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

[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?

 

[mfb]

Why would you aim at the axis of rotation for a peripheral docking?

I don't say you aim for it. The direction of approach is basically given by the launch site and the orientation of the space station. If you are unlucky, then that's the direction of your approach.

But I see that the axis of rotation of the station would need to be parallel with the orbital axis.

That orientation is probably unstable, but it depends on the size, mass and so on of the space station, and I guess a detailed analysis is beyond the scope of this thread.

 

[sophiecentaur]

I don't say you aim for it.

This comment b seems to be based on that assumption.

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.

All that presupposes that you would be using the conventional approach and then change for a tangential approach. Is that really a likely strategy? If your system would involve slowing down and then speeding up in a different direction then it is clearly not optimal. You seem to be making a lot of assumptions about using what would be old technology in such a system. 30m/s is a small fraction of the overall orbital speed so

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.

Sorry - wrong guy but the level of damage in the case of a failure is inherently less than is being implied here. The fail condition will only involve the ship leaving the contact point. I would have thought that a 'weak link' would be included, to limit the stress on ship or station. Perhaps AT could contribute here.

 

[mfb]

All that presupposes that you would be using the conventional approach and then change for a tangential approach.

No it is not.

The direction of your approach to the space station is determined by orbital mechanics: your launch site and the orientation of the space station. You might be lucky and have an approach where you can use the approach velocity to dock at the rim. In that case, you save a bit of fuel. But if you are not lucky, you need extra fuel to make that work out.
In general, your approach velocity will have some component along the spin axis. You have to remove that in both docking approaches. But only in one you also have to speed up in the other plane.

30m/s is a small fraction of the overall orbital speed so

It is a part that has to be delivered in a much more controlled way. Big rocket stages can deliver 9 km/s, but they won't do that with 1m/s precision.

 

[sophiecentaur]

No it is not.

The direction of your approach to the space station is determined by orbital mechanics: your launch site and the orientation of the space station. You might be lucky and have an approach where you can use the approach velocity to dock at the rim. In that case, you save a bit of fuel. But if you are not lucky, you need extra fuel to make that work out.
In general, your approach velocity will have some component along the spin axis. You have to remove that in both docking approaches. But only in one you also have to speed up in the other plane.It is a part that has to be delivered in a much more controlled way. Big rocket stages can deliver 9 km/s, but they won't do that with 1m/s precision.

I am now more aware of the importance of getting lined up with the plane of the station. I still don't see where 'speeding up in the other plane' has to come into it. Your planned rendezvous can be made on the basis of 30m/s speed difference from the start as easily as planning for a suitable speed of approach before using retros, prior to conventional docking. Yes, the calculations are harder but that is hardly a problem these days.

 

[A.T.]

I can also imagine components of a docking mechanism to break if docking doesn't work properly.

Yes, that's what I meant.

 

[sophiecentaur]

Yes, that's what I meant.

Hyperbole is not really helpful in Engineering discussions. The term "ripping" was over the top. You could also have said that the mountings of landing gear, thrusters and other engines should be strong enough to avoid them being "ripped out". Isn't it obvious that any system would be designed to accommodate stresses? The cost of such design would be included in a detailed study.

It is a part that has to be delivered in a much more controlled way.

Oh yes. But you are presupposing that it would inherently involve more fuel for a peripheral docking. From what I understand from your useful previous points about orbital considerations, you aim to be 'near enough' (energy-wise as well as positional) with your main engines and then fine tune with thrusters. What would make a different approach speed involve using more fuel?

That orientation is probably unstable,

Is that based on some figures, or is it just an assumption? Does the ISS orbit involve keeping the hull pointing tangentially to its course? That would require some fuel to keep it 'rotating' correctly as the internal masses are re arranged. A wheel, orbiting in the way I suggest, would need no such correction energy.
Please don't just argue against this as a matter of principle and give it some unbiased thought.

 

[Al_]

I'm not sure why your concerned with a reaction wheel.

Consider the arrival of new people on the station. Imagine it's a small station, at 1g, with a risk of nausea due to rotation. To quote the pdf in the link:
"The results of this study may be summarized:
1. 1.71 rpm: very mild symptoms.
2. 2.2 rpm: one subject threw up (he had a history of seasickness) but otherwise similar to 1.71 rpm.
3. 3.82 rpm: mild symptoms and subjects adapted within a day; adaptation was longer for the less resistant subject.
4. 5.44 rpm: highly stressful (except for the deaf subject) but most adapted in a day or so. Subjects with prior rotation experience did better than those without.
5. 10 rpm: highly stressful (except for the deaf subject); subjects could not complete all tasks. There was some adaptation over the two day run."

