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In 1962 Dr. Krafft Ehricke, who was one of the most authoritative
       astrodynamicists of that time, carried out one of the most detailed
       analytical investigations of the design of free-fall multiplanetary
       trajectories ever conducted.  He concluded that the effects of 
       planetary gravitational perturbations had to be canceled out by
       using an onboard rocket engine.  This procedure of canceling out
       the effects of planetary gravitational perturbations was essentially
       identical to the method described by Walter Hohmann in 1925.  
       See page 1062 in Ehricke's investigation entitled "Interplanetary
       Flights Involving Several Planets."

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