Complex sentence: Furthermore, design specifications called for the use of linear aerospike engines that maintain thrust efficiency at all altitudes, whereas the Shuttle relied upon conventional nozzle engines which achieve maximum efficiency at only a certain altitude.
Split into simpler sentences: Furthermore, design specifications called for the use of linear aerospike engines that maintain thrust efficiency at all altitudes. Whereas the Shuttle relied upon conventional nozzle engines which achieve maximum efficiency at only a certain altitude.