Correct answer is... 0 . It is equal to 0 lbs. Empty weight of Northrop X-21 is total weight of the airframe, engine, avionics and all other systems installed by manufacturer. It is lower weight then empty speed of other aircrafts of a similar size. Average is about 5694 kg (12554 lbs).
Disclaimer: Our articles and data connected to empty weight of Northrop X-21 are based on public data from various sources. We are very picky to select an reliable data providers, but we cannot guarantee that everything is 100% error-proof. We recommend for any serious purpose - as a rule of thumb - to request offical data about empty weight from manufacturer of Northrop X-21.