Realistic, near-term, rotating Space Station

[mikelepage]

Shielding is a surface area to volume ratio problem, so the larger scale your station (assuming the volume isn't nearly entirely empty space like in an O'Neill Cylinder), the easier is to shield. There's also a geometric effect due to shielding thickness in linear units... like, even if a slab of hydrogen is technically the best shield per unit mass, at small scale, it'll actually be worse for shielding a spherical or cylindrical volume than other stuff like water or polyethylene for a given total mass.

For a large station, liquid hydrogen is probably the best shielding material for now.

This is getting somewhat off topic, but it will be relevant to space station geometry and design.

I wasn't sure I believed your last statement, even in a large station, so made an effort to investigate. Attached graph shows how hydrogen is better *per gram*, but you have to account for the density of LH2 at 20K, which is only 70.85g per litre (i.e. 1/14th that of water). Also AIUI, it's the hydrogen nuclei that count, and LH2 has 70.85g of hydrogen nuclei per litre, while water has 111g of hydrogen nuclei per litre.

So for a given amount of shielding, water is ~64% the volume of LH2, while being 9x the mass. The trick will be in containing the liquids. Quick google searches show that a PET (Polyethylene Terephthalate) "slimline" rainwater tank for containing 5000L of water masses roughly 180-200kg, while state of the art tanks for containing liquid hydrogen hold 150kg, massing 67kg (double walled steel duwars can be 5x or more the mass of hydrogen contained). So making the assumption that those are representative, the mass of the water tank material is  conservatively 4% of the weight of water contained, while the LH2 tank equivalent is, so far at best, 44% (i.e more than 9x).

All of which is to say, that by the time you include the mass of LH2 tankage plus cryocooler, I don't think there's any size where you're not ahead by using water instead of LH2. There might not be too much in it, but you can also keep STP water inside the habitat, which you can't do with LH2, so I'd be surprised if LH2 ever comes out on top, all things considered.

[LMT]

I don't think there's any size where you're not ahead by using water instead of LH2.

What does your plot indicate, quantitatively, about the water / LH2 depth required to limit annual GCR dose to a suggested 20 mSv?

[mikelepage]

I don't think there's any size where you're not ahead by using water instead of LH2.

What does your plot indicate, quantitatively, about the water / LH2 depth required to limit annual GCR dose to a suggested 20 mSv?

I didn't mean to give the impression the plot was mine. I've seen many variations of this graph online, but chose this one because of the inclusion of Boron Nitride nanotubes - which seems interesting given the adjacent "White Graphene" thread. I also didn't realise until just now that the source paper wasn't paywalled - it's an interesting one.
http://virtualsim.nuaa.edu.cn/file/up_document/2020/12/oi41zzhD2wviXZ_U.pdf

The paper said they used OLTARIS, so it should be possible to replicate the conditions and answer your question quantitatively, but I'll leave that to you. I did find the following quote - from the intro - explaining why the relationship between shield thickness and dose is highly non-linear:

There is a small but significant component of GCR particles with high atomic number (Z > 10) and high energy ( E > 100 GeV). 1 These high-atomic number, high-energy (HZE) ion particles comprise only 1–2% of the total GCR fluence, but they interact with very high specific ionizations and thus contribute about 50% of the longterm space radiation dose in humans. 2

That references this encyclopedic tome here, although I can't find the specific quote.
https://spaceradiation.larc.nasa.gov/nasapapers/RP1257.pdf]this reference

The closest answer I did find to your question on page 409 in the document (is page 421 of the pdf) at solar minimum:
On page 412 of the doc it looks at water, with the last line being 50g/cm2 (50cm) water = yearly dose of 220mSv (22cSv).
On page 418 of the doc it looks at LH2, with the last line being 100g/cm2 (14 metres) LH2 = yearly dose of 38mSv

From the first paper I quoted here:
https://superdarn.thayer.dartmouth.edu/downloads/JSR09.pdf
They estimated 10⁸ amps to deflect 1GeV GCRs using that system, and that needing to keep superconducting wires cool enough would require significant apparatus (maybe LH2). They finished saying that clearly more advancements were needed - but the paper showed the principle that a toroidal shaped craft could project a strong magnetic field externally, whilst nulling the field inside the torus.

