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(Before I begin, I would like to assure everyone present that, yes, I'm fully aware of the lunacy of this idea.)

Let us assume this takes place on a parallel Earth sometime in the early 1900s, during the dawn of the great age of airships. You, an aspiring aerostat engineer of great creativity and little self-preservation, have been approached by a organization with (relatively) limitless funding and are told that to create a better airship with better lift than what is previously had by everyone.

Now, there's an issue - you can't just create lift out of nowhere for an airship. Sure, you can keep making the envelope and the gas sacks bigger and bigger, but anyone can do that. Suddenly a flash of inspiration hits you - you can make a better lifting gas than anyone else, all you need to is simply fill it with the lightest lifting gas known to man (hydrogen) and then heat the hydrogen cells. I mean, it works for air, right? And hydrogen is lighter than air, right? So it stands to reason that heated hydrogen is the going to be the greatest lifting gas of all time! ...Once you work out the kinks of figuring out how to heat it without killing everyone onboard, anyway, anyway.

The Challenge: Figure out a way to build an airship that safely* uses super-heated hydrogen cells instead of normal ones. Any technology up to 1920 is considered fair game, and alternate technology that could have been discovered but haven't (i.e. metal alloys) are also permitted. The airship dimensions preferably should be modeled after the LZ 104, though if a smaller airship / larger airship is needed, that's fine.

Now, conventionally, this is probably considered to be a "foolish" idea given hydrogen's incredibly flammable (and explosive) properties, but in theory, this should be fully possible, even with early 1900s technology. Now, some people would argue to use helium instead, but those people clearly don't understand science - we aren't trying to be safe and go for second best. The only acceptable solution here involves heated hydrogen cells.

Some people would also point out that the increase of lift would be marginal at best (probably no greater than 3% over normal hydrogen under normal conditions, this depends heavily on how much you can heat the hydrogen) and that whatever methods used to heat the hydrogen would consume all that weight, and to that I respond that it just sounds like an engineering challenge to me.

*For some, but not all degrees of safe. Ultimately, you're flying an airship while attempting to heat giant sacks of hydrogen. Things can go horribly wrong, but the goal here is a design that you can be reasonably sure won't spontaneously ignite.

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    $\begingroup$ Three cheers for your humor and thorough defense of the noble endeavor of inventing a Very Intelligent Bad Idea. Your foolhardy engineer sounds like the beginning of an excellent story! $\endgroup$
    – BoomChuck
    Commented May 31, 2022 at 4:17
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    $\begingroup$ I'm a Lunatic and I endorse this idea! 😜 $\endgroup$ Commented May 31, 2022 at 14:18
  • $\begingroup$ It would probably be easier to invent a lighter and stronger material for the gas-bags because, of all disciplines, Materials Science has a very high density of "could've been discovered earlier with luck"-inventions. A surprising amount of it was discovered by brute force or chance and if someone stumbled upon a way to produce a lot of metallised film in a feasible manner instead of having to slaughter hundreds of thousands of cows to get enough intestine for a single airship, that's already a fantastic technological leap, $\endgroup$
    – Dragongeek
    Commented May 31, 2022 at 17:15
  • $\begingroup$ Also, should be noted that a pure hydrogen environment does not burn. If you lit a flare that provided it's own oxidizer inside a pure hydrogen environment, it would probably be fine. $\endgroup$
    – Dragongeek
    Commented May 31, 2022 at 17:17

5 Answers 5

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I got your hot hydrogen right here!

Roziere Balloon

rozier balloon and roziere

https://balloon.hu/new/balloons/roziere-balloon/

The Rozière balloon (or simply Rozière) is a type of hybrid balloon that has separate chambers for a non-heated lifting gas (such as hydrogen or helium) as well as a heated lifting gas (as used in a hot air balloon or Montgolfière). This type of aircraft takes its name from its creator, Jean-François Pilâtre de Rozier.

The advantage of a Rozière is that it allows some control of buoyancy with much less use of fuel than a typical hot air balloon. This reduction in fuel consumption has allowed Rozière balloons and their crew to achieve very long flight times, up to several days or even weeks.

It makes sense. There was a problem with Roziere's balloon, chiefly that he died when it crashed and so I guess other people were chicken to try again. People make them today with helium.

