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I think you're vastly overestimating the significance of satellite material cost and underestimating how much larger assembly, testing, and operating costs are going to be. Four thousand satellites sounds like a lot and it is in the single unit volume world of aerospace, but it's not even close to enough to reach the kind of economy of scale everyone thinks about when they hear the phrase mass manufacturing. The cost of parts will be tiny compared to the cost of assembly and testing. To give you some perspective, you can get a 80mmx80mm 28% efficient GaAs solar cell for about $600 on Alibaba and the equivalent Si cell is about $6 with about 20% efficiency. At the volumes needed for a constellation of thousands, from a reputable vendor like Spectrolab, the price difference is in the 50-100x range so assuming a $1 Si cell the equivalent GaAs cell will cost $50.

Do you see how small the difference is in absolute terms? That means that only a few hours of labor per solar cell or 100g of launched payload mean the difference between a 50x and 1x difference. Two GaAs panels ($100) will requires tens of grams less support structure than the three Si panels producing the same power ($3) so already the extra launch cost (at $5k per kg, assuming free payload support structure) for the Si panels is eating away at their benefit. Each solar cell will need hours of inspection and testing by people paid $25+/hr so even if each extra Si cell and support structure costs nothing to launch, the final cost once fully assembled, installed, and tested will be in the range of $1k-10k per solar cell.

Especially at the scale of disposable satellites, GaAs is likely to be cheaper, more efficient, and free more volume and mass in the satellite design. 4000 satellites worth of Si solar cells would be the equivalent of a few dozen decent sized residential installs so it would be a drop in the bucket for SolarCity. I don't think keeping such small scale business in-house is much of an advantage, especially when there are many other suppliers with lots of experience in using solar cells in space.

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