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The destruction of the "Hindenberg" - a new theory.


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The destruction of the airship Hindenburg was probably the first disaster reported by public radio broadcast and filmed as it happened. Initially there were thoughts that it was an act of sabotage, but both the USA and Germany (at that time Nazi Germany) agreed that sabotage could be ruled out. Since then a number of hypotheses have been propounded as to the cause of the ignition. 

 

 The Hindenburg  the Graf Zeppelin we well-used aircraft with lots of safe hours of operation. The only real safety concern was that the lifting gas was hydrogen. The USA was the World's producer of the inert gas, helium, but would not sell it to Germany. The Zeppelin company had over 30 years' experience with handling hydrogen, so they had covered the foreseeable risks.

 

It is known that the Hindenburg was out of trim as it approached its destination. It was down at the rear. The obvious cause of this was that it was loosing the lift from hydrogen in the rearward gas bags

May 1937 - The Hindenburg disaster | The Engineer The Engineer

It doesn't seem that the rate of leakage was anything like a bursting balloon, more like a slow leak in a tyre for a nail. So within the body of the aircraft, there was a mixture of air and hydrogen, which is highly combustible. But for that to happen, there needs to be an ignition source.

 

Passengers and crew were carried in an hermetically sealed area, under the gas bags.  In any case, one would expect the leaked hydrogen to concentrate in the upper regions of the body. Crew members could move through the aircraft as they carried out their duties, but they would have been trained in safe work practices.

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 As the Hindenberg was approaching its destination, it had to go into a holding pattern because wind and rain were preventing an arrival. So, the outer covering was wet from the rain. Also, any object moving through the air, and the Hindenburg's flight had been about two and a half days, will develop a static charge on its surface. 

 

As the aircraft approached the mooring tower, the crew dropped two ropes to the ground crew. It is the electrical behaviour of these ropes that is a factor in the creation of the ignition source.

 

The most recent hypothesis for the creation of the ignition source is based on the properties of an electrical capacitor. A capacitor is basically two conductors separated by an insulator. 

 

Internal capacitor view

In the case of the Hindenberg, one of the conductors was the airframe, and the other was the aluminium powder contained in the dope that was applied as a final coat to the aircraft covering. The hypothesis is that when the ropes were dropped, they completed and electrical circuit between the statically charged airframe covering and the ground. 

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We can describe what happen this way. The voltage, Vo comes from the static electricity present on the aircraft. The "1" is a "switch". The white rectangle is the outer covering, and the black rectangle is the airframe. The orange rectangle is the ropes going to the ground crew. 

 

While the aircraft is in the air the static electricity can't go anywhere. As soon as the switch is closed in position 1 the battery is connected across the capacitor, current flows and the potential difference across the capacitor begins to rise but, as more and more charge builds up on the capacitor plates, the current and the rate of rise of potential difference both fall. . Finally no further current will flow when the potential difference across the capacitor equals that of the supply voltage Vo. The capacitor is then fully charged.

 

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As soon as the switch is put in position 2 a 'large' current starts to flow and the potential difference across the capacitor drops.. As charge flows from one plate to the other through the resistor the charge is neutralised and so the current falls and the rate of decrease of potential difference also falls. Eventually the charge on the plates is zero and the current and potential difference are also zero - the capacitor is fully discharged. Note that the value of the resistor does not affect the final potential difference across the capacitor – only the time that it takes to reach that value. The bigger the resistor the longer the time taken.

 

And therein lies to clue to what actually happened. The landing ropes were made of Manilla hemp, not highly conductive, so their initial resistance was high. As the ropes were wetted by the light rain, their resistance decreased until at some point the voltage stored in the "capacitor" was high enough to jump the air gap between the outer aluminium-containing outer covering and the metal airframe which was attached to the landing ropes. 

 

The spark ignites the hydrogen/air mixture adjacent to the spark and the subsequent flame front moves thought the airspace around the gas bags, igniting the hydrogen/air mixture in the same way that a flame front moves across a cylinder in the combustion stroke of an internal combustion engine.

 

This edited film shows the lead up to the start of the destruction with the dumping of water ballast and the dropping of ropes. Unfortunately, it doesn't show the escape of the flames from the rear of the fuselage before the flame front moves forward as the front of the aircraft pitches up due to the buoyancy of the forward gas bags. 

 

This "capacitor" hypothesis was the subject of a documentary which was broadcast in Australia on 11/4/21. As part of that documentary, after displaying that the hypothesis was sound, the scientist conducting the investigation calculated how long it would take a capacitor the size of the Hindenburg to charge up. He obtained a time of 4 minutes and a few seconds. Coincidentally, that was the time between the dropping of the ropes and the first signs of fire.

 

There have been other hypotheses as to why the aircraft was destroyed in just 90 seconds. That's a tale for another time.

