/u/JeffHoffman on I am Jeff Hoffman, MIT Professor of Aerospace Engineering and former NASA Astronaut. Ask Me Anything!19 Feb 2015, 19:13 UTC
I just talked about this today in my MIT "live" 16.00 class. Given our current propulsion technology, to build a single stage to orbit rocket would require that the entire structure (rocket, engines, tanks, payload, etc.) be less than 10% of the total mass on the launch pad. i.e. Well over 90% of the launch mass is propellant. Right now, we can't build structures that are light enough and still have the strength to withstand the rigors of launch. But material technology is improving, and if carbon nanotube technology develops sufficiently, perhaps we will someday be able to realize the dream of "single stage to orbit". For now, though, we have to stage our rockets.