I don't have solid proof, i.e. a functioning low-cost space vehicle. I do have my own experience with many aspects of design in low-cost space transportation, including propulsion, avionics, systems design, concept evaluation, and other things. Obviously, we have yet to see a low-cost space transportation system be successful, but this is due far more to financial roadblocks than technical ones.

This is a difficult subject. While I don't agree with SpaceProjects.com on a number of things, one core fact is true:

NASA does not primarily exist to develop and complete space missions. It primarily exists to hire NASA employees and expand NASA facilities.

For this reason, it is almost impossible for them to consider concepts that do not require vast amounts of research, personnel, new technology, and long-term time frames. Do they come up with successful missions? Sure they do. Are NASA employees for the most part skilled and dedicated? Sure they are. But that doesn't change my premise.

An example:

I worked on Gravity Probe B, a NASA project that has devoured almost $700 million dollars on a marvelous scientific instrument carved from solid quartz to nearly-atomic perfection and operating at near-absolute-zero, and whose technology and science is so far beyond me that I can't even see it in the distance. My work was more conventional engineering involving gas systems, valves, and normal things like that.

While I was there, an engineer and physicist much more brilliant than I am developed an idea to accomplish the same science results using a simple instrument of his own design. For orders of magnitude less money and time. I sat in on the evaluation. Would it work? Probably. Would they use it? Hell no, although they were very polite about it. They were too far along to change, and frankly, I think it was embarrassing to them. I guess they deserve credit for at least evaluating it at all, but I could see the handwriting on the wall. This was quite a few years ago, and that engineer left soon after.

Another example:

SpaceX has developed a conventional rocket capable of placing a payload in orbit. The company was founded, I believe, in June of 2002. The rocket, using all new construction, will see its first launch less than two years later. If it fails, they will fix it and relaunch in less than another year.

NASA, faced with exactly the same project, would still be pushing Powerpoint slides around, along with lead times of five years or more. NASA's cost of the project is left to your imagination. Gpod God, how much are they going to spend to reinvent the Apollo capsule?

If humanity is ever going to get into space in a big way, NASA is not going to do it, any more than IBM was going to launch the personal computer industry. Oh, IBM came running into the market later, but does anyone doubt that if someone else hadn't blazed the trail they would never have even considered the concept?

Beyond that, I don't know who is going to have the breakthrough, or exactly how. I'm still hoping I may play a hand in it--I'm not out of the business yet--but there are a lot of people in the private space arena now, and anything could happen.