Same physics issue as the 'old' meaning of shell-worlds (wrap a solid surface around an arbitrary point-mass at such a separation to get a 1g surface gravity on the outer surface of the shell): the setup is unstable, the 'contained' mass will drift into the shell wall (or the shell wall will drift into the mass, depending on your point of view) without constant active control and manipulation to keep it centred. On top of that, the ~2x10^18 kg mass makes it uncompetitive in mass-per-unit-habitable-area compared to a rotating station (e.g. Island 3 on the order of 4.5x10^12) until you want to build more than a million stations.
Quote from: edzieba on 12/31/2019 06:47 amSame physics issue as the 'old' meaning of shell-worlds (wrap a solid surface around an arbitrary point-mass at such a separation to get a 1g surface gravity on the outer surface of the shell): the setup is unstable, the 'contained' mass will drift into the shell wall (or the shell wall will drift into the mass, depending on your point of view) without constant active control and manipulation to keep it centred. On top of that, the ~2x10^18 kg mass makes it uncompetitive in mass-per-unit-habitable-area compared to a rotating station (e.g. Island 3 on the order of 4.5x10^12) until you want to build more than a million stations.Mass efficiency is probably not a major concern if you're working on the surface of a planetary body. A more modest air supported structure would be more readily assembled, and is a proven, if out of favor, technology.
You will need to acquire that mass from somewhere & refine it, as well as loft it above your target body. As the 'pressure supported' structure only works once the structure is complete and pressurised, lifting it to altitude either requires enormous amounts of scaffolding structure (to support a planetoid-wide peta-tonne roof several kilometres high), or launching your shell into orbit in sections before joining and despinning once the atmosphere is pressurised. Either are enormous energy and mass expenditures that could also be served launching a fraction of that mass into orbit and assembling into stations instead. This is true even if you;re harvesting the host body for mass, but much moreso of you need to import your mas from another body.
the setup is unstable, the 'contained' mass will drift into the shell wall (or the shell wall will drift into the mass, depending on your point of view) without constant active control and manipulation to keep it centred.
On top of that, the ~2x10^18 kg mass makes it uncompetitive in mass-per-unit-habitable-area compared to a rotating station (e.g. Island 3 on the order of 4.5x10^12) until you want to build more than a million stations.
Digging up, digging out, and back-filling a canyon in Mariner Valley is probably a quicker and easier approach than bootstrapping an atmosphere on the entire planet.
Quote from: RotoSequence on 12/31/2019 04:29 amDigging up, digging out, and back-filling a canyon in Mariner Valley is probably a quicker and easier approach than bootstrapping an atmosphere on the entire planet.It doesn't make sense to do it on a planet (unless the planet doesn't have a usable surface.) You only save part of the required mass of atmosphere, and nothing on the required water, etc.
IMO, the issue with a shell world is the construction. Unlike other types of paraterraforming, or building individual rotating habitats, it doesn't lend itself to natural expansion, it's all or nothing.
That doesn't work this this technique: it relies on the mass of the shell to counter the internal pressure. If you have a dome, its own mass cannot do this (as the dome surface normal is only aligned to local gravity at the peak) so the dome must be built to mechanically resist the pressure, defeating the entire point of the concept and leaving you with a regular old dome.
Quote from: edzieba on 01/03/2020 05:07 pmThat doesn't work this this technique: it relies on the mass of the shell to counter the internal pressure. If you have a dome, its own mass cannot do this (as the dome surface normal is only aligned to local gravity at the peak) so the dome must be built to mechanically resist the pressure, defeating the entire point of the concept and leaving you with a regular old dome.If you have a high aspect ratio dome (that is, much larger in diameter than in height), then gravity can supply the vast majority of the containment force. The radial force does need to be mechanically contained.
Quote from: edzieba on 12/31/2019 06:47 amthe setup is unstable, the 'contained' mass will drift into the shell wall (or the shell wall will drift into the mass, depending on your point of view) without constant active control and manipulation to keep it centred.Incorrect. If the gap between the gravitational mass (in this case, acting as the surface) and the shell is large enough to permit a pressure gradient, then there will be more pressure on low points of the shell than on high points. Given that the gravity on a sphere is uniform, even a slight difference in the pressure will continually centralise the shell (or the contained mass, depending on your point of view.)
The shell would bounce around the planet like on springs and and would create shear stresses with huge component forces. These things are not even in dynamic equilibrium without restoring force
Quote from: Mark K on 01/03/2020 07:24 pmThe shell would bounce around the planet like on springs and and would create shear stresses with huge component forces. These things are not even in dynamic equilibrium without restoring force How is a spring not in dynamic equilibrium, providing a restoring force?