[LMT]

What does your plot indicate, quantitatively, about the water / LH2 depth required to limit annual GCR dose to a suggested 20 mSv?

(14 metres) LH2 = yearly dose of 38mSv

14 m for 38 mSv...

From the first paper I quoted here:
https://superdarn.thayer.dartmouth.edu/downloads/JSR09.pdf
They estimated 10⁸ amps to deflect 1GeV GCRs...

Notice Slough's 2022 improvements over Shepherd's 2008 toroidal design.  E.g., 2.2 T vs. 10 T requirement. 

[Coastal Ron]

...
All of which is to say, that by the time you include the mass of LH2 tankage plus cryocooler, I don't think there's any size where you're not ahead by using water instead of LH2.

Polyethylene baby!

Water is a blocker, but if you want your radiation protection to also do double duty, consuming the water that is blocking the radiation isn't a good tradeoff.

At least with polyethylene you can use it to build your station, so not only is it blocking radiation, but it is also providing structural duty of holding atmosphere in, the station together, or both.

[mikelepage]

What does your plot indicate, quantitatively, about the water / LH2 depth required to limit annual GCR dose to a suggested 20 mSv?

(14 metres) LH2 = yearly dose of 38mSv

14 m for 38 mSv...

From the first paper I quoted here:
https://superdarn.thayer.dartmouth.edu/downloads/JSR09.pdf
They estimated 10⁸ amps to deflect 1GeV GCRs...

Notice Slough's 2022 improvements over Shepherd's 2008 toroidal design.  E.g., 2.2 T vs. 10 T requirement.

Another good paper. Thanks.

The figure with 14m LH2 (or 9m water) for 38mSv/year (skin) is at 100g/cm² shielding.

Looking at the numbers in the RP1257 paper I linked earlier, we can (and might have to) quarter the shielding size and aim for around the 100mSv/year threshold in which negative impacts are equivocal.

Looking up the Ramsar wikipedia page took me to studies of several high natural background radiation (HNBR) areas, which was interesting. More in this paper. Even though there are tens of thousands living in Ramsar, there are less than 2000 people living in the HNBR area. Even then, it was only by combining the exposure from the spring water (up to 131mSv/year) and the radon in the air (up to 72mSv/year) do you get the "up to 260mSv/year" figure quoted. Also, apparently almost everyone who lives there smokes, so it is difficult to dis-entangle the causes of cancer rates that exist. Interesting that the mean estimated exposure among those residents in the HNBR was 6-10mSv/year, and would fall well below established limits for radiation and airline workers.

That said, there are clearly family groups that live in areas that receive much higher exposures, and there is a lack of obvious negative impact in the form of radiation sickness as one would expect if the linear no dose threshold hypothesis was true. That gives some support to the radiation hormesis hypothesis and the idea that chronic radiation exposure is significantly more tolerable than acute doses. Elsewhere the article even equivocates on much higher doses, saying even acute doses of 100mSv "may" increase lifetime cancer risk by ~0.8%. IIRC the figure NASA uses is for 1000mSv lifetime exposure to increase lifetime cancer risk by ~5%.

Given that shielding costs go up significantly to take exposure below 100mSv/year, it seems reasonable to me that a near term rotating space station could justify limiting shielding to one quarter of the above, to 2.25m LH2 or (preferably) 1.44m of water which can be kept inside the habitat, taking us into the range of figures we designers can work with.

Suppose the craft takes the form of a torus which can run a sufficiently powerful field to deflect the lowest energy GCRs below 0.5GeV (probably still requiring MegaWatt scale solar arrays, but power requirements are still unclear). Put it on a duty cycle of 16 hours a day, reducing dose equivalent to about 70% when it is on. During the other 8 hours, everyone can be assumed to be in the crew quarters section of the habitat, which is surrounded by 50cm tanks of water and would double as a storm shelter. In their private quarters, crew have additional water stores in the form of a "water bed", which could consist of a 30cm soft polyethylene water-filled mattress. Also, the walls of the torus itself would contain 50cm of water.

