To get into orbit a huge amount of energy is required in about 8 minutes. So far the only way we manage to do this is with chemical rockets. To drastically reduce the cost to orbit, we need to have totally reusable vehicles. Reentry at orbital speeds makes full re-usability difficult. A tether and an SSTT can solve both of the two big problems just mentioned. A tether can share a huge amount of energy in a short period, and make it so the launch vehicle does not have to reenter from full orbital speed.
Many people interested in space have recognized that the cost to LEO is the most important issue. Some examples are:
On page 274 of Spacefaring The Human Dimension Albert A. Harrison says, "The greatest challenge is finding less expensive ways to lift material and people into orbit. If we can reduce launch costs from a few thousand dollars to a few hundred dollars per pound we will jump the greatest hurdle to space settlement. Low-cost lifts to orbit are the closest thing to a "magic bullet" for accelerating our progress into space."
On page 311 of Space Resources Breaking the Bonds of Earth John Lewis says, "However, there is one area that must be mentioned first because it is of extreme importance, because it is one of the very few areas that would not immediately benefit from space resources, and because it is the major expense in many future programs. That is the cost of launching payloads from the surface of the Earth into Low Earth Orbit. Indeed, even those programs which bring back space materials to LEO at much lower cost will for decades to come be dependent upon Earth launch of all of their complex and high-technology components. It is likely that the expense of extraterrestrial materials in LEO will still be dominated by costs of launch from Earth. Thus, the reduction of launch costs is fundamental and contributes to the economic feasibility of all competing sources of materials in comparable ways."
On page 5 of The High Frontier - Human Colonies in Space - 3rd Edition Freeman Dyson says, "O'neill's dream makes sense if, and only if, the cost of launching stuff from the ground into space can be drastically reduced." And then on page 7, "Skeptics need to be convinced that the O'Neill dream can still make sense as a realistic future for our species, whenever in the fullness of time we shall deploy a new technology that reduces launch costs by a factor of a hundred."
In an interview Elon Musk said, "It has been freak'n decades ago that we got to the Moon. That is very disappointing. And the number one issue is the cost of access to space. If you can nail that one, then I think that's the key to the log jam."
We believe any money currently spent for a Space Station, Trip to Mars, Lunar Base, Mining Asteroids, etc before we solve the LEO problem is mostly wasted money. We would be better off to use that money solving the LEO problem and then do these other things once they were far cheaper. For example, if it currently costs $100 billion to go to Mars, we would be better off to first spend $1 billion developing cheap LEO. Then it might cost only $10 billion to go to Mars. In fact, we would be better off to cancel the Space Shuttle and use the money saved to develop better launch systems.
Both the Single Stage To Tether and the Two Stage To Orbit approaches seem reasonable.