LZ-129 HindenburgFor a few years in the late 1930s airships flew passengers around the world in luxury, and some aviation experts speculated they would be the future of worldwide passenger aviation. When the Hindenburg was built in 1936, the revived Zeppelin company was at the height of its success. Zeppelins had been accepted as a quicker and less expensive way to travel long distances than ocean liners provided.
Longer than three Boeing 747s, the Hindenburg and her sister-ship, Graf Zeppelin II, were the two largest aircraft ever built - 804 feet (245 meters) long and 135 feet (41 meters) in diameter. Four 1,050-horsepower (783-kilowatt) Daimler-Benz diesel engines provided a top speed of 82 miles per hour (132 kilometers per hour). They had cabins for 50 passengers (upgraded to 72 in 1937) and a crew of 61. The airship had a dining room, library, lounge with large windows, and even boasted an aluminum grand piano.
The Hindenburg was designed to be filled with helium, but a US embargo forced the Germans to use highly flammable hydrogen. The Hindenburg held over 7 million cubic feet (200,000 cubic meters) of hydrogen in sixteen gas cells. Sixteen compartments each contained a gas cell made from a cotton fabric, then a hydrogen impermeable layer of rubberized 'film', then another cotton fabric layer. Hydrogen gas pressure in each cell was on the order of 0.5 to 1 inch of water gauge (124.3 to 248.6 Pa above atmospheric pressure). The diffusion rate was about 1 liter of hydrogen per square meter of gas cell fabric per 24 hours. The airship outer skin was a linen fabric that was then coated with cellon 'dope' and aluminum powder mixture for weather protection. The outer fabric skin protected the gas cells and made the airship more aerodynamic. The dope (i.e., varnish) was used to waterproof and tauten the fabric cover. For ultra violet light protection, the inner surface of the fabric on the upper part of the airship was coated with red paint.
The frame was built with aluminum-copper alloy girders. Girder rings were connected by straight section girders to form the characteristic ellipsoid shape. Wire cables were used to provide internal support for each ring. The gas cells were located between each girder ring. The frames of the Hindenburg were numbered in meters from the aft forward; frame 2 was near the tail and frame 247.1 was at the nose.
Miscellaneous inflation possibilities undoubtedly existed in the prospect of new gases to be discovered or in the utilization of ones now known but not employed, but whatever the advantages thus left to be secured it was certain that among them there will not be any material increase in lifting capacity, since hydrogen already affords nearly 44 of all the lift there is to be had, this factor being limited, as has been previously emphasized, not by the lightness of gases, but by the weight of air displaced.
Ammonia gas, which is almost as light as some illuminating gases -.04758 pound to the cubic foot - might appear to have some possible application to the inflation of balloons designed to be proof against incendiary projectiles. Its cost, difficulty of preparation with present portable facilities, its extremely irritating effect when respired, even in very small quantities, and its deleterious action on envelope coatings, are among the greatest objections to it.
Should helium, which is almost as light as hydrogen (11 units of lifting capacity against 12 with hydrogen), ever be commercially produced in quantity it was possible that it would be of advantage to use it because of its chemical inertness, which in general as well as military uses certainly would contrast favorably with the dangerous inflammability of hydrogen. By 1911, practically all the isolated helium in the world was the quantity of about 14 cubic feet in the possession of the University of Leyden.
In 1907 Dr. H. P. Cady, of the University of Kansas, found more than 1 per cent of helium in some natural gas from Kansas. Up to April, 1918, helium had been obtained only in extremely small amounts, as a curiosity in scientific laboratories, so that the total quantity separated in the whole world probably did not exceed 100 cubic feet, the cost of production being about $1,700 to $2,000 a cubic foot. By the end of the Great War the large-scale production of helium was one of the great scientific and technical triumphs resulting from the application of exact knowledge and inventive genius to problems of military importance, making this hitherto exceedingly rare gas available for use in balloons and dirigibles.
The Hindenburg was originally designed for helium usage, but Germany was unable to purchase helium from the US. The US had passed a Helium Control Act in 1927. The helium supply was small and the US government did not believe that the new German government at that time had strictly peaceful intentions. The Zeppelin Transport Company of Germany was forced to use hydrogen to fill the dirigible. The ship was fitted with precautions for hydrogen use. All crew ladders and catwalks were rubber encased to eliminate possible sparking, crew uniforms for use when inspecting the gas cells and ship's structure were made from asbestos without any metal (no metal fasteners, buttons, zippers, snaps, tabs, etc.) to eliminate static electricity generation and discharge. The crew shoes were either sneakers or felt boots to preclude static buildup. The axial passageway along the length of the airship and vertical shafts between gas cells were well ventilated, including use of natural draft air movement. Passengers were allowed to smoke tobacco, but only in designated smoking rooms that were under positive air pressure to ensure no hydrogen admission. Crew members helped passengers light their smoking products, and no passengers were allowed to possess matches or lighters during their voyage.
