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
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 that some people want to name the unit the “Yuri” due to its high relevance to space tether applications.
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.
Carbon nanogels.
@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.
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.
Leo Bloom, that’s exactly what Francis Vaughan was talking about (and which he found was insufficient).
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. 
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.
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.
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.
There are several different issues being discussed.
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?
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
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?
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.
In the best case, it’s linear. You could of course make something that’s worse in some nonlinear way.
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.
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.
Your balloon within a balloon design would make it not hard, but impossible, to evenly circulate the hot air.
You didn’t answer my question
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.
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