Space Exploration Technologies (SpaceX) may someday build just one large rocket, rather than their current rockets of different sizes. This could be very economical if the vehicle is fully reusable, which could happen when SpaceX finishes developing their new Raptor rocket engine based on methane fuel.

Here’s an article on the new SpaceX rocket:

http://spacenews.com/musk-offers-more-technical-details-on-bfr-system/