There is something wrong when i try to calculate the Mechanical Energy


I’m working on a orbital simulator with using physics. But i got some problems.

In free fall motion, mechanical energy E = Ep + Ek. and Ep=mgh ,Ek=mv^21/2

The value E should be consistent until it hit ground. But…

The value getting smaller and smaller.

In general, it is not a big deal,but in orbit system any small deviation will make huge mistake.
I’m pretty sure that linear damping has be set to 0.

Could you Help me solve this problem ?
Thanks a lot