After his resignation from the army in 1891 at age 52, Zeppelin devoted his full attention to airships. He hired the engineer Theodor Gross to make tests of possible materials and to assess available engines for both fuel efficiency and power-to-weight ratio. He also had air propellers tested and strove to obtain higher purity hydrogen gas from suppliers. Zeppelin was so confident of his concept that in June 1891 he wrote to the King of Württemberg's secretary, announcing he was to start building, and shortly after requested a review from the Prussian Army's Chief of General Staff. The next day Zeppelin almost gave up as he realized he had underestimated air resistance, but resumed work on hearing that Rudolf Hans Bartsch von Sigsfeld made light but powerful engines, information soon shown to be overoptimistic. Whereupon Zeppelin urged his supporter Max von Duttenhofer to press Daimler-Motoren-Gesellschaft for more efficient engines so as not to fall behind the French. Duttenhofer wrote to Gross threatening to withdraw support, and Zeppelin shortly afterwards sacked Gross, citing Gross' lack of support and writing that he was "an obstacle in my path".