You slow the station down for the new arrivals, then build the speed back up slowly over several days. Everyone stays at peak performance.

 

[sophiecentaur]

Consider the arrival of new people on the station. Imagine it's a small station, at 1g, with a risk of nausea due to rotation. To quote the pdf in the link:
"The results of this study may be summarized:
1. 1.71 rpm: very mild symptoms.
2. 2.2 rpm: one subject threw up (he had a history of seasickness) but otherwise similar to 1.71 rpm.
3. 3.82 rpm: mild symptoms and subjects adapted within a day; adaptation was longer for the less resistant subject.
4. 5.44 rpm: highly stressful (except for the deaf subject) but most adapted in a day or so. Subjects with prior rotation experience did better than those without.
5. 10 rpm: highly stressful (except for the deaf subject); subjects could not complete all tasks. There was some adaptation over the two day run."

You slow the station down for the new arrivals, then build the speed back up slowly over several days. Everyone stays at peak performance.

That suggests that all the crew would need to go through this procedure - unless there were two wheels; one for newcomers and one for the existing crew. The outer wheel could have zero rotation for docking, making my idea irrelevant except that the docking ship would not need to rotate itself for docking.

 

[mfb]

Oh yes. But you are presupposing that it would inherently involve more fuel for a peripheral docking.

Yes, but that statement was purely about fuel for an aborted docking maneuver. Keeping that reserve will inherently need more fuel.
For the first approach, I said that it can need more fuel, depending on the orbits. It does not have to.

Is that based on some figures, or is it just an assumption?

It is based on some basic orbital stability considerations.

The ISS keeps a fixed orientation with respect to its flight direction, which means it rotates once per orbit. It has its largest principal axis in vertical direction. That configuration is stable. Any object where this axis is not in vertical direction all the time will feel a torque. A torque on a rotating space station will (in general) make its axis of rotation change.

I guess a space station could stay stable in any orientation if it has some mass at the end of a long tether. Otherwise things get more complicated.

 

[sophiecentaur]

Yes, but that statement was purely about fuel for an aborted docking maneuver.

OH, right. That makes sense. I would have to ask what percentage of a normal 'contingency' fuel load, that this could represent.

It has its largest principal axis in vertical direction

So would a ring, orientated with its plane in the same plane as its orbit.

 

[mfb]

OH, right. That makes sense. I would have to ask what percentage of a normal 'contingency' fuel load, that this could represent.

I posted the numbers a few pages ago. It is significant.

So would a ring, orientated with its plane in the same plane as its orbit.

It would have two axes with nearly the same moment of inertia, constantly changing their orientation. Adding some mass suspended from the hub that rotates once per orbit would stabilize it.

 

[A.T.]

Isn't it obvious that any system would be designed to accommodate stresses?

It is obvious that your system in general (the station, the ships, the connection) would have to accommodate higher stresses. This leads to heavier structures, which is bad for space travel. And for the connection in particular it's also the combination of very limited time to engage properly with immediate full loading that tries to pull it apart.

 

[sophiecentaur]

is obvious that your system in general (the station, the ships, the connection) would have to accommodate higher stresses.

Why would they be greater than those acting on the outer parts of the station anyway? We are already discussing a rotating structure with all the implied stresses it would entail. I can't show that the forces on a coupling mechanism would be greater than you'd expect from 'local g'. The station structure would need a lot of built in redundancy to deal with internally generated accidents.

immediate full loading that tries to pull it apart.

Not 'immediate' in any way. The peripheral and tangential paths would part at a very leisurely rate; actually, the separation would increase only at the rate of a body falling under local g. That is hardly immediate. Can you actually support your hyperbolic description with numbers? You are ignoring the great feature of this fail safe manoeuvre. If you miss docking you move clear. If you don't quite latch on, you still move away, relatively. What sort of mass are you envisaging for a suitable coupling? A length of kevlar webbing that can tow a large yacht behind a Severn class lifeboat (under storm conditions) is only a bit thicker than a standard car seat belt. Are you being misled by the dramatic clunking sounds that we always hear on blockbuster Space movies? Perhaps you could take a look at pictures of cable car suspensions that are also designed for storm conditions and full g. The sort of strap that would probably suffice could be only a bit more massive than Tim Peak's guitar.

