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Permalink Reply by MikeInFrance on February 25, 2010 at 3:27am

At these altitudes, you got to be cautious!
Don't know if you're not cruising at flight altitude of commercial lines ==> NoNo
The winds up there are very different from the winds at ground level (speed / direction) --> how will you control position?
Pressure is very different from ground level, so you have to control that too!
Therefore I wonder if a balloon is the real solution to your problem.

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Permalink Reply by Martin Seven on February 25, 2010 at 4:54am

At 100 kft altitude the temperature is low, but so is the density and therefore the heat transfer rate (a radio transmitter designed for sea-level ops frying itself is not uncommon at that altitude). A solar balloon (a black envelope with air) can reach these altitudes and at the same time hold its gas (almost) indefinitely (it's mostly N₂, a large molecule). But it won't stay afloat indefinitely, because sunlight is not permanent.

There are a number of legal issues with uncontrolled, permanent, high altitude flight. FAA has strict regulations wrt ballooning and it usually requires you to notify your local air control tower before and after the launch and supply it with trajectory predictions. A balloon floating indefinitely would be an issue I guess.
It could also cross into foreign airspace (although jetstreams over US would push the balloon east and over the ocean, and not into say Canada or Mexico). In WW2 Japan used to send balloon bombs over the Pacific to the US, for example. The receiving country might not permit high-altitude ballooning.
Another issue is with your balloon's comm system. From that altitude the radio signal covers huge areas (10s of miles with generic ~50mW xbees) and might cause interferences. Your 900 MHz system would also violate local regulations if it passed over the Atlantic into the EU (a matter of days and not an issue with a truly permanent balloon).

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Permalink Reply by Martin Seven on February 25, 2010 at 5:55am

It gave me an idea though. Nitrogen itself is a lifting gas, albeit with piss poor performance. One cubic meter of N₂ will lift about 24 grams of payload. That's about 42 m³ of it to lift a single kilogram (a sphere with a diameter of approx. 5 m). But it should stay afloat reasonably indefinitely, even with materials that wouldn't fly with helium or hydrogen.

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Permalink Reply by Jack Biggs on February 25, 2010 at 10:10am

By the way, I made a mistake on my pdf: By saying D-STAR instead of APRS, I meant D-STAR instead of 900MHz. If the payload were to have GPS, I think it could work just fine (disable the radio should it pass over the EU). Although there is a problem with passing over non-high-altitude-ballooning nations. I suppose that a propeller system could be implemented (I guess turning it into an actual blimp)

The only problem that I could see with the blimp idea is that it might not be powerful enough for the winds going on at that altitude. And that the FAA may not really like the idea of such a balloon staying at 100,000 feet consistently with no predictable flight pattern.

But I really appreciate your advice! I'm going to look into that solar balloon idea. Thanks a lot! :)

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Permalink Reply by Martin Seven on February 25, 2010 at 11:54am

Yeah, the steering part sounds like a pipe dream (unfortunately). Jet streams go anywhere between 100 km/h and 400 km/h, depending on temperature differences. A propulsion system that could push a balloon/blimp with several square meters of cross section at almost Mach 0.4 would definitely not fit within the unmanned free balloon definition as per FAA Part 101. And you'd never pull it off with solar panels, you'd need a bad-ass powerplant to do the job, cutting through your perpetual mobility requirement. It would also tear the crap out of that thing :))

The radio issue could be solved by either having the balloon listen for your tracking station's beacon and broadcast to downlink telemetry only after some kind of handshake has been done. Or you could pay a buck and get an iridium phone (satellite, global-ish coverage) and a modem. Or simply dump 900 MHz and pick a globally accepted ISM frequency. You'd still have to be careful about output power though. While you guys can push several watts of microwaves over a stretch of desert here in EU a 500mW transmitter is considered to be "high powered".

