Slide 1
Challenging the Assumptions that led to
the Orbital Settlement Designs of the Past
Design Considerations
for an Orbital Settlement
Asteroid Mining Group
-Resources for the Future of Humanity –
Stephen D. Covey, Chris Cassell, Al Globus, Jim Luebke,
Mark Sonter, Bryan Versteeg, and James Wolff
ISDC 2012, Washington DC
Welcome to Design Considerations for an Orbital Settlement. My name is Stephen
Covey, and I’m a member of the Asteroid Mining Group.
Space settlement is our common goal, and we believe that asteroid resources are the
major enabler for Space Settlement, Space Solar Power, and the ultimate expansion of
humanity throughout the solar system.
Slide 2
Designing an Orbital Settlement
OVERVIEW
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Historical Assumptions
Rotational Stability
Thermal Management
Recycling
Population Density &
Social Dynamics
• Proposed Design
– Architecture
– Scalability
• Research Topics
This presentation challenges several of the underlying assumptions made in a number of
past proposals for orbital settlements.
While I won’t take the time to review those designs, I will discuss a number of the
assumptions made, and the changes I suggest.
This will lead to a specific proposed design, and I’ll conclude with a discussion of the
R&D areas that need to be addressed.
Slide 3
Designing an Orbital Settlement
HISTORICAL ASSUMPTIONS
• Designed for Visual Appeal (Artistic)
• Large Open Space (Pastoral Setting)
– Low surface population density
• Radiation Shield
– Soil and/or non-rotating external shield
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Crops Don’t Need Gravity or Shielding
Natural Sunlight
Ignore Social Dynamics
Ignore Rotational Stability
Inadequate Attention to Thermal
Management
We’ll start with a little history. Examples of past designs include the Stanford Torus,
Bernal Sphere, and Island Three (aka O’Neill cylinder), and there are many others. They
make for great magazine covers. They are typically spacious, often with suburban or
even rural settings, with pastures, forests, lakes, and streams. They all use huge
expanses of windows to admit natural sunlight – which was considered a requirement in
the 1975 NASA study.
While some early designs ignored the need for radiation and meteorite shielding, others
assumed a simple thick layer of soil or even non-rotating external shields, estimating
that the shield mass was too heavy to rotate. Some, combining the natural sunlight and
shielding requirements, specified complex chevron and mirror shields to provide both.
Another common theme is external agricultural sections, sometimes without gravity,
often times with only minimal radiation shielding. But some designs simply have the
farms and housing sharing the 1-G areas of the habitat.
A final common theme seems to be a requirement for a “low stress” environment to
counter the presumed isolation of living in space, generally implemented via a low
population density, long sight-lines, and a sense of variety generated by multiple housing
and recreational options.
Two vital aspects of the design of orbital settlements are rotational stability and thermal
management – primary issues in this presentation.
Slide 4
Designing an Orbital Settlement
ROTATIONAL STABILITY
• Spinning Structures Stable ONLY
About Axis With Greatest
Moment of Inertia
– Long Cylinders Bad
– Spheres Bad
– Wheels Good
• Kalpana One Revised (Al Globus,
et al.) Is Right
– Cylinder with Width <= 1.3xRadius
– Shield Mass (10 Tons/m2) Rotates
Let’s start with rotational stability.
Many of the popular designs are inherently unstable – their axis of rotation is not
orthogonal to the largest moment arm. Without continuous dynamic stabilization, they
will begin to tumble, and ultimately spin with the maximum mass away from the axis.
Think of a long O’Neill cylinder, becoming unstable and ultimately tumbling end-overend. This is non-optimal.
Of the popular designs, only the wheels are stable.
Al Globus et al., in their Kalpana One Revised design of 2007, derived that a cylinder
resulted in the most efficient use of shield mass, and that to be rotationally stable, a
hollow simple cylinder must have a width no longer than 1.3 times its radius. Their paper
also noted that incorporating the shield mass in the surface of the cylinder eliminated
failure modes, which is always a good idea.
My early settlement designs failed to address rotational stability, and when I read about
Kalpana One, Al’s arguments were so compelling I immediately adapted it as the basis
for my subsequent settlements.
