Pros/cons of launching at higher altitudes vs higher latitudes

[Eisen Feuer]

I've seen the radical differences in spacecraft design required to land at sea level or on the high plains of Eve, I was wondering what might be the implications of launching from higher altitudes here on earth, specifically relating to the advantages to be gained by a USA-based space program attempting the build a space station/LEO vehicle assembly station in the equatorial plane of the solar system. So we're talking about establishing a long term goal with many launches, the heavier we can make each launch the better. Less pieces = less assembly = less to go wrong in LEO.

Let's say right now we launch from southernmost Texas at 26° latitude, at only 2.5° above the Tropic of Cancer, only 2.5° of adjustment in the orbital inclination would be required to enter the equatorial plane. But, we're at sea level.

Let's increase the altitude, which for staying inside the continental USA, requires us to move north. At about ~31.5° and ~10,000ft/2900m we have Miller Peak in far southern Arizona. If a peak is too much, there are flatter areas in the Big Bend of Texas that average 4,000ft/1200m.

Then disregarding geopolitics, shipping costs and the challenges of launching from a peak/volcano, there is the ultimate advantage, picking a peak in the tropics (no inclination adjustment) like Pico de Orizaba outside of Mexico City at ~18490ft/5636m, or Mauna Kea at 13795ft/4205m

I guess now that I've said all that what I'm really looking for is the math to make a comparison chart of ∆V saved per altitude increase and ∆V required per degree of inclination adjustment.

[sgt_flyer]

You will face other problems launching from high altitudes for not much gains. Earth's orbital speeds are way higher than KSP's planets - so simply launching from an higher altitude won't change a lot of things. Whereas on eve, the pressure is so strong at sea level that it's very useful to launch from high altitude (so you lose much less delta-v due to drag)

Finally, the main problem will be the way higher logistical and infrastructure costs to launch from high altitudes : bridges and tunnels will put severe constraints on your rocket designs (ex : Baikonour cosmodrome : the Proton rocket design was limited by the railroad tunnels.)

Whereas they can easily transport huge rocket stages on boat in florida or guyane. (Like the space shuttle's external tank, etc)

Basically, i don't think the gains in delta-v you would get by launching at high altitudes would offset the additional costs.

[Justin Kerbice]

@Eisen Feuer: I thought something similar, but in game.

I've add a launch point at the highest Kerbin altitude, but (didn't make the calculation) getting a proper inclination may cost more than saved dV, unless using a carefully planned ascension path to get into the orbital plane.

Feel free to experiment by yourself on Kerbin (click my signature logo, go to buildings, Chalice Tower).

[Kerbart]

It's called delta-V for a reason, not delta-H. The real "killer" is speed. With a circumference of 40,000 km and a rotation period of roughly 24h, launching at the equator gives you 463 m/s for free. At a latitude of 45° that would be reduced to 327 m/s and reduce rapidly as you move closer to the poles. Conversely, when at a 45° latitude, moving to the equator for a launch fill get you around 140 m/s extra for free.

On the other hand, moving to Mt. Everest (around 9000m above sea level) will only get you about 0.008 % further away from the center of the earth, resulting (starting at 45° latitude) in a measly 0.25 m/s extra speed. Add the logistic nightmare of lugging everything up the mountain and it's obvious why a tropical location is preferred over high up in the mountains.

[Kryten]

You might be only slightly higher, but you'd be starting at 1/3 atmospheric pressure, which'd take a big chunk off of losses to drag. Nowhere near enough to justify lugging a rocket up a mountain, but it's what make airlaunch potentially viable.

[R0cketC0der]

You might be only slightly higher, but you'd be starting at 1/3 atmospheric pressure, which'd take a big chunk off of losses to drag. Nowhere near enough to justify lugging a rocket up a mountain, but it's what make airlaunch potentially viable.

IRL rockets only loose about 100m/s due to drag. At 9300m/s needed to get into orbit that makes barely a difference.