Although the crossing into enemyforeign airspace part is what's probably the show stopper here. I just can't imagine that happening, not with a government aircraft, let alone a civilian, DIY one. I'm starting to think that a LEO satellite would be a safer, cheaper and easier way to go :))

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Permalink Reply by David on February 25, 2010 at 3:21pm

There's a lot of stuff to consider here, but it's a great idea...

First off, the FAA doesn't have a problem with high-altitude balloon (HAB) operations, as long as they don't violate the US Code of Federal Regulations Title 14 Section 101. I'm not a lawyer or an FAA official, so I'm not going to post a translation of what's being said, but I can tell you to look at sections 101.1-101.7 specifically for exemptions and operational restrictions. Note that it is possible to get waivers for stuff that falls outside of the specs... I've done it, and flown, but it wasn't fun to do.

Also, there are a lot of people who do HAB stuff as a hobby. The ARHAB page has links to most hobby HAB programs in the US. There is a yearly conference of HAB enthusiasts called the Great Plains Super Launch, and an associated email list, full of people who enjoy talking about their favorite hobby. Also, a number of universities have HAB programs as well; I was responsible for Taylor University's High Altitude Research Platform team for roughly 5 years; there's a link to that program on the ARHAB site, and a video that I put together can be found on youtube. Also, check out JP Aerospace and Space Data; both of which are highly-organized teams.

Some radio suggestions for you: the 900 MHz ISM band is awesome for these applications; FreeWave and Maxstream are two suppliers that I worked with, and both do a great job (FreeWave is much more expensive, but has better performance). Also, there are other opportunities on the ham bands (think what could be done with a repeater at 100k' - it's pretty cool, just coordinate with your local clubs/repeter coordinator).

Recovery via parachute is not difficult at all; look for high-powered rocket parachutes. At the weights that my team flew (no more than 12 lbs, in no fewer than 2 packages), 4-6' diameter chutes were perfect. Plus, deploying them is not hard, either; if the order of your system is (from the top) balloon -> load line -> parachute -> payload, the chute will be collapsed on the way up, and open on descent. At 100k', there isn't enough air to appreciably slow the chute (it inflates, but terminal velocity is extreme), but the rate of descent evens out as you get lower.

I believe that JP Aerospace did some experiments with propulsion, but they involved really big props to grab what little air exists up high (you're in a near vac at 100k'; low pressure testing of your systems can be helpful if you have access to a bell jar), and I don't recall if they were successful at all. However, in your area, there is generally a weak western flow above about 90k' in the summer (the dominant jetstream runs east), and some station keeping can be achieved by changing your altitude through layers of air - vent lift gas to descent, dump ballast to ascend.

I would encourage you to strongly consider including a small APRS system in your payload. Don't broadcast too often (to avoid the wrath of the APRS community), but if you transmit on one of the two national frequencies (depending on which direction you're predicting the payload to go; you're pretty close to the divide between the use of 144.39 and 144.34 MHz), you'll have the assistance of any APRS-equipped ham in tracking your payload.

Also, there have been some attempts by the HAB community to fly a trans-atlantic mission that I'm aware of, but they have all failed (one by only ~150 miles, if I recall correctly). Information on that project is here.

This is probably way more information than you were looking for, but hopefully some if it helps. If I can be of further assistance, feel free to let me know. I may be able to dig up some of the actual temperature / pressure data from some of my team's flights, too, if you're interested.

-D

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Permalink Reply by Jack Biggs on February 25, 2010 at 4:51pm

Thanks David! The only major concern that I had was having it float over a not-so-tolerant country, the absolute worst case being North Korea. I was considering using just APRS and potentially 900MHz or D-STAR for downlinking the photos/data taken. I've decided to follow the advice of Martin, going with an N2-filled solar balloon. The problem with a solar balloon, though, is that when the sun goes down that thing will undoubtedly start its descent... and a pure-N2 balloon would require such a huge balloon (solar is pretty big already) that it would undoubtedly catch a paranoid eye or two. What are your thoughts on this?

