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Cylindrical Colonies - by

Critical Trajectories for the Human
Settlement of the High Frontier

by Lee S. Valentine
Executive Vice President Space Studies Institute,
P.O. Box 82, Princeton, NJ 08540

Presented at the New Trends in Astrodynamics 2006 Conference,
Princeton University, August 16-18, 2006
Copyright 2006 Space Studies Institute and AIP

Space for All Blog
Space Transport News blog
Space Activism
Life in Space, Space Settlements

Lee Valentine is a physician, a long time space advocate, and a director of the Space Studies Institute. Hear the The Space Show interviews with him on March 14, 2006 and on March 16, 2004.

See also this earlier article by him: A Space Roadmap: Mine the Sky, Defend the Earth, Settle the Universe.


Abstract: If preservation and prosperity of humanity on the Earth and human settlement of space are our goals, we should concentrate on a commercial path to get there. Commercial enterprise has a long history of fortuitously aiding scientific progress. We expect radical changes in the cost of earth to orbit transportation, and in the methods and efficacy of deep space transportation, within the next two decades.

A successful space tourism industry, beginning with suborbital tourism, will greatly drive down the cost of access to orbit over the next 15 years. Inexpensive transportation to low Earth orbit is the first requirement for a great future on the High Frontier. Inexpensive means the cost associated with a mature transportation system. A mature system has a cost of three to five times the cost of the propellant. The first cheap, reliable and highly reusable rocket engines are just now appearing in vehicles. With an assured market and high flight rate, the heretofore glacial progress in reducing the cost of space transportation is likely to become rapid. This is the first critical enabling example of synergy between free market economics and scientific and technical progress in space. It will not be the last.

New high power switches and ultracapacitors developed for the automotive market make possible cheap, robust and reliable mass driver engines. In space construction, using masses of nonterrestrial materials make the gravity tractor technique much more capable than previous schemes to maneuver asteroids. Ion propulsion will continue to improve and the first solar sails will be flown. Advanced robotics will allow remarkable improvements in productivity. The computing power available to robots began to follow the exponential Moore's law less than decade ago. The first commercial autonomous mobile robots appeared in late summer 2006. Humans, however, will be required for the foreseeable future in repair and supervisory roles, particularly in unstructured settings such as asteroid mines.

The evolution from small tourist stations of the next decade to large space hotels will make economical the use of fully closed life-support systems. These could be considered the first space colonies. Derivatives of these commercial space hotels may form suitable Moon and asteroid mining habitats.

Using nonterrestrial materials is a key to opening the space frontier. Dozens of rendezvous missions to Near Earth Objects will be needed to assay their resources and to plan rational NEO diversion. The development of NEO mining techniques serves two purposes, raw materials supply and planetary defense. We need economical trajectories to and from these bodies. These trajectories must not only be economical in terms of delta V or time, but in dollars; and in the time value of money, factors not generally considered by the OMB.

Satellite solar power stations may be a $500 billion per year market worldwide and cheap nickel steel from asteroids may be an enabler of power satellite construction. One asteroid of the right size and composition in a suitable orbit could open this market. Platinum group metals may be an important export, either as a primary product, or as a byproduct of nickel steel alloy production. Other products, derived from carbon, may also be important. The first economical product from an asteroid mine is likely to be water, for propellant or life-support and radiation shielding in space hotels.

Keywords: Space Transportation, Asteroid Mining, Solar Power Satellites, Space Settlement, Private Space Travel, Space Hotels, Nonterrestrial Materials, NEO, Composite Tanks, Rocket Engines. PACS: 00


I would like to argue that the human space enterprise has three interrelated goals; the first is preservation of the human race. Preservation means two things: first, it means assured defense against catastrophic comet and asteroid impacts. Defense against cosmic impacts is something we must do in space, even if we do nothing else. Second, preservation means expanding the ecological range of humankind into the wider universe. Expansion of ecological range will provide insurance against whatever other extinction event may come our way. It makes no difference whether it is thermonuclear war or natural or artificial pestilence. With an ecological range extending across the solar system, some humans will almost surely survive any cataclysm. And thirdly, we would like to use the energy and material resources of circumsolar space to ensure prosperity on the Earth. We would not only like to survive, we would like to thrive. The transmission of space solar power to the Earth and the delivery of essential metals to the terrestrial economy promise to improve living standards globally, to mitigate global warming, and to increase the sustainable carrying capacity of our home planet.

