I'm doing a research on relativistic time dilation and I'm analysing context and results of Hafele-Keating's experiment performed in 1971/72.
It looks not easy to retrieve the papers cited in the different articles, in particular this one (the oldest):
"Relativistic Behaviour of Moving Terrestrial Clocks", published on Nature vol 227 in 1970. Any (legal) suggestion/site for not spending a fortune for buying them?