Turbofans for takeoff, rockets for spped boost, then scramjets..
Presumably the "zero emmissions" wil come about because it uses hydrogen as fuel, so it's emissions will be water vapour........even though water
vapour is the main greenhouse gas in the atmosphere, and possible climate effects of contrails are under investigation.
Hydrogen was used as a fuel in some of the earliest jets (Hans v. Ohain used it in his early experiments), and perhaps by then mineral oil based fueld
will be uneconomic for various reasons...so maybe the propulsion technology will be available.
but at the very least the environmental effects of pumping tons of water vapour into the stratosphere will undoubtably come in for much scrutiny
before anyone lets it hapen.
it's an interesting kite to fly