the ground before the 79-second interval. Fall time at was approximately 10 seconds. Burst height in retarded airburst depended on release height. The weapon was unpredictable at releases between as either the freefall or retarded fuzing would be randomly selected. Variants Twenty different versions of the B28 were made, distinguished by their yield and safety features. The B28 used the "building block" principle, allowing various combinations of components for different aircraft and roles. The principal configurations were: B28EX – (EXternal), streamlined external-carriage for free-fall delivery. Design specifications called for carriage by the Air Force's B-47, B-52, B-57, B-66, F-84F, F-100, F-101 and F-105; and Navy's AJ-1, AJ-2, A2U-1, A3D-1, A4D, F2H, F3H-2, F7U-3 and P6M. B28IN – (INternal) unstreamlined internal-carriage version for free-fall delivery, primarily for the Republic F-105 Thunderchief and B-52. B28RE – (Retarded External) streamlined external-carriage version with a parachute retarder for low-level delivery. B28RI – (Retarded Internal) unstreamlined internal-carriage laydown weapon. Weapon specifications call for carriage by the B-47, B52 and B-66. B28FI – (Full-fuzing Internal) unstreamlined internal-carriage version for laydown delivery and full-fuzing option (i.e. with parachute retarded laydown, retarded air burst, free fall air burst and free fall ground burst fuzing). Specifications called for carriage by the B-47 and B52. The B28FI did not use the Mod 0 and Mod 1 warheads, as they were not strengthened for laydown delivery. The following mods were produced: Mod 0 – Used on TM-76 Mace surface-to-surface missile and the GAM-77 Hound Dog cruise missile. Mod used an internal initiator while subsequent mods used external neutron initiators. Mod 1 – TX-28-X2 design. Also carried by Mace and Hound Dog missiles. Mod 2 – TX-28-X3 design. Mod was hardened for use with the B28FI configuration. Mod 3 – Mod had a Cat A PAL. Mod 4 – Mod had a Cat B PAL. Mod was hardened for use with the B28FI configuration. The yield variants were: Y1 – . Y2 – . Y3 – Y4 – Fission weapon, may have been the W34 warhead. Y5 – A total of approximately 4,500 B28s were produced. The last weapons in use were retired in 1991. Related designs The W49 warhead for the Thor, Atlas, Jupiter, and Titan I ballistic missiles was a W28 Y1 warhead with internal power systems removed. It came in two yield options; the Y1 with a yield of and Y2 with a yield of . Mods 0 to