TL;DR
On a circuit connection to the rocket silo it should be possible to read the number of rocket parts built.What?
The rocket silo circuit control should either output the number of built rocket parts as part of "Read contents" or have an option for "Output rocket parts as <X>". The former would be more in line with how assemblers handle "Read contents" and the rocket parts signal would not conflict with any contents.Why?
There is currently no way to check the progress of the rocket silo building a rocket. That makes it impossible to control when a rocket is build or request just enough items to finish a rocket or any other fine control.Knowing how many rockets are available for launch would allow producing items to ship in a just-in-time method instead having to buffer full rocket loads ahead of time.
It would also make it possible to build 49 rocket parts, then load items into the silo via inserters and finally finish the rocket when the rocket should be launched. This would allow the planet to prevent an early launch when the space platform hub has set a "minimum amount".