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Copyright 1978 by the American Institute of Aeronautics and Astronautics.

As an outgrowth of the 1977 NASA-Ames Summer Study, artist R. Guidice was asked to paint the most recent engineering concepts depicting the building-block phases of a program of space-based manufacturing from non-terrestrial materials. The paintings on this issue of A/A's cover, and on the opening pages of the following article, were produced in close consultation with the study group. For example, the paintings of the mass-driver reaction engines and the modular habitats are based on engineering sketches drawn by one of the study participants, J. Shettler of General Motors.

The paintings depict two of the scenarios treated in the summer study: retrieval of lunar materials and retrieval of asteroidal materials. Both scenarios assume mass drivers used for low-to-high orbit transportation along the lines described in G. K. O'Neill's article following here. Mass drivers would also propel lunar materials into free space for processing and pull Earthapproaching asteroids on gravity-assisted retrieval missions into high Earth orbit. In his article, O'Neill describes a "low-road," bootstrap program based on the lunar option.

The summer study also made considerable progress in exploring the asteroid alternative. An asteroid study

IMPRESSIONS OF SPACE MANUFACTURING

group found that the retrieval of asteroidal material may be cost-competitive with the retrieval of lunar material. Some Earth-approaching asteroids, if gravityassisted by Venus, Earth and Moon, were calculated to be as accessible (in terms of energy) to space manufacturing sites as the lunar surface.

Scientists and engineers at both the 1977 summer study and at a NASA Office of Space Science-funded workshop on near-Earth resources held later in the summer of 1977 in La Jolla unanimously recommended an increased effort in the telescopic search for Earthapproaching asteroids, follow-up work in determining their orbits and chemical classification, and a Fiscal Year 1980 new start on a program of rendezvous missions for the mid-Eighties to prime asteroid candidates for chemical assay.

Advantages of the asteroid case include the probable availability of water, carbon and free metals, the availability of continuous solar energy and freedom from the need for soft landings. Disadvantages include poorer knowledge of available materials and greater distances and times required for retrieval. At this early stage it seems wise to keep both options alive by vigorous research.

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At the lunar base's operations center, controllers monitor the mining of lunar soil and its emplacement into mass-driver buckets. The mass-driver accelerates the lunar material to the Moon's escape velocity of 2.3 km/sec. The throughput of material, estimated to be initially tens of thousands of tons per year, and later hundreds of thousands of tons per year, would permit constructing several satellite power stations and space habitats per year.

In this view, three massdriver reaction engines have been attached to a 100-m-diam (1-million-ton) asteroid and are taking it on a gravity-assist trajectory past Venus. A bag encloses the asteroid to contain debris as it is mined. Some debris supplies reaction mass to make the necessary velocity changes. A solar, furnace and storage tank

for extracted water,

carbon, and free metals ride the center axis of the assembly.

This interior view of a habitat module shows the living accommodations and office of a crew member. A cylindrical liquid-hydrogen container from a Shuttle external tank forms the pressure shell of each module. In this design, generated by a 1977 NASAAmes Summer Study group, modules are divided into some seven levels, with three pieshaped living accommodations on each level. These modules would be clustered, shielded, tethered by cables, and rotated to provide Earth-normal gravity during the buildup period of space-based manufacturing from non-terrestrial materials. All illustrations in this section courtesy of NASA.

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Plant and animal agricultural areas, shown in this cross-sectional view, occupy toroidal rings at the ends of an early space settlement fabricated from nonterrestrial materials. The rings supply food for 10,000 residents of the settlement, a sphere about 500 meters in diameter.

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SPECIAL SECTION

The Low (Profile) Road to Space Manufacturing

By GERARD K. O'NEILL

Professor of Physics, Princeton University

Rendering the Shuttle's external tank into powdered propellant for a mass-driver would constitute a breakthrough in space transportation economics, in effect, a quicker route to largescale space manufacturing

GERARD K. O'NEILL chairs the Task Group on Large Space Structures of the Universities Space Research Association in addition to holding down his Princeton post. At the time this article was written he was on sabbatical leave at MIT as Hunsaker Professor of Aerospace. His book, The High Frontier, won the Phi Beta Kappa award as the best science book of 1977. O'Neill received a Ph.D. in physics from Cornell Univ. and specialized in high-energy particle physics.

Early in 1977, an exciting new possibility for speeding space industrialization came up. Several of us at MIT checked out working-model hardware for it in the spring of that year. An independent NASAsupported study that summer wrung out the concept much more thoroughly, with similar results. In this article, I will outline the concept in its original brief form, and note significant changes made as a result of the detailed review.

