Comment by dkokelley | Hacker News Reader

Comment by dkokelley | Hacker News Reader

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While burning off excess speed is one major concern for this approach, I've always wondered what it would take to use a balloon like this one to get past some of the tricky parts of rocketry like dealing with atmosphere and carrying extra fuel to carry the extra fuel you are carrying. If you could launch a rocket from 40km (roughly the height of the balloon), you're 25% into the minimum LEO, and ~10% of the way to the ISS. More importantly, the fuel savings from launching at a higher altitude are exponential, because each meter of altitude is one meter of less atmospheric drag (with diminishing returns) and the corresponding fuel/weight savings from not having to carry fuel to travel the first 40km.

I would imagine that the current limitations on doing something like this are related to the maximum weight a hot air balloon can lift, in addition to the untested physics and engineering of a mid-air space launch. Imagine if SpaceShipOne (which was launched at 13.3km) could be taken to 40km and then launched.

Perhaps someone with a better understanding of lighter-than-air lift physics could explain what the current limitations on this are.

Suppose you could use a balloon to get to LEO altitude. You're still not in orbit. Anything that actially is in orbit will come hurtling at you faster than a bullet. Most of the fuel used in getting to LEO is used to achieve orbital speed, not orbital altitude. So the question of ground launch vs space launch is really one of where your engines operate most efficiently, or if you can avoid air friction in the dense lower atmosphere, or that sort of thing. Baumgarnter was nowhere close to "10% of the way to the ISS."

You're right about acceleration being an issue. It's possible that altitude is negligible compared to orbital velocity. I'm not sure what you mean by Baumgartner wasn't 10% of the way to the ISS. The ISS is at 370km, and the jump was from almost 40km. Of course there are other technical hurdles to getting there, but I was only referring to pure altitude.

Considering kinetic and potential energy together, adjusted for weight, he was about .5% there. 6.310^7 (m/s)^2 vs 3.810^5 (m/s)^2

A good way to sort of wrap you mind around exactly LEO means is to consider that at LEO altitudes the strength of gravity is, for most intents and purposes (9.0m/s^2 vs 9.8m/s^2), the same as on the surface. If you get up that high then let go, you are going to drop like a rock.

If you want to not fall, you have to keep thrusting straight up, which obviously is not practical. The next best thing is to try to move sideways. Sideways fast enough that you clear the horizon before you hit the ground. You have to move fast enough that when you fall you miss the earth.

Moving that fast is pretty damn hard. Any rocket capable of doing it, even if it is already "up there" is going to be far to large to practically lift with a balloon.

> You have to move fast enough that when you fall you miss the earth.

So what you're saying is that there is an art, or rather, a knack to flying, and that the knack lies in learning how to throw yourself at the ground and miss?

took a while to get that reference.

I didn't figure I'd get downvoted for quoting Douglas Adams, though.

And it wasn't even "just" a joke, finding out there was actually a grain of truth to that classic, I felt worth pointing it out.

I thought it was hilarious, thank you. Never figured Douglas Adams was right on that one until you pointed it out.

Without doing any math, I would think the main constraint is the sheer size of the balloon that you would need to lift any significant mass. That said there are some people who are proposing things like this with at least some seriousness:

http://en.wikipedia.org/wiki/Orbital_airship

Interesting. I wonder what the theoretical maximum height/weight is achievable with current LTA materials. I was thinking that the LTA vehicle would act as a miniature space elevator, with a launch craft carrying the traditional rocket part of the way through the atmosphere, and then launching the rocket the rest of the way up and into orbit.

One of the entrants in the first X Prize competition was going to try this: http://www.davinciproject.com/beta/Technical/TechnicalFlight...

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