... 30a. The HARP cannon . ... in the tube on top, the gun barrel is on bottom. ... Suppose the barrel of the cannon is a mile long (—1600 meter) and the final velocity v , the one with which the shell emerges, is the escape velocity from the surface of the Earth . ... The solution is to use a gun with two connected barrels, an auxiliary barrel as well as a main one in which the payload is accelerated (see drawing ). ... Next Regular Stop: #31 Far-out Pathways to Space: Nuclear Power . ...