Yes, however, for completeness: an explanation of why we must limit designs to chemical rockets ought to include an explanation of why the dozen or so fusion projects underway around the world will all fail, i.e. let's inject some rational optimism. Note that the Apollo programme began before its tech was ready.
NASA is currently working with John Slough's company on a fusion rocket for interplanetary travel.
http://www.nasa.gov/directorates/spacetech/niac/2012_phaseII...
http://www.washington.edu/news/2013/04/04/rocket-powered-by-...
Orion gets its momentum transfer from the plasma debris of the nuclear explosion, and the plasma speed comes from the explosion itself. Orion carries the source of the plasma, hence it is limited by the rocket equation.
For instance, in a NERVA rocket, the "fuel" is the uranium rods in the reactor, the "propellant" is the liquid hydrogen that is heated and shot out the exhaust bell.
Basically any spacecraft that does not carry its own propellant. Most of these have either pathetic thrust or are way beyond our current state-of-the-art.