Gravity Plane

[Johnny Pneumatic]

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

[AWPrime]

It is quite likely that it will float quite far in the other direction, before it can glide forwards.:D

[BillC]

Hunter estimates a gravityplane that can carry the same payload as a Boeing 747 would be roughly 50% larger than the current 747.(my italics)

I would have expected a plane designer to be calculating his design parameters, rather than estimating them.
Particularly if he wants the likes of me to get on board it.

[The Don]

The tanks required to displace enough air to cause the thing to float are enormous. Assuming that the thing weighs about 1/2 a 747 (200 tons empty) and that perfect vacuum is achieved then you'd need around 80 000 cubic metres of storage.

Or put another way two sausages 10m in diameter and 1 kilometer long would be needed at sea level, even more at altitude. I think the gentleman's having a laugh. It also looks like he hasn't bothered to do spme basic buoyancy calculations. Aero engineer my big fat ass.

This design is complete garbage, and it doesn't even consider the enormous amounts of energy required to generate the vacuum

[Benguin]

Looks like an unfortunate mating attempt between two zepplins and a stealth fighter.

He doesn't explain properly how he'd contain a vacuum of that 'volume' (or should that be displacement?). You'd need some pretty hefty infrastructure to get anything that low pressure without it collapsing.

That isn't addressed at all in the discussion about the 'challenge' of making the pontoons.

[richardm]

To help test and refine his designs, Hunter plans on building a scaled-down, three-man submarine version of his gravityplane over the next five months.

As anyone will tell you, building a submarine is an important part of testing any new aircraft design.

[Khonshu]

I, for one, have no desire to ride in a plane that was made to be a submarine, nor a submarine that was supposed to be a plane.

[MRC_Hans]

OK, that article is old hat. Now for dismantling :D:

1) As already mentioned the buoyancy sections must be dimensioned MUCH larger than depicted. You get about 1.3 kilograms of lift per cubic meter helium at sea level, but og course you also need to lift the tanks.

2) Making vacuum tanks with positive lift is impossible with known technology.

3) Making wind generators with 4 times better efficiency than present designs is probably impossible with current technology (otherwise why does he not sell the idea to the wind generator industry).

4) Even if all snags could be ironed out, the craft needs a power source. Making vacuum and compressing helium requires exactly the same amount of energy as can be gained from the changes in lift obtained, plus losses. This is because buoyancy is really about pressure. A body that is lighter than its surrounding medium has lift because the pressure difference between its lower and upper survafaces exceeds its weight. To alter that by changing the pressure inside (part of) it, you have to excert a corresponding pressure on the medum in it, and presto, you loose what you gain.

Hans

[The Don]

He'd have gotten away with it too...........if it wasn't for those pesky laws of physics

[Benguin]

Originally posted by MRC_Hans
3) Making wind generators with 4 times better efficiency than present designs is probably impossible with current technology (otherwise why does he not sell the idea to the wind generator industry).


Totally OT, but I saw a fascinating attempt in Peter's port harbour on Guernsey a few weeks ago ... a full size catamaran with the mast removed and replaced with a danish style wind turbine. Presumably it was driven by some sort of electric motors, unless the whole get up was mechanical.

I was trying to work out whether the power generation would be sufficient to get any sort of speed, but it gave me headache so I went for more beer instead. I suppose a boat like that would need about 150 hp to move at a leisurely 12-15 knots, that would be about 11kW. I don't know how much those turbines generate ... 3 blades, about 15 m diameter.

I would have taken a photo, but I think everyone would accuse me of having photoshopped it together. I'd imagine it would be one hoopy sight coming towards you in the channel.

Edited to add: Ah well here it is (http://www.multihullcentre.co.uk/news/news.htm)

[MRC_Hans]

To move a sailboat at moderate speed does not require 150hp. I used to have a 26ft yacht weighing in at two tons. It could run 6 knots with a 6hp outboard.

I don't think a wind turbine to propeller arrangement (whether electric or direct drive) is nearly as efficient as a sail, but it should be able to move a boat. An advantage would be the ability to sail directly into the wind.

