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# Physics/O'Neill Cylinders

Question

ANSWER: Ummmm...you neglected a few things!

What thickness is the material?  What is the atmospheric pressure inside?  What is the mass/unit surface area of human civilization inside that these cylinders are supposed to support?

All of these factors are very important.  Also the type of steel and grade of titanium.  Also the rotation rate, are they supposed to provide 1 standard Earth gravity?  People can live with lower gravity and lower air pressure just fine, or we wouldn't have Denver (the air pressure more than the gravity).  Are you supposed to make assumptions about these parameters?  If you just want the last sentence answered as a question, then you need to answer these questions (thickness, gravitation from spin, pressure, surface density) and choose just one material.

---------- FOLLOW-UP ----------

QUESTION: OK Let's make thickness 5 meters, gravity 1g, pressure 90% of what we have on Earth at sea level and the civilization inside would be 50 million population and they would live in multi story buildings maximum height 300m. I don't know anything about the mass/unit factor you mentioned so if these all figures are needed could you think of some mass/unit of surface area?

There are still so many factors, like what you'd use for the window sections to let sunlight in.   Also you never picked a material at all, so we'll go with the ridiculous choice of titanium because it's lighter and stronger.  You propose to support 300 meters of buildings on only 5m of titanium at 1g.  That alone seems unlikely, even with lightweight materials like carbon fiber you're talking about buildings in the 15x atmosphere range for the pressure that their weight would exert on the surface.  Even with only 0.9 atm and the weight of titanium itself (no people) you get 2 atmospheres from the weight of the spinning titanium and the atmospheric pressure.  Using Barlow's formula for bursting pipes as an approximation and assuming total rupute (ultimate strength of titanium), I get 1.3 km diameter for the rupture diameter of this structure.  This ignores the window sections, which presumably would need to have enough tensile strength to keep the titanium segments from flying apart.  Making the titanium thicker (see the pressure section, above) won't help much, as you'll be adding weight to the walls.  Also, there's only so far you can lower the buildings the people live in and so much mass you can remove before the people can't sustain themselves.  And you need about 8 million metric tons of titanium per kilometer of tube, so I'm not sure where you get the tube material in the first place...or how you get it to space.

In short, leave these to science fiction.  The engineering challenges of our biggest projects in history are staggering...but not staggering on this scale.

Physics

Volunteer

#### Dr. Stephen O. Nelson

##### Expertise

I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.

##### Experience

I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Education/Credentials
Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.