but the key is, a lot of (read: "most of") the power is to come from fission.
I really can't think that Boeing would be so daft as to think that anyone would ever use this on Earth.
I think you may have missed that only the fusion products come out the back. The fission products are contained, and its relatively easy to contain the fission chamber in the event of a crash, unlike nuclear-powered spacecraft, which re-enter with 1000 x the energy of a subsonic aircraft.
Surely the point of the patent is to use it for exploration of other planets. Right? I hope so...
It has the advantage of operating without fuel or oxygen, but why would a planetary explorer need a jet engine? Whats the hurry?
They can just float around with a small solar or RTG-powered propeller.
Even if the technical challenges can be overcome, its hard to imagine this being economically viable over a conventional jet engine, even if you have to synthesise the fuel on the ground with nuclear power.
I miss the days when to get a patent you needed a working model, instead of just a vague idea that might never work.