In the autumn of 1976 the articles from the 1976 NASA-Ames Summer Study on Space Manufacturing from Non-Terrestrial Materials were completed. They gave us for the first time necessary formulae for optimizing the design of magnetic "mass-drivers." In addition, they gave us detailed numbers on space-manufacturing plants: mass, throughput of materials, power requirements, and workforce needs.

The new data suggested to me that, within the lift constraints of the Shuttle era, we might be able to reach the "takeoff point" at which space manufacturing will begin to regenerate and grow exponentially in its culture-medium of full-time solar energy and lunar-derived materials. Although caution is always advisable, it may not be an overstatement to describe the new work as a substantial advance. It has several components:

-A method by which the Shuttle could be upgraded to geosynchronous capability.

-Planning of a minimal facility on the lunar surface, from which materials could be transferred at low cost into free space.

-Evaluation of a processing and fabrication facility in space.

-Calculation of the inputs required in order for an initial facility to grow to the takeoff point. Combining these building-blocks, I find that the total requirement for lift from the Earth may be equivalent only to a few dozen Shuttle flights per year for several years.

For both technical and political reasons, we must squeeze the most performance, payback, and productive lifetime out of the Shuttle operational era. Our first problem is that the missions with a high potential economically-satellite power system deployment and the utilization of non-terrestrial materials-go to geosynchronous or lunar orbits; but the Shuttle is a low-orbit machine. Is there a way to bypass this problem and give the Shuttle geosynchronous-orbit capability?

The conventional answer has been no, other than by accepting a penalty in payload transferred, through the necessity of carrying propellant as payload during the Earth-to-low-orbit lift. That penalty would reduce the Shuttle's modest payload (29 metric tons per flight) by a factor of 2 to 4-to a point too low for the seeding of a space-manufac

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Copyright © 1978 by the American Institute of Aeronautics and Astronautics.

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A mass-driver reaction engine several kilometers long being assembled in low Earth orbit from Shuttle-carried sections. The astronaut in the foreground is adjusting the solar collectors that provide power for the mass-driver. Spent oxygen-hydrogen fuel tanks, in powdered form, will be fed into the mass-driver from a container in the background and expelled as reaction mass at 8-10 km per sec. Mass-drivers like the one shown are being studied as orbit-transfer vehicles for large payloads. Painting courtesy of NASA.

turing program even of minimal type.

The problem in fundamental terms: the Shuttle can accumulate payloads in low Earth orbit; to raise them to geosynchronous orbit would take energy, reaction mass, and an engine able to use them.

Solar energy is available, and has long been considered the prime candidate to power ion or plasma engines for deep-space missions. But to raise accumulated Shuttle payloads, several hundred tons at a time, from low to high orbit would take an engine of substantial thrust. In order to escape the loss in payload incurred by transporting propellant to orbit, a geosynchronous system would also need another source of reaction mass.

As designed, the Shuttle Orbiter carries no fuel for its main engines. It must bring its external tank (35 metric tons empty weight) almost to orbital energy. But either by the sacrifice of a few percent in payload to additional OMS (orbital maneuvering system) propellant or by storing hydrogen slush rather than liquid hydrogen, the external tankage could be brought into orbit to serve as reaction mass. To prevent the reaction mass from becoming a hazard to orbiting spacecraft, it should be ground into a fine powder, and after acceleration by and release from the carrier bucket, dispersed by being charged electrostatically. From 60 Shuttle flights per year, the reaction mass powder entering the atmosphere would amount to less than 1% of natural micrometeorites.2

The electromagnetic mass-driver (see the Oct

1976 A/A, page 23) seems to be the best, and perhaps the only, reaction engine with adequate specific impulse, thrust, and efficiency to use so unlikely a substance as powdered Shuttle external tankage as reaction mass. Essentially a linear electric motor, the mass-driver uses small buckets, constrained laterally by magnetic forces, to accelerate liquid or solid payloads to high velocity before releasing them and returning for reuse.

Once the processing of lunar materials in space begins, almost surely liquid oxygen will become the optimum material for reaction mass. Oxygen constitutes 40% of lunar surface soil, is likely to be a waste product from its processing, should be easy to handle as a liquid, and after acceleration would disperse by boiling to molecular form.

The third of the three articles contains formulas and numbers on which designs for a massdriver reaction engine can be based. 3.4.5 Later articles summarize without derivation formulas for optimizing system mass, based on the most recent theoretical work. 67 We have now established as a baseline a system in which each bucket has just two coils, the drive circuits are of two phases in quadrature, and the current in each drive coil oscillates through one complete sine wave as each bucket coil passes.

The 1977 NASA-Ames Study concluded that total system mass changes little with cross-sectional dimensions. For constant throughput, system mass changes by only 50% as payload varies by a factor

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