Slide 5
Designing an Orbital Settlement
THERMAL MANAGEMENT
• Life in a Thermos Bottle
• Natural Sunlight is 2/3 invisible, 15% bioavailable
• Stefan-Boltzman Law is Not Your Friend
• 6 kw Per-Person Energy Budget
– 3.5 kw Agriculture (optimized LED lights)
– 0.5 kw Common Areas (sulfur microwave for
park)
– 1.0 kw Home Use
– 1.0 kw Office/Retail/Services/Transportation
• Spin Axis Points To Sun
• Settlement should stay in the shade!
Thermal management is critical.
While everyone recognizes that an orbital settlement shares many characteristics with a
thermos bottle, other orbiting settlement designs generally handle heat dissipation with
a wave of the hand or a simple radiator without addressing how the excess heat gets
carried to it.
Admitting natural sunlight is the first of the problems, as it is extremely inefficient as far
as heat is concerned, more so when used for agriculture. Chlorophyll is rather inefficient,
using primarily red light plus some blue. It has no use for green, so it reflects it! There
are commercial red & blue LED assemblies that waste little energy yet grow crops
extremely well. Using these LED panels, and allowing for 50% more “farm area” than the
1975 NASA study required, it works out that 3.5 kilowatts is adequate, on average, to
grow essentially any crop, even assuming the kinds of high-intensity agriculture that
O’Neill described. These LEDs produce just the light plants can use. For homes, hallways,
and offices we can use white LEDs, but for the park we’ll need broad-spectrum lighting,
even including UV. The highest efficiency is currently sulfur microwave lamps, and my
allowance is just enough for grass, shrubs, and trees to thrive. Adding rather average
total power allowances for home, office, and retail uses results in a total energy budget
of 6 kilowatts per person.
Slide 6
Designing an Orbital Settlement
SOME HABITAT IMPLICATIONS
• 6 kw/person is a Design Constraint
– More needed for typical American diet
• At 25% Efficiency, Need 33 m2 Per Person of
Solar Panels for Energy
• 20 m2 Per Person External Area Required
– Assumes all shaded surface, painted black!
• 10 tons/m2 Shielding Necessary
• => 200 tons Shield Mass Per Person
I want to emphasize some of the implications for all habitats.
Energy and shield mass requirements result in several design constraints as per-person
constants.
It’s difficult to get by with less than 6kw/person unless significant efficiencies in highintensity farming are achieved, as 60% of the energy is used in direct agriculture. This
allowance supports a largely vegetarian diet, with goat milk and cheese, eggs and
chickens, tilapia, and bunny burgers. These are all efficient vegetarian critters. Goats are
twice as efficient as cattle for producing milk per pound of feed.
At 25% efficiency, we need 33 m2/p of solar panels to collect the 6kw/p, all of which
needs to be radiated, and at 0 degrees C 19 square meters per person for a perfect
blackbody is required.
We can get by with an external radiating surface of 20 m2/p if we paint the habitat flat
black and keep it in the shade, easily done by hiding behind the solar panels.
Also, note that the external surface area and shield mass requirement implies a fixed
200 tons of shield mass per person. In a really large habitat the shield can, of course, be
provided by the shell of the habitat. More on that later.
lide 7
Designing an Orbital Settlement
POPULATION DENSITY
• Low Density
– Scenic, Private, Peaceful, Relaxed,
Sedentary
• High Density
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Bustling, High-Energy Social Interaction
High Personal Productivity
Improved Health
Efficient Retail & Services
• www.WalkScore.com
The popular designs of the past often present a low-density environment with
agriculture, homes, parks, and shops sharing the limited 1G surface, and sometimes
even sharing floor space with windows to admit light.
However, numerous studies have shown significant benefits to living in high-density,
walkable communities. There, we weigh 6-10 pounds less and live 1.5 – 2 years longer.
Business productivity is significantly higher. Even in the suburbs our favorite shopping
centers and meeting places tend to be in bustling, high-density areas, where there is
something for everyone.
