Hello Vince
Your original question about the Popular Mechanics issue from 1952 featuring a scale model build of the s/s United States has bugged me for a month now. I wanted to help out, since I grew up with those vintage PMs. I remember the cover first featuring it very well.
So I wanted to find it too. The reason I haven't until now just came to me when I looked at the date of your original posting - 19 April of this year. Turns out that the day after I broke my hip - or thigh bone rather - and I'm only now getting to the point where sitting at the computer is reasonably comfortable and fun again. So here goes.
From this search I've learnt that the place to start is here:
The Internet Archive: Digital Library of Free Books, Movies, Music & More.
Whenever you find something there it will most likely get you to an e-book kind of thing in your browser. Try the search option on that site for "Popular Mechanics 1952" and you will see what I mean. You will find one item (N:o 209). Clicking that you'll get to a layout which is very nice to browse, but not so easy to download specific pages, nor to copy them into a graphic programme.
By some kind of freak accident or luck I didn't get there the first time, but to another layout,
namely this one:
https://archive.org/download/PopularMechanics1952.
Having downloaded each and every copy of the twelve issues for 1952 in pdf-format, I of course found that the much sought for United States issue didn't appear until the last issue for that year, N:o 12, December 1952.
Having found and browsed through the article about the United States, it turned out that it continued the next issue, which of course would be January 1953. So the link now had to be manually changed to:
https://archive.org/download/PopularMechanics1953.
On that site I had to download the first four issues for 1953 in order to get the complete set of articles about the S/S United States. So, in short, you will have to click on these five links:
Popular_Mechanics_12_1952.pdf
Popular_Mechanics_01_1953.pdf
Popular_Mechanics_02_1953.pdf
Popular_Mechanics_03_1953.pdf
Popular_Mechanics_04_1953.pdf
I'm glad I found this way to access free archives of PM and many other magazines and books. Hope you'll enjoy the S/S United States material. Also glad about finding links that easily adapt to download-friendly links for any year of PM, and by inference to other publications.
Leif