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Comment by wantoncl | Hacker News Reader

I can't find it now, but there was previous discussion on HN, about the amount of atmospheric drag a balloon-launched rocket would save as not being worthwhile. Most of the orbital velocity is spent going horizontally, the vertical part is <20% of even the most aggressive orbital trajectory, and the drag is virtually nil past 150K feet. And the delta-v needed to reach orbit is some very high percent of the total fuel and thrust.

Basically balloon launches would only benefit small sub-orbital sounding type rockets.

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JP Aerospace has an interesting concept where the entire balloon would get slowly accelerated to orbital velocity using electric/chemical hybrid propulsion: http://www.jpaerospace.com/atohandout.pdf

That could potentially be viable, as it'd save fuel over more traditional rockets by allowing for the use of high efficiency, low thrust engines which would otherwise be infeasible for an orbital rocket due to gravity losses.

Not sure though, I haven't done the math. It might be that the losses due to drag exceed what a more traditional rocket would lose to gravity.

Pretty cool that they’ve built some prototypes

There is a big benefit... Rocket nozzles that are most efficient in a vacuum don't work at sea level (oscillations cause the whole thing to shake apart). Therefore launching from high up means you can skip the less efficient rockets and just have a single type of more efficient rocket engine.

This is a very interesting point. Is there a counterargument to this?

Vacuum optimised rockets require a vacuum. Balloons require atmosphere… so there is a gap where the balloon can’t go higher and the engine isn’t in vacuum yet

Thin-atmosphere-optimized is a thing, and lets you get most of the benefits.

Combine balloons with skyhooks. https://www.youtube.com/watch?v=dqwpQarrDwk

Edit:

According to proprietary imtringued numbers the hydrogen for a balloon capable of lifting 1kg to 50km would be $800 if you do not reuse the hydrogen. The skyhook would accept vehicles at an altitude of 100km so no, keep balloons out of the space launch industry.

Rockets use stages partially to solve this problem, also to deal with rocket equation issues.

So already a solved problem

1. Stages

2. Adaptive rocket nozzles are a thing.

I researched this pretty comprehensively, at least as a curious person in an armchair, and this is 100% correct.

There's one possible way it may pencil out, which is you gain some flexibility in terms of launch site. There's one company pursuing airplane based launches for this reason, but I'm still skeptical. Elon et all haven't had much trouble securing land for launch facilities.

There's an XKCD blog (?) that explains space launches in an intuitive way, with pictures:

https://what-if.xkcd.com/58/

Getting space isn't about getting "really high" (although of course you need to do that too). It's more about going "really fast" (relative to the ground). Like wantoncl mentioned, most of the fuel getting into orbit is spent on building that speed along the ground, not the height into the sky.

Think of it this way: space is only 100km away. ("If you're in Sacramento, Seattle, Canberra, Kolkata, Hyderabad, Phnom Penh, Cairo, Beijing, central Japan, central Sri Lanka, or Portland, space is closer than the sea.") You can drive that distance in a car in a moderately short time. The challenge is getting to the speed to stay in space and not fall back to earth.

Leaving Earth orbit for other astronomical bodies is another very interesting problem entirely. If you'd like to learn more about this, I'd encourage you to explore the video game Kerbal Space Program which has very accurate orbital mechanics.

somewhat unrelated but wouldn't this also defeat the purpose of a space elevator? It seems that also would only save on the vertical part. I realize I'm almost certainly wrong but I'm curious why.

The idea of a space elevator usually requires an orbital anchor point, in which case lifting yourself up the tether would pull you fully out of the gravitational well.

I've not done the mechanics in a long time (not since college), but I believe the tether itself would be providing all horizontal propulsion. Essentially by riding the tether up, it starts pushing you faster and faster. Since the payload's mass would be small compared to the anchor, the drag on the anchor would be negligible.

There's likely some need for thrust compensation on the anchor over time to counteract the delta V lost to the lifting of payloads, but that would all be part of station-keeping and would be there for tether drag as well.

