[...]

I don't think what you describe would work. The ballon, once inflated in the ground, maintains a similar ammount of lift for the entire flight, because the maths end up roughly cancelling out between the outside, inside pressures and air density. If you look at the graphs of altitiude over time for these things, they're very close to a straight line. Thus - a ballon that was under inflated at the ground ( and thus not providing any real lift ) would be larger but still providing no lift @ 100kf.DougI don't think what you describe would work. The ballon, once inflated in the ground, maintains a similar ammount of lift for the entire flight, because the maths end up roughly cancelling out between the outside, inside pressures and air density. If you look at the graphs of altitiude over time for these things, they're very close to a straight line. Thus - a ballon that was under inflated at the ground ( and thus not providing any real lift ) would be larger but still providing no lift @ 100kf.Doug

[...]

Yup - for one of those utterly enormous balloons that looks like what happens when a kitten gets hold of a roll of cling film

KISS / BISEOGE etc etc. Get to 80-100kf, get down. That's the principle here.

Doug

Apparently I wasnt paying attention when this thread evolved......

Anyway, I'd be happy to help - I may have some relevant skills and experience (see, e.g.
my instrumented frisbee http://www.lpl.arizona.edu/~rlorenz/frisbee_mst.pdf)

I have also been hacking cameras to be PICAXE-controlled (for study of dust devils)

If I can make a couple of plugs for nonimaging data, I'd love to see at least 3-axis acceleration
data (ideally gyros too), maybe sun/horizon sensing etc. too. I can make design suggestions,
or build/provide (depending on timing).

Some very nice SD-card or USB-stick datalogging boards that can be stamp- or picaxe driven
these days. Could have a very nice instrument package for 100g all-up.

Ditto - the entire turbulance-thru-cloud issue that we've talked about would be an interesting way to go. With SD / CF then we could have a high enough data rate to make it usefull, whilst having the space for a long flight (longer than a frisbee anyway )

Perhaps LDR's might work as inverted sun sensors? One on top/4xsides - then could technically interpolate a sun position..ish?

Doug

Actually LDRs would not be ideal since they are rather nonlinear (not sure on their temperature
dependence too, which given nonlinearity would be a tricky data reduction problem) ; if the package
were to be spinning during parachute descent, the response time of LDRs could become a factor too.

Much better to use a planar photodiode (only $1 or so) in current mode - photocurrent is linear with
flux so you get a very near to cosine angle response (deviates slightly due to reflection/matching effects
with the plastic cover). Since it is a current device, you can use a microcontroller digital channel to time
a capacitor charge-up, without even needing an Analog-to-Digital converter.

With my frisbee work on planar photodiodes I don't think I could do much better than 5 degrees (and spin
is helpful, gives you a lot of datapoints). To do better you'd want a slit-mask type or something that
images the sun position.

I know Ralph was trying to plug non-imaging data but I would have thought that the simplest way to track the sun would be to have a small camera pointing at a sundial.

James

That's quite a heavy and data-expensive way of doing it though - as well as requiring a lot processing after the flight...and assumes a clear day for the bottom, say, 1/3rd of the flight. Sensors + maths would, I'd have thought, done a better job

I have a Picaxe data logger working - with Temp, LDR and Humidity (a 'maybe' on the humidity at the moment)I have a Picaxe data logger working - with Temp, LDR and Humidity (a 'maybe' on the humidity at the moment)

Just raw figures, not values yet - but I held the temp sensor, breathed on the humidity, and 'showed' my laptop screen to the LDR a few times - and stuff changed

Doug

An overnight run.

Blue line is the LDR - you can see a couple of overnight triggers of our security light, followed by the increase of the sunlight kicking in at dawn

Green is temp - chilled within 10 minutes when I put it in the conservatory, then it rose slightly when the sun rose.

The orange is humidity..I'm not sure what, if anything, is going on there. 9/10ths of 4/5ths of nothing. To be honest, I may have cooked it last night when wiring it up - there's no indication which way round it should be wired up. I'll do some testing using the kettle - see if I can switch up to 100% humidity etc etc.

Doug

Just an update - things have been slow, real life has been getting in the way. For <£10 I've just picked up a Vaisala RS80 radiosonde - the typical weather balloon payload launched hundreds of times a day around the world. Thought it'd be an interesting research artifact to have a look at, maybe even steal it's sensors

http://www.vaisala.com/weather/products/so...adiosondes/rs80



Doug

Just an update - things have been real busy (AGU - managed to work 'All your base are
belong to us' into my Sagan lecture!)

I did just get a few of those $20 1.3MP keychain cameras from Walmart. Took one apart
(even this tiny package is mostly plastic case) and will let you know when I get it under
PICAXE control.....

Any chance this was recorded for web broadcast?
I know we are getting spoiled.....

It was - I signed a release form, at least. But I don't know when AGU will
put it on-line...

Woo hoo! Between AGU proceedings and the US DVD release of the SkyOne Hogfather movie, I am going to be set for winter break this year.

