Courtesy of the Smithsonian National Air and Space Museum’s Udvar-Hazy Center, it’s a Navaho rocket engine (strictly speaking, it’s two engines, but they were used in pairs):
As you may recall from a previous post, Navaho was an early (1940s / 1950s) attempt at a two-stage cruise missile, in which a liquid-propellant booster would get a ramjet-powered missile up to its operational speed and elevation. It wound up being exorbitantly expensive, and was never fielded, but proved to develop many of the technologies of the space age.
This engine design, built by Rocketdyne (a division of North American Aviation), burned alcohol and liquid oxygen and produced 120,000 pounds of thrust from each of the pair’s two engines. Later generations of the power plant used three upgraded engines, producing a total of over 400,000 pounds of thrust.
From the placard:
This two-chambered, liquid-fuel rocket engine served as the booster for the Navaho missile, which was powered by two ramjets. The huge, vertically launched intercontinental cruise missile was designed to strike a target up to 8,850 km (5,500 miles) away. However, the Navaho never became operational. Its unsuccessful testing program and enormous development cost, which had reached almost a billion dollars, caused the program to be cancelled in 1957.
The Navaho was an important stage in the evolution of American large-scale liquid-fuel engines, including those for the Redstone, Jupiter, Thor, and Atlas missles, Saturn V Apollo launch vehicle, and the Space Shuttle.