I started doing some very rough math on this config (I came up with 72-180mSv per year, depending on solar max or min), but I'm realising I don't have any good indication of whether, or how well, the two forms of shielding would combine. It's clear that GCRs with energies above a certain threshold will make it through the dipole shield, but to what extent will additional water/polyethylene shielding be able to capture more of those GCRs with energies just above that threshold? Or is it just that the 50cm of water shielding and/or dipole shield will take out the same low-energy GCRs, and the rest will just sail on through?

[LMT]

Given that shielding costs go up significantly to take exposure below 100mSv/year, it seems reasonable to me that a near term rotating space station could justify limiting shielding to one quarter of the above, to 2.25m LH2 or (preferably) 1.44m of water which can be kept inside the habitat, taking us into the range of figures we designers can work with.

Well, no shielding is needed for AG stations in EVLEO:  deep space settlement is a hard sell, comparatively.

Again, NASA career limit is 600 mSv.  And ICRP recommends 20 mSv/yr, with [effective] dose in "no single year exceeding 50 mSv".  Corporate budget doesn't override such standards.

Toroidal HTS cabling gives best AG GCR shielding today.  The thread might explore potential toroidal improvements for higher GCR energies.  Note logarithmic drop-off of GCR energies above 1 GeV/n, Simonsen et al. 2020.

Fig. 3a: "GCR particle spectra at solar minimum conditions (June 1976) denoted by solid lines, and solar maximum conditions (June 2001) denoted by dashed lines..."

Refs.

Simonsen, L.C., Slaba, T.C., Guida, P. and Rusek, A., 2020. NASA’s first ground-based Galactic Cosmic Ray Simulator: Enabling a new era in space radiobiology research. PLoS biology, 18(5), p.e3000669.
 

[mikelepage]

Again, NASA career limit is 600 mSv.  And ICRP recommends 20 mSv/yr, with the public dose in "no single year exceeding 50 mSv".  Corporate budget doesn't override such standards.

Not immediately no. But sometimes it's good to step back and think about why certain rules exist, and in what circumstances they can change.

Do we think the settlement of space will be the start of an epochal change or not?
If so, what do we think about the demographic changes that will inevitably come with it?
Whole nation states were founded on the basis of such migrations, and the laws of a nation are designed in response to the economic and social factors that drive migrations, not the other way around.

600mSv corresponds to an additional 3% lifetime risk of cancer.
On top of the average population's 40% lifetime risk of cancer.

An increase to lifetime risk of 3% is acceptable in the context of industry, in a first world country, but is actually pretty minor overall, especially when you compare it to the baseline 40% risk that everyone already deals with. (N.B. it used to be about a third, but has risen with the increase in life expectancy of the general population)

I think it's a legitimate question to ask 'what increase in risk should we tolerate in the context of an epochal change that space settlement signifies'? (and where overall risk of death is much higher anyway). I'd hypothesise that if there is actually a viable means for thousands of people to migrate off Earth to deep space every year (because rotating space stations can be economically produced), this will be in a context where lifetime cancer risks of 50%-60% will be considered acceptable. This would correspond to 2000mSv - 4000mSv lifetime doses, with no more than 200mSv in a year, giving people a 10-20 year deep-space career, perhaps followed by retirement in EVLEO.

This is what I think we should be designing for. I do think Twark Main went too far in his minimisation of shielding requirements (as 30cm water/polyethylene is clearly nowhere near enough), but I do think we can afford to relax our exposure guidelines considerably from what is currently the norm, because the context we're designing for is not normal in any sense of the word.

Attached is a graph showing the average age of onset of cancer (from NIH). With an increased acceptance of cancer risk, we'd expect average cancer onset age to come forward maybe a decade or so, and average lifespan will decrease similarly. But even so, we'd still be doing far better than in every previous age of exploration.

