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Author Topic: Airship material  (Read 5867 times)
Professor Fzz
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« Reply #25 on: May 31, 2009, 01:01:44 am »

Just to come back with some maths on why 1 bar doesn't work (I know you've abandoned the idea, but I got curious anyway and looked into it a bit more).

If you want a pressurized sphere (for the sake of the simplicity of the maths) of 200m3 volume (i.e. enough hydrogen to lift 200kg),  then from v =(4/3) pi r3, we get a radius of 3.5m or about 23 feet diameter.  That's pretty big.  

Now, if you compare this with a party balloon of 10cm radius (which is on the limits of the tensile strength of rubber of that thickness).  A party balloon is inflated to somewhere vaguely around 50cm H20 (had to look in a medical journal to figure this out - it comes from how hard you can blow!), which is roughly 5KPa.  1 bar is 100KPa.  So you need your 10cm balloon to be 20 times thicker than a party balloon to withstand 1 bar - that's roughly twice as thick as an *uninflated* regular party balloon, so if you use rubber you need it to be around 10mm thick before inflation, and 1mm thick when inflated to withstand 1 bar.  I'm not convinced thick rubber can stretch like that, but we'll ignore this for now.

Now that's for a 10cm radius balloon.  A 3.5m radius rubber balloon must be 35 times thicker as tension is proportional to radius for a pressure vessel, so 35mm thick when inflated (or about 1.4 inches if you prefer old money).  

How much would that weigh?  Area of a sphere is 4 pi r2,  so a 3.5m sphere has an area of 154 m2.  Volume of rubber in a 3.5m sphere of 35mm thickness = 154* 0.035 = 5.4m3.  Density of rubber is about 1500kg/m3, so the sphere would weigh 8100kg, or 8 tonnes.    It won't fly, not even close.


So what can you do?  As you scale up, the volume (and hence lifting capability) grows with r3.  The area of the balloon grows with r2.  The thickness needs to grow with r for the same pressure.  So the mass of the balloon grows with r3.  Basically you can't win by making it bigger.


Your only hope is to go with something that has a lot better tensile strength than rubber, but which is also light.  Rubber has a tensile strength of around 15MPa according to wikipedia.  Nylon is about 5 times better, but you'd still need a 7mm thick envelope. Nylon is also slightly less dense, but it would still weight 1.2 tonnes.  Still not useful

The only thing I can think of that might work is carbon fibre.  According to wikipedia (source of all truth :-)  carbon fibre has a tensile strength of 5600MPa.  But I've also seen sources say 1500MPa, and most places say three times stronger than  steel, so I'm inclined to believe 1500.   But you've got to have the fibres aligned both ways for an envelope, so the effective tensile strength would be halved.  So now you could decrease the thickness by a factor of 750/15 = 50.  This gives an envelope of 0.7mm.    But would it fly?  Carbon fibre is slightly denser than rubber 1700 vs 1500, so this gives an envelope mass of 154*0.0007*1700 = 183kg.  It would
fly, but it wouldn't lift you.


Caveats:  all this is from extrapolation from a party balloon, based on some pretty shakey numbers :-)  But unless I messed up the maths (entirely likely!), it makes it pretty clear why airships aren't pressurised to anything significant above atmospheric.








« Last Edit: May 31, 2009, 01:03:19 am by Professor Fzz » Logged

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« Reply #26 on: May 31, 2009, 01:18:40 am »

Another point of note is that the higher above atmospheric pressure, the less the lifting capacity you get per m3 gas: by increasing the pressure you increase the density. The low density is what gives you lift ie. lighter than air flight  -in effect 1m3 at high pressure will then lift far less than 1Kg. Wink

Also have you considered building a smaller prototype first to the tune of 1m3 of H2; to even get a small home made lighter than air craft is quite an achievement, and would give a good idea of the scale of the project, what are the issues to look out for,  &c. Grin
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« Reply #27 on: May 31, 2009, 01:38:29 am »

Quote from: M. Gladstone
Indeed. However, we still face the issue that all of the ideas proposed arn't currently legal (in the sense of being registered airworthy by the CAA), and as we have different rules to the U.S
Oh yes. I'd forgotten about that.  (Well, actually I'd forgotten to notice where the poster was from.)
You might wind up having to be tethered to the ground to avoid legal problems. Or worse, indoors. Any Zeppelin hangers nearby that you could borrow?

Well, I'm not worried about altitude limits. At least not on the low altitude flights, as I want to be flying around above people anyway and scaring them.
I'm not sure if you've thought this through. You're going to be strapped to a balloon the size of a house, and you're going to be totally drifting. There's no way this is going to be a powered craft if your payload is under 200kg.  And the people nearby are going to be the people who helped you inflate and launch your craft.  
So ... you're not really going to be sharking anyone, you're going to be more of a helpless jellyfish.

Quote
Would Mylar be strong enough to survive free flight?
You're thinking of a single enclosure?  I'm not sure I'd trust mylar to hold my weight. How will you connect it to yourself? Some sort of netting?
I don't know about your skill level, but I wouldn't really trust a single enclosure that I'd built myself.  If that seam gives way, or if your harness rips the mylar, you're going to plummet.
That's the advantage to the weather balloon plan. Not only do you have redundancy, but your gasbags are built by people who know what they're doing.

