I just do not see why people keep trying to eliminate the weight of oxidizer and propellent. IE scram jets or space elevators when the cost of propellants is so small. The propellent cost for a Falcon 9 launch is only $250,000. The Falcon 9 can put 50,000 lbs of cargo in low earth orbit that's just $5 per pound for propellent. Propellent does not matter. What matters is the cost of the vehicle. We need 100% spacecraft reusability for 1,000's of launches to get the cost of space travel down. Space X is working towards that goal. The current engines that use Kerosene will wear out rather quickly because the engines need to run with excess oxidizer to prevent them from being fouled by carbon build up. The oxidizer causes the engine to rust out(titanium rust). The new Space X engine uses liquid methane fuel. It can be run with a more perfect oxidizer/ fuel ratio and eliminate the corrosion problem so much longer life.