 

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Just now, Chris SS said:

Very interesting. Thank you for sharing.

 

My pleasure. Too bad the documentary is not readily available. It was one of the better US produced ones.

 

A point to note: sometimes people say that the Hindenburg collided with its mooring mast. That is not correct. The error comes from the camera viewing the airship with the mast between the camera and the airship. If you watch most of the films, the airship was out in the open field when the fire started and it remained out there.

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I reckon that is a sound theory, supported by the dumping of a lot of ballast to try and get the tail up so there would have been a pretty volatile hydrogen/air mix somewhere around the leak. I have seen the video before but not taken a lot of notice. The speed of the destruction is incredible.

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I have read that the hydrogen gas has been given an unfair amount of the blame and that it was the nitrate-doped linen which was the worst culprit. Nitro-cellulose is an explosive, and nitrate-doped linen is pretty close to this.

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OME I watched the Documentary the other night, apart from the 4 minute period of charge the Investigation 80 years ago was pretty much on the money. I wonder why the skin was intentionally isolated from the frame with the dowel. Maybe not to discharge through ground crew handling the ropes ??

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1 hour ago, Bruce Tuncks said:

I have read that the hydrogen gas has been given an unfair amount of the blame and that it was the nitrate-doped linen which was the worst culprit. Nitro-cellulose is an explosive, and nitrate-doped linen is pretty close to this.

That was one hypothesis, but during the documentary is was said that if the doped covering was the cause of the spread, then the fire would have burnt for many hours. If you watch the spread of fire you will see that the flame front moves internally from the rear to the nose. In fact, in the latter part, you can see the flame front venting out of the tip of the nose before the skin rearward  is engulfed in flames.

 

Can you recall back in high school science generating hydrogen in a test tube, then igniting it? The hydrogen and oxygen combined as the flame front moved down the test tube. The combustion products and inert content of the air were pushed out, creating a vacuum at the bottom of the tube. As the last of the gases left the tube, air under pressure rushed in, making a popping sound that is what you hear when you pull a cork from a wine bottle. The airframe covering, being relatively tough and tear resistant, acted like the walls of the test tube, confining the flame front inside the body. That's why you see the flame front moving inside the body. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C (932 °F). The plasma temperature of a spark discharge is almost 5,000-6,800 C. 

 

1 hour ago, Cosmick said:

why the skin was intentionally isolated from the frame with the dowel.

The dowels, were placed on the metal frame in order to keep the fabric from chaffing. Much like the way we put tapes on the top edge of wing ribs before fitting the covering. An unforeseen consequence was that the dowels created an air gap between the metalised covering (due to the aluminium powder in the final dope coating) and the metal frame. That air gap was the dielectric that allowed the whole airship to become a capacitor. 

 

Internal capacitor view

 

What benefits did we get from study of this disaster?

Although not recognised  as a factor in the disaster until this hypothesis, we have long had a recognition of static electricity, and this has resulted in the use of anti-static wicks on the trailing edges of wings and tailplanes. Another is that we ensure that when refuelling, we attach a grounding lead to the aircraft to discharge any static electricity.

 

If you have a fabric covered aircraft, then it would be wise to attach anti-static wicks, and maybe even drag an anti-static wire from an undercarriage leg. Back in the day when the harbour Bridge toll was collected manually, there were earth wires sticking up from the roadway that scrapped the underside of vehicles, discharging static electricity before the toll collectors got zapped.

 

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9 hours ago, Bruce Tuncks said:

Quite often, we get stuff about how the airship will have a resurgence but it doesn't seem to happen.

I was told recently that a project to develop high-altitude communication blimps/dirigibles has been abandoned.

Can’t find a link, but perhaps others know the name of the company involved.

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10 hours ago, Bruce Tuncks said:

it was the nitrate-doped linen which was the worst culprit.

During the documentary they hit aluminised doped fabric with a spark. It did catch fire, but the burning was slow, much like the amount of burning you'd get from an ember dropped on a carpet. It is true that a mixture of aluminium powered, cotton fabric and dope could be explosive, but only if all the ingredients were powdered so that there was a massive surface area for combustion to occur over. 

 

When you add aluminium powder to dope and apply it to fabric, the dope will react with the fabric, but the aluminium is locked in the mixture with the dope. The air can't get into the mixture quickly enough to have it combust (explode) instantaneously. Think what happens if you leave a snag on the barbie. The outside will burn to charcoal, but if you cut that away the filling is still edible. 

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1 hour ago, old man emu said:

...When you add aluminium powder to dope and apply it to fabric, the dope will react with the fabric, but the aluminium is locked in the mixture with the dope. The air can't get into the mixture quickly enough to have it combust (explode) instantaneously...

As a kid in the 60s I could buy all sorts of chemicals by mail order, including aluminium powder.

One of my rockets went off prematurely and severely singed my hair. Others toughened up the entire neighborhood...

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