...but remembering that at the least the sphere should contain enough space to do/store/accomodate everything you need it to. So it's minimum size will be governed by its intended uses and duration, rather than engineering ;-)
Plus enough for expansion across its intended lifespan.
Well, yeah. That's why I said "the smallest possible sphere".
Somebody famous once said "If you can get a space-craft into orbit, you're half way to anywhere".
For the moment, at least, the biggest constraint we have is that, on a journey of, possibly, millions of miles, it's the first 60-odd miles, through our atmosphere, that's the awkward part due to aerodynamic drag and the effects of gravity and that's why size and weight are key issues, and why there's no room to swing a cat inside the ISS.
Personally, I reckon one of THE biggest steps we need to be taking in the field of space-exploration is to figure out how to build stuff in orbit.
And, by that, I mean stuff that actually has engines attached and that can actually do stuff once it's built.
Once we start doing that we'll get better at it and we'll only be in a position to explore other planets once we get really good at doing that stuff cos, let's face it, there's no way that a ship that could reach, say, Mars, is ever going to be launched from Earth in one lump.
Well, technically we could do that now....in that we could loft pressurize-able tankage modules - with engines attached - and put a modular vehicle together in orbit. It's what they SHOULD be doing for Mars...but it's Nasa, you just know they'll try to do it in one jump...
....or at least, as with Apollo, at most a single change of attitude in Earth orbit to put the lander in the right place on the Earth-Mars stack.
After all - don't forget the whole ISS is "dirigible" It has a small amount of tankage for propellant gases for the station's thrusters, which are used periodically to raise its orbit. And the ISS has been designed to withstand the stresses of this manouvering.
The structure of a space-station does have to be self-supporting, against internal stresses.
You don't need to worry about gravity so much in space. That's why a pontoon bridge was a suitable analogy because its own weight is not an issue. Also the pressure at each point is localised.
But with both pontoon bridges and space-station shells, if you want to manoeuvre the whole thing by applying force at a small number of points, then you have worry about how that force is distributed through the whole structure.
In space at least, you have the choice of applying very low acceleration to minimise the forces. Although you still have to worry about some astronaut putting his foot through the outer wall, you can't get away with too much.
However, consider that an aircraft fuselage has a skin only 1/8" or so thick, and is also pressurised. That seems to work reasonably well, with tremendous forces during take-off and landing, and not many people manage to kick through it either.
Problem is, IMO, that we've kind of "abused" all these wind-up toys that we're currently so fond of.
I've got nothing against them, in-particular, but I can't help thinking that every time one of these probes lands on some other planet and reports back that there's rocks and dust, somebody crosses that planet off a list and puts their chequebook back in their pocket.
What we really need is for one of these probes to find diamonds the size of coconuts (or, more usefully, some kind of hydrogen or electricity-producing rock, perhaps?) lying around on the surface of Titan or something.
I bet that'd get things moving again.
ISS also uses multiple gyroscopes spinning to help maintain orbit.
Many satellites such as those used for tv have tiny rocket motors to maintain and adjust position - these can be powerfull enough to move them to a different position in orbit, such as when a satellite gets replaced and is placed in a parking orbit.
Similarly, the now obsolete space shuttle had little puffer jets which they used to alter the pos of the shuttle while on orbit. Because of the mico g or zero g environment, it doesn't actually need massive rockets for these kinds of small changes. Leaving Earth orbit for the moon or other planet is a whole different story though.
You don't need to worry about gravity so much in space. That's why a pontoon bridge was a suitable analogy because its own weight is not an issue. Also the pressure at each point is localised.
Eh?
With a pontoon bridge, the water is what supports the weight bearing down on each section of it.
With a regular bridge, the structure, itself, has to support the weight.
When you're considering the effects of differential-pressure, it's the latter analogy that applies.
In space, there's no "water" to offset the internal pressure and any differential-pressure has to be contained by the structure, itself.
However, consider that an aircraft fuselage has a skin only 1/8" or so thick, and is also pressurised. That seems to work reasonably well, with tremendous forces during take-off and landing, and not many people manage to kick through it either.
