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I don't recall the details. But there was a plan back in the day to launch rockets under huge balloons. The plan was they would float up (like giant weather balloons) to high altitude without using any fuel and then the rocket engine would ignite and blast the rocket through the balloon into space from 60,000 feet - climbing the first 60k without fuel was supposed to save a lot of fuel. I don't think much came of it. Possibly because the complexities of doing so far outweighed any savings in fuel.