Why an orbital ring?
In summary an orbital ring is a space elevator for grown-ups and allows the kind of logistical endeavours a true solar system spanning civilisation would face daily better than any other launch asssist concept. Space elevators have a terrible throughput of only about 15 t/week according to recent papers and only bring one up to geostationary orbit with a litte push allowing one to bring out stuff to the asteroid belt on a slow transfer orbit. If you want to launch a 150 t space craft it would take 10 weeks to do so. Most other launch assist concepts face similar issues and are inferior to the orbital ring in most points but construction cost and energy demand (this will become irrelevant quickly as the energy cost is mostly upfront and the ring is a great base for off-world solar,
Helium-3 or fissile material import). In terms of scalability, throughput, interplanetary launch assist and general utility no other launch assist method can rival the ring. With that out of the way...
Crucial to an orbital ring is the concept of active support. Whenever you build a structure you want it to stay stable. Buildings do this by relying on passive support, i.e. their own structure can carry their weight. This approach is limited by the ability of the building materials to resist the force the rest of the structure exerts on them. This is Newtons third law, actio = reactio. Actio is the force the structures weight delivers and reactio is usually the force the material must be able to muster. Yet nowhere it is said that reactio must be provided passively. Imagine a friend of yours is walking over a thin plank, which would break under his weight. The passive support of the plank isn't sufficient to counteract the force your friend exerts. Now you go under the plank and push it up, so that it can hold your friends weight. You are providing active support. The great thing about active support is that you aren't limited by puny compressive or tensile strength, you are dumping energy into the system to keep it stable. And you can dump infinite ammounts of energy into a system.
Secondly some basic orbital mechanics are important for the ring. An object moves through space on an elliptical trajectory corresponding to it's speed. The faster the object moves (i.e. the more energy it carries) the higher it will move. If the object is restrained from moving into a higher orbit appropriate for it's speed it will exert a upward force on the restraint.
Constitution of the Ring
Now imagine bringing a stream of orbiting, magnetic slugs in a circular orbit arround earth, lets say 200 km high. They will move at orbital velocity. Arround each slug we have a a metal ring containing electromagnets. The rings are loosely connected and under power. They too move at the same orbital speed as the slugs. Then we activate the magnets, forcing the slugs to move though the center of each ring.
Finally we start decelerating the rings with their magnets. This will transfer their momentum to the slugs speeding them up. The now faster slugs want to move to a orbit higher than their current 200 km. The now slower rings want to move to a lower orbit. But the magnets force both to interact. To be precise the rings want to fall down with the same force the slugs want to go up. Like on the plank with your friend, the situation is stable and the structure stays in place. We continue this until the outer structure sits statically over the earth. The rings, also called stator, sit fixed 200 km over the earth and levitate magnetically over the slugs, also called rotator, which keep the structure in the sky.
At this point you can drop tethers down from the ring to ancor it and to install elevators. One of the rings many advantages is that normal nylon ropes will work for this and that no fancy carbon nanotubes are required. You can send tethers down to everywhere within a ca 500 km distance from the ring.
On top of the ring you install several maglev rails. Since the ring spans arround the planet and the velocity an object can achieve on a mass driver is given by this formula.
$v = d/\sqrt(d/(0,5*a))$
$v$ = velocity
$d$ = distance (track length)
$a$ = acceleration
As the track is circular, $d$ can be considered to be infinite, thus setting the theoretical limit of $v$ to $c$.
The best thing is, that this subverts the Tyranny of the Rocket Equation and allows for practically free transfer of goods between two planets with orbital rings. Just strap some ion drives on the cargo pods to allow the to adjust their Brachistochrone trajectories to correct for the orbital inclination differences. Regenerative breaking will make this more efficient than any rocket.
This video will give you some more insights and this paper deals with the construction of the ring base. As soon as such a base has been established the ring can be expanded at an extremly low cost. While the ring can be constructed with current day technology, high or even room temperature superconductors would be nice to have.