Hans

[Benguin]

Originally posted by MRC_Hans
To move a sailboat at moderate speed does not require 150hp. I used to have a 26ft yacht weighing in at two tons. It could run 6 knots with a 6hp outboard.

I don't think a wind turbine to propeller arrangement (whether electric or direct drive) is nearly as efficient as a sail, but it should be able to move a boat. An advantage would be the ability to sail directly into the wind.

Hans

I'm a bit ropey at that, I'm used to steel and wooden things without sails, not grp hulls. This looked more like 40ft twin hulled, but I suppose it would still lightly skip through the water like a yacht.

Last boat I piloted was more like 18 tons, 63 ft with about 40hp that struggled to get it to 5 knots. All the hydrodynamics of a rusty bathtub.

I suppose with buffering batteries they could get smoother progress, with none of that irritating listing you yachties like so much.

[Art Vandelay]

Originally posted by MRC_Hans
To move a sailboat at moderate speed does not require 150hp. I used to have a 26ft yacht weighing in at two tons. It could run 6 knots with a 6hp outboard.

I don't think a wind turbine to propeller arrangement (whether electric or direct drive) is nearly as efficient as a sail, but it should be able to move a boat. An advantage would be the ability to sail directly into the wind.

Hans
Seems like there'd be conservation of momentum issues. Suppose you're sailing into the wind. You want to increase the energy of the boat, so you have to decrease the energy of the wind. The wind is blowing towards you, so you need to impart a forward momentum on the wind. To do this, you must impart a backwards momentum on the boat. I think you'd have to have the boat go at a fraction of the wind speed to counter this.

[StaticEngine]

buoyancy, a form of gravity
Really? Hmm, I never learned THAT at my Engineering University...

[Johnny Pneumatic]

Originally posted by MRC_Hans
2) Making vacuum tanks with positive lift is impossible with known technology.


So I see I was right about this craft violating entropy.

#2: Couldn't a balloon be made that has two layers; one inside the other and tied together at many points with kevlar cords. The inner sphere is airless and the space between the two cloth spheres is pressurized to two or three bar?

[CurtC]

Originally posted by SkepticJ
#2: Couldn't a balloon be made that has two layers; one inside the other and tied together at many points with kevlar cords. The inner sphere is airless and the space between the two cloth spheres is pressurized to two or three bar? I'm not sure what you mean here. If it's a balloon, the atmospheric pressure would collapse it.

[MRC_Hans]

Originally posted by Art Vandelay
Seems like there'd be conservation of momentum issues. Suppose you're sailing into the wind. You want to increase the energy of the boat, so you have to decrease the energy of the wind. The wind is blowing towards you, so you need to impart a forward momentum on the wind. To do this, you must impart a backwards momentum on the boat. I think you'd have to have the boat go at a fraction of the wind speed to counter this. Mmm, I'm not sure what you mean by this. The winf blows against the turbine, providing energy (at the cost of locally slowing down the wind, there will be a zone of relative lee behind the turbine). This energy you use for driving a propeller, which makes the boat move forwards. If you steer directly into the wind, you will, of course, experience maximum resistance from the head-wind, slowing down the boat, but at he same time, moving against the wind, part of this (but NEVER all) is compensated by your relative wind strenght becoming higher (wind speed + boat speed). I don't see where conservation of momentum comes into it.

It is really not different from a conventiona sailboat tacking into the wind, except that a conventional sail loses efficiency when going too close to the wind, whereas the turbine does not.

Hans

[Ziggurat]

Originally posted by SkepticJ
So I see I was right about this craft violating entropy.

#2: Couldn't a balloon be made that has two layers; one inside the other and tied together at many points with kevlar cords. The inner sphere is airless and the space between the two cloth spheres is pressurized to two or three bar?

