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Author Topic: Airship material  (Read 5917 times)
19th Century Space Pilot
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« on: May 29, 2009, 08:15:23 pm »

Any ideas? It has to be strong enough to withstand at least one bar pressure differential, but it can't be too heavy (100g/m^2 perhaps?).
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Matthias Gladstone
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« Reply #1 on: May 29, 2009, 09:31:48 pm »

Hmmm... need a bit more than that i'm afraid. What gas are you using and how big is your ship going to be?
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19th Century Space Pilot
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« Reply #2 on: May 29, 2009, 09:50:12 pm »

Well, I intend to use Hydrogen. Which was why I bumped the thread in Metaphysics about extracting Hydrogen.

I just want a small airship, capable of transporting 200kg (officially a Micro/Ultralight aircraft).
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Matthias Gladstone
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« Reply #3 on: May 29, 2009, 11:07:17 pm »

Ooh, for yourself to fly in? It's a marvellous idea - however, I think the rules on hydrogen filled airships are very stringent. In fact, I doubt it'll be allowed and you won't be able to get a certificate of airworthiness.

However, that all up in the air anyway (get it? i'll get my coat). Oddly, seeing as it's got a smaller molecule/atom size, hydrogen is easier to contain. The first blimps were made from goldbeater's skins - apparently the lining of ox stomachs - but you probably won't be able to get them any more. I'd imagine some sort of rubberised cloth is your best bet - I think the goodyear blimps use silk coated in rubber, or at least they did back in the day. There must be specialist suppliers for the stuff. Otherwise, no idea i'm afraid.
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« Reply #4 on: May 30, 2009, 02:30:24 am »

Are you looking for what to hold the gas or what to build the structure out of?

If for the gas look towards using a modern synthetic rubber, they are usually less permeable than natural rubber and will hold your hydrogen much better.

If you are looking for what to build the superstructure out of. I would say that an alloy of titanium and Aluminum would be in order. Very good strength to weight ratio. The bonus is that they are pretty common today. I think they make tennis rackets out of it.
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Mr. Boltneck
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« Reply #5 on: May 30, 2009, 02:51:50 am »

Possibly I am missing something, but isn't the lifting gas in the ballonets of an airship at something close to ambient pressure? Or is the pressure differential spec to allow for altitude and temperature?
Some old formula books suggest things like natural rubber dissolved in various unpleasant solvents and coated onto silk, or even several coats of cooked linseed oil, which oxidizes to a tough film, but modern materials like heat-sealed Mylar may perform a bit better.
If Mr. Gladstone is right, and the UK won't let you have any fun with hydrogen-lifted ultralights, it may be worth checking the regs in the US (I've no idea what the state of play is here right now, but there is a fair amount of latitude in ultralight and sport aviation for heavier-than-air), and maybe doing your flights in one of the states with big empty flat spaces and a tendency to officially ignore you until forced to do otherwise.
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« Reply #6 on: May 30, 2009, 03:08:34 am »

You're not going to be able to build an airship of any significant size with 1 bar of overpressure.

For a cylinder, T=PR
T is the tension in the material, P is pressure, R is radius of the cylinder. 

For a sphere, T=PR/2

So, lets take nylon, which is a fairly strong lightweight material.  According to wikipedia, this has a tensile strength of around 75MPa.
To work out the pressure you can contain, you just need to know the thickness of the skin.  Say 1mm for something fairly thick (soda-bottle thick).

So, 75MPa*0.001m = 1bar*R/2. 

We want to know R.

1 bar = 0.1MPa.    So R = 2*75*0.001/0.1 = 1.5m

Make your balloon larger than 1.5m radius, and 1mm thick nylon cannot contain 1 bar. 


But as Mr Boltneck said, I don't really see why you would want such a large overpressure?  My hydrogen reconnaissance balloon used very close to atmospheric pressure.
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« Reply #7 on: May 30, 2009, 04:05:25 am »

I've been told of a balloonist's rule of thumb that one cubic meter of hydrogen can lift one kilogram. If you are planning to build a 200 m3 airship, I do hope your workspace is large enough.
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« Reply #8 on: May 30, 2009, 12:39:26 pm »

Mylar to contain the gas backed with something else for strength
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« Reply #9 on: May 30, 2009, 01:42:14 pm »

When I say Airship material, I'm refering to what to contain the Hydrogen with. I'll use inflated structures for the rest.

I was thinking of Mylar. I don't want to make it massive, just a microlight.

The reason I want 1 bar of overpressure is so that I can cut the weightlifting ability in half by using balloonets. I don't like this messy issue with ballast, especially since I want to be able to land and take off again within a few seconds. I was also thinking of using tanks that I can put the excess Hydrogen into.

