If you have a nuclear reactor, the power vs mass loss could end up working in favor of the emdrive, especially for long range interstellar flights. None of this is, of course, practical anytime soon...
Nuclear reactors are incredibly massive though, especially with all of the required shielding. We've never launched anything even as close to as massive as a space-borne nuclear reactor would be. I agree with you that it won't be practical anytime soon (or at least not politically), so as long as we're positing some future level of tech, why not nuclear fusion?
Due to political and environmental concerns alone no modern terrestrial reactor design would be allowed to fly. That's usually not what nuclear propulsion proponents argue for.
There are several other reactor designs that have been researched over the last half century that can operate in a closed loop with much smaller amounts of nuclear fuel and more manageable radiation emissions. Designs like the nuclear lightbulb, which was researched by United Aircraft Corporation and NASA for almost a decade before the plug on the Mars mission was pulled in the 70s, are much better suited and are what proponents of active nuclear propulsion most often have in mind. Once there is some political will, we have decades old research to start from to build a flight capable nuclear reactor.
The nuclear lightbulb doesn't generate power though -- it's a rocket engine, and "generates" reaction mass leaving at high velocities. So it's not suitable for use powering an emdrive, the whole point of which being indefinite flight without needing to use up any reaction mass, which a nuclear lightbulb can't do.
No, the nuclear lightbulb generates energy in the form of heat and electromagnetic radiation. How you use that energy output depends on the design of the reactor and propulsion system.
You can seed expanding liquid hydrogen in the outer cavity with tungsten nanoparticles which absorb the UV radiation to heat the hydrogen for use as a propellant. You can pump the outer cavity with a UV transparent coolant and line the walls with parabolic photovoltaics (that convert UV instead of visible spectrum) for direct conversion of the black body radiation to electricity. You can theoretically even create a magnetohydrodynamic "turbine" in the outer cavity that is coupled to the spinning nuclear fuel (which can be charged plasma).
You may be thinking of an earlier that was mistakenly called nuclear lightbulb or another design that was lumped into the concept. The variation I have in mind is just as general purpose as any other nuclear reactor, it just uses a lot less fuel and has to actively maintain temperature and pressure to keep the neutron cross section energy high enough to sustain power positive fision.
Do you have a link with more information to the specific concept that you're talking about? It looks like I was led astray by the Wikipedia article. I'm super interested.
Nuclear reactors have actually been launched into space multiple times [0][1]. Note, however, that their power output was on the order of a few kilowatts.
Wow, that's awesome. I hadn't realized. Unfortunately those systems were fraught with problems (that persist to this day in their orbital debris), but they flew and they worked. Looking at the power generation capacity of those reactors vs their mass, however, they weren't even that much better than plutonium RTGs! The TOPAZ generated 5 kW using a 320 kg reactor, and was only good for five years. That's 15.6 W/kg. Meanwhile, the standard RTG design we've been using in recent space probes, including New Horizons, generates 300 W at 57 kg, or 5.3 W/kg, but the half-life is a long 87.7 years and the system doesn't suffer the kind of wear problems that a nuclear reactor does, so the total lifetime energy output is substantially greater. This would matter for interstellar probes with near-present levels of technology, assuming a working emdrive but nothing else.
I don't really see a huge difference between a spaceborne reactor and the kind that have been in constant use for decades in submarines. We just need a new Admiral Hyman Rickover to make it happen safely and effectively.
1. Details on submarine nuclear reactors are classified, but five minutes' Googling shows that a reasonable guess for their total mass (including shielding) is 1,000 tons. Meanwhile, the entire payload to LEO of the largest rocket ever successfully launched, the Saturn V, is only 155 tons -- and that's for the entire top stage.
2. Submarine nuclear reactors use water for cooling. Water that is not available in space. Cooling would be a huge problem.
So, existing submarine designs are not practical. You'd need to design something from the ground up that is much lighter, and the cooling system would be entirely different and likely more massive, since you don't have all of that free water available to dump heat into. Instead you're talking massive radiators.
Good point re: the mass of the reactor, that's a hell of a lot more than I'd have guessed.
But any mission involving humans is likely to carry a large amount of water beyond the crew's personal needs, because it makes such a good radiation shield. So presumably the same water would be used for cooling the reactor.
It all depends on the size and design of the reactor, though. A reactor that you put on a plane just for the hell of it (which is what that was) is going to be a lot smaller and lighter than a nuclear reactor that needs to power an entire submarine. The point to my reply was that submarine nuclear reactors were in no way suitable for space use because they aren't optimized for weight at all. Reactors optimized for plane use would be a closer fit. The reactor in that plane could be lifted to orbit on a Saturn V, so we're making progress, but, and this is a huge but, it was air-cooled.
A 3 MW reactor puts out a hell of a lot of heat, and without the benefit of air-cooling in space, I'm not sure what exactly you would do with all of that waste heat. Consider how massive the space shuttle orbiter's radiators were (they are on the inside of the cargo bay here: http://i.stack.imgur.com/Flgzb.jpg ), and all of that is only capable of shedding waste heat in the amount of ~6 KW! We can put a much more capable reactor into space than we can possibly cool, so we haven't bothered. Cooling is the real problem. The total mass of the radiators and the structure required to support them ends up being way more than the reactor itself.
So for good long distance transportation in space, not only do we need a working, efficient emdrive, but we also need better power generation that is much more efficient from a waste heat perspective. These are really hard problems.
