Throughout recent history there were several attempts to build Jules Verne's vision of a space cannon, from Krupp's wartime Paris Gun as a weapon to the 60's Harp project as a space launch system.In recent decades the SHARP light gas gun and the current successor Quicklaunch project are all still unable to affordably and reliably achieve their goal.
My idea is to use a common old technology, the air gun, and some additional elements to propose an alternative way to reach success.
Normal cannons use chemical based propellants that provide high initial acceleration.This is bad because of the high G force tolerant components required to make any good use of it. An air gun can be regulated to discharge a number of pressure vessels sequentially and provide a lower initial acceleration but maintaining it for a longer period thus keeping or increasing the resulting exit velocity.
Since the maximum working pressures can be significantly lowered, it should be possible to use a much cheaper and lighter barrel to reach the same exit velocities. It will only take a longer barrel to reach higher velocities.
A common steel structure can be built to support the barrel along two parallel circular rail tracks for positioning purposes. A small scale prototype is relatively cheap to produce and a full scale system would be far cheaper to build and operate than rocket based ones.
Since this system could be exclusively powered by electricity it would be much less pollutant than a rocket.
Though large pressure vessels capable of sustaining 100000Psi are hard to build, they must be easier than rockets. Since time and resources to investigate this idea further are scarce for me, i hope those with better knowledge and resources can verify if this idea has any merit or probability of success.