Buoyancy compensation

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The static buoyancy of airships is not constant during a journey. Measures must therefore be taken to control the lift and thus the flight altitude, the so-called lift compensation .

Various effects are responsible for the changes:

Airships can compensate for these changes in lift in several ways:

  • Use of dynamic lift or downforce (see under: Dynamic lift )
  • Buoyancy increase through ballast release. This is usually done by draining ballast water specially carried for this purpose. With balloons sand is often thrown
  • Buoyancy reduction by releasing the lifting gas or taking on additional ballast
  • By changing the density of the lifting gas through a change in temperature (heating increases, cooling reduces the buoyancy) or - only to a small extent - through a change in pressure (due to ballonnets in the rigid airship and in the area of ​​the impact height on bulging aerostats).

Buoyancy through fuel consumption

Particularly in the case of the historical large airships (especially in the case of zeppelins ), some attention was paid to the problem of increasing buoyancy due to the consumption of fuel.

LZ 126 , for example, used 23,000 kg of gasoline and 1,300 kg of oil for the transfer from Friedrichshafen to Lakehurst (average consumption 290 kg / 100 km). Therefore, about 24,000 cubic meters of hydrogen had to be released before landing in order to land with a statically balanced ship.

A trip from Frankfurt am Main to Lakehurst in an airship the size of the Hindenburg consumed around 54 t of diesel oil . This corresponds to the lift produced by 48,000 cubic meters of hydrogen. If this value is compared with the total lifting gas volume of almost 200,000 m³, it becomes apparent that this makes up almost a quarter of the total volume. This amount then had to be replaced by new lifting gas at the destination airport.

Buoyancy compensation

Zeppelin pursued two strategies to avoid releasing the lifting gas:

  1. Use of a fuel that had the same density as air and therefore does not cause an increase in fuel consumption.
  2. Extraction of ballast while driving. In a practical way, one dealt with the production of water as ballast.

The Zeppelin NT does not have any special devices to compensate for the gain in lift due to fuel consumption. On the one hand, it compensates for this with a take-off weight that is above the lift, so that part of the lift is generated by the motors during take-off and during flight (dynamic lift). In the same way, if it becomes lighter than air during the flight, it can land with the help of the swiveling motors and then take up ballast again on the ground. The relatively small size and a range of only 900 kilometers (compared to the historical zeppelins) made it possible to dispense with a ballast extraction system.

Power gas

Only a gas can be used as fuel with a density similar to or equal to that of air.

hydrogen

There have been attempts to burn part of the hydrogen carrier gas in the engines as a fuel gas , for example in the case of LZ 129. However, the attempts were not very successful and this possibility of reducing lift was no longer possible with the intended use of helium as a carrier gas.

Power gas

So-called blue gas was therefore used as the fuel gas . The name blue gas goes back to the Augsburg chemist Hermann Blau , who first produced blue gas in the Augsburg blue gas factory in Auerstrasse in 1905. Various sources suggest that it is propane , butane or a mixture that is normally known in the form of liquefied gas .

In the Zeppelin case, however, a mixture of propylene , methane , ethene , acetylene (= ethyne ), butylene and hydrogen was used.

LZ 127 “Graf Zeppelin” made some trips with gasoline. Twelve material gas cells were used for this, which could achieve a total volume of up to 30,000 cubic meters. This amount was sufficient for 100 hours of driving at cruising speed. The fuel tank volume was sufficient for a maximum of 67 hours of driving. On long journeys, a supply of petrol and motor gas was carried for up to 118 hours of driving or a range of 13500 km. The volume that was taken up by the fuel gas and was therefore not available for the hydrogen carrier gas could be used, since no additional buoyancy had to be provided for the liquid fuel to be consumed.

Motor gas was also tested in the US Navy. A 1464 m³ (51,700 ft³) power gas balloonet was installed on the K-1 impact airship.

Ballast water recovery

There were four sources of water in airship operations:

  • humidity
  • Precipitation on the envelope
  • Water on the ground (sea, rivers, lakes, ...)
  • Water vapor in the combustion occurs of the hydrogen contained in the fuel with oxygen in the air

Dew and rainwater of the envelope

In the case of the airships LZ 127 "Graf Zeppelin" and LZ 129 "Hindenburg" , rain gutters were attached to the hull on a trial basis in order to collect rainwater during the journey and thus fill the ballast water tanks. However, this method is heavily dependent on the weather and can therefore not be used reliably.

Water absorption from the ground

Water from the ground can be absorbed from overflown bodies of water such as the ocean or lakes.

In 1921 a ballast generator was tested on Lake Constance with the airships LZ 120 “Bodensee” and LZ 121 “Nordstern” before the airships had to be delivered as reparations. However, these tests did not show satisfactory results.

Silica gel process

The granular desiccant silica gel (silica gel), dried by heating before use, can absorb water from the air humidity . This chemical process increases the weight of the airship. This procedure was tested at LZ 129 Hindenburg , but was rejected again.

Condensation of the exhaust gases

The most promising method of ballast recovery while driving is the condensation of the exhaust gases from the engines. Fuels generally consist of hydrocarbons . When they are burned, mainly water (vapor) and carbon dioxide are produced . Normally, these combustion reaction products are released into the environment through the exhaust . However, if the exhaust gases are cooled, the water condenses and can be collected. Theoretically, more mass can be gained in this way than is lost through fuel consumption. The main influencing factors for the amount of water that can be obtained are the type of fuel used (hydrogen content) and the air humidity.