There are two senses in which I would like to use the word trajectory. The first trajectory is that of a spaceship or probe traveling in space governed by gravitation, light pressure, and engine thrust. The second sense of trajectory is the path of progress in time and space of a commercial enterprise which would use the first trajectory to best advantage. The second kind is governed by the laws of supply and demand and return on investment and technological capability and, not least, fiduciary responsibility. To determine whether the trajectories of the second kind will be interesting at all, we must examine the reasons for the present high cost of space transportation.

Cheap Reliable Transportation Is A Prerequisite

The prerequisite to achieving these goals is cheap Earth to orbit transportation. We should concentrate on an evolutionary, commercial path to achieve that. The previous approaches, converting ICBMs and expensive politically motivated projects to win battles in the Cold War, have failed.

Toroidal Colony - Don Davis
"Part of the rim including many nearby space vehicles" (large)
Don Davis - NASA Space Colony Artwork
The technical maturity of one or other kind of transportation, historically, has been critical to the success of new commercial enterprises. With the immature space transportation of 2006, the trajectory of commercial enterprises other than satellite communications and remote sensing, and the occasional orbital tourist, will be a horizontal line. The reason is straightforward, with transportation costing thousands of dollars per kilogram to low Earth orbit, solar power satellites, and asteroid mining and space hotels cannot turn a profit. (1) Space settlement will remain a dream.

It is a sad commentary, as well as an indictment of government space programs, that a vehicle designed in Russia more than 50 years ago provides the cheapest manned flight to low Earth orbit.(2) It is as though the WWI Sopwith Camel were the most advanced aircraft flying in 1965. Unfortunately, the past half century's stagnant Earth to orbit launch cost has convinced people that high space launch costs are intractable and must be a necessary result of the challenging physics of the problem. This high cost is mistakenly viewed by some as a necessary consequence of ascent to orbit using chemical rockets.

For most booster designs, the flight rate is so low that production economies of scale are imperceptible. An empiric rule of thumb is a 15% reduction in unit cost for every doubling of the number of units produced. The absolute cost of space transportation has fallen by only 30% over the past few decades. That number agrees with the rule of thumb, and reflects a pitifully low flight rate.

The High Cost Of Space Transportation Is A Legacy Of Artillery Rockets

The argument is this: the present high Earth to orbit launch cost is an artifact of using rocket artillery in a transportation role. Transportation to low Earth orbit is expensive because the rocket vehicles have been expendable. It is plainly impossible to reduce the cost of transportation below a few thousand dollars per kilogram if the discarded launch vehicle costs that much itself. During the Cold War, rocket engines and structures were optimized to deliver the maximum warhead mass with a single use. The lack of time during the Cold War space race forced the conversion of intercontinental ballistic missiles into manned space launchers.

Incremental Flight Test Saves Money And Increases Reliability

It is more expensive to design a spaceship that cannot be incrementally and repeatedly flight tested. Incremental flight test saves design engineering hours. Aircraft designers know that it is much cheaper to develop and flight test a piloted vehicle with all altitude save- the- vehicle capability than it is to build a series of vehicles that must be tested to destruction, or ascend flawlessly to orbit on first flight. (4) Compare the six million dollar loss of the Falcon 1 twenty-eight seconds into its maiden flight, because of a minor fuel leak, to the uneventful landing of the piloted EZ Rocket following a similar minor problem. Because test flight opportunities are expensive and hence few, design flaws may go undiscovered for years. The space shuttle's foam shedding problem is a case in point.

A corollary is that infant mortality of traditional launchers is high. Incremental flight testing prevents early mortality from faulty construction. Every commercial airliner is test flown to reveal manufacturing defects so that those can be corrected before the airliner enters passenger service. Expendable launchers cannot be test flown before their single mission; therefore, manufacturing defects must be inspected out. That strategy is expensive and doesn't work particularly well.

Spaceships must be reliable to be cheap: if vehicles are unreliable, the replacement costs and insurance rates make them expensive. Ted Taylor pointed this out four decades ago. (5) The idea of separating cargo and people for safety's sake is nonsense; in the commercial world, cheap transportation must be safe transportation. The suborbital launch companies understand that the inexpensive development and high reliability enabled by incremental flight test are crucial to market success.