Dunno about the 3MP version, but I have the 1.3MP $20 version now being triggered at
why whim by a PICAXE 08M. Just needed a bit of hotwiring and a pair of 2N3904
transistors....
My guess is that these things go out of stock and get replaced by newer technology faster
than we get round to designing/building balloon payloads. The Flycam-1 units I played with
just 3 months ago have already been upgraded..

I have been wanting to build my own balloon, and I have been designing, and coming up with some idea's on what type of science payload I should fly. Upon my research, I found these interesting products that I might use.

0.4 MP OEM camera's, these are not 3 MP but HEY they seem like great cameras anyways .
http://www.oemcameras.com/

Maxstream OEM XTend long range radio telemetry modules.
http://www.digi.com/products/wireless/long-range-multipoint/

A mini GPS module from Futerlec.
http://www.futurlec.com/GPS.shtml

Also looking into building my own balloon. So very interested in your suggestions.

However i think there is a potential problem with the GPS module as the specification says it is limited to 18000m or 60000 feet. As i understand it, this is one of several limitations enforced by the US govt (to prevent non-friendly use). I did read somewhere (sorry can't remember where) that there were several GPS solutions that didn't have this limitation. Perhaps units manufactured outside the states are better?

Also found an ebook (well several pdf documents) describing the construction of high altitude balloons that you may not have seen:

http://www.parallax.com/Resources/Customer...67/Default.aspx

I haven't managed to read it all yet, but it does seem a comprehensive resource.

Graham

I had not yet read till now the technical specifications of the GPS module, thanks for pointing that out.

Your right Graham, a VERY good source of usefull information that I am reading. I was particularly interested, and needed more information on the FAA guide lines and regs, these PDF's has covered that.

Thanks!! for the link.

Ohhh, and Good luck on your Near Space Exploration Efforts .

I've also been looking into this with regard to the UMSFB project and I believe that the general limit is >18000m AND >515m/s simultaneously, i.e one can be exceeded but not both. Hence there should be no problem with balloon applications.

I don't know if some GPS units differ from this or if it's just sloppiness in writing the spec sheets.

Can anyone else confirm if this is true, or have regulations got me confused again?

James

I think you may be right about the regs, however i am guessing that for simplicity of implementation it seems as if some manufacturers may just choose to implement the altitude part of the regs and ignore the others. I guess for most applications 60,000' limit is not a problem.

I have now found the list of GPS's that apparently do work above 60,000':

http://showcase.netins.net/web/wallio/GPSrcvrsvs60kft.htm

I would be interested to know if anyone knows anymore about this.


Graham

ps what is the UMSFB project? I tried google but to no avail.

This thread, basically.

I've been totally sidetracked from it - I'd love to see it happen, but I no longer have any time to spend on it, I've got a very interesting project that's come up and I'm focussing on that a lot. If anyone in Europe wants to take this on - I'd be happy to bundle up all the kit assembled so far and send it over to you.

Doug

oops yes i really should have realised.


i am in the uk, and may be interested. However, i wouldn't want to take over as it seems as if others have done lots of work so far. Anyone interested in working together on this?

Graham

I'm in the UK, manchester to be exact, and I'd be happy to give up some time at the weekend to it. My knowledge of electronics is minimal, but I'd be happy contribute any way I can.

I'm certainly still very interested in seeing this through, I want to fill in that white space in my simulations. I also don't have that much to spare at the moment and that is likely to get worse in the next few months, however I'm hoping that if I can find the time for some research and ideas now then come the summer when I have a bit more time I can get on with some proper testing.

James

Just so no one thinks I've totally forgotten about this - Helen and I went out to Cambridgeshire today to see James et.al. launch one of their payloads. I was able to track it with good binos for about an hour. The comms died after about 50 mins, probably at 15-18km altitude, somewhere just north of Cambridge.

Some pics and a movie here : http://www.rlproject.com/launch_feb_2008

Can someone explain me (on easy way) why 'burst' balloon happend on 30-40 km altitude?

The balloon grows as the atmospheric pressure outside decreases. This maintains it's buoyancy - but eventually it just expands so much, the rubber/latex can't cope and it breaks. For balloons of this type, that typically happens at about 100,000 ft.

Thx for explanation.

Is there any other balloons wich can rich more then 100,000 ft??
And what about gravity on 100,000 altitude? is less then on the Earth surfice ??

The difference in gravity is negligible. The radius of the Earth is 6378km, and gravity inversely scales with the square of that radius. At 30km altitude, you're at 6408. The difference in gravity would be approx 1%. There are balloons that can go higher, but they tend to be much larger, and much much MUCH more expensive. If you're lucky and have a small payload, a typical met-balloon might reach 120,000ft, but not significantly more than that.

Doug

Local gravity at 33km altitude is less than it is at the earths surface but only by about 1%. You can find out all you need to know about how local gravity changes with altitude (and latitude, since the earth is not a perfect sphere) at the Wikipedia page on Earth's gravity.

