Quote: "an in-depth analysis of the rocket design and spacecraft integration as well as mission architectures enabled by the FDR need to be performed. Fulfilling these three elements form the major tasks to be completed in the proposed Phase II study. A subscale, laboratory liner compression test facility will be assembled with sufficient liner kinetic energy (~ 0.5 MJ) to reach fusion breakeven conditions." {emphasis added]
Without a net energy gain, the project would be better off using an ion thruster, which has the advantage of already existing and being quite efficient (but with a power gain < 1).
To repeat what others have told you several times in this thread:
Why do we still use chemical rockets instead of ion thrusters for boosters? Answer: thrust vs. specific impulse. Ion-thrusters are efficient in use of (high specific impulse), but are weak in how much thrust they create. Your ship would have to be mostly ion-thruster to get it anywhere, thus ruining the efficiency advantage with respect to payload of the engine.
http://en.wikipedia.org/wiki/Ion_thruster "Given the practical weight of suitable power sources, the accelerations given by ion thrusters are frequently less than one thousandth of standard gravity."
What does linear kinetic energy have to do with fission reactors? It's not as simple as "attach a turbine to the kinetic energy conversion process." Or rather, it is that simple, and that's why it doesn't work, because it's another point at which one loses power to inefficiency. But if all you want is kinetic energy in the first place, then you don't lose efficiency to that extra step.
There is something you are not getting. Assuming a set of solar panels like those on the ISS, able to deliver 200KW near Earth and 100KW near Mars, one must still convert that power to useful thrust. That means achieving fusion break-even, a requirement that the NASA documents explain.
Obviously ion thrusters aren't practical for this kind of heavy lifting and short mission time, but that doesn't usher in fusion power as an obvious substitute -- unless the project creates a useful reaction, meaning a gain > 1.
