GAGA-2 | Hacker News Reader

GAGA-2

blog.jgc.org

158 pointsjgrahamc21 comments

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.)

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 ?

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...), 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.

> balloons capable of traversing large distances safely and reliably.

How would you do that? Once it is in the air isn't it at the mercy of the air currents (can you control the altitude?). Then is it a matter of analyzing and predicting the flow of air currents and the weather?

Japanese for example attempted to send balloons over the pacific that were intended to drop on US and start forest fires.

http://en.wikipedia.org/wiki/Fire_balloon

They even claimed one human victim.

Prediction tools are a big part of it. The US government publishes global wind data every 6 hours, as well as tools that can be used to parse and process this data. We've done a bunch of scripting on top of that to automatically email, sms, and tweet launch opportunities. The team doing the predictions is too busy/lazy to upload their code to our github account(https://github.com/whitestarballoon/), and that's the only reason it isn't open source.

We can drop ballast to maintain altitude, but we don't have any other control. As a result, we must wait for the winds.

(You can see an example prediction feed here: https://twitter.com/WSBFlightPath Cryptic, but it's meant for the launch team).

The Japanese balloons were examples of exquisite analog engineering. The Army made a great video documenting their construction and operation (available here: http://www.archive.org/details/gov.archives.arc.13084).

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")

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.

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.

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.

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

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.

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

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

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?

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.

It seems like extending the operating life of the device is ideal since then it could tolerate a much slower descent and burst much higher.

On #highaltitude, what were the highest altitudes others have achieved?

I believe the record is just > 40km

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.

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.

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.

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.

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