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

Copyright 1978 by the American Institute of Aeronautics and Astronautics.

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. 6.7 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

of over 1000. To keep the driver length within practical limits, it must use high acceleration (several hundred Earth-gravities).

The mass-driver employs superconductors only in the buckets; the stationary drive windings are of ordinary aluminum. For commercially available superconductors, field-strength limits are not reached even at accelerations of more than 1000 g. (Mainly for historical reasons, the earlier calculations on mass-drivers were based on rectangular geometries similar to those of an MIT Magneplane transportation concept. Calculations indicate tight-coupled axial structures, advocated by H. Kolm of MIT, to be lighter by more than a factor of 2 for the same performance.")

At a nominal 60 flights per year, the Shuttle can bring to low orbit approximately 1700 tons of payload and 2100 tons of reaction mass (i.e., external tanks). For spiral orbits, missions to lunar distance require an engine with specific impulse in the general range of 1000 sec, if the engine is to be returned and a substantial payload carried. A massdriver of that sort would have these characteristics based on the latest 1977 Study results:

The 1977 Study identified as a major necessary development for reaching this performance reducing the weight of a silicon-controlled rectifier to only a few times the weight of the silicon wafer alone. Present silicon rectifiers, designed for entirely different service with high duty cycles, weigh far more."

Until the 1977 Study, the mechanical stability of the long, thin mass-driver structure as buckets accelerated and decelerated remained unclear. After a great deal of work the Study concluded that the machine would just remain stable. Therefore, the system mass includes a yardarm and guywire structure with slow-acting servomechanisms to maintain straightness. (The Stanford Two-Mile Electron Accelerator, located almost on top of the San Andreas earthquake fault line, is given occasional adjustments based on a laser-beam sight line. Its aperture is about 1 cm.)

The nominal design just outlined would give more than twice the specific impulse of the Space Shuttle Main Engine, which is the state of the art for large chemical rockets.

To the best of my knowledge, no physical upper limit on mass-driver output velocity exists (other than c); but the optimum I, for missions in the

Earth-Moon system is 800-1000.

So we should be able to lift about 1300 tons/yr of Shuttle payloads from low-Earth to lunar orbit, without having to devote additional Shuttle flights to carrying propellant. What minimal facility would then be necessary on the Moon to convey lunar materials into space?

My original estimates centered on 1050 tons. A more detailed evaluation carried out during the 1977 study concluded with a figure of just under 1000 tons. The axial mass-driver for the lunar surface contributes less than a quarter of the total, excluding its foundations. That machine would be sized for an eventual throughput of 600,000 tons/yr, but the photovoltaic array first sent to the Moon could handle only a twentieth as much throughput. The machine parameters are:

F-2 In mission scenario for mass-driver reaction engine (MDRE) operating as an interorbital transfer tug, total mass in low-Earth orbit (LEO) would be 2075 tons. The engine would expend 1000 tons of powdered Shuttle tankage in the spiral-orbit climb to the Moon. After dropping a 730-ton payload in lunar orbit, the unmanned tug would spend the remaining reaction mass in a relatively quick return to low-Earth orbit for maintenance, refurbishment of solar-cell arrays, and reloading with payload and reaction mass.

Photovoltaic power supply (at lunar surface),

Life-support resupply (50 people, 1 year, at 10 kg/d). 180 Silicon and glass-fiber plant for materials encapsulation.... 30

In addition, resupply for a 10-person crew using 10 kg per person per day would total 37 tons per year. To soft-land that equipment, an equal quantity of propellant must be brought to lunar orbit. For operations both on the lunar surface and in space, we must ante up about 2800 tons of Shuttle payloads-about 100 Shuttle flights-to commence transporting materials from the lunar surface to a precise point in space. The 2800 tons breaks down as follows:

Total lunar surface equipment and supplies.. 1085

Interorbital tug (LOX-H2 engine, tanks, controls) 8 Supplies for personnel in space (for 1 yr at 10 kg per person) 110

The 1977 Summer Study obtained a similar total from a slightly different mix. Lunar landers and personnel transfer tugs had more mass, but converting spent Shuttle external tanks to 21-person habitats for use on the Moon and in space saved substantial mass. Those habitats could be spun on cables to create normal gravity, and would be divided into private apartments, one for each person in the original workforce.

At the first plateau, a number of milestones would be reached: proof of the concept of lunar materials transport, provision of large quantities of reaction mass for all later operations, and opening of a large reserve of mass for shielding manned operations at geosynchronous or higher orbit.

In reaching this plateau it will not have been necessary to learn how to chemically process or fabricate from lunar materials, nor to learn how to set up agriculture in space.

Once at the plateau, with the availability of lunar soil as reaction mass, the throughput of equipment from the Earth to geosynchronous and higher orbits will no longer depend on powdered Shuttle tankage. (That might be a good time to deploy a Shuttlederived lift-vehicle, so that subsequent launch operations can be of lower cost.)

The next step requires chemical processing, making oxygen in lunar materials (40% by weight) available as propellant for lunar landers, as

Throughput/power, tons/year per Mw... 650 Process and fabrication plant throughput (includes radiators but not power supply), tons/year-ton of plant 200 Photovoltaic power in space, tons/Mw... 5.3 Labor utilization (Si plus metals, fabricated), tons/year per person 60 Resupply needs (after oxygen extraction becomes practical), kg/person per day... .....2 LEO to high orbit mass penalty (personnel transfer)...

Totals for process and fabrication:
Throughput, ton/yr....

. Factor of 4

30,000

The 1977 Study updated these numbers, but the major conceptual breakthrough on chemical processing during 1977 was laying out a new basic process-flow chart depending on a carbo-chlorination reaction rather than on carbothermic reduction of aluminum-bearing minerals. The change reduced the peak temperature in the system by a thousand degrees to bring it in line with normal industrial practice here on Earth.

Once chemical processing is tested and working, the next step is fabricating a limited range of products from chemically separated lunar materials, such as photovoltaic arrays (lunar material, 20% silicon by weight) and thermal radiators to upgrade both the lunar launcher and the processing/fabrication plants in space. Products might later include structural components. We estimate an output of about 9000 tons per year of fabricated components-of these, about 20% would be photovoltaic arrays and 80% would be habitat components.

Together with 1800 tons/year from the Earth (21 flights/year of a Shuttle-derived freight rocket) and 750 people per year (13 Shuttle flights), this system would grow at a rate of 150,000 tons of additional throughput per year. When that growth reaches 630,000 tons throughput per year, or 190,000 tons/year of fabricated output products, the system output would equal in mass two 10,000Mw satellite power stations per year.

Minimum transport investment up to that point comes to just under 300 flights from Earth to low orbit, or about 42 flights per year averaged over seven years; of these, over half would be Shuttle flights (STS) and the remainder Shuttle-derived lift vehicles (SDLVs).

vestment.

F-3 summarizes recommended transport inIt includes additional safety factors-mainly 96 additional SDLV flights to provide the 3000-person workforce in space with 8400 more tons of supplies. These may be in the form of 1 ton of organics per person to stock later space farming, of 2 kg per person per day of emergency resupplies adequate for two years, and 4300 tons of Earth

built components for habitats or for products exportable to geosynchronous orbit.

If (as assumed) space agriculture has not yet been developed, 25 SDLV flights per year will be

required to transport food to the 3000-person work force, at 2 kg per person per day. At $20 million per flight that will add about a dollar per pound to the cost of fabricated products. (The rate of developing agriculture in space will likely be affected less by direct lift-cost economics than by the need to

YEARS FROM START OF LIFT.