Complex sentence: Because the payload is accelerated to velocities greater than required for orbital velocity by the time the payload has left the device, either very high accelerations are required, or very large structures (thousands of kilometres long) are needed.
Split into simpler sentences: Because the payload is accelerated to velocities greater than required for orbital velocity by the time the payload has left the device. Either very high accelerations are required, or very large structures (thousands of kilometres long) are needed.