If an anti-proton collides with a proton, they are supposed to totally annihilate each other, correct? Then how is conservation of momentum preserved?
Could you launch both protons and antiprotons toward a common point, where they annihilate, and give their energy back, while you keep the gained the momentum from the launch of the particles? This would allow for a closed engine and would seem to violate the conservation of momentum. What would prevent this from working?