Surprisingly, NASA did not enforce any particular flight rules and the astronauts could do acrobatics, including dangerous one-on-one air combat training over the Gulf of Mexico. Non-pilots could take the controls from the trainer's back seat during these sessions. The likelihood of being killed in a T-38 crash was one reason a scientist-astronaut left the program in 1968. In fact, five astronauts would be killed in T-38 crashes over the years. Successive heads of the astronaut office had to constantly fight to keep the T-38's against headquarters pressure after each accident or astronaut high-jinks.