And how much internal bracing do planes have in order to maintain their structural integrity?
And how many planes do you know of that are bigger than they need to be?
Simple fact is that the bigger you make something, the more bracing it requires to prevent it from bending (especially when you're considering things that are intended to have a useful internal space) and that means adding weight.
You don't need to worry about gravity so much in space.
To quote Si....eh?
For many Earth satellites and orbiters the effects of non-Keplerian forces I.E. the deviations of the gravitational force of the Earth from that of a homogeneous sphere, the influences of gravitational forces from the Sun and the Moon, solar radiation pressure and in the case of Low Earth Orbit air-drag all need to be counteracted.
In other words - there's more than just the Earth's gravity acting on an object in orbit to worry about. And the ISS' structure has to be strong enough to handle both minor and major orbital corrections.
Simple fact is that the bigger you make something, the more bracing it requires to prevent it from bending and that means adding weight.
Because of gravity, or because of considerable forces being applied.
What are the forces acting on something that is built in space and spends its life in orbit? Virtually none (well, unless it rotates like something from 2001).
It has to hold in the air-pressure inside, but that is a rather different problem. You can imagine a vessel with flexible but airtight walls that is small when deflated, but expands when filled with air. No rigid bracing need, and bending doesn't matter.
Similarly, the now obsolete space shuttle had little puffer jets which they used to alter the pos of the shuttle while on orbit. Because of the mico g or zero g environment, it doesn't actually need massive rockets for these kinds of small changes. Leaving Earth orbit for the moon or other planet is a whole different story though.
To be fair, that's a bit of a fallacy.
Thrusters such as those used in the shuttles OMS are fine as long as you're planning on going somewhere roughly close to where your previous orbit was going to take you anyway but if you want to make any significant changes to direction or velocity, you still need a pretty big engine (or a big fuel supply for a little engine and a lot of patience).
Course, the need for those type of engines can largely be offset with a bit of planning and the use of some clever computers to ensure that you achieve an initial orbit that's very similar to your target and then you can get away with using "little puffer jets" to make corrections.
As long as you never find yourself doing something in space which requires any deviations from the predetermined plan.
What are the forces acting on something that is built in space and spends its life in orbit? Virtually none (well, unless it rotates like something from 2001).
First of all....it's IN "orbit" because it's still within the influence of the Earth's gravity. As I noted above, this is not a homogenous sphere anyway, but there's ALSO the effects of the Sun's and Moon's gravities on an object in Earth orbit. Then there's air-drag....which is still a significant factor even on the 160-200-mile orbital band of the ISS. And the "pressure" from solar radiation.
Because of gravity, of because of considerable forces being applied.
What are the forces acting on something that is built in space and spends its life in orbit? Virtually none (well, unless it rotates like something from 2001).
The forces acting on an object in space are, as I said, the result of the air-pressure inside the object.
It has to hold in the air-pressure inside, but that is a rather different problem. You can imagine a vessel with flexible but airtight walls that is small when deflated, but expands when filled with air. No rigid bracing need, and bending doesn't matter.
Well, yes. I can imagine such a thing but, clearly, they're not actually a real thing thus far (though they would, undeniably, be a terrific way to go).
When considering things that do exist, the fact remains that you have to brace a cylinder to avoid it swelling up into an ovoid and the bigger the cylinder, the more bracing it'll require.
First of all....it's IN "orbit" because it's still within the influence of the Earth's gravity. As I noted above, this is not a homogenous sphere anyway, but there's ALSO the effects of the Sun's and Moon's gravities on an object in Earth orbit.
Not significant. And those are forces acting on each particle of the structure at the same time. (The sun and moon are also acting on me at this minute, but I can't see it's causing me any problems!)
Then there's air-drag....which is still a significant factor even on the 160-200-mile orbital band of the ISS.
Yeah, I've noticed how streamlined the space-station is!
I has been reading about the problems of highly relativistic spaceflight.
Beyond the problems of carting a big enough energy source to facilitate acceleration to useful relativistic speeds and to slow down again, there's the very difficult problem caused by "interstellar hydrogen H, [which] although only present at a density of approximately 1.8 atoms/cm3, turns into intense radiation that would quickly kill passengers and destroy electronic instrumentation."
