This is the RTV-N-15, also known as Pollux, a post-World War II U.S. Navy pulsejet-powered research vehicle for missile development and testing piloted aircraft components. The last and largest vehicle of the important Gorgon series of post-war experimental Navy missiles, it was unusual in having an internally-mounted pulsejet and was an attempt to increase the normally slow operating speed of a pulsejet vehicle by streamlining.
The design range was 75-100 nautical miles, with guidance by active radar and heat-seeking homing. It made only three test flights from 1948-1951 and then was cancelled due to its slow development. This is probably the only extant example of the RTV-N-15 and was donated to the Smithsonian in 1971 by the U.S. Navy.
This object is on display in Rockets & Missiles at the Steven F. Udvar-Hazy Center in Chantilly, VA.