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I read this essay by Neal Stephenson recently, which makes the case that, had we not been locked-into chemical rocket technology through years of investments, other approaches might have been more viable.
As of today, what non-chemical rocket technologies appear to be the most promising for the future of space travel?
There has been a lot of researching regarding Non-Chemical Rocket propulsion Systems. The most recent one being beamed thermal propulsion. It involves propelling a rocket by using microwaves from the ground. Here you can find more Systems which have been researched on. Some have already been used, such as the Air Launch, where rockets are launched from conventional horizontal-takeoff aircraft, to carry satellites to low earth orbit. Also some other examples are: Static Structures: Space Elevators, Skyhook Dynamic Structures: Space Fountain, Orbital Ring And Mechanical Systems such as Pneumatic launchers.
This table gives the estimate build cost, the technological readiness for most of these Non-Rocket Space launch Methods.
