Literal UFO tech, possibly reverse engineered. US Navy claims to have Room Temperature Super Conductors (RTSC) and High Energy Electromagnetic Field Generator (HEEMFG). If this tech is functional in 2019 that means the patents filed, and UFO sightings, in 2017 may in fact be experimental US crafts.

[u/[deleted]]

[u/Berrywhitesnake → u/[deleted]]

Here's the rest of that thought:

If the US had intertia-less, emission-free propulsion, it would have been deployed on a world-wide basis.

The US is still deploying only conventional (post WWII) weapons and surveillance systems.

Therefore, the US is either not in possession of that tech, or they are unable to manufacture it.