The D class blimp was a patrol airship used by the US Navy in the early 1920s. The D-type blimps were slightly larger than the C-type and had many detail improvements. The Navy continued the practice of dividing the envelope production between Goodyear and Goodrich. The control cars were manufactured by the Naval Aircraft Factory. The major improvements over the C-type blimps were a better control car design and easier, more reliable controls and instrumentation. The engines were moved to the rear to reduce noise and allow better communications between crew members. The fuel tanks were suspended from the sides of the envelope. The envelope was identical to the C-type, except an additional six-foot panel was inserted for a total length of and a volume of . The last of the D-Class, D-6, had a different control car designed by Leroy Grumman who later founded the Grumman Aircraft Engineering Corporation. Operational history The D-1 caught fire and burned the day of its first flight in the Goodyear hangar at Wingfoot Lake, Ohio. The D-2, D-3, D-4 and D-5 were transferred to the United States Army which the Air Ship Board had given the primary role of operating non-rigid airships after World War I. D-3 participated as an observation and photography aircraft at the famed "Mitchell" bombing test of 1921. D-3 also participated in the Mitchell bombing trials and tested experimental mooring masts. D-3 also participated in early "hook-on" experiments to see if it was possible for an airplane to fly up to and hook onto a trapeze hanging from an airship. No actual hook-ons were achieved, but approaches were practiced. D-4 also participated in the Mitchell trials, for observation and photography. The D-5 was never operated by the Army with that designation. After the loss of D-2, the D-5 was erected with more powerful 180 hp Wright V engines and flown as the D-2 (no. 2) D-2 (no 2 is a designation created by historian James Shock, never used by the Army). There is no evidence the airship flew for the Army with the designation "D-5". D-5 is believed to have been renumbered "D-2" after the loss of that airship. The Navy retained one additional D-type, the D-6. The D-6 was built by the Naval Aircraft Factory, Philadelphia, Pennsylvania, but her design was sufficiently different that she was distinct from the other five D-class airships. It featured a further improved