Float into space?

[Savage117]

Sorry guys, I totally didn't take buoyancy into account, total brainfart there. But would I not still be right? I got my concepts messed up but wouldn't the end result be the same? No matter what material you use, it would either collapse under the pressure of the air ,because there is nothing to counteract the pressure on the outside, or be too heavy to float, because of the weight of the material needed to contain a vacuum at sea level. Doesn't the idea of a super light and super strong material defy some other physical law? I'm obviously out of my depth (no pun intended) on this one, but it sounds almost like a "what came first?" dilemma, the chicken or the egg.

Also, if vacuum is so buoyant, why don’t they use it in submarines? Would it not make ascents faster than the current method of pushing air into the ballast tanks? Plus you wouldn’t have to store pressurized air aboard the sub, you would only need stronger pumps to suck the water out and push it into the surrounding water, leaving a vacuum behind. They may already do this, I did a quick search but didn't find anything. I feel like there is something being missed in this topic, a known physical law that would be broken by this concept. I'm no scientist, I could be wrong, but you can't learn if your always right.

Thanx for the brain food OP and other posters, although I totally made a fool of myself I still learned something.

[Adent]

If it was a perfect sphere, the pressure would be equal along the outside surface, I think. It couldn't collapse if pressure was even on the perimeter.

[Savage117]

If it was a perfect sphere, the pressure would be equal along the outside surface, I think. It couldn't collapse if pressure was even on the perimeter.

As far as I know, a "Perfect" sphere is impossible, but I am usually the one to say "Nothing is impossible" so you never know. But if the pressure were perfectly even on all sides then it wouldn't shoot up or sink or do anthing right? It would just float there forever until some sort of force acted on it from the outside, a push, a breez, or an attached engine of some sort, I think.

[Yasmy]

As far as I know, a "Perfect" sphere is impossible, but I am usually the one to say "Nothing is impossible" so you never know. But if the pressure were perfectly even on all sides then it wouldn't shoot up or sink or do anthing right? It would just float there forever until some sort of force acted on it from the outside, a push, a breez, or an attached engine of some sort, I think.

Nope. You can't have the pressure perfectly even on all sides if there is a gravitational field anywhere nearby. (Or a heat source, either.) If the sphere is lighter than the displaced air, the sphere will rise.

Due to gravity, the air pressure is higher at the bottom than the top of the sphere. Air pressure in the atmosphere at some height can be thought of as the weight of the air (per unit area) above that height. The sphere is pulled down by gravity, and pushed up by the pressure difference between the bottom and top, and the difference determines which way the sphere moves.

BTW: Most smartphones come with pressure sensors these days. If you get an app to read the sensor, you can see the difference in the air pressure at your head vs. your toes.

Edited November 20, 2013 by Yasmy

[Ralathon]

Also, if vacuum is so buoyant, why don’t they use it in submarines? Would it not make ascents faster than the current method of pushing air into the ballast tanks? Plus you wouldn’t have to store pressurized air aboard the sub, you would only need stronger pumps to suck the water out and push it into the surrounding water, leaving a vacuum behind. They may already do this, I did a quick search but didn't find anything. I feel like there is something being missed in this topic, a known physical law that would be broken by this concept. I'm no scientist, I could be wrong, but you can't learn if your always right.

The reason they use air instead of vacuum in submarine ballast tanks is to keep the pressure difference down.

Say you have a ballast tank that's 500 meters below the surface. That's 50 bars of pressure trying to collapse the tank. But, at the same time the water inside the ballast tank is also at 50 bars. So the net force on the tank is minimal. This is why humans can survive just fine when we go diving, the internal pressure is the same as the outer pressure, so there is no net force to rip our fragile bodies apart.

If you fill the ballast tank with air to make it rise, you simply pump in 50 bars worth of air and you'll still have a net force of about zero. So you only need minimal structural integrity and thus weight to do this. If you try pull the tank vacuum however, there is suddenly a massive difference in pressure between the inside and the outside. So you need a really strong ballast tank to resist the pressure. The extra reinforcements would negate any potential benefit a vacuum pump would have.

[peadar1987]

This reminds me of an idea I once had for a "space gun". You'd have a floating platform, filled with hydrogen (or helium, or you want to make Jebediah cry), with enough buoyancy to hold up a large railgun, powered through a tether from the ground. Payloads get lifted up to the platform, loaded into the railgun, and fired into a suborbital trajectory, whereupon they can be placed into a stable orbit with greatly reduced delta-V.

