[PeterisP]

How would that heat radiate?

Electromagnetic radiation has momentum - it's small, but not zero. If you're emitting a bunch of photons (e.g. infrared photons from blackbody radiation of a hot object) more to one side than the other, then the remainder of that device will have some thrust. But that does not violate conservation of momentum, as the momentum gained by the device will equal the momentum of these photons; and the total momentum of the whole system (the device plus all the photons radiated) will be constant.

[aaronblohowiak]

Thank you!