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Gravity Plane

Yes, great post. We were typing at the same time, obviously.

Hans
 
SkepticJ said:
I'm thinking about this and something doesn't seem right. It seems like it would violate entropy. A solar powered version would be nice though. http://www.machinedesign.com/ASP/strArticleID/56635/strSite/MDSite/viewSelectedArticle.asp
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Well it sounds like this guy may know his history.

Sorry, but I do not recall the exact details, but before the Montgolfier brothers started doing their work with hot air ballons in the 1780's there was another person some decades earlier who proposed that one could build a flying craft using large, hollow copper spheres that had all of the air removed from them.

In one sense his idea was valid in that a large and very light weight vaccuum bottle could indeed have substantial lifting properties.

However, his idea was impratical since the spheres would collapse because they were not strong enough to withstand the pressure differential.

Anyway, I looked at the web site and it looks like he will have the same problem (to say nothing of the other formidible problems that will have to be overcome as well):
he did not explain how such a large vessel nearly 100% air tight and,
be strong enough not to collapse (withstand about 1.06 tons/ft^2) and,
be light enough so that it could carry its own weight plus a decent amount of payload.

Such a thing may make a good plot device for a science fiction story, however!
 
Re: Re: Gravity Plane

Crossbow said:

In one sense his idea was valid in that a large and very light weight vaccuum bottle could indeed have substantial lifting properties.
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I must admit I don't really understand this. Surely if you had a vacuum bottle then the pressure would be equal on all sides. This is why Magdeburg Hemispheres act the way they do, isn't it? Why would it have lifting properties? Please feel free to laugh at my puny understanding of physics.
 
Re: Re: Re: Gravity Plane

richardm said:
I must admit I don't really understand this. Surely if you had a vacuum bottle then the pressure would be equal on all sides. This is why Magdeburg Hemispheres act the way they do, isn't it? Why would it have lifting properties? Please feel free to laugh at my puny understanding of physics.
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The same reason a bubble of air floats upward in water: Its bulk density is less than that of the surrounding fluid. The "missing air" in the sphere reduces its total weight, and if that total weight was less than that of the missing air it would float. Same principle as a helium balloon, too.
 
Re: Re: Re: Re: Gravity Plane

garys_2k said:
The same reason a bubble of air floats upward in water: Its bulk density is less than that of the surrounding fluid. The "missing air" in the sphere reduces its total weight, and if that total weight was less than that of the missing air it would float. Same principle as a helium balloon, too.
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And the same reason why giant concrete hull ships that were used in WW1 or maybe it was 2 worked. Oh, and huge steel aircraft carriers. All it is is displacement of a more massive fluid. Gases are fluids to. They take the shape of the container they are in(in a gravity environment).
 
Re: Re: Re: Re: Re: Gravity Plane

SkepticJ said:
And the same reason why giant concrete hull ships that were used in WW1 or maybe it was 2 worked. Oh, and huge steel aircraft carriers. All it is is displacement of a more massive fluid. Gases are fluids to. They take the shape of the container they are in(in a gravity environment).
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You'd be thinking of the huge artificial 'mulberry' harbours manufactured along the English south coast and floated over to North France after D-Day.

They are still there, at arromanche.

You'd need a cracking bit of infrastructure to contain a vacuum that size, I think we are a long way from having a material that could do it and not weigh significantly more than the mass of air it manages to displace.
 
Re: Re: Re: Re: Re: Re: Gravity Plane

Benguin said:
You'd be thinking of the huge artificial 'mulberry' harbours....You'd need a cracking bit of infrastructure to contain a vacuum that size, I think we are a long way from having a material that could do it and not weigh significantly more than the mass of air it manages to displace.
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No, they were ships; big ships. Cargo carriers I think.
Once we had such a material though the shell could just be made thicker and thicker for making really huge spheres?. For smaller spheres it can be much thinner. Their wouldn't be an upper limit on sphere size would there? Imagine 10km in diameter ones floating in Jupiter's upper levels lifting hundreds of thousands of metric tons of machines that collect hydrogen and compress it into tanks for use as fusion fuel in future intrasolar craft. Feasible?
 
Re: Re: Re: Gravity Plane

richardm said:
I must admit I don't really understand this. Surely if you had a vacuum bottle then the pressure would be equal on all sides. This is why Magdeburg Hemispheres act the way they do, isn't it? Why would it have lifting properties? Please feel free to laugh at my puny understanding of physics.
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No, pressure varies with altitude. The higher you go, the less the pressure is. The pressure at the top of a balloon is slightly less than the pressure at the bottom, and that difference creates buoyancy. If you do the math, it turns out that the total buoyancy force comes out to be the weight of the displaced fluid. If this is larger than the weight of the object, it will float.

Magdeburg Hemispheres are a different issue, since that deals with compression and tension forces, not buoyancy.
 
Re: Re: Re: Re: Re: Re: Re: Gravity Plane

SkepticJ said:
No, they were ships; big ships. Cargo carriers I think.
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Well these were just floating harbours, I don't know if they put anything in them but they did sail them across the english channel. Not heard about cargo ships, I'm not sure concrete would be a particularly good material choice for a sea going commercial vessel but that doesn't mean it wasn't tried.

