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Russia is currently developing nuclear powered cruise missiles. These presumably use nuclear ramjet propulsion in the same way that the US's Project Pluto would have. Is there enough similarity between nuclear ramjets and nuclear thermal rockets, that research into the first is likely to lead to breakthroughs for the second?

Are the two technologies similar enough that there is the possibility of a hybrid nuclear engine that operates as a nuclear ramjet in atmosphere and then switches to a NTR mode once in space? Similar to how SABRE uses air as the oxidiser in atmosphere and switches over to being a chemical rocket engine in space.

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    $\begingroup$ Since the "AB" in SABRE stands for "Air Breathing", I'm not sure what "SABRE-like hybrid nuclear engine" would even mean. I'd remove that part of the question, and if you were interested, ask a separate question about it where you can explain. The rest stands as a complete question by itself. $\endgroup$ – uhoh May 25 '18 at 9:13
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    $\begingroup$ @uhoh: It's not completely off-topic. SABRE is a chemical engine which can run with atmospheric air as ramjet, or own oxidizer, like a classic rocket engine. The hybrid nuclear engine would work as nuclear ramjet in air-breathing mode and classic NTR with own propellant once air becomes unavailable. $\endgroup$ – SF. May 25 '18 at 9:49
  • $\begingroup$ @SF. Oh I see, thanks for the explanation! The only ramjet I'm really familliar with is Roger: youtu.be/SIbFJmCUxsA $\endgroup$ – uhoh May 25 '18 at 10:10
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    $\begingroup$ Maybe I should remove the SABRE part of the question, I only meant the comparison in the broad sense of an engine that can either be air breathing or internally fueled (though 'fueled' is the wrong word as it is oxidizer in SABRE's case and propellant in the nuclear case). The point of having this in the question was an extension of the main question regarding the similarity of the two nuclear engine technologies. Are the two nuclear engine technologies similar enough for a single engine to operate in both modes? $\endgroup$ – Lex May 25 '18 at 20:08
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    $\begingroup$ @Lex there's no limit to the number of questions you can ask, and a new question can link to the old one, so if you think this is asking for more than one answer, I'd still recommend to split it up. That will keep answers to each question laser-focused on a single issue. You're in the driver's seat! $\endgroup$ – uhoh May 25 '18 at 20:13
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Yes, they are similar that there is the possibility of a combined cycle nuclear engine.

This was proposed by John Bucknell as the nuclear thermal turbo rocket (NTTR) in 2015.

https://drive.google.com/file/d/0B52GlVTiDp1AU2JxRlFxNFNXUXM/view

As of July 2018 he has introduced updates to the design as well as a chemical version to be used as a transitional version so development can be parallelized.

https://drive.google.com/file/d/11WcdsIT3RX_3idEBYouKFdWVjb8PybFb/view

The design goes a step past combining ramjet and rocket flight modes. It starts as a nuclear turbojet, then ramjet, then scramjet, and finally rocket. This design allows a claimed average ISP of between 1400 and 1800 seconds to LEO.

Note that this implementation seems to use propellant in all cycles, augmenting it with air in the non rocket cycles, but never running fully on external air, so different the Project Pluto.

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