Orion is a great project ! I also have heard of another way to propel the vessel, it uses black holes but I don't know how they would use it, still they would create the tiny black hole thanks to very very very veryyyy powerful lasers to create it and then the black hole would evapore it in seconds but it would be enough for the vessel to absorb huge quantity of energy and thus reach a maximum speed of 60 % of the speed of light. That way the crew would reach the nearest star in only 7 years !