How to say "It's not rocket science" before rockets existed
Solution 1:
You need not be a wizard.
wizard:
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a person who practices magic; magician or sorcerer.
-
a person of amazing skill or accomplishment: a wizard at chemistry.
(Random House)
But one need not be a wizard to foresee by now that the outbreak of a revolutionary movement on the Communist order, in a Europe laid waste by a long war, will result in an era of anarchy, misery and extermination. (Google Books, 1940)
Financial geniuses are rare. Many men get wealthy through persistent plugging. You need not be a wizard. Just common sense will put you where the wolf will never molest your door. (Virginia Chronicle, 1921)
Solution 2:
You don't have to be a genius appears to have been used in the early decades of the 20th century. Ngram shows examples of its usage before the 1930's.
From Popular Science. June 1919:
- You do not have to be a genius. If you have a liking for drawing and develop it intelligently, there are many opportunities for you in this profitable professian.
Solution 3:
It's not a Herculean task.
i.e. not requiring tremendous effort, strength, etc.
by reference to the twelve labours of Hercules (latin) or Herakles (greek). - Wikipedia
1748, Tobias George Smollett, The Adventures of Roderick Random
"He replied in a dry manner, that I would find it a Herculean task to chastise everybody who should laugh at my expense".
2006: Jeremy Clarkson, Top Gear, commenting on the Bugatti Veyron
"The guys at Volkswagen have a Herculean task".
For VW, we can also say the same in 2015!
An authentic quote with "not" in Justice of the Peace and Local Government Review dated 1841, citing Douglas Fox, Esq. surgeon and child labor pundit, regarding the Derby Silk Mill:
Running 20 miles a day is not a herculean task for a child of 10 years of age.
Times have changed!
Solution 4:
I think the phrase you are looking for is "It's Greek to me" but used in the negative, "It's not Greek." This phrase predates Shakespeare's use in Julius Caesar, and in my opinion is the closest fit.
If this kind of exchange happened in Victorian London I don't think it would be anachronistic:
"I'm thinking of purchasing a motor carriage but I'm not sure. It looks rather difficult to operate."
"Nonsense. It's not Greek, you know. Anyone with the least bit of perseverance could master that contraption. Why, I have heard that even a lady might be successful in its operation."