Why is NASA planning to deorbit the ISS instead of reusing its newer modules, like the Russians will?

Question

I was reading about Russia's plans to detach their modules from the ISS before its deorbit and use them to begin a new space station. The version I read said Zvezda (DOS-8) and Poisk (MRM-2) are planned for inclusion in this new station, and there are variants on that plan. The Nauka (FGB-2) would go, along with the hub module (Uzlovoy module) that will later dock to it, neither of which have yet launched. This can't be done with the other modules. The estimate is that sometime in the 2020s it will be necessary to do a controlled deorbit and these modules will be lost, even though some of them will still be quite functional at that time and could have continued service.

Why was the choice made not to include mechanisms for disassembly in the design of these modules? They were designed to be modular so the ability to swap out old ones for new ones must have been discarded for specific reasons.

Answer 1

It will be several years before we get to see what actually happens here. However the overall driver for the currently stated NASA plan is money. (In contrast, the currently stated Russian plan seems disconnected from what one might assume about their budget realities.)

If the NASA budget remains flat, and there is all expectation that it will, indefinitely, then any plans to send humans to the moon, asteroids, or Mars will need the money currently being used to operate ISS to even get a running start at accomplishing those objectives. ISS currently consumes over \$3B a year, growing to \$4B by 2020. Plus civil servant salaries, which are not included in that. (It's a little hard to track real costs at NASA.)

There are many other excuses for ditching ISS, such as the control function mentioned, aging systems (e.g. hatch seals), etc. But if there were some truly compelling reason to retain the ISS function, it seems likely that solutions could be developed. So in the end it will come down to priorities and budget.

Answer 2

The difference between the US Operating Segment (USOS) and the Russian segment is one of self sufficiency.

The first module launched, Zarya, was enough to base a station on. Sure not a ton of living and working space but it had all the orbital control, solar, thermal radiators needed to be a space station.

After that, everything grew off that initial kernel with larger more competent versions of those subsystems being added. The obvious two are the huge solar arrays and the huge radiator fins.

Thus the Russian segment, has the solar, thermal, and orbital management it needs on its own at a small scale. But the US side relies on the bigger systems except it lacks the orbital management part. I.e There are no engines on the US side to reboost the station. But there are on the Russian side.

Since the USOS relies on the larger heat and power systems, that could not be provided on each node individually, it did not make sense to provide 100% self sufficiency on every module, since it is better to save mass/performance for the real purpose of the module.

As Nickloai notes in another answer, this is a difference between 1st, 2nd, 3rd, and 4th generation systems. ISS is a hybrid that started as a 4th gen station, and the USOS acts as add on modules to it.

Now if the US and international partners decided to go off on their own, they potentially could launch something like the ICM (Interim Control Module) and potentially move existing modules and components to a new base unit for a new station. They were not planned to do that from the outset, but if the new base unit provided the proper power, heat, and orbital control infrastructure they could unberth from one CBM port and berth to a second new one. If done while all attached to the station, they could likely even use the CanadArm to do the work. But the US, unlike the Russians, never seem to have planned for this.

Having said all that, your question is missing a subtle nuance. The Russian segment that would potentially separate and form a new Russian station post-ISS service life would be the newest modules, not yet delivered to the station (as of Mar 2015). They do not plan on taking Zarya or Zvezda with them. If they did, the US has the Interim Control Module (ICM), developed in case Zarya failed on launch to consider refurbishing to replace them.

The Pirs module docked to Zvezda's earth facing port is scheduled to be removed by a Progress departing the station, and the Nauka module is due to dock there. This is a similar module to Zarya. Then at the bottom of Nauka is the OM module, which is a 6 port docking sphere. It is ultimately the OM module that becomes the heart of the new Russian station, where modules come and go, but the OM remains.

There are two modules planned to dock to the OM that are not yet complete and may still change, as they have changed many times over the past decade.

There are some great images of the US and Russian segments in a question about whether the ISS will need more docking ports.

Answer 3

I'm speculating here, but I think that the broad 'reason' is that that's how they've designed the station. Let me expand on that: the first stations (early Salyut, Skylab), were launched all as one piece. All their maneuvering fuel, consumables, any spare parts, were launched in one go, and that was all you had.

These were the so-called "first generation" space stations.

The second generation begins with Salyut 6, which was the first station to have two docking ports. Now the station could be resupplied with fuel, consumables, and replacements for broken parts. This drastically increased the lifespan of the station.

The third generation begins with Mir, and in this generation you have a main 'base block' of your station and then attach modules to it. The base block itself is one of the modules, however, and this will limit the lifespan of the station.

Now comes the real speculation: I think that what Russia is doing with their 'node module' will be the fourth generation of space stations. As far as I can tell, the node module will be relatively simple, and so can be designed to have a very long lifespan. Modules would be able to dock to the node module, and undock when their lifespan or technology has run its course. The longer-living node module would remain in space and see many modules dock and undock. But I'm just speculating here, I have no idea if this is Russia's intent or if its how they plan to use the node module.

To answer your question in this context, I see the ISS as a third generation space station. So it is modular, but a lot of the components are still interconnected, and as such the station itself had a very finite lifespan by design. Modules could still be disconnected (I think they've run a lot of cables between the modules, so they would have to disconnect all those first. Not impossible, but a lot of work I'd imagine), but with no 'core' or 'node' to reattach them to, the question then becomes what to do with them, and that's where geoffc's answer goes into more detail than mine does.

Ultimately I think the 'real' answer to your question is a combination of my answer and geoffc's answer

Answer 4

This is partially speculation, but I do know that the original plans for the ISS were much grander and more complex than the end result turned out to be, due to a number of reasons... Cost being the biggest one, but also the difficulties after the space shuttle crash that slowed down construction by years.