Trends in space station design, weight versus volume

Here's an interesting table, showing the mass (in metric tonnes) and
habitable volume (in cubic metres) of various space stations, and hence
how many cubic metres of habitable volume you get for each kilogram of mass.

Now, IANARS, but I would have thought that in simple terms mass would be
very roughly proportional to surface area and so would scale as a
square, while volume would scale as a cube. Obviously this ignores
stuffing the habitable volume full of massive machinery, but I would
have thought the principle would hold as a first approximation of the
relationship.

Yet:

Mass Vol m3 per kg
Salyut 7 20 90 4.5
Skylab 76 361 4.75
MIR 124 350 2.82
ISS 246 425 1.72

So ISS is 12 times as massive as Salyut 7, but only provides about 5
times as much habitable volume. It's three times as massive as Skylab,
but only provides 18% more volume.

The trend seems to be that the newer or larger the space station, the
poorer the relationship of volume to mass.

So what is going on here? Perhaps the modern craft are stuffed full of
more goodies (scientific equipment, coke machines, etc), but surely
1990's technology is more weight-efficient than 1960's technology.
Doubtlessly for electronics, but presumably for other things too.

I understand Skylab was exceptionally spacious, but there's four data
points here with a consistent trend.

Obviously there's some very unfortunate scaling going on which would
have ramifications for even larger stations. It also implies that for
the "space hotel" style projects, you'd be much better off launching 5
Salyut -style craft bolted together than one ISS-style.

Seems to me IIS really just a bunch of Salyuts bolted together (with the
odd CMG thrown in, valves for replenishment, etc), so why the
extraordinary difference in volume efficiency ?

Cheers

-- Ian

Ian Davies wrote:
Here's an interesting table, showing the mass (in metric tonnes) and
habitable volume (in cubic metres) of various space stations, and hence
how many cubic metres of habitable volume you get for each kilogram of
mass.

Mass Vol m3 per kg
Salyut 7 20 90 4.5
Skylab 76 361 4.75
MIR 124 350 2.82
ISS 246 425 1.72

So ISS is 12 times as massive as Salyut 7, but only provides about 5
times as much habitable volume. It's three times as massive as Skylab,
but only provides 18% more volume.

The trend seems to be that the newer or larger the space station, the
poorer the relationship of volume to mass.

So what is going on here?

More infrastructure. Look at the mass of the ISS truss, which provides
solar power and thermal control for the rest of the station. If you
compare power capacity in KW between the above stations, you'll see a
very different story.

Perhaps the modern craft are stuffed full of
more goodies (scientific equipment, coke machines, etc), but surely
1990's technology is more weight-efficient than 1960's technology.
Doubtlessly for electronics, but presumably for other things too.

I understand Skylab was exceptionally spacious, but there's four data
points here with a consistent trend.

It just goes to show that it is possible to draw erroneous conclusions
even with multiple data points.

Skylab had enough power for the science they attempted on it, but on the
other hand it was launched with all the science it ever did - it wasn't
"assembled" per se.

The Salyuts and Mir were, by most historical accounts, power-limited.
They literally had more habitable volume than they could put to use.

ISS was the first station designed with power to spare. It originally
had more habitable volume, but some (the US Hab module, most of the
Russian research modules) were cancelled, so now it has an even more
impressive power surplus (or habitable volume deficit, depending on how
you look at it).

Habitable volume is not the only, or even the best, figure of merit here.

Obviously there's some very unfortunate scaling going on which would
have ramifications for even larger stations. It also implies that for
the "space hotel" style projects, you'd be much better off launching 5
Salyut -style craft bolted together than one ISS-style.

Seems to me IIS really just a bunch of Salyuts bolted together (with the
odd CMG thrown in, valves for replenishment, etc)

Wow. What an extraordinarily ignorant statement.

On Feb 21, 7:31 pm, Ian Davies wrote:
Here's an interesting table, showing the mass (in metric tonnes) and
habitable volume (in cubic metres) of various space stations, and hence
how many cubic metres of habitable volume you get for each kilogram of mass.

Now, IANARS, but I would have thought that in simple terms mass would be
very roughly proportional to surface area and so would scale as a
square, while volume would scale as a cube. Obviously this ignores
stuffing the habitable volume full of massive machinery, but I would
have thought the principle would hold as a first approximation of the
relationship.

Yet:

Mass Vol m3 per kg
Salyut 7 20 90 4.5
Skylab 76 361 4.75
MIR 124 350 2.82
ISS 246 425 1.72

So ISS is 12 times as massive as Salyut 7, but only provides about 5
times as much habitable volume. It's three times as massive as Skylab,
but only provides 18% more volume.

The trend seems to be that the newer or larger the space station, the
poorer the relationship of volume to mass.

So what is going on here? Perhaps the modern craft are stuffed full of
more goodies (scientific equipment, coke machines, etc), but surely
1990's technology is more weight-efficient than 1960's technology.
Doubtlessly for electronics, but presumably for other things too.

