Perhaps this:

Henry J. Kelley (1960). Gradient Theory of Optimal Flight Paths.

[1] https://claude.ai/public/artifacts/8e1dfe2b-69b0-4f2c-88f5-0...

Thanks! This might be it. I looked up Henry J. Kelley on Wikipedia, and in the notes I found a citation to this paper from Stuart Dreyfus (Berkeley): "Artificial Neural Networks, Back Propagation and the Kelley-Bryson Gradient Procedure" (https://gwern.net/doc/ai/nn/1990-dreyfus.pdf).

I am still going through it, but the latter is quite interesting!

Count another in the win column for the USA's heavy investment into basic sciences during the space race.

So sad to see the current state. Hopefully we can turn it around.