 After a short retirement, Mueller became the president of the International Academy of Astronautics, in addition to consulting and other volunteer work.Yearning to return to full-time employment, he became the CEO of Kistler Aerospace in 1996 and remained with the company for a number of years.Mueller died of congestive heart failure in Irvine, California, on October 12, 2015, at the age of 97.