Yup, people do not understand how stable water is (hence the low energy density). Phase change on the other hand, is a extremely effective method to harness work from heat.
They're going all the way to plasma. The idea is to pump as much energy as possible into small amounts of reaction mass, to maximize momentum while minimizing reaction mass consumption. They can get more energy from solar panels over time, but not more reaction mass.
Remember, not only do rockets need something to push against, you have to bring that something with you.
Remember, not only do rockets need something to push against, you have to bring that something with you.
Rockets do not need to push against anything... they expel reaction mass out one end ensuring an equal and opposite reaction. Hence the term “reaction mass” in the first place. Rockets would work just as well in a vacuum as in atmosphere, but it is true that you have to carry your fuel with you.
Downvote if you must, but Newton’s third law still exists. ;)
Probably mode of energy transfer. Microwaves are extremely efficient with transferring energy to water since you do not need to have something with higher temperature to produce heat flow. Also, you can heat up a contained area (especially water vapor) easily where as heating elements are limited to surface area (and the heat flow will be extremely low since the mass is low).
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.