Transparent Monocoque Shell

Mikek and I have been talking about a transparent airship shell for a while so I thought I would start a thread specifically for that topic. I don't know about all the different plastics out there, but I think lexan (polycarbonate) is a good candidate for a stiff, transparent envelope material.

I don't know if there are any mechanical advantages to a hard shell construction over a normal rigid or a non-rigid, but I can really "see" the advantage in having a clear envelope. Incorporating the gondola into the envelope would inprove aerodynamic efficiency, but makes visibility a problem. Only a clear forward envelope solves this problem in a simple way. I also believe that a stiff envelope would be aerodynamically superior to a flexible skin because it would not deform as much in flight. Perhaps only the forward section should be monocoque?

With that in mind, I will post a link to a site that describes how to work with polycarbonate. If we are going to consider a hard polycarbonate shell, we need to find a construction method that works for the material. 

We need a plan for forming this material into an envelope. We should also consider the case of an envelope that can't be cut into identical gores.

http://www.plasticsmachining.com/magazine/199609/polycarbonate.html

I also found a bunch of home thermoforming videos on youtube.

Vacumforming is good for small parts and complex shapes, like ends and corners. For most of the ship, I'm hoping sheets can be used, similar to airplane construction. Using a known and accepted construction technique will be better for kit builders and inspectors. Once the material is approved and accepted we'll be fine.
The first unit may be a bandit sort of item, not acceptable in the USA until it gets some airtime to prove itself. That doesn't bother me, I'm thinking somewhere like Mexico or Australia  which have wide open spaces and little air traffic. Also a cheap transportation device that runs on sunshine may be just what folks need in those places.
Jungles, yuch, just want to keep out of the snakes and spiders. No place to land an airplane, our unit can settle in a treetop if necessary. Sit there and make more gas, if you know what I mean.

Well, maybe vacuum forming should be restricted to small parts. But, vacuum forming tends to thin out the material being worked. That may be an advantage if we can't buy our material as thin as we would like to. Vacuum forming could be done on a larger scale if we could apply enough suction and heat. I think applying enough heat would be the most difficult part.  

We could dig a form in the ground in the shape of half of the envelope, then we would have to either place a heating element in the hole or pipe in hot air. It would have to be hot enough to get the material loose. Then we would hook up a vacuum and pump all the air out of the hole.

Or alternately, we could heat up a liquid (probably water) and pour it on top of the material and let it press the material down into the form. 

Or we could do some combination of the above. 

But, that might not work...

If we could produce the envelope via a one step process like thermoforming, we could pump out a lot of them, cheaply.

Vacumforming is a lot tougher than you think. You are limited by the size of the sheet, and some has to be left in the clamped area. The heat has to be perfect or the plastic gets brittle. If you're worried about your object getting too thin, you can start with thicker stock. The vacuum has to happen as fast as possible, while the plastic is at optimum temperature.
Early in the design I entertained a one piece ship. A cotton mold would be waiting around a blow-mold nozzle like sausage casing. Start the machine and stand back. When the mold was filled to the end, shut it off and terminate the end. Unzip your mold and, ta da. Lots of ways to go wrong, waste lots of plastic. The blow machines cost tons of money, and only blow objects no bigger than a large nalgene drum. Fun to imagine, but not practical.
As a kid, I used to get this goop in a tube and a plastic straw. Blew bubbles from this stuff that dried into gross little balloons. Some kids will buy anything, if it's gross enough.

I was worried about the stock being too thick rather than too thin. We may not be able to buy it thin enough for our purposes. 

Do you think the process could go slower if the temperature were maintained? And do you think that we could weld together multiple sheets into one large sheet before forming it.

I know the 2 piece concept is ambitious, but the payoff of simplifying the construction to such an extent would be huge. 

