maintenance of the orbital velocity

Maintenance of the orbital velocity of orbiting objects is needed, called "station keeping" as applied to the communication satellites currently in GEO. 

In the novel's Clarke Belt Cities, the orbiting velocity of all the cities has to be kept within a narrow range. With a KESTS to GEO in place, it would have the ability to provide some coupling down to the earth's rotating mass. But in the stories where the KESTS has been destroyed, other temporary means for maintaining the orbital positioning of the vast numbers of space settlements forming the Clarke Belt City Ring; and using reaction motors would be needed as emergency station keeping means. Obtaining expendable mass for the reaction motors would be part of the solution, as in the story "It's Down to Earth."