After briefly serving as a member of the Apollo 8 support crew as well as crew leader during vacuum-chamber Lunar Module testing, Bull resigned from the astronaut corps after learning that he was suffering from pulmonary disease. He attended Stanford University graduate school and received a Ph.D. in Aeronautical Engineering in 1973.
After receiving his Ph. D., Bull returned to NASA and worked at the Ames Research Center from 1973 to 1985, where he conducted simulation and flight test research in advanced flight systems for both helicopters and fixed-wing aircraft. From 1986 until his retirement in 1989, he managed NASA-wide research programs in autonomous systems technology for space applications. He maintained an office at NASA Ames until at least 1997.
Bull died on August 11, 2008 at the age of 73  in South Lake Tahoe, California.