The TDC was designed to provide firecontrol solutions for torpedo launches against enemy ships. It was most effective against surface vessels, but also helped with hostile subs too. The machine had a wide array of complicated dials and switches and required the operator to input navigation readings from his own boat and the coresponding data from an observed target, and the TDC would report the correct firing angles and velocity. The TDC also had a sophisticated scope that allowed the operator to get an accurate fix on the target's range, speed and heading.
The TDC was a rather bulky addition to the sub's conning tower and required two extra crew, one as an expert in its maintenance, and the other as its actual combat operator. Despite these costs, the use of the TDC dramatically boosted the kill rates for American subs during the war, most notably in the Pacific theatre where the technological disparity was more pronounced.