I don't have a massive workshop to store it, so I want to be able to collapse the envelope. Meaning I'll need to find tanks to store the Hydrogen. A propane tank could do the trick?
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« Reply #10 on: May 30, 2009, 02:21:08 pm »

It all depends on what the specifications of the tank are- I don't see why not, however it also depends what pressure you want to keep it at. The best way to do that would be to look at what you want, then look at the specifications on the tanks. Also depending where your workshop is you may want to check what you are allowed to store. Large quantities of pressurised highly flammable gas are a little bit of a liability! Grin

Also it would seem like balast is an easier option, and probably wise to have even if you don't intend to use it. Remember its always safer to over-engineer, and give yourself tolereance. Plus if you go wrong it gives you more leaway to ajust and alter.
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19th Century Space Pilot
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« Reply #11 on: May 30, 2009, 03:35:30 pm »

Well, I don't actually have a workshop Embarrassed

I'm 15! What do you expect? It's why I want a collapsable structure.

I'll probably relax the wieghtlifting constraints down to 100kg. To start with. I wonder if I could store Hydrogen in plastic tanks?

I still don't know what the best design would be. I want it to be twin envelope, but...

I might just go with a design like this:
Spoiler (click to show/hide)

I've got a dingy Cheesy I just need to make the envelopes. I've been thinking of simply having a long tube that is connected together, like an innertube filed with Hydrogen. Then hook the gondola on to that.

I really need to get hold of some material and Hydrogen, so I can start making models.

I just want to fly!
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Matthias Gladstone
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« Reply #12 on: May 30, 2009, 04:11:45 pm »

That looks rather dangerous, and I really wouldn't recomend it. Also, theres just not enough space in those balls for enough hydrogen to lift the boat and people inside. That airship was designed with a vacuum in mind, a technology which is sadly beyond us.
If you really want to fly, join the ATC, or if you can afford it, a ballooning or gliding club. I was in the ATC, and depending on what your squadrons like, you expect to fly about once every 2 months doing aerobatics and the like.
-Matt
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19th Century Space Pilot
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« Reply #13 on: May 30, 2009, 04:13:55 pm »

I'm going gliding soon, but... I want to fly on demand.

Hmmm... I've been thinking of a bouyant (but not enough to float) base, hoisted by a ring envelope. Bouyuncy could then be controlled by varying the amount of Hydrogen in the base.

Originally I concieved the idea for a flying house, but it could also apply to airships. Thoughts?
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« Reply #14 on: May 30, 2009, 04:36:05 pm »

Reality check : Has anyone ever created an airship light enough to qualify as an Ultralight?

The only one I've ever heard about was the lawn chair balloonist, and his 'aircraft' wasn't exactly controllable.


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« Reply #15 on: May 30, 2009, 04:47:34 pm »

I might just go with a design like this:
Spoiler (click to show/hide)

What you've got there is a balloon.  A rather inefficient one,too because of the way the gasbag is broken in to four parts. 
There is no way to steer that craft, and it looks like the artist has neglected to even draw a way for the pilot to control whether the craft is rising or falling!
And, of course, there's no way that those gasbags are lifting that gondola. They'd need to be about a hundred times bigger.


If you insist on trying this, you should try to base your craft on something out of reality not fantasy. (Surely, you suspected this?)

There is a lot of math that goes into the design and construction of any aircraft. On an ultralight the margin of error for that math becomes razor thin.  If you're not prepared for a lot of math-heavy engineering you absolutely should not attempt to build a manned craft.
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Matthias Gladstone
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« Reply #16 on: May 30, 2009, 04:49:10 pm »

Yeah, here we go:
http://www.personalblimp.com/
Not entirely of it's classification yet, but it doesn't matter what the FAA register it as as we're in the UK and we have a completely different system.
I belive you can get ultralight hot air balloons too, and there is of course: http://en.wikipedia.org/wiki/Cluster_ballooning
As for the design your quoting, it's a very old design for a flying machine. I think it's unfair to say it's fantasy - it was a serious effort let down by the technology of the day. The spheres were designed to be a vacuum, which is why there are four small ones. I don't know enough about the lift of vacuums to say if they're large enough though, but there is promise in the thoery at least.
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« Reply #17 on: May 30, 2009, 04:59:30 pm »

That Personal blimp is far too heavy to qualify as an Ultralight, so you'd need to be a licensed pilot to fly it here in USA.


I did not realize it was an a seriously proposed design.  Still, I think the point still stands, you need to base off of something that works.
Vacuum's lift would not be significantly more than hydrogen.   The difference between Helium and Hydrogen is greater than the difference between Hydrogen and Vacuum. (And the mechanism required to create a vacuum balloon would more than swamp the minor improvement.)

The photograph on wikipedia's Cluster Ballooning page is awesome.

Edit : Seems I was a little wrong about the vacuum balloon.  Struck the incorrect statement.

« Last Edit: May 30, 2009, 05:52:30 pm by VRAndy » Logged
19th Century Space Pilot
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« Reply #18 on: May 30, 2009, 05:05:47 pm »

The wouldn't have been enough lift even with a vacuum, I know that, and I know that the craft wouldn't have been very maneuverable. Butr the basic concept - multiple envelopes attached to a gondola - is... sound. Sort of.

