The space station has had a chequered history. Much discussed in the 1920s by the pioneers of astronautics -- notably Hermann Oberth, Guido von Pirquet and Willy Ley -- in the 1960s it was bypassed in favour of direct flights to the Moon.
The first station, the Russian Salyut 1 in 1971, was a success in every respect except that its first intended crew failed get on board, and the second, while completing their mission on the station, failed to come home alive. The first American station, Skylab, successfully hosted three astronaut crews in 1973-1974, but was then allowed to fall back to Earth without any attempt to replace it.
During the 1970s and 1980s there followed a succession of increasingly capable Salyut stations, culminating in Mir, which saw two major periods of continuous occupation totalling over 12 years. Mir was deorbited and destroyed in 2001 after a possible rescue by private enterprise in the form of MirCorp failed to save it. Russia had switched its attention to the American-led International Space Station, whose first Russo-American crew had recently taken up residence there.
One would have thought that space station design and operation would be well understood by now. And yet the clear design logic behind these stations has been violated in the past, and seems set to be violated again in a few years time in the case of the ISS.
Take Mir. The heart of the station, the core module or base block (bazovyj blok in Russian), was launched in February 1986. To this was added the Kvant-1 module, launched in March 1987 (docked, April). Kvant-2 arrived in November 1989 (docked, December), Kristall in May 1990 (docked, June), Spektr in May 1995 (docked, June), and finally Priroda in April 1996. Kvant-1 docked to the rear axial port, while the add-on modules from Kvant-2 onwards docked initially to the front axial port of the base block, and were later manoeuvred to one of the four forward radial ports by means of a robot arm.
When Mir was finally deorbited in March 2001, the various modules had therefore been in space for:
|Base block:||15 years 1 month|
|Kvant-2:||11 years 4 months|
|Kristall:||10 years 10 months|
|Spektr:||5 years 10 months|
(towards the end of Mir's life, damaged and depressurised)
|Priroda:||4 years 11 months|
The modules were originally only intended to last for 5 years in space (according to Astronautix.com). But in practice they were serviceable for between twice and three times that period (barring accidents, such as the collision which disabled Spektr). MirCorp must have believed that the oldest units would last even longer.
This means that, when Mir was finally scrapped, the Kvant-2 and Kristall modules should still have had four years of life left in them, if they had been pushed as hard as the base block. Priroda should have been serviceable for another decade yet, as should Spektr, if it could have been repaired.
The design logic suggests that what should have happened at some point during the 1990s is that a programme of replacement of the older modules should have begun, starting with the base block itself.
Since all the other modules are attached to the base block, this may seem to be a problem, but it is not so. After launch, the new base block could have been attached to the old one, joining up at their front docking ports. It could have remained in this position for weeks or months, with its own rear docking port available for visiting Soyuz and Progress craft. Equipment could have been transferred from the old unit to the new one at leisure.
During this time, the four radial modules are detached in turn from the old base block, using a robot arm similar to the one which attached them in the first place, moved across the few metres to the new one and redocked. Finally, Kvant is undocked from the old base block and backed off. Spin the complex 180 degrees and Kvant can be reattached to the new core module. The old base block is then removed, no doubt with a Progress craft to act as a tug, and trashed over the Pacific Ocean.
Replacement of the add-on modules is trivial, since each old one can be removed and its replacement attached without disturbing any of the other modules.
In other words, if a total of X modules are used for a particular station and the rated lifetime of modules is Y years, a regular launch rate of one module every Y/X years will ensure that the station is maintained indefinitely. This is surely the fundamental equation governing modular space station design.
For example, the Mir station had 6 modules and each one was serviceable for at least 12 years. So, once the station had been completed, a constant launch rate of one module every 2 years or so would have kept the station running forever.
With experience, of course, the rated lifetime of modules should increase, allowing a reduction in the replacement rate or an increase in the number of modules in use in orbit.
The problem is that space agencies apparently think of a space station as a mission, not as infrastructure. So, in their view, the station must inevitably come to the end of its mission and be trashed. And in the case of Mir, there was the additional problem that the Soviet/Russian economy was collapsing, and the later add-on modules were much delayed.
But if a station is thought of as permanent infrastructure instead, then there would be an interest in maintaining it indefinitely.
We are now coming up to the same problem in the case of the ISS. People are already suggesting that at some future date the ISS will come to the end of its life. For example, in his essay on the next 50 years in space, Michael Griffin ventures to say: "by 2022 the ISS will be definitely behind us" (Michael D. Griffin, "Human Space Exploration: The Next 50 Years", Aviation Week, 14 March 2007).
Again, Claude Lafleur recently wrote in The Space Review: "In 2018, the first elements of ISS will be 20 years old and, if everything went well, dozen of crews would have lived aboard for 6,500 days. The station would probably be at the end of its useful life."
But what these people clearly fail to understand is that the ISS, like Mir, is modular. A well-designed modular construction need never come to the end of its life and need never be abandoned -- except in the case of the collapse of its economic or political base. (I am not necessarily claiming that the ISS is "well-designed", but it could have been.)
On the contrary: progress in manned spaceflight demands continuity and growth. Technologies need the chance to mature through growth in their use, and the customer base for space stations needs to be able to expand. This is hindered when flights are haphazard and occasional, as they are at present, and when space stations are liable to be lost and not replaced.
Without the economically stimulating factors of continuity and growth, manned spaceflight will forever remain an uneconomic, elitist, occasional activity of marginal significance to the broader economy.
Although the construction of the ISS was, of course, originally planned to be finished by now, the way it has turned out is that Zarya and Unity are already 10 years old, having been launched in Nov. and Dec. of 1998. Zvezda is 8 years old as of July 2008. Meanwhile, Columbus and Kibo are almost brand-new, and modules will still be being added up to 2010.
Therefore around 2018 we will have a situation where modules like Zarya and Unity are 20 years old, while Columbus and Kibo are only 10 years old, and elements such as the viewing cupola (due for launch in 2010) will be younger still.
The logical procedure here is to replace Zarya and Unity, and to continue replacing the other modules in order of age, so that Columbus and Kibo get replaced in 2028, assuming a 20-year lifetime per module.
Unfortunately, a decade from now space agencies seem more likely to be losing interest in maintaining the ISS, and focusing on manned exploration of the Moon and Mars.
Whether governments will stump up the money for these flights, and if so then for how long, are open questions -- ones that will only be answered in the fullness of time against a background of continued environmentalist panic as the global population and resource consumption continue to multiply, a continued squeeze on energy resources, ageing populations demanding more pensions and healthcare, disruptive new technologies in computers, biotech and nanotech, and very possibly renewed conflict between the West and the Arab/Iranian world, or between the West and a resurgent Russia or China.
If the space agencies cannot lead the way to sustainable growth in low Earth orbit, then entrepreneurial NewSpace will have to do the job on their own, without the government support they need to reduce the risks and underwrite their investments.
I hope they have enough time and enough private support to complete the job.
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