4 ms·

But it's not just altitude, right? Acceleration and Speed are important factors to getting to orbit (not that I'm a rocket surgeon).

alonmower 8y ago
Drag at low altitudes is much worse, so perhaps just bringing it to a higher altitude is that much more efficient?
- sitkack 8y ago
  Build a 3 mile tall launch platform that is also an EM rail. Or better yet, BORE a tunnel through a really tall mountain. https://en.wikipedia.org/wiki/Pico_de_Orizaba https://en.wikipedia.org/wiki/Pico_de_Orizaba
throwanem 8y ago
Right. The closer to sea level you are, the denser the atmosphere you're pushing out of the way as you accelerate - thus, the more force you need to use in doing so, and the lower the acceleration you obtain from the propellant you expend. So if you can get above the thickest part of the atmosphere, say attached to a giant goofy-looking airplane, before you light off your engine - rather than having to push your way through that same thick air on your own - you need less propellant, overall, to reach orbit, which means you can devote a larger fraction of your launch vehicle's mass to payload.
- Retric 8y ago
  Rockets are not moving very quickly in the lower atmosphere, which makes a real difference. Space shuttle was 3 g - 1 from gravity = 2g net acceleration, but that's kept low for passengers. Even then 2 g net ~= + 44 MPH every second. So, first 15 seconds your going under 660 MPH which is not that fast, but you get ~1.4 miles up. Because drag increases with speed very quickly the next bit is harder, but you very quickly get above what an aircraft could take you with ~5.6 miles at 30 seconds. Note: Higher g's mean more drag in denser atmosphere, but less gravity drag it's a meaningful trade-off. However, aircraft's velocity is very useful essentially saving those first 15 seconds of full burn, but comes at the price of needing more structural elements to support the hanging rocket.

alonmower 8y ago

Drag at low altitudes is much worse, so perhaps just bringing it to a higher altitude is that much more efficient?

sitkack 8y ago
Build a 3 mile tall launch platform that is also an EM rail. Or better yet, BORE a tunnel through a really tall mountain. https://en.wikipedia.org/wiki/Pico_de_Orizaba https://en.wikipedia.org/wiki/Pico_de_Orizaba

throwanem 8y ago

Right. The closer to sea level you are, the denser the atmosphere you're pushing out of the way as you accelerate - thus, the more force you need to use in doing so, and the lower the acceleration you obtain from the propellant you expend. So if you can get above the thickest part of the atmosphere, say attached to a giant goofy-looking airplane, before you light off your engine - rather than having to push your way through that same thick air on your own - you need less propellant, overall, to reach orbit, which means you can devote a larger fraction of your launch vehicle's mass to payload.

Retric 8y ago
Rockets are not moving very quickly in the lower atmosphere, which makes a real difference. Space shuttle was 3 g - 1 from gravity = 2g net acceleration, but that's kept low for passengers. Even then 2 g net ~= + 44 MPH every second. So, first 15 seconds your going under 660 MPH which is not that fast, but you get ~1.4 miles up. Because drag increases with speed very quickly the next bit is harder, but you very quickly get above what an aircraft could take you with ~5.6 miles at 30 seconds. Note: Higher g's mean more drag in denser atmosphere, but less gravity drag it's a meaningful trade-off. However, aircraft's velocity is very useful essentially saving those first 15 seconds of full burn, but comes at the price of needing more structural elements to support the hanging rocket.