The literal meaning of rocket science is the science of rockets and rocket propulsion; the earliest instance that I have found is from a paragraph foreseeing low-cost airlines, published in The Minneapolis Tribune (Minneapolis, Minnesota) of Tuesday 9th December 1930:
VON OPEL SEES ROCKET PLANES IN 20 YEARS
New York, Dec. 8.—(AP)—The prediction that “rocket science” planes 20 years hence will carry tourists from America to Europe in three hours at less than the present fare from New York to Chicago, was made here tonight by Fritz von Opel of Germany. Von Opel, who is here to study American automobile manufacturing methods, was the first to drive the famous rocket car and later a rocket plane.
The word rocket science came to be used ironically as a generic term for anything requiring a high level of intelligence or expertise. All the early instances of this generic use that I have found are connected with sports; on Tuesday 14th May 1985, the Sun-Sentinel (Fort Lauderdale, Florida) published Nuggets’ Moe would fit in with Larry and Curly, in which Bob Ryan wrote the following about Doug Moe (born 1938), an American professional basketball player and coach:
Just because he likes to laugh, and just because he doesn’t believe in treating basketball as rocket science, don’t start thinking he isn’t a tremendous competitor, or that he doesn’t care about winning.
In The Press Democrat (Santa Rosa, California) of Thursday 25th June 1987, Ira Berkow wrote how a newspaper reporter, after attending a “Horse Course” teaching how “to handicap to win in 4 free lessons”, bet on the wrong horse in each of three successive races:
Enough. The reporter decided that handicapping was for other kinds of students, probably Ph.D.’s in rocket science, and quit throwing away his life savings.
The earliest instance of rocket science used in a negative construction and implying that something is simple that I have found is from The Philadelphia Inquirer (Philadelphia, Pennsylvania) of Saturday 25th July 1987; in The cannons of the left, by Bill Lyon, Del Unser, the Phillies’ hitting coach, commented on the fact that all the best baseball hitters, including Ted Williams, “the last player to hit .400”, were left-handed:
“It’s probably something as simple as the majority of pitchers being righthanders,” he said. “The very best hitters do have a couple of things in common, though — compact strokes, and their hands are always back. And they’re students of the game. They don’t just study hitting but pitching, too. Ted Williams always preaches that pitchers are creatures of habit and tend to repeat their sequences. Just pay attention.”
It may not be rocket science exactly, but .360 doesn’t come about by accident, either.