Your character can surely make them. I like the idea of heating the hydrogen internally with a resistance wire powered by a handcrank that the pilot turns. Your character cramps up at the prospect of physical labor (and also the prospect of falling from great heights) and so hires out the job to pair of brawny pilots.

More reading along more speculative lines and a scheme by the inimitable bungston... https://www.halfbakery.com/idea/Internal_20flame_20hydrogen_20balloon#1232972746


addendum - balloon schematics. The Roziere does not heat the lift gas directly but heat is conducted to the lift gas balloon from the external hot air balloon.

schematic

https://twitter.com/balloonarchives/status/1348562796674502656

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  • $\begingroup$ Roziere balloons use heated air and non-heated lifting gas, so it's not exactly what I'm looking for. I do like the idea of a resistance wire heating element, but it's liable to trigger hydrogen's flashpoint, so that seems like an accident waiting to happen. $\endgroup$
    – Halfthawed
    Commented May 30, 2022 at 18:36
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    $\begingroup$ @Halfthawed - the heated air is how they heated the lifting gas. The gas was not heated directly but it still heats up. Schematic attached. $\endgroup$
    – Willk
    Commented May 30, 2022 at 19:18
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    $\begingroup$ Hydrogen in very high concentrations is not flammable, it also can diffuse very very quickly, so in some ways the risk of heating the hydrogen may be less than first thought. The Roziere approach the hot air might also hear the hydrogen bags giving more lift. The bags however will need to expand with the heat or there would be no additional lift. It can also be hard to contain hydrogen. But heating pure hydrogen with a hot wire should be ok unless there is a leak… $\endgroup$
    – UVphoton
    Commented May 30, 2022 at 19:22
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Oxygenation

Burning things requires 3 things. A material to burn. Sufficient heat for that material to readily start a reaction with oxygen. Finally you need oxygen. The fire triangle. You have two out of three, namely heat and a material that can start an oxygen reaction. Now we just need to prevent oxygen from touching the heated material and no fire reaction can start!

That means that if we contain the hydrogen with no or too few oxygen, we can heat it to very high temperatures without starting a fire.

Of course there is the crux. How do we store the heated hydrogen safely inside the cells? Short answer is we can't. It would have been used if we could. But what gets us close enough?

The problem with hydrogen is that it's the smallest element in existence. It is hard to contain even with modern materials. In 1920 it'll get out anyway. It doesn't go terribly fast, but if heated hydrogen ignites the moment it leaves the containment and touches oxygen you'll have a fireball. The envelope would heat and deteriorate, giving more and more chance for hydrogen to escape and ignite.

What we can do is compartmentalise. Make a heated hydrogen cell. Surround the hydrogen cell with a non flammable material that is still light enough. A thin layer of helium for example. This allows for some cooling of the hydrogen if it escapes the first layer and if it finally diffuses to the outside it can be so little it's not a problem anymore. Maybe you can pressurise the helium enough that it becomes very difficult for hydrogen to leave the internal cell as well, but might defeat the purpose (being light).

The biggest problem is that such technique is most likely offsetting your gains of heating the gas. You need extra stuff to make it safe. However, you might be able to sell it as a more safe alternative. Making multiple compartments for the gas is a good practice anyway, so compared to a similarly structured airship you have an advantage. Damages and ruptures will not instantly damage your hydrogen cells, so safer! We'll campaign so hard the population doesn't realise that if any rupture occurs they are this much closer to heated hydrogen touching oxygen.

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Your engineer needs an indirect heating element. That is, a thermal conductor which is itself not flammable, and can heat the hydrogen in a diffuse way so that no localized part of the hydrogen reaches ignition temperature. Willk's answer is correct in that, but it seems that using a significant volume of regular heated air is distastefully conventional for such an avant-garde and reckless inventor.

Hot oil heaters and steam heating systems are a couple conventional examples of such thermal conductors, but would add significant weight. Perhaps thin sheets of a thermally conductive solid within the cells could act like the fins of a processor heat sink. Copper would be the best choice because silver doesn't grow on trees. Beryllium oxide deserves a notable mention for being both highly thermally conductive and electrically insulating (non-sparking for safety!), with bonus idiot points for being carcinogenic when crumbled and generally poisonous.