The anchor is past geosynchronous orbit so it’s applying a constant upwards force. The energy for horizontal motion comes from the rotation of the earth, much like how a rotating ice skater slows down when they spread their arms.

Station keeping may be used to dampen oscillations, but managing climbing rates works just as well.

So the constant upwards force is providing enough tension to keep the cable in the realm of a rigid body approximation? I would have assumed the force applied to accelerate the payload would also deflect the tether and anchor backwards (in a miniscule amount) and would have built up over successive payloads.

Not arguing, just curious. It's been at least a decade since I've messed with orbital mechanics.

It does defect the anchor from pure vertical, but like a ball on a pendulum the vertical force from the anchor translates into a horizontal force.

EX: https://www.researchgate.net/publication/312934526/figure/do...

Of course if you really dig into things it’s more complicated, but the basic principle is very similar.

Space elevators get you all the way out to geosync orbit (and beyond) - as you climb you accelerate as you move away from the surface of the earth, and so by the time you reach geosync height you are traveling at orbital velocity.

To be effective, a space elevator would have to deliver the payload all the way out to geostationary orbit altitude. At that hight, the payload would already be in orbit without adding any additional horizontal velocity. But almost all of the height is higher than you can float a lighter than air craft (42164km vs ~40km for lighter than air craft).

My guess would be: top of the space elevator needs to stay in orbit so it already needs to have appropriate "horizontal" part.

With the space elevator, you can get to space easily, but getting to orbit is much harder. Basically, you only achieve orbital speed once you have crawled to the geostationary orbit, e.g. 35,786 km above the Earth. Crawling that far from the Earth costs a lot of energy.

If you crawl only to the LEO altitude (e.g. 300 km) on the space elevator, you will be in space, but not in orbit. If you let go of the elevator there, you will immediately start falling to a gruesome death.

It could become a thing.

https://en.wikipedia.org/wiki/Space_diving

If it’s any comfort, you’d asphyxiate before the fall killed you.

Assuming one didn't asphyxiate or freeze to death on the way up, unfortunately, if during the day, you'd likely suffer immolation (due to high temperatures in the thermosphere) before losing consciousness from asphyxiation. Also, even if not either, the fall doesn't kill you; it's the impact.

Apropos:

https://everything2.com/title/Desiderata+of+Time+and+Elevati...

Huh? Isn't the air too thin up there for that? One high temperature particle isn't going to set you on fire

Because a real space elevator would go all the way to geostationary orbit where the orbital speed is the same as the rotation of the planet. Remember that the further up you go, the slower the orbital velocity, i.e. the "horizontal part" shrinks until the "vertical part" becomes the whole thing.

In fact, the way to build a space elevator is not to build a "tower to space", but to put an anchor rock into geostationary orbit and then hang a cable down to earth from it. You can then connect the cable to terra firma so that it doesn't sway, but in terms of the main forces involved it's hanging down, not standing up.

The tether needs to go past geosynchronous orbit to act as a counterbalance

A space elevator not only raises payloads, but also accelerates them to geostationary orbital velocities (~3000 m/s). Luckily these velocities are much much lower than LEO orbits (7600 m/s).

Ideally the energy to perform this acceleration comes from the rotation of the planet below, transmitted via the tether. The anchor station would have to be stabilized to prevent oscillation and rotation. However that technical challenge is minor compared to the tether material itself.

Another advantage of a space elevator is that we can use electrical power from a base station instead of onboard power on the vehicle alone. Also the power can be much lower than a comparable rocket. To ascend on the rail/tether you can accelerate much slower. It doesn't need to lift fast or fall. It can just slowly accumulate altitude and velocity.

A space elavator would also lend alot of centrifugal force past the atmosphere due to the tethered rotation that you wouldn't get with a balloon, if it's high enough it will be able to escape just by virtue of that when released

>Basically balloon launches would only benefit small sub-orbital sounding type rockets.

So you're saying Jeff Bezos should look into this for Blue Origin since you essentially described his capabilities? ;-)

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