The X-39 engine was a project in 1956 to make a nuclear-powered airplane, and it succeeded to fly! Unluckily, on the tests the X-39 engine went too fast to control. Also, the radiation produced by the plane was big enough to kill the passengers and there was a 53% chance it was going to explode! Even without this trouble the plane would have to be carried to air by electric engines because it couldn’t take off properly. That’s why the project got canceled. We found these information from YouTube channels such as  P3naz89 and illusive 0ne and from the website www.realclearscience.com.