Airship Gases - Hydrogen - Military Applications - 1875-1938
One of the first uses of large quantities of hydrogen gas was in providing lift for dirigibles and zeppelins at the dawn of the 20th Century. Hydrogen was chosen because it has 7% of the density of air and thus provides excellent lift to balloons and airships. Hydrogen also had the advantages of being much more plentiful and much less expensive than the next most likely candidate gas, helium. During World War I, the German Army and Navy used zeppelin rigid body airships for bombing cities in the United Kingdom. The German airshipmen maintained gas safety by keeping the hydrogen lift gas separated from air, precluding any combustion reactions. The zeppelins were rigid aluminum frame body structures, the interior was compartmented with gas cell membranes and the exterior was covered with fabric. The zeppelins flew very high, 3 to 4 miles in altitude, and although large, they had speeds on the order of 40 mph. They were hard to hit with anti-aircraft fire. A few German zeppelins that were hit by antiaircraft fire did burn, but several of the German airships hit by anti-aircraft shells during bombing raids over the United Kingdom did not burn or explode (e.g., the L-13, L-33).
The battle damage allowed hydrogen gas to escape gas cells into the air, but the hydrogen did not ignite. The damaged airships lost lift because of the escaping gas; some crashed while others were barely able to make port. When the allied forces developed fast climbing, night fighting interceptor aircraft and began using incendiary (phosphorous coated) bullets in machine guns on those aircraft, the zeppelins began to suffer losses from hydrogen fires and explosions. The bullets breached the gas cells, and localized, sustained gunning with incendiary bullets would ignite the fabric and the escaping hydrogen gas.
The German airshipmen had strict safety rules that they could not vent off hydrogen for airship control during a storm, so they continually watched for weather changes. The airships withstood lightning strikes; some zeppelins, such as the L-11 airship, were lightning damaged to the point of fusing the aluminum girders in the bow. The storms also brought atmospheric pressure changes, and also the occasional sudden change in airship altitude caused by a storm could force the airship to vent some lift hydrogen so that the gas cells were not overpressurized. Vented hydrogen gas could be ignited by a nearby lightning bolt, and hydrogen in flare gas stacks can be ignited by atmospheric electrical discharges; a lightning strike within one or two kilometers is sufficient to ignite the flare gas. This phenomena is consistent with the low ignition energy of hydrogen in air.
The German airship base at Tondern suffered several hydrogen fires. Five zeppelins were lost because of accidents in handling hydrogen; these had nothing to do with enemy action. The L-18 zeppelin is an example of these accidents. The L-18 was a brand new airship being filled with hydrogen. A spark developed in a gas fill line, and the line exploded. This was a minor explosion, but it caused a nearby liquid fuel tank to explode. The fuel tank explosion spread flames to one of L-18's already-filled gas cells. The airship caught fire and was lost. Flames burned vigorously for over an hour.
These wartime experiences show that if the hydrogen is kept isolated or confined; that is, unmixed with air, then there is little danger of fire or explosion. The hydrogen must be kept above its upper flammability limit for airship usage. The wartime experiences showed that even energetic scale events (i.e., anti-aircraft shells striking or exploding within zeppelins, machine gun bullet impacts, lightning strikes) did not always cause gas fires or explosions. However, allowing the gas to leak out into air and then subsequently providing an ignitor allowed hydrogen fires and explosions.
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