Astronaut Group 6 (the 'XS-11') was announced by NASA on August 11, 1967, the second group of scientist-astronauts. Only five of the eleven were given formal assignments in the Apollo program and these were all non-flying support crew positions. (Chapman was a support crew member on Apollo 14 and Apollo 16; Parker was assigned to Apollo 15 and Apollo 17; England was assigned to the Apollo 16 support crew; and Henize and Allen rounded out the Apollo 15 support crew.) Assignments for the group were delayed by the requirement to spend a full year at UPT to become qualified as jet pilots like the Group 4 scientists before them), a process that resulted in the initial attrition of O'Leary and Llewellyn in 1968. Subsequently, Musgrave and Lenoir were assigned as backup Science Pilots in the Skylab program while Chapman and Holmquest resigned due to the dearth of NASA flight opportunities in the era. The requirement for scientists to be trained as jet pilots was eventually lifted with the creation of the Mission Specialist position in the Shuttle Program. The seven members of Group 6 who stayed with the program after Apollo went on to form the core of Shuttle Mission Specialists, accomplishing a total of 15 flights.
A chart showing Group 6 assignments in relation to other astronaut groups up through the Apollo era. This shows how Group 6 scientists earned their nickname "XS-11", with all eleven members being "excess" to the program.
Completed initial training and work on Skylab habitability systems and medical experiments, but left NASA in September 1973 following two extended leaves (encompassing postdoctoral training in nuclear medicine at Baylor University) due to lack of spaceflight opportunities.