America's reliance on space systems drives a requirement for rapid access to space toreplace failing systems or add new transformational capabilities so the U.S. maintains its spacedominance. There are currently studies in progress for a new spacelift vehicle that can rapidlyrespond to a launch callup, but this paper will show how it is possible to have responsive spacelaunch with current systems. This study discusses why only horizontally processed systems suchas the Delta IV can accomplish the seven day turnaround which meets A.F. Space Command'sresponsive launch criteria.An upgraded Delta IV launch system could respond to America's need for replenishmentand expansion of its space capabilities. Delta IV's horizontal processing allows multiple vehicles to be processed and stored simultaneously.