Sounds like a nice idea, but the forces won't balance unless the inner balloon is a lot smaller than the outer sphere, because the cords will pull inwards on the outer shell along with the outside atmosphere. If the inner balloon has a surface area A, and the outer ballon has a surface area B, and the pressure in the intermediate region is P, then the net outward pressure from the intermediate gas is going to be P*(B-A)/B, not P. So if you pressurize it with 2 atmospheres, you need the inside area to be less than half the outside area to be stable. If it's half the area, it's sqrt(2)/2 times the radius, for a sphere. And that means the volume of the inside sphere is less than 1/2 the volume of the outside sphere, which means that the air you need to keep the shell stable is actually going to weigh MORE than the air it displaces. So even with a weightless membrane for your balloons, this idea won't end up working. It's a neat problem to figure out why it doesn't work, though :)

[MRC_Hans]

Originally posted by SkepticJ
So I see I was right about this craft violating entropy.

#2: Couldn't a balloon be made that has two layers; one inside the other and tied together at many points with kevlar cords. The inner sphere is airless and the space between the two cloth spheres is pressurized to two or three bar? Yeah, back on topic, heheh. Yes, that might be one way to create a light but strong container, but the atmosheric pressure is 1kg per square centimeter, so even on a normal CRT, the pressure is several tons. The pressure a large container (and we are talking thousands of cubic meters) would have to withstand would be astronomic.

So, in your double-layer structure, pressure perpendicular to the surface would be translated to pressure parallel to the surface, and countered by the pressure between the two layers. But while the perpendicular pressure from the surrounding air adds up with increasing area, your parallel pressure does not (because the cross section of the space between the layers is constant), so you would have to increase the pressure between the layers as the container got bigger. Even with a very moderate container, this pressure would soon become impossibly high. I even suspect the weight of the thus compressed air might outweigh the buoyance of the vacuum.

Hans

[Ziggurat]

Originally posted by MRC_Hans
I even suspect the weight of the thus compressed air might outweigh the buoyance of the vacuum.

It does indeed. I gave a specific example above (2 atmospheres in the intermediate region of a spherical balloon), but it's actually true for any shape and pressure arrangement.

[MRC_Hans]

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

Hans

[Crossbow]

Originally posted by SkepticJ
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

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!

[richardm]

Originally posted by Crossbow

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


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.

[garys_2k]

Originally posted by richardm
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.
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.

[Johnny Pneumatic]

Originally posted by garys_2k
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.

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).

[Benguin]

Originally posted by SkepticJ
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).

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 (http://cruises.about.com/library/pictures/baltic/blnormandy09.htm).

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.

[Johnny Pneumatic]

Originally posted by Benguin
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.

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?

[Art Vandelay]

Originally posted by richardm
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.
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.

[Benguin]

Originally posted by SkepticJ
No, they were ships; big ships. Cargo carriers I think.


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?

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!

[MRC_Hans]

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

[fishbob]

Such a thing may make a good plot device for a science fiction story, however! Edgar Rice Boroughs used this plot device about 80 years ago. Tarzan at the Earth's Core.

[Benguin]

Originally posted by MRC_Hans
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

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?

[fishbob]

Hypothetically, would hydrogen be more advantageous if we could find a material to contain it properly? 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.

[MRC_Hans]

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

[CurtC]

Originally posted by MRC_Hans
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.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.

[Johnny Pneumatic]

Originally posted by MRC_Hans
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.

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.

[MRC_Hans]

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

[Benguin]

Originally posted by SkepticJ
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.

Well, technically atmospheric gases would leak into it ...

[Johnny Pneumatic]

Originally posted by Benguin
Well, technically atmospheric gases would leak into it ...

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.?:)

[The Don]

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.

[AWPrime]

Originally posted by The Don
To do this requires energy, and the indications here are that it is a considerable amount of energy.

Especially if you consider that it should be done in a very short time.

