Maybe when spaceships will not be based on rocket engines.
That said, at the moment it seems difficult to see what could do better than a ionic engine, given the huge margin of progress there exist in this area.<p>Also the main problem is not a new propulsion technology, but a new energy source, passive like in solar sails or active like in liquid/radio-active rockets.<p>After thinking about it, the origin of this expression may be the difficulties there was to understand how the combustion chamber works. Actually they had to create instabilities to obtain a stable exhaust jet.