Why do we launch space satellites and vehicles from sea level and not from a higher locale like Denver? Is saving a mile not more fuel efficient? Gravity is less the further away from center or is this negligible in a mile? Atmosphere thinner so less friction and less heat over less time? Must be something simple I am missing.
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1$\begingroup$ The summit of Mt. Kilimanjaro would be great -- but getting the equipment up there would probably be more effort than it's worth. $\endgroup$– Keith ThompsonAug 29, 2016 at 21:48
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$\begingroup$ Latitude is more important than altitude, and although there are tall mountains near the equator, they're in places like Ecuador and Tanzania. A country like the US doesn't want to build a spaceport in a place like Ecuador, because NASA is about pork-barrel politics. There's a reason that the Johnson Space Center is in Lyndon Johnson's home state. Launching from a mountaintop would also have to be cheaper or better than an air launch to orbit, en.wikipedia.org/wiki/Air_launch_to_orbit $\endgroup$– user687Aug 29, 2016 at 23:46
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2 Answers
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At the surface, this seems like a decent idea. Why not launch from higher up, so you don't have to go as far?
The answer is that when choosing a launch location, we are not so much concerned with height as we are with speed. To get into a Low Earth Orbit, you need to be traveling at about 7.8 km/s. At the equator, the Earth is spinning at 1670 km/hr, or ~0.5 km/s. This means that we need 7.3 km/s of delta-V
According to NASA, we can calculate the Earth's rotational velocity at any latitude using the formula $1670*\cos(\theta)$. Denver is at 39 degrees North, so the rotational velocity there is ~1298 km/hr, or ~0.36 km/s. So to launch from Denver we need to increase our speed by 7.44 km/s. This is a difference of 0.14 km/s, which is a significant saving.
Of course, we don't actually launch from the Equator, but we try to get as close to it as we can. All the different countries around the world try to launch as close to the equator as possible.
The other thing to consider is that the Earth is not perfectly spherical. It has an equatorial bulge formed by its rotation. This means that the radius at the equator is about 26 miles larger than at the poles. According to Wikipedia, this results in the equator actually being taller than Mount Everest. Of course, the atmosphere and ocean also bulges out like this, so you are still under the same amount of atmosphere, but it does make it easier to get to space. Since you're further from the center of the Earth, you experience more centrifugal force. This helps counter the force of gravity by a very tiny amount, but every little bit helps when trying to get to space.
As @OrganicMarble points out, it also allows us to drop our boosters in the ocean, to be retrieved later.
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$\begingroup$ "At the surface" -- Was that intentional, or just a happy coincidence? $\endgroup$– userAug 29, 2016 at 21:07
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1$\begingroup$ Boosters generally speaking aren't retrieved for future use, and it would be easier to retrieve them in many ways if they landed on the ground. But it has the effect of not dropping a large object on a city! $\endgroup$– PearsonArtPhoto ♦Aug 29, 2016 at 21:15
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1$\begingroup$ This answer is written as if the only issue were delta-v, but atmospheric drag is also an issue. $\endgroup$– user687Aug 29, 2016 at 23:37
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1$\begingroup$ @EvanSteinbrenner No, you see it's how much deltaV you need. You need a total of 7.5 km/s. Launching from the equator gets you 0.5, and launching from Denver gets you 0.14. So you need an to accelerate by 7.3 from the Equator and 7.44 from Denver. In this case, a smaller required deltaV is better. I'll change "acceleration" to "Delta-V" to clarify things. $\endgroup$– PhiterosAug 30, 2016 at 0:12
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Space launch is speed, not altitude. (If you climbed a 200 mile high tower based in Denver, and stepped off, you would NOT be in orbit and you'd fall to the ground). The closer your launch site is to the equator, the more free rotational speed you get from the Earth (as long as you launch eastwards).
In addition, it is nice to have less populated areas to the east (the ocean, for example) for your boosters to fall in.
answered Aug 29, 2016 at 20:31
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2$\begingroup$ "If you climbed a 200 mile high tower based in Denver, and stepped off, you would NOT be in orbit and you'd fall to the ground" But if you climbed a 20,000 mile high tower based in the appropriate spot, and stepped off, then you might be. :-) $\endgroup$– userAug 29, 2016 at 20:59