The Little Joe was used to test the Mercury spacecraft launch Escape System by accelerating the capsule to launch velocity and simulating an abort.  The booster was produced specifically for this purpose, and consists of four Pollux or Castor motors grouped with four smaller Recruit motors.  The cross section diagrams highlighted the four larger motors, and suggested the name 'Little Joe' for the dice playing term for  a pair of twos.  During the Apollo program a larger booster, named Little Joe II, was used for similar purposes with the Apollo spacecraft.