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Wasted Weight in Space?

04/11/2011 1:12 AM

Can anyone guess why the International Space Station needs such heavy trusses? I've tried a query to the NASA site that reported this without success.

"To function, the ISS needs huge trusses , some over 15 meters long and with masses over 10,000 kilograms, to keep it rigid and to route electricity and liquid coolants."

Since the inertial loads on the assembly are very low, the main threat to rigidity seems to be thermal cycling. Have we lifted all that material just for thermal mass? From what I've seen, the solar panels are seldom aimed within ten degrees, and the components should be fine either independently or with compliant joining. A rigid assembly of thin shells attached to a strong, rigid truss, with cyclic thermal stress sounds more risky than a monocoque with loosely bundled wiring and plumbing.

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#1

Re: Wasted Weight in Space?

04/11/2011 1:20 AM

Keeping it rigid makes sense, notably accommodating the acceleration to send it into orbit. Heavy mass would also limit movements of the space station when people move around within it. The part about routing electricity and liquid coolants makes little sense; those would not entail heavy structural support.

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#2
In reply to #1

Re: Wasted Weight in Space?

04/11/2011 1:33 AM

The various modules all have enough strength to maintain pressure and survive launch forces. What I don't understand is the function of a separate frame, which only gets used under zero G conditions to keep the assembly aligned. Most aerospace construction is monocoque, even for high G loadings. The Atlas rocket can't even support its payload when not pressurized by fuel.

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#3

Re: Wasted Weight in Space?

04/11/2011 1:35 AM

What pressure do they keep the internal atmosphere at in the station?

A little PSI creates a lot of force when applied over large areas.

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#4
In reply to #3

Re: Wasted Weight in Space?

04/11/2011 1:44 AM

They maintain the same pressure we enjoy on earth in the ISS, although other space vehicles have gone down to 1l3 of that, and even airliners use reduced pressure.

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#5

Re: Wasted Weight in Space?

04/11/2011 2:11 AM

It might be essential to reduce the effect or contain damage in a small area in the event of hits by other orbiting objects or passing objects.

Essential services like electricity or coolants also need special protection.

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#6

Re: Wasted Weight in Space?

04/11/2011 4:10 AM

The ISS structure would require to be well supported. Despite being in "weightless" space there are many stress that it has to endure. Aside from the thermal ones, whenever a orbit correction is required the whole structure has to deal and absorb the inertial forces of the atitude rockets.

While it may not have "weight" it certainly has mass. Newtonian forces still apply.

There are other better qualified CR4 members who could supply the math for you.

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#7
In reply to #6

Re: Wasted Weight in Space?

04/11/2011 5:58 AM

The ISS usually gets its velocity restored with a boost from a shuttle, but the standard rockets provided accelerate it at under .004g, which is appropriate for the interior storage arrangements. Total thrust is under 800 kg, which seems like an easy incidental load to add to such massive pressure vessels. If it did pose any problem, the thrust could be reduced by a couple of orders of magnitude without any problem over the results.

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#8

Re: Wasted Weight in Space?

04/11/2011 4:28 PM

I think that is for thermal mass. I have seen a piece of iron that have passed through several million cycles, lets say from -40deg C to 50 deg C, without or with very small load, and it was like eaten on the surface, in some places missing 1/16, in some places more. On the space station, the cycling can be much deeper in temperature.

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#9
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Re: Wasted Weight in Space?

04/11/2011 11:14 PM

The effect of thermal cycling erosion would be more important on the relatively thinner material of the pressure-containing inhabited modules. My question really is about what loads does the frame carry, apart from those it generates itself by being of different thermal characteristics? If some intrepid spacewalker were to disentangle the wiring from it, and unbolt the connections, would it not just fly formation, or slowly drift away if pushed? How would the other astronauts know, if the windows were blocked, etc.? Presumably, the renovations would include some reinforcing at the joints, but much weight would be saved.

I should put in a caveat here that not all of my references are well dated, so various quantities quoted might be out by 50%, but we are talking about much bigger differences. The weight of the frame for the pickup-truck style box-on-frame assembly comes to at least 2% of the finished weight. I wonder if that is 100% wasted. One reference noted that the planning was haphazard, so the frame might be a cost of limited thinking. The shuttle's solid-fuel boosters were added to take care of unplanned weight.

Anyway, the outward air pressure on the end of the Proton module is over 135,000 kg, so it should be able to handle the 8,000 kg inward force due to normal acceleration from a single rocket location on itself. Even if things were loaded in bending, there seems to be plenty in hand. A few lines of rigging could triangulate anything nicely.

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#11
In reply to #9

Re: Wasted Weight in Space?

04/12/2011 7:53 AM

if it ever becomes a gas station, refuelling to craft at once, as in the movie "Armageddon"... it might have to experience shocks and rotations...

from a modular point of view, a rigid backbone kind of makes sense, for R and R of components.

If it was just mass they needed, the could use water as mass. the specifiy strength..

I don't know.

Chris

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#13
In reply to #9

Re: Wasted Weight in Space?

04/12/2011 11:16 AM

"The shuttle's solid-fuel boosters were added to take care of unplanned weight."

Not true. The boosters provide over 80% of lift-off thrust and were a planned component from the very beginning.

And, to stay on topic, IIRC the structure was designed to handle the stresses of thrusting while adjusting orbit when and if necessary.

Hooker <-- who machined the NASA Langley brass thermal wind tunnel models for the initial shuttle design contest way back when. BTW, Langley lost, and yes we had SRB's too.

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#17
In reply to #13

Re: Wasted Weight in Space?

04/12/2011 7:42 PM

GA and nicely said - saved me typing a diatribe on center of gravity and inertia too

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#10

Re: Wasted Weight in Space?

04/12/2011 2:26 AM

Hi Easyway, Maybe they build like this to withstand the forces that we have on earth, transportation of the modules to the launch site and the launch itself would have incredible stresses on modules. I suppose that they cant afford any stress fractures in the transporting stages, so they build it like a brick shit-house, but I do understand where you are coming from, but i am no expert, but i have been to Cape Canaveral a few years ago, but that makes me no expert.

But now with the development's in nano technology's, they are probably looking at new materials for outer-space modules!

Cheers

Joe

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#12

Re: Wasted Weight in Space?

04/12/2011 8:46 AM

What are the rotational forces on the ISS?

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#14
In reply to #12

Re: Wasted Weight in Space?

04/12/2011 11:53 AM

The rotational forces are presumably about nil, to keep the solar panels lined up. They are not using spin for artificial gravity. We have all seen video of ISS crew floating around, and manipulating stuff, and of spacewalks, so why would a frame not also just fly along? The normal maneuvering loads, even if applied at only one point, are far below the loads being handled by the modules as pressure vessels.

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#15
In reply to #14

Re: Wasted Weight in Space?

04/12/2011 12:12 PM

You've obviously looked into this in more detail than I have so I'm just throwing brainstorming questions out right now. That being said, the ISS is rotating about the Earth right?

You might be right that the rotational forces are close to zero but these forces seem to add up quick.

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#16

Re: Wasted Weight in Space?

04/12/2011 7:30 PM

We may be missing the most obvious. Could this have been built in for a future expansion to the space station? May be to build a larger structure at a later date. They would have to plan for that possibility in advance.

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