> Perseverance was originally a twin of Curiosity, but it cost more than Curiosity.
I can't imagine how that could come about.
> There's no way that a twin of JWST would cost only 10%.
It's kind of the way building things works. The cost of the prototype is enormous compared to the next one. For one thing, the additional R+D cost is $0. The additional cost of the software (and I bet the custom software is a big chunk) is $0. The additional cost of committee meetings to discuss competing alternatives is $0. And on and on.
When cutting parts, the cost isn't in cutting the parts. The cost is setting up the machine to cut the parts. The cutting cost is trivial.
I still can't figure out why this "economies of scale" misconception is so popular on HN with respect to JWST. The major costs of building JWST are in the testing + validation + refinement phases, which must be done meticulously for every unit that is built. A JWST "out of the box" is guaranteed to fail: literally, you could launch a million "unrefined" JWSTs and every single one of them will experience a critical mission failure.
Most components of the JWST are not within spec as they leave the factory floor; for many components, the precision required cannot be achieved with machining metrology alone. Remember that system error compounds with every new component that is incorporated. Components have to be constructed, integrated, and then measured/validated with sophisticated metrology equipment after full assembly. If you're lucky, you can modify the components you have to achieve the desired overall tolerances. But a lot of the time, you have to bin the same component a dozen times until you get a batch which happens to be correct (much like in microchip manufacturing).
And this is just for physical manufacturing -- there are multiple other dimensions which are impossible to get right the first time, requiring multiple iterations until your integration tests pass. Many of these test scenarios are extremely expensive to simulate (e.g. full-size vacuum chambers, launch and zero-g simulators), and must be done to validate every single phase of a 5-year mission to an extremely high chance of success (from transport to launch site -> launch -> full deployment -> science operations). Something as simple as a wrongly-tensioned cable is enough to scrap an entire mission -- the validation is absolutely essential to ensure that anything from a manufacturing defect to a simple human error doesn't make it through to launch.
Even in spite of all the lessons learned from JWST 1, I would be surprised if JWST 2's cost was less than 50% of JWST 1 (realistically, I'd peg it at ~80%). The testing costs are a very high fixed cost that must be paid for every unit you make. There's no other way around it.
I remember seeing a documentary on designing the parachute for one of the Mars landers. JPL build this huge building solely for the purpose of testing the parachute designs. Design after design after design failed, and the engineers were worried that they'd never figure out how to make a working Mars parachute.
But they did come up with a design that worked. Phew!
The cost of that special building, the building's design, the special machinery that filled it, and all those months of testing various parachute designs must have been enormous, and all count for the cost of parachute #1. The construction of parachute #2, after all that, was likely insignificant in comparison.
Again, you'd have $0 in research and development and software and test rig costs of #2.
> The testing costs are a very high fixed cost
I'm sure they are. But you won't have to design the tests and build the test rigs and validate the test procedures a second time. Secondly, you'll inevitably learn from the first test runs to need less iteration.
For example, the full-size vacuum chamber. You would already have it on hand, and not need to build another one. Having already just run #1 through it, you'd know just what to do to get #2 through.
For example, the first time I took the heads off my Mustang it took 4 hours. The second time 2 hours. The third time 20 minutes. The procedure was already all laid out for me in the shop manual. But knowing just what to do cut the time enormously.
> When cutting parts, the cost isn't in cutting the parts. The cost is setting up the machine to cut the parts. The cutting cost is trivial.
When you're building a one-off, you aren't setting up a machine to cut the parts. You're just cutting the parts more or less by hand. It'd be too expensive to set up the machine, and calibrate, and run all of the prototypes to make sure it works, if all you need is a couple of pieces out of it.
I can't imagine how that could come about.
> There's no way that a twin of JWST would cost only 10%.
It's kind of the way building things works. The cost of the prototype is enormous compared to the next one. For one thing, the additional R+D cost is $0. The additional cost of the software (and I bet the custom software is a big chunk) is $0. The additional cost of committee meetings to discuss competing alternatives is $0. And on and on.
When cutting parts, the cost isn't in cutting the parts. The cost is setting up the machine to cut the parts. The cutting cost is trivial.