I'm super passionate of space habitats and have done extensive reading about them. This is not a habitat in the real sense of the word. It provides a more spacious environment for the ISS. Therefore, it's just an extension to the already existing science laboratory (not habitat).
For humans, a habitat should replicate earth's environment. Gravity is the first thing that comes to mind, but there are other things it should provide.
I hope in the near future we work on actually building/sending habitats in space with the goal of actually living there, and not only for scientific research.
Development of inflatable volumes is a reasonable step towards us becoming a spacegoing species. If you have a decent bag and enough air to pressurise it, you can have as much room as you want in space. There's no external pressure working to collapse it.
Of course, there are questions to answer about safety, shielding against debris and radiation protection, but that's the point of doing the research. I love this kind of stuff. It serves to remind people that space travel isn't just launch vehicles. Part of, as you say, providing a place in space for humans to live, is providing spacecraft that aren't utterly cramped.
Inflatable habitats actually scale worse than ridged structures. The problem is as you inflate a structure the air pressure is constant but the surface area increases. And you must carry the load around the edges.
The advantage is actually in mid-sized structures where you can more easily compact them for transport where rigid structures are sent up in a single piece for strength.
PS: This is also why you need buttressing around above ground pools. For comparison 1ATM = 33.9 feet of water. Above ground pools rarely go above 1/3 ATM at there base. Granted, they are also not made out of Kevlar, but the same basic problems apply in space.
Interesting. I would expect that both air pressure and the tensile strength of your enclosure would scale directly in proportion with the area of the enclosure.
But some thought experiments about enormous pools - ponds, really - make me think that you're correct.
Indeed, looking it up, in a sphere, the stress (in units of pressure) is equal to (pressure x radius) / (2 X thickness). In a cylinder, it's worse - axial stress is the same as a sphere, but hoop stress is (pressure x radius) / (thickness)! You also have to think about increased stretching in the hoop direction compared to the ends of a flat-plate cylinder, which should probably be a sphere or complicated ellipsoidal shape to best deal with the stress.
Applying these equations, an ideal steel (assume an alloy with tensile strength 700 MPa) sphere with 1cm thick walls would have a maximum radius of 138.2m. A cylinder would have half the radius. Using Kevlar (tensile strength 3620 MPa) increases the radius by 3.62.
Doubling the thickness doubles the allowable radius but also increases the mass of our sphere from an already staggering 136 metric tons (3 Falcon Heavy payoloads) to an astonishing 546 metric tons, or more than 10 Falcons Heavy.
Kevlar is, of course, lighter than steel by about 5 times, but that many tons of Kevlar will not be cheap. That's about 0.5% of the total world annual production of the stuff!
We won't be going up to inflatable planets anytime soon.
You could have multiple membranes wrapping one another, each with a lower pressure than the last until vacuum. That way you substantially reduce pressure forces, and improve ballistic resistance.
Make it big enough, and vaguely cylindrical, and you could spin it up to provide simulayed gravity - you'd just need internal webbing to prevent it bulging from centripetal forces.
This all adds mass of course, but it still be lighter and more compact than an equivalent rigid structure.
Your mistake is in assuming we're simply optimizing for volume and nothing else.
Factor in each of size, cost, and deployment complexity, and it's clear many feel that inflatable structures represent huge potential wins.
Besides, it's not like you actually need to build single, huge volumes. Space-based habitats built of multiple, smaller linked structures makes more sense, anyway, as it enables module isolation in case of failure.
We are still thinking in terms of tiny structures. Picture a space station with 100,000+ people.
From a safty standpoint you want modular structures. But, you also want to minimize the areas directly linked to space. Thus you end up with something like a nuclear submarine vs an ever expanding ant hill.
Remember apartment builds give every room a view, but that's high risk in space. You can also reuse walls, between areas to cut down on mass without compromising if there is a leak.
It should also be noted that this will not be actually inhabited by the resident astronauts - it contains a heap of sensors to monitor retention of pressure and atmosphere etc, and astronauts will enter it a handful of times (wikipedia suggested around 30-40 times during its mission last time I looked) - so it's just laying the groundwork for a potential further attachment in the future
Then what is the point of having the habitat in space in the first place? Should we block off the windows, too, and replace them with posters of the side of some terrestrial apartment? Hamstring the solar panels because the atmosphere would block a lot of energy?
Living in space should be different than life on earth. Humans need some things to be similar to survive, so give them those things - possibly including some time in gravity - but we don't need a few more square feet of living space equivalent to a vacant lot.
I agree that living in space does not need to perfectly replicate the feeling of living on Earth. Humans should be supplied with the things they need to live and gravity might be one of those things, but only when it comes to long term health concerns.
On the other hand, the word "habitat" literally means the natural home of an animal or plant, and a gravity-free inflatable balloon in space is about as far from the natural living space of a human. It's going to be a long, long time before anyone can recreate Earth-like conditions in space.
Living in zero gravity tends to be very bad for long term health because bone and muscle mass start disappearing.
Radiation shielding is a huge problem. Inflatable habs provide almost none, so they're dangerous in LEO and potentially fatal further out.
And then there's danger from micro-debris.
So inflatables could be good for short trips - space hotels, especially - and for extra storage. But they're really not a solution for long-term occupation.
How about inflating them with water, and then living inside another air inflated bubble inside? Like a giant cell. Radiation shielding, micrometeorite shielding, plus water is pretty important too. Other gear can float inside the water behind their own membranes like organelles, some use the water to radiate heat away, some use water for other things like making macronutrients (carbs, fibre, protein, & fat), or processing waste.
I've seen people propose this before, and it's always bothered me, because when they say they're "storing" water it suggests the water will be there until the needed for some other purpose.
But if you're using water for radiation shielding and you then dedicate it to some other purpose, you no longer have radiation shielding.
That's only the cost if you use disposable rockets. About the same cost/kg of flying if you're throwing away the 777 after one flight. Reusable rockets are magnitudes cheaper per launch.
Or there's robotic in-situ water mining - Ceres has a shallow gravity well and probably has as much water as Earth. Easy? No, but space is always going to be hard.
Probably not what you had in mind, but I found Neal Stephenson's novel Seveneves to be very thought provoking on the subject. I had never thought of inflatables before I read that.
For humans, a habitat should replicate earth's environment. Gravity is the first thing that comes to mind, but there are other things it should provide.
I hope in the near future we work on actually building/sending habitats in space with the goal of actually living there, and not only for scientific research.