This is the YLR45-AJ-1 liquid fuel rocket motor developed by the Aerojet Engineering Corp. from 1948 for use as a JATO (Jet-Assisted-Take-Off) to boost the B-47B jet bomber. The JATOs were used in pairs for a total thrust of 10,000 pounds for 60 seconds.
Because of the YLR45's complexity, however, it suffered several developmental problems. This caused delays that led to the decision not to produce it in quantity production although it was briefly operational in the early 1950s. The motor was donated to the Smithsonian in 1968 by the Aerojet General Corp.