

GAGA-2 - jgrahamc
http://blog.jgc.org/2012/01/gaga-2.html

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malandrew
In your experiments with spray polyurethane foam, did you use open or closed-
cell foam? I ask because you said that in your experiments it didn't insulate
that well. AFAIK, open cell foam has a lower R-value than standard Styrofoam
polystyrene (~3.5 vs. ~5.0). Closed-cell polyurethane foam on the other hand
has an R-value of about ~6.5. The problem with closed cell foam is the weight
(2.0 per cubic foot vs. 0.5 for open-cell.

Even if it were too messy to use for the outer construction, it might be
interesting to use it only for those components that are most temperature
sensitive, such as the battery. This would also solve the battery dislodging
problems you had with your first flight.

I'm wondering if anyone is creating a stereolithography machine capable of
printing structures using materials designed for insulation such as
polyurethane/polystyrene foams and silica aerogels (R-value ~10.0.

I'm also curious if using stereolithography to print honeycomb structures in a
vacuum would be a good way to produce vacuum insulated structures (AFAIK
R-values of 30 to 50 should be possible.)

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meow
Theoretically, will the balloon be able to escape the atmosphere if it is
provided with a valve to control the pressure based on altitude ? does it
always have to burst at some point ?

~~~
Zuph
The balloon will never escape the atmosphere. It simply drifts higher until it
either reaches neutral buoyancy, or pops. As the balloon rises, air pressure
decreases. The result is that the gas inside the balloon is compressed less as
the balloon rises, causing the balloon to expand.

In a flight like jgc's, the latex balloon expands until it pops. There are
other balloons which can float at a steady altitude. One of these is called a
"Zero-pressure Balloon." This balloon operates with a valve on the bottom. The
helium will expand until it reaches this valve, then spill out. When the
balloon loses helium, it loses lift. The valve will tend to dump helium until
the balloon reaches neutral buoyancy. These balloons are so named because the
pressure at the valve is the same as the ambient atmospheric pressure.

Another type of balloon is called a "Superpressure Balloon." These balloons
maintain altitude by pressurizing internally. Unlike latex, these balloons
stop stretching at a certain altitude. As soon as the balloon stops expanding,
it maintains a constant volume. Buoyancy is based on the volume of fluid
displace, so a pressurized balloon maintains the same altitude.

I work with a group of hackers attempting to cross the Atlantic Ocean using a
Zero-pressure balloon. (<http://www.whitestarballoon.com>). A group of radio
amateurs actually beat us to the punch ([http://www.arrl.org/news/amateur-
radio-balloon-flight-crosse...](http://www.arrl.org/news/amateur-radio-
balloon-flight-crosses-atlantic-sets-records)), their crossing was involved a
large quantity of luck. We seek to develop systems and methods for engineering
balloons capable of traversing large distances safely and reliably.

~~~
malandrew
Very interesting, but why won't it ever leave the atmosphere or at least rise
far enough that it could reach the Kármán Line (~100km). The record according
to jgc is ~40km. Why is it so difficult to go from 40km to 100km if these
zero-pressure valves exist?

(PS: I am aware the Kármán line actually has to do with aerodynamic lift (and
therefore may not really be applicable to lighter than aircraft). It just
chose it because it's the commonly accepted "edge of space")

~~~
Zuph
In order to go higher, you must enclose a larger volume of lifting gas.
Eventually, the weight of your envelope will exceed the amount of lift you're
able to generate with the enclosed volume.

The Japanese currently hold the world altitude record (among all professional
and amateur balloons), IIRC. Their balloon was 3.4 micrometers thick, 60,000
m^3 of helium. They reached 53 km.

~~~
malandrew
Out of curiosity, why use helium instead of hydrogen? It's not like human
lives are at risk in these experiments.

I imagine you'd get more lift with hydrogen.

~~~
russss
People do use hydrogen, but most balloons still use helium because it does
provide a slight safety advantage when launching. (It's also sometimes an
insurance requirement.)

I expect to see an increase in the number of hydrogen-filled balloons as
helium prices rise in the next few years.

~~~
malandrew
That makes sense. Helium seems to be too precious a natural resource to be
using in applications where hydrogen can be used safely.

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dolbz
Looks like great fun. I'm always tempted to have a go at things like this
myself when I see reports like this. One day...

One question. Did you choose the video cameras based on size or are did you
assess the quality too? I've seen some pathetic 'HD' videos from cheap cameras
before but things seem to have progressed a lot over the last couple of years.

~~~
jgrahamc
I spent a lot of time investigating small cameras and these ones were the ones
that looked best. You can read more than you want to know here:
<http://www.chucklohr.com/808/>

The ones I am using are what he called #16:
<http://www.chucklohr.com/808/C16/index.html>

~~~
joshu
I have a source for #3s if you want it.

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zeteo
I see you've gone to great lengths to make it lighter this time. Any idea what
the relationship is between payload weight and balloon burst altitude?

~~~
jgrahamc
I think the burst altitude is far more related to how full the balloon was.
Fill the balloon a lot and you get a fast ascent but a lower burst, underfill
and you'll get higher for the same payload weight, but of course you get blown
further.

~~~
seats
You could design a two balloon system with a secondary balloon that is nearly
initially empty that will take over lift after the first bursts.

~~~
zeteo
Or an automated valve controlled by a manometer. The people who build these
awesome contraptions seem to put a lot of effort into the payload and
comparatively a lot less into the balloon.

~~~
jellicle
The design goal isn't usually to maximize time at altitude. If you do that,
your balloon lands 10,000km away and is difficult to retrieve.

~~~
zeteo
But a common design goal is to maximize altitude, and an automated valve would
help with that. You can also have it wired to open if the balloon stops
ascending, or strays too far.

