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Comment by WalterBright | Hacker News Reader

> Imagine a cigar-shaped balloon as large as a skyscraper filled with explosive gas. Combine that hydrogen with oxygen from the air, and a source of ignition, and you have "literally a bomb," Giapis says.

Imagine a tin cigar filled with explosive gas! Combine that with oxygen from the air, and a source of ignition, and kaboom! Today we call those "airliners".

If people had continued to develop Zeppelins, the problems with safely handling hydrogen would have been solved, just like the problems with handling aviation gas and jet fuel have been solved. There's a lot of consideration with airliners about dealing with lightning strikes and all sorts of possible sources of sparks so the fuel is not ignited.

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I agree. Hydrogen and jet fuel are not "explosive". True explosives contain all the components they need for the reaction. Hydrogen and jet/diesel/gasoline fuel require injected oxygen to burn, and you can create an explosive mixture by mixing it with oxygen or keep it non-explosive if you can keep the oxygen out.

Yes, if exposed to air, hydrogen will mix itself more readily than jet fuel will, but it is also much less energy dense, so it will burn out quickly and cause much less damage. A Hindenburg could not have brought down the World Trade Center buildings.

If you have hydrogen sealed in a fireproof bag of some sort, you can heat the outside with a blowtorch, send giant electric sparks through it, etc., and no explosion if the bag doesn't rupture. Because it's not explosive. If it does rupture, no explosion, but the gas will burn as it rises into air with oxygen that hasn't yet been consumed.

I imagine (would have to be rigorously proved, of course) that if you compartmentalized hydrogen in lots of bagged "cells", and the cell membranes were designed so that even if the neighboring cells all ruptured, encountered air, and burned, the heat would not be enough to rupture an unruptured cell, you could have a safe hydrogen airship. It might even be less vulnerable to certain dangers (ex: shoulder-launched terrorist missile) than an airliner.

That said, the detonation speed of perfectly mixed hydrogen / air is amazing. If you fill a weather balloon with a mix - get ready for a loud bang. Not good for your hearing (even a small balloon)

https://www.youtube.com/watch?v=MMB2VR0087w

https://www.youtube.com/watch?v=l9CI6KSV560

Sure. Imagine a perfect mix of jet fuel and air in a weather balloon. Or metal shavings. Or a perfect mixture of wheat and air in a silo. BANG!

Now imagine a metal aircraft full of fuel with a cargo of wheat--if you dare. Oh the humanity!

Hydrogen leaks into confined spaces have done the boom thing. My impression is wavefront speed may be supersonic or somehow drive a lot of intensity (not my area)

Wheat is scary in a silo, but in a 50 lb bag not so much. I liked the cell engineering idea with hydrogen - not sure of weight / etc tradeoffs.

Though secondary explosives (TNT, etc.) are usually safer than fuels like jet fuel / hydrogen because they don't explode without a primary explosive percussion and don't ignite as easily.

You're safer crashing a glider full of TNT and lit sparklers than a jet full of fuel.

If it does rupture, no explosion, but the gas will burn as it rises into air with oxygen that hasn't yet been consumed.

All the videos on YouTube of hydrogen-filled balloons (yes, pure hydrogen, not HHO) disagree with you.

Not saying you're necessarily wrong, but it could be that the balloon exploding is mistake for an actual combusting explosion

It was already solved. Helium instead if hydrogen.

"Despite a U.S. ban on the export of helium under the Helium Control Act of 1927, the Germans designed the airship to use the far safer gas in the belief that they could convince the US government to license its export. When the designers learned that the National Munitions Control Board refused to lift the export ban, they were forced to re-engineer Hindenburg to use hydrogen for lift."

And even with helium-buoyed airships, the US could not make them work safely and effectively.

Explody lift gas is only one of numerous failure modes of lighter-than-air craft. Handling in winds, structural stability (the frames are at the very limits of material science), limited lift capacity (the only very slightly greater density of helium has a disproportionate impact on payload capacity reduction), all impose severe limits.

Wishing away problems doesn't.

Jet fuel is flammable, but hardly explosive in the same sense that hydrogen is.

The passengers and crew aboard TWA Flight 800 would probably disagree if they'd lived.

https://en.wikipedia.org/wiki/TWA_Flight_800

The probable cause was a spark in the fuel tank.

It's disingenuous to present the parent comment as saying "jet fuel isn't explosive". Here's some more info on a hydrogen accident involving a relatively small amount.

https://www.powermag.com/lessons-learned-from-a-hydrogen-exp...

Whether its explosive or not is irrelevant when it comes to aircraft/Zeppelins. The problems is it burns very hot and any airplane that catches fire has only seconds before it is consumed, whether or not it explodes.

https://www.britannica.com/topic/Air-France-flight-4590

and this:

https://www.youtube.com/watch?v=vVyZeSgxmsw&t=74s

The thing Lindbergh was most afraid of in his transatlantic attempt was the fuel load. His predecessor died in a huge fireball on takeoff.

Lindbergh was flying with a fuel much closer to gasoline than jet fuel. Jet fuel is on the kerosene/diesel end of the flammability spectrum (which is to say low and much lower than gasoline).

