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Nuclear Engines: What am I missing?


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So my meager monkey brain is starting to work with me on this game. I'm planning a trip to the Jool system probably, but I'm building the engine for this thing and I'm using KER to know my delta-v. I use a big ol fuel tank and put the big ol nuclear engine on the bottom of it and I get 2397 m/s (vacuum obviously) and then I put the mainsail on the same craft, same fuel tank. 3033 m/s... So I'm confused. I thought the better efficiency would give me more delta-v. The nuke engine weighs 3 tons more, which is a third more but would I really lose that much delta-v as a result, even with the extra efficiency? If so, when does it actually become beneficial to use a nuclear engine?

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I think what you're missing is that nukes and mainsails use different fuels: The mainsail uses liquid fuel and oxidizer while the nuke uses liquid fuel only. So all that oxidizer is just dead mass for the nuclear version of the rocket. 

You can improve the nuclear version's performance by tweaking out the oxidizer in the right click menu or, better yet, use tanks that only contain liquid fuel and no oxidizer (this will reduce dry mass for a given fuel mass).

In answer to your last question, the nuclear engine becomes more mass-efficient when the added mass of the engine is less than the amount of fuel mass saved. Versus the Terrier (the common comparison), this happens at about 4t of fuel mass for the chemical engine (roughly one 800 unit tank). More fuel than that and the nuclear engine becomes more mass efficient.

Cost efficiency is a whole other thing, nuclear engines are really expensive and really only work cost wise if they are reused.

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In vanilla KSP (no mods), I often use the NERV engine(s) with the Mk3 liquid fuel tanks as they have a huge capacity, 10km/s delta-v is easy peasy this way.

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