area alone, a station that's ten times the cross section of the ISS would need ten times the docking time. Otoh, a tangential approach would reduce the consequences of a slight mis-registration because the closing speed could be less; the approaching ship would only need to deflect by a few metres to avoid a collision, compared with needing to veer off by the total radius of the station.

You are conflating two independent issues:
A) Docking to an inertial part vs docking to a non-inertial part
B) Approaching while aiming the center vs the periphery

 

[sophiecentaur]

You are conflating two independent issues:
A) Docking to an inertial part vs docking to an non-inertial part
B) Approaching while aiming the center vs the periphery

I deal with the two issues separately. I think you are assuming that all the same problems apply to both methods. They don't. Of course, I wouldn't fancy speeding towards the hub at 30m/s and rely on stopping at the last minute. But, in the peripheral approach, I can be going 30m/s faster than the station CM and be at just the right speed and on the right course to latch onto the rim. It doesn't matter when the actual contact is made because there is always a piece of the periphery right next to me as I go past. Why ever wouldn't a half decent nav system do that for me?
Aircraft manage it every day so why do you see it as such a problem - apart from an understandable gut reaction?

 

[OldYat47]

Interesting thread. I was struck by some of the assumptions around peripheral docking. Let's suppose that you have an approaching craft that will synch with the docking port. That is, the linear velocity of the approaching craft is equal to the tangential velocity of the docking port. The craft lines up and "grabs" on to the port as the two line up. But the docked craft must be accelerated (centripetally) to make it travel in a circular path. That costs angular momentum. Alternately, the approaching craft could use its own navigation system and rockets to make it travel in a circular path of decreasing radius until docking is achieved. Sounds energy intensive. Either way, unless some mass balancing takes place very quickly the mass of the docked craft will cause the center of rotation to move. The more massive the docked craft, the more it will make the center of rotation wobble. I think the 2001 docking method is the most practical.

 

[A.T.]

I deal with the two issues separately.

No you aren't:

I wouldn't fancy speeding towards the hub at 30m/s and rely on stopping at the last minute. But, in the peripheral approach,...

Docking to the hub and peripheral approach are not mutually exclusive.