Shellworld

Shellworlds, also known as hollow worlds, shell planets or - when referring to larger versions - mega earths (although this term is more commonly used for a class of massive planet) are a type of megastructure built to house large numbers of sentients in their natural, biological form, as well as to house ecosystems of non-sentient life, in an environment designed to simulate that of a terrestrial planet. Shellworlds are designed to emulate natural planets, but require far less mass to create.

Shellworlds are typically composed of a surface layer of rock, soil, water and living things sitting on an immensely strong spherical shell made of materials capable of holding the surface in place for timescales on the order of billions of years. Beneath the shell is a source of gravity, typically a large amount of mass in the form of a gas giant planet, brown dwarf, white dwarf, neutron star or black hole.

Although artificial gravity can be used, this is undesirable on a planet sized structure since failure of a system based on tractism could result in the sudden release of huge amounts of mass from gravitational pull, with dire consequences.

If the gravity source is large enough, shellworlds can be built inside each other to form multi-shell-worlds, with little decrease in gravity being detectable.

Construction
Shellworlds have the advantage over traditional planets in that they do not contain large amounts of valuable heavy elements in their interior (as gravity can be generated form hydrogen/helium based gas giants, brown dwarfs, stellar remnants or dark matter), and thus can be built en-masse with the same amount of material that would normally be contained in a single planet.

Many thousands of Earth sized shellworlds can be constructed using the material present inside the Planet Earth.

Typically, a terrestrial shellworld consists of a layer of rock and dirt anywhere between 50 meters to 10,000 meters thick. Beneath this is the shell; the component responsible for holding up the immense weight of the megastructure. These are usually composed of incredibly sturdy exotic materials. Magnetic accelerators can be used to generate centrifugal force to hold up the surface, and nectism can be used to artificially increase the toughness of the structure. Tractism based repulsors can also be used to help hold up the surface.

All machine based methods, however, face the issue of continuous power consumption and risk of failure due to mechanical fault or power outage.

Gravity generation
Unlike smaller spacecraft, which use tractism to simulate gravity, or rotating habitats, which use centrifugal force, shellworlds almost always use genuine gravity from a large source of mass. There are several types of mass used.

Planet based
A common approach is to use large planets such as gas giants, ice giants, or dense rocky planets as a source of gravity. In the case of a rocky planet, the material used to build the shellworld itself may be sourced from within the planet, but the shellworld's surface would be positioned away from that of the planet for any number of reasons, including to be the correct distance from the gravitational source or avoid a toxic atmosphere or unstable geological activity. In the case of a gas giant, the pressure from the gas itself would assist in supporting the surface of the shellworld, thus allowing it to be made of lighter, less sturdy materials than otherwise.

Any shellworld built above a planet has the added benefit of being able to mine the planet for resources, use the planet's own magnetic field for defence against stellar wind and potentially use heat from inside the planet as a source of power.

Brown dwarf based
Brown dwarfs are often smaller and denser than gas giant planets but have greater gravity, meaning any shellworld built around them has to be built away from their surface, which negates the possibility to use internal pressure from gas to stabilise the shellworld's surface.

However, brown dwarfs have the added benefit of being far more massive, allowing for the shellworld to be larger and have a far greater surface area with the same amount of gravity. The heat found in the interior of brown dwarfs can also be used to generate power.

Star based
The immense heat released by stars usually makes them impractical for use as a shellworld gravity source. However, if infrastructure is in place to divert that energy to a propulsion system, weapon or other power intensive application, a shellworld can be possible. Generally, this is a dangerous option, and a star's powerful magnetic fields and solar wind release add to this problem. Shellworld built around stars can generally survive without orbiting their own star, since the heat from the interior can keep an atmosphere warm. The internal pressure from a star can also help support the shellworld.

Remnant based
White dwarfs and neutron stars are very compact and thus do not provide any additional support for the shellworld's superstructure. White dwarfs, however, can provide energy due to their latent heat, enough for substantial quantities of photic energy to be harvested but far less than a living star. Neutron stars, in the form of pulsars and magnetars, can be more problematic due to their radiation release and powerful magnetic fields, respectively. The latent rotation of pulsars and magnetars can be used to generate energy, however.

Black hole based
Black holes are ideal gravity generators due to their small size and great stability. They can also be used for the disposal of waste heat or unwanted debris. Black holes can be used to create the very largest and very smallest shellworlds (hawking planets and birch planets).

One disadvantage of black hole based planets is the immense bursts of radiation commonly emitted should any matter fall into them. This issue can be avoided by increasing the size of the black hole. Another issue is the incredible difficulty inherent in recovering any mass energy consumed by the black hole.

Multi-layered shell worlds
These are shellworlds with multiple layers stacked on top of each other. They work best on a large scale, with a large gravity source, since gravity is consistent throughout layers and tidal forces are very low. Large numbers of shells can house more people but can develop a serious problem with waste heat. Multi-layered shell worlds around a black hole can use said object to help dissipate heat.

Size range
The smallest shellworlds are hawking planets, so named due to the fact that they can subsist on hawking radiation from their interior black holes for power. A black hole of this size weighs about 10 billion tonnes. The planet can be as small as 100 meters in diameter, at which point the radiation from the interior becomes too powerful and tidal effects become very noticeable. Such a planet has a surface area of about 7 acres (3 hectares).

The largest shellworlds are birch planets, which are built around supermassive black holes. The largest possible size of a birch planet is one built around a black hole with an event horizon with equal surface gravity to Earth. Such a black hole has a mass 1500 billion solar masses (the mass of an entire galaxy) and a diameter of 1 light year. Beings living on a birch planet experience the pronounced effects of time dilation. However, due to the extreme mass of the object, tidal forces would be extremely small, meaning many thousands of shellworld layers can be built before loss of gravity becomes an issue.