cylinder with convex endcaps?

Concave bases are commonly used in aerosol cans; because it allows the can
to stand up on its own.

The seam is not particularly an issue, it's roughly the same loading as the
convex case.

And nor are normal *materials* weaker in compression than tension, actually
they're very usually stronger in compression than tension (in the case of
concrete, enormously so). Compression tends to close up cracks.

The problem isn't a material one, it's to do with the way the material is
laid out.

Although the concave shape is in principle actually every bit as strong as
the convex one, the big problem is buckling- the concave shape tends to
want to invert, turn back into a convex shape, and if it's thin walled, if
you press on a point, it can suddenly do this, catastrophically.

This is a huge issue at large sizes, where the diameter is very large
compared to the wall thickness. At small sizes like aerosol cans the metal
can just be thick enough it won't invert.

To prevent it doing that at larger sizes, you have to thicken the structure
so it's not simply a thin wall.

As in, not simply making it a lot thicker and heavier, you can even
actually thin the wall material down, but then add girders and supporting
fins or whatever on the outside so that overall there's about the same
amount of material, but the structure has strength against inverting.

And this is done, concave propellant tank endcaps are sometimes done in
rocket propellant tanks, to reduce the length of the tank in the interstage
section, one design is like an egg box.

On 16 May 2014 14:28, 'Combs, Mike' mikecombs@... [spacesettlers] <
spacesettlers@yahoogroups.com> wrote:

>
> I think you're right about the stress at the seams. Another issue is
> that in the existing design, every part of the endcap is in tension. With
> convex endcaps, every part of it would be under compression. And all
> building materials are stronger in tension than under compression.
>
> That's also one of the reasons why designing structures for space habitats
> might always be easier than structures for habitats on the ocean floor.
>
> Regards,
>
> Mike Combs
>
> *From:* spacesettlers@yahoogroups.com [mailto:
> spacesettlers@yahoogroups.com]
> *Sent:* Thursday, May 15, 2014 9:48 PM
> *To:* spacesettlers@yahoogroups.com
> *Subject:* [spacesettlers] cylinder with convex endcaps?
>
> Does it make any sense at all for a pressurized cylinder (space habitat)
> to have hemispherical endcaps that extend into the cylinder, instead of
> out?
>
> I've certainly never seen a balloon shaped that way... it seems like the
> forces trying to burst that shape at the seams would be tremendous. I
> also can't think of much reason such a shape would be particularly
> desirable.
>
> But before I dismiss the possibility entirely, I thought I'd check here.
> Can anybody think of a reason such a shape wouldn't be silly?
>
> Thanks,
> - Joe
>

--
-Ian Woollard

The seam is not particularly an issue, it's roughly the same loading as the convex case.
And nor are normal *materials* weaker in compression than tension, actually they're very usually stronger in compression than tension (in the case of concrete, enormously so). Compression tends to close up cracks.
The problem isn't a material one, it's to do with the way the material is laid out.
Although the concave shape is in principle actually every bit as strong as the convex one, the big problem is buckling- the concave shape tends to want to invert, turn back into a convex shape, and if it's thin walled, if you press on a point, it can suddenly do this, catastrophically.
This is a huge issue at large sizes, where the diameter is very large compared to the wall thickness. At small sizes like aerosol cans the metal can just be thick enough it won't invert.
To prevent it doing that at larger sizes, you have to thicken the structure so it's not simply a thin wall.
As in, not simply making it a lot thicker and heavier, you can even actually thin the wall material down, but then add girders and supporting fins or whatever on the outside so that overall there's about the same amount of material, but the structure has strength against inverting.
And this is done, concave propellant tank endcaps are sometimes done in rocket propellant tanks, to reduce the length of the tank in the interstage section, one design is like an egg box.
On 16 May 2014 14:28, 'Combs, Mike'
mikecombs@...
[spacesettlers]
<
spacesettlers@yahoogroups.com
>
I think you’re right about the stress at the seams. Another issue is that in the existing design, every part of the endcap is in tension. With convex endcaps, every part of it would be under compression. And all building materials are stronger in tension than under compression.

That’s also one of the reasons why designing structures for space habitats might always be easier than structures for habitats on the ocean floor.

Regards,
Mike Combs

From:
spacesettlers@yahoogroups.com
[mailto:
spacesettlers@yahoogroups.com
]
Sent:
Thursday, May 15, 2014 9:48 PM
To:
spacesettlers@yahoogroups.com
Subject:
[spacesettlers] cylinder with convex endcaps?

Does it make any sense at all for a pressurized cylinder (space habitat)
to have hemispherical endcaps that extend into the cylinder, instead of out?
I've certainly never seen a balloon shaped that way... it seems like the
forces trying to burst that shape at the seams would be tremendous. I
also can't think of much reason such a shape would be particularly
desirable.
But before I dismiss the possibility entirely, I thought I'd check here.
Can anybody think of a reason such a shape wouldn't be silly?
Thanks,
- Joe
-Ian Woollard