However, complex exhaust gas coolers are necessary for these processes . In the early years there were also problems with corrosion .

A system designed by Wilhelm Maybach on behalf of Graf Zeppelin was tested on the DELAG Zeppelin LZ 13 “Hansa” (1912–1916) . However, the attempts were unsatisfactory, so that they were given up again for the time being.

ZR-1 USS Shenandoah (1923–1925), the first helium-filled rigid airship, was, according to the US Navy, the first airship in which ballast water was obtained from the condensation of the exhaust gases. In the case of the LZ 126 / ZR-3 USS Los Angeles , the hydrogen lifting gas was replaced by helium after the ship arrived in the USA. In order not to have to drain the precious helium unnecessarily, a ballast water recovery system was also retrofitted.

The water on board the airship (for example LZ 130) should be used as service water . ( Hindenburg , LZ 130 , USS Akron , Cargolifter CL160 , LoftyCruiser)

Change in temperature of the carrier gas

Changes in the temperature of the lifting gas compared to the surrounding air result in an increase in lift (lifting gas preheating) or a loss of lift (lifting gas cooling). The technical implementation requires a lot of energy, as the gas cells are only isolated from the environment by the gas cell wall, a layer of air and the airship hull .

In practical operation, however, this procedure has already been more or less consciously applied to almost all rigid airships by using the temperature differences between day and night , the surroundings and the airship hangar, as well as the differences in different layers of air .

Carrying gas preheating

In order to compensate for the higher take-off weight, Zeppelin also experimented with preheating of the lifting gas . At LZ 127 Graf Zeppelin , for example, warm air was blown past the gas cells to warm them up. The aim of the preheating was to gain lift for takeoff. The gas could then cool down again during the journey. The decrease in lift was first compensated for by dynamic lift. At the destination airport, a large part of the fuel was then used up and a static increase in lift was achieved.

The hybrid rigid airship prototype Aereon III (early 1960s) had propane gas burners installed inside the hulls in order to be able to heat the helium in the gas cells and thus achieve a higher lift value. There was even the idea of ​​heating and cooling to achieve a wave-shaped flight profile that should also enable forward movement without additional propulsion.

Like hot air balloons, hot air airships generate all their buoyancy using heated air, which also absorbs the exhaust gas from the heating flame. You do not use any special lifting gas.

Carrying gas cooling

So far, no technical systems for carrying gas cooling (lift reduction) have been used in airships. Aereon III had ventilation flaps in the outer shell in order to be able to cool the previously heated lifting gas by "ventilation". With the exception of the German LoftyCruiser project, no concrete considerations in this direction are known. However, weather effects were used to maintain a lower temperature in the airship than in the surrounding air. So airships landed very often in the evening. That is why they often circled over the landing site or took "detours" during the approach to their destination.

In the evening hours, the air cools down and with it the lifting gas. However, near the ground, the air stays warm longer because the ground releases the heat it has absorbed during the day. In this way, it was possible to land in warm air layers with reduced buoyancy thanks to a cooler lifting gas. If this was not possible or if the lift was still higher than the weight of the ship, the remaining difference in lift had to be compensated for with dynamic downforce. Furthermore, ropes were dropped with which the ship was pulled to the ground. This was done by the holding teams, but there were also attempts with motor-powered winches ( e.g. LZ 130 ) to reduce the need for personnel. The ship was then moored on the ground and immediately weighted down with ballast. Of course, lifting gas could also be released.

Other forms of propulsion

Another way of avoiding the consumption of fuel and the problems that arise with it is to simply do without it and use other forms of energy.

  • Solar airships store the energy in accumulators. Their mass therefore does not change.
  • There were also various concepts that used nuclear reactors as a source of propulsion . They mostly come from the 1960s / 70s and didn't get beyond the drawing board.
  • Another possibility is to supply the airship with energy from the ground, for example with microwaves . Such an airship model with a length of 17.5 m and a 10 kW beam was developed by Onda in Japan in 1995 and tested in practice ( HALROP ).

See also

Literature and Sources

  • F. Sturm, G. Molt: Ballast water production in the airship LZ 130 "Graf Zeppelin" VDI-Zeitschrift Vol. 83, No. 15, April 15, 1939 (as reprint in "The Great Zeppelins" ISBN 3-540-21170-5 )
  • Organizational Hubris - Rise and Fall of a Celebrity Firm using the example of CargoLifter AG Inaugural dissertation to obtain the academic degree of Doctor of Economics from the Department of Economics at the Free University of Berlin; Diplom-Kaufmann Philipp Hermanns; Disputation day: November 16, 2012. Print edition: Kölner Wissenschaftsverlag, ISBN 978-3-942720-33-5 . See u. a .: Appendix E9 “Unresolved fundamental technical issues in the CL-160 project”; available online as a PDF at: [1] ; last accessed July 3, 2015.

Web links

Individual evidence

  1. ↑ Kite Balloons to Airships ... the Navy's Lighter-than-Air Experience; (Edition on 75 Years of US Navy Aviation); Published by the Deputy Chief of Naval Operations (Air Warfare) and the Commander, Naval Air Systems Command, Washington, DC, Edited by Roy A. Grossnick, Designed by Charles Cooney, US Government Printing Office: 1983-187-029; Page 34
  2. ^ A b Douglas H. Robinson: Rigid-airship Venture: Details of the highly unorthodox "Aereon III" . In: Flight International . 82, No. 2797, October 18, 1962, pp. 648, 650.