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

[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.

 

[OldYat47]

Continuing my idle observations, counter rotating parts means that some form of bearing surfaces are required. Even if the total mass is all "payload" the inevitable friction at whatever joint is used will take some angular momentum to overcome. Even rotation of the entire mass in one direction will have to be "adjusted" by some energy inputs over time.

 

[A.T.]

Aircraft...

See post #74.

 

[A.T.]

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. .

The other half of the problem are the requirements on the docking mechanism: It has only a fraction of a second to engage and will be immediately loaded with a huge force, so a incomplete engagement would rip it apart.

 

[sophiecentaur]

The other half of the problem are the requirements on the docking mechanism: It has only a fraction of a second to engage and will be immediately loaded with a huge force, so a incomplete engagement would rip it apart.

Have you never come across the way an arrestor wire on an aircraft carrier works? It can stop a medium size bomber at 150kph. They have a far shorter window for making contact. And what are there relative speeds involved in the tangential docking? Also, what "fraction of a second" is involved? The ship and periphery are traveling at the same tangential velocity and the lateral rate of separation is not high. I was originally assuming they would use 1g but it seems that a lot less than that would be used - reducing the tangential speed to perhaps 10m/s. Where is the big deal in that? The maximum acceleration needed from the suspension would be the same as the station's g level. The actual numbers are important here, as ever.
You are now finding other partial arguments rather than dealing with the numerical questions I have been asking.
People have brought up the problem of wobble. That could be relevant for a big ship and a small station mass but I have been assuming a big mass ratio and the wobble would be no worse than effects on a large ship at sea, these days. Subjectively, it would be very much like dealing with conditions on board ship and, actually, much more predictable than when making way through an irregular swell at sea.

 

[A.T.]

Have you never come across the way an arrestor wire on an aircraft carrier works?

Would arrestor wires exist, if aircraft could hover for free?

 

[sophiecentaur]

Would arrestor wires exist, if aircraft could hover for free?

Irrelevant because it doesn't answer my response to your point. You keep doing this, instead of answering the actual question. You implied it can't be done. I pointed out that it can, Then you say it's not necessary. That isn't a conversation, it's a Trumpism.

 

[mfb]

It can be done. It just adds unnecessary risk, needs more delta_v capacity, and would need literally many billions of dollars of development costs for a fast secure docking system.

According to this website, logging had 132 fatalities per 100,000 in the US in 2015, this website says 111 for 2014. Over a 30 year career, that leads to 3-4% fatality rate.

The 3% fatality rate is for astronauts who went to space. There are also astronauts who don't go to space.

 

[A.T.]

You implied it can't be done.

No, I never did. I'm merely pointing out the obvious downsides, and still waiting to hear a single plausible and relevant benefit.

 

[sophiecentaur]

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

They are, on the same flight.

needs more delta_v capacity,

Explain, please. I can't get a different answer for the two.

There are also astronauts who don't go to space.

Seems like a matter of definition. You could include all the ground team if you wanted a really low risk number. It sounds like sailors who don't sail and tightrope walkers who don't actually go on a tightrope.

immediately loaded with a huge force, so a incomplete engagement would rip it apart.

A statement like the implies it can't be done but it is hopelessly over egged. With a very small amount or resilience, the maximum acceleration encountered would be little more than the on board g. Is that not clear? The parts of the station that the ship would contact would be going very close to the same speed as the ship and the effect of the docking would not involve any great stress. Without telling me, yet again, that it's not worth doing (and I do not disagree strongly with that) give me your argument that makes the forces great enough to "rip" things apart, which implies catastrophic failure. We do need to get the mechanics right here and not rely on intuition.

 

[A.T.]

A statement like the implies it can't be done

No it doesn't. That's just a straw man that you keep putting up, no matter how many times people clarify this.

 

[sophiecentaur]

No it doesn't. That's just a straw man that you keep putting up, no matter how many times people clarify this.

I have already stated that, at the moment, the idea is not worth implementing so what more do you want in that direction? But, apart from that, can you clarify the two problems I had with your previous post? The "rip apart" phrase needs some justification and so does the "delta v" assertion. They are matters of Physics and not semantics and I'd appreciate some numbers or a citation, perhaps.
It struck me that a rotating station would actually need to have much more robust construction because its 'floor' would have to withstand heavy loads and be able to deal with 'falling' objects. That's something that the ISS doesn't need.

 

[mfb]

Explain, please. I can't get a different answer for the two.

Contingency for aborted approaches.
And more delta_v even for the first approach if the orbital maneuvers don't lead to a suitable relative velocity.

Seems like a matter of definition.

Going to space is the most important part, but by far not the only thing astronauts do. A tightrope walker is still a tightrope walker when they work on installing the tightrope. If someone is trained to work on the ISS, and does all the stuff astronauts do on the ground, they are still an astronaut even if their mission gets canceled or whatever. This is not my definition, it is the NASA definition and I would expect other space agencies to have similar definitions.

 

[sophiecentaur]

Contingency for aborted approaches.

The importance of that will depend upon the actual numbers involved - what fraction of the total fuel carried is involved. It's something that would be decided on a bit further down the line in system design, I would have thought.n Is it not true (?) that the fuel used for a stationary docking would be equivalent to -30m/s more than for a peripheral docking; the ship would use the same amount of fuel to basically get to the vicinity of the station, I assume. As far as the ship is concerned, once docked at the periphery, its momentum would be shared (parked) but that momentum would be returned once it parts from the station on the return flight. Leaving from the hub would require a similar magnitude of delta v to what the retro's gave it. I don't know enough about the mechanics of this and I have been after some numerical help from you as you seem to have accessed more information about this topic than I have (I have had difficulty in finding any actual numbers). But you haven't been helping me.
Such an open definition of an 'astronaut' would not count for a guy trying to get life insurance for an actual space flight and that's the risk that interests me. I am sure that NASA would be quite prepared to present risk figures in a way that would avoid putting off suitable recruits. (Same as the adverts for joining the military). Suffice to say, its a pretty risky business.

You still haven't replied to my request to justify the (somewhat loaded) "rip things apart" statement. Have you tried some actual figures? I can't produce any extreme forces on the back of my envelope.

still waiting to hear a single plausible and relevant benefit.

You have had my view on this already. Irrespective of any possible benefits or otherwise, the Physics of the situation still applies and that is what interests me. For the nth time, I wish to make it clear that I don't claim it is the best thing to do at present. There could be some organisational benefits - particularly for a visiting un-manned shuttle that could just dump a cargo pod and get on its way. The whole procedure could take place in a very short time window (an advantage and not a disadvantage). I assume that re-usable vehicles will eventually be the way to go.
PF has discussed many hairy possibilities for space travel (space elevator, for instance) and they are not always practicable or even desirable at present - nevertheless the Science behind them has been of interest. You seem to have a problem with this particular idea and I can't see why.

 

[mfb]

The importance of that will depend upon the actual numbers involved - what fraction of the total fuel carried is involved.

We had this before, I won't repeat the numbers again.

Space is three-dimensional. If your orbital maneuver to approach the space station leads to an approach aligned with the rotation axis, you have to slow down, and then start flying tangentially. Extra delta-v. If you want to leave in the same direction, you have to slow down again and speed up in a different direction. Even more delta-v.

I didn't write "rip things apart" anywhere in this thread. A.T. wrote something like that. I can also imagine components of a docking mechanism to break if docking doesn't work properly.

 

[sophiecentaur]

Why would you aim at the axis of rotation for a peripheral docking? You would aim along a tangent. But I see that the axis of rotation of the station would need to be parallel with the orbital axis. Would that be a problem?