Noting that "the annual dose limit for a radiation worker is 0.05 Sv (NRC, 2011)", the interstellar H proton radiation at 0.999999995 of light speed would amount to about 346,000 Sv per second! Which would clearly be somewhat hazardous!!
Slow down a bit and it's less hazardous. A 10 m diameter spherical vessel with 0.10 m thick aluminium outer hull (which is within the range of shielding that has been considered for interplanetary travel within the solar system) would weigh approximately 85 metric tons and provide enough shielding against the radiation at speeds up to about 0.5c, but even at that relatively slow speed there's still a considerable amount of heat to dissipate (in the order of 570kW) and the relativistic time dilation effect is right down to about 15%.
So it's looking like the space travel speed limit will be about 300,000,000 mph and our journey times will be reduced by just a couple of months per year at best.
I has been reading about the problems of highly relativistic spaceflight.
Beyond the problems of carting a big enough energy source to facilitate acceleration to useful relativistic speeds and to slow down again, there's the very difficult problem caused by "interstellar hydrogen H, [which] although only present at a density of approximately 1.8 atoms/cm3, turns into intense radiation that would quickly kill passengers and destroy electronic instrumentation."
Noting that "the annual dose limit for a radiation worker is 0.05 Sv (NRC, 2011)", the interstellar H proton radiation at 0.999999995 of light speed would amount to about 346,000 Sv per second! Which would clearly be somewhat hazardous!!
Slow down a bit and it's less hazardous. A 10 m diameter spherical vessel with 0.10 m thick aluminium outer hull (which is within the range of shielding that has been considered for interplanetary travel within the solar system) would weigh approximately 85 metric tons and provide enough shielding against the radiation at speeds up to about 0.5c, but even at that relatively slow speed there's still a considerable amount of heat to dissipate (in the order of 570kW) and the relativistic time dilation effect is right down to about 15%.
So it's looking like the space travel speed limit will be about 300,000,000 mph and our journey times will be reduced by just a couple of months per year at best.
I guess that's one of those things where you have to just trust that if we ever figure out how to travel near the speed of light, we'll also figure out how to build suitable radiation shielding.
I read an interesting (if rather speculative) article in Popular Science a few years ago, where they asked a bunch of boffins about it and they came up with about half a dozen different things that'd need to be solved before it could happen.
On the one hand it was kind of disappointing to read of all the problems that'd need to be solved but, OTOH, it was encouraging to know that we do, at least, seem to understand what needs figuring out.
Not significant. And those are forces acting on each particle of the structure at the same time. (The sun and moon are also acting on me at this minute, but I can't see it's causing me any problems!)
Yeah, I've noticed how streamlined the space-station is!
You don't seem to get it. Up there, where the ISS is, the remaining somewhat tenuous attraction of the Earth's gravity is usually working at cross purposes and tangents to that of the Moon and the Sun; the structure can in effect be affected by "tides", as the gravitic influences pull in different directions.
You'd be affected by the Moon's gravity - twice a day - if you were walking your dog along a beach and didn't notice the tide coming in!
I guess that's one of those things where you have to just trust that if we ever figure out how to travel near the speed of light, we'll also figure out how to build suitable radiation shielding.
I read an interesting (if rather speculative) article in Popular Science a few years ago, where they asked a bunch of boffins about it and they came up with about half a dozen different things that'd need to be solved before it could happen.
On the one hand it was kind of disappointing to read of all the problems that'd need to be solved but, OTOH, it was encouraging to know that we do, at least, seem to understand what needs figuring out.
If we ever developed the technology to travel near the speed of light in Einsteinian space...we'd have the energy to spare to loft enough radiation shielding!
That is....if we don't actually end up using something like the Bussard ramjet...which would anyway depend on magnetic fields to shovel said interstellar hydrogen out of our way and into the drive as our fuel source;-)
You don't seem to get it. Up there, where the ISS is, the remaining somewhat tenuous attraction of the Earth's gravity is usually working at cross purposes and tangents to that of the Moon and the Sun; the structure can in effect be affected by "tides", as the gravitic influences pull in different directions.
