Moon Miners' Manifesto

#106 June 1997

Concept Papers from Seattle Lunar Group Studies: Part 1

About SLuGS
Sheet Piled Lunar Excavations
Sheet Piled Lunar Pressure Hulls
Lunar Base Construction by Regolith Tunneling
Vacuum Operated Lunar Excavator
Balloon Launch of Small Rockets
Replenishment of an Orbital Propellant Depot by Means of a Coil Gun
An Energy Factory Near the Sun

[A Major Contribution of Seminal Concept Papers to MMM. These are the work of a significant brainstorming group in Seattle which has continued
over a span of many years. MMM thanks David Graham and Hugh Kelso for permission to reprint these papers.]

[About the acronym - if you have ever been in the Pacific Northwest, where slugs (cousins of snails, but without shells) are abundant, and individuals up to half a foot in length are common, this totem / mascot is quite appropriate. - MMM]

SLuGS is an acronym for the Seattle Lunar Group Studies, an association of people from many walks of life that share a common interest in the advancement of the human species into extra-terrestrial space. Our primary focus is Lunar colonization and the technology needed to develop from LEO to the full range of Cis-Lunar space. The primary assumption of the group is that once permanent Lunar bases are established, the Moon will become a jumping off place for the exploration and development of the entire solar system.

SLuGS is a research group patterned after an engineering think tank. We are pathfinders on the leading edge of current aerospace R&D technology. We bring together multi-disciplinary knowledge, thinking, methods, and research to solve problems relating to Lunar development and colonization. Our emphasis is on current, off-the-shelf technology that is proven, well understood, and generally lower cost than the technologies traditionally associated with aerospace flight hardware.

Once a question or topic of interest has been investigated and researched by the group, the results are written up for publication. SLuGS papers have been accepted and presented at the Lunar Bases; Space Activities of the 21st Century Conference of the Lunar Planetary Institute, the Engineering, Construction, and Operations in Space Conference of the American Society of Civil Engineers, and similar professional conferences.

Recent papers of note are "Comparing Structural Metals for Large Lunar Bases" presented at the American Society of Civil Engineers conference and "Lunar Base Design Concepts" presented at the Lunar Planetary Institute conference. (Reprints of these papers are available from SLuGS, see Appendix C.)

Additionally, SLuGS is looking at political, economic, legal, cultural, sociological, psychological and human factors affecting Lunar colonization. Our "projects we would like to tackle" list typically consists of four to five dozen subjects that need investigation. We are always interested in expanding our membership and thus our ability to investigate more topics. If you have an interest in the area of Lunar and Cis-Lunar space development, we invite you to check us out. There are plenty of interesting questions to investigate and another good mind is always welcome.

Weekly meetings are held in Seattle each Monday evening from 7:30 PM to 9:00 PM. Special working groups meet as needed to complete projects
and prepare papers for publication. For information, contact David Graham at (206) 440-1255 or Hugh Kelso at (206) 789-3906.

What follows is a series of concept papers that SLuGS members prepared for the Space Exploration Initiative (SEI) in 1990. These concept papers are still relevant after the passage of seven years. Some of the concepts have become "hot topics" in current space development circles while a few shine less brightly than they did in 1990. Most are still very good food for thought and excellent jumping of spots for further research and discussion. There are twenty-five papers in the series. Most were submitted to both the SEI (AIAA) and the Stafford Report (Rand Corporation). The first six are republished in this issue of MMM.

Introduction - the 1990 Space Exploration Initiative At the request of the National Space Council, NASA prepared a 90 day study on its internal mission planning capabilities and methodology. This report was presented to the National Space Council on November 20, 1989. Upon review of the report, the NSC determined that NASA's current planning policy and methodology was too internalized and too limited in scope. The NSC felt that NASA would benefit from an infusion of new, non-NASA developed innovation and new conceptual approaches to the problems of developing an aggressive space program.