I understand Skylab was exceptionally spacious, but there's four data
points here with a consistent trend.

Obviously there's some very unfortunate scaling going on which would
have ramifications for even larger stations. It also implies that for
the "space hotel" style projects, you'd be much better off launching 5
Salyut -style craft bolted together than one ISS-style.

Seems to me IIS really just a bunch of Salyuts bolted together (with the
odd CMG thrown in, valves for replenishment, etc), so why the
extraordinary difference in volume efficiency ?

Cheers

-- Ian

Frail human DNA needs all the surrounding mass it can get. Perhaps 10
meters of beer in all directions would do quite nicely, especially if
going for station-keeping within the Earth-Moon L1.
.. - Brad Guth

Jorge R. Frank wrote:


Wow. What an extraordinarily ignorant statement.


Wow. What an extraordinarily redundant observation.

There's probably a whole range of things you're extraordinarily ignorant
about. I had stated I was not a rocket scientist. Of course I'm
ignorant - that's why I asked the question. The arrogance is unbecoming.

As it happens, I'm still ignorant as to the reason. All you've told me
is ISS is a bunch of bolted-together modules with over-engineered power
supply.

You observe that MIR and Salyut were power limited but Skylab was not,
so if the answer really related to power, you'd expect to see a ranking
such as:
Salyut ... MIR ........... Skylab.......... ISS

Instead the ranking is:
Skylab..Salyut..................MIR........ ISS

Mass Vol m3 per kg
Salyut 7 20 90 4.5
Skylab 76 361 4.75
MIR 124 350 2.82
ISS 246 425 1.72

It is hard to prove anything from only 4 data points (which have
hard-to-control-for differences such as nationality, whether the
purpose was quick-and-dirty or "optimally" designed, etc), but if I
had to guess, I'd say that people have gradually figured out that big
stations have a lot of air drag and thus require a lot of reboost
fuel. The ideal station, from this point of view, would have very low
frontal cross-section (in the limiting case, some kind of cigar shape
pointed in the direction of the orbit).

Volume is an imperfect proxy for frontal cross-section but I suppose
they would tend to correlate somewhat.

High air drag is one of the problems that most shuttle external tank
based proposals had.

Ian Davies wrote:


As it happens, I'm still ignorant as to the reason. All you've told me
is ISS is a bunch of bolted-together modules with over-engineered power
supply.

If you think that's all I've told you, you've understood nothing.

Ian Davies formuleerde de vraag :
Jorge R. Frank wrote:


Wow. What an extraordinarily ignorant statement.

Wow. What an extraordinarily redundant observation.

There's probably a whole range of things you're extraordinarily ignorant
about. I had stated I was not a rocket scientist. Of course I'm ignorant -
that's why I asked the question. The arrogance is unbecoming.

As it happens, I'm still ignorant as to the reason. All you've told me is ISS
is a bunch of bolted-together modules with over-engineered power supply.

You observe that MIR and Salyut were power limited but Skylab was not, so if
the answer really related to power, you'd expect to see a ranking such as:
Salyut ... MIR ........... Skylab.......... ISS

Instead the ranking is:
Skylab..Salyut..................MIR........ ISS

The power of Skylab was also limited, one solarpanel was damaged during
launch, and could not be unfolded.

André

On 22 Feb 2008 01:11:32 -0500, in a place far, far away, Jim Kingdon
made the phosphor on my monitor glow in such a way
as to indicate that:

Mass Vol m3 per kg
Salyut 7 20 90 4.5
Skylab 76 361 4.75
MIR 124 350 2.82
ISS 246 425 1.72

It is hard to prove anything from only 4 data points (which have
hard-to-control-for differences such as nationality, whether the
purpose was quick-and-dirty or "optimally" designed, etc), but if I
had to guess, I'd say that people have gradually figured out that big
stations have a lot of air drag and thus require a lot of reboost
fuel.

Only if at low altitudes. This is a function of launch costs. If
costs of reaching it weren't a consideration (and earth observation
weren't an issue), a station would like to be much higher than ISS is,
with an upper limit as it gets into the belts. The altitude is
constrained by the increasing reduction of payload to reach it with
the Shuttle (a problem that could be solved by a station-based tug).

André wrote:

The power of Skylab was also limited, one solarpanel was damaged
during launch, and could not be unfolded.


Actually, one side panel fell clean off during ascent.

Pat

"Pat Flannery" wrote in message
...


André wrote:

The power of Skylab was also limited, one solarpanel was damaged during
launch, and could not be unfolded.


Actually, one side panel fell clean off during ascent.

The thermal/micrometeorite shield ripped off during launch, taking one main
solar array with it and jamming the other main solar array so it wouldn't
deploy. Skylab made it to orbit with far less power than originally
intended and with an overheating orbital workshop.

The story of how NASA, specifically the astronauts on the first Skylab
mission, fixed these serious problems makes for good reading.

Jeff
--
A clever person solves a problem.
A wise person avoids it. -- Einstein