My best recommendation is that your engineer concede to using a minimal amount of helium, heated and circulated through a narrowly enveloping balloon outside the hydrogen cells, and/or a web of tubing within the cells. As a side benefit, this design will also mitigate the loss of hydrogen, which more readily diffuses through materials - at the heating location, any hydrogen that has diffused into the helium will combust with oxygen impurities and further heat the helium.

Side notes: Inducing a convection current in the hydrogen would improve heat diffusion as well as provide another wonderful point of failure. And finally, with this level of ingeniousitude, it behooves one to at least attempt burning some of the hydrogen to heat the hydrogen. Bringing other heavy fuel along would be pointless! What could go wrong?

Sources:

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I'm going to build off of BoomChuck's answer. They note that a steam heating system would add considerable weight, which of course is true. However, the LZ 104 used water-cooled engines. We still need to carry those radiators somewhere, right? What if we put them inside the hydrogen cells?

I would assume that airship engines of the time used non-pressurized cooling systems; but I'm not aware of a technical limitation that forces that to be the case, since there clearly were pressurized steam systems at the time. (I'm invoking the "alternate technology that could have been discovered" clause here.) If we seal and pressurize the cooling systems, we can run that cooling water over 100°C.

As far as practical (hah!) considerations go, the distributed radiator system would certainly be heavier than a conventional radiator, but would allow us to reduce or eliminate the conventional cooling system. (Let's just eliminate it entirely, because that gives us the great story opportunity of potentially having to decide whether to reduce engine power or overheat the hydrogen.) In addition to the fact that the piping would have to run the entire length of the Zeppelin, you'd also probably want some sort of valving so you could increase or decrease the heat in individual cells, as well as being able to shut off flow entirely if a pipe or radiator broke. (...assuming that spraying steam all over the inside of your hydrogen cell somehow didn't just send you to a flaming death anyway.)

You'd definitely have some engineering questions as far as how quickly the heat dissipated given the large surface area of the Zeppelin. It's possible that the cooling water alone wouldn't be enough to heat things up sufficiently. No problem! We have another heat source with us, after all: I mean, that engine exhaust has to go somewhere, right? In fact, let's just use it anyway—the more heat, the better!

In case you're wondering if I'm suggesting what it sounds like I'm suggesting, the answer is yes: take the cooling water from all five engines, superheat it with the engine exhaust, then run that superheated water through radiators in each of the hydrogen cells, where we hopefully dissipate enough heat to keep the engines from overheating when we run that water back into them. (...thus drastically increasing number of points of failure and potential ways to die, all for a maybe 10% lift increase that is partially offset by the weight penalty for the whole system.) What could possibly go wrong?

Edit: Halfthawed's comment made me curious, so I did some searching and found some numbers, so let's try some back-of-the-envelope calculations.

An engine with a liquid-cooled exhaust manifold produces about 645W/hp; the LZ 104's engines were rated for 240hp; so about 154,800W per engine or 774,000W total for the five engines. We want to maintain a temperature of 100°C, so let's call that 85°C warmer than the ambient air (cruise altitude for a Zeppelin was only around 200 meters, so you're close to sea-level temperatures). LZ 104 was 23.9m diameter, 226.5m long; let's approximate it as a cylinder, so we have 17,000m2 surface area. (An overestimate, of course, but close enough for the accuracy we need here.) To maintain the 85°C difference across 17,000m2 of surface area, we need an (SI) R-value of 1.87, which is about an inch-pound R-value of 10.6, so to maintain temperature we need to wrap the whole Zeppelin (or, more accurately, its hydrogen cells) in R-11 fiberglass. Modern R-13 fiberglass is about 1kg/m2, which is a nice round number, so let's just use that figure... and we need 17,000kg of fiberglass insulation in order to gain less than 2400kg of lift. Looks like we do, indeed, need moar boosters.

(BTW, I'm once again invoking the "technology that could have been discovered" clause; fiberglass was discovered by accident in 1932 when a jet of compressed air was directed at a stream of molten glass, so there's no fundamental reason that couldn't have happened a few decades earlier.)

