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For some purposes it's understandable - we send the probe, get the science it's designed to collect, and we're done. And even in those cases if the mission fails we do not attempt again - we collect the crush data and scrape the mission. For example the schiaparelli lander on mars or the long tether in orbit experiment.

Why don't we launch few Hubble or Kepler telescopes each costing about 500 million to develop and probably 50 - 100 to build now that we have the design. NASA spends 9 billion on James Webb, why not spend few hundred million to launch more of those observatories? Why no more shuttles are built if it's such a great design?

And why don't we ever launch few probes for a backup if one fails?

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    $\begingroup$ See the Mariners, Surveyor, Voyager, Viking, Mars Exploration Rover. $\endgroup$
    – Mark Adler
    Apr 5 '17 at 2:28
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    $\begingroup$ My guess:each vehicle is largely hand-built - lots of labor making it very expensive, no significant opportunity to cut "per-copy" cost by making multiple. There's no opportunity to save on launch costs - each vehicle takes the same amount of fuel, effort and equipment to get it off the ground, and each vehicle requires its own staff to fly the mission. For one research objective, barring malfunction or mishap, one vehicle is enough and all that needs to be budgeted. Some missions do use multiple vehicles, but it's a requirement of the mission objective. $\endgroup$
    – Anthony X
    Apr 5 '17 at 3:01
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    $\begingroup$ "First rule of government spending: Why build one when you can build two, at twice the cost!" $\endgroup$
    – Cort Ammon
    Apr 5 '17 at 4:31
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    $\begingroup$ The shuttle wasn't such a great design. The asymetric form made everything very complicated. The reusability never materialized, and it was seriously overbudget. So it missed most of its design goals, but NASA was committed to that program and had to fly them, but didn't bother building more. Now everyone is flying and designing symmetric rockets again. $\endgroup$
    – Innovine
    Apr 5 '17 at 6:10
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    $\begingroup$ Because new discovery requires instruments with new capabilities. Unique stuff never done before, and which will never be done again since it just filled a temporal gap in the exploration. Another copy of the original Hubble space telescope wouldn't revolutionize much. One must create unique new instruments to see that which has never been seen before. The latest astronomical instrument will always be as expensive as is possibly affordable. (But for other purposes than extreme exploration, multiplication makes alot of sense) $\endgroup$
    – LocalFluff
    Apr 5 '17 at 15:28
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The Russians often build multiple copies. The Nauka module, yet to be launched, was the backup for the Zarya module.

The Zarya module was the Mir backup. (Might be Zvezda, I always get those two mixed up).

There is often an engineering model that is kept back on earth for testing changes in software as a full up simulator before uploading code. (I.e. Bricking a billion dollar probe orbiting Saturn is VERY VERY VERY VERY bad form).

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    $\begingroup$ I'd also add that communication satellites are often produced almost in batch. SS/Loral has they LS-1300 platform, Boeing their 702, Orbital the Star2, Airbus the Eurostar 3000 and Thales Alenia the SpaceBus 3000. Between different spacecraft, only the payload changes, and even then, it'll be the frequency and number of amplifiers. $\endgroup$
    – ChrisR
    Apr 4 '17 at 20:55
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    $\begingroup$ It's also worth noting that we used to follow this practice, with the Voyagers, Mariners, etc. $\endgroup$
    – Phiteros
    Apr 4 '17 at 22:33
  • $\begingroup$ @Phiteros True. I suppose a point could be made that previously we sent several probes to increase the chances that either arrived successfully to complete its primary mission; these days we build singles, and do everything we can both before and after launch to ensure that it arrives successfully to complete its primary mission? $\endgroup$
    – user
    Apr 5 '17 at 11:43
  • $\begingroup$ @geoffc Wasn't it more the Mir replacement than backup? $\endgroup$
    – user
    Apr 5 '17 at 11:44
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Why we always build only one spacecraft?

We don't always build only one spacecraft. A newly launched environmental or communications satellites oftentimes is a carbon copy of existing environmental or communications satellite. For example, 17 Geostationary Operational Environmental Satellites have been built (one suffered a launch faiure), but there have only been five GOES designs. The newest design is expected to be reused at least four more times. That design will be obsolete by the time the satellites using this newest design start failing in the mid 2030s.

That said, building four or five copies does not constitute mass production. Production costs are reduced with those copies, but not by orders of magnitude.

NASA spends 9 billion on James Webb, why not spend few hundred million to launch more of those observatories?

I suspect the cost of a carbon copy of the James Webb Telescope would be well over a few hundred million dollars, and more likely over a billion dollars. The mirrors are one of the biggest costs in developing a new large telescope, whether space-based or ground-based. Pseudo mass production doesn't help much here. Moreover, that design will be obsolete by the time the Webb fails in the 2030s (or so we hope).

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    $\begingroup$ "The mirrors are one of the biggest costs in developing a new large telescope, whether space-based or ground-based." – Even that may be an understatement. Often, they are scraping the very edges of our capabilities to build them. I wouldn't be surprised to learn that the mirror alone was worth on or even several PhDs. $\endgroup$
    – Jörg W Mittag
    Apr 5 '17 at 14:27

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