Therefore, Vice President Dan Quayle, as Chairman of the National Space Council, directed NASA to conduct a major outreach program in search of new and innovative approaches to space program planning. This outreach was to extend beyond the usual aerospace and scientific communities and extend to all interested parties.

The National Space Council also approached the American Institute of Aeronautics and Astronautics (AIAA) requesting aid in the outreach program for new ideas and innovative approaches. The AIAA mounted an intensive effort to collect ideas from all interested parties. The format of the AIAA effort was a single page paper describing each concept submitted. These were then reviewed by a "synthesis" group charged with the responsibility of condensing all the incoming data into a set of mission goals, objectives, and policy guidelines.

NASA responded to the NSC directive with both in house programs and contract services by the Rand Corporation of Santa Monica, CA. The Rand outreach format was a two page concept description with an option of submitting a supplemental background proposal. The supplemental proposal was limited to ten pages. The Rand Corporation would then create a database of the ideas collected and submit the processed data to the "synthesis" group for final review and action.

The "synthesis" group was headed by former Astronaut Lt. General Thomas P. Stafford, USAF (Ret). It is the responsibility of this group to assess and evaluate this body of suggestions and ideas and develop specific recommendations regarding future space program objectives and policy. This information is to be presented to NASA and the NSC in mid-1991. The evaluation and assessment of the results of the outreach programs is ongoing at the time (1/1991) of publication of this group of SLuGS concept papers.

SLuGS responded to the solicitations of both the AIAA and the Rand Corporation. Thirteen papers were submitted to AIAA and twenty-five papers were submitted to the Rand Corporation. There was some overlap between the two submittals, thus a total of twenty-five concepts were presented. These submissions are collected here in their entirety.

Most of these concepts are currently the subject of deeper study by various SLuGS investigative teams. Others will be taken up as study projects in the near future. It is the collective opinion of SLuGS that these concepts deserve further investigation.

SLuGS continues to develop and explore new concepts in space exploration and development. These concepts are periodically offered to interested parties in hopes of stimulating investigation by other participants in the space development community.

SLuGS welcomes comments. To comment on current or past projects, suggest new ones, or join one of our investigative teams, contact us at:

Woolly Mammoth Co./ECS
Attn: David Graham (SLuGS)
15254 Densmore Ave N.
Seattle, WA 98133
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Sheet Piled Lunar Excavations
(SEI; Stafford) by David D. Graham

Virtually all current lunar base designs involve substantial excavations of lunar regolith. The proposed structures are either wholly or partially buried and the regolith is mounded over the structure for radiation shielding. One popular design even has bagged regolith protecting the structure in "sand bag" fashion.

The problem with these designs is that all of them assume excavation and other handling of the lunar material will be done by pressure suited crews with or without the aid of lunar versions of bulldozers and backhoes. The number of EVA hours necessary to perform, in pressure suits, the amount of excavation and recompaction envisioned by most of these designs is unreasonable and entails exposure to dangerous amounts of radiation.

It is critical a construction methodology be developed that will minimize the amount of work performed in vacuum and on the surface where radiation exposure is a factor. A popular Earth construction technique minimizing the amount of excavation is sheet piling. Sheet piles are interlocking sheets of material (steel generally, but also aluminum, concrete and various plastics) that are driven into the ground by an impact or vibratory hammer. The native material is then excavated from between the sheet piles.

This would minimize the amount of material to be excavated as sloped trench walls would not be needed to prevent caving. The Handbook of Lunar Materials puts the angle of repose of regolith at 22 to 35 degrees. Without the sheet pile technology, considerable additional excavation would be required to create slopes not subject to caving in an open trench.

Once the regolith has been excavated from between the sheet piles, the Moon base structure is then placed in the excavated trench and the trench backfilled with regolith.

