i don't know if Launch loop is possible with 1970s technology
even now is on edge of feasible, more on this here
You need allot of machine in orbit to build a O'Neill habitats out of stuff you find in space
If the Launch loop is not a option, then you need big reusable Launch Rocket
Like SpaceX BFR, Convair NEXUS, Douglas Rombus or Boeing HLLV /
MLLV and Seadragon
about O'Neill habitats
by the way
on Issac Arthur You tube page has allot answers for this discussion
https://www.youtube.com/channel/UCZFipeZtQM5CKUjx6grh54g/featured
That's where I've been getting half my sources from (having discovered Mr. Arthur in the past few weeks), the other half of my sources is either what I already knew or content linked from Isaac Arthur's videos.
Now, as Isaac Arthur points out, there are many alternatives to launch loops, although Keith Lofstrom himself has some
criticisms for alternatives to rockets (as a side note, I love Lofstrom's writing style, he comes across as a crazy old mad scientist). I'm not an electrical engineer (or any sort of engineer) like he is, so I can't exactly say if he's right or wrong on his criticisms.
His response to Isaac Arthur's video is a pretty interesting read.
From what I get, we'd get a lot closer to getting our launch loop if we start small first. And in the context of launch loops, that means using
smaller versions for power storage, which Lofstrom calls a power loop. This is a relative of the flywheel, which is already used for power storage. For renewable energy, power storage is even more important since the sun can't shine all day (except in space) and the wind doesn't blow all the time. Lofstrom even proposes a few scaled-up power loops which serve as a global power grid. Related to this, my favourite idea for a launch loop is to build it in the vast and mostly unpopulated Australian outback, where you have extensive amounts of solar energy for powering the thing (and nuclear backup using locally mined uranium). This could also be used for transport, like from a southern Australian city (Perth, Adelaide, Melbourne) to Darwin (with stops on the way, which an Adelaide - Darwin loop could easily put a stop at Alice Springs, say), which once that market develops, will in of itself be a motive for building further launch loops (Denver to Washington launch loop, or assuming Africa is stable and no potential for terrorism against a launch loop, a Cape to Cairo launch loop even)--new towns will sprout up along the "ports" alongside the loop. I know Lofstrom says to build it over an ocean, but I can't imagine you'd lose too much potential by building it away from the equator, and using it for transit around Earth seems too good to pass up. Launch loops also make for great telecommunications platforms (indeed, the same "active support" sort of structure can make for fantastic radio masts, and I fully expect any launch loop to be used for such).
Let's put this together. We have massive advances in solar power, and likewise flywheel energy storage (thanks to materials science advances) which develops into something like the power loop. The idea of "global warming will destroy everything" is hyped during the 50s/60s, and this flows nicely into a solar energy/renewable enegy wank. We have a buffed environmentalist/green movement which promotes renewable energy, and even behind the Iron Curtain, communist nations get a lot more green and likewise invest in renewable energy. Overpopulation and the "population bomb" is an even more massive concern, and the idea of consuming all of Earth's resources and destroying the beauty of the world becomes a public concern. Nuclear energy is still used, but the anti-nuclear movement still exists thanks in part to atomic weaponry. During the 60s, the leading nations of the world decide to hold a conference on global warming and what they can do to stop it. The idea of millions of small mirrors in space to block sunlight is proposed at the conference. The question becomes "how do we launch these mirrors?", and the launch loop is devised as the solution, as a logical extension of the power loops. During the 1970s, the first launch loop is built in Australia with global funding, with even communist nations like the Soviet Union and China providing money and engineers to the project (in part to develop their own). The mirrors are launched starting in the late 70s, but lots of other space industry ends up developing at the same time with this revolutionary new launch tech, including skyhooks to make launching easier.
Other launch loops are under construction globally starting in the late 70s, including America's where ATL's Keith Lofstrom (yes, butterflies, but his parents were probably alive before our 1929 POD) plays a key role in the design (TTL he becomes most famous for his
Server Sky concept, launched using launch loops). Many launch loops are fueled by solar and wind power (and the supporting power loops). As I noted earlier in this thread, by the 90s we still have ecological concerns (ocean acidification, caused by the same things as global warming), yet have moved lots of resource extraction to near-Earth asteroids and the moon.
At the start of the 21st century, a new space race is underway, a five-way space race between the US, USSR (reformed to be more like PR China TTL so it never falls), China, India (far more developed TTL), and a federal European Union, each competing to build the grandest space colonies which are far better than the small colonies in space where miners on the Moon live with their families. Other nations like Brazil and Japan aren't far behind, while the Arabian Republic (a united Arab peninsula initially under Nasserist ideology) uses what's left of its oil money and its vast solar energy money to likewise compete. Global initiatives (supported by the UN, major businessmen, and every nation) are promoted starting in the late 2010s that by 2050, every nation will own at least one space colony like the American Washington Colony or the Soviet Lenin Colony. Although the public's attention has always been captured by rotating space habitats, far more than the idea of living on Mars, thanks to pop culture TTL since the 60s including numerous popular movies featuring space colonies, popular novels (including ones not thought of science fiction), popular TV shows and animation (TTL's version of Mobile Suit Gundam is just as much a pop culture fixture globally as it is in Japan), and later video games (a cliche sort of mission is often "defend the launch loop from the bad guys"). Floating space habitats are as commonplace in the media TTL as Moon/Mars habitats are OTL--moon habitats (using spin gravity mixed with the Moon's gravity) are commonly seen TTL, but Mars is thought of similarly as Venus as being bad for long-term human habitation since it's a dreary, frigid desert far from home.
Basically, we have a renewable energy wank and environmentalism wank which results in a launch loop wank which results in a space colonisation wank. In TTL's 2018, I could easily be writing this post from space, either in a space hotel or even from my house in a O'Neill cylinder.