1. 1) K. E. Tsiolkovsky: Izbrannye Trudy (Selected Works), Ed. Acad. Blagonravov, Izdatel'stvo Akademii Nauk SSSR, Moscow, 1962, p.222 (in Russian)
2. Edited Machine Translation prepared by Translation Division, Foreign Technology Division, WP-AFB, Ohio, on May 5th, 1966 p.307.
3. 2) G. A. Askarian and E. M. Moroz: Soy. Phys.-JETP 43 (1963) 1638.
4. 3) G. Marx: Nature 211 (1966) 22.
5. 4) W. E. Moeckel: J. Spacecraft 9 (1972) 942.