Originally Posted By: paul
internal force can change the momentum of the iss.

I can't believe you're having trouble with this. Although most of the ISS changes its angular momentum, the total angular momentum of the ISS and its contents doesn't change. This is analogous to the total linear momentum of the tube and it's contents, which also doesn't change.

Gyro goes one way, rest of ISS goes the other way.

Masses go one way, tube goes the other way.

Gyro keeps spinning indefinately, rest of ISS keeps spinning indefinately.

Masses must reverse direction within the length of the tube, tube reverses direction.