[balloon-makers] RE: Solar Balloon

Rochte, Robert rrochte at gpacademy.org
Mon Jun 30 09:51:19 CDT 2003


>Bill wrote:
>Needless to say, we are all waiting for your results from the 
>flight. But, we can wait. 

I am working on the final write-up now.  The web site should have it in a
few days.  The data analysis has been quite a bit more involved than I had
expected, mostly because I have to interpret everything *without* explicit
altitude data above 8km.

>In the meantime, while crewing for a hot air balloon, I
>noticed some of the construction similarities and differences. 
>As expected there is no need for a crown opening, Since (I assume) that you
want to
>retain as much heat as possible.

I had originally intended to have a parachute valve at the top and use it
for cut-down.  In the end, it turned out to be a very difficult idea to
implement given the scale of the balloon and the materials involved.  I
ultimately decided to skip the cut-down system altogether (since the flight
was FAR 101 exempt, it wasn't legally required) and the top panel was
permanently attached.

>
> Now for the questions:
>
>How did you do your initial inflation on the ground? For 
>example did you
>use a fan to cold inflate like they do in hot air ballooning?

I used a gas-powered leaf blower for the cold inflation.  (Not exactly
elegant, but it worked!)

>Once inflated did you wait for the sun to heat the system or 
>did you pump in some heated air.
>How long did it take for the sun to provide sufficient heat for the
>balloon to rise with the payload?

There was no external source of heat - all heating was solar.  Also, no
helium was used for initial lift (as done by CNES with MIRs and proposed by
JPL for solar balloons on Mars).  The entire flight was hot air with all
heat provided by the sun.  It took about 15 minutes for the internal
temperature to rise to 90 F or so, at which point the balloon was aloft.
This was done in 3 mph winds - which sound very light until you try to
launch a solar balloon in them!

>Did you wait for the hottest part of the day or did you launch early
>morning to take advantage of the daylight.

The launch was in the morning to assure cool ambient temps.  The temp at
launch was about 64 F.

>What did you use for the glue to hold the seams together?

Seams were thermally welded.

>At what altitude do you think that the outside air temperature 
>would be so low that any heat derived from the black envelope would not be 
>sufficient to have it continue to climb. In other words, do you think your
balloon
>can make it above 60000 feet flight level? 

Well, if you run some calculations you'll see that the limiting factor
(theoretically - and yes, this is grossly over-simplified and not an
entirely accurate statment) is the volume of the balloon, not the ambient
temperature.  Convective cooling is actually *reduced* above the tropopause.
In the real-world (as you'll read in the next paragraph), the high
temperature characteristics of the envelope material are probably the
limiting factor....

My latest calculations show that the balloon reached a maximum altitude of
almost 49,000 feet - above the tropopause (so it's not getting any colder
outside) - with an ambient temp that day of -76 F.  At this level the
difference between internal and external temps was about 130 F and the
balloon was at equilibrium.  Due to the greatly reduced atmospheric density,
however, the temperature of the envelope continued to climb and it
eventually melted and initiated descent.  The envelope had to reach at least
160 F to produce the thinning and melting that I have observed.

>Did you use off the shelf radio equipment for the payload or 
>build it all yourself?

Mostly OTS ham stuff but some custom cables, etc., had to be built.  The
radio was an old Radio Shack HTX-202 2 meter handheld, the GPS was a
Magellan 315 (hacked apart to add NMEA output lines) and the TNC (basically
a modem) was a KPC-3+ from Kantronics.  Power was supplied by a mil-surplus
lithium battery pack.  Note that FRS and GMRS radios can't legally be used
for telemetry like this (including the RINOs - they cannot send data in an
unattended mode).  MURS could be used, but you wouldn't have the APRS
infrastructure to support you - you would have to directly receive all of
the data yourself.

>I bet all your students have asked these questions, but I 
>would appreciate any response you care to give.

No worries - thanks for writing!

-R
-
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