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.
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.
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.
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.
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.
It's worth more than any existing or previous theoretical engine. Put this rotating around a center point. Now put 1000s of them. Now we have power generation for outposts in space. The implications of a working EM drive are staggering.
That's another good reason to think it doesn't work.
By special relativity you can't break the momentum conservation principle without breaking the energy conservation principle.
Somehow most people feel more comfortable dismissing the momentum conservation principle, but they are intertwined.
A carousel with Emdrives like the described in the previous comment should be a perpetual mobile and create energy if it's spinning in the right direction. (It will destroy energy in the reverse direction, so be careful.)
[Anyway, I think that the emdrive doesn't work and the conservation of energy and momentum are safe.]
However, any true reactionless thrust is equivalent to a perpetual motion machine, because reactionless thrust drives produce the same acceleration regardless of speed, but the kinetic energy produced by the same acceleration goes up as the speed goes up.
If you accelerate one kg from 0m/s to 1m/s, you impart 0.5joules of energy on it. If you accelerate it from 1m/s to 2m/s, you impart 1.5joules of energy on it. If you accelerate it from 2m/s to 3m/s, you impart 2.5 joules of energy on it, and so on and so on. Each added m/s costs more in energy.
A reactionless drive working in a system with no preferred frame would add the same amount of acceleration for the same amount of cost, regardless of how much energy there already is. This would mean that eventually it would be going fast enough that the added kinetic energy would be more than however much energy it draws in. Then you can build a gigantic carousel that is spinned on the rim and takes energy from the middle and feeds some back in to move it, and now you have perpetual motion.
This is why no real physicist actually thinks that this will be reactionless thrust. However, that does not necessarily mean it's useless. If it, for example, allows you to push against the earth's magnetic field more weight-efficiently than current magnetic propulsion systems, it would be a major win for satellite stationkeeping.
A) Where is that in the paper? Or am I missing something elementary to this?
B) We don't even know if it produces the same level of thrust as speed increases. If it produces the same level of thrust across all speeds, then we have a problem, because as velocity increases, so does mass, and the same level of thrust would produce less acceleration over time, because of the increase in mass. Of course, that may only effect things at relativistic speeds.
Rockets do that too, but they get away with it because they stored the necessary extra energy into the kinetic energy of the fuel.
If there was a reactionless thruster that did, in fact, get less efficient at higher speeds, then that would also be world-shattering physics news, because then you could measure it's performance after accelerating in different directions and eventually get a true rest frame that is privileged over the rest of the reference frames out of it.
That's the implication of the EM Drive. I didn't invent it, it's the very first thing that you think of once you violate the momentum conservation principle. This is why it's unlikely to be true, but at the same time an alchemists dream scenario.
The advantage of the emdrive is that it does not require reaction mass, at the cost of incredibly high power consumption. Unfortunately that makes it not really viable for anything; ion drives are limited by power, not reaction mass, so the emdrive is actually worse because it uses way more in power than it saves in not needing reaction mass.
Since violation of conservation of energy is very unlikely, I'd say that this is a sign that the drive doesn't actually work. Or at least that there is some gotcha that we haven't understood yet.
The EM Drive is an energy reaction engine, rather than a mass reaction engine (if it works as described), so instead of pushing off wake, it uses energy itself to "deepen the fold", as it were, and increase speed (and mass, since the two are exchanged).
That's interesting. I've never considered that the very act of increasing the velocity of the EM Drive would actually increase its mass. Hrm.
And anyway, kinetic energy isn't a conserved quantity. It can be converted into potential energy in a gravity well, or lost entirely in inelastic collisions. Momentum is the quantity that is always conserved, and that is what the emdrive violates directly (though still not in a way that allows for perpetual motion or free power, near as I can tell).
The first internal combustion engines were quite inefficient as well.