# Why no vacuum or hydrogen based airships?
**URL:** https://boards.straightdope.com/t/why-no-vacuum-or-hydrogen-based-airships/763811
**Category:** Factual Questions
**Created:** [August 24, 2016, 2:38pm UTC](https://boards.straightdope.com/t/why-no-vacuum-or-hydrogen-based-airships/763811 "2016-08-24T14:38:10Z")
**Posts on this page:** 20
**Page:** 14
### Author:  [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)
#### Post date: [August 30, 2016, 5:26am UTC](https://boards.straightdope.com/t/why-no-vacuum-or-hydrogen-based-airships/763811/261 "2016-08-30T05:26:21Z")
> [@Dr.Strangelove](#):
>
> Not quite true. See my post above. Your material needs a compressive strength/density of \>~124 kJ/kg. Plenty of common materials achieve this–a decent aluminum alloy might reach 600 MPa/2700 kg/m[sup]3[/sup] = 222 kJ/kg. Obviously, something like diamond is much better…
Scores of “specific strength” (strength/density) charts/graphics for different material types, individually and in groups (woods, ceramics, etc.) with some lay commentary.
Good page.
[http://www-materials.eng.cam.ac.uk/mpsite/interactive\_charts/strength-density/basic.html](http://www-materials.eng.cam.ac.uk/mpsite/interactive_charts/strength-density/basic.html)
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### Author:  [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)
#### Post date: [August 30, 2016, 5:36am UTC](https://boards.straightdope.com/t/why-no-vacuum-or-hydrogen-based-airships/763811/262 "2016-08-30T05:36:21Z")
> [@Leo\_Bloom](#):
>
> Scores of “specific strength” (strength/density) charts/graphics for different material types, individually and in groups (woods, ceramics, etc.) with some lay commentary.
Thank you! I knew there had to be a name for that property, but couldn’t remember/find it. “Specific strength”–good to know.
Granted, we want to know compressive and not tensile strength here. They tend to be about the same for metals but certainly not for ceramics, etc.
I see from the [Wiki page](https://en.wikipedia.org/wiki/Specific_strength) that some people want to name the unit the “Yuri” due to its high relevance to space tether applications.
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### Author:  [@Tired\_and\_Cranky](https://boards.straightdope.com/u/Tired_and_Cranky)
#### Post date: [August 30, 2016, 11:30am UTC](https://boards.straightdope.com/t/why-no-vacuum-or-hydrogen-based-airships/763811/263 "2016-08-30T11:30:49Z")
> [@Scylla](#):
>
> If I miscalculated in post 79 and it’s only 33 pounds per 1% reduction, than it’s clearly a nonstarter. I need to see where Tired and Cranky and I diverge, to see which of us is right. I took the volume of the blimp’s envelope directly from the Goodyear website.
I didn’t recalculate your numbers but your assumption in post 79 is patently wrong. For your weight reduction assumption, you use the density of air. The Goodyear Lightfoot One is filled with helium. If it were filled with cold air like you assume, it would never get off the ground. Each cubic foot of helium only weighs roughly 14% as much as air, so you start off with an assumption that is roughly 7 times too generous to your idea. Second, in that post, you assume a reduction in pressure of 20% instead of 1%. That is a more generous reduction than you assumed elsewhere or that I assumed in my post. If you want to bear negative pressure of 20% of atmospheric pressure, you will need a structure that is 20 times stronger and thus will be 20 times heavier. That is just as hard as the apparently impossible 1%. It means you need a 660 pound structure that supports something like 13 million pounds of pressure. I’m doing this on a tablet so I may not have remembered the numbers exactly but that’s the gist of it.
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### Author:  [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)
#### Post date: [August 30, 2016, 12:06pm UTC](https://boards.straightdope.com/t/why-no-vacuum-or-hydrogen-based-airships/763811/264 "2016-08-30T12:06:52Z")
> [@Chronos](#):
>
> …
> But the best we can do in that direction, with current tech, is those graphene aerogels that **Francis Vaughan** already looked into, which were still only 2% of the needed strength.
Carbon nanogels.
