It was a canceled USAF project called SLAM (Supersonic Low Altitude Missile) with a nuclear engine intended to fly below the radar and deliver thermonuclear warheads...
My turn??
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I told you it was easy.
The SLAM, which was part of Project Pluto, was never built, but the nuclear ram jet engine was tested...mounted to a railroad car. The longest burn was for five minutes or more. Clearly the engine was going to work. But the problem was, the reactor was unshielded and when the thing flew, it was going to spew out radioactive waste. That made the testing very difficult. They would have to test it at sea...but when the fuel was expended, it was not made to land, so would have to crash into the sea with a few tons of dirty radiaoctive reactor on it. So, they cancelled it. Besides, ICBMs were coming into play by that time, which were able to deliver the wepaons much faster.
Oh well, my next one will be harder.
But now, it is your turn. Go for it!