As I understand it, movement of two objects is relative to one another. Velocity is always in relation of one thing to another. A car travels at 100km/h (60mi/h) relative to the rotating Earth surface it's driving on. A rocket propels upwards relative to the rotating Earth surface.
If we choose a different reference point, things change. If we use the center of the Earth as a reference point, the car, parked at the side of the road, is technically rotating with the Earth. If you were to drive east at the same speed as earth rotates, we would now be going twice as fast. And if we drove west at the same speed as the earth rotates, we would be stationary relative to the center of the Earth.
Similarly, if we launch a rocket in the direction Earth is rotating, at the same speed as Earth is moving around the sun, then relative to the sun it is moving twice as fast. If we launched the rocket in the opposite direction, then relative to the sun it wouldn't be moving (other than starting to fall straight into the sun).
Thus, there really isn't one speed for any object but multiple speeds depending on what you use as the frame of reference. And right now, even staying still, we are moving in all sorts of ways from the universe, the sun, the Earth around the sun, and the Earth's rotation.
We know that no object can travel at the speed of light. As an object goes to approach the speed of light, the amount of energy increases exponentially until it reaches infinity. However, a velocity must be in relation to something. That frame of reference must be defined somewhere.
If an object is already moving at half of the speed of light, and from it we launch a smaller object as fast as we possibly can, the speed of that object relative to us could only ever get to half the speed of light, because the total speed could never reach the speed of light.
And if we launched that smaller object in the other direction, could it not therefore move at up to 1.5 times the speed of light relative to our frame of reference?
From this then, we could see that the object we launched in the same direction would take substantially more energy to accelerate than the object launched in the opposite direction. Furthermore, if we slow down or speed up our spaceship, the relationship would change.
Could it not then be possible to launch objects in all direction and be able to determine the exact speed we are moving at, and from there, calculate what the reference frame of the universe is?