Force projectors ground Newton's third law to its room without supper. They consist in a molecule-thin sheet of crystalline material that experiences a reactionless force normal to its surface when a voltage is applied across it. The higher the voltage, the stronger the force.

Force projectors convert surrounding oxygen to ozone when running, which is why very large projectors that run continuously like you see in spacecraft cannot be used in atmosphere. The projector plates on the paws of large mechs are the largest practical terrestrial application for them.

From a Doylist perspective, I can remember vehemently not wanting force projectors to be used for aircraft or to allow things to hover in midair, while still using them for powered armor and spacecraft, but I honestly can't remember why. I think it had to do with the absence of artificial gravity in this universe, but if anyone has any suggestions I'd love to hear them. I also don't want them to be what keeps Welkinstead's floating cities aloft.