1. Optimum transfer between non-coplanar elliptical orbits;Eckel;Astronautica Acta,1962
2. Optimum Transfer in a Central Force Field with n Impulses;Eckel;Astronautica Acta,1963
3. Fundamentals of space navigation;Lawden;J. Br. Interplan. Soc.,1954
4. Numerical solutions of non-coaxial optimum transfer problems;Eckel;J. Astronautical Sciences,1963