Design Requirements Of A Mature Space Transportation System

The design requirements for a mature space transportation system, in which each vehicle is to be used thousands of times, are much different from those of missiles. Rocket engine lifetimes until recently have been short, partly by design. Engineering a long lifetime in a missile really is undesirable since the increased weight needed to yield a longer service life reduces the warhead mass. And the rockets themselves are fragile for the same reason. The more robust structure needed for reuse would be uneconomical: they are throwaway articles designed to deliver a warhead and be destroyed in the process. Present day rockets could theoretically be reused, if provision were made for recovery. (6) Their fragile structure would limit their maximum service life to tens of flights. Engineering a duty cycle of thousands of flights, as opposed to the theoretical tens of which present expendable rockets might be capable, exacts a mass penalty of about 20%. The Saturn boosters, I and V, were the first designed explicitly for reuse and they were designed to be reused fewer than 50 times. In that sense, they represent the pinnacle of reusable rocket engineering up to now.

The Importance of New Rocket Technology

Rocket engines for cheap transportation must be designed for thousands of flights. Rocket engineering has improved dramatically in the past five years. XCOR has designed and tested cheap, robust and reliable, high Isp rocket engines capable of thousands of full duration burns. The new engines have orders of magnitude better price performance than Cold War legacy engines. XCOR has also built a new composite LOX tank which is both stiffer and stronger than previous designs and capable of thousands of flights. The new composite LOX tanks weigh 65% as much as the best previously available tanks. Since the LOX tank is more than half the typical vehicle weight excluding the engines, the large weight saving means increased margin or increased payload. Adoption of this LOX tank design may increase the payload by a factor of as much as three. The new tank system is easily repairable, has a very low thermal coefficient of expansion and does not burn in high pressure oxygen.

The second major contributor to high space transportation costs is the low flight rate, since both labor cost and the capital cost of facilities must be amortized over a small number of launches. (7) To achieve minimum cost, we need robust reusable vehicles and a high flight rate. Southwest Airlines makes money by keeping its airliners' wheels in the wheel well. Contrast a 747 which flies 500 flights per year and has a ground crew of twelve with the Space Shuttle which flies once a year and has a ground crew of twelve thousand.

Mature transportation technologies such as airlines, shipping lines or trucking companies have the peculiar characteristic that the transportation price is within a factor of three to five of the fuel cost. Design, construction, operator's wages, amortization, depreciation, profit and insurance comprise 60 to 80 % of the price. The Zenit booster has a propellant cost of about ten dollars per kilogram of payload orbited. The idea that the rocket equation and implied cost of propellant prohibit reducing launch costs is just false. A mature space transportation system might be expected to have a cost per kilogram to orbit of $100-160 since making the vehicles more robust may increase the required propellant to a quantity greater than today's most fuel efficient launchers. That is a fraction of one per cent of space shuttle costs and a just few per cent of the cheapest launch available today. It would be useful to point out that launch demand is predicted to become elastic at a launch price of $1200 per kilo.

Private Space Transportation Provides Market Pull

Fermi's Paradox: where are they? The skeptic may say, "Well, if it is really possible, why hasn't the private sector done it already?" The new technologies are wonderful but not sufficient. A problem for companies planning to build reusable space transportation is that the present demand for space launch is only a few hundred tons per year. The greater demand necessary to justify a fully reusable launcher may not appear rapidly enough to amortize the development cost, and yield the necessary return on investment. It appears that the first vehicle will have to be small, to allow the market to grow, and cheap to develop and build, to give investors an adequate ROI. The demand for space flights must increase by orders of magnitude before the private capital markets will finance expensive new spaceships. The new launch companies understand this, too.

Of course, a capitalist has to believe that there is a market for his product or service. Bankers have a fiduciary responsibility to their investors to avoid overly risky investments. Until Dennis Tito's flight and the successful flights of SpaceShipOne, few people believed there was a space tourism market. Luckily for would be space settlers, dynamic American entrepreneurs have increased the wealth of our society so that now many people now are able to afford the projected price of a suborbital flight. Market studies by the Futron Corp. and others project a vigorous suborbital space tourism market. With an assured market and high flight rate, and free market competition, the heretofore glacial progress in reducing the cost of space transportation is likely to become rapid.

