MMM Classics Year 7: MMM #s 61-70

engines generate hundreds of horsepower, plenty for a car sizedvehicle if it has adequate energy efficiency, about which seebelow. As to control at high speed, cheetahs travel up to 120km/hr over broken terrain. Mechanical legged vehicles withelectronic control should be able to do at least as well.All vehicles expend energy to raise the weight of thevehicle against gravity when ascending large terrain features.Properly designed vehicles can recover some of this energywhen they descend. However, regenerative braking systems arestill experimental for wheeled vehicles and research has barelybegun on downhill walking.Both wheeled and legged vehicles expend energy toaccelerate the vehicle body. A rough ride, aside from beinghard on passengers and cargo, wastes the energy that is used toaccelerate the body in directions other than the direction oftravel. Legged vehicles have the potential for a smoother rideat all velocities, but it is not clear whether this produces asignificant energy saving. A wheeled vehicle must climb overan obstacle all at once, requiring high peak power. Leggedvehicles can move one leg at a time, if necessary, using asmaller, lighter power plant.Wheeled vehicles use energy to angularly acceleratetheir drive train and wheels, which uses little energy for usualdesigns. (The rover designs that are mostly wheel use muchmore.) Legged vehicles must accelerate their legs. On levelground, the legs oscillate in regular patterns and a properlydesigned mobile (such as most mammals) expends little energyto keep the oscillations going, but much more than comparablewheeled vehicles. Current mechanical walkers dissipate the legkinetic energy at each stroke, and much more research needs tobe done in this area. On rough ground, the irregular patterns ofleg motion increase the energy loss significantly at high speeds,so picking one’s way across a boulder field is not just safer, itis more efficient as well. Energy dissipated in leg/wheel motionis the area where wheeled vehicles do significantly better.Soil interaction is where legs do much better. On softground, wheels compact the soil ahead of the wheel, expendingenergy to dig a rut in the ground. Wheeled vehicles are continuallyclimbing out of their own rut (on soft soil), reducingtheir traction. As a leg pushes back, the soil behind the foot iscompacted, increasing traction. Hard ground reduces thepenalty to wheels, but this usually requires prior paving or raillaying,at great expense, and is only economic for high trafficcorridors - common on Earth and nonexistent elsewhere.In summary, properly-designed legged vehicles canoffer efficiencies within an order of magnitude of wheeledvehicles on smooth, paved surfaces and do better than wheelson rough terrain.Where are these properly-designed legged vehicles?They don’t exist (yet). Serious research on mechanical leggedvehicles is less than two decades old, while the automobile hasbeen in development for almost a century, and wheeledvehicles for millennia. Animals demonstrate excellent mobilityand good efficiency for their materials. With higher-strengthmaterial, higher energy densities, and the speed of electronics,we should be able to do at least as well, if not better, thanprotoplasm technology. In addition, mechanical walkers do notneed to be fed when not working, can run all day and all night,and do not have desires of their own.The most efficient and practical legged vehicle so faris the Ohio State Adaptive Suspension Vehicle (ASV) whichmasses 1700 kg and carries a 220 kg payload at up to 13 km/hr.It is powered by a modified motorcycle engine.Outlook for Legged VehiclesOn Earth, legged vehicles will find a niche, but willnot replace wheels and roads and rails. We have a great investmentin wheel technology and our society is set up around it. Inaddition, population is high and transportation routes areheavily used.On the Moon, Mars, and other bodies, the reverse istrue: we have no infrastructure of roads and rails, and traveldensities will be low for a long time. If suitable legged vehiclesare available by the time colonization is starting, colonies canbe designed around the use of legs instead of wheels.What would be different? Primarily, you don’t need topave anything. No unsightly and expensive roads and parkinglots. Trails need only be cleared of the largest boulders and canascend steeper slopes than are practical for roads.Wheeled off-road vehicles and rovers also eliminatethe need for roads, but offer a much rougher ride which is hardon people (reducing the amount of work they can do) and ondelicate scientific gear. Legged machines still need bridges forgaps larger than a few meters. (Ohio State’s ASV can crosstrenches up to 2.7 meters and climb cliffs up to 2.1 meters,capabilities far beyond that of any wheeled vehicle.)A legged vehicles can carry heavy equipment right upto its final location whether that is in a canyon or on top of amountain, and hold it level! You don’t need to drive a road to asite before you develop it. Boulders falling on a trail for leggedvehicles don’t block the trail, but merely it to the side.Legged vehicles are mechanically more complex, andprobably will require more maintenance than wheeled vehicles.You won’t have two lanes of traffic going opposite directionswithin a meter of each other, almost eliminating vehiclecollisions. An interesting visual effect is that a large expeditionwould resemble a stampede with diesel engine sounds. Agood thing there isn’t any local fauna to terrify!Legged vehicle travel will follow natural routes acrossmountains (valleys, ridges, passes) but on the plains travelerswill head straight for their destination, instead of along the roadgrid we use on Earth. Putting up a fence and saying NOTRESPASSING might be considered downright hostile.You could drive your legged truck right into themiddle of the greenhouse to load it, with it carefully steppingon the walkways, Feet do far less damage to soft ground andvegetation than wheels, reducing erosion, and kick up less dustin dry soils. If terraforming ever covers the dead sea bottoms ofMars with ocher moss, feet will be kinder to the plants introducedwith great effort.Legged vehicles offer the potential to significantlyimpact the way other planets are explored and developed. DRREFERENCES:D. J. TODD, Walking Machines: an Introduction to LeggedRobotics, Chapman & Hall, New YorkShin-Min Song & Kenneth J. Waldron, Machines that Walk: TheAdaptive Suspension Vehicle, The MIT Press, Cambridge, MAMoon Miners’ Manifesto Classics - Year 7 - Republished January 2006 - Page 13