Gas pressure sensors indicated pressure in each gas cell to the control car. The sensor elements were diaphragm pressure onto a spring loaded plunger within a wire coil (24 VDC, 100 ohms). A cell could lose only about 200 to 300 cubic meters of gas before the sensor readout would show a problem; this is only a very small portion of cell volume (a cell was about 12,500 cubic meters).
The Hindenburg's May 1936 launch inaugurated the first scheduled air service across the North Atlantic between Frankfurt am Main, Germany, and Lakehurst, New Jersey. Its first trip to the United States took 60 hours, and the return trip took only a quick 50. In 1936, it carried more than 1,300 passengers and several thousand pounds of mail and cargo on its flights. It had made 10 successful roundtrips between Germany and the United States. The Hindenburg performed well, making thirty-four Atlantic crossings to Lakehurst, New Jersey and Rio de Janiero, Brazil in 1936 at an average crossing time of two and three days, respectively. The ship weathered storms well; passengers did not generally know how severe the weather was and they did not suffer from motion sickness because the airship had good stability in rough weather.
On May 6, 1937, the Hindenburg was due at Lakehurst, New Jersey at 6:00 am, having departed Frankfurt Germany two days earlier. Wind and rainy weather slowed the airship, it diverted around the worst of the storm. The Hindenburg passed New York City at 3 pm. The airship hovered along the coast to wait out a thunderstorm and allow for ground preparations. Because of the size of ground crew needed, only two times were scheduled to receive airships, 7 am and 7 pm. The airship approached the landing field at 6:00 pm, maneuvered to dock. Part of the docking procedure was to vent hydrogen and ballast as needed to steady the craft at the docking mast altitude and maintain gas cell pressure for differing barometric pressures at changing altitudes. The Hindenburg vented hydrogen from the five forward gas cells twice for 15 seconds while still decelerating from 73 to 27 mph, and decreasing from about 590 feet to 390 feet altitude. Hydrogen was vented once for 5 seconds while traveling at about 27 mph on approach to the landing field. There was a 6 knot variable wind from the east. The airship then dropped a total of about 2,425 pounds of water ballast from frame 77 to trim the ship in preparation to dock.
The airship passed through a small rain shower and a light rain was falling; the airship had successfully maneuvered around the heart of the thunderstorm. Spectators on the ground noted that the ship appeared to be slightly tail heavy as it approached. Then six members of the airship crew were sent to the bow to use their body weight as a means to help trim the craft even after the water ballast had been dropped. Crew movement for trim was an uncommon, but not unknown, practice for a large zeppelin. The airship captain noted some light rain and that there was a distant storm with lightning activity to the south and southwest. The sun had not set, so there was ample light for docking.
As the ship approached the docking mast, it performed a sharp turn (varying testimony about how sharp the turn was) to face into the wind, which was shifting. Some ground crew noted that when Dr. Eckener piloted airships, he brought them in straight into the wind, slowed and "stopped on a dime". This captain performed a spectacular high speed, low altitude pass over the crowd of spectators and then a sharp turn to line up with the docking mast. The engines were run at full speed astern for about a minute to slow the airship. Then, with the Hindenburg about 200 feet above the ground, two 2-inch diameter manila rope lines from frame 244.5 (i.e., near the bow) were dropped to begin securing the airship. Dust came from the lines as they dropped to the ground, but the lines wetted in the light rain.
The procedure was that a steel cable would moor the airship nose to the docking mast, and the manila ropes hold the airship into the wind and also prevent it from over-riding the mast. Only the port line had been attached to a ground winch used for drawing the airship down to ground level for disembarking passengers and cargo. The starboard line was being handled by ground crew.
The nose cable had been lowered about 50 feet but was not connected to the docking mast when the fire began. The airship never got closer than 700 feet (horizontal) to the docking mast. Ground crew saw the outer cover at the tail of the airship fluttering, and the skin of the airship seemed to be rippling. Since the propeller slipstream was far below that area, the ship had little headway, and the wind was light, the only reasonable explanation for the cover flutter is that hydrogen was leaking from a gas cell and causing the cover to move.
