JOHT 89

[stickmaker]

 

 

The Joy of High Tech

 

by

 
Rodford Edmiston

 

 

Being the occasionally interesting ramblings of a major-league technophile.

 

 Enter your cut contents here.

 

Please note that while I am an engineer (BSCE) and do my research, I am not a professional in this field. Do not take anything here as gospel; check the facts I give. If you find a mistake, please let me know about it.

 

 

 

 

Why Did the Hindenburg Burn?

 

 

 

 

Airships were already on the way out before the disaster at the Lakehurst Naval Station, near Manchester Township, New Jersey. Though they were much faster than steamships, they were slower than airplanes. They were also unpressurized. Now, airplanes from that era were also unpressurized - with a few exceptions - but designers saw the future coming and airplanes were easier than airships to pressurize. Part of the problem was the added weight required to pressurize at least the passenger volumes. Keeping the weight low was important to both types of craft, but airships were especially sensitive to this concern. Pressurization of passenger aircraft was coming, for several reasons. If the Second World War hadn't intervened, there could have been routine transoceanic flights of pressurized passenger airplanes by 1940. 

 

Note that even during the Second World War most airplanes were unpressurized. In fact, there was a huge amount of work done by the participants in that conflict to find oxygen masks which would work with the wide variety of face sizes and shapes possessed by aircrew. (Harking back to something from a previous JOHT.) Still, the way to get long range in an airplane is to fly very high. The energy cost of climbing into the thinner air was more than recovered on long flights by the reduction in drag during cruise. Above a certain altitude supplemental oxygen just isn't enough. So pressurized aircraft were the obvious improvement needed for future aviation. 

 

Not that airships couldn't do high altitude. During the First World War there were times when German Zeppelin bombers over Great Britain were immune from artillery and the airplanes sent against them. The Height Climbers and the later Super Height Climbers (two sequential classes of Zeppelin bombers lightened and otherwise optimized for high-altitude operation) had crew dressed in electrically heated flight suits and supplied with supplemental oxygen. This created great hardships for the airship crews - the technologies were very primitive compared to what became available just a few years later - but the discomfort and inconvenience were minor when compared to getting shot down. This concern was presumably not something airship passengers would need to put up with, of course. 

 

Because of the difficulties of bringing the fight to the enemy with artillery and early airplanes during the Airship bombing raids of the Great War, many people - including Sir Rider Haggard - advocated for Britain to build its own airship fleet. These would be dedicated to hunting down and destroying the German airships, since artillery and airplanes - though only early on in the conflict for the latter - could not reach the altitude where the Zeppelins (after a few low-altitude attacks) operated. (In fact, the German airships could outclimb the crude airplanes of the day, and some were faster horizontally than the early British interceptors; at least at high altitude.) However, technology soon led to improved airplane interceptors. Even with the increased operational height of the later Zeppelin bombers, the airplanes eventually rose to the challenge. 

 

Early on, the most common cause for the loss of an airship were operational mistakes. (This also applied to the early airplanes, a few years later.) People learned, and flights became safer. Then came the Great War, during which those attacked by airships (most of the attacking being done by German Naval Zeppelins) devoted considerable resources to developing ways to destroy them. When enemy action was not a consideration the two main causes of airship loses were structural failure and weather. Often, running into bad weather led to structural failure. Airships were large, so they caught a lot of wind, and they had to be built light, which reduced strength. (One reason so many non-German, post-war rigid airships - such as the _USS Macon_ - failed was that many were based on the lightweight height-climbers, which were even more fragile than most Zeppelins.) As design and construction methods and materials improved pure structural failure became a rarity. However, weather continued to be a problem. German Zeppelin captains were trained to recognize hazardous weather conditions and avoid them. Which is one reason those airships had such a good overall safety record in their years of passenger service.

 

Speaking of materials, note that during the First World War the Germans had _two_ rigid airship programs; one by the Army and one by the Navy. The smaller program of the German Army used craft built by the Schütte-Lanz Company, rather than by Zeppelin. The Schütte-Lanz airships used glued, laminated wood, including plywood, for their main structural material. The Navy program used Zeppelins with duralumin (aka duraluminum, duraluminium, duralum, durallium, or dural; a series of strong, light aluminum-copper alloys, today considered obsolete) for their structures.

