Is it? How much fuel?

Even using a conservative estimate of 100 km for the thickness of the atmoplane and assuming 100% efficiency in terms of the conversion of the heat of combustion into kinetic energy, it would require 111 tonnes of hydrogen/oxygen fuel to elevate an object with the mass of a Saturn V rocket into space. That's not accounting for the fuel required to accelerate the fuel to be used ahead of it, either. If, as I suspect, the atmoplane is ten times that thick or more, it would require ten or more times that amount of fuel - again, this isn't accounting for the fuel required to move the fuel itself, which would make the calculation more complicated as the mass would vary with time.

Taking into account all the shortcomings of the calculation, I'd say we'd be looking at ten kilotonnes or more of fuel.