I am not a fan of re-posting work that has been done by others, however I recently watched a video by Dark5, a popular Youtube channel, that covered Project Aurora in depth. In their traditional style of video you must read the details as the video plays, however it is definitely worth the six minutes of your life if you are interested in alternate propulsion that may be in use by our governments.
In a document that was unveiled in a Freedom of Information Act request there are talks about the United States Air Force operating routine nuclear powered space flights into local space as well as to the moon. The document even speculates that the U.S.A.F. is operating a base on the moon in which we regularly travel to and from. The document suggests that the amount of nuclear fuel needed is no longer measured in pounds, but grams.