RADIOISOTOPE FUELED AUXILIARY POWER UNIT. Quarterly Progress Report No. 6, April 1958-July 1958, report, October 31, 1963; (https://digital.library.unt.edu/ark:/67531/metadc1034430/m1/1/: accessed April 21, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; .