Morgan (l982) estimated that the exhaust velocity of a pion- relecting matter/antimatter rocket could be in excess of 0.9c. If the spaceship is a sphere, and one antimatter rocket is aimed in the X axis, and one antimatter at 90 degrees to X 1/4 the way across the sphere is aimed in the Y axis...
If a spaceship is traveling at 90% the speed of light on the X axis. And it is also traveling 90% the speed of light on the Y axis. What is its velocity?