Walkscore.com ranks cities, neighborhoods, and addresses according to how close and
walkable the amenities of everyday living are, from groceries to restaurants to libraries,
schools, and parks. Their algorithms suggest values that should be considered in the
design of any community.
Slide 8
Designing an Orbital Settlement
SERVICES DENSITY
Walkscore.com
•20% 1 Grocery
•20% 10 Restaurants/Bars
•13% 5 Retail Shops
•13% 2 Coffee Shops
•6.7% each for 1 bank, 1
park, 1 school, 1 library, any
entertainment
Why?
•Increased Life Expectancy
•6-10 Pounds Lighter
•Lifestyle Satisfaction
2010 Census
Services Per Thousand
•3 Physicians
•5 Dentists
•10 Nurses
•2 Grocery Stores
•2.5 Restaurants
•1 Tavern/Sports Bar
•0.2 Coffee Shops
Area = 2.0 m2 Per Person
Services density.
Walkscore.com assigns a value to the proximity of basic services such as groceries and
schools, but also considers variety. Few of us eat at the same fast-food restaurant every
meal, or do 100% of our shopping at Walmart. A perfect walk score results when all of
the required amenities are within a quarter-mile walk. As you might expect, the highest
scores are for urban areas such as Manhattan, San Francisco, Seattle, and Miami, but
nearly all cities have some areas that are highly walkable.
For a different approach, I examined some selected services as reported in a census
bureau analysis of the 2010 results. They reported services per thousand population,
and by considering the average square feet per applicable business, I deduced that 2
square meters per person provided sufficient space for all of these retail and service
businesses. Note that the differences in ratios are attributed to different definitions, and
the 2 square meters per person includes additional services not listed above such as
retail shops.
Slide 9
Designing an Orbital Settlement
COMPLETE RECYCLING
• All Organic Material Must Be Burned!
– All roots, stems, leaves must be recycled
– Feeding to animals is a form of burning
– Fungi, bacteria, & insects are too slow
• Don’t want buildup of organic matter in soil
• Sludge Burned Via Supercritical Water
Oxidization
• Conserve Carbon, Hydrogen, especially
Nitrogen
– Industrial processes (ie, smelting &
refining) must recover all gasses
Recycling. Everyone realizes that total recycling is vital for the permanent settlement of
space.
But few recognize that the plants we depend upon for food and oxygen need a great
deal more CO2 than we exhale. Our metabolism converts food into CO2, consuming a
corresponding mass of oxygen in the process. But we only eat perhaps 25% of a typical
high-yield crop plant’s biomass, and to grow a crop, we need to provide sufficient CO2 to
grow the entire plant. Further, while converting CO2 and water into hydrocarbons and
oxygen, the plant also produces much more oxygen than we can consume. The answer is
to capture 100% of the uneaten plant matter, leaves, stems, and roots, and burn it. That
will ultimately restore the balance – except for when we use that plant matter in
products such as wood furniture, or paper books, or clothing, in which case we still have
to deal with excess oxygen.
Note that feeding plant matter to animals is another form of burning. The method we
use to convert uneaten hydrocarbons into CO2 is not as important as the end result, and
that means we can afford to raise goats, chickens, rabbits, and some kinds of vegetarian
fish without a significant increase in agricultural area.
On Earth, we have so much environment to trash that we bury sludge, and use wasteful
industrial processes that pollute the air and water. We assume that Mother Earth will
absorb our wastes. We’ll have no such luxury in our settlement, but there doesn’t
appear to be any show-stoppers, only different ways of conducting ourselves.
Slide 10
Designing an Orbital Settlement
PROPOSED DESIGN
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Kalpana One Revised Cylinder
Outer Endcaps for Agriculture
Inner Endcaps for Offices, Hotels, Medical
11 m2/p One-G Cylinder Area
3 Floors for Residential => 300 sf/p
3 Floors for Offices, Schools, Overhead
Cylindrical Park (one huge volume)
Park Edges for Restaurants & Retail
Axial Tube for Zero-G Research,
Manufacturing, and Entertainment
The proposed design: Structurally, it seems clear that the Kalpana One Revised design is
optimal in many ways, especially for rotational stability and efficient use of shield mass.