Actually the gravity at the ISS orbit is not much less than it is down here. (A quick calculation makes it about 15% less, as it's quite high up.)
But I still don't think these forces are as significant as you make out. Consider a 'space-station' consisting of two loosely coupled blobs of matter, at the gravitational half-way point between earth and moon. It's not as though one blob is pulled towards the earth, and other towards the moon! Gravitational forces will apply equally to both.
There might well be tide-like forces at work, but on something the size of the space-station, they will have about the same effect as the moon will have on the liquid in your glass of beer.
Actually the gravity at the ISS orbit is not much less than it is down here. (A quick calculation makes it about 15% less, as it's quite high up.)
The point is that the various gravitational and other influences can be at a tangent to one another - air drag will be at an angle to the Earth's gravity, for example...and its the torsional stresses that require the ISS' construction to be strong enough to withstand them. And also to resist the "fatiguing" effect of the constant change in the balance of conflicting influences 15.54 times per day...
But I still don't think these forces are as significant as you make out. Consider a 'space-station' consisting of two loosely coupled blobs of matter, at the gravitational half-way point between earth and moon. It's not as though one blob is pulled towards the earth, and other towards the moon! Gravitational forces will apply equally to both.
The factors are enough to cause 2 kilometers of orbital decay per month unless corrected, in addition to the torsional issues. As it orbits in a band between 330 and 435 kms up...i'll leave it to you to calculate how many months of such decay the ISS could have withstood before returning involuntarily to Earth...
The factors are enough to cause 2 kilometers of orbital decay per month unless corrected, in addition to the torsional issues. As it orbits in a band between 330 and 435 kms up...i'll leave it to you to calculate how many months of such decay the ISS could have withstood before returning involuntarily to Earth...
So about 13 feet loss of height after each 26,000-mile orbit (and your figures suggest the height varies by 100km or 300,000 feet anyway).
I'm not disputing there are a number of forces at work, but I'm saying most of them (the gravitional ones) will apply to each atom of the space-station at the same time. There will be little stress between the various parts of the structure due to gravity, which forces will all combine to form a single gravitational vector acting in one direction.
(Just as it would work down here, with that one vector acting downwards, for a structure that is in freefall. For something resting on the ground however, the ground will exert uneven upward forces on parts of the structure, so that a large ungainly structure could collapse. That won't happen in space.)
The astronauts moving about inside and pushing themselves off the walls will likely have more impact, as will any waste vented or expelled from the space-craft.
I'm not disputing there are a number of forces at work, but I'm saying most of them (the gravitional ones) will apply to each atom of the space-station at the same time. There will be little stress between the various parts of the structure due to gravity, which forces will all combine to form a single gravitational vector acting in one direction.
Except it doesn't....or else we wouldn't have tides on Earth. We wouldn't have specific Trojan points and Lagrange points where the various competing gravitational influences balance each other there and nowhere else....
(Just as it would work down here, with that one vector acting downwards, for a structure that is in freefall. For something resting on the ground however, the ground will exert uneven upward forces on parts of the structure, so that a large ungainly structure could collapse. That won't happen in space.)
Yes it will and yes it does....such as when the Zvezda Module's manouvering engines are used to lift the ISS to a higher orbit. Which is why the ISS is constructed to be strong enough to absorb the torsional stresses from THAT particular manouver as well as all the incidental stresses and external influences.
The astronauts moving about inside and pushing themselves off the walls will likely have more impact, as will any waste vented or expelled from the space-craft.
No...because they're not being expelled from the ISS - and thus they're not acting as expelled reaction mass (any action having an equal and opposite reaction, etc.) Whereas vented waste can....but only if vented in large amounts and with force behind the venting.
I don't know why they don't have inflatable space stations. They could have an attachment on the front of the rocket that blows up a big sort of bubble and then the astronauts would climb in and close the entrance flap. They could float around all day then.
Can't remember but I think I mentioned Bigelow up the thread....?
