With Apollo 10 having mitigated the risks, Armstrong and Aldrin were able to focus their training on the powered descent and lunar lift off. However, because Apollo 10 had first call on the simulators until early May, Clifford Charlesworth initiated training in April with the Saturn V launch phase. Two months then remained in which to conduct the specialised training because, with a target launch date of 16 July, the most intensive training using the simulators would be completed about 10 days earlier in order to enable the crew and flight control teams to finish other activities. Simulation explored two basic scenarios: ‘nominal’ and ‘contingency’. The nominal part occupied only a few days, and defined the Go/No-Go decision points, the procedures, and the timings for the interactions between the crew and the flight controllers. The first full set of mission rules for Apollo 11 was issued on 16 May, but was preliminary pending methodical testing by simulation. Because the nominal powered descent was to last only 12 minutes, it was possible to perform many runs and debriefings during a single day’s training. While Apollo 10 was performing its rehearsal in lunar orbit, Armstrong and Aldrin were routinely landing by flying the nominal profile. Contingency training was designed to test how the crew and flight controllers dealt with departures from the nominal profile involving trajectory and systems problems. The Simulation Supervisor (SimSup) for the powered descent was Dick Koos, an early recruit of the Space Task Group to train control teams. As there were then no graduates with computer degrees, NASA had hired engineers with experience, and his background was the computerisation of ground-to-air missiles for the Army Missile Command at Fort Bliss, Texas.