Here's a quote from Wikipedia :
Quote
Modern hot air balloons are usually made of light-weight and strong synthetic fabrics such as ripstop nylon, or dacron (a polyester).[13]
A hot air balloon is partially inflated with cold air from a gas-powered fan, before the propane burners are used for final inflation.

During the manufacturing process, the material is cut into panels and sewn together, along with structural load tapes that carry the weight of the gondola or basket. The individual sections which run from the throat to the crown (top) of the envelope are called gores or gore sections. Envelopes can have as few as 4 gores or as many as 24 or more.

Envelopes often have a crown ring at their very top. This is a hoop of smooth metal, usually aluminum and approximately 1 ft (0.3 m) in diameter, to which vertical load tapes attach.
So there you go.  If lighter materials were available they're surely be using them.
« Last Edit: May 31, 2009, 02:17:56 am by VRAndy » Logged

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« Reply #28 on: May 31, 2009, 09:52:25 am »

This might give you some perspective on how big your craft is gonna have to be, these are helium, but using hydrogen wont substantially shrink your craft, especially with the added weight of a solid structure to lift, i say, as a 15 year old, try something safer!

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« Reply #29 on: May 31, 2009, 03:45:25 pm »

Also have you considered building a smaller prototype first to the tune of 1m3 of H2; to even get a small home made lighter than air craft is quite an achievement, and would give a good idea of the scale of the project, what are the issues to look out for,  &c. Grin
That *was* what I have in mind. I still need to know what material to use, though.

I'm considering geodesic shapes. All I need are triangles in that case, then to assemble them in the correct shape... somehow.
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« Reply #30 on: May 31, 2009, 03:48:56 pm »

I'd quite like to build something about the size of/smaller than No.1. Unfortunately, I don't have shed or much room.

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« Reply #31 on: May 31, 2009, 04:54:07 pm »

that personal blimp thing is a hot air balloon, However they do have a good design for a type of envelope though. I would hang the gondola a little lower under it and attach balloonetts directly to the gondola through the envelope then attach the propulsion to the gondola. That would make for a pretty decent personal airship.
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« Reply #32 on: May 31, 2009, 07:11:43 pm »

I'm considering geodesic shapes. All I need are triangles in that case, then to assemble them in the correct shape... somehow.
Think about failure modes.  A geodesic balloon would have approximately a zillion seams. If one of them breaks you're going to plummet.

You also need to consider how you're going to distribute your weight evenly across the balloon. You can't just attach a string to the bottom of a giant balloon, it would tear right out.

But seriously consider the legal problems here if you leave the ground, even a little.  There is no way you could do this secretly.  And someone is bound to call the police.  Someone always calls the police.
Besides, what good is an airship if you're too afraid of the cops to gain any altitude? Even professional made airships have terrible maneuverability. They're simply no good close to the ground.

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I'd quite like to build something about the size of/smaller than No.1. Unfortunately, I don't have shed or much room.
Perhaps building a 75ft airship is a little ambitious for a first project?
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« Reply #33 on: June 01, 2009, 01:17:38 am »

Everybody else seems to have a handle on the engineering.  Let's talk about why you don't want to use hydrogen.

First, hydrogen leaks even faster than helium.  Not only does it rapidly leak through actual holes, it will leak right through many materials -- most thin plastics and many metals included.  Second, it makes metals brittle.  For example, storing it in propane tanks is right out -- those are plain carbon steel, which will turn brittle on that much exposure to hydrogen.  Brittle tanks are bad, they break in normal handling.

Second, saying that hydrogen is flammable is only the start of that warning.  Propane is flammable and needs caution when handling; hydrogen is in a whole different league.  It catches fire more easily than most other gases -- both in terms of temperature required and energy required.  It takes around 20 microjoules of energy to ignite hydrogen in air.  That energy in the form of a static electric spark is something you might not even feel.  Hydrogen is *really* flammable.  Once you've lit it with a minuscule spark, hydrogen burns with a clear flame.  Ever seen alcohol burn in full sunlight?  Hydrogen is like that, but far more so.  If you suspect a hydrogen fire or leak, one standard safety precaution is to carry a broom in front of you and wave it slowly about -- so that the bristles will visibly catch fire before you walk into the flame.  Similarly, there's very little IR output, so you won't feel the heat from the fire until you stick your hand into the flame.  If you're working with pressurized hydrogen, you get a whole new can of worms.  Under the wrong conditions, hydrogen leaks can self-ignite.  I've seen it recommended to wrap paper towels around all plumbing joints and tape them in place.  That way, if you get a hydrogen fire, you can see the paper towels burning.

If this doesn't sound like the level of precaution you're used to dealing with, I strongly recommend you use helium, even if it is a bit more expensive and a bit less period.  For small demonstrations in chem class, the hydrogen safety is fairly simple -- don't make all that much, wear safety goggles, etc.  But when you start talking about hundreds of cubic meters, you should start looking at industrial safety practice -- and that's not something you want to deal with.
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« Reply #34 on: June 28, 2009, 05:34:53 am »

The lightest fabric at present I could find suitable is called Cuben Fabric and is used to make high performance sails for racing. It is ridicuously light a 10'x12' tarp only weighs 8 oz. but it is also VERY expensive.
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