Edit: I was also planning on letting the APRS radio act as a repeater as well, and potentially even an AMSAT relay to help out with ground-to-space satellite communications :)

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Permalink Reply by David on February 25, 2010 at 7:14pm

In my experience, upper atmosphere air (something like 60k' +) - especially in the summer - is fairly calm, on the order of 0-25 mph. Not to say that I haven't seen it faster, but that, generally, it's going to take a long, long time to get from the middle of the country to the west coast. Things happen pretty fast at launch, but, during flight, you'll have plenty of time to react before you lose touch with the balloon (command a payload cutdown, etc.). If you are worried about the payload running out to sea, I have to ask: are you planning on designing a "disposable" payload, or do you want it back? I'm working under the assumption that you are planning on recovering the system (I wouldn't want to throw a Gumstix-based payload away), but correct me if I'm wrong.

I promise I’ll answer your question, but some further background would make my replies much more useful, I think, so bear with me…

OK, so let’s start with some balloon types: there are three major types of balloon envelopes: the latex (stereotypical “weather balloon”), zero-pressure, and super-pressure. Latex balloons are generally used for launch->burst->land operations, but it is possible to extend their mission through the use of gas valves and ballast tanks. One of the biggest drawbacks to a latex balloon (aside from its permeability), is that it will suffer from UV degradation, which is only accelerated at altitude. Latex balloons are inflated to a small percentage of their burst volume, and expand as they ascend: the balloons that we used to launch most of our smaller payloads (we used He and H2 as our lift gas) were about 6-8’ dia. on the ground, and had a spec. burst diameter of about 40’ (at an altitude of between 90-110k’, generally). Both zero-pressure and super-pressure envelopes are more designed for altitude hold, but they accomplish this goal in different ways. A zero-pressure envelope never changes size – it starts out with the capacity to hold the volume of lift gas necessary to achieve neutral buoyancy for the desired altitude and payload weight, and a slit to dump excess helium out of the bottom. The zero-pressure envelope is filled with a “bubble” of lift gas in the top – sufficient to lift the payload – and, as it ascends, the bubble of gas expands as the exterior pressure goes down until it begins to “leak” out of the vent in the bottom of the balloon. The gas dump process continues until neutral buoyancy is achieved, but the important thing is that the gas pressure inside and outside the envelope is (as close as you can get to) equal. I believe that the Spirit of Knoxville team developed their own zero-pressure envelope for their flights. I know that Aerostar makes zero-pressure balloons, but their sales policy is pretty stringent. The zero-pressure approach does much better at long-duration flights than the latex, but, by design, you are losing lift gas, which can be detrimental to staying aloft for a long time.

A super-pressure envelope is similar, but is designed to ascend until the gas fills the envelope, and then stop at the point that the density difference between the lift gas and the environmental gas causes a lift equal to the payload weight, at a pressure differential between the lift gas and the environment. This envelope is tricky to design and build, because even a small pressure when distributed over the surface area of a balloon, can result in forces difficult for seams to handle. However, this is the envelope technology that is used by NASA for their polar-orbiting balloons in Antarctica, which have stayed up for a month plus. (NASA also uses zero-pressure balloons for float flights, but I can’t think of a mission that lasted longer than about a week and a half with that tech.) However, even with the relative impregnability of the super-pressure envelopes, ballast is a significant component of the payload for the long-duration flights.