There will be destinations. Bigelow Aerospace is testing its prototype orbital hotel that was launched from Russia a few weeks ago. In many details, not the least of which is cost, it promises a space habitat much superior to the ISS. Mr. Bigelow has plans to deploy a station composed of his modules on the Moon. Lockheed Martin and Bigelow have undertaken a joint investigation into the conversion of the Atlas V launcher for private spaceflight.

Space Hotels: The First Off World Colonies

The evolution from small tourist stations of the next decade to large space hotels will necessitate the incorporation of fully closed life-support systems. According to a conservative analysis by the Boeing Corporation, a largely closed life-support system for a low Earth orbit space hotel should pay for itself within five years. (8) The resupply mass per day per person is about 3 kg and the total mass of a closed environment life-support system sized for one person is estimated to be 4 metric tons. The closed systems have an advantage for low Earth orbit locations since, in that location, they do not require massive radiation shielding. Orbital locations above the Van Allen belts will require massive radiation shielding. In the case of an asteroid mission that shielding may be obtained cheaply from the asteroid itself, and of course, on the Moon the regolith can provide radiation shielding that is very cheap indeed. The CELS systems do require, however, some natural or artificial gravity to separate gases from liquids, and to allow reliable pumping of liquids from one subsystem to another.

Despite several decades of effort, including several attempts at a completely closed system sized for people, a simple, reliable, robust closed environment life-support system has yet to be developed. Considerable effort and expense has gone into the design of small systems to grow plants in zero gravity. Effort has also been expended on algal bioreactors to remove carbon dioxide, unfortunately, algae is not an acceptable human food stuff. NASA has made considerable effort to minimize the area requirements for food plants. A serious disadvantage of optimizing for minimum area is that very tight control over the quantity and quality of nutrients delivered to the plants is required.

Crop And Animal Waste Recycling Is The Critical Problem

For Closed Systems Relatively little effort has been expended on the key problem of converting crop waste and animal wastes into nutrients suitable for growing plants. SSI's investigators believe that a largely, or even wholly biological, closed environment life-support system will be the optimum choice. It is possible that we would choose the Haber Bosch process for nitrogen fixation and/or a backup supercritical water oxidation unit to recycle refractory crop waste. Both require high temperature and pressure and generate highly corrosive chemicals. Those two physicochemical subsystems do offer the possibility of decreasing required surface area of the CELS system by about one third.

Prof. William Jewell, SSI's PI for CELSS has been developing a system at Cornell that would avoid the need for complicated monitoring systems tuned to precisely control delivery of nutrient mixtures to plants. It relies on robust biological systems, in the experience of the Russians, much more reliable than their artificial gear. The system would allow the recapture of much of the chemical energy in the crop wastes. The methane produced could be used as rocket fuel or as a chemical feedstock. The waste management subsystems are useful on Earth to recycle manure and crop wastes on farms and to produce energy and byproducts. Prof. Jewell has demonstrated, at a bench level, a digester capable of returning 85% of the chemical energy in low-quality hay as high-pressure methane. (9)

Nonterrestrial Resources: Mining The Sky

It appears obvious that nonterrestrial materials are key to opening the space frontier.(10) The minimum cheap rocket transportation cost to low Earth orbit is an order of magnitude greater than the price of even the most expensive raw materials in common use. The Moon's resource potential has not been fully explored, but, for the purposes of this conference we should also think about the NEOs. In terms of delta V, some are the easiest bodies to reach in the solar system. These bodies taken together contain the full range of elements and are, in some cases, highly differentiated. (11) From the examination of meteorites, we have some working knowledge of their compositions. Many have higher concentrations of platinum group metals than the best terrestrial ores. Many contain volatiles that are in short supply on the Moon. There are at least 20 spectral types and close rendezvous missions to a representative of each type should be undertaken to assay their chemical and physical properties. Physicochemical characterization of NEOs will allow development of asteroid mining techniques.

The ability to mine NEOs is also implies the ability to deflect them. The optimum method could be a gravity tractor (12) with most of the mass of the tractor to be obtained from the asteroid itself. Using mass drivers for propulsion instead of ion engines would allow orders of magnitude greater acceleration as well as much lower total electrical energy requirement. Whether a private enterprise could afford the insurance to swing one around the Moon or persuade the U.N. that it would be safe to return anything over 30 meters, is hard to say.