However, instead of end cap windows to admit natural sunlight, I specify using the outer
end caps for agriculture, since even LED lights produce heat and are best located near
the outer wall. The inner end caps are an ideal location for hotels, offices, hospitals, and
light manufacturing.
The Kalpana One geometry results in a maximum population with inadequate perperson space in the 1-G cylinder area, only 11 m2/p. For the smallest habitat, I allocate 3
floors for residential living, resulting in over 300 sf/p, 600sf/couple, or 1200 sf for a
family of 4.
An additional three floors would serve for offices, schools, libraries, and overhead areas
for ductwork, conduits, and storage.
The vast bulk of the volume is occupied by a central cylindrical park, which serves for
outdoor recreation, exercise, and provides a bit of Earth in space. Note that the sides of
the park are the ideal locations for shops and restaurants, providing the kind of shopping
and dining experience found in the newer outdoor walkable shopping centers.
However, for the larger settlements I increase the allowances for residential living space.
Slide 11
Designing an Orbital Settlement
SCALABILITY
Spin
RPM
Radius
(m)
Width
(m)
Popula
tion
Shell
(cm)
Park
Width
Park
Ceiling
Park
Acres
Aster.
Diam.
2.99
100
130
8k
1.3
100
160
13
120m
1.73
300
390
67 k
3.7
350
550
160
240m
1.00
900
1,170
590 k
11.3
1,125
1,750
1,500
480m
0.58
2,700
3,500
5.3 M
38.5
3,400
5,300
14,500
850m
0.33
8,100
10,500
47.5M
118.5*
10 km
16 km
130,000 1500m
•Standard engineering formula for pressure vessel (+weight of shield and structure)
•Assumes high strength (1,200,000 kpa = 180,000 psi) weldable 9Ni-4Co steel
•* (largest structure) reduces design pressure from 250 kpa to 150 kpa
•=> 1.25 m of steel provides full 10 T/m2 shield
•Largest settlement constructed (almost) entirely from nickel-iron asteroid
•The largest nickel-iron asteroid (16 psyche) could build 1,250,000 of these
•Steel can build Kalpana One cylinders up to 16 km radius; glass or basalt fiber cables
can built them over 100 km in radius; carbon nanotubes over 1,000 km.
This architecture scales over two orders of magnitude, supporting populations from 8
thousand to roughly 50 million people.
I use standard engineering design formulas for pressure vessels, having recognized that
10 tons per square meter of shield results in precisely the same stress as 1 atmosphere
of pressure. I specify using a very-high-strength weldable steel whose composition
happens to be close to that of the metal grains in asteroids. The shell thicknesses are
calculated for a total of 2.5 atmospheres of pressure, except the largest for which the
shell itself is adequate shield mass.
The largest settlement depicted is the biggest one that is relatively efficient in its use of
steel for a shell. Larger structures are possible up to a maximum theoretical 16 km radius
cylinder for steel, or over 100 km for more exotic materials such as glass or basalt fibers.
We can build the smallest settlement from the resources in a 120 meter ordinary
chondrite asteroid massing 2 million tons, and have enough steel left over to build a
dozen 5-gigawatt solar power satellites (we need the slag for our shield).
But even the largest settlement requires only a 1.5 km nickel-iron asteroid, of which
there are many. Note that the largest iron asteroid, 16 Psyche, contains sufficient iron to
build a staggering 1.25 million of these habitats – enough to support a total population
of 60 trillion people – 7,500 times the Earth’s current population. But then again, I’m
preaching to the choir!
Slide 12
Designing an Orbital Settlement
RESEARCH TOPICS
• Complete Closed-System Recycling
– Burn all wastes into CO2 and ash
– High-intensity farming
– Ecosystem balance
• Low Carbon/Nitrogen/Hydrogen
Products for Clothing, Furniture,
Insulation, Carpeting, Packaging
• Oxygen-Fueled Ion Thrusters
• Airline-Scale Reusable Launch
Systems
• Space Suits Appropriate for
Everyday Workers
Important research topics.
Chief among these is closed-system recycling, including high-intensity agriculture which
itself is full of unknowns, such as ecosystem balance requirements.
