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So, in looking at Air-Augmented ("ducted") rockets recently, and considering nuclear propulsion, this idea came to mind (I think I've read about it before- will post links for background when I have time) Basically, it combines the features of a nuclear thermal rocket (aka. 'NERVA' aa the most famous example) or a nuclear thermal turbojet, with a ramrocket (itself the hybrid of a Ramjet and an Air-Augmented Rocket). So, it looks something like this: air enters into intakes (and probably then a pre-cooler passing some heat to Liquid Hydrogen as a heat-sink: ala. "SABRE" intakes in real life) and enters a mixing chamber. Separately, Liquid Hydrogen is passed over a nuclear reactor's heat-exchanger, just like in NERVA or any similar design. The exhaust from the nuclear heat-exchanger then enters a mixing-chamber with the intake air, exactly like how any ducted rocket works (basically, ducted rockets mix the exhaust from a Combustion Chamber with intake air BEFORE ejecting it through a rocket nozzle- this is all there need be to such a design... If the air is pre-cooled first, heat management becomes easier and you can use more heat-resistant materials or a lightweight heat-vulnerable Aerospike Nozzle, without them melting, but this is not usually done...) However because there is Oxygen in the intake air, and the nuclear reactor exhaust is superheated Hydrogen, there is the possibility for Ramjet-style combustion. This is how a ramrocket normally works (the ONLY difference here is that instead of Hydrogen-rich combustion products from a conventional rocket combustion chamber entering the mixing/ secondary combustion chamber, you have 100% Hydrogen from a nuclear reactor's heat exchanger entering it instead...) This Hydrogen is afforded the opportunity to combust (this is also why you want pre-coolers: to allow compression and cooling of the intake airflow by Compressors to speeds/temperatures usable for a Ramjet even in very high speed/altitude flight), and you get additional energy from this Ramjet-style combustion, which gets you extra Thrust after you pass the exhaust through a rocket nozzle as with any ramrocket... Basically, it's a nuclear thermal turbojet, but with the addition of an integral ramjet for extra Thrust and higher Exhaust Velocity (which also means the whole thing is useful at higher speeds than a normal nuclear turbojet). The secondary combustion/mixing chamber could even be designed as a Scramjet: although that creates problems with no longer having any static thrust on the runway... (due to having low TWR, very high Effective ISP, and being optimal for high-speed atmospheric use: this is best used on a horizontal-takeoff spaceplane...) Other notables: - The airflow would be possible to close off at the point of the intake, or possibly also downstream, allowing for operation as a pure (but very heavy) nuclear thermal rocket once you leave the performance envelope for airbreathing propulsion. Thos is similar to the Rocket-Based Combined Cycle ("RBCC"- basically a ducted rocket, ramrocket, and closed-cycle rocket all in one...) propulsion system, except that the initial heat source is a nuclear reactor rather than a primary rocket combustion chamber. - This works best with lighter/more powerful next-gen nuclear reactors, like Molten Salt Reactors (not coincidentally, previous investigation into nuclear thermal turbojets looked at using MSR's for the reactor componemt...) - Nuclear Reactors with TODAY's tech actually eject COLDER exhaust than a rocket combustion chamber. The reason for their high ISP is due to using a pure exhaust of nothing but Hydrogen... This means that, if you further diluted this already-lower heat with colder-still atmospheric air, you would have exhaust temperatures very usable for an Aerospike Nozzle: these nozzles normally having tendency to melt without a lot of extra weight for heat-managenent.