> **[Carbon nanofoam](https://en.m.wikipedia.org/wiki/Carbon_nanofoam)**
>
> Carbon nanofoam is an allotrope of carbon discovered in 1997 by Andrei V. Rode and co-workers at the Australian National University in Canberra. It consists of a cluster-assembly of carbon atoms strung together in a loose three-dimensional web. The fractal-like bond structure consists of sp2 graphite-like clusters connected by sp3 bonds. The sp3 bonds are located mostly on the surface of the structure and make up 15% to 45% of the material, making its framework similar to diamond-like carbon f Ea...
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### Author:  [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)
#### Post date: [August 30, 2016, 12:37pm UTC](https://boards.straightdope.com/t/why-no-vacuum-or-hydrogen-based-airships/763811/265 "2016-08-30T12:37:55Z")
> [@LSLGuy](#):
>
> …
> My thought process went like this: according to our dear misguided OP vacuum is lighter and therefore better than helium as a lifting medium. So given that, obviously something even lighter than vacuum would be even better. …
> [@Scylla](#):
>
> So, vacuum is not lighter than helium? All other things being equal it would not be a better lifting medium?
>
> You condescend and say I’m misguided. Guide me correctly. Which of the above statements is untrue?
> [@watchwolf49](#):
>
> It’s misguided to think a vacuum is **better** than Helium … since all other thing are profoundly _ **not** _ equal … find your magical material and we can then figure if it will work … using diamonds may not be cost-effective …
@ **Scylla** : **watchwolf49** quoted here covered my POV almost perfectly.
Your first sentence above is absolutely factually true. But …
Your second sentence jumps from “lighter” to “better”, with the caveat that “all other things being equal.” Which introduces judgment about what criteria apply to arrive at “better” and what exactly are “all other things”. With no explanation or justification for the jump. And that’s the spot where “and then a miracle occurs” jumps in. “And then a miracle occurs” is fine marketing. It’s crappy engineering.
Through 6 pages now you’ve alternated between:
A) Steadfastly ignoring the tradeoff in envelope structural weight to support any negative pressure, much less ~15 psi.
B) Arm-waving it away using your math-free intuition that the issue is relevant but readily surmountable (ref **Francis Vaughn** a few posts above here).
and
C) Conceding that it would require unobtanium at least using 21st Century tech.
Note I chose the word “misguided”. I mused on that word choice for some time. I didn’t say ignorant, etc. Because IMO you’re not.
To persist in this intellectual bobbing and weaving is misguided as I understand the term. About 150 posts by various folks have tried in various ways to provide guidance of varying quality and relevance. You’ve shaken off almost all of it.
I don’t have the structural / materials engineering chops to argue the details. But I respect the expertise of those who do. This isn’t a situation where the deal almost closes and we’re just a few percent short of having the materials to build this thing. Instead folks I trust say we’re short by several orders of magnitude. And that’s before we consider the cost.
My suggestion for anti-gravity gas was clearly flippant. If you feel insulted I’m sorry; that wasn’t my intent.
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### Author:  [@Manlob](https://boards.straightdope.com/u/Manlob)
#### Post date: [August 30, 2016, 1:36pm UTC](https://boards.straightdope.com/t/why-no-vacuum-or-hydrogen-based-airships/763811/266 "2016-08-30T13:36:53Z")
Hexply IM7/8552 carbon fiber composite has density of 0.0574 lb/in^3. With a quasi-isotropic layup, the open hole compression strength is 48900 psi. Compressive elastic modulus is roughly 7 or 8 million psi.
If air has density of 4.42e-5 lb/in^3 and 14.7 psi air pressure, using formula others have given earlier for a sphere, that gives strength needs to be 28600 psi, which is less than the open hole compression strength of 48900 psi.
Using the buckling formula others have given earlier (the conservative one with buckling load knocked down by factor of 5 from ideal case), gives buckling pressure on the order of 0.1 psi
for a sphere. There is enough margin on compressive strength to add about 14 times as much foam as the thickness of the composite (say FR-4300 which is 1/20th the density of the composite) to form a sandwich wall. That would increase bending stiffness of the shell by a factor of over 500, raising buckling pressure to over 50 psi.
