The static buoyancy of airships in flight is not constant. It is therefore necessary to control the altitude of an airship by controlling its buoyancy: buoyancy compensation.
Changes which have an effect on buoyancy
For example, on a flight from Friedrichshafen to Lakehurst, the rigid airship LZ 126, built in 1923-24, used 23,000 kg gasoline and 1300 kg of oil (an average consumption of 290 kg/100 km). During the landing the airship had to release approximately 24,000 cubic meters of hydrogen to balance the ship before landing it. A Zeppelin of the size of the LZ 129 Hindenburg on a flight from Frankfurt am Main to Lakehurst consumed approximately 54 tonnes of diesel with a buoyancy equivalent of 48,000 cubic metres of hydrogen, which amounted to about a quarter of the lifting gas used at the start of the flight (200,000 cubic metres). After the landing, the jettisoned hydrogen was replaced with new hydrogen.
The Zeppelin NT has no special facilities to offset the extra buoyancy by fuel consumption. Compensation takes place by using a start-weight that is higher than the buoyancy lifting level at the start and during the flight, the extra dynamic buoyancy needed for lift-off and flight is produced with engines. If, during the trip, the ship becomes lighter than air because of fuel consumption, the swivel engines are used for down 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.
With a rigid airship two main strategies are pursued to avoid the venting of lifting gas:
Fuel with a density close to air
Only gasses have a density similar or equal to the air.
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.
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. In density it was 9% heavier than air. The Zeppelins 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 gas as a propellant. Twelve of the 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 could give 118 hours cruise.
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 would land on the sea and pick up temporary ballast water. 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.
The silica gel method was tested on the LZ 129 to extract water from the humid air to increase weight. The project was terminated.
Water from fuel combustion
The most promising procedure for ballast extraction during the journey is condensation of the engines' exhaust gasses, which consist mainly of water vapour and carbon dioxide. The main factors affecting gainable water are the hydrogen content of the fuel and humidity. The necessary exhaust gas coolers for this method had repeated problems with corrosion in the early years.
The first trials on the DELAG-Zeppelin LZ 13 Hansa (1912–1916) were conducted by Wilhelm Maybach. The trials were not satisfactory, resulting in the project's termination.
The USS Shenandoah (ZR-1) (1923–25) was the first airship with ballast water recovered from the condensation of exhaust gas. Prominent vertical slots in the airship's hull acted as exhaust condensers. A similar system was used on her sister ship, USS Akron (ZRS-4). The German-made USS Los Angeles (ZR-3) was also fitted with exhaust gas coolers to prevent jettisoning of the costly helium.
Lifting gas temperature
Changes in the lifting gas temperature in relation to the surrounding air have an effect on the buoyancy balance: higher temperatures increase buoyancy; lower temperatures decrease buoyancy. Artificially changing the lifting gas temperature requires constant work as the gas is barely thermally isolated from the surrounding air. However, it was common to make use of natural differences in temperature such as thermal updrafts and clouds.
Preheated lifting gas
Preheated lifting gas was tested to offset the higher weight of the Zeppelin. One variation tested on the LZ 127 Graf Zeppelin was to blow heated air on the lifting gas storage cells with the aim to gain buoyancy for launch.