- #71
sophiecentaur
Science Advisor
Gold Member
- 29,066
- 6,945
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.
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!"Baluncore said: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.
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 said: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?
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 . . . .A.T. said: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.
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.Baluncore said:With today's navigation technology the docking process would be automatic and use minimum fuel.
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.Baluncore said:With the arrival of the visiting ship's linear momentum, the rotating station would move onto a new trajectory.
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 said:So there is no basic principle that says it can't be done.
Hmm. That's not how I have been interpreting your posts.A.T. said: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.
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 said:Safety in such matters is based on numbers and actual risk and that's what I'm after at the moment.
So I take it, you have no answer and that you did not read my last post.A.T. said: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.
Did we ever disagree about that?mfb said: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 .
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 said:as pointless as people in 1800 discussing how advanced mechanical calculators might be in 2000.
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.mfb said:I'm sure someone studied that.
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.mfb said:without advantages.
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 said:In a tangential approach, you do not need to 'slow down' your linear approach speed
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 said:Retro failure in the present system involves a major impact.
Oooh, I don't know about that. The ISS has done maybe 2.7 billion miles already without a fatality on board.sophiecentaur said:Non risky space travel is an oxymoron.
Orders of magnitude higher than the overall power consumption of the space station, probably.Al_ said:This would mean windage losses between the hull and the spinning structure
A vacuum-tight connector for different rotation speeds is certainly not the easiest component, but I would expect it to be possible.Al_ said:crossing between parts rotating at different speeds or in different directions would be much easier.
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.sophiecentaur said:You can't have it both ways.
~3% fatality rate for astronauts, not from in-orbit operation but from launches and landings.Al_ said:Oooh, I don't know about that. The ISS has done maybe 2.7 billion miles already without a fatality on board.
What is the rate so far for docking maneovers? I can't think of one fatality.mfb said:~3% fatality rate for astronauts, not from in-orbit operation but from launches and landings.
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.sophiecentaur said:I have already acknowledged that the time factor is not that important these days.
On the other hand, a navigational error or malfunction risks the same impact on a two lane highway every day of the week.csmyth3025 said: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. .
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.mfb said:~3% fatality rate for astronauts, not from in-orbit operation but from launches and landings.
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. said:You are obsessing about a non-issue.
You are conflating two independent issues:sophiecentaur said: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.
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?A.T. said: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
No you aren't:sophiecentaur said:I deal with the two issues separately.
Docking to the hub and peripheral approach are not mutually exclusive.sophiecentaur said:I wouldn't fancy speeding towards the hub at 30m/s and rely on stopping at the last minute. But, in the peripheral approach,...