pressure and landing. The relatively small size of the Zeppelin NT and a range of only 900 kilometers compared to the historical Zeppelins allowed the waiver of a ballast extraction device. Buoyancy compensation With a rigid airship two main strategies are pursued to avoid the venting of lifting gas: 1. The use of a fuel with the same density as air and therefore no increase in buoyancy caused by consumption. 2. Adding water as ballast by extraction during the trip. Fuel with a density close to air Only gasses have a density similar or equal to air. Hydrogen Different attempts were made on hydrogen airships: the LZ 127 and LZ 129 to use part of the lifting gas as a propellant without much success, later ships filled with helium lacked this option. Blaugas Around 1905, Blau gas was a common propellant for airships; it is named after its inventor the Augsburger chemist Hermann Blau who produced it in the Augsburger Blau gas plant. Various sources mention a mixture of propane and butane. Its density was 9% heavier than air. Zeppelins often used a different gas mixture of propylene, methane, butane, acetylene (ethyne), butylene and hydrogen. The LZ 127 Graf Zeppelin had bi-fuel engines, and could use gasoline and Blau gas as a propellant. Twelve of the vessel's gas cells were filled with a propellant gas instead of lifting gas with a total volume of 30,000 cubic metres, enough for approximately 100 flight hours. The fuel tank had a gasoline volume of 67 flight hours. Using both gasoline and Blau gas, one could achieve 118 hours of cruise time. Water as ballast Dew and rainfall on the hull In some airships rain gutters were fitted to the hull to collect rainwater to fill the ballast water tanks during flight. However, this procedure is weather dependent and is therefore not reliable as a standalone measure. Water from the ground Captain Ernst A. Lehmann described how during World War I Zeppelins could temporarily remain at the sea surface by loading ballast water into tanks in the gondolas. In 1921 the airships LZ 120 "Bodensee" and LZ 121 "Nordstern" tested the possibility on Lake Constance to use lake water to create ballast. These attempts, however, showed no satisfactory results. Silica-gel method The silica gel method was tested on the LZ 129 to extract water from the humid air to increase weight. The project was