[sgt_flyer]

You might be only slightly higher, but you'd be starting at 1/3 atmospheric pressure, which'd take a big chunk off of losses to drag. Nowhere near enough to justify lugging a rocket up a mountain, but it's what make airlaunch potentially viable.

Well, Airlaunch speeds is even more interesting than the altitude at which they make the launches a turbofan powered aircraft carrier (like whiteknight 2) can easily reach and maintain speeds between mach 0.8 and 0.9 - and do it at the equator. (Near 254m/s of additional speed) - besides, they fly at altitudes way higher than the highest mountains

Now, that's still not much compared to orbital speeds

[cpast]

The main advantage of air launch is flexibility - the rocket can fly to almost any launch site, without having to build the expensive ground infrastructure there. The downside is very limited payload.

[Kryten]

Except you still need the specialised range-safety and telemetry infrastructure. Hence Pegausus only using the existing ranges at Vandenburg and Kwalajein.

[Eisen Feuer]

OK so what I'm getting is a maximum of 100m/s ∆V for starting in a dragless environment, a maximum of 463m/s for launching at the equator instead of the poles, no losses to inclination adjustment by launching in the tropics...

How much ∆V is expended in altitude increase rather than contributing to orbital speed? This is fuel being expended when the rocket is at its heaviest, the fuel spent at the beginning is what really makes the rocket heavy when it could be swapping out that fuel for increased payload.

[Idobox]

OK so what I'm getting is a maximum of 100m/s ∆V for starting in a dragless environment, a maximum of 463m/s for launching at the equator instead of the poles, no losses to inclination adjustment by launching in the tropics...

How much ∆V is expended in altitude increase rather than contributing to orbital speed? This is fuel being expended when the rocket is at its heaviest, the fuel spent at the beginning is what really makes the rocket heavy when it could be swapping out that fuel for increased payload.

The delta V to reach 100km is about 1400m/s.

A scenario where high altitude launch facilities are useful is for mass drivers. You definitely don't want your craft travelling at 8 or 9 km/s at sea level.

[DerekL1963]

A scenario where high altitude launch facilities are useful is for mass drivers. You definitely don't want your craft travelling at 8 or 9 km/s at sea level.

You don't even want your craft traveling at 8-9 km/sec at say, two or three times the height of Everest.

[YNM]

Altitude doesn't really save anything. Once you're traveling faster than the terminal velocity (is it gas thermal velocity, actually ?) of that particular height, the air molecules are coming towards you like a brick that stays still - hence we have fairings. And I guess that this is why launchers often spend it's second stage circularizing for a quite a long time (look at Ariane 5 launches - looks silly to me too). Launching from a greater height doesn't really relax this - most high place that's reasonable will still have enough oxygen for us to breathe there - and even, you need to go fast quicker (least so in my opinion, due to reaching greater heights faster).

What save you most is latitude - using the fact that the Earth is spinning and doesn't want to slow down, and it's a sphere, makes the equator to have the highest linear velocity; hence, free prograde dV.

Edited July 19, 2014 by YNM

[Sky_walker]

It's quite common question:

https://space.stackexchange.com/questions/744/effect-of-atmospheric-drag-on-rocket-launches-and-benefits-of-high-altitude-laun

http://www.reddit.com/r/askscience/comments/1xl7iv/escaping_earths_gravity_takes_29810_n_of_thrust/

TL;DR: Mt Everest sux for a launch pad.

[xcorps]

How would a higher launch altitude affect max q? Higher or would it occur at the same speed? Would the forces be lower?

Just something that popped in my head

[magnemoe]

The main advantage of air launch is flexibility - the rocket can fly to almost any launch site, without having to build the expensive ground infrastructure there. The downside is very limited payload.

Yes, you can fly to equator or other places for launch, also if you stage at 10 km you can design the rocket first stage with a vacuum specified engine, you don't have to compromise on the engine.