I stand corrected. That is an interesting concept.

> Direct cycle nuclear engines would resemble a conventional jet engine, except that there would be no combustion chambers. The air gained from the compressor section would be sent to a plenum that directs the air into the nuclear reactor core. An exchange takes place where the reactor is cooled, but it then heats up the same air and sends it to another plenum. The second plenum directs the air into a turbine, which sends it out the exhaust. The end result is that instead of using jet fuel, an aircraft could rely on nuclear reactions for power.

A direct cycle design (which is the only one that was ever thought practical to use) passes outside air through the core for cooling purposes, then sends it through the exhaust.

> Indirect cycling involves thermal exchange outside of the core. The compressor air would be sent to a heat exchanger. The nuclear reactor core would heat up pressurized water or liquid metal and send it to the heat exchanger as well. That hot liquid would be cooled by the air; the air would be heated by the liquid and sent to the turbine. The turbine would send the air out the exhaust, providing thrust.

An indirect cycle design doesn't expose outside air to the core, but "never came anywhere near producing flight-ready hardware", presumably because the amount of water needed for cooling would have required an absurdly large plane and even more power, leading to recursive issues.