Simplified Force Diagram for a Conventional Mine Shovel

The cutting force of a shovel is a function of hoist line pull, crowd effort, and front-end geometry. A large machine weight is needed to provide horizontal resistance to slippage during digging.

Courtesy of J. D. Humphrey, Dresser Industries

Traction is an important operational aspect of most vehicles. It is particularly important for vehicles that need to exert large horizontal forces; e.g., for excavating, loading, and hauling. Many types of mining equipment are very dependent on the development of adequate traction. This equipment includes excavation equipment such as bulldozer-mounted rippers and scrapers, front-end loaders, shovels, and drills, especially those for drilling angled or horizontal holes.

Comprehensive studies have been performed of vehicle traction on the Moon (among them, Karafiath 1970a,b; Nowatzki 1972). Even though these have addressed the operations of primarily small, lightweight roving vehicles, they provide fundamental insight into the traction of larger, heavier lunar mining vehicles. Moreover, experience with the lunar Rovers has provided an operational record by which to validate the traction models and predictions made for them.

Traction deserves attention because it is a major force needed for many mining operations. Because it is a function of gravity and of friction, the latter affected by vacuum, it will be affected by the space environment. Considerable experience is available to guide further research into this aspect of lunar mining.

Muck Pile

This is an example of a good muck pile, well-fragmented and largely remaining in one heap. Loading would be much more time consuming if the rock were widely dispersed, as it might be by conventional blasting in a low-gravity environment, without air resistance. The loading machine must have sufficient traction (created by both friction and weight) to be able to push the loading bucket into the muck pile.

Lunar mining may involve the removal of various types of ground, ranging from massive solid rock to loose, granular soils. This possibility suggests the need to investigate a range of material- removal technologies. It may be desirable, at this early investigation stage, to distinguish between the fundamental mechanics underlying the technologies and the technologies themselves. Both will be affected by operations on the Moon, but in different ways.

a. Hard rock excavation mechanics
In earthbound mining, hard rock is removed primarily by explosive excavation. Lunar blast design is likely to require significant changes from conventional blasting. An obvious consideration will be the need to control the broken rock pile. It is usually assumed that gravity plays no role in actual rock breakage by conventional blasting, but it plays a significant role in displacement of the broken rock (and thus dominates the shape of the muck pile). Low gravity could result in extremely wide scattering of rock fragments, even more so in the absence of air resistance, and hence lead to exceedingly inefficient loading operations. An interesting challenge may be posed by the need to adjust blasting patterns from the traditional ones to those designed to minimize scatter in a low-gravity and high-vacuum environment.

It is possible that vacuum might affect blasting performance, although it may not be a significant factor in low-permeability rock, at least at greater depths. The breakage induced by blasting is usually attributed in part to seismic effects and in part to gas pressure effects. Presumably gas pressure effects could attenuate much faster in a space environment than on Earth. This could affect fragmentation and almost certainly would affect heave and throw; i.e., rock movement.

Potential impacts of low gravity on mechanical excavation have been discussed under topic 1. Drastically different excavation technologies are summarized by Maurer (1980), and they deserve intense scrutiny for lunar applications.

b. Soft ground excavation mechanics
Mechanical excavation of loose, granular material on the lunar surface is likely to be facilitated by the lower gravity in terms of actually lifting the material, although this improvement may be partially offset by increased friction between particles. It is likely that the most significant detrimental effect will be on the forces that can be delivered by the equipment. Reduced equipment weight will reduce breakout forces and sliding stability. It is quite possible that even a simple force analysis of excavation systems will shed considerable light on lunar soil loading requirements and potential problems. Gertsch has suggested that we add mass to lunar equipment by building into it large volumes to be filled with lunar rocks. However, as he notes, the added mass would add to the problem of inertia in mobile equipment.