Free momentum WAS claimed. Hence the space engine applications talked about everywhere. Momentum is conserved independently, it can not be created from energy

Free momentum WAS claimed. Hence the space engine applications talked about everywhere. Momentum is conserved independently, it can not be created from energy without spilling some the other way (into whatever you're pushing against). The vacuum is a translation invariant state so you can't push against it to get momentum either, but you can create particles out of it and then push against those. That's what electro magnetic thrusters do. They generate photons. The effect is just super tiny.