[Johnny Pneumatic]

I've done some more thinking about what you've said about the vacuum spheres and I don't get it. Once you had a shell that could hold up to 1 bar/14psi of air pressure the shell wouldn't need to be made any thicker as the volumes of spheres was increased would it? If so, why? The titanium sphere that is the cab where the people stay in DSVs when going deep in the ocean holds up to 14,000+psi without failing. If the sub were only less than a mile closer to the surface it would be possible to pump the air from the sphere and it wouldn't implode. What am I not getting? Titanium isn't nearly the
strongest know material (http://en.wikipedia.org/wiki/Carbon_nanotube)

[CurtC]

Originally posted by SkepticJ
[B]I've done some more thinking about what you've said about the vacuum spheres and I don't get it. Once you had a shell that could hold up to 1 bar/14psi of air pressure the shell wouldn't need to be made any thicker as the volumes of spheres was increased would it? If so, why?I think you're saying that if you have a 2 mm thick wall of "Krell metal" or whatever, which can hold a near-perfect vacuum around a 0.1 liter volume, could you scale up that volume to an arbitrarily large size, still with only a 2 mm thick wall. Is this right?

It doesn't work. The curvature of the container acts like an arch mechanically, and if it's scaled up, that arch flattens out so can't hold as much force without deforming.

Anyone remember what "Krell metal" refers to?

[Benguin]

Originally posted by CurtC
I think you're saying that if you have a 2 mm thick wall of "Krell metal" or whatever, which can hold a near-perfect vacuum around a 0.1 liter volume, could you scale up that volume to an arbitrarily large size, still with only a 2 mm thick wall. Is this right?

It doesn't work. The curvature of the container acts like an arch mechanically, and if it's scaled up, that arch flattens out so can't hold as much force without deforming.

Anyone remember what "Krell metal" refers to?

Also, a more slender surface with shallower curve is much more prone to deflection, deflection is critical in this application from a point of view immediate failure under load and resonation problems (particularly from wind forces).

Edited to add: though reinforcing it with lightweight T section ribs with extended webs could probably negate the problem without adding nearly as much mass as thickening the membrane. I'm pretty sure the zepplins held their shape using a similar structural method. I'll look into it.

[MRC_Hans]

Originally posted by SkepticJ
I've done some more thinking about what you've said about the vacuum spheres and I don't get it. Once you had a shell that could hold up to 1 bar/14psi of air pressure the shell wouldn't need to be made any thicker as the volumes of spheres was increased would it? If so, why? The titanium sphere that is the cab where the people stay in DSVs when going deep in the ocean holds up to 14,000+psi without failing. If the sub were only less than a mile closer to the surface it would be possible to pump the air from the sphere and it wouldn't implode. What am I not getting? Titanium isn't nearly the
strongest know material (http://en.wikipedia.org/wiki/Carbon_nanotube) Same problem as the double-walled container with pressure in the wall: The pressure perpendicular to the surface (air pressure) has to go somewhere and it is transferred to a pressure along the surface. So, the larger the surface, the more pressure along the surface that must be taken by the container. Therefore, the pressure anf thus the strength of the container must is proportional to the surface area of the container.

So, you may see small out here: The VOLUME of a container rises with the radius to the third power, whereas the surface only rises with the radius squared. Unfortunately, the volume, and thus the weight, of the container walls will also rise with the surface to the third power :(.

Hans

[Johnny Pneumatic]

Originally posted by MRC_Hans
So, you may see small out here: The VOLUME of a container rises with the radius to the third power, whereas the surface only rises with the radius squared. Unfortunately, the volume, and thus the weight, of the container walls will also rise with the surface to the third power :(.

Ouch!:( And I thought I had come up with a good idea for an SF story. Physics gets you every time.

[MRC_Hans]

Cool. Just got me a new sig.

:D

Hans

[Crossbow]

Originally posted by SkepticJ
I've done some more thinking about what you've said about the vacuum spheres and I don't get it. Once you had a shell that could hold up to 1 bar/14psi of air pressure the shell wouldn't need to be made any thicker as the volumes of spheres was increased would it? If so, why? The titanium sphere that is the cab where the people stay in DSVs when going deep in the ocean holds up to 14,000+psi without failing. If the sub were only less than a mile closer to the surface it would be possible to pump the air from the sphere and it wouldn't implode. What am I not getting? Titanium isn't nearly the
strongest know material (http://en.wikipedia.org/wiki/Carbon_nanotube)

Ah, but the DSRV is an example of compressive loading.

Whereas in the vaccuum plane, one would have deal with tension loading. The problems to contend with are very different indeed!