There’s physically not enough infrastructure (clean rooms, testing facilities, vacuum chambers, etc…) nor skilled manpower available to NASA to be able to build more than one in parallel, so that would add to the cost significantly.
Nothing?
Of course, the sensible thing to do is to build the test equipment, procedures, train the testers. Then test #1. Then test #2. Then test #3. That's what everybody does.
The cost of the Saturn V rocket was amortized over many launch vehicles, despite a host of new technologies that had to be developed for it, despite much of it being hand-built.
All that adds to the cost of the first mirror, and adds $0 to the second.
I don't think these NASA/JPL guys are quite as dim as you seem to suggest (but you have more info) and they may have done a little cost/benefit analysis before embarking on the project.
BTW, I am not suggesting they are dim. But one can be the finest physicist in the world and not be familiar with cost accounting and manufacturing.
Successful launch and arrival at the observation point? The 6 month setup period before observations can be made?
I contend that success here is a full mission that yields science data over decades of observation. If we cut down the acceptable error rate so we can launch early, how does that impact longevity?
2x — failed to deploy, mission lost
4x — early operation error, 6mo lifetime
3x — lower performance, 3 year lifetime
1x — mission successful, 10 year lifetime
…do we get a better deal building them with larger faults, at a lower cost?
A lot of expense is in developing technologies, assembly tooling, and test rigs — all of which is easier if we don’t need them to be as assured because only 1 in 10 needs to succeed.
No idea. Do you? Genuinely interested, particularly regarding methodology. Not intending at all to be insulting.
My dad told me that Pratt&Whitney made the most reliable aircraft engines by putting them on a test stand and running them at full power until they broke. The engineers would figure out why it failed, redesign the part, stick that on the engine, and continued running them at full power.
It's straightforward, inexpensive, and it works.
Then there's all sorts of complex failures that aren't addressed by single engines: " Prior to this crash, the probability of a simultaneous failure of all three hydraulic systems was considered as low as one in a billion. However, statistical models did not account for the position of the number-two engine, mounted at the tail close to hydraulic lines, nor the results of fragments released in many directions. Since then, aircraft engine designs have focused on keeping shrapnel from puncturing the cowling or ductwork, increasingly utilizing high-strength composite materials to achieve penetration resistance while keeping the weight low"
(I'm not disrespecting P&W- I'm sure they have more tests than just "100% power until it breaks, fix and repeat")
Keep in mind that my former job was designing gearbox parts for the Boeing 757. This included doing the math, and devising a test plan. I've spent a lot of time on "what could go wrong" scenarios in the real world.
Also, when I prepare slides for a coding presentation, the implementation code for a concept gets trimmed way down to what will fit on a slide.
What if 10 were built, each with only a 10% chance of success? What would that have cost? After all, it doesn't need the expense of being man-rated.