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [August 30, 2016, 1:46pm UTC](https://boards.straightdope.com/t/why-no-vacuum-or-hydrogen-based-airships/763811/267 "2016-08-30T13:46:28Z")
**Leo Bloom** , that’s exactly what **Francis Vaughan** was talking about (and which he found was insufficient).
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### Author:  [@Pleonast](https://boards.straightdope.com/u/Pleonast)
#### Post date: [August 30, 2016, 1:53pm UTC](https://boards.straightdope.com/t/why-no-vacuum-or-hydrogen-based-airships/763811/268 "2016-08-30T13:53:50Z")
It seems to me a better approach to making a more buoyant structure is to fill it with something lighter than H[sub]2[/sub].
The obvious choice would be monatomic H. With the obvious drawback that it’s unstable.
The next choices would be subatomic particles: protons, neutrons or electrons. Each of these have the problem of being unstable and difficult to contain.
I’ll leave it to the engineers to work out the details. 😃
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### Author:  [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)
#### Post date: [August 30, 2016, 2:28pm UTC](https://boards.straightdope.com/t/why-no-vacuum-or-hydrogen-based-airships/763811/269 "2016-08-30T14:28:34Z")
> [@Chronos](#):
>
> **Leo Bloom** , that’s exactly what **Francis Vaughan** was talking about (and which he found was insufficient).
Ok–sorry if he mentioned it. I was on a cell and didn’t want to page through for the original cite.
Fun thread, in that among other things I get to think/learn about exotic materials.
The best from here: “syntactic foam.” Couldn’t imagine a better word for much of _Finnegans Wake_. (Etymology of “syntax” becomes re- and back-formed, ironically enough to its original. A standard Joycean act itself…)
I will introduce this term to the FW cabalists. Just a matter of time before it’s picked up.
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### Author:  [@Scylla](https://boards.straightdope.com/u/Scylla)
#### Post date: [August 30, 2016, 2:47pm UTC](https://boards.straightdope.com/t/why-no-vacuum-or-hydrogen-based-airships/763811/270 "2016-08-30T14:47:04Z")
> [@Pleonast](#):
>
> It seems to me a better approach to making a more buoyant structure is to fill it with something lighter than H[sub]2[/sub].
>
> The obvious choice would be monatomic H. With the obvious drawback that it’s unstable.
>
> The next choices would be subatomic particles: protons, neutrons or electrons. Each of these have the problem of being unstable and difficult to contain.
>
> I’ll leave it to the engineers to work out the details. 😃
We do know that subatomic particles spontaneously create and then annihilate themselves. Such activity could cause pressure in a vacuum, obviating the need to reinforce the vessel. We would simply need to increase the probability of such events within the envelope, to get enough of them, using some sort of field, since these events occurring at any specific time or place are highly improbable.
I’d call it the Infinite Improbability Drive.
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### Author:  [@Scylla](https://boards.straightdope.com/u/Scylla)
#### Post date: [August 30, 2016, 3:31pm UTC](https://boards.straightdope.com/t/why-no-vacuum-or-hydrogen-based-airships/763811/271 "2016-08-30T15:31:02Z")
> [@LSLGuy](#):
>
> @ **Scylla** :
>
> Through 6 pages now you’ve alternated between:
> A) Steadfastly ignoring the tradeoff in envelope structural weight to support any negative pressure, much less ~15 psi.
> B) Arm-waving it away using your math-free intuition that the issue is relevant but readily surmountable (ref **Francis Vaughn** a few posts above here).
> and
> C) Conceding that it would require unobtanium at least using 21st Century tech.
There’s a quote function, and you could use that rather than tell me your interpretation of what you think I’m saying. That would be nice.
A.). I haven’t done that. The first step is to find out how much you could gain, by reducing lifting gas, the second step is to figure out how much weight you have to add to meet net that gain. If the first number is bigger than the second number by a lot, it’s worth looking at, but you have to have that first number.
B.). Again, not cool. “Arm waving” is a derogatory characterization, a dismissal.
C.). That has been apparent and agreed upon since page 1. I’m not sure how you make a complaint out of that.
