W-56 Warhead 17.4 47.3 600; 680 1.2 Mt  Airburst or surface Manufactured 3/63 - 5/69;
retired 9/66 (early mods), Mod-4 retired 1991-93;
1000 produced (all mods), 455 Mod-4s produced  Minuteman I and II warhead, based on UCRL W-47, competitor with the W-59 for Minuteman; 4 mods, retrofit of early mods required to fix reliability problem, blast and radiation 

Test: Arkansas
Time: 18:00.00.16 27 April 1962 (GMT)
Location: Christmas Island
Test Height and Type: B-52 Airdrop, 5,030 Feet
Yield: 1090 kt
Device Diameter (inches): 17.36
Device Length (inches): 47.3
Device Weight (lb.): 600

This test was the first parachute-retarded device to be dropped at Christmas Island, it missed its intended air zero point by 600 ft. The aim point was over a raft moored off the southern end of Christmas Island.

Arkansas was a highly successful LRL test of the XW-56X2 (Fife-I) warhead for the Minuteman missile. This warhead was derived from the breakthrough LRL W-47 warhead developed for the Polaris missile. The characteristics of the two warheads are generally similar although the W-56 kept the same general yield (usually given as 1.2 Mt) as the high yield W-47Y2 variant, while trimming 133 pounds of the weight. This test used a Fife secondary stage. This test demonstrated a yield-to-weight ratio 4.00 kt/kg (remarkably close to the effective practical limit of 6 kt/kg for such a light weight device). The test device included a mockup war reserve firing set. This was similar to the devices (also W-56s) fired in Swanee and Bluestone. The mushroom cloud rose to about 60,000 ft.

About 1000 W-56s were eventually produced for the Minuteman I and II missiles. Some 450 remained in service until the Minuteman II was finally retired in the 1990s. The last W-56 was retired sometime between October 1992 and March 1993.