10. Potential asteroid fragmentation technology
Thermomechanical Boulder Breaker
A mobile thermomechanical boulder breaker could fragment rock by first weakening it by applying heat (in this concept by means of a burner) and then hitting it with a mechanical impactor.
From Thiruma/ai, Demou, and Fischer 1975, quoted by Maurer 1980, p. 653.
Technology for fragmenting rock particles has been researched and developed over many decades. Conventional fragmentation is primarily mechanical. Its effectiveness on a virtually gravity-free asteroid will depend in part on the degree to which the mechanical fragmentation system depends on gravity. We can conceptualize mechanical fragmentation systems that are independent of gravity; i.e., those that work by splitting or pinching. Also available are a variety of explosive, electrical, chemical, and thermal disintegration methods. These methods will impose different logistical requirements, depending on what supplies they need and on how operations are carried out. For example, the efficiency of several fragmentation methods would increase if the fragmentation took place in drill holes. But drilling holes in asteroids will pose unusual problems (see topic 6).
It may be desirable to distinguish between two classes of fragmentation problems, those where a single fragment (or a small number of fragments) is to be removed or reduced to certain dimensions, and those where a large number of particles are to be reduced in size. The latter class of applications is discussed under topic 15, crushing and grinding. The choice of technology most readily applicable to removal or controlled-size reduction of a single large block might well benefit from an evaluation of quarrying practice for building stone. Advanced rock disintegration techniques, some of which should have direct applicability to space operations, are summarized by Maurer (1980).
11. Automation, operator proficiency, and excavation efficiency
Eliminating the need for human operators would significantly enhance the economic attractiveness of nonterrestrial mining. Few attempts have been made at developing fully automated mining excavation cycles; i.e., operations without human intervention. The economic incentives for doing so on Earth are marginal, at best.
Fully automating the mechanical excavation and loading of broken rock is likely to result in drastic productivity losses. It is well established that the productivity of virtually all excavation and loading equipment is highly sensitive to the expertise of the operator. Human judgment and fast response to seemingly minor aspects of rock loading operations are significant production and safety factors. Of particular concern in this context is that misjudgment by an operator can result in serious, even disastrous, consequences, such as cables breaking and machines overturning. Control engineering will have to preclude such occurrences as well as assure a reasonable production level.
The importance of human judgment in excavation technologies suggests a number of avenues for research aimed at identifying candidates for automation and nonterrestrial application. Questions that can be raised include the following: Will the implementation of automatic operation be most difficult for equipment that is most sensitive to operator handling? Should automation be preferentially applied to excavation technologies that are robust or insensitive to operator errors? What tradeoffs are acceptable between automatic control and productivity?
To allow automation, operations should be as simple as possible. This fact, explicitly recognized in the space program (e.g., Firschein et al. 1986, p. 103), unquestionably underlies the mining industry's reluctance even to attempt to automate most excavation methods. The few notable exceptions (Iongwall mining, tunnel boring) for which automation is being investigated are already fully mechanized (involve minimal human intervention during normal operations). These exceptions tend to be high-production systems. They are prone to frequent breakdown and require preventive maintenance. Maintenance is recognized as a major difficulty in implementing automation (e.g., Firschein et al. 1986, p. 355); it will require major developments in artificial intelligence software and robotics. The need for human reasoning capability is again apparent.
12. The influence of gravity on slusher mining
Gertsch identifies slusher mining as one of the more promising lunar mining methods. The performance of a slusher on the lunar surface (or in underground operations on the Moon) will be affected by the low gravity.
The lighter weight of the scraper (bucket) on the Moon may lower the loading efficiency of the slusher bucket, because the weight influences the vertical penetrating force into the material to be loaded. Conversely, the lighter weight lunar material may flow more easily up into the bucket. It is conceivable that artificial weighting down of the bucket, or a reconfiguration of the cable force system, might be required in order to assure adequate penetration into the lunar soil and to avoid riding of the (empty or partially filled) bucket over the material to be loaded. Conversely, friction, abrasive wear, and power requirements during both inhaul and outhaul may be significantly reduced by the low gravity.
The reduced effective weight of the bucket, which is likely to have a detrimental impact on the efficiency of the all-important bucket-loading phase, might also adversely affect the performance of the bucket as it is hauled in to the unloading point. Assuming a relatively rough and bumpy ride during inhaul, the bucket may not retain its full load. An analysis might suggest a reduction in hauling speed, but this might also affect production adversely. It is possible that bucket redesign and cable reconfiguration might compensate at least partially for the reduced effective bucket weight.
Given the interest by this group in the application of slusher mining to the lunar program, it may be appropriate to outline in some detail steps that could be taken to reduce the need for speculation about the performance of such systems on the Moon.
Obtaining a clear understanding of the mechanics of bucket loading would be a desirable step. This step could be initiated with a comprehensive literature survey. It is unlikely that much fundamental information is available about slusher bucket mechanics, but considerable analysis has been made of the mechanics of similar excavation elements, such as dragline buckets, bulldozer blades, front-end loader buckets, and scrapers. Integrating this knowledge in a framework emphasizing the mechanical differences between terrestrial operating conditions and lunar operating conditions would go a long way towards identifying potential problems. Such an integrating effort should be made by a group with a clear understanding of the fundamental mechanics of the machine (bucket) and material (broken rock). At a minimum, meetings should be organized with experienced bucket designers from various manufacturers. In order to obtain maximum contributions from such personnel, it may be preferable to formally contract for their technical services. Equally important would be information exchanges with operators; e.g., by means of visits to mines.
On the basis of the initial analyses, it should be possible to make preliminary estimates of the influence of gravity on bucket loading performance. This information could in turn form the basis for designing experiments (for example, experiments using centrifuges) to verify the analyses. Similarly, it may be possible to instrument buckets and their cables and chains in order to obtain a better understanding of the distribution of forces during loading. An appropriate iterative sequence of bucket analyses, experiments, and design modifications should provide a considerably improved understanding of bucket mechanics, ultimately leading to adequate bucket designs for drastically different operating conditions.
While I have emphasized slusher bucket development, I should point out that any studies of this type, aimed at an improved understanding of the mechanics of loading broken rock, will be beneficial for eventual redesign of other systems that might be considered for nonterrestrial loading operations. These would include hydraulic excavators, electric shovels, front-end loaders, bulldozers, scrapers, draglines, and clamshells.
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