One thing I appreciate about Elon is his pointing out of the need to accept a greater risk of death in the pursuit of something grand. That applies here too.

EDIT for people skimming: You could argue we're getting pretty far off topic at this point, but I've justified it in my head because of the earlier debate between bola and torus geometries - and the fact that toruses lend themselves to magnetic dipole radiation shielding techniques. So bola geometries (with spherical habitats) minimise habitat mass if radiation shielding is to be done passively, while torus geometries minimise habitat mass if radiation shielding is to be done magnetically.

[Coastal Ron]

Again, NASA career limit is 600 mSv.  And ICRP recommends 20 mSv/yr, with the public dose in "no single year exceeding 50 mSv".  Corporate budget doesn't override such standards.

Not immediately no. But sometimes it's good to step back and think about why certain rules exist, and in what circumstances they can change.

Do we think the settlement of space will be the start of an epochal change or not?
If so, what do we think about the demographic changes that will inevitably come with it?
Whole nation states were founded on the basis of such migrations, and the laws of a nation are designed in response to the economic and social factors that drive migrations, not the other way around.

600mSv corresponds to an additional 3% lifetime risk of cancer.
On top of the average population's 40% lifetime risk of cancer.

An increase to lifetime risk of 3% is acceptable in the context of industry, in a first world country, but is actually pretty minor overall, especially when you compare it to the baseline 40% risk that everyone already deals with. (N.B. it used to be about a third, but has risen with the increase in life expectancy of the general population)

...

Attached is a graph showing the average age of onset of cancer (from NIH). With an increased acceptance of cancer risk, we'd expect average cancer onset age to come forward maybe a decade or so, and average lifespan will decrease similarly. But even so, we'd still be doing far better than in every previous age of exploration.

One thing I appreciate about Elon is his pointing out of the need to accept a greater risk of death in the pursuit of something grand. That applies here too.

Yes, the bottom line is what PEOPLE will determine to be their risk level, not governments or organizations. We all take calculated risks every day, some more risky than others. And then there are people that are paid to take risks, such as law enforcement, firefighters, nurses, those in the military, flight attendants, etc.

As rotating space station designers we should be pursuing all possible designs that could mitigate radiation issues, but ultimately it will be PEOPLE that decide what they are willing to risk - we just need to be honest with them about what those risks are so they can make informed decisions.

EDIT for people skimming: You could argue we're getting pretty far off topic at this point, but I've justified it in my head because of the earlier debate between bola and torus geometries - and the fact that toruses lend themselves to magnetic dipole radiation shielding techniques. So bola geometries (with spherical habitats) minimise habitat mass if radiation shielding is to be done passively, while torus geometries minimise habitat mass if radiation shielding is to be done magnetically.

Radiation protection affects the design of rotating space stations, so it is very germane.

[Twark_Main]

state of the art tanks for containing liquid hydrogen hold 150kg, massing 67kg (double walled steel duwars can be 5x or more the mass of hydrogen contained).

You only need the double wall because Earth has an atmosphere. In space you can eliminate the second wall, which (to the first approximation) halves the mass of the hydrogen tank.

So making the assumption that those are representative, the mass of the water tank material is  conservatively 4% of the weight of water contained, while the LH2 tank equivalent is, so far at best, 44% (i.e more than 9x).

"More than 9x" is a misleading figure to conclude with, as it ignores the tremendous baseline mass savings of hydrogen vs. water.

Separately, per above it's really 22% not 44%.

All of which is to say, that by the time you include the mass of LH2 tankage plus cryocooler, I don't think there's any size where you're not ahead by using water instead of LH2. There might not be too much in it, but you can also keep STP water inside the habitat, which you can't do with LH2, so I'd be surprised if LH2 ever comes out on top, all things considered.

Maybe so, but size (i.e. thickness, i.e. curvature penalty) is not highly relevant, especially with decently scaled stations. The primary metric (as usual) is mass per area vs. radiation dose, and hydrogen still comes out way ahead.