Once we had such a material though the shell could just be made thicker and thicker for making really huge spheres?. For smaller spheres it can be much thinner. Their wouldn't be an upper limit on sphere size would there? Imagine 10km in diameter ones floating in Jupiter's upper levels lifting hundreds of thousands of metric tons of machines that collect hydrogen and compress it into tanks for use as fusion fuel in future intrasolar craft. Feasible?
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In theory I think it is fine, however you need something with enough structural integrity to spread the force about from the fixing points carrying the load. It also needs to be able to resist the oscillating forces placed on it by wind and atmospheric pressure changes.

I like your vision though, and maybe one day!

edited to correct stupid geographical goof!
 
The vaccum sphere idea is simply unworkable. Even with some putative exotic material that might be light and strong enough to build a vacuum container with lift, it would be more effective to use a light gas as helium. The structural weight of container+gas will always be smaller than the weight of a vacuum container, because if you get this super light weight material, you can also make the helium container much lighter.

Hans
 
MRC_Hans said:
The vaccum sphere idea is simply unworkable. Even with some putative exotic material that might be light and strong enough to build a vacuum container with lift, it would be more effective to use a light gas as helium. The structural weight of container+gas will always be smaller than the weight of a vacuum container, because if you get this super light weight material, you can also make the helium container much lighter.

Hans
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Good point.


Would it be possible to gain an advantage with lower pressure helium, or are we screwed the minute we try?

Edited to add: Thinking about it, I can't see how it is possible to design a sphere that would be lightweight and resist these pressures and still provide a lift advantage over a mere balloon.

Hypothetically, would hydrogen be more advantageous if we could find a material to contain it properly?
 
Hypothetically, would hydrogen be more advantageous if we could find a material to contain it properly?
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Hydrogen weighs less than helium and is cheaper to produce. That nasty explosive tendency is a real (bad pun) downer.

Most likely, accidents would happen filling containers and in general handling of the H2.

Speaking of which, our local libertarian candidate for US Representative is running on a platform of:
Using blimps to move good to remote Alaska and Taxi reform.
I think he overlooked the high winds and great big mountains.
 
Yes, hydrogen has about one quarter of the mass of helium for a given volume, so it has better lift. Not four times the lift, though, as both have masses a fraction of that of atmosperic air.

No, lowing the pressure is not the way. The trick of a gas is exactly that it provides the internal counterpressure so the container needs not withstand any air pressure. A way to go would be to use hot gas. Actually, early in the balooning era, somebody did build a hot hydrogen baloon :eek: ! ..... The outcome did not encourage further experiments in that particular direction.

Hans
 
MRC_Hans said:
Yes, hydrogen has about one quarter of the mass of helium for a given volume, so it has better lift. Not four times the lift, though, as both have masses a fraction of that of atmosperic air.
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Wouldn't Hydrogen be half the weight of Helium, since Hydrogen is diatomic as a gas?

Continuing with your idea of comparing masses, here are the weights of different gas molecules:

Normal air: 80% N2, weight = 28; 20% O2, weight = 32, plus some CO2 (weight = 44) and H2O (weight = 18 - notice that water vapor is significantly lighter than air). Average somewhere around 29.

Helium: 4

Hydrogen: 2

Vacuum: 0

So you can see that He and H get you close enough to the vacuum number, compared to regular air's weight, to make the structure to support a vacuum unnecessary.
 
MRC_Hans said:
The vaccum sphere idea is simply unworkable. Even with some putative exotic material that might be light and strong enough to build a vacuum container with lift, it would be more effective to use a light gas as helium. The structural weight of container+gas will always be smaller than the weight of a vacuum container, because if you get this super light weight material, you can also make the helium container much lighter.
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True, but I've started a thread sometime back about helium leaking and was informed that because it's so small it will eventually leak through anything. http://randi.org/vbulletin/showthread.php?s=&threadid=35432&highlight=helium So a vacuum sphere would be useful for keeping things up for how ever long you need and wouldn't explode when it's high in the air like a gas balloon will. There is nothing in it to leak out. It would take years before an airship needed to be refilled so I'm talking about keeping things up for centuries.
 
I think it wil be simpler to reduce leakage to a negligible level than to make a vacuum container that has lift. Also, the larger the structure, the less problems with leakage, because if you increase volume by 4 you only increase surface by 2.

Hans
 
SkepticJ said:
True, but I've started a thread sometime back about helium leaking and was informed that because it's so small it will eventually leak through anything. http://randi.org/vbulletin/showthread.php?s=&threadid=35432&highlight=helium So a vacuum sphere would be useful for keeping things up for how ever long you need and wouldn't explode when it's high in the air like a gas balloon will. There is nothing in it to leak out. It would take years before an airship needed to be refilled so I'm talking about keeping things up for centuries.
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Well, technically atmospheric gases would leak into it ...
 
Benguin said:
Well, technically atmospheric gases would leak into it ...
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Have small onboard vacuum pumps? Isn't it neat how this thread has drifted from debunking a gravity powered plane to looking at problems with creating lifting vacuum spheres.?:)
 
Well the reason for that was that we had to find a way of generating lift and for that lift to be negated for the gliding phase. The two ways of of doing this are:

Filling the pontoons with a lighter than air gas to generate the lift (this gas then has to be compressed and relaced with air in the pontoons to negate the lift). This process needs to be reversed to generate lift in the second cycle. To do this requires energy, and the indications here are that it is a considerable amount of energy.

The other alternaive is that there is no lighter than air gas and the lift is provided by a vacuum.

Neither is a feasible approach.
 
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