The master programmer controlled execution of programs. While most of the programming was performed manually by setting switches and cable connections, the master programmer unit allowed for altering the program and iteration. They were set with numbers and functions for the problem before hand, but could not be changed during program execution. This had to be done for every problem and made using the machine very tedious. However, the speed of the computation made up for this.
Ballistic trajectories can take someone with a hand calculator twenty hours to compute.
The Bush differential analyzer reduced this time down to fifteen minutes. Another drawback on the machine was a side effect of its reliance on vacuum tubes, which tended to burn out very often. This caused the machine to go through some ups and downs. It may have a few days of problems, but once they were all worked out it could run error free for weeks. When errors did occur, they were found by stepping through the program one instruction at a time.