Several space terms and concepts are abused by NewSpace supporters and used as deceptive P.R. devices. A recent comment from Australia needs to be addressed;
“-(also as far as I aware there’s nothing wrong with calling a Bernal Sphere a space station).”
That’s no space station, it’s a moon. An artificial moon.
Let us start with defining what space itself is. Space, as differentiated from Earth Orbit, should be demarcated at Geostationary Earth Orbit (GEO) 22,236 miles up. NewSpace has pushed much of their business plan for years with propaganda identifying LEO, a couple hundred miles up, as some kind of new frontier. LEO is actually a dead end for Human Space Flight and should not even be classified as such. Comparing LEO to deep space is like comparing a duck pond to the North Atlantic.
Continuing on to spacecraft vs spaceships, what I call the Parker-Dyson-Spudis Continuum is I believe the best guide for understanding the difference between the two. And with the preliminaries out of the way we can now address the subject of space stations. The first problem is that 40 years of LEO space stations have been a tremendous waste of time and resources and proof of why we should abandon LEO as a complete dead end in terms of Human Space Flight. For many years the space station vacation tourist fantasy was NewSpace dogma and has always been a joke to true space enthusiasts. If one goes by certain criteria, the Space Station and the Lunar Cycler are the interesting comparison, since a Shielded Lunar Cycler would be the intermediate step to a true spaceship.
The ISS and those before it are what should be more accurately called “Orbital Platforms” or “OP’s” since they cannot really support long duration missions anywhere except LEO as they lack a cosmic ray shield. A “Space Platform” might have a cosmic ray shield but not artificial gravity and does not have much utility so I doubt they will emerge. A true Space Station would have both a cosmic ray shield and artificial gravity. Shielding and artificial gravity almost certainly equate to lunar water and tether systems as other methods are not very practical. A capable enough propulsion system would make a true Space Station into a Lunar Cycler and a nuclear propulsion system would make it a Spaceship. Such basic practical realities and distinctions need to be understood. A Habitat, as visualized by the space colonization movement of the late 1970’s, is large enough for people to raise families and actually live in indefinitely. The Habitat is thus orders of magnitude larger so tether systems would be replaced by the torus, sphere, or other configurations. In this context calling a Bernal Sphere a Space Station is wrong.
If you look into this NASA study on habitat design you find some mention of material strengths and how strength is a determining factor of size due to artificial gravity stresses. What type of alloy and pattern of construction would be ideal remains to be seen. http://large.stanford.edu/courses/2016/ph240/martelaro2/docs/nasa-sp-413.pdf
In my view the benchmark, after looking at the various concepts, is an artificial moon, a hollow sphere, several miles in diameter, spinning to provide 1G on the inner surface at the equator (a Bernal Sphere). One question of interest is how many miles in diameter? The trick is to provide cosmic ray shielding as well as structural strength, as well as making the structure able to be gradually repaired and likely entirely rebuilt over time so that it will last several centuries or, essentially, as long as structure is being remade over time. These three requirements; 1G, cosmic ray shielding, and gradual regeneration, are the factors to focus on when converting a Habitat design into a Starship design.
A Bernal Sphere converted into a Starship would be accelerated to some percentage of the speed of light out of the solar system by an immense solar powered beam propulsion system. Upon reaching another star several centuries later it would likely use pulse propulsion nuclear devices (bombs) to slow down. Due to the acceleration and deceleration, the more severe interstellar radiation environment, and degradation of structural materials over time, these Habitat/Starships would need to be built to a much higher standard than Bernal Spheres used only as habitats. The smart species would build them to the higher standard from the start.
The missing piece of the puzzle in regards to Star Travel is freezing people. A population reproducing over generations on a “world ship” does not appear desirable and I place it in the category of space elevators as being a lost cause. Suspended animation is, however, not a losing proposition at all since it is a straightforward problem preventing damage to cells from ice formation. We can likely freeze people without damage and revive them so we should start building the infrastructure now to allow missions to other stars to be launched within a century.
Of course being able to freeze people, as I have written about on this blog, is the ultimate game-changer and would be the most significant event in the history of humankind. Besides having no option except to find a cure for aging, suspended animation also means Star Travel because humans will not stop having children. While it has been calculated the resources are available in the asteroid belt to support space colonies in this solar system with a total population in the tens of billions, interstellar migration is of course the better path to plan on from the beginning.