Lower air pressure. Rocket engines have to be optimized for a range of pressures for maximum performance. Small nozzles near sea level and large nozzles in vacuum. Too narrow, and the exhaust expands sideways and you lose thrust. Too wide, and you get back flow around the rim of the nozzle and lose thrust.
That first mile gets you down to 0.82atm already, so you can optimize for a much larger range of altitudes on your first stage.
While this is true, our ICBM program showed us the differences are extremely minute and easy to compensate for.
The same Nike for example can launch from Pacifica California (100ft) or Mount Gleason (6,500ft).
The difference is almost completely ignored beyond tracking needs, because the point of the rocket is SPEED. It's actually about the rockets ability to get to speed, not just to climb altitude. That's where the fuel goes, it's for acceleration, not just raw thrust.
The difficulty for a rocket is not the ascent, or to get over the hurdle of atmosphere, it is to gain speed second by second. You have to be at or exceeding 17,000mph by 250 miles high to stay in low earth orbit. Shaving a mile off the launch and making it 17,000mph by 249 miles high, effectively doesn't change the math at all because that first 5000 feet on launch is completely negligible regardless of altitude, it's simply establishing forward inertia.
The height wouldn't matter much, but it's true that launching in thinner air would save a significant amount of fuel weight fighting drag, and the drop in pressure from sea level to the tops of mountains is decent enough to matter.
The real reason we don't do this is just that there are like a dozen other more important things that make a good launch site and it's not practical to find a place that checks all the boxes.
Actually if we launch from the top of a tall mountain near the equator I believe it saves like 10% fuel which isn’t nothing. The problem is 1) those mountains aren’t in countries with space programs and 2) infrastructure up mountains is hard. There are some chilean mountains that would actually make pretty good candidates though.
It would actually make a bit of a difference. Not because of the height, but because the air is thinner so you'd get better efficiency. There's a reason rockets fly up first, the original guy is just being obnoxious. A lot of the comments here don't seem to realise that, the main reason is logistical and having a nearby ocean to drop things into.
That's the main reason. If launching rockets from top of mountains would make a huge difference, I'm sure governments and billionaires would build mountain railways and pipelines to transport parts and fuel to the top.
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u/rysy0o0 11d ago
It would cost a lot of money to haul all the rocket parts and fuel on top of a mountain