> Too slow for transportation, maybe. But leisure travel? Imagine a one week air safari over the Serengeti, Ngoro Ngoro crater, and Kruger Park. It could be awesome.
Well, if we're talking about weird Sci-Fi ideas (that might be possible), lets just go all in on it.
Rocket launches are extremely expensive, and most of it is in the lifting costs. Would it be cheaper to "lift" a rocket with hydrogen slowly?
Yeah, we also need to get enough kinetic energy to enter orbit. But surely getting rid of a huge chunk of "Gravity" costs could lead to substantial rocket-fuel savings?
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.
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.
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.
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
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.
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.
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.
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.
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.
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).
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.
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.
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.
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
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.
This is fun to think about. You only need that horizontal velocity to stay in orbit, right? But what about getting to the moon? Turns out it only goes about 1km/s. Still bloody fast, but it's a bit less scary. Daydreams of capturing that energy on descent to power a moonbase... but then the moon is quite a long way away, and you need to escape over 99% of the earth's gravity, opposed to the 10% needed to reach space. Phooey, lunch is never free.
The learning journey has been more good than bad. My favorite was that orbit seems far away, until you look out from an aircraft and realize you're 30% of the way there. Usually, from where you live, the next major town is further than LEO.
Well, if you could go high enough, you could be in a geosynchronous orbit without moving horizontally at all. I think that's how space elevators would work. But then to move to a lower orbit you would actually need to speed up.
Theoretical geosync means zero velocity in any direction relative to ground. Zero up, down, forward, back, left, or right. You're stuck above a single point of dirt/water, over the equator, at a fixed altitude.
The only way to "fall" (lower your altitude) from here would be via some sort of acceleration force towards ground (like a burn). So yes, your speed would have to increase.
Well fine. I meant that orbital speed (the "along track part") would increase if you lowered the altitude of a geosync object with rockets, string, or any other force, I think.
That's my understanding, geosync is the point at which you continually fall toward the Earth and miss, closer and you'd eventually hit, farther and you would move away.
Geosync is the altitude at which a circular orbit takes 24 hours - meaning it takes as long to complete one orbit as it takes the earth to complete one revolution.
But there’s an orbital velocity at EVERY altitude, and therefore there’s an orbital period at every altitude too.
If you are at a particular altitude and you are going at a different speed than the one needed for a circular orbit, you are either going too fast (in which case you are in an elliptical orbit and you will gradually increase altitude and lose speed until you reach the top of that ellipse) or you are going too slow to stay in a circular orbit (in which case you will follow an ellipse down to a lower altitude and gain speed).
In that ‘too slow’ case, if it’s much too slow then the ellipse you follow will take you low enough to hit the surface or atmosphere of whatever you’re orbiting (ie you will crash into it)
I think you knew this but it wasn't obvious from how you wrote it. Your orbit is not required to be a circle. The diagrams we draw for children are nice circles, but most things that we know are orbiting something do not travel in a circle, e.g. the Earth -- if the orbit was circular our seasons would be slightly different† and the insight that the orbits can be non-circular ellipses was critical to the reasoning that eventually got us a heliocentric model of our solar system.
Geostationary communications satellites do have a basically circular orbit, as you said, but many other birds do not.
Russia has a bunch of stuff in orbits that "linger" very high over Russian territory for much of their orbital period then shoot right around close to the back side of the planet quickly and linger again, these are called Molniya orbits.
The US has a bunch of secret (presumably spy) satellites that fly less obvious orbits like this too, for presumably similar reasons.
† Edited: This post originally said "very" different, but in fact the difference would be modest since our orbit just isn't that elliptical. I wasn't able to find out how modest, but certainly if you think summer in New Zealand (in December) is pretty similar to summer in England (in June) then it's reasonable to say at least that similar.
I think GP means that if you are stationary relative to the ground (such as on a space elevator or a rocket going straight up), 35786 km up is the only height which is a stable orbit. If you let go of the elevator too early, you'll fall to the ground; too high and you'll fly off into space.
What about doing the 2-stage system similar to Virgin Galactic? Use the "balloon" to lift the system to upper altitudes, and then fire off the rocket to do orbital insertion type stuff.
>Getting to space[1] is easy. It's not, like, something you could do in your car, but it's not a huge challenge. You could get a person to space with a small sounding rocket the size of a telephone pole. The X-15 aircraft reached space[2] just by going fast and then steering up.[3]
Loved that line.
Puts the Branson/Bezos thing into perspective.
The idea is that this gets you above the densest atmosphere and that could save you fuel. I used to think this, but if you run the numbers it doesn't work out.
From linked xkcd what if: Gravity in low Earth orbit is almost as strong as gravity on the surface. The Space Station hasn't escaped Earth's gravity at all; it's experiencing about 90% the pull that we feel on the surface.
The majority of energy expended in getting to orbit is spent in getting to orbital velocity (going sideways), rather than getting to orbital altitude (going up). Balloons only help in going up, and only to where the density of the atmosphere is so low that the balloon is no longer lifting, they are still far short of the altitude necessary for orbit. Balloons top out at around 20-30 miles while LEO is ~100 miles.
So launching using a balloon only really gives you a very small fuel savings
That's the issue though. The hard part about achieving orbit isn't the height, it's the speed. In both a ground launch and a balloon launch you are starting at 0.
This article does a better job explaining it than just about anything else on the web:
https://what-if.xkcd.com/58/
So complex schemes to lift the rocket to the edge of space don't end up buying you very much.
Nope. I don’t have numbers but according to Elon Musk (who we probably at least agree on has a grasp of the basic principles) “space is easy, orbit is hard”.
Well, if we're talking about weird Sci-Fi ideas (that might be possible), lets just go all in on it.
Rocket launches are extremely expensive, and most of it is in the lifting costs. Would it be cheaper to "lift" a rocket with hydrogen slowly?
Yeah, we also need to get enough kinetic energy to enter orbit. But surely getting rid of a huge chunk of "Gravity" costs could lead to substantial rocket-fuel savings?