Sheet piling lends itself readily to automation. In all probability, the sheet piling operation could be accomplished by a tele-operated robotic machine controlled either from Earth or an orbiting lunar station. Thus crews need not be sent to the lunar surface until the sheet piling is complete. Sheet piles have an additional benefit of high packing density, thereby lowering transport costs.
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Sheet Piled Lunar Pressure Hulls
(SEI; Stafford) by David D. Graham

Sheet piling is a commonly used construction technique where interlocking sheets of steel (or aluminum, concrete, or various plastics) are driven into the ground forming either a retaining wall or a closed structure. Bridges are sometimes built on artificial islands created from sheet piled caissons. After driving the sheet piles to create a caisson, water is pumped out of the enclosed space and the space filled with sand or other structural fill. On land, sheet piled excavations minimize the amount of material excavated and often become part of the structural wall. Sheet piles can be driven with sealants in the interlocks to form water tight walls.

On the Moon , sheet piling could both minimize excavation and serve as the pressure hull for large structures. The sheet piles could be driven in square, rectangular, or circular configuration. By incorporating a suitable sealant in the interlock, the sheet piled wall becomes the primary pressure hull.

Once the enclosed regolith is excavated to the desired depth, a floor and roof section is attached to the sheet pile walls. (Incorporation of a modular, prefabricated air lock assembly is assumed.)

The roof is then covered with sufficient rego-lith to provide radiation shielding and the structure pressurized for use. Additional sealant is then applied to the sheet pile joints as necessary from within the structure. The internal structures can then be assembled in a shirt sleeve environment at much greater worker efficiency than assembling the base in a vacuum with a pressure suited work force.

The internal habitats and equipment of the lunar base are shipped to the Moon in kit form allowing greater packing densities than would be possible with assembled, prefab units. This simplifies transport logistics and helps lower costs by reducing the number of flights necessary to deliver the lunar base components to the Moon.

The result of combining the sheet pile technology with dense pack kits is the ability to rapidly assemble habitats and research or work space structures of much greater volume than would be practical to transport to the Moon as prefabricated units. Work space could now be planned to accommodate the needs of the tasks and/or experiments rather than compromising the task or experiment to fit the volume allowed by the shuttle cargo bay.
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Lunar Base Construction by Regolith Tunneling
(SEI; Stafford) by David D. Graham

The information brought back by the Apollo missions regarding subsurface characteristics of the lunar regolith indicate very high compaction of this material. Further studies performed on lunar simulant by Bernold & Sundareswaran (Laboratory Research on Lunar Excavation, Space 90 proceedings published by ASCE) indicated that the lunar regolith is almost totally lacking in voids and is compacted to the maximum possible for that material.

NASA examination of the lunar regolith samples from the Apollo missions reveals that grain structure is very angular and sharp. The granules are completely devoid of any weathering or rounding of the individual grains. At high compaction percentages, the internal friction of this material would be enormous.

This suggests an interesting method of lunar base construction. Without voids and with a compac-tion density approaching 100%, the lunar regolith at depth would be relatively impervious to an atmosphere. An atmosphere to regolith interface would be essentially "air tight." With little or no penetration (leakage) of the atmosphere across the air/regolith interface, tunneling through the regolith suddenly becomes a very attractive possibility.

A bore pit would be constructed (sheet piling is a common technique on earth) in the regolith to gain access to the regolith at a depth of approximately 17 meters. At this depth, the overburden weight of regolith on a 5.5 meter bore at 1/6G would resist an atmosphere of about 9 psi. The overburden would also provide radiation shielding.

A boring machine could then begin tunneling through the regolith. A tunnel size of 5.5 meters (18') in diameter would allow ample living and working space (10' ceiling by 15' clear span floor). The air pressure in the tunnel would balance the weight of the surrounding regolith and, because the regolith is nearly air tight, would allow the crew to work in a shirt sleeve environment.