 

Early Zeppelins generally had ceilings of under 2000 meters. The first German airship bombing raids against Britain were usually made from even lower altitudes, usually below a thousand meters. Mostly this low altitude was used to improve accuracy. The first Zeppelin bombing raid on London was made from about 4000 meters. Note that the limits of altitude and the time which airships could spend at altitude were generally determined by the endurance of the crew. That is, going higher and staying higher put a huge amount of stress on the humans due to the thin, cold air. 

 

After the first downing of a Zeppelin by a British airplane on September 2, 1916, the Germans developed the Height Climbers. (Note that "Height Climber" and "Super Height Climber" were British designations. The Germans used model numbers.) These routinely operated at altitudes of 6000 meters, at over 110 kilometers per hour. Trying to intercept these meant that British airplane crews fought oxygen deprivation and extreme cold, as well as the enemy.  The thin air stole engine power. The cold led to snapped oil lines, congealed oil, frozen radiators, and cracked windows. The reduction in oxygen intake led to mental and physical impairment of the pilots, which made dealing with anything - especially an emergency - more difficult. 

 

For a time, these higher-flying Zeppelins proved immune from interception, except when returning to base. As they descended to approach their airfields in Germany, many were shot down by French planes, and British planes based in France. The Zeppelin captains quickly learned to maintain altitude until over their bases, where they could be defended by German airplanes and ground artillery. Which added to the strain on the crews.

 

One such airship waited too long. The rising Sun warmed the hydrogen and the Zeppelin shot up to 7460 meters. Much of the crew lost consciousness, and if those remaining functional hadn't been able to force the craft to a lower altitude when they did, all might have died. The unconscious crew eventually revived and the frozen engines were restarted, but the ship had to make a forced landing in central Germany. This remains the altitude record for a rigid airship. As well as the record for the most time spent above 6060 meters by an airship with crew members actually surviving.

 

The British responded to the higher-altitude bombing raids by dressing pilots more warmly and taking measures to protect their aircraft against the cold. This included using airplanes with air cooled engines. As far as I have been able to determine, they did not provide supplemental oxygen. Since their pilots spent much less time above 4800 meters they were much less affected that German airship crews, who were spending tens of hours "up there." However, they _were_ up there, and the British were desperately trying to reach them. 

 

The Germans dealt with the problem of the effects of thin air by providing tanks of supplemental oxygen for their aircrews. However, the gas was usually contaminated by things such as pump oil (including glycerine) which caused days of side effects after the airships returned from their high-altitude missions. Of course, some of those physiological effects were purely due to spending so much time at high altitude. (Mountain climbers knew about "altitude sickness." Did no-one think to consult them?) 

 

Passenger airships stayed low enough that those on board didn't need pressurization or oxygen supplementation. However, while having a good turn of speed when compared to steamships or even steam locomotives, they were slower than passenger airplanes (which, well into the Forties, were also mostly limited to lower altitudes). Neither class of passenger aircraft from that era would normally go above storms. Still, Zeppelin commanders were trained to avoid storms, even if that meant taking a longer route. For the most part this worked well. They could afford the extra time for this due to their much greater endurance.

 

In many ways the _Hindenburg_ (German airship LZ-129) and similar craft served the same purpose as the later Concords. They were luxurious, intended to replicate the feel and style of upper class steamship accommodations, and intended mainly for the wealthy. There was no steerage class on airships. The _Hindenburg_ actually had a lounge, with a custom piano made largely of aluminum. There was even a smoking room, with an airlock. Outside air was pumped into the room, keeping it at a higher than ambient pressure, so that hydrogen couldn't enter. The airship's sole cigarette/cigar lighter, an electric one, was in this room, on a chain. This room was also the location of the bar. 

 

There, by the way, you have indirectly the reason why the _Hindenburg_ fire was so catastrophic. The airship was originally intended to use helium. However, then as now the US was the main source of helium (extracted from natural gas wells in some parts of the country) and a 1927 Act of Congress forbade its export. So the _Hindenburg_ instead used hydrogen, like the bomber airships of the Great War. Hydrogen is not hard to acquire, and can be made through such chemical operations as reacting iron filings with acid. This flammable gas provided slightly more lift than inert Helium, but greatly increased the risk. 

 

Then there was the fabric coating used on most airships and many airplanes of the time.