Stopped by the sign shop, cutie wasn't there. The lady I talked to wasn't very helpful. She said one pattern print four feet by eight feet could cost up to three hundred dollars. Obviously not interested in helping me find plastic either. There's another one in town I know who does big jobs, I'll talk to him.
No replies from suppliers, I'll go back to the one who did reply, get some more info.
 I like the idea of a model to test the system. Do everything on a small, affordable scale to get the feel of it.
Looking at Marvin's ship gives one the scale of envelope size to payload. He did have a bunch of motors. I would like to get some specs from Marvin. His fabric seemed very light.

Now I'll answer your questions. I'm as bad as the lady at the sign shop. As far as stock thickness, that will have to be determined by trial and error, hopefully the model will give us a clue. Too thin and the ship will crumple when uninflated, making construction more difficult.
Temperature control while thermoforming is a beast. I really only want to form small parts, use flat sheet for the big areas.
Keeping the kit idea in mind, the customer receives pre-printed sheets he cuts and folds to make the panels. Pre-formed parts come nested and packed in a large box. Add motor, avionics,mylar gas bags, etc.
I think I can get the design of the spot welder down simple enough for easy production, one may already be available. The builder would want to keep it for repairs, modifications, etc.

I finally ordered enough plastic to make a nine foot model. It will probably take a month to get here. I will order the spot welder, too. An old friend is working in the sign shop, so I'll go back there and set up a deal for the plotter.
I'm going to need a pattern. It's too risky to just draw out some curves and start cutting, I found that out with my lenticular model, which started to get very round on me. I need a gore pattern for an eight gore football shape. According to my plans, each gore will start at 12" wide, but seam material will take up one inch on one side, and an inch and a quarter on the other, leaving 9-3/4" for each actual gore. This should make a model about 24" in dia.with a four to one ratio, roughly.
Since the sheet is 8 foot, and the ship is 9 foot, the pattern will be cut off on the ends. That allows me to make vacuum-formed ends. I don't need a pattern for that, it will be done to fit after the ship is assembled.
I'm hoping somebody can do this for me in some autocad program, I'm terrible at that stuff. The pattern I need will have four gores on a 4X8 foot sheet. I'll double check the plotter, if it is less than 4 foot, I only need one gore, using 12" of plastic for the gore and the seam material. It would be nice if the lines for the seam were on the pattern. If somebody needs a sketch, I'll try to upload one.

Did they tell you how to format your pattern?

I'll double check, but it seems they can work from just about anything. The best bet would be to find out what program is going to be used, and then I can see if they can work with it.

Well can you just hand them a list of coordinates in excel?

They can work with Adobe Photoshop and Illustrator, Corel Draw, Flexi Sign-Pro, EAS, TIFF, JPEG, BMP, and GIF. No mention of Excel. Maybe Excel produces a document in one of these formats.

Sounds like they need an image file. Maybe you could trace a gore in a math program, then take a screenshot and save it as a JPEG.  

Describe the shape to me. I may be able to calculate your gores for you. I did a piriform (sp?) that is teardrop shape in excel.

This is one gore, not to scale. The end tabs are 3/4", I left it out, oops. The extra material for the seams add weight, but make the ship monocoque. I expect the ship to hold it's shape ininflated, and be very rigid inflated.

Attachment: monocoque model gore.odt (733.0KB)

I hope this upload comes through as a jpeg. odt is an open office file, some mistake.

How thick is your plastic? Given your dimensions, and the attached file, What will your envelope weigh?

Attachment: lexan_polycarbonate_9030_technical_properties_data_sheet.pdf (219.0KB)

How did you arrive at the curve in your gore pattern?

The available petg is .020", weighs .14 lbs/sq.ft., or 2.335 oz/sq.ft. I expect my model will be nine foot long, about two foot in diameter, and weigh about nine pounds. It is not expected to lift, not enough volume. It's just to test the monocoque design.
This is the same size plastic I would be using in a full size ship. I've estimated a White Dwarf sized ship would have an envelope weight of about 160 lbs.
The curve in the gore pattern is just for example, not to any scale. I didn't put a dimension for the end, it should be around four inches, making a twelve inch opening on the ends.
I'll be vacuum-forming the ends, a round nose and a pointy tail.

So your feelings won't be hurt if I provide a gore based on an ellipsoid?