You can get Ultralight hot air balloons; they're basically hot air balloons where the humans are attached to them with harnesses. Hmmm::: would Santos-Dumonts craft have qualified as Ultralights?

I've been thinking of a bouyant (but not enough to float) base, hoisted by a ring envelope. Bouyancy could then be controlled by varying the amount of Hydrogen in the base. Zero-pressure for the base (except for the platform) and slight overpressure for the ring (to maintain it's shape).

I've always loved Airships, even before reading Philip Reeves and Kenneth Oppels stories...
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« Reply #19 on: May 30, 2009, 05:30:07 pm »

In an article someone linked to recently it gave the relative lifts for 1 cubic meter of gas as being

Helium - 800 grams
Helium/Hydrogen, 40/60 mix - 840 grams
Hydrogen - 864 grams
Helium/Hydrogen mix heated to a couple of hundred degrees- 960 grams
Vacuum - 1,200 grams

The big problem with vacuum is that the pressure really is going the other way.  And anything rigid enough to hold it's shape with a vacuum inside it is too heavy to be lifted.    Assuming that you are using 50% of the lifting capacity to lift the balloon itself (and that's just a random figure pulled from nowhere) then to lift 100kg you will need 250 cubic metres of helium, or 232 cubic metres of hydrogen.  In other words 250 cubic metres of gas (a cubic balloon, 6.3 metres per side and weighing not more than 100kg would  do the job.  To give you some idea of scale a London double decker bus is 8.4m long.  So, no it's not a balloon one double decker bus long in each direction... it's only 3/4 of that size.  

If you had the equipment to make something that size, could you keep the weight low enough?  From a quick calculation a cube of untreated 1.1oz ripstop nylon that size would weigh about 11kg.  Now, that's probably not going to hold the gas in, doesn't give you anything except an enclosure.  and is going to cost you close to $3,000.

Z.
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19th Century Space Pilot
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« Reply #20 on: May 30, 2009, 05:41:19 pm »

I was the one linking to it, I think. Was it Lab Notes?

I don't know where he got those figures from. The density of air at sea level is 1.25kg/m^3. Hydrogen is roughly 2/27 of that, so it has a lift of approx. 1.16kg/m^3. Other people have said a rule of thumb is 1kg/m^3, but is that Helium and are the including the envelope?

We can do better than using 50% of the power to lift the envelope itself.
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« Reply #21 on: May 30, 2009, 06:16:40 pm »

1000gm-3 is a reasonable rule of thumb, it might be erring slightly on the light side but rather that than assume that you can lift more than you want to.  So, you will need a minimum of 100m3 of hydrogen to lift your load, plus enough extra to lift the weight of your craft.  Assuming that the craft consists of a lawn chair, a canopy and some straps you can keep the weight pretty low.  We already know that you're looking at around 10kg for the canopy, let's be generous and include the straps in that (We were talking about a 250m3 canopy at that point) add another 5kg for the chair and you have a total weight of 115kg for the canopy to lift.  For safety's sake we probably want to increase the capacity of the canopy to 125m3.  That's about half the previous volume.  That drops your canopy cost to $1,500.  That's assuming that most of the weight you are lifting is the load, I'm allowing 1/4 of that weight for the craft itself.  It's still not a simple proposition.  Mylar might well be a better choice to hold the gas in, and might well be lighter, but it's also more expensive. 

Z.
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« Reply #22 on: May 30, 2009, 06:24:05 pm »

Ok, here's a table of balloon lifts, (With Hydrogen.)

(And, for the sake of argument, I'm assuming that the 200kg includes the pilot, the harness/gondola, the ballast, and all the cables needed to hold this mess together. )

A 1.2m balloon gives a net lift of 800g.   So you'll need a whopping 250 of them To reach your target lift of 200kg. At a cost of $3500  Which isn't really that bad, considering.

You'd only need 100 1.9m balloons, which I was able to find on shopping.google for $22.50 each, for a total of $2250. Even better.

However, Larry Walker was able to do it with only 45 8ft(2.4m) balloons.  So obviously, bigger is better.

And the price is surprisingly not prohibitive.  It ain't pocket change, especially after you come up with the lifting gas. But you could probably pull off a insane stunt like this with the money you'd make on a summer job.

Of course, uncontrolled cluster ballooning doesn't have the same romance as your own personal Zeppelin, but it has the advantage of being possible. 
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Matthias Gladstone
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« Reply #23 on: May 30, 2009, 07:01:20 pm »

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, and all aircraft are classified I'm pretty sure you can't get around it by having an ultralight aircraft, although I need to check up on that.
As for the vacuum idea, I wasn't saying it was the solution to this problem - merely a commendable idea in an age where even hot air balloons hadn't been flown yet (16th century I believe)
Sadly, I think it's just not going to be possible. I think you'd face legal proceedings if you did try i'm afraid (or at least went over a certain height).
-Matt
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« Reply #24 on: May 30, 2009, 08:28:24 pm »

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.

So, any ideas for materials now that I've decided it isn't required to surive 1 bar of overpressure? Would Mylar be strong enough to survive free flight?
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