Which engineer conceptualized the Grand Tour mission that led to Voyager?

Aerospace engineer Gary Flandro conceptualized the Grand Tour mission that led to Voyager.

In 1964, while working at NASA’s Jet Propulsion Laboratory, Flandro recognized that a rare alignment of Jupiter, Saturn, Uranus, and Neptune could let one spacecraft visit the outer planets using gravity assists. The geometry would occur only about once every 175 years, making the late-1970s launch opportunity unusually valuable.

The original Grand Tour was an ambitious proposal involving multiple spacecraft and visits to the outer planets. Its cost led NASA and Congress to scale the idea back, but the resulting Mariner Jupiter-Saturn project retained trajectories that could be extended. Those spacecraft became Voyager 1 and Voyager 2. Voyager 2 ultimately visited Jupiter, Saturn, Uranus, and Neptune, fulfilling the core outer-planet vision.

William Pickering was a major JPL leader, Carl Sagan helped shape scientific and public interest in planetary exploration, and Edward C. Stone later led Voyager science. None of them, however, was the engineer credited with identifying the Grand Tour opportunity.