"Oh, the humanity," cried a radio announcer on May 6, 1937, as the German airship, the Hindenburg, exploded in flames while landing outside New York City. For many people this is all they know of airships - that category of "lighter than air" craft which includes dirigibles and blimps. The age of the airship ended on 6 May 1937, the day of the Hindenburg disaster.
At 6:25 pm, just after the port line had grown taut, a small tongue of flame emerged from where the skin had been fluttering. Ground crew then noted a red glow. Seconds later, burning hydrogen burst from the top of the airship. Photographs and witness testimony allowed investigators to conclude that the fire did start at the top of the craft, near gas cells 4 and 5. The fire spread down the rear sides of the craft. The stern of the craft was engulfed in flame and began dropping. As the airship bow began pointing skyward, hydrogen flames shot up through the bow "like a blowtorch". The entire craft was afire and the frame collapsed.
About 32 seconds had elapsed from the time the ground crew noted the red glow until the ship lay smoldering on the ground. Secondary fires, mainly of diesel fuel used for the propeller engines, burned for another three hours. The combustion was a rapidly burning fire, not a deflagration explosion. Post-accident photographs show that the docking mast was intact, and the few injuries to the nearby ground crew and spectators show that it was not an overpressure event, only a fire. While the flames "shot up through the bow like a blowtorch", this is to be expected when a fire is introduced on a slope, since the flame radiant heating more easily preheats combustible materials (the hydrogen and the fabric) on a slope. Pre-heating combustible materials allows faster flame spread, and flame upward buoyancy would also allow easy propagation into unburned material.
As the Hindenburg continued to settle by the stern, the slope continued increasing, allowing faster and faster flame spread. Hydrogen does not radiate as much heat as other materials, but the Hindenburg cover fabric and gas cell materials were also burning, which contributed to pre-heating the unreacted hydrogen. Fortunately, Captain Pruss allowed the airship to settle. The first instinct was to release ballast to trim the craft. Had he done so, the airship would have remained at too high an altitude and more lives would have been lost.
There was some speculation that the Hindenburg was sabotaged but there is no conclusive proof that any sabotage occurred. The German investigation of the accident concluded that, while definitive causes could not be found, the most probable cause was that a leak developed in gas cell 4 or 5, possibly caused by a failed support wire inside the airship, possibly due to the sharp turn during landing. Sometimes support wires did fail, but they usually did not cause any leaks. The wreckage was too damaged to determine anything about the hull internal support wires. The leak caused a flammable hydrogen-air mixture to form in the upper part of the ship's stern. The gas mixture was ignited either by (a) a brush discharge after the ship was electrically grounded or (b) the airship frame grounded quickly through the port manila line but the wet outer fabric cover did not ground as quickly, allowing a voltage potential difference to develop and create a spark between the fabric and the aluminum alloy frame. The German investigators, including Dr. Eckener, favored scenario (b).
The American investigation included witness testimonies and photographic coverage of the accident, as well as investigation of the remains on the field. One witness from the port bow landing party of the ground crew discussed the flutter he saw in the fabric on the top port side of the airship. The German airship design and operations expert (and member of the German investigation team), Dr. Hugo Eckener, stated that a leak of hydrogen on the order of 40 to 50 cubic meters per second could cause the sort of fabric flutter described by the witness. He also stated that this leakage rate would not immediately be noticed on instruments in the control car.
US investigators constructed an airship master diagram and marked eyewitness locations of fire origin. From that diagram the investigators concluded that the first open flame was on the top of the ship forward of the entering edge of the vertical fin over gas cells 4 and 5. The investigators believe that there was no detonation explosion after the fire began, just a very rapid burning of the hydrogen as it escaped from the gas cells. The investigators determined that the gas cell was not damaged by a propeller blade fragment; the propellers were recovered from the wreckage and shown to be intact when the craft impacted the ground.
A plausible cause for the leakage was the fracture of a shear wire in the airship hull; a wire might have snapped during the last sharp turn to line the airship nose up with the mast and face into the wind. Possible gas combustion ignitors were discussed, including the pressure sensor, outgoing radio transmissions (transmissions ended 15 minutes before docking), mechanical friction heat from the airship structure, chemical reactions, electrical energy, and drive engine exhaust.
Electrostatic energy seemed the most promising cause. Tests on the manila (hemp) rope showed that for conditions existing at the time of the accident, the airship could electrically discharge by 90% in a period of 0.6 to 170 seconds of the rope contacting the ground. A theory was advanced that a brush discharge from the airship fabric to the grounded part of the airship occurred because of the voltage potential gradients that existed at the field after thunderstorm passage. A brush discharge was proven to be able to ignite hydrogen in tests, and this discharge could not be seen in daylight. The American investigation concluded that a leak in the vicinity of gas cell 4 or 5 formed a combustible mixture, and it was probable that a brush discharge ignited the mixture.