The REAL problem with space travel is....there's nowhere to travel TO at the minute. if we're restricted to Earth orbit....there's nowhere else in Earth orbit except the ISS...and look at the horrendous cost they charge for tourists! Thus nothing really driving an impetus to bring down the cost of getting a pound of mass into orbit - except the occasional "prize" award...that are not really worth the overall costs of development, testing etc. needed to win them. Now that the X Prize is won....look how space tourism and "private carriers" are languishing...
Hence Bigelow's idea of an inflatable "space hotel"...
That doesn't follow. Whatever size it is, each square inch only needs to be able to withstand the same 14.5 pounds of pressure. (Although I'm not sure why it needs to be as high as sea-level pressure; what's the pressure inside an aircraft at 40,000 feet?)
In fact, if you double the size of the structure, you have eight times the volume, but only need four times the surface (ie. materials, and at the same thickness). Although it might need to be a spherical shape to maintain rigidity.
(Disclaimer: I haven't tried building a giant space station myself, so I could be wrong...)
Hoop stresses increases considerably the large the diameter of a cylinder, requiring thicker wall thickness for a given pressure.
However I would think that cost effectiveness or a similar measure of efficency is a primary driver in the space industry.
Comments
Well, yeah. That's why I said "the smallest possible sphere".
Somebody famous once said "If you can get a space-craft into orbit, you're half way to anywhere".
For the moment, at least, the biggest constraint we have is that, on a journey of, possibly, millions of miles, it's the first 60-odd miles, through our atmosphere, that's the awkward part due to aerodynamic drag and the effects of gravity and that's why size and weight are key issues, and why there's no room to swing a cat inside the ISS.
Personally, I reckon one of THE biggest steps we need to be taking in the field of space-exploration is to figure out how to build stuff in orbit.
And, by that, I mean stuff that actually has engines attached and that can actually do stuff once it's built.
Once we start doing that we'll get better at it and we'll only be in a position to explore other planets once we get really good at doing that stuff cos, let's face it, there's no way that a ship that could reach, say, Mars, is ever going to be launched from Earth in one lump.
....or at least, as with Apollo, at most a single change of attitude in Earth orbit to put the lander in the right place on the Earth-Mars stack.
After all - don't forget the whole ISS is "dirigible"
But with both pontoon bridges and space-station shells, if you want to manoeuvre the whole thing by applying force at a small number of points, then you have worry about how that force is distributed through the whole structure.
In space at least, you have the choice of applying very low acceleration to minimise the forces. Although you still have to worry about some astronaut putting his foot through the outer wall, you can't get away with too much.
However, consider that an aircraft fuselage has a skin only 1/8" or so thick, and is also pressurised. That seems to work reasonably well, with tremendous forces during take-off and landing, and not many people manage to kick through it either.
I've got nothing against them, in-particular, but I can't help thinking that every time one of these probes lands on some other planet and reports back that there's rocks and dust, somebody crosses that planet off a list and puts their chequebook back in their pocket.
What we really need is for one of these probes to find diamonds the size of coconuts (or, more usefully, some kind of hydrogen or electricity-producing rock, perhaps?) lying around on the surface of Titan or something.
I bet that'd get things moving again.
Many satellites such as those used for tv have tiny rocket motors to maintain and adjust position - these can be powerfull enough to move them to a different position in orbit, such as when a satellite gets replaced and is placed in a parking orbit.
Similarly, the now obsolete space shuttle had little puffer jets which they used to alter the pos of the shuttle while on orbit. Because of the mico g or zero g environment, it doesn't actually need massive rockets for these kinds of small changes. Leaving Earth orbit for the moon or other planet is a whole different story though.
Eh?
With a pontoon bridge, the water is what supports the weight bearing down on each section of it.
With a regular bridge, the structure, itself, has to support the weight.
When you're considering the effects of differential-pressure, it's the latter analogy that applies.
In space, there's no "water" to offset the internal pressure and any differential-pressure has to be contained by the structure, itself.
And how much internal bracing do planes have in order to maintain their structural integrity?
And how many planes do you know of that are bigger than they need to be?
Simple fact is that the bigger you make something, the more bracing it requires to prevent it from bending (especially when you're considering things that are intended to have a useful internal space) and that means adding weight.