I'm no expert, but it looks like there is a misconception
that the fact that astronauts in orbit are weightless means
that Earth's gravity at that elevation is zero. In fact, the
weightlessness is the result of the orbital path that allows
the astronauts to fall toward the Earth at the same rate
that their craft descends around the curve of the Earth.

A misconception held by whom? I doubt anyone here would think that.

WIKIPEDIA



I don't understund this quite well.
On 400 kilometers is 90% of Earth greavity.??? That is a little bit strange because i read some text on Internet "everybody who rich 100 km is astronaut".

There is Karaman Line (http://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line) at 100 km

Ok thx for explanation but can you tell me why we have a limit at 120k ft? only because of lower pressure?
Do you have some example what ballon they use for that altititude? ('much larger, and much much MUCH more expensive.")

thanks again

The key to understanding this is free-fall, Vladimir. An object that was stationary 400 km above the Earth would experience 90% of the surface gravity. However, an object in orbit is in effect falling around the Earth (i.e., there's a substantial vector of motion transverse to gravity), so there's very little acceleration towards the Earth...certainly far too little to be noticed by a human observer.

Free fall is the key way of describing it. A satellite in orbit is always falling towards the Earth, at the standard rate at which any object falls toward the Earth from that height. It's just going fast enough in a vector, as Nick pointed out, transverse to "down" that the Earth's curved surface falls away from the satellite as fast as the satellite falls toward it.

You can experience free-fall without being in orbit -- most well-known is the KC-135 "vomit comet" trainers that NASA flies, which basically have the airplanes fall freely along a diving arc for about 40 seconds at a time. But you can observe the same effect in an elevator that falls down a shaft without touching the walls. Both you and the elevator would be falling at the same rate, so you would experience "weightlessness" while both of you fell. (You'd experience very short-term but extremely high G-forces when you got to the bottom of the shaft, though...)

-the other Doug

Interesting now i understund much better.
Can you tell me why then if we have some balloon at (for example) on 30-40km altitude (when burst) payload must fall on Earth? And then something above 100km never fall?

Objects that are orbiting the Earth at an elevation of
many hundreds of killometers will fall if their
forward orbital motion is stopped or slowed.

...and if something is launched upwards to 100 km, but does not have orbital velocity, it will come back down.


(if I put in my 2 cents to a comment from centsworth_II, does that equal four cents?)

There's a transatlantic balloon attempt underway right now.

The plan is to get a small balloon across the Atlantic. It launched a few hours ago from Knoxville.

Don't think it has any cameras but seems to be well instrumented.

The telemetry is available at: http://tracking.spiritofknoxville.com/dash...ashboardus.html

(I had better luck at getting all the displays to run with IE than with Firefox but YMMV.)

My concern, looking at the telemetry, is that they have dumped most of the ballast and are still not high enough. If that's the case, come nightfall they won't be able to maintain altitude (I believe the lift is coming from a combination of Helium and solar). OTOH, I may have wrongly interpreted things -I've only just found the site!

Rob

Look on the bright side for that spirit of knoxville attempt: it seems they have a good chance of recovering the flight hardware. :^) (altimeter showing 4094 feet and falling, location over virginia heading farther inland)

(OMG, it's coming right for me! )

It is cloudy, cold and dreary all over the Piedmont: 48 F, and solar energy less than 150 W m-2. I don't think there's much chance of any solar help.

But a really cool attempt. I hope they try it again!!

[967 ft as of 15:00 EDT, not a good trend]

-Mike

I think the 967 number is meters, it says 3172 ft on the US version. That value hasn't updated in a while, though the climb rate is about zero now. Really like the layout of that website. It makes it very exciting so see this live

They found it !!

http://www.pegasushabproject.org.uk/wiki/d...firefly:fhalp-2

And...looking at the imagery ( which is very cool ) - lesson learnt... if you want those big ass panoramas....you've got to take multiple images at the same time. There's so much chaotic motion, it doesn't work on its own.

Doug

I don't there needs to be. Aren't the answers to this:

A longer bridle.
A fishing swivel.
A vane on the instrument housing to stabilise the unit in any (apparent) wind?

Andy

Hmm.

I think the lesson I'm taking away from the imagery is that the motion of the platform is too great for the camera to deal with. The payload must have twisting on the line connecting it to the balloon quite severely which ruins many of the images.

I'm sure the balloon itself is a much more stable platform. I think it would be a good idea to try and couple the payload to the balloon better to stop this rotation, using multiple lines or a stiff connecting rod rather than cord (I think there are laws restricting the strength of the connection so this may be a no-no)

Cross posted with Andy - That's a good idea as well - anything to reduce the rotation of the camera housing.

To be fair - they really did set up that camera to be a troublesome child - it wasnt decoupled very well - if you look at the pics I took of the stack, it was going to be a nightmare from the get go.

They might be launching again - sat or sun, from Cambridge.