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[aprssig] Balloon warmth

AD4BL ad4bl at arrl.net
Wed Mar 24 23:14:51 UTC 2010


My friend Dan who works on our balloon launches had this to say about 
the hand warmers......which we use......

Hand warmers use oxygen for the chemical reaction so when the O2 ends, 
the reaction stops producing heat. So... they will work from ground 
level through 40,000 feet (or so), then the air thins out so the hand 
warmers gradually stop functioning. On the way back down, they will 
start to function again at about the same 40,000 foot level, down 
through ground level. You can test this easily enough by starting a hand 
warmer, let it heat for 30 minutes or so, then put it in a clean glass 
jar with a air tight lid. After a few minutes, it will stop functioning. 
It's also a good way to save some money on hard warmers you're only 
going to be using for a couple of hours. Most of them will last 6-12 
hours. When you are done with your "quick" ski lesson or 45 minute 
winter run, just stick them in an airtight glass jar and you can reuse 
them next time. Saves money !
Still... there are some advantages of using the hand warmers as 
temporary heaters for a balloon. They will keep the inside of the 
capsule very warm (sometimes too hot) up thru the above mentioned 40,000 
ft. but keeping everything warm for awhile also means it takes time for 
things to cool off once the hand warmers stop. There's a pretty 
good-sized thermal mass residing in the on-board equipment, cameras and 
the hand warmer itself. My experiments indicate the interior of the 
capsule will stay above freezing for about 30 - 45 minutes after the 
hand warmer fails which means everything keeps on working inside the 
capsule for about 80 minutes of total flight time - 40 minutes (with a 
working hand warmer) to reach 40,000 ft. plus 40 additional minutes (non 
working hand warmer) to reach 80,000 ft. At a lift speed of 1,000 
ft/min, that calculates out to about a height of 80,000 ft. for 
everything to reach outside thermal equilibrium. BEAR #3 flight 
effectively proved this to be true.
Additionally... if you remember the presentation on the last BEAR #3 
flight and Dr. Thorson's comments about height / temperature 
variability, you will remember that it is colder for the early part of 
the flight, then at about 25,000 ft, it actually begins to warm a bit 
(relatively speaking). So from 40,000 ft. (where the hand warmer 
gradually quits), the outside temperature warms from minus 40 to about 
minus 20 to almost 20 above. There is a very definite temperature change 
between the Troposphere and the Stratosphere atmospheric components.
If you go to the BEAR website and look at the BEAR #3 HOBO data logger 
graph (3rd one down)
http://www.bear.437am.com/bear3data.htm
you will see that for this flight, the internal temperature of the 
capsule remained at 28 degrees F or warmer for the entire flight (2 hr. 
37 min). Note the starting temperature was 80 degrees. Bear in mind (pun 
intended), I use one of the BIG JUMBO hard warmers because of it's 
bigger thermal mass and ability to operate for 14 hours. Also, look at 
the external temperature component of the graph, you will see the 
outside temperature gradually dropped to about minus 40 (about 35,000 
ft) and then started gradually warming up, reaching to almost +25 (at 
85,000 ft - almost the top of the flight).
So again, considering the change in temperature when going up in 
altitude and resident thermal warmth even when the hand warmers stop, 
you really only need to keep the capsule above freezing between 25,000 
ft. (hand warmer still working) through 50,000 ft. (plenty of remaining 
thermal mass).
Obviously, there are other ways to keep the interior of the capsule warm 
such as using battery heating elements or passive solar heating but 
considering how easy it is to put a Jumbo hand warmer into the capsule; 
it’s the cheapest and simplest method and with the on-board data logger 
for the BEAR 3 flight, the facts prove it out. You can't dispute the data.
Dan (KL1JP)

-- 
Linda <><  AD4BL

DEC Fairbanks Northstar Borough

He came to pay a debt He didn't owe
Because we owed a debt we couldn't pay
Life is the Childhood of Eternity




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