NUMEROUS PROPERTIES MAKE SERPENS X-1 THE BEST CANDIDATE FOR CONSTRAINTS ON THE INNER ACCRETION FLOW AND NEUTRON STAR RADIUS VIA DISK REFLECTION SPECTROSCOPY. WE PROPOSE A JOINT NUSTAR (40 KS) AND NICER OBSERVATION OF SERPENS X-1 IN ORDER TO OBTAIN THE BEST POSSIBLE LIMITS ON THE NEUTRON STAR RADIUS THE BEST MEASUREMENTS OF THE INNER ACCRETION FLOW AND TO EXPLORE CHANGES IN THE IONIZATIONOF THE DISK WITH RADIUS. THE EXTRAORDINARY SENSITIVITY OF NICER AT AND BELOW 1 KEV WILL BE ABLE TO REVEAL RELATIVISTIC NE X AND FE L LINES PREDICTED BY THE PRIOR NUSTAR EXPOSURE OF SERPENS X-1. WE WILL MAXIMIZE THE RETURN OF THESE OBSERVATIONS BY UTILIZING NEW DISK REFLECTION MODELS OPTIMIZED FOR NEUTRON STAR X-RAY BINARIES.