To quote Si....eh?
For many Earth satellites and orbiters the effects of non-Keplerian forces I.E. the deviations of the gravitational force of the Earth from that of a homogeneous sphere, the influences of gravitational forces from the Sun and the Moon, solar radiation pressure and in the case of Low Earth Orbit air-drag all need to be counteracted.
In other words - there's more than just the Earth's gravity acting on an object in orbit to worry about. And the ISS' structure has to be strong enough to handle both minor and major orbital corrections.
Because of gravity, or because of considerable forces being applied.
What are the forces acting on something that is built in space and spends its life in orbit? Virtually none (well, unless it rotates like something from 2001).
It has to hold in the air-pressure inside, but that is a rather different problem. You can imagine a vessel with flexible but airtight walls that is small when deflated, but expands when filled with air. No rigid bracing need, and bending doesn't matter.
To be fair, that's a bit of a fallacy.
Thrusters such as those used in the shuttles OMS are fine as long as you're planning on going somewhere roughly close to where your previous orbit was going to take you anyway but if you want to make any significant changes to direction or velocity, you still need a pretty big engine (or a big fuel supply for a little engine and a lot of patience).
Course, the need for those type of engines can largely be offset with a bit of planning and the use of some clever computers to ensure that you achieve an initial orbit that's very similar to your target and then you can get away with using "little puffer jets" to make corrections.
As long as you never find yourself doing something in space which requires any deviations from the predetermined plan.
First of all....it's IN "orbit" because it's still within the influence of the Earth's gravity. As I noted above, this is not a homogenous sphere anyway, but there's ALSO the effects of the Sun's and Moon's gravities on an object in Earth orbit. Then there's air-drag....which is still a significant factor even on the 160-200-mile orbital band of the ISS. And the "pressure" from solar radiation.
How many forces do you want?
The forces acting on an object in space are, as I said, the result of the air-pressure inside the object.
Well, yes. I can imagine such a thing but, clearly, they're not actually a real thing thus far (though they would, undeniably, be a terrific way to go).
When considering things that do exist, the fact remains that you have to brace a cylinder to avoid it swelling up into an ovoid and the bigger the cylinder, the more bracing it'll require.
Yeah, I've noticed how streamlined the space-station is!
Beyond the problems of carting a big enough energy source to facilitate acceleration to useful relativistic speeds and to slow down again, there's the very difficult problem caused by "interstellar hydrogen H, [which] although only present at a density of approximately 1.8 atoms/cm3, turns into intense radiation that would quickly kill passengers and destroy electronic instrumentation."
Noting that "the annual dose limit for a radiation worker is 0.05 Sv (NRC, 2011)", the interstellar H proton radiation at 0.999999995 of light speed would amount to about 346,000 Sv per second! Which would clearly be somewhat hazardous!!
Slow down a bit and it's less hazardous. A 10 m diameter spherical vessel with 0.10 m thick aluminium outer hull (which is within the range of shielding that has been considered for interplanetary travel within the solar system) would weigh approximately 85 metric tons and provide enough shielding against the radiation at speeds up to about 0.5c, but even at that relatively slow speed there's still a considerable amount of heat to dissipate (in the order of 570kW) and the relativistic time dilation effect is right down to about 15%.
So it's looking like the space travel speed limit will be about 300,000,000 mph and our journey times will be reduced by just a couple of months per year at best.
http://www.scirp.org/journal/PaperInformation.aspx?paperID=23913#.VQY0q-F5b8M
Aww don't feel bad about being jobless and bored, think happy thawts. 😝
I guess that's one of those things where you have to just trust that if we ever figure out how to travel near the speed of light, we'll also figure out how to build suitable radiation shielding.
I read an interesting (if rather speculative) article in Popular Science a few years ago, where they asked a bunch of boffins about it and they came up with about half a dozen different things that'd need to be solved before it could happen.
On the one hand it was kind of disappointing to read of all the problems that'd need to be solved but, OTOH, it was encouraging to know that we do, at least, seem to understand what needs figuring out.