 

[sophiecentaur]

It would have two axes with nearly the same moment of inertia, constantly changing their orientation. Adding some mass suspended from the hub that rotates once per orbit would stabilize it.

That doesn't tie in with what I thought I understood about gyroscopes (??) The ring is constantly rotating around its axis.

 

[mfb]

Not 'immediate' in any way.

The forces acting on the connecting structure go from 0 to ( spacecraft mass)*(local acceleration) within fractions of a second. That is a very high force gradient. Plug in numbers as suitable.

If you miss docking you move clear.

Or collide with the space station. It will be a soft collision if you don't miss it by a large margin, but that applies to a hub docking maneuver as well.

It would have two axes with nearly the same moment of inertia, constantly changing their orientation. Adding some mass suspended from the hub that rotates once per orbit would stabilize it.

That doesn't tie in with what I thought I understood about gyroscopes (??) The ring is constantly rotating around its axis.

Which part is unclear?

 

[sophiecentaur]

The forces acting on the connecting structure go from 0 to ( spacecraft mass)*(local acceleration) within fractions of a second. That is a very high force gradient. Plug in numbers as suitable.

That is not right. It is only the equivalent of going from a straight road into a 100m radius bend at 70mph. (Still using the extreme 30m/s for 1g local) The sort of stress is less than for a skater dropping down onto a half pipe and in that situation. the maximum g at the bottom will be 3 or 4 g, depending on the initial drop height. I would be grateful to see your sums that produce more than local g on the ship. It would be quite possible to have some resilience in the coupling (or perhaps in the crew seats) to take care of the transitional forces - not unlike going over a humped back bridge in a motor car or even applying the brakes (1g is a realistic value for good brakes on a good road). What sort of g forces do they experience at takeoff from Earth? Where is the 'extreme' situation that you are painting?

Which part is unclear?

If the station is a symmetrical ring and it is spinning in the same plane as its orbit then is that not a stable situation? The station's axis would always point to the same star. Did you include some other factor in your model?

 

[mfb]

It is only the equivalent of going from a straight road into a 100m radius bend at 70mph.

Within fractions of a second. A roller coaster might do that along the vertical axis (and only there), with a car you certainly wouldn't drive like that.

I said force gradient, not force. The rate of change of the force is very high. See jerk.

If the station is a symmetrical ring and it is spinning in the same plane as its orbit then is that not a stable situation?

See earlier posts: Not necessarily if the wheel is not perfectly symmetric.

 

[sophiecentaur]

with a car you certainly wouldn't drive like that.

Maybe not every day but everyone is (I hope) prepared to apply the brakes when necessary. Are you really claiming that the forces (step change included) would upset anyone who's fit for a trip to that space station? What happened to The Right Stuff? In a practical space station, the local g would be much lower than 10m/s², in any case. Can we just put that "ripping" word to bed please? A bit of resilience in the suspension would reduce the trauma to a very acceptable level - We can hit a bump in the car and the suspension takes care of some really significant impulses.
Someone mentioned a Straw Man and this looks like another one.

Not necessarily if the wheel is not perfectly symmetric

Right - but that would apply to any toroidal space station and you have not given a reason that would make my idea for orientation any worse than any other. I have assumed all along that the arriving ship's mass would be comparatively low. Moreover, its arrival would be expected and could be compensated for - just as the movement of large objects within the station.

 

[mfb]

Maybe not every day but everyone is (I hope) prepared to apply the brakes when necessary.

Car brakes don't lead to jerk as fast as the docking mechanism needs.

Are you really claiming that the forces (step change included) would upset anyone who's fit for a trip to that space station?

In the right orientation they would be okay, but that was not my point. They lead to high stress and design challenges for the docking mechanism.

Right - but that would apply to any toroidal space station and you have not given a reason that would make my idea for orientation any worse than any other.

Every orientation would need calculations for stability. My point is that you cannot simply choose the orientation to be convenient for docking. The stability is the main consideration.

 

[sophiecentaur]

You are assuming that full docking and initial coupling have to be at the same time. That's just not the case. You never seem to take Earthbound examples as valid but the hooking on procedure can be done on a resilient link. (Arrester wire) After a convenient time, during which the ship could move itself along the rail it can dock at a port.

The station would need to be stable or a wobble would be felt by everyone, like a rolling ship. A tolerance for contact by the visiting craft wouldn't be far different from what the station staff would need.