The new PanSTARRS telescope array in Hawaii, of which the prototype saw first light in June, 2006 will increase the detection rate of NEOs. Most will be main belt asteroids but tens of thousands will be Earth crossers. Eventually, one will be found on an Earth impact trajectory. We need economical trajectories to and from these bodies. The trajectories must not only the economical in terms of delta V, but also eventually in dollars. NASA and the OMB do not generally consider the time value of money. If we are successful in our quest to expand the ecological range of the human race, the time value of money will be an additional factor in calculating trajectories.

Satellite Solar Power: A Huge Potential Market

In 1968, Peter Glaser proposed that solar power satellites to beam energy from geostationary orbit to the Earth could be a major economic boon from advanced space technology. (13) Solar power from space is one of the few technologies that appear to be able to produce the quantity of high-quality electric power needed to bring a first world standard of living to all inhabitants of our planet. To illustrate the size of the potential market, the worldwide cost to build electric power stations is projected to exceed $500 billion annually for the next quarter century. Former astronaut Dr. Philip Chapman calculates that ground launched power satellites could be economical at a price of $200 per kg. In 1974, Gerard K. O'Neill proposed using the resources of the Moon to build solar power satellites in geostationary orbit to supply energy to the Earth. (14) According to his analysis, and confirmed by subsequent work at the Space Studies Institute, the capital cost of a system based on nonterrestrial resources could be much lower than the capital cost of an Earth launched power satellite system. One 80 meter nickel steel asteroid would provide enough metal to construct ten five gigawatt solar power satellites. Or a handful of O'Neill Island One space colonies.


In summary, radically cheaper Earth to orbit transportation is on the horizon. The near term economic driver is private space travel; longer term drivers are likely to be space solar power and asteroidal metals. CELSS has a near term economic benefit and will enable long duration operations far from Earth, including permanent settlement. New technologies, many under commercial development for terrestrial applications, and the invisible hand of the free market will make things even cheaper. So that the words of the poet, Alfred, Lord Tennyson:

"Men, my brothers, men the workers, ever reaping something new:
That which they have done but earnest of the things that they shall do:

For I dipt into the future, far as human eye could see,
Saw the Vision of the world, and all the wonder that would be;

Saw the heavens fill with commerce, argosies of magic sails,
Pilots of the purple twilight dropping down with costly bales;"

may soon represent more than dreams.


The author would like to acknowledge the insights of Eric Anderson, Philip Chapman, Len Cormier, Samuel Dinkin, Freeman Dyson, Peter Glaser, Jeff Greason, Klaus Heiss, William Jewell, John S. Lewis, Ed Lu, John Mankins, Hans Moravec, Rusty Schweikart, Henry Spencer, Gordon Woodcock and Edward Wright.


1. Commercial Space Transportation Study, NASA Langley May, 1994
2. Futron Corp., Space Transportation Costs: Trends in Price per Pound to Orbit, 1990-2000 Sept. 6, 2002
3. General Public Space Travel and Tourism, Volume 2 Workshop Proceedings NASA CP 1999209146 Feb., 1999
4. " Reducing the Cost of Space Transportation" AAS Science and Technology Series Vol. 21, 1969
5. T.Taylor, Propulsion of Space Vehicles in R.Marshak: Perspectives in Modern Physics, New York 1966
6. Lockheed internal study9-2006 proposes reuse of Atlas V booster as part of a study examining the feasibility of an Atlas V tourist vehicle
7. B. Kutter, Commercial Launch Services: An Enabler for Launch Vehicle Evolution and Cost Reduction, Lockheed Martin Space Systems Company
8. E. Gustan and T. Vinopal, Regenerative Life Support/ Controlled Ecological Life Support System Contract NAS 2-11148, November, 1982 Boeing Aerospace Co.
9. Prof. Wm. Jewell, personal communication
10. K.E. Tsiolkowskii, "Dreams of Earth and Sky" 1895
11. J.S.Lewis and M.Matthews, "Resources of Near Earth Space" Univ. of Arizona Press April, 1994
12. E.Lu and S. Love, Gravity tractor for Towing Asteroids, Nature, vol. 438, 10 Nov, 2005
13. P.Glaser, Power from the Sun, Its Future, Science vol.162, no. 3856, 1968 pp. 857-861
14. G.K.O'Neill, Space Colonization and Energy Supply to the Earth, Science, vol. 190, no 4218, Dec 5,1975

Copyright 2006 Space Studies Institute and AIP

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