dichro: (Default)
One of my pre-roadtrip notions was to broadcast an APRS beacon from the car, for tracking our progress across the country. Having a radio in the car, I further justified, was also a useful emergency communications tool for, you know, emergencies. Without further ado, I bought a Yaesu VX-8DR, advertised as an APRS/GPS/Bluetooth radio.

This description turned out to be taking certain liberties with the truth.

It did Bluetooth in the sense that you could buy a separate $80 Bluetooth radio to plug into it.
It did GPS in the sense that you could buy a separate $75 GPS radio to plug into it (with an additional $40 adapter).
It did APRS, to be sure, in that it had a built-in 1200/9600 baud modem, which must be, what, a hundred lines of code to replicate in software?

Affronted, I refused to spend the money, deciding that I had a perfectly good GPS in my phone that is running ~100% of the time in the car anyway, and there's no reason to buy all that extra hardware when I could just send it to the radio's serial port via a $11 Bluetooth adapter. There's even a range of GPS mouse apps out there ("GPS mouse" being a term that made a lot more sense before Bluetooth).

Unsurprisingly, I ran out of time to do anything with this before we set out, and didn't find much more en route, beyond determining that the plug on the radio was finicky and it was too hard to convince the adapter's cable to mate with it. I bought a breakout cable for it–grumbling–and then took advantage of an electronically inclined friend's hack night to connect the parts together.

Pretty much everything went wrong from that point onwards.

The Bluetooth adapter was entirely happy to relay GPS data as long as somebody else connected to it first. But that's not the way that GPS mice (and apps that mimic them) work: a Bluetooth client must connect to them before they'll respond with GPS data. Trying to convince the adapter to initiate a connection was a multi-hour journey through inconsistent datasheets and discordant firmwares (firms-ware?); frustrating despite being a well-trodden path. The closest match–at least, the one whose sample output matched–was mostly untranslated and then the module didn't work as documented anyway, though I suspect here that it may have been attempting to connect to a particular UUID/port, and I still have no idea how to mess with Bluetooth at that level on a Mac.

Eventually I gave up, dug up somebody else's GPS mouse Android source, and talked it into making outbound connections. Then it became clear that the VX-8DR has NFI how to parse standards-compliant GPS output and demands fixed-width formatting because lazy firmware programmers. Also, the code was so reminiscent of my own approach to hacking together minimally functional Android apps that I spent approximately equal time feeling faintly nostalgic and wrestling with the urge to rewrite imprudently large chunks of it and send pull requests.

But with all that done and working, it was finally time to go for a drive! At which point I discovered that my Nexus 4 can't push data down an RFCOMM link without destroying the audio quality of the podcasts it's streaming through the car's speakers. Argh. Then it also became clear that the radio largely failed to punch a signal through the car's skin anyway.

So, back to the drawing board. On the upside, I rediscovered a lovely parkable perch for admiring the city lights from above.


Nov. 20th, 2013 03:36 am
dichro: (Default)
I haven't given up on the whole floating thing. The notion that's been rolling around my mind of late is aerogel-filled balloons. It isn't a new idea, and even SEAgel is easily floatable at STP.

But the more interesting compound is the recently synthesized aerographene, which not only boasts an absurdly low density, but remarkably high compressive strength.

One of the things mentioned by smarter balloon people than I was the magnitude of the thermal stresses caused by solar heating. You have an envelope floating around, filled with gas, when the sun comes up. In short order, the sun heats the envelope and gas, raising the pressure inside significantly. Your envelope either bursts or vents. If it vents, then when the sun subsequently goes down again, envelope temperature will return to equilibrium, and the envelope will shrink, causing its lifting power to be dramatically reduced, thence likely plummeting to earth, dragged down by the weight of your payload.

If the envelope is filled with aerogel, two things come into play. One, the aerogel resists being compressed, meaning that the envelope will be at a slight negative pressure to the outside air, boosting buoyancy. The aerographene paper is irritatingly vague on compressive modulus, but it does cite two useful facts: compressive modulus is 8 kPa at a density of 5.1 mg cm-3 and varies with the square of the density. Since the paper claims that the lowest synthesizable density is 0.16 mg cm-3, one could handwave, but it's too hard to run up a spreadsheet at 3am to verify my suspicion that that's too small an effect to make much difference. I think the density of aerogel required to make a meaningful difference at any particular pressure significantly outweighs the air it's saving.

The other potentially useful property is insulative. These payloads all have electronics inside them, and electronics generate heat. Nesting the electronics inside an aerogel balloon might result in a high enough temperature to make a meaningful difference. 3am is once again preventing me from contemplating Stefan-Boltzmann's law and the diurnal stratospheric temperature curve with the focus those calculations require, insomnia notwithstanding.

There's a few other interesting points in the aerographene paper. The synthesis doesn't appear enormously complex to my ignorant eyes (although the synthesis of giant graphene oxide sheets, a precursor, doesn't fill me with enthusiasm). The raw materials aren't enormously expensive for experimental quantities. Lastly, the paper claims that production can be easily scaled to volumes of m3.

I dream of people using their swimming pools to grow aerographene skybergs, attaching a lawn chair, and floating away.


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Miki Habryn

April 2017



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