I'm not sure how useful thruster would be that produces barely measurable thrust. The criteria should be that the thruster has obvious thrust. The other questi

I'm not sure how useful thruster would be that produces barely measurable thrust. The criteria should be that the thruster has obvious thrust. The other question is how efficient it is or what is the power to weight ratio. Not throwing out reaction mass is nice, but it needs power and that power needs to be generated by fuel. If it doesn't save overall fuel or is heavy, it doesn't matter if doesn't need propellant.