[ExI] trust the fake science?
spike at rainier66.com
spike at rainier66.com
Mon Jun 10 22:42:20 UTC 2024
From: extropy-chat <extropy-chat-bounces at lists.extropy.org> On Behalf Of Adrian Tymes via extropy-chat
Sent: Monday, 10 June, 2024 1:53 PM
To: ExI chat list <extropy-chat at lists.extropy.org>
Cc: Adrian Tymes <atymes at gmail.com>
Subject: Re: [ExI] trust the fake science?
On Mon, Jun 10, 2024 at 1:02 PM spike jones via extropy-chat <extropy-chat at lists.extropy.org <mailto:extropy-chat at lists.extropy.org> > wrote:
The Airlander's shape is not an option for a vacuum balloon, which must be
spherical and enormous because its shell is a structural element under
enormous compression stress.
As Mike said, think outside the balloon. A modern airship has multiple balloons. So would an airship using vacuum balloons.
Consider, for instance, the classic cigar-shaped blimp with four or six vacuum balloons taking up most of the internal volume, the cigar shape being essentially light stiff armor (since the vacuum balloons are under considerable mechanical stress already) and streamlining around the whole set. It would use airscrews, just like modern airships…
Ja, but when I ran the numbers, I came away unconvinced of this idea. Adrian I hope you can make it work, and we are all cheering for you.
Do let us try a thought experiment, which would illustrate. Imagine a spherical tank containing positive pressure inside, kinda the opposite of the vacuum balloon idea, and assume isotropic material for the tank, such as steel. Imagine away any stress concentration factors anywhere (such as by imagining it in orbit.) OK for any given pressure contained in that vessel, a skin thickness can be calculated. So far so good.
Now if we imagine the pressure doubling inside the tank, the skin thickness must also double in order to keep the material stress constant. Make sense?
Now imagine keeping the pressure constant and doubling the tank radius. The volume of the tank increases by a factor of eight and the surface area of the tank increases by a factor of 4. So we could just square/cube our way to being a hero, ja?
YeeeeeeahNo. With those assumptions, when we double the radius of the tank with constant pressure inside, the thickness of the skin must also double. So if we double the radius of the tank, the volume of the tank increases by a factor of 8 but its mass also increases by a factor of 8.
So now the next question: does that cube/cube relationship apply to a negative pressure vessel? Well, not exactly. But in general, it kinda does. We get some bonus with compressive stress, but it is harder to model than tensile strength. Consider for instance tensile/compression testing machine, the kind made by Tinius Olsen for lab use. Of all the tests I have seen in the lab, a general rule is that tensile strength of a number of samples of the same material will produce similar results. But compression tests of the same material are scattered all over the map. I don’t know why that is, but it means that compression stress is harder to model because we need bigger margins. The result is that a vacuum balloon needs to be huge.
Adrian I didn’t post about it because I want to see if you can make it happen.
spike
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