This sounds terrific, but it doesn't seem self-consistent between wanting companies with the scale of ambition of SpaceX and Tesla, but insisting on early momentum being critical and avoiding a big and expensive initial project.

Tesla took four years to introduce their first product, the Roadster. When they went public two years later, they had gone through a net loss of around 300 million dollars, and were still two years away from introducing the Model S. Even then, it had a few near-death experiences. SpaceX took 200 million dollars of investment, and took four years to launch their first flight, which failed. Over the next two years, their second and third flights also failed. Neither company really showed momentum until seven to nine years after founding.

It would be nice to see a clear path from a three-month-to-demo-day minimum viable product to a breakthrough company that overturns a capital-intensive industry, but it's hard to think of good examples of this so far. At the least, it would probably require substantial development before applying to YC, and/or starting with a very limited prototype.