By the time Walther Gerlach took over the uranium project in late 1943, it was clear that Germany could not build atomic bombs before the end of the war, which was now going very badly for Germany. Work continued under Paul Harteck on isotope separation and heavy water production. Small but steady improvements were made with the centrifuges, but only very small samples with low levels of uranium 235 enrichment were ever achieved. Heavy water production failed, despite great efforts. The work on model nuclear reactors continued, culminating in a final experiment using a lattice of uranium cubes immersed in heavy water. This came close but fell short of achieving a self-sustaining nuclear fission chain reaction. During this period the scientists were focused first and foremost on the survival of themselves and their families. Both Gerlach and Werner Heisenberg sought to facilitate this by continuing to suggest to powerful members of the Nazi elite that their research might lead to an unexpected breakthrough and win the war.