The popular perception is that the Hindenburg was destroyed on its maiden voyage. Unlike the Titanic, the zeppelin had flown for over a year before the disaster. In 1936 the Hindenburg flew 191,583 miles carrying 2,798 passengers and 160 tons of freight and mail. In that year the ship made 17 trans-Atlantic flights with 10 to the United States and seven to Brazil. It also made a record-breaking Atlantic double-crossing in five days, 19 hours and 51 minutes. The Hindenburg was just one of a number of hydrogen airships that met with an accident involving the hydrogen lift gas. Of 129 hydrogen airships in the early 1900's, over one-third met with that type of accident. The most well known accident event with a hydrogen dirigible was the Hindenburg disaster. Until that time, commercial German airships had a flawless safety record. The Graf Zeppelin airship alone had flown around the world, and had logged over a million passenger miles with no injuries to passengers. This value is perhaps small by today's standards, but in that era most dirigibles carried on the order of 50 passengers, so the value did represent many airship miles flown.
The Hindenburg did not explode, even though the average person believes that it did. As stated earlier, this was a severe flash fire, not an explosion. If the Hindenburg had exploded, then the loss of life would have been much greater than actually occurred. Certainly the crew and passengers would have perished in the air. The ground crew would have suffered more than one fatality (wreckage did fall on one man) from the blast heat, overpressure, and debris missiles when 7 million cubic feet of hydrogen exploded. Spectators at the landing field would have had injuries and fatalities from the blast overpressure and heat exposure.
The Hindenburg accident claimed the lives of 13 of the thirty-six passengers, 22 of the sixty-one crew members, and 1 ground crewman. The 13 passengers were the only fare-paying passengers ever lost in 20 years of commercial travel by airship. While more people perished in the R-38 and R-101 accidents, the public outcry was worse with the Hindenburg disaster.
The tragedy became one of the greatest news events of its time. Stark public memory of the big dirigible going down in flames at its mooring at Lakehurst, New Jersey, and of the extraordinarily emotional live reporting of an eyewitness radio announcer, guaranteed the death of derigible flight as the losses of the Roma, Shenandoah, Akron, and Macon had not. While the Hindenburg was not the largest loss-of-life accident with an airship, it is the most memorable. Some reasons for this are the motion picture camera footage, the live radio broadcast, and the sensitivity of a foreign flag vessel having a catastrophic accident at a US port during a time of failing political relations with the country.
In the aftermath of the Hindenburg fire, all airships were grounded, and commercial airship travel ended later in May 1937 because of the perception on the part of companies and passengers that it was too dangerous. Germany grounded the Graf Zeppelin, but also did construct the D-LZ130 airship in 1938. Again, the US did not supply helium for airship use, and the D-LZ130 used hydrogen. The D-LZ-130 was named the Graf Zeppelin, but it never flew passengers. After a few trial flights, the DLZ-130 was disassembled for parts in 1940 as Germany was involved in World War II.
The only significant past commercial airship operations were those of the Zeppelin Company and its subsidiary DELAG. None of these commercial operations can be considered a financial success and most were heavily subsidized by the German government. For example, the transatlantic service with the Graf Zeppelin in 1933-1937 required a break-even load factor of 93-98%, a value seldom achieved, despite carrying postage at rates over ten times higher than 1975 air mail rates. Throughout most of these commercial operations, there was little or no competition from heavier-than-air craft. However, airplane technology was making rapid strides and airplane speed, range, and productivity were rising steadily. Airships and airplanes are difficult to compare because of the remoteness of the time period and the limited operational experience.
By the time of the Hindenburg disaster in 1937, it seems clear that the most advanced airplane, the DC-3, had lower operating costs as well as higher cruising speeds than the most advanced airship, the Hindenburg. Of course this tended to be offset by the Hindenburg's luxury and longer range. Nevertheless, it is clear that although the loss of the Hindenburg hastened the end of the commercial airship era, it was not the primary cause; the airship had become economically uncompetitive.
By 1939, the first airplane carried a paying passenger across the Atlantic, although transatlantic travel was not routine until after World War II. Despite the fact that nearly two-thirds of the people aboard survived the Hindenburg airship fire, the name Hindenburg came to mean any sudden, tragic technological disaster.
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