Although these resistance values, coupled with those of several of the 

 excavation candidates, result in total drawbar force within the target 

 limitation of 10,000 pounds, it would be desirable to reduce the skid 

 resistance to a minimum. It is reasonable to expect that a 50% reduction 

 in motion resistance might be attained by forcing water between the skids 

 and the soil to lubricate the interface. With a nominal skid area of 

 60 ft^, each of two skids can be 4 x 8 feet long. The following assumptions 

 are made 



(a) Water flow from back of skids = machine speed 



(b) Water flow from sides of skids = 50% machine speed 



(c) Water layer =1 in. thick 



(d) Supply pressure =10 psi 



P = Q p = ((0.083 ft) [8 ft + (32 ft) (0.5)] (1 .69 ft/sec)| 

 (10 lb/in. ^) (144 in.^/ft^)[hp/(550 ft-lb/sec)] 



P = 9 hp 



Thus, an additional 9 hp may reduce the drawbar force required by 1,750 

 pounds for sand and for clay. 



The major advantage of using skids to support the burial machine is 

 their simplicity and reliability. However, their ability to negotiate 

 obstacles is limited, the resistance prediction outlined above will be 

 affected greatly by how much soil is pushed ahead of the skids, and the 

 water lubrication technique is not proven technology and must be tested. 



Rolling Elements. One means of reducing motion resistance due to the 

 running gear/soil interface is to provide passive rolling elements such 

 as tracks or wheels. Rolling elements have the capability to negotiate 

 some obstacles, as discussed in the propulsion section. Liquid-filled 

 tires can be designed to vary the nominal unit ground pressure, which 

 could prove beneficial when different soil conditions are encountered. 

 Bekkar [43] predicts that the motion resistance for wheels or tracks 

 is 1,000 to 1,500 pounds, significantly less than that for unlubricated 

 skids. Also, differential braking or articulation of the rolling elements 

 can be an aid in machine steering. The drawbacks of rolling elements 

 include their complexity (relative to skids) and the uncertainty in motion 

 resistance and traf f icability predictions. 



Water Cushion Vehicle (WCV) . A WCV is similar in concept to an air 

 cushion vehicle, except that water flow and pressure is used to support 

 the platform at the water/soil interface vice air at the air/water 

 interface. A distinct advantage of the WCV concept is that the machine 

 frame/soil interface is separated by a layer of low-pressure water. Thus, 

 the motion resistance is reduced to that force necessary to shear the 

 water layer, or essentially zero. Reference 45 describes a WCV concept 



31 