Never got round to crunching the numbers on it though, so I have no idea if it is in any way practical

[magnemoe]

The reason they use air instead of vacuum in submarine ballast tanks is to keep the pressure difference down.

Say you have a ballast tank that's 500 meters below the surface. That's 50 bars of pressure trying to collapse the tank. But, at the same time the water inside the ballast tank is also at 50 bars. So the net force on the tank is minimal. This is why humans can survive just fine when we go diving, the internal pressure is the same as the outer pressure, so there is no net force to rip our fragile bodies apart.

If you fill the ballast tank with air to make it rise, you simply pump in 50 bars worth of air and you'll still have a net force of about zero. So you only need minimal structural integrity and thus weight to do this. If you try pull the tank vacuum however, there is suddenly a massive difference in pressure between the inside and the outside. So you need a really strong ballast tank to resist the pressure. The extra reinforcements would negate any potential benefit a vacuum pump would have.

Yes add a lots of other benefit as having stores of high pressure air you can quickly dump into the ballast tanks to come up fast in an emergency. Even with power out you can simply open the valve to get up, the compressed air can also be use for breathing, breathing has health benefits.

Ballast tanks on subs is essentially open at bottom to make sure they keep at the outside pressure.

This has one downside, if you dive the air pocket compress, you get less buoyancy making you drop faster so you have to inject more air to compensate, however they learned this early then making subs so this is more annoying if you go scuba diving and have to correct buoyancy every time you change attitude.

[christok]

The idea of getting to orbit with a balloon/airship isn't extremely far fetched. While it doesn't take you to orbital velocity in itself and there is also still substantial aerodynamic drag at any altitude an airship can reach, it does have the advantage of letting you go to orbital velocity with low thrust (since you're less inclined to fall).

The JP Aersopace guys have been talking a good fight for some time and I dearly wish they could get their concept off the ground.

For your amusement, almost everything in the current thread was discussed previously in the "Airships" thread (May 2013).

[K^2]

it does have the advantage of letting you go to orbital velocity with low thrust (since you're less inclined to fall).

No, that's not one of the advantages. 30km off the ground, optimal TWR is still about 2, and you are going nowhere but down with anything less than 1.

This reminds me of an idea I once had for a "space gun". You'd have a floating platform, filled with hydrogen (or helium, or you want to make Jebediah cry), with enough buoyancy to hold up a large railgun, powered through a tether from the ground.

Entirely workable. Except for the size of the "railgun". (You don't actually want a railgun, but rather a maglev linac. But that's semantics.) It's not quite as bad if you want to shoot cargo, but say you want to launch humans. That puts the cap at about 8G. So lets say 8km/s at 80m/s². Acceleration will require 100s, which is survivable at 8G by trained crew, but in that time your ship will travel 400km. So you are going to need at least 400km of rail. This is a big deal even on the ground, but getting that to float in the upper atmosphere is just way too challenging. Also, if it gets even a little bent due to the winds, at 8km/s that can be catastrophic.

Launch loop follows the similar idea, but it has some added advantages in terms of stability and wind-resistance.

[christok]

No, that's not one of the advantages. 30km off the ground, optimal TWR is still about 2, and you are going nowhere but down with anything less than 1.

I think I stated that poorly. I don't refer to a balloon-launched rocket but to a spacecraft that is a balloon. It doesn't fall because of bouyancy, allowing you to accelerate at any pace as long as thrust > drag.

[peadar1987]

No, that's not one of the advantages. 30km off the ground, optimal TWR is still about 2, and you are going nowhere but down with anything less than 1.

Entirely workable. Except for the size of the "railgun". (You don't actually want a railgun, but rather a maglev linac. But that's semantics.) It's not quite as bad if you want to shoot cargo, but say you want to launch humans. That puts the cap at about 8G. So lets say 8km/s at 80m/s². Acceleration will require 100s, which is survivable at 8G by trained crew, but in that time your ship will travel 400km. So you are going to need at least 400km of rail. This is a big deal even on the ground, but getting that to float in the upper atmosphere is just way too challenging. Also, if it gets even a little bent due to the winds, at 8km/s that can be catastrophic.

Launch loop follows the similar idea, but it has some added advantages in terms of stability and wind-resistance.

I was thinking along the lines of a circular maglev system, so for your 8km/s you'd have a centripetal acceleration of (v^2)/r, which works out at a radius of just a shade over 800km to keep the astronauts from being pulped. Hmm... Really should have done the maths before posting about it on the internet! It could still be a possibility for rocket launches I suppose, a floating launch platform 20 to 30km above the ground to get above the worst of the atmosphere. Doubt it would be worth the effort though.