It all hinges on your (unstated) assumptions about cryocooler mass. If you're picturing a gang of coolers on each tank—ie just linear scaling up numbers from existing systems—you're probably overestimating the mass by a lot. Instead this might require a little cleverness, but the reward seems more than worth it!



My baseline shield material has always been polyethylene, possibly salted with neuron absorber[​s] like boron. Perhaps it's worth reconsidering liquid (or even solid) hydrogen....   

[Coastal Ron]

state of the art tanks for containing liquid hydrogen hold 150kg, massing 67kg (double walled steel duwars can be 5x or more the mass of hydrogen contained).

You only need the double wall because Earth has an atmosphere. In space you can eliminate the second wall, which (to the first approximation) halves the mass of the hydrogen tank.

The double wall protects from heat sources, regardless of what they may be.

Here on Earth the heat sources are the warm atmosphere and sunlight, but out in space you have this thing called "the sun" that is a HUGE heat source, and we already know it causes boil off in space.

A variation on the double wall is a sunshade, which may be an option depending on your needs. ULA has proposed using sunshades for their propellant depots, as can be seen in some of the images in this paper (with our own Jon Goff as the first person listed):

Realistic Near-Term Propellant Depots: Implementation of a Critical Spacefaring Capability - United Launch Alliance

The bottom line is that while you could get away with a single walled tank in a permanently shaded part of a moon, if you are in space and rotating to show part of your surface to the sun you will need a barrier layer, whether that be a sunshade or tank double wall.

[Barley]

You only need the double wall [for LH₂] because Earth has an atmosphere. In space you can eliminate the second wall, which (to the first approximation) halves the mass of the hydrogen tank.

For the best geometry the shielding should be conformal to the habitat, which does contain atmosphere.  Perhaps to first approximation a one-and-a-half walled tank?

My baseline shield material has always been polyethylene, possibly salted with neuron absorber[​s] like boron. Perhaps it's worth reconsidering liquid (or even solid) hydrogen....   

A thick-walled polyethylene tank could be filled with water.  Depending on the relative availability of oxygen and carbon, the desire to shift mass around for a storm cellar, etc.  I don't think you can pull that off with hydrogen.

There are advantages to using materials that are less incompatible with life.  There will be a tension between the engineers who operate the station and the theorists who want to optimize each structure for some particular property.  Compare and contrast the issues of a hole between the habitat and a shield made of LH₂, H₂O, or [CH₂]_n.

[JohnFornaro]

There will be a tension between the engineers who operate the station and the theorists who want to optimize each structure for some particular property.

Indeed.  We all know that a sphere is more "efficient" in some ways, but it is the case that living spaces in space so far are shaped nothing at all like a sphere.  Propellant tanks are another matter. 

As to your nested quote:

Perhaps it's worth reconsidering liquid (or even solid) hydrogen....

Solid hydrogen is not easy to come by and probably not worth considering, much less re-considering, as a shielding material.  Same goes for liquid hydrogen.

[Coastal Ron]

An interesting study about indoor air that has relevance to rotating space stations:

Indoor plants are surprisingly good at devouring carcinogenic toxins - New Atlas

Relevant quote:

Researchers have demonstrated how effective plants are at ridding the air in your home, school, or workplace of toxic, carcinogenic pollutants, providing a sustainable, low-cost way of ensuring that the air you breathe is cleaner.

Different rotating space stations will have different use cases - different uses of the indoor space such as living spaces only, manufacturing only, or some combination of both and more. Some of them might have a high use of volatiles that will need to be scrubbed from the air system, so designing air pathways to take advantage of the ability for plants to help clean the air might make sense.

Not sure what the density of the plants need to be though...

[Robotbeat]

NO ONE has enough facts to know for sure what humans will experience once we leave the various degrees of radiation protection the Earth provides. Studies can provide hints, but we need MORE data - which we may not have until we start sending people out into the harsh radiation environment of BEO space.