As the boring machine tunneled through the regolith, a light coating of a pliable, latex like material would be sprayed on the tunnel walls forming a positive seal. This seal layer would both prevent what little leakage of atmosphere naturally occurs while also preventing sloughing of regolith from the tunnel wall. The result would be an inflatable structure in situ. A possible refinement would be to slip form lunar concrete behind the boring machine for a rigid structure not dependent on air pressure for its structural integrity.
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A Vacuum Operated Lunar Excavator
(SEI; Stafford) by David D. Graham

Excavating material on the Moon will prove very difficult because of the environment (vacuum and high radiation) and because of the very abrasive
nature of the lunar regolith. Coupling these problems with the weight and bulk of excavation machinery makes the mobilization logistics of a lunar excavation operation needlessly expensive. There may be a light weight alternative to transporting the lunar equivalent of backhoes and bulldozers to the Moon.

By enclosing the area to be excavated with sheet piling, a pressure tight containment wall could be built around the site. The sheet piles would be left extending above the surface three to four meters. The area inscribed is then capped with a roof attached to the sheet pile walls. Placed near the center of the roof is a rotating pipe penetration of twenty to thirty centimeters in diameter. To this rotating pipe is attached a flexible hose on the inside of the structure and a semi-flexible pipe on the outside. The internal flex hose is fitted with handles and several scarifying teeth similar to a rototiller. The structure is then connected to a large reservoir of gases scavenged from the lunar regolith via a solar furnace.

Pressure suited crew then excavate the structure floor by pressurizing the structure and using the pressure differential to move loose regolith through the pipe. Other workers on the outside of the structure direct the placement of the excavated regolith by pointing the semi-flexible pipe. A protective layer of regolith is gradually built up on the roof as a result of the interior excavation.

This method consumes a large quantity of volatile gases but requires little mass for excavation machinery transported from Earth to the Moon. The gases could be cooked out of the regolith by a brute force solar furnace. It is not necessary to separate and purify the gases. Any mix of gases will suffice. All that is necessary is a pressure differential.

This may not be the method of choice for all lunar construction, but it may be the best and quickest way to excavate the first large structures. Less equipment would be brought up from earth and probably fewer men would be required to complete the excavations.
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Balloon Launch of Small Rockets
(SEI; Stafford) by Stan Love and Jeff Klein

The greatest problem with current attempts to develop space is the tremendous cost of putting mass into orbit. Present technology sets strict limits on the amount of propellant required to lift an object from the surface to orbital altitude, and to accelerate it from Earth rotation velocity to orbital velocity. Additional costs arise from construction and maintenance of complicated ground facilities for launching rockets of even modest size. Both of these costs can be reduced to some degree by launching from altitude, a technique which the recent success of the Pegasus vehicle proves to be workable.

Additional benefit could be gained by using a balloon rather than an airplane as a launch platform. Unlike airplanes, balloons use buoyancy to lift, and hence consume no fuel. The altitude limit for balloons is roughly 25 km, twice the ceiling of a B-52. High altitude launch has several advantages. It reduces the amount of fuel needed to reach orbit, and cuts down on the energy lost to atmospheric friction during ascent. In addition, launch from 25 km avoids the environmental issue of having lit rockets ascend through the ozone layer which lies at 20 km altitude.

Balloons have a critical disadvantage in that they cannot be steered (although the payload can be mounted on a steerable platform,) but tethering the balloon would solve most drift problems. Generally, a balloon is expendable, and cannot carry more than a few thousand pounds aloft. In advanced scenarios, large tethered balloons could be flown with hydrogen gas for buoyancy. Some of this hydrogen could be used to fuel the rockets carried aloft, so that the propellant in effect lifts itself to 25 km altitude! Replacement hydrogen could be piped up the tether from the surface. If weight permitted, an apparatus for extracting liquid oxygen from the atmosphere could be attached to the balloon, so that none of the oxidizing portion of the rocket's fuel would have to be lifted from the ground.
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Replenishment of an Orbital Propellant Depot by Means of a Coil Gun
(SEI; Stafford) by Rodney Kendrick

For trips to geosynchronous orbit, the Moon , or beyond, a low Earth orbit fuel depot is essential. This proposal describes a method for resupplying an orbiting depot with up to 14,000 kg of propellant (produced from water ice) daily. Water is dense and inert, yet when electrolyzed and liquefied, it can become a high energy propellant.