 

Aircraft dope is a special type of paint. It bonds with fabric, then shrinks as it cures. This pulls the outer skin of an aircraft taught, making it strong and smoothing the surface. The dope also protects the fabric from weather and other sources of potential damage, such as gnawing insects or light impacts. Today, all aircraft dope (which is not only used on the old aircraft which are still around; there are some of current manufacture which are made with fabric coverings) is safe once it cures and the highly flammable solvents evaporate. However back then the shiny appearance of airship hulls (the silver reflecting sunlight to reduce heating, which could cause the lifting gas to expand and change the craft's buoyancy) was accomplished by adding powdered aluminum to the dope. 

 

Powdered aluminum is very flammable. When combined chemically with oxygen it releases a huge amount of energy. It is one of the major components in traditional thermite. It is also used in some solid rocket propellants, such as the boosters for the Space Transportation System. The dope used on the _Hindenburg_ and several other Zeppelins of that era had an aluminum-containing dope which also had a small proportion of iron oxide, in a cellulose acetate butyrate binder. The cured dope resembled in general chemical makeup modern, aluminum-based solid rocket propellants. Though there was too little iron oxide in airship dope to provide oxygen for combustion - as occurs in thermite - a trace of that substance is included in aluminum-burning solid rocket propellants as a catalyst. In the doped skin of the _Hindenburg_, once combustion started there would be plenty of oxygen available from the air.

 

A few years back, someone who had a piece of unburned fabric covering from the unlucky airship (souvenir seekers quickly took away anything small they could grab) volunteered a small portion of it for testing. When this was touched to a flame, it quickly ignited and burned very energetically. The Zeppelin company knew this dope was not only flammable but highly so, and was phasing it out. However, they hadn't recovered the _Hindenburg_ yet. All it needed was a spark. 

 

Early on, airships had many advantages over airplanes (which came along only after the airships had been around for a while). Besides greater altitude, they had the combined benefit of much greater endurance and range, plus greater stability. Despite the amount of gas needed to lift even a small weight they could also carry greater loads than the early airplanes. Hence their use as bombers. By 1937, though, their only real advantages were endurance and stability. Airplanes had grown larger, faster and longer-legged. They could routinely cross great distances without refueling, and do it faster than the airships. The Zeppelins, though, were not in a hurry. They could spot a luxury liner on the ocean below and, if the Captain of the airship was so inclined, drop down to shout greetings or even exchange small packages via lines lowered from the airship. 

 

On that last trip, the _Hindenburg_ fought strong headwinds much of the way across the Atlantic. When it finally arrived in the US on May 6, 1937, debarking was further delayed by thunderstorms over and around the destination at Lakehurst. The airship flew over Manhattan Island to kill time and keep the passengers from getting antsy about the additional delay. With the weather at Lakehurst still iffy the huge airship then did a tour of the New Jersey seashore. When it finally arrived at Lakehurst fitful winds required multiple, sharp changes in direction to perform a high mooring. That was to be followed by winching the _Hindenburg_ to the ground. 

 

Unfortunately, the airship now also had trim troubles, and came in tail low. Water ballast was valved from the rear, and hydrogen from the forward lift cells. Some witnesses reported fluttering of the airship's fabric covering in front of the upper tail fin just before the fire. There was definitely hydrogen in the air from the forward venting. If that fluttering was caused by hydrogen leaking from a rear gas cell (there was later speculation that some plumbing or even a gas bag was sprung by those sharp turns) that would explain why the _Hindenburg_ was tail heavy. Others reported a dim, blue discharge, also in front of the top fin, probably due to a buildup of static electricity.

 

Mooring lines were dropped and the ground crew pulled the great craft towards the mooring post. A light rain began to fall. People on board heard a muffled detonation from the rear of the ship. Flames were observed at the front and rear of the upper fin. Some witnesses later said they first saw flame elsewhere at the rear of the _Hindenburg_.

 

The fire spread rapidly. The aft end of the ship quickly lost lift and crashed into the ground. The duralumin frame bent and broke, which allowed the still-bouyant nose to swing upwards. This caused the forward part of the airship to act as a chimney, channeling much of the fire forward through the inside. From the first sighting of flame to the nose crashing into the ground took under 40 seconds. 