> [@](#):
>
> Note I chose the word “misguided”. I mused on that word choice for some time. I didn’t say ignorant, etc. Because IMO you’re not.
>
> To persist in this intellectual bobbing and weaving is misguided as I understand the term. About 150 posts by various folks have tried in various ways to provide guidance of varying quality and relevance. You’ve shaken off almost all of it.
There are several different issues being discussed.
1. Can you make a blimp that will lift on negative pressure alone?
2. Can you make a blimp where heated helium and negative pressure lift it in combination.
3. Is there any advantage to trying to reduce the weight of an airship through negative pressure?
4.Can you have a “balloon within a a balloon” and ad adjust the diameter and lift of the inner balloon by heat and or negative pressure within the outer balloon
Unfortunately some people jump in and think we are talking about #1 when we are talking about #3 or #4.
No. I don’t think I’m misguided. I’m trying to respond to people appropriately who are sometimes speaking about different things, or who haven’t bothered or read carefully enough before they feel the need to say “A vacuum ballon doesn’t work,” when that’s not currently under discussion.
Your belittling this effort, by calling me “misguided” or accusing me of “bobbing and weaving” and unfaithfully characterizing my arguments rather than quoting them, isn’t helpful.
[quoteI don’t have the structural / materials engineering chops to argue the details. But I respect the expertise of those who do. This isn’t a situation where the deal almost closes and we’re just a few percent short of having the materials to build this thing. Instead folks I trust say we’re short by several orders of magnitude. And that’s before we consider the cost.[/quote]
I don’t understand it either. Let me clue in you case you don’t get it. I don’t actually think I’ve revolutionized lighter than air travel with a new concept. I walked into this knowing that if it were easy or even workable, somebody would probably be doing it by now. If I just wanted to accept the experts word for it, I needn’t start the thread, as the evidence that it doesn’t work is because nobody is doing it.
However, I just don’t want to accept it blindly, I want to understand why and learn something. As for the “experts,” I don’t think any actual lighter than experts are posting in this thread. There are a lot of knowledgable people, but I don’t think we have any experts. We also have a smattering of folks jumping in to try to make themselves look smart, but who really don’t know what they think they know. There are good objections to things I have been proposing, but not all of the objections being put out there have been good.
Trying to separate this out, and understand it, is the point of this thread. What exactly is “misguided,” about this?
> [@](#):
>
> My suggestion for anti-gravity gas was clearly flippant. If you feel insulted I’m sorry; that wasn’t my intent.
I have no issue with that. It’s calling me “dear misguided op” saying I’m “bobbing and weaving” which implies disingenuousness, and mischaracterizing my statements that I’m unhappy with
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### Author:  [@Scylla](https://boards.straightdope.com/u/Scylla)
#### Post date: [August 30, 2016, 3:37pm UTC](https://boards.straightdope.com/t/why-no-vacuum-or-hydrogen-based-airships/763811/272 "2016-08-30T15:37:10Z")
> [@Tired\_and\_Cranky](#):
>
> I didn’t recalculate your numbers but your assumption in post 79 is patently wrong. For your weight reduction assumption, you use the density of air. The Goodyear Lightfoot One is filled with helium. If it were filled with cold air like you assume, it would never get off the ground. Each cubic foot of helium only weighs roughly 14% as much as air, so you start off with an assumption that is roughly 7 times too generous to your idea. Second, in that post, you assume a reduction in pressure of 20% instead of 1%. That is a more generous reduction than you assumed elsewhere or that I assumed in my post. If you want to bear negative pressure of 20% of atmospheric pressure, you will need a structure that is 20 times stronger and thus will be 20 times heavier. That is just as hard as the apparently impossible 1%. It means you need a 660 pound structure that supports something like 13 million pounds of pressure. I’m doing this on a tablet so I may not have remembered the numbers exactly but that’s the gist of it.
I calculated those numbers using air, when I was thinking about a hot air/negative pressure hybrid. Than several pages later I reached out to grab them mentally when we were speaking of helium. Clearly, they do not apply to helium. My mistake.
You are correct, and thanks for clearing it up. If the weight saving per 1% of negative pressure is only 30 pounds or so, it’s obviously a nonstarter.