To grow our food, each person requires a half-ton each of hydrogen and carbon, and
50kg of nitrogen. We’ll want larger allowances to have some excesses for paper,
clothing, swimming pools, the central park plants, and the like. We’ll need much more
nitrogen if we want to breathe the 50% nitrogen atmosphere that the NASA 1977 study
recommended – the smallest habitat alone requires a half-ton per person.
Note that we presently depend upon hydrocarbons not only for organics such as wood,
cotton, and paper, but also as paint binders, insulators for electric wire, plastics for
household goods and packaging, elastics, bearings, seals, even toilet seats. I cringe
thinking about stainless steel toilet seats.
When we convert iron ore into iron, the bi-product is oxygen - about a ton for every
three tons of steel produced. Building the habitat itself frees up enough oxygen to fill it
600 times over. The best use of that surplus is as fuel in ion thrusters.
We all know that a major need is low cost launchers, and reusability is key. Enough said.
Also, today’s space suits take forever to get into and out of, and are difficult to work in.
This needs to be fixed.
ide 13
Designing an Orbital Settlement
CONCLUSIONS
• Kalpana One Revised Style Orbital
Settlements
• High Population Density
• Artificial Lighting (primarily LEDs)
• Shield Incorporated in Shell
• Per-Person: 75m2 total area (30 residential),
6kw, 20m2 cylinder surface, 200 tons shield
• Design for Walking Efficiency & Social
Interaction
• Restaurants & Retail Borders Cylindrical Park
• Scales 100m & 8k People to 8km & 48M
People
To summarize, I recommend the stubby cylinders proposed by Globus et al. in the
Kalapana One Revised paper.
I recommend the adoption of high-density walkable neighborhood approaches to both
reduce costs and improve health, socialization, and productivity.
Artificial LED plant lighting for agriculture is a no-brainer, as is incorporating the shield
mass in the rotating shell of the habitat.
This design results in a relatively simple, scalable architecture, whose greatest flaw is
that a tin can is visually boring, so an artist is needed to add visual details to make it
appealing!
Slide 14
Designing an Orbital Settlement
QUESTIONS?
• Shape: a cylinder with width 1.3 x radius
• Energy:
– 6 kw/person input and output
• 20 m2 / person external area at 0°C
• 33 m2 / person solar panels
– Settlement to be shaded!
• High-density urban population with walkable
community dynamics & >= 30 m2 residential space per
person
• Recycling: Plants require much more CO2 and
nitrates/ammonia/urea than we exhale or excrete, and
produce much more oxygen than we consume
Slide 15
Designing an Orbital Settlement
MOST IMPORTANT R&D ISSUE
 Design and implementation of complete closed
system recycling facilities to efficiently convert
human waste and crop residues into food,
oxygen and water
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Burn All Wastes Into CO2 and Ash (SCWO)
High-Intensity Farming – Is it Maintainable?
Ecosystem Balance – What Else is Needed?
Continuous Loop or Store Excesses?
How Close to Perfection?
Will Poisons Prevent Success?
The most important R&D issue: Recycling.
High intensity farming currently relies on heavy use of fertilizers, and may be susceptible
to drug-resistant pathogens.
Ecosystem balance may be critical; what protozoa, fungi, nematodes, mites, worms, or
insects will prove necessary?
What critters will force themselves on us and affect the ecosystem (i.e., will bacteria
thrive on something unanticipated or consume something vital?)
Excess oxygen is produced by growing plants, oxygen that will be consumed when the
plant is eaten and/or burned. To start the farms, we’ll have to pump in CO2 and H2O
from somewhere, likely the smelters, and we’ll have to extract and store the excess
oxygen. But once we’ve reached a stable population, can we keep it smoothly in balance,
or will we always have to control the gasses?
I described a goal of complete closed system recycling, but I suspect we’ll never quite
achieve it. Something will vanish, I don’t know what, selenium or vanadium or
manganese or something else needed in trace amounts.
Another thing I worry about is poisons, especially heavy metals like mercury, but
potentially anything that is much more common in asteroid regolith than in the Earth’s
crust, such as chromium, cobalt, nickel, and selenium.