 

Luckily, the ship was carrying only about half a full load of passengers. Thirteen of those thirty-six died, along with twenty-two of the sixty-one crew, with most of the rest in both groups being badly burned. An additional three passengers and six crew died of their injuries later. Some of those on the ground also died.

 

One survivor was saved by water ballast spilling onto him. Many of the survivors were on the port side, which burned last, giving them time to let the ship settle before they jumped out windows. (Some of the fatalities were from people jumping or falling while still too high to survive the drop.) However, the main reason so many survived is that hydrogen burns quickly, and being so light flows upwards. The bulk of the fire burned out in about a minute and a half, most of it above the actual airship.

 

What started the fire? Sabotage was suspected at first, and is still held responsible by many. However, most of the investigators eventually decided on static discharge. Due to the way the outer covering was attached to the duralumin frame a charge could temporarily build up between those parts of the _Hindenburg_. If the rain wetted the mooring ropes - which were anchored to the frame and may have been impregnated with salt from the ocean crossing, increasing their conductivity when wet - the frame would have been grounded while the hull was still charged. The weather was certainly amenable to generating a static charge on an object in the air. However, the hull and frame _were_ electrically connected (if not thoroughly) and the _Hindenburg_ had flow many times through similar weather without incident. This included being struck by lightning!

 

The cause will likely never be determined for certain.

 

What is certain is that if the ship had been lifted by helium instead of hydrogen that the fire would likely have at most consumed the doped fabric and possibly a few other things, and deaths and injuries would have been greatly reduced. 

 

The era of airships did not end instantly. Many - including the _Graf Zeppelin_ - continued to fly for a while. The _Graf_ (LZ-127) was the most successful airship in history, even making a trip around the world in 1929. There was even a successor planned for it, which would have the same name. However, their era did end. Partly because Hitler saw them as symbols of a previous German regime, the one which had surrendered during the First World War. (Hermann Göring - though a fan of the Zeppelin - ordered the _Graf_ dismantled in 1940. He ordered the sister-ship of the _Hindenburg_ - LZ-130, _Graf Zeppelin II_ - dismantled in 1940. It did fly but never carried passengers; its last flight was on 20 August, 1939.) Partly because the Germans simply could not get helium for the remaining Zeppelins. This abandonment being despite continued popular and press (and even some high ranking Nazi Party member) support for the airship program in Germany and elsewhere. They were not even to be preserved in aviation museums, some of which already housed extremely large airplanes. Even most of the infrastructure was destroyed, the installations at Frankfurt being blown up by Wehrmacht demolition specialists on 6 May, 1940.

 

So, why did the _Hindenburg_ burn? As usual, the disaster was due to a combination of factors. Some of them, being absent, might have prevented the disaster. Others, if different, might have made it less bad... or worse. However, the airship did burn. The conflagration didn't cause the immediate end of airships, but it sure didn't help.

 

There have been multiple attempts to revive airships through the decades since the _Hindenburg_ burned. Some of these are currently ongoing. With helium available for lift and modern, lightweight structural materials, such craft are more practical than ever. (Though some people claim they were never practical. A few say they never can be.) Electric motors - powered by one or more on board generators with battery supplementation and maybe photovoltaic cells on the top of the envelope - would make for efficient operation. These lighter-than-air craft could be used for cargo which needs to travel faster than by ship but which does not need the extra speed - and expense - of being shipped by airplane. Though slower than airplanes, airships are more fuel efficient, since there is little fuel spent providing lift. With modern resources  pressurization is even practical. This would allow higher cruise altitudes, for more-rapid flight. Including passenger trips.

 

Most of these new airships are either not rigid or only semi-rigid. That is, they don't have a full internal framework containing the cells of lifting gas, with a separate cover for streamlining and weatherproofing. Many are hybrids, with enough gas to counter all or most of their weight, but which still require forward motion through the air to leave the ground with cargo. 

 

As well, there is a strong nostalgic factor involved in reviving airships. Lighter-than-air craft are seen as emblems of a vanished past, an age which was somehow better than the current one. Despite all the problems rampant back then. They are also seen as emblems of wealthy travel and government fiat, during a time of kings and queens and conspicuous consumption by the nobility, which some people see as better than what they have now. The great airships are also possessed of an inherent elegance, which can be appreciated by anyone of any age or level of realistic historical awareness. 

 

None of those are good engineering reasons, of course. However, even engineers can have a sense of esthetics.