University of Washington's Ram Accelerator (RAMAC) (https://en.wikipedia.org/wiki/Ram_accelerator https://en.wikipedia.org/wiki/Ram_accelerator) is what I think

University of Washington's Ram Accelerator (RAMAC) (https://en.wikipedia.org/wiki/Ram_accelerator https://en.wikipedia.org/wiki/Ram_accelerator) is what I think will ultimately win. It's basically a long tube with mixtures of natural gas, designed to work like a ramjet.