Next question. Can we rule it out in a hot air only balloon?
Perhaps there is another gas who’s weight and thermal absorption characteristics are much better. can we rule out the utility of negative pressure in a hot air balloon filled with such a gas?
Even if it it doesn’t work, what gas do we think would come closest?
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### Author:  [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)
#### Post date: [August 30, 2016, 4:19pm UTC](https://boards.straightdope.com/t/why-no-vacuum-or-hydrogen-based-airships/763811/273 "2016-08-30T16:19:33Z")
> [@Scylla](#):
>
> Unfortunately some people jump in and think we are talking about #1 when we are talking about #3 or #4.
I think this is where you are still missing the point.
The idea has no sweet spot - the mass needed to make it work scales with the pressure differential linearly. We talk about #1 (full vacuum) because it is the case everything flows from. If you want 1% of the vacuum, you need 1% of the extra mass of the full vacuum case. If you can’t make the full vacuum version buoyant it follows immediately that there is no partial (even 1%) vacuum version that does anything other than make matters worse.
Other ideas are worth pursuing, but anything at all that involves ideas of lower than ambient pressure in the balloon are a non starter.
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### Author:  [@Scylla](https://boards.straightdope.com/u/Scylla)
#### Post date: [August 30, 2016, 4:42pm UTC](https://boards.straightdope.com/t/why-no-vacuum-or-hydrogen-based-airships/763811/274 "2016-08-30T16:42:56Z")
> [@Francis\_Vaughan](#):
>
> I think this is where you are still missing the point.
>
> The idea has no sweet spot - the mass needed to make it work scales with the pressure differential linearly. We talk about #1 (full vacuum) because it is the case everything flows from. If you want 1% of the vacuum, you need 1% of the extra mass of the full vacuum case. If you can’t make the full vacuum version buoyant it follows immediately that there is no partial (even 1%) vacuum version that does anything other than make matters worse.
>
> Other ideas are worth pursuing, but anything at all that involves ideas of lower than ambient pressure in the balloon are a non starter.
I don’t think that’s correct. It’s almost certainly not linear. To build a vessel that can handle some negative pressure is going to make it inherently different, and almost certainly heavier than a positive pressure envelope (I say almost certainly to proclude the possibility that their is some sort of dark magic geometry or unexpected discovery that makes negative pressure easy.)
So, let’s say you have to add 500 pounds to make something that can handle .0005% negative pressure, basically you’ve just rigidized the balloon. But maybe if you add two carbon fiber cross races that number jumps to .5% but only adds 40 pounds. That’s not linear. Maybe next you spray the outer shell in carbon nanotubes, and that adds 200 pounds, but gets you safely to 2%.
I don’t know, but I don’t think it’s accurate to say it’s a nonstarter for sure. It probably is, but we might be getting close enough with lightweight materials to make it interesting enough to look at.
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [August 30, 2016, 6:24pm UTC](https://boards.straightdope.com/t/why-no-vacuum-or-hydrogen-based-airships/763811/275 "2016-08-30T18:24:49Z")
In the best case, it’s linear. You could of course make something that’s worse in some nonlinear way.
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### Author:  [@Spiderman](https://boards.straightdope.com/u/Spiderman)
#### Post date: [August 31, 2016, 1:24am UTC](https://boards.straightdope.com/t/why-no-vacuum-or-hydrogen-based-airships/763811/276 "2016-08-31T01:24:50Z")
> [@Tired\_and\_Cranky](#):
>
> The energy to run that electric heater doesn’t come from nowhere. It comes from the fuel that you carry on the blimp.
>
> Hot air balloons don’t carry electric generators and then use electric space heaters to heat the balloon because that’s less efficient and would add weight. Hot air balloons burn lightweight fuel to heat their gas. You should probably do the same.
>
> I’m not an engineer so I’m just doing a first order approximation of how much fuel you might need. I’ll let the real engineers here correct my numbers if they are interested.
