Ok, this will be awkward but i somehow got lost in this thought, For low velocities, the formula for energy boils down to the classical E=1/2m*v^2 So let's say two astronauts A and B are in empty space absent of gravity. B has a jetpack and accelerates to 5m/s which require E=1/2m*v^2= 1/2*100kg*(5m/s^2) = 1250 joule When B is finished accelerating, he finds another astronaut C right next to him (doesn't matter how he got there, he just is). B and C are moving at 5m/s relative to A. They can both consider themselves at rest according to SR. So B decides to accelerate again to 5m/s relative to C, requiring again 1250 joule. For such low velocities, we can add the velocities in the classical sense. So B would see A moving at 10m/s relative to himself with a total of 2500 joules used. But it requires 1/2*100kg*(10m/s)^2 = 5000 joules to accelerate to 10m/s. So where is my confusion?