You can put a match out in a jar of jet fuel at room temp. (Start at 1m30s: https://m.youtube.com/watch?v=7nL10C7FSbE )

See my other links in this thread showing airplanes engulfed in fireballs.

The idea that jet fuel is safe and doesn't burn in crashes is very, very false. Remember those fireballs when jets crashed into the World Trade Center?

Literally nobody here is making argument that jet fuel is 100% safe and we should fill fire extinguishers with it.

There is a clear spectrum of flammability/explosiveness in which hydrogen, gasoline, and jet fuel are sorted in that descending order. That's what several posters are saying.

A crash and deflagration is not an explosion.

An explosion has a reaction front moving faster than the speed of sound. A deflagration is a rapid combustion, but without the shockwave.

Sufficient quantities of aviation fuel --- in the case of piston-powered craft, petrol rather than the heavier and less volatile kerosene of jet fuel --- especially when mixed with or dispersed through air will burn quite vigorously. Unless specifically contained, they won't actually explode. Even combustion at high pressures within a piston's cylinder is more a combustion than explosion.

The interesting thing about hydrogen is that it is flammable in almost any concentration (4% to 74% in air). Jet fuel, on the other hand, is only flammable over a small concentration range (0.6 to 4.7%).

I once tried getting a plastic sack of hydrogen to ignite -- it was basically impossible. Since the gas was not pressurized there was nothing to force the mixing with outside air, and whenever I managed to burn a hole in the bag, it would just burn slowly until it melted shut.

In the end I folded the bag around a military-grade flashbang (which has magnesium). That worked.

> If people had continued to develop Zeppelins

Not sure I'd want to cross an ocean in a Zeppelin, but I think I'd enjoy a quiet leisurely one week flight safari over the Serengeti, Ngoro Ngoro, Kruger, and Okavango Delta, for example. Particularly with panorama windows and a lounge and fine restaurant and bar. Pity.

A much more difficult problem with Zeppelins is their susceptibility to bad weather.

IIRC, the US navy had 2 rigid airships in the 30ies, Akron and Macon (both filled with Helium).

Both crashed after a few years in service, in weather related accidents (with significant loss of lives in Akron case).

These two examples are not isolated cases, several zeppelin crashed in similar circumstances.

Replacing H with He solves part of the safety issues with zeppelin, but not the main source of accidents. Given how high the susceptible to weather is, due to the inherent size an light weight of zeppelins, I don't think we will see a large zeppelin being built in the foreseeable future, despite all the projects around these.

Wouldn't we have much (!) better weather monitoring and prediction capabilities today, so that one could avoid bad weather (or land in time, preemptively)?

Don't we have Helium anyway?

We don't have an unlimited supply.

Helium is extremely limited and cannot be manufactured at any appreciable scale (you're not dealing with typical chemistry, but physics. Helium is an element.)

What we have naturally comes from radioactive decay deep in the earth. This will continue for some time, but it's a fixed, slow rate.

Helium evaporates into space. Once it's gone, it's gone for good.

The other options we have are hydrogen (extremely reactive), heated oxygen (also reactive), methane (reactive), ammonia...

It's a hard problem.

One more problem that will be solved by fusion energy!

We can theoretically do vacuum zeppelins, which is of course lighter than helium and unlimited tho it has its own problems too.

I was curious about this and it looks like it's currently impossible to have a material that is both strong enough and light enough to support a vacuum balloon:

https://physics.stackexchange.com/questions/71027/is-it-poss...

> Helium is an element

Nitpick: So is hydrogen.

Yeah, but only one of them is a noble gas that is rarely bound into molecules that would be found in solid form. As a consequence, it tends to just leak out of the atmosphere without a lot of sources for replenishment.

Yes.

More precisely, helium is an element that doesn't readily form compounds. Hydrogen does, and as such can be extracted from, say, water via electrolysis.

The secondary issue with helium is that it's much lighter than air -- this is why helium and neon are both much rarer than, say, argon (which is actually the third-most common element in the air after nitrogen and oxygen).

> The secondary issue with helium is that it's much lighter than air

Actually, hydrogen is lighter, and so a better lifting gas, than helium.

https://www.wikiwand.com/en/Lifting_gas#/Hydrogen

Yes, but while hydrogen (H) has half the mass of helium (He), H is only 8% better than He in providing lift in the air, if I computed correctly (as that is generated from displacing much heavier air), so it's not a huge deal:

He: 0.1786 kg/m^3

H: 0.08988 kg/m^3

Air: 1.225 kg/m^3

Air vs He: 1.0464 kg/m^3

Air vs H: 1.13512 kg/m^3

To put it differently, for helium, the lift is about 86% of the weight of the displaced air, while for hydrogen it's 93%.

Both very good, and both are much better than hot air!

