This is the Super Sprite, a British liquid propellant JATO (Jet-Assisted-Take-Off) rocket motor, or known to the British as a RATO (Rocket Assisted-Take-Off). It was designed by the De Havilland Engine Company to be either permanently or temporarily fixed on to a military or civilian aircraft for short bursts of extra power. The engine was good for 50 firings.
The Super Sprite had a maximum thrust of 4,200 pounds for 40 seconds, but both the thrust and duration could be adjusted to fit different mission needs. The unit was first used on the Vickers Valliant bomber. In 1955, it also became the first British rocket motor to receive approval for mass production. This engine was donated to the Smithsonian in 1970 by Rolls-Royce Ltd.