[K^2]

I think I stated that poorly. I don't refer to a balloon-launched rocket but to a spacecraft that is a balloon. It doesn't fall because of bouyancy, allowing you to accelerate at any pace as long as thrust > drag.

There is a very small range of altitudes over which it will help, and you'll be paying for it with increased mass and drag. There is simply no way to use this to make a launch cheaper.

[christok]

There is a very small range of altitudes over which it will help, and you'll be paying for it with increased mass and drag. There is simply no way to use this to make a launch cheaper.

Perhaps or perhaps not. I do think it's the closest thing to the original post which gets serious press. I honestly haven't punched the numbers but it remains a cool idea if someone can figure out a way to use it. Even if it isn't cheaper it would reduce the stresses of launch and reentry, which seriously compromise the validity of many biological experiments.

[Adent]

Try this for thought...

Imagine a wire mesh, similar to one of the lunar rover wheel designs, but forming a sphere. At equal 1:1 pressure, the mesh is loose enough to be just that, a mesh. Now when pressure outside the mesh is greater than inside, the mesh is contracted into a smaller radius and more tightly woven mesh, to the point that its a solid ungapped surface. Maybe something as small as a 0:0.00000000001 (no idea what measurement units im trying to say, just a ratio I pulled out my rocket bell) pressure ratio would contract the mesh to a solid surface state.

[Anton P. Nym]

That may help hold a vacuum, but it won't help make a bouyancy chamber. You want a *big* volume evacuated to lift a cargo, but this mesh is being compressed.

-- Steve

[Adent]

If the mesh was tight enough, it wouldn't lose that much radius when contracted by pressure. Like %10 tops. The volume would scale with the size of the sphere, non-linear. Lifting any kind of cargo would complicate things immensely, the only cargo would be the mesh structure itself.

If it were to hold its shape in 1 atmo, and be lighter than the atmo it displaces.

Again thanks for the brain food, helps me get though some boring days

[Savage117]

This kinda went down another branch from the same tree slightly with all the talk of pressure and material strengths, when the core of the debate is, would a vacuum balloon work? After reading many of your posts I think it just might, might not be cost effective or very useful, but it would work.

I really like that mesh idea. What if we take it a bit further? Surround this mesh with a very light, extremely elastic, material that is airtight. In a vacuum the "Vacballoon" is fully inflated due to the mesh pushing against the inside of the elastic material, but when the "Vacballoon" is brought down into the atmosphere the elastic material collapses and compresses the mesh until it is so compressed it is also airtight and extremely strong, attach the "Vacballoon" to a payload of some type, release, and it would work in the opposite direction until the buoyancy vs weight equalized, then the payload would have to get to orbital velocity from there under its own power, likely a rocket.

@The submarine vacuum ballast thing, I understand that it is a pressure issue with the current design and a vacuum would cause the current design to collapse, but I was wondering why they designed it with air instead of vacuum. If the sub is designed with vacuum in mind instead of air, you would probably save space, the ballast tanks would not need to be so big due to the extra buoyancy, you wouldn't have to worry about the deeper you go the more pressure needed inside the ballast to counter thing, and the ballast tanks would probably be denser and therefor stronger than current tank types. I'm not sure if that extra density would counteract the gain in buoyancy. But there were two things that make air a better choice that I can't really counter with vacuum, the ability to provide breathing air for the submariners, and a powerless ascent method.

Just some more food for thought, and it makes me wonder if they actually tested some of these ideas somewhere, maybe just in a computer simulation, but actually tested them and found one better for the current situation than another and never looked back.

[akhmeteli]

This kinda went down another branch from the same tree slightly with all the talk of pressure and material strengths, when the core of the debate is, would a vacuum balloon work? After reading many of your posts I think it just might, might not be cost effective or very useful, but it would work.

Just some more food for thought, and it makes me wonder if they actually tested some of these ideas somewhere, maybe just in a computer simulation, but actually tested them and found one better for the current situation than another and never looked back.

Together with my coauthor, I've spent quite some time "engineering a vacuum balloon":-). Our conclusion: difficult, but possible with current materials. Although an evacuated homogeneous spherical shell cannot be both light enough to float and strong enough to withstand atmospheric pressure, no matter what existing material you use (e.g., diamond's elasticity modulus is at least three times too small), a sandwich shell can meet these conditions with a reasonable safety factor. See details in our US patent application 20070001053 (11/517915) (you may google "layered shell vacuum balloons").

I agree that a vacuum balloon will not have greater lift than a helium balloon, but it can have some other strong points, such as easier altitude control.