Complex sentence: A modern civil turbofan has a low specific thrust (~ 30lbf/lb/s) to keep the jet noise at an acceptable level, and to achieve low fuel consumption, as a low specific thrust helps to improve specific fuel consumption (SFC).
Split into simpler sentences: A modern civil turbofan has a low specific thrust (~ 30lbf/lb/s) to keep the jet noise at an acceptable level. A low specific thrust helps to improve specific fuel consumption (SFC).