If you are used to fly you certainly have listen to the captain saying:"just finishing to load your bags and we are ready to go. Relax and enjoy your flight".
Now, according to a patent filed by Boeing, in the next decade we might hear the captain saying: "we are about to to blast a nuclear device in the place and we are ready to go. Relax and enjoy your flight". The question, of course, is how much relaxed you are likely to be if you are sitting on a nuclear bomb getting ready to explode.
Boing researchers idea is to feed a stream of pellets of hydrogen (deuterium and tritium) into a cavity in the nuclear engine and heat them to extremely high temperature with laser beams. This causes a nuclear fusion (a thermonuclear bomb) that releases neutrinos in large quantity (and heat...). The neutrinos will hit the wall of the engine that is covered by uranium 238 leading to fission (a nuclear bomb) that again generates enormous amount of heat that is absorbed by a coolant enveloping the engine chamber that in turns will spin a turbine thus creating the energy required to power the laser beams. No, it is not a more complex version of perpetual motion: here you are using deuterium tritium and uranium and after a while they will be "consumed" and you will have to replenish the nuclear fuel.
Of course the energy density is very high (since you are transforming mass into energy) and a simple tank of nuclear material would be able to power the plane for a long time.
Still, being seated on a thermonuclear and a nuclear bomb may not be the most relaxing way to fly.
Additionally, as someone pointed out, in the "unlikely" case of a crash all the uranium 238 will be disperse into the environment, and this is not good...
Get a look at the clip to understand better the Boing proposal. And for the time being, and few more years, enjoy the jet engines working on "normal" fuel.