The top shape UFO uses liquid hydrogen propellant location to shield astronauts from neutron exposure but the same can be said for the aerospace shuttle that was canceled over the fuel tank design production problem. What the nuclear fission rocket really does is raises isp from 360 to over 800. Add to that laser 3D that melts the highest refractory metal oxide powders and the fission rocket can attain an isp about 2000 with no significant loss of static thrust. The refractory lift blades can lift the ufo nuclear top into the stratosphere before firing the nuclear rocket. Plus soft land on Mars. On Mars with nitrogen extracted from the atmosphere it can go more than orbital. It can go well past escape velocity because mars gravity is one third G. What NASA is trying to develop is a no throw away reusable space shuttle because of a dangerous accumulation of space junk. The result is a variety of UFO sightings because to go nuclear one has to first flight test a variety of jet and chemical burn prototypes.