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Just changing the code on one computer won't do it.

I'm not sure what exact configuration they're using now, since I left NASA in 1987, and the shuttles have been upgraded several times, but in the past, the shuttles had 5 flight computers on board. At any one time 3 computers ran in parallel, checking with each other to be sure they all agreed on the commands. If one of them started to disagreed with the others, it would taken off-line and one of the remaining two would be brought into the voting.

4 of the computers were identical, with identical software. 1 of the computers was a completely different design with completely different software. Designed by different people at a different company with the software written by different people at another different company.

This was to eliminate the problem of a built-in error in the computer or software design causing unexpected problems.

With the 5th computer a different design with different software, it was thought that this would cancel any errors in hardware or software.

Also the software loaded in the 4 identical computers was not the same. Each computer has its own software load, with separate storage and separate compiling.

That way a random error in one computer's software load would not be duplicated thru all 4 computers.

BTW when the shuttle first flew, the flight computer was about the power of an Apple II.