An alternative to a full space elevator is the rotovator or rotating sky hook. Instead of building a single tethered cable that reaches up to geostationary orbit, you instead build shorter, untethered cables that rotate end over end in their orbit.

These structures can then be used to assist conventional rockets. Place a rotovator in low orbit, then send up a suborbital rocket to hook onto the end of the cable when at its lowest point. Detach half a rotation later, and the rocket is flung into a higher orbit.

There's a more detailed discussion here:

https://www.reddit.com/r/technology/comments/2h07ku/japanese...