Their Zeal thruster uses a heating element: http://momentus.space/services/ http://momentus.space/services/ To answer your question, I would guess that a micro

Their Zeal thruster uses a heating element: http://momentus.space/services/ http://momentus.space/services/ To answer your question, I would guess that a microwave can get the reaction mass hotter than just a resistive heating element, and thus get higher exhaust velocity.