>
> This site says that a typical hot air balloon has about 90,000 cubic feet of air, or about 2548 cubic meters ([http://socalballoons.org/ballooning/faq/](http://socalballoons.org/ballooning/faq/)) and it needs 30-40 gallons of propane, or roughly 126 to 168 lbs of fuel for a flight. So from what I can tell, this is about the amount of fuel necessary to keep 2548 cubic meters of air at about 100 Celsius. At 100 Celsius, the mass of the air in the hot air balloon is about 2411 kg by my estimate (using the air gas constant from this site [Gases - Specific Heats and Individual Gas Constants](http://www.engineeringtoolbox.com/specific-heat-capacity-gases-d_159.html) and a density of air of 1.225 kg/m^3).
>
> By comparison, the envelope in the Goodyear Lightfoot One contains about 3.3 times more volume of gas. It is also full of helium which has less mass than a comparable volume of air but which has a higher specific heat – that is, it takes more energy to raise the temperature of a kilogram of helium by one degree than it takes to raise a kilogram of air by the same amount. As I noted before, the mass of your hot helium is 1,101 kg. This means mass of the helium in the dirigible is only about 45.7% of the mass of the hot air in the balloon (1,101 kg of helium versus 2411 kg for hot air). But, it takes about 19% more energy to raise the temperature of a kg of helium than it does to raise the temperature of a kilogram of air. In the end, that means you need only about 54% of the energy (fuel) to heat the helium in the dirigible than you need to heat the volume of air in the hot air balloon. So, you need roughly 68 to 91 lbs of fuel for heating the helium (or about 16 to 22 gallons of propane). You also need the burner and the tanks. A 20-gallon titanium tank apparently weighs about 40 lbs empty. Wikipedia says a burner weighs about 50 pounds.
>
> So, with minimum fuel and equipment, you’ve already used up roughly 180 lbs of the theoretical 880 extra pounds that you get to carry. But you’ve created another problem. Hot air balloons can burn their fuel right in the envelope. You can’t because your envelope is filled with helium that won’t burn. So you need to find a way to move the heat from 7 million BTU burner into your helium-filled envelope. That’s more weight, and if the heat transfer process is less than perfectly efficient, more fuel that you would need, adding yet more weight. I don’t have a good idea how you would accomplish the heat transfer. All of the helium heating adds complexity with seemingly modest theoretical benefit. The hot helium idea seems worse and worse the more I look at it and I thought it was terrible to begin with.
Pretty good on your numbers, except your burner size is almost ½ of a 12 million BTU burner.
Also, just a note, flight will take off with 30-40 gallons (depending upon tank configuration) but not use all of that in a flight; one wants to land with a reserve as you can’t be certain where/when you’re going to land.
> [@Scylla](#):
>
> Ok. That one is trounced. The version they used in the Breitling orbiter essentially had a hot air balloon surrounding a helium balloon. Don’t think that would work in a blimp.
> How about this. According to [fueleconomy.gov](http://fueleconomy.gov). 58-62% of an automobile engines efficiency is lost as waste heat. I could not find figures for blimp engines, but assume that there is some correlation.
>
> How about instead of putting the engines outside the gondola, we put them on top of the gondola inside the pressure envelope. We run the air intakes back outside. Next we run the exhaust through a heat exchanger before venting it outside. We run the propellers out on drive lines.
>
> That should add very little to no weight.
>
> Using Kyles converter (found on web) that converts hp to btu I find that 400 hp equal over 1,000,000 btus/hr. If the twin 200 hp engines on the GY blimp are equivalent in their thermal efficiency that means they throw off 1,200,000 btus of waste heat per hour.
>
> Let me try to figure out what that does for us.
Two things you’re forgetting.
1. 12 million BTUs heats things up rather quickly. If one wants to ascend, how long do you think it’ll take your relatively low-temped waste heat to do anything of value?
2. Heat rises. The traditional hot air tear drop or Rozière shape are the ideal shape for maximum hot air. When you get into special shaped balloons, the appendages don’t typically get a lot of hot air in them. Your airship is essentially a special shape in that it’s much longer/wider than it is high. You’re not going to efficiently heat the ends with a single burner. That means multiple burners (with the added weight). I’ve also not seen a remote burner. While it _could_ be done technically how safe would that be? If the pilot light goes out, you can’t relight it, if it sticks, either open or closed, you couldn’t do anything while in flight as there’s no body under it. You could shrink the size of the main cabin/flight deck & build a second one with little weight penalty but I don’t think it’s worth it.
