Now we agree what happens a little more:

Before starting:
pPipe = 0
pMass = 0
pTotal = 0

After the acceleration:
pPipe = 4000Ns
pMass = -4000Ns
pTotal = 0

After the 1st turn:
pPipe = -4000Ns
pMass - 4000Ns
pTotal = 0

After the 2nd turn:
...