Now, moving closer to your question, most payload-carrying solar balloons are subsets of zero-pressure envelopes, designed to vent the solar-heated gas out of the envelope as the gas warms and the balloon ascends. However, super-pressure solar envelopes exist, though mostly as toys (the long black tubes that sometimes make an appearance outside of science museums). As you pointed out, a solar balloon will descend after the sun goes down and it’s no longer being heated, but it may be possible to design a solar super-pressure envelope that “oscillates” between two altitudes (for example 60k and 100k): it carries enough lift gas that it will hover at the lower altitude, but has the envelope capacity to handle volumes to get to the higher altitude with the assistance of solar heating. Some math will be required to figure out how big the envelope would need to be, and how much gas would need to be in it. The envelope will be huge, you’re right… but the zero-pressure type can be built out of cheap trash bags and a heat gun (I had a class build one with a ~3000 cu.ft. volume which flew for about 8 hours with a 1lb payload and topped out at about 60k’). As long as the balloon envelope is all a dark color, lift will be generated (I wouldn’t recommend trying to use solar-generated electrical power for a heater… you are correct in assuming that your heating efficiency will be abysmal). Will it create a stir? Maybe… when ours came down in rural OH the local Sheriff was called, but there was no problem from anyone about the mission. I can also tell you that, on a clear day with the right light (morning or late afternoon) a standard latex balloon is visible at 100k’ just before burst to the unaided eye as a bright light a little larger than a very bright star at night.

My final thought would be to avoid the temptation to “go for the gusto” on the first flight, and lay out a schedule of flights starting with a standard He or H2-filled latex balloon to test your system. Any critical mission safety devices (cutdowns, etc) could be tested with a much lower risk, and you’ll gain some experience tracking and managing the mission. In addition, latex envelopes are fairly cheap, you can get them through Kaymont. Make sure that you have the math right for an N2 solar balloon before you launch it, and then make sure that you have some way to build the envelope, or the budget to have one built… (yikes). The solar balloon that my students built took up most of a large classroom floor to build, and it was folded over on itself as the seams were being fused. Build up your flight times, and see how far you can go… the Spirit of Knoxville is the current record holder for amateur HABs at better than 48 hours.

Hope that helps…

-D

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Permalink Reply by Jack Biggs on February 25, 2010 at 7:26pm

Thanks so much! That really helped me out! The only question I have with regards to that is how porous are trash bags are? N2 is a large molecule (at least, compared to helium) but is there a risk of the gas gradually leaking out of the balloon? I really like your idea of creating a balloon with an oscillating height, but I was curious as to how long that could be maintained before the N2 leaks out. Furthermore, you mentioned that the 60,000 ft winds are fairly calm, but then talk about going up to 100,000 feet, and fairly heavy trade winds are characteristic at that altitude. With an envelope that big, what's the danger of the winds potentially ripping the envelope to shreds?

Also, you asked if I was intending to make this a disposable-type unit. I don't necessarily want it to be disposable, but I certainly don't count on getting it back again if I want an indefinite-flight setup. I want something that will last, furthermore, something sophisticated enough to gather lots of readings and lots of photos, and have the ability to relay them down over RF.

I really appreciate this! I might end up sending this link to some of my interested friends... :-)

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Permalink Reply by David on February 25, 2010 at 7:43pm

So, as far as how porous trash bags are, the best that I can tell you is that it depends on the trash bag: they come in different weights (thicknesses) and plastics, but beyond that I don't really have a whole lot more info for you. Aerostar makes their envelopes out of polyethylene... but I know that from their website. I would look up a place that specializes in plastic film production and ask the specialists about that... sorry I can't be of more help with that question.

The probability of wind being the cause of envelope failure is pretty remote... there can be turbulence, but keep in mind that wind is experienced only when the air is moving relative to the frame of reference. From the frame of reference of the balloon, wind speed is almost always low because (in the absence of propulsion) the force of the wind on the balloon encounters little to stop it from just moving the balloon.

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Permalink Reply by Jack Biggs on February 25, 2010 at 7:47pm

I recently had an idea: could it be possible to have a cage within the solar balloon that is approximately the size of the balloon at ground altitude, so that should the balloon cool down and head for Earth, could it stay inflated and potentially rise the next sunup?

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Permalink Reply by David on February 25, 2010 at 8:32pm

I've heard that idea thrown around to greater or lesser degrees, but I am not aware of an implementation that has managed sufficiently low weight to be functional... it is an interesting concept, though.

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