(which gives a lift of only about 25% of the weight of the displaced air, at typical temperatures of about 120 deg C)

According to the wikipedia page I linked to:

> In a practical dirigible design, the difference is significant, making a 50% difference in the fuel-carrying capacity of the dirigible and hence increasing its range significantly

But I wouldn't know :)

Oh, very interesting. The linked TIME article from 1924 (!) gives a bit more detail:

> While helium is exceedingly light as compared with air, it is somewhat heavier than hydrogen. The total lift of a helium-filled dirigible is accordingly some 10% less than that of the hydrogen-filled airship. The difference does not appear important at first sight, but the total lift of the gas carries the structure, the motors and the crew. It is only the last 20% or so that is available for carrying fuel, and hence a difference of 10% in the gross lift may spell a difference of 50% in the fuel-carrying capacity. On long-distance flights this difference is vital.

> Nor is the danger of fire totally eliminated with the use of helium; the gas-tanks and the fuel system generally are still vulnerable. But when a ship is properly designed and carefully handled, the danger of fire is comparatively small, even with hydrogen.

https://web.archive.org/web/20110209024932/http://www.time.c...

Note that a contributing problem to the Hindenburg disaster was the exterior canvas paint was chemically about the same as rocket fuel.

That's what I remembered, so I was surprised it wasn't mentioned in TFA.

> Both very good, and both are much better than hot air!

No.

Hydrogen is highly explosive/flammable — that is why Helium is preferred.

Yes, both helium and hydrogen are much lighter than air.

But that doesn't significantly impact rarity of hydrogen, as it reacts very readily and is typically found in compounds.

You don't find much hydrogen gas in nature (~1ppm in the atmosphere, vs 5ppm for helium).

Don’t forget about “null” gas.

Better the hydrogen, a few small technical problems aside.

https://en.m.wikipedia.org/wiki/Vacuum_airship

That won't work on earth due to weight of materials and material strength requirements. It might work on a dense atmosphere, such as Venus.

> It might work on a dense atmosphere, such as Venus.

Gravity also matters here.

Implosions would be much worse imo.

iirc, the germans wanted to use helium at the time, but the US more or less had a monopoly on it and would not sell to germany

It's not the same thing. Airliners are not filled with explosive gas, they're filled with breathable air. They have these specialized things called "fuel tanks" that are purpose-designed to safely hold combustible fuel.

On the other hand, zeppelins like the Hindenburg were literally just filled with Hydrogen. It was a fundamental part of the design; they hydrogen provided the buoyancy needed for the vessel to float. The vessel and the fuel tank were one and the same. Moreover, there isn't a way to design a hydrogen-based zeppelin that can both use hydrogen for buoyancy and hold it safely; even today containers for gas hydrogen are bulky and heavy. The solution is simply not using hydrogen.

Airliners carry a lot of fuel. The wings are fuel tanks, and there are more in the belly. There's so much fuel they cannot even land after takeoff without dumping it.

> Moreover, there isn't a way to design a hydrogen-based zeppelin that can both use hydrogen for buoyancy and hold it safely; even today containers for gas hydrogen are bulky and heavy.

Of course there is a way. The Hindenburg's dialectric problem could have been solved. It's frankly incredible how good we have gotten at making airliners safe despite being pretty much a flying bomb. See the 9-11 films of what happens when two of them, loaded with jet fuel, hitting a skyscraper. Those fireballs weren't from materials in the buildings.

It depends on the plane but airliners can land with full fuel loads. Juan Browne does a great job explaining this on his YouTube channel: https://www.youtube.com/watch?v=4D2Kj0t4t9s.

The video explains that when they do land overweight, they have to be inspected for damage. They will require a much longer stopping distance, too, and the runway may not be long enough depending on weather conditions.

> There's so much fuel they cannot even land after takeoff without dumping it.

This is a false statement.

and the 911 planes were at best half full when they hit as they already had flown around half of their programmed flight distance.

To be fair, it would be more than half full when at half distance, because airliners carry a significant reserve in case they have to divert.

> zeppelins like the Hindenburg were literally just filled with Hydrogen

As I understand it this is inaccurate -- there is an aircraft frame (made of aluminum) with canvas stretched around it. Then inside that frame there are gas bladders filled with hydrogen. Short of a very intense electrical discharge that would jump the gap, there is no electrical connectivity between the outer skin/frame and the inner bladders. And the bladders in turn were designed to vent upwards in the event of an emergency to prevent exactly this situation, since hydrogen is not an explosive gas unless it is sufficiently mixed with oxygen, which is hard since it will disperse rapidly in air.

One thing often overlooked about the Hindenburg and its reputation as being a dangerous bomb with people hanging below it is that there were a substantial number of survivors -- out of 97 passengers/crew, 62 survived.

I think the GP is probably correct that had not development stopped, we would likely have very safe storage for hydrogen at that scale; like the hybrid helium/hydrogen mechanisms originally intended for the Hindenburg class.

> One thing often overlooked about the Hindenburg and its reputation as being a dangerous bomb with people hanging below it is that there were a substantial number of survivors -- out of 97 passengers/crew, 62 survived.

Hindenburg wasn't the deadliest airship disaster, it was just the final straw. It's the best known of the crashes because it was highly publicized and the last. The USS Akron crash killing 73 of 76 crewmen. There were no survivors from the 52 people on the Dixmude. 44 of 49 died in the R38 crash. 34 of 43 died in the Roma crash. 48 of 54 died in the R101 crash.

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