Rocket launch locations?

Question

Since most of the fuel is consumed by a rocket in within the first few miles of flight, why aren't rockets launched at higher altitudes where air density is lower?

I know rockets are launched over the ocean to prevent pieces of the first stage from causing land debris, but i'd imagine launching over a desert would not be too hazardous.

Answer 1

Some rockets are launched from the air.

But there are limitations on what you can haul into the air with a 747 or bomber.

Answer 2

Launching from high altitude would not be a terrible idea, but the linear speed is probably the most important issue--rockets use the most fuel in the first few miles not to get up to altitude, but to get up to _speed_.  The energy it takes to lift a payload to 100 miles is minor compared to the energy it takes to accelerate it to a 17,500+ mph _orbit_, and every extra 1% of speed requires much more than 1% more fuel, because you have to accelerate that extra fuel as well.  If I remember my astrophysics homework from college (which was 20 years ago, so don't quote me on this), but an extra 1% of orbital energy will require an additional 3-4% of fuel in to reach orbit.  So you can imagine an extra 2-3 km of _stationary_ elevation is not all that helpful.

Answer 3

Or maybe from Mauna Kea or Mauna Loa in Hawaii.  High altitude, ocean to the east, and low latitude -- all good things.  But one must compare the expense of launching single rockets from less than ideal sites, to the cost of taking all the materials you would need to build a launch facility way up on a mountain.

Anyway, your idea makes sense and we do have a good place for it.

Answer 4

Your idea makes sense.  They launch from coastal areas for range safety and as close to the equator as possible because the Earth is moving faster there. A high altitude launch site would complicate a launch and would not offer much of an advantage.  Denver is the mile high city, but it would be hard to find a high altitude launch site that was also easy to get to, and launched over a unpopulated area.  When launching a rocket to orbit it doesn't go straight up.  They aim the rocket up and towards the east or in the direction of orbit.

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Answer 5

LAUNCH ON WARNING

Answer 6

Well, I think the lower latitude ends up being a bigger cost saver than the altitude of launching from the Sierra Nevada over the desert. At lower latitudes, the linear velocity of the Earth's surface is higher, so you're getting a pretty big boost from that.

Also consider that the rocket is past the altitude of even Mt. Everest in less than a minute. The difference in air pressure at Denver shouldn't make that big of a difference in fuel consumption, because the main drain on the fuel is overcoming the momentum of the craft itself.

It's sort of a pyramid situation - the more weight you want to launch, the more fuel you need to launch it, which adds more weight which requires more fuel to cover the added weight of that fuel..... It's the launch weight that is the main drain on fuel consumption, not air resistance. NASA's approach to dealing with the air is to not go to full throttle until after about 90 seconds into the mission.

They save fuel by going slower through the thicker part of the air. This is a two-pronged solution because wind resistance increases on the square of the velocity, so going slower helps. It also helps to slow down because the lower speed requires less strength in the structure of the rocket to resist the leading air pressure. If they went faster in thick air, the pressure would be higher, which would require a stronger, heavier structure. (now see the fuel vs. weight problem again)

All in all, I'm quite certain that the cape is our most ideal location to launch from, given that when it was established as our launch site, NASA pretty much had carte blanche to the entire nation's resources, and that's where they chose to launch from.

Not only these reasons, but I forgot to mention that a low-inclination orbit takes less energy than a high-inclination orbit. Low inclination orbits are most easily reached from low latitudes.