text: Production was expected to begin in March 1945, full production to be achieved in August; the schedule had slipped but production was to start within two months. One Lancaster had returned soon after take-off and another turned back over the target with engine-trouble, ditching the Grand Slam in the North Sea. Moderate to heavy flak was encountered by the gaggle of Lancasters but there was no fighter opposition. The bombs were dropped in about a minute and despite 1-hour delay fuzes, three bombs exploded immediately. Two of the fourteen hits on the pens were by Grand Slams and one by a Tallboy, the hits landing on the un-thickened part of the roof on the west side. The Grand Slams penetrated  into the concrete causing about  of the roof to fall in bringing down two moveable cranes. The thickened part of the roof and a periscope-testing tower were also damaged. One bomb damaged power stations, concrete mixing facilities and shelters on the north side. Marines who had been inside the building reported later that there was a huge concussive effect, which severely depressed their morale; no aircraft were lost.
  Hamburg, 9 April
  Seventeen Lancasters from 617 Squadron were dispatched to bomb the Finkenwerder U-boat pens in Hamburg, two with Grand Slams and the rest with Tallboys. The pens were inside a -building with a roof  thick, reinforced by steel beams and trusses. Forty Lancasters from 5 Group were to bomb oil storage tanks nearby. The weather was clear with some haze, a thin layer of cirrus clouds above the city but the visibility was very good. A fighter escort from 11 Group consisted of 120 Mustang Mk III, 30 Mustang Mk IV, 59 Spitfire Mk IX and 24 Spitfire Mk XVI to counter any attempts by Me 262 jets to intervene.  fighters, including jets, attempted to intercept the formation and there was much flak, which damaged six Lancasters before they bombed. Buildings to the north and west were hit and a reconnaissance photograph showed seven hits, four penetrating the roof.