We also don't know for sure what the use cases are for having humans in space post ISS, which means we don't know for sure what the radiation protection levels truly need to be, don't know the duration of stays for humans in space, or what the economic limitations will be for constructing rotating space stations with radiation protection.

In other words, we don't know enough to eliminate ideas yet.

Yelling, "We don't know," while ignoring info is just shout-down.

Look, no one is getting ready to build a rotating space station, …

Well… VAST is. Ish.

[Coastal Ron]

NO ONE has enough facts to know for sure what humans will experience once we leave the various degrees of radiation protection the Earth provides. Studies can provide hints, but we need MORE data - which we may not have until we start sending people out into the harsh radiation environment of BEO space.

We also don't know for sure what the use cases are for having humans in space post ISS, which means we don't know for sure what the radiation protection levels truly need to be, don't know the duration of stays for humans in space, or what the economic limitations will be for constructing rotating space stations with radiation protection.

In other words, we don't know enough to eliminate ideas yet.

Yelling, "We don't know," while ignoring info is just shout-down.

Look, no one is getting ready to build a rotating space station, …

Well… VAST is. Ish.

I meant of the people on this forum. 

Though who knows, maybe someone from vast has been reading and otherwise contributing to this thread, though if they were they'd see the discussion about their 100-meter spinning stick station that they hope to build next decade... 

And if someone from Vast does drop by, any background on how they plan to deal with docking, specifically how they plan to take into account the intermediate axis theorem.

Of course there is a specific NSF thread for Vast, so that conversation could go there too.

[LMT]

An interesting study about indoor air that has relevance to rotating space stations:

Indoor plants are surprisingly good at devouring carcinogenic toxins - New Atlas

A common source of indoor air pollution is gasoline vapor,
which contains the 'big four' volatile organic compounds
benzene, toluene, ethylbenzene and xylene...

"Gas station in space" is just a metaphor.   

[Coastal Ron]

An interesting study about indoor air that has relevance to rotating space stations:

Indoor plants are surprisingly good at devouring carcinogenic toxins - New Atlas

A common source of indoor air pollution is gasoline vapor,
which contains the 'big four' volatile organic compounds
benzene, toluene, ethylbenzene and xylene...

"Gas station in space" is just a metaphor.   

You're missing the big picture here, which is that plants can rid the air of toxic, carcinogenic pollutants.

The interiors of future rotating space stations (and non-rotating ones too) will likely have many materials that can produce air pollutants, so there will likely be a constant need for removing such pollutants from the air.

One method I'm considering is to use the excess power of the solar electric generation system to power a complete recycling of the air by liquifying it, separating out the components, and then only reintroducing the purified oxygen, nitrogen and other useful gasses. Everything else will be discarded. But using plants to do some of that scrubbing would reduce the overall cost and complexity of keeping the inside air "fresh".

[Paul451]

One method I'm considering is to use the excess power of the solar electric generation system to power a complete recycling of the air by liquifying it, separating out the components, and then only reintroducing the purified oxygen, nitrogen and other useful gasses. Everything else will be discarded.

No need to liquify the air itself. All the VOCs have much higher boiling and freezing points than nitrox, so if you're willing to spend the energy to liquify air itself, you can spend much less just freezing out the target pollutants. Ditto water vapour, CO/CO₂, etc.

(Also, you wouldn't discard them. The benzenes/toluenes/etc are important organic solvents. It's why they are in everything in the first place. So recycling them will be useful to industry in space.)

[MickQ]

I have another idea for docking to a rotating station.

Visiting ship approaches the docking port and noses into a de-spun framework which has various clamps that hold the ship in place. The frame then rotates up to match the station spin rate and lines up the ship’s docking port with a crew access arm that attaches to the ship’s hatch.  Pressurise and transfer.

No rotating seals. Slight adjustments of the ship position by the clamp mechanism could fix any balance problems.

If the central hub of the station was actually cup or tube shaped then the ship would be effectively inside the station and in the plane of rotation, on the axis of spin.  Would this solve any intermediate axis problems ?