This question already has an answer here:
Scramjet technology allows for propulsion once Mach4 is reached, without needing oxidizer thus reducing the weight up to 80% (as read in https://medium.com/@rsn/spacex-falcon-9-vs-isros-reusable-launch-vehicle-c52d4d56f87d#.5sjgv3mv1)
Why doesnt spaceX plan to use scramjets for propulsion ?
Would a combination of maglev mass-driver (to reach mach3) with scramjet (to continue to space) make sense ?
How much could that reduce the cost of rocket launching ?