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NASA SP-413 — SPACE SETTLEMENTS — A Design Study
The rotary pellet launcher is a heavy tube rapidly rotating to accelerate and eject small pellets of rock. (See fig. 5-21.) Velocities as high as 4000 m/s may be attained, equal to the exhaust velocities of the best chemical rockets. The pellets themselves are sintered or cast directly from lunar rock, with no chemical processing required. The launcher uses 5 percent of the mass received as propellant. (For further analysis see appendix I.) This rotary pellet launcher is mechanically driven by an onboard nuclear power system rated at 20 MW. The power plant radiator is situated in such a manner as to radiate freely to space while being shielded from impacts of stray masses. The inner surface of the radiator is insulated and made highly reflective, so as to avoid heating the catcher. The transport of lunar material from the catcher to the colony is accomplished using a space ore-carrier. The trip from L2 to L5 requires some 2 months. The rotary pellet launcher is the primary propulsion system since a thrust of only several thousand newtons must be obtained over a period of weeks to perform the mission. Of course, the rotary launcher cannot be used in the vicinity of either the colony or the mass-catcher, because of the danger from its exhaust of high-velocity pellets. For low-velocity maneuvers in these vicinities chemical rockets can be used.
You express concern to the operator of the mass catcher over the possible hazard of using high velocity pellets as propellant mass because they constitute artificial meteoroids. They are ejected with high velocity, not so high as to escape the Solar System, but sufficiently high to escape the Earth-Moon system and take up solar orbits. Typically, they will range inward as far as Venus and outward to Mars’ orbit. The operator’s response is reassuring. He reminds you that the astronomer George Wetherill (ref. 21) studied the lifetimes of meteoroids in such orbits, or the times before collision with Earth He found a mean lifetime of 107 years. The Earth presents a surface area of 5 X 108 km2, while a colony’s area is some 1 km2 or less, and a spacecraft’s area much less. Using a standard that no more than one impact from a pellet per square kilometer every 10 yr may be allowed, then 5 X 1014 pellets may be permitted to orbit the Sun following ejection. If each has a mass of 10 g, the allowed mass of ejected pellets is 5 X 109t. This is some 10,000 times the mass of pellets to be ejected in the course of carrying material for building the colony. He assures you that the rotary pellet launcher will be a useful propulsion system for many years, before the environmental effect of ejected pellets becomes noticeable in comparison to the effect of meteoroids naturally present in space.
HOME TO EARTH
space. Homesickness is inevitable. It is time to leave the realms of the colonists Their tasks and their will to do them are enormous, and only those people can be colonists who have a large capacity to work hard and long when, as soon happens, tedium replaces the initial excitement. You speculate that it will be mostly their children and grandchildren who will master space The great mass of mankind will remain in the cradle of Earth; only a few will go into space. You are fortunate to get a berth in one of the ships that brings supplies to the Moon and rotates personnel from the Moon base directly back to Earth. In the early years all the men and women of the base went straight back to Earth and so the personnel transporter was full to capacity. Now increasing numbers choose to spend their rotation time at L5 instead of on Earth and berths are available on the run to Earth. You wonder whether this seed of human society planted in such an unlikely environment will flourish, and settling back into your seat to read a terrestrial news magazine you conclude that only time will tell.
APPENDIX A STRUCTURAL DESIGN CONCEPT FOR A SHELL STRUCTURE A section from a symmetric structural shell transmitting only normal stresses in orthogonal directions may be designed either as a stressed skin or a rib system. The stressed skin is the most efficient in that the same material carries the stress in both directions and there is integral resistance to secondary torsional and bending loads. In addition, both fabrication and construction generally are simplified and problems with sealing joints, finishing, and maintenance are reduced. For a rib system, such as shown in figure 5-22, each orthogonal set must entirely carry the membrane force (N1 or N2) in that direction. This increases the mass required to carry the membrane stresses by the factor (1 - 2) (where 1 > 2 and 1 and 2 refer to the membrane stresses in directions 1 and 2) plus the intermediate plates required to bridge between the ribs. Moreover, if the ribs are made of cable and are flexible there is no resistance to secondary torsion, bending, or buckling. There may, however, be some advantages in fabrication and construction to include some cables encased in the ribs. The obvious requirement for any shell configuration is to avoid ribs whenever possible. Therefore, this design assumes a stressed skin structure except in the windows where the required ribs flare into the skin at the boundaries.
Design Formulas for Torus
It is now 2 months since you left Earth. In that time you have traveled over 750,000 km, you have another 386,000 to go to get home to Earth. You have seen a tiny community of 10,000 men and women crowded into the colony and in small bases on the Moon and at L2 separated by vast distances which are in turn dwarfed by the immensities of
For a stressed skin design of a torus the required skin thickness in the meridional and hoop directions, respectively, are given by
Chapter 5 — A Tour Of The Colony