See page 13 of: Dowling, Richard L.; Kosmann, William J.; Minovitch, Michael A.; and Ridenoure, Rex W., “The origin of gravity-propelled interplanetary space travel”(IAA paper no. 90-630), presented at the 41st Congress of the International Astronautical Federation, which was held 6-12 October 1990 in Dresden, G.D.R. Available on-line at: http://www.gravityassist.com/IAF1/IAF1.pdf (页面存档备份,存于互联网档案馆) .
See page 13 of: Dowling, Richard L.; Kosmann, William J.; Minovitch, Michael A.; and Ridenoure, Rex W., “The origin of gravity-propelled interplanetary space travel”(IAA paper no. 90-630), presented at the 41st Congress of the International Astronautical Federation, which was held 6-12 October 1990 in Dresden, G.D.R. Available on-line at: http://www.gravityassist.com/IAF1/IAF1.pdf (页面存档备份,存于互联网档案馆) .