Suppose we have a flashlight. Itself it is non-mass losing engine. But still there is an impulse which is carried by photons. Now suppose "black body" is in the opposite to flashlight and it collecting all emitted by flashlight photons. In this system flashlight produces impulse which is 2 times bigger than opposite impulse of black body because at first mirror "eat" photon and then emitting in the opposite direction. So the overall system will have some uncompensated impulse and will actually move in space without any emissions outside.
I am not specialist in physics so my conclusions maybe look like too naive. But still I am asking you to dispel my difficulty in this matter. Thank you all who deliver knowledge.