I have to wonder, though—could we heat the hydrogen less and still produce a usable amount of additional lift? Maybe only a 35°C temperature difference? That would still be ~1000kg of extra lift, and would require a low-enough amount of insulation that the basic construction of the Zeppelin might suffice. (After all, you don't have perfect conduction from the hydrogen cells to the skin.) Also, the figure I used for engine cooling was for a liquid-cooled exhaust manifold, not for a design intentionally trying to capture as much heat from the exhaust as possible, so it's possible we could push the heat output of each engine a little higher. (Ultimately, though, the total heat you're getting out of each engine is the amount of energy in the fuel burned, and some of that energy is going to, y'know, actually moving the thing.)

One more complicating factor arose while doing research for this. It turns out that actual dirigibles would try to limit their heating because otherwise the gas cells would exceed their pressure limits—there were safety valves for that. I have to wonder just how much heavier the cells would need to be to hold the higher-temperature, higher-pressure gas. I wouldn't even know where to start to calculate that, though.

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    $\begingroup$ Of course, if I were actually a reasonable engineer, I'd make sure to have supplemental external radiators. On the other hand, if I were a reasonable engineer, I'd say "Oh hell no" and run away from this project as fast as possible the moment I heard about it. $\endgroup$ Commented May 31, 2022 at 18:08
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    $\begingroup$ The only problem I see with this answer is that I don't think that zeppelins carry enough engines to produce sufficient thermal heat to use to heat an entire ship's worth of hydrogen gas - typically zeppelins carried between four to six engines. Of course, you could 'solve' this problem by adding more engines, and use them as turbines to add even more lift. (Who says 'moar boosters' doesn't solve every problem?) $\endgroup$
    – Halfthawed
    Commented May 31, 2022 at 21:28
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    $\begingroup$ @Halfthawed, I approve of the Kerbalness of adding moar boosters! To address your point, though, I think it really comes down to how quickly heat dissipates from the hydrogen cells, and whether you could reach a steady state, and that would require more information on Zeppelin construction than I have. I did figure you'd almost certainly need to run up the engines on the ground well ahead of takeoff in order to heat the hydrogen, but that's ok—you can refill all the fuel you use doing that before launch. (Alternatively, have an external heat source for the initial heating.) $\endgroup$ Commented May 31, 2022 at 22:40
  • $\begingroup$ Incidentally, I have since realized that the whole point here would be that the gas wouldn't be at higher pressure; the reason to heat it is to decrease the amount of gas required to fill the volume of the cells at atmospheric pressure. That leads to the issue, though, of the cells being below atmospheric pressure when heat is removed, meaning they need to accommodate that. Maybe you could have a bladder inside the cells that is open to atmosphere, which would inflate to take up the lost volume of the cooled hydrogen and deflate as the hydrogen heated up and displaced it. $\endgroup$ Commented Sep 7, 2022 at 23:51
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There would be no significant difference in lift.

There was a question just asked on a vacuum blimp, Would a "Ridged" balloon "filled" with a vacuum work better than a hydrogen balloon in an earth atmosphere?

A vacuum gives 88 milligrams/litre better lift.

For hydrogen at 0°C to 100°C, (273 kelvin to 373 kelvin), the mass of 1 litre of hydrogen would decrease by 100/373, or 26%.

1 mole of a gas occupies 22.7 litres at 0°C. One mole is the molecular mass in grams. Thus one mole of hydrogen weighs 2 grams, and one mole of air is (28 * 0.8 + 32 * 0.2) = 28.8 grams. Therefore a hydrogen blimp at 0°C gives 26.8 grams/22.7litres lift (1.18 grams/litre) and a hydrogen at 100°C blimp gives 26 grams/22.7 litres (1.14 grams/litre), a difference of 35 milligrams/litre.

https://en.wikipedia.org/wiki/Molar_volume#Ideal_gases

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    $\begingroup$ Are you sure about the "no significant difference" part? Sure, 35mg/L isn't much... unless you're talking about a Zeppelin with 68,470,000 liters of capacity, which we are! 68,470,000L * 0.035g/L = 2396450g = 2396.45kg, right? Zeppelins were BIG. $\endgroup$ Commented May 31, 2022 at 16:57

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