You don't seem to get it. Up there, where the ISS is, the remaining somewhat tenuous attraction of the Earth's gravity is usually working at cross purposes and tangents to that of the Moon and the Sun; the structure can in effect be affected by "tides", as the gravitic influences pull in different directions.
You'd be affected by the Moon's gravity - twice a day - if you were walking your dog along a beach and didn't notice the tide coming in!
If we ever developed the technology to travel near the speed of light in Einsteinian space...we'd have the energy to spare to loft enough radiation shielding!
That is....if we don't actually end up using something like the Bussard ramjet...which would anyway depend on magnetic fields to shovel said interstellar hydrogen out of our way and into the drive as our fuel source
Actually the gravity at the ISS orbit is not much less than it is down here. (A quick calculation makes it about 15% less, as it's quite high up.)
But I still don't think these forces are as significant as you make out. Consider a 'space-station' consisting of two loosely coupled blobs of matter, at the gravitational half-way point between earth and moon. It's not as though one blob is pulled towards the earth, and other towards the moon! Gravitational forces will apply equally to both.
There might well be tide-like forces at work, but on something the size of the space-station, they will have about the same effect as the moon will have on the liquid in your glass of beer.
The point is that the various gravitational and other influences can be at a tangent to one another - air drag will be at an angle to the Earth's gravity, for example...and its the torsional stresses that require the ISS' construction to be strong enough to withstand them. And also to resist the "fatiguing" effect of the constant change in the balance of conflicting influences 15.54 times per day...
The factors are enough to cause 2 kilometers of orbital decay per month unless corrected, in addition to the torsional issues. As it orbits in a band between 330 and 435 kms up...i'll leave it to you to calculate how many months of such decay the ISS could have withstood before returning involuntarily to Earth...
So about 13 feet loss of height after each 26,000-mile orbit (and your figures suggest the height varies by 100km or 300,000 feet anyway).
I'm not disputing there are a number of forces at work, but I'm saying most of them (the gravitional ones) will apply to each atom of the space-station at the same time. There will be little stress between the various parts of the structure due to gravity, which forces will all combine to form a single gravitational vector acting in one direction.
(Just as it would work down here, with that one vector acting downwards, for a structure that is in freefall. For something resting on the ground however, the ground will exert uneven upward forces on parts of the structure, so that a large ungainly structure could collapse. That won't happen in space.)
The astronauts moving about inside and pushing themselves off the walls will likely have more impact, as will any waste vented or expelled from the space-craft.
Except it doesn't....or else we wouldn't have tides on Earth. We wouldn't have specific Trojan points and Lagrange points where the various competing gravitational influences balance each other there and nowhere else....
Yes it will and yes it does....such as when the Zvezda Module's manouvering engines are used to lift the ISS to a higher orbit. Which is why the ISS is constructed to be strong enough to absorb the torsional stresses from THAT particular manouver as well as all the incidental stresses and external influences.
No...because they're not being expelled from the ISS - and thus they're not acting as expelled reaction mass (any action having an equal and opposite reaction, etc.) Whereas vented waste can....but only if vented in large amounts and with force behind the venting.
So I will just post the trailer for the re-release of a film that showed how things could have been to cheer everyone up.
https://www.youtube.com/watch?v=XHjIqQBsPjk
Funny you should say that.
http://www.hngn.com/articles/77454/20150316/international-space-station-expanding-inflatable-habitat-module-testing-starts-year.htm
The REAL problem with space travel is....there's nowhere to travel TO at the minute. if we're restricted to Earth orbit....there's nowhere else in Earth orbit except the ISS...and look at the horrendous cost they charge for tourists! Thus nothing really driving an impetus to bring down the cost of getting a pound of mass into orbit - except the occasional "prize" award...that are not really worth the overall costs of development, testing etc. needed to win them. Now that the X Prize is won....look how space tourism and "private carriers" are languishing...
Hence Bigelow's idea of an inflatable "space hotel"...
Hoop stresses increases considerably the large the diameter of a cylinder, requiring thicker wall thickness for a given pressure.
However I would think that cost effectiveness or a similar measure of efficency is a primary driver in the space industry.