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Comment by SonicScrub | Hacker News Reader

Using a simple back of the envelope calculation on energy requirements can tell you why. A 500km orbital altitude requires about 7.66 km/s of velocity. That's 29,337 kJ of kinetic energy per kilogram, and only 4,905 kJ per kilogram of potential energy. You don't perform an air-launch to for the potential energy savings: you do it for the reduced air-loads and energy loss due to drag from beginning your trajectory in a thinner atmosphere. This trade-off is almost never worth the added engineering complexity, as seen by vertical launch completely dominating the launch space compared to air-launch.

As for a balloon launch, well, balloons have absolutely dismal maximum payloads, so there's no way you're going to be able to hoist a vehicle with enough propellant to reach upwards of 7 km/s underneath a balloon.

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Excuse me but I really appreciate your replies, so my reading of what your saying is the launch is less about reaching a height but instead reaching a velocity that can escape earth’s gravity and launching from a higher altitude just decreases the distance the rocket has time to reach this speed?

Things in orbit like satellites and the moon have not escaped earth's gravity at all. There's lots of gravity in orbit. Being in orbit is when the centripetal force from an object's tangential velocity matches the force of gravity. In other words, get thrown so fast that the arc of your trajectory continually misses the Earth as the surface curves away from you.

The only reason why you need to go up at all is the pesky atmosphere works to slow you down, making it difficult to maintain such high speeds. Satellites have to be up high enough that drag becomes less of an issue. If it weren't for the atmosphere, you could achieve orbit 10 feet off the ground if you moved fast enough.

What about vertical launch from 10k feet up, or even higher on a mountain or something, would that provide a better energy savings?

That savings has to be worth the building of expensive infrastructure at altitude, which is unlikely to happen. Air density at 10k feet yields is only 25% less than sea-level, so you're not going to see substantial performance gains. It's much better to focus efforts on choosing a launch location as close to the equator as possible where you can shoot due East (to take advantage of Earth's rotation) without flying over-top of people. This will yield much greater energy savings. There's a good reason why the world's launch locations are where they are.

you get more savings being near the equator than being high. if you can manage both you get a very small win that might not be worth it if you are far from infrastructure. better to add a little more fuel than make the satellite get shipped to your remote equatorial mountain

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