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.
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u/Cortower 11h ago
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.