The moon, 384,400 km away from the earth, is the first heavenly body to have been trodden upon by human foot. Therefore, it is but natural to expect it to host man's first space settlements. Is that really feasible, given our present knowledge and technology?
Articles in this rubric reflect the opinion of the author. -Ed.
By Leonid BOBYLYOV, Cand. Sc. (Tech.), member of the International Society of Soil Mechanics, Geotechnics and Groundwork Construction; Gennady PROKHORENKO, head of the Commercial Project Department of Production Center (named after Pavel Voronin); and Andrei BOBYLYOV, senior specialist of the same Center
Let us first remind you of the key parameters of our natural satellite as compared with those of the earth: diameter -0.27; mass -0.01; force of gravity acceleration -0.61; temperatures drop from +130 o C in the day-time down to -170 o C at night.
The surface of the moon is not protected either from the impact of micrometeorites or from ultraviolet radiation or space rays. Besides, it "floats" in hyperhigh vacuum. Still, in spite of such aggressive environment, some scientists believe that in not so distant future the moon may host research stations and bases, and, henceforth, enterprises for the excavation of mineral resources and production of various construction materials.
To resolve problems of such magnitude one has to be in command of a solid body of evidence about the physical and mechanical properties of lunar soil: solidity, compressibility, shift-resistance, density, carrying capacity, mineralogical and granulometric composition, and so on. Until 1966 evaluation of the above parameters was based solely upon astronomic observation and radio-physical probing yielding rather rough results; hence, hypotheses of lunar soil properties put forward by specialists of different countries varied widely.
The tide turned after a Soviet space probe successfully landed on the moon on February 3, 1966, followed by serial missions, both Soviet and American. The moon was visited by manned craft, special-purpose unmanned probes and lunar vehicles, both remote-controlled from the earth and manned.
But that is just a preface. And the story goes: manned moon-rovers were delivered to the moon on July 6, 1971, July 6 and September 7, 1972, respectively, by Apollo 15, 16 and 17 spacecraft. Their maximum velocity amounted to 13 km per hour. They could easily climb slopes with a pitch of up to 20 and get a firm hold on them probing vast spans of our closest space neighbor's surface with terrain of varying roughness at that.
In 1972 American astronauts surveyed the surface of the moon using cine equipment, photo cameras, radio isotope instruments, drills, sample takers and manual geological survey instruments. They took soil samples from depths down to 3 meters using a core drill 25 mm in diameter, delivering 0.5 hp. It looks like a tube with bits on the outside surface to take soil samples in the process of well drilling. After the withdrawal of the tool no sloughing-in of well walls was observed which was indicative of soil carrying capacity. Soil solidity was measured with the use of the so- called penetrometer with a recorder - a cone with a tensometer placed inside*.
To probe the soil at large depths the astronauts put on the surface of the moon a seismograph, 19 explosive charges and a triggering device. The readings thus obtained showed that in some lunar areas the homogenous loose layer with no solid inclusions reached down to almost 45 meters.
In addition to the field tests of lunar soil, 418 kg of its samples from nine different moon areas were brought back to the earth for close examination in laboratories of different countries. The bulk of the material was left in hermetically sealed containers for prolonged storage, pending the appearance of higher technologies facilitating
* Tensometer, or strain gauge, is an instrument measuring deformation of solid bodies. -Ed.
more sophisticated test methodologies with the use of new instruments.
So, what is the practical value of all information thus obtained?
Primarily, it was used by Russian and American designers for engineering new automatic lunar probes, spacecraft, spacesuits and space shoes and sophisticating research equipment. However, those are but current tasks, while there are long-term ones. These data are crucial for designing special machines and mechanisms for excavation on the lunar surface with maximum protection of operational personnel.
The above said, essentially, boils down to the following: in engineering any underground structure on the moon provisions should be made for the safe thickness of the "roof. Studies carried out by researchers of different countries, including the well-known Russian soil scientist Igor Cherkassov, Dr. Sc. (Tech.) (1912-1988), prove that a soil layer 3 meters thick provides ample protection of occupants from sharp temperature fluctuations, penetrating radiation and meteorite impacts.
What remains is but a "trifle": to determine a way of constructing the "roof. In our view, the widely used mundane method called "wall in the soil" best suits the purpose of engineering on the moon. This method has two options. The first one consists in digging along the chamber's perimeter a trench 20-30 m deep which is filled with concrete, and then the soil is removed from the inside contour. In the second case the so-called jet technique is used, whereby wells are drilled along the required contour with a certain pitch, subsequently filled with concrete.
However, neither of the above techniques is applicable in the lunar conditions in its authentic form for lack of atmosphere, low gravitation and sharp temperature fluctuations. And if so, does the above said make any sense or is it just another bluff?
As it appears, no. But before explaining, we shall have to make a brief digression.
Back in 1980 at the Institute of Mining of the Siberian Branch of the USSR Academy of Sciences a group led by Valentin Svirshchevsky, Dr. Sc. (Tech.), developed a new technique-rolling-out of wells-by creating a unique device. The invention was immediately appreciated in USA, Canada, Germany, France, Japan, UK and Sweden and patented in 10 countries.
Its technological solution looks as follows: consecutively strung on a metallic shaft through rigid cam couplings and pendulum bearings are several conic rollers positioned angle-wise to the shaft and to each other (know-how). The diameter of the rollers increases from the first to the last one, to be more precise, depending on the angle and the number of rollers, it varies within the range of 30-1,500 mm.
For the purpose of well drilling on the moon we propose a similar well
rolling machine additionally provided with a rotating course with a twisted blade fastened to it, which, through continuous contact with the soil, maintains a permanent rate of sinking. Each roller is provided with electric supply and a sinking mechanism. Digging into the soil the tool produces a well with wall material packed to 2-3 well diameters.
We would like to point out that the suggested technology has been tried at underground construction sites of Manezhnaya Square in Moscow and for driving gas pipeline and communication cable crossings under highways. The soil rollers were granted top awards of the International Fair of Inventions and Technological Innovations in Geneva in 1991 and the World Show of Inventions, Scientific Research and Industrial Innovations "Brussels-Eureka95".
Back to drilling of wells on the moon, we previously mentioned packed soil around them. This is indicative of the absence of moisture and air in soil pores resulting in the maximum density and solidity of well walls, superior even to those produced in earth conditions. Thermal conductivity of lunar soils is way below that of those on earth, therefore the thickness of the "roof protecting future lunar dwellings may be around 2- 3 meters.
Let us attempt some specific recommendations. We believe that the construction of an underground structure on the moon after the wall-in-the-soil methodology, with the use of the described rollers, should begin with rolling out the projected structure's contour. Next, wells are sunk, or rolled out, to its full depth, into which modular fiber- glass epoxy tubes are stuck, armored with tough steel ribbon. On the earth such tubes are broadly used for oil and gas pipelines. By way of toughness they are not inferior to steel pipes but are 3-4 times lighter.
Further, a sheet piling is constructed, and the soil is extracted from the inside of the contour. Equipment for that operation is still to be engineered.
As a result, a subsoil ("sublunar") chamber is built which, following installation of the required equipment, is ready to accommodate the first moon settlers.
By way of conclusion, let us stress again: due to the low gravitation on the natural satellite of our planet the pressure of soil on the sheet piling will be reduced drastically, a favorable factor for the construction of sublunar shelters.
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