A recent article (Breck, Henderson, "Sandia Researchers Test 'Coil Gun' For Use in Orbiting Small Payloads." Aviation Week and Space Technology, May 7, 1990, pages 88-89) described the capabilities of a "coil gun" for launching payloads into low Earth orbit. This coil gun would replace costly rocket propulsion with cheap electricity.

This proposal calls for building a coil gun and rocket combination capable of placing a 10 kg payload of water ice in orbit. The gun would be sited on the equator and fired due east. A very small maneuvering motor would circularize the orbit at 277 km altitude. Firing one shot every minute would thus produce a ring of orbiting payloads about the Earth.

The depot would orbit at 300 km at an inclination of zero degrees. It would consist of tankage, electrolyzer, liquefaction machinery, power plant, and a 23 km tether. This would extend down to the 277 km orbit with a large net at its end. The end of the tether would not be in orbit, and the orbiting payloads would pass through the opening of the net at a closing velocity of 40 m/s where they'd decelerate and be captured. Thus a difficult rendezvous maneuver would be avoided. The captured water ice would then be pumped up the tether to the depot.

Economies of scale could come into play. With up to 500,000 shots a year, the price per shot should be quite low. The per year payload equivalent will be that of over 40 Saturn V's.
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An Energy Factory Near the Sun
SEI; Stafford) by Mark Lawler

Large geosynchronous solar power satellites have been envisioned to convert sunlight into electricity and thence into microwave transmissions to Earth. A perfectly efficient 1-GW solar power satellite would be 8.5 km square. Lofting the construction materials for such a large spacecraft into geosynchronous orbit would be very costly. Solar energy collection will be even more expensive at Mars and beyond because of the great distance from the sun and the need for much larger collectors.

Building a solar collector in Mercury's orbit instead, or even closer to the Sun, can achieve significant savings in the amount of necessary materials. Mercury's semi-major axis is 0.387 AU, so the solar input there is on average 6.6 times that near Earth. Thus a 1-GW solar collector near Mercury would have 1/7 the area of a collector near Earth - with correspondingly lower mass and expense required.

Mercury, with its abundance of metals, might economically supply much of the construction material for such a power station. Launch of materials from Mercury would be relatively cheap, since escape velocity from Mercury is 4.2 km/s, compared to 2.4 km/s from the Moon and 11.2 km/s from Earth.

No feasible means exist so directly transmit energy across vast distances such as between Mercury and Earth. Instead, antimatter could be produced at the power station and launched toward users elsewhere in the solar system. 175 grams of antimatter reacting with a like amount of matter will produce the raw energy equivalent of a year's output from a 1-GW power plant. Costs to transport such small masses would be minimal. Robert Forward has studied the antimatter production problem extensively and while finding significant technical obstacles to efficient antimatter production; he has also identified possible paths to overcome them through a vigorous research program.

An inexpensive and inexhaustible supply of antimatter shipped about the solar system in tiny transfer spacecraft could supply the energy needs for asteroid belt missions, lunar bases, space stations and industrial facilities, Earth-Mars transfer vehicles, and Earth itself. Antimatter could be used to generate relatively clean electrical or thermal power, and in propulsion systems as suggested by Forward.

A major drawback of antimatter is its potential use in weapons of mass destruction, so shipments may have to be guarded or monitored. Yet antimatter produced close to the sun could become the principal fuel for developing a spacefaring infrastructure and help to solve energy needs on Earth.
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Additional SLuGS papers are reprinted in MMMs # 107 and 108

Contents of this issue of Moon Miners' Manifesto