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### Author:  [@Scylla](https://boards.straightdope.com/u/Scylla)
#### Post date: [August 31, 2016, 2:13am UTC](https://boards.straightdope.com/t/why-no-vacuum-or-hydrogen-based-airships/763811/277 "2016-08-31T02:13:38Z")
> [@Spiderman](#):
>
> Pretty good on your numbers, except your burner size is almost ½ of a 12 million BTU burner.
> Also, just a note, flight will take off with 30-40 gallons (depending upon tank configuration) but not use all of that in a flight; one wants to land with a reserve as you can’t be certain where/when you’re going to land.
>
> Two things you’re forgetting.
>
> 1. 12 million BTUs heats things up rather quickly. If one wants to ascend, how long do you think it’ll take your relatively low-temped waste heat to do anything of value?
> 2. Heat rises. The traditional hot air tear drop or Rozière shape are the ideal shape for maximum hot air. When you get into special shaped balloons, the appendages don’t typically get a lot of hot air in them. Your airship is essentially a special shape in that it’s much longer/wider than it is high. You’re not going to efficiently heat the ends with a single burner. That means multiple burners (with the added weight). I’ve also not seen a remote burner. While it _could_ be done technically how safe would that be? If the pilot light goes out, you can’t relight it, if it sticks, either open or closed, you couldn’t do anything while in flight as there’s no body under it. You could shrink the size of the main cabin/flight deck & build a second one with little weight penalty but I don’t think it’s worth it.
You’d use the engine exhaust heat with exchangers, not burners. You can’t burn in a helium environment. I bet the burners would work really well though if your blimp was hydrogen filled.
You make goods points that you’d need more than one exchanger or need to circulate the air to keep things even, and that the normal blimp shape is not the most efficient for a heated design. You’d have to alter the geometry or Ho back to my original balloon within a ballon suggestion to negate this.
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [August 31, 2016, 2:45am UTC](https://boards.straightdope.com/t/why-no-vacuum-or-hydrogen-based-airships/763811/278 "2016-08-31T02:45:21Z")
Your balloon within a balloon design would make it not hard, but impossible, to evenly circulate the hot air.
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### Author:  [@Spiderman](https://boards.straightdope.com/u/Spiderman)
#### Post date: [August 31, 2016, 12:19pm UTC](https://boards.straightdope.com/t/why-no-vacuum-or-hydrogen-based-airships/763811/279 "2016-08-31T12:19:25Z")
> [@Scylla](#):
>
> You’d use the engine exhaust heat with exchangers, not burners. You can’t burn in a helium environment. I bet the burners would work really well though if your blimp was hydrogen filled.
>
> You make goods points that you’d need more than one exchanger or need to circulate the air to keep things even, and that the normal blimp shape is not the most efficient for a heated design. You’d have to alter the geometry or Ho back to my original balloon within a ballon suggestion to negate this.
You didn’t answer my question
> [@Spiderman](#):
>
> 1. 12 million BTUs heats things up rather quickly. If one wants to ascend, how long do you think it’ll take your relatively low-temped waste heat to do anything of value?
Inside of a hot air balloon is roughly 100° hotter than ambient air temp. How long would it take your exhaust gas heater to _meaningfully_ heat the airship? What about cooling to come down at the end of a flight; how do you accomplish that? I think your design fails if the answer is hours.
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### Author:  [@Scylla](https://boards.straightdope.com/u/Scylla)
#### Post date: [August 31, 2016, 12:39pm UTC](https://boards.straightdope.com/t/why-no-vacuum-or-hydrogen-based-airships/763811/280 "2016-08-31T12:39:56Z")
> [@Chronos](#):
>
> Your balloon within a balloon design would make it not hard, but impossible, to evenly circulate the hot air.
Yes, but then the inner balloon can have the classic and more efficient teardrop shape Spider-Man referred to. In any event, it doesn’t have to be perfect or even very good. I think things just need to be mixed up enough to let entropy do the work, like heating a building. It’s never even, but it’s good enough
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