Posted: 09/02/2010 22:09 PM Post subject: Vacuum Balloon
Resident Tinkerer
Joined: 07 Aug 2007 Posts: 1202 2200.81 Spud Bux
Ok, here is a question in the exact opposite direction of what we deal with normally.
I want to make a negative pressure chamber.
I am trying to figure out a way to build a vacuum lighter than air ship.
STOP. Don't tell me it is stupid.
I know, helium and hydrogen, and just about everything is more practical, but this is purely for the experiment, to show it is possible.
Many people have come up with ideas of a metal frame with a cloth stretched between that holds the vacuum, like this:
I want to try going about it in another direction. Rather than relying on tensile strength, I want to try one that is based off compressive strength.
My idea is to create a hollow sphere out of Styrofoam, (blown with hydrogen to make it extra light maybe?) so that the shell can be very thick, and the smallest imperfection won't cause it to implode. Also, with thickness, if I am not mistaken, comes exponential strength.
Do you guys have any suggestions, or relevant equations, or anything of the like to point me in the right direction?
Yet again, please don't tell me it can't be done. That is exactly the point. I want to try something new.
Just take a rubber balloon and take the air out of it.
Edit- ... never mind.
haha just kidding, but keep us posted if you make it, i've never heard of that concept before. i think the problem would be having a truly air tight balloon/frame. Maybe having a vacuum constantly suck air out would be a solution?
Won't the atmosphere around the chamber compress it across it's entire surface consistently? I realize that vacuum means no air, and thus is lighter than air, but taking a balloon and removing the air from it does not make it more likely to float.
Yes, but the point is to make a rigid, lightweight chamber which can hold a fairly high vacuum... thus lowering the overall density and causing it to float.
Ie a helium balloon floats because helium is lighter than air, and if you've got enough helium its buoyancy overcomes the weight of the balloon itself. A vacuum is lighter than helium, so if you could 'fill' a balloon with a vacuum, it too would rise. The issue is stopping the balloon from collapsing.
Joined: 15 Mar 2007 Posts: 18051 32498.46 Spud Bux
Gaderelguitarist wrote:
Won't the atmosphere around the chamber compress it across it's entire surface consistently? I realize that vacuum means no air, and thus is lighter than air, but taking a balloon and removing the air from it does not make it more likely to float.
Simple buoyancy, same volume and less weight - if it works for helium, it will work even better with vacuum. Not much better, really, but Lentamentalisk doesn't want us to talk about that
So let's see, 4 foot diameter sphere, 0.9" wall thickness, made from 2lb/cubic foot styrofoam, weighing almost 14lbs - but only displacing around 3.5lbs of air...
Simple buoyancy, same volume and less weight - if it works for helium, it will work even better with vacuum. Not much better, really, but Lentamentalisk doesn't want us to talk about that Wink
Granted. I must be weakened in my more tired state.
The best I can suggest is to work on it as an engineering problem.
How much air must you displace to get the desired lift. I assume you want as much lift as the airship weight.
With a known volume, the tough part begins. Find the force on the shell from atmospheric pressure and find the shell and support structure that is within your weight allowance. This is where most designs fail as you are trying to build a structure to support several tons of force but weighs only a few pounds.
Note the amount the rail car collapsed. The volume shrank to nearly 10% of the original volume. How much did the tank car weigh? How much weight of air does it displace?
Somehow even without wheels and other non essentials attached, I highly doubt the rail car was anywhere near equilibrium.
Designing a container with the strength of a rail car, but light as a hot air balloon is where the trouble arises.
A typical hot air balloon weighs about 600 lbs with the gondola and fuel. The balloon contains about 2.5 tons of air inside. If you can make a tank that size able to hold a vacuum, and the size of a hot air balloon and weigh under 2.5 tons, you are close to flight with a vacuum.
If I had to give it a go without trying any calculations before hand, I'd try a carbon fibre structure fitted inside a an airtight envelope. And then try and make it very big.
So let's see, 4 foot diameter sphere, 0.9" wall thickness, made from 2lb/cubic foot styrofoam, weighing almost 14lbs - but only displacing around 3.5lbs of air...
this will be your biggest problem.
as jack stated, your first issue will be finding out how much weight i air you will be able to remove for a give volume....
I suggest starting there.
once you have that done, you can determine what your chamber can weigh.
Joined: 15 Mar 2007 Posts: 18051 32498.46 Spud Bux
Let's go with a 1 metre radius sphere.
If the total weight is more than 5kg, it won't float.
Let's say the foam has a density 150 kg/m3
Going with the 5kg limit, ie for enough buoyancy for the sphere to lift only itself, even with a wall thickness of less than 1/8", it will still be far too heavy to float - and at that thickness, a 1 metre foam sphere is guaranteed to implode in a spectacular fashion.
I'm not saying it can't be done, but the grade of high strength and low weight material required for such a project suggests it's beyond the reach of the average individual.
edit: fine, yeesh :p
Last edited by jackssmirkingrevenge on 09/03/2010 10:28 AM; edited 1 time in total
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