HAB Design Considerations

[PhilipM]

Hi,

I am new to HAB and am just thinking through what I'd like to design. Please forgive the noob questions!

I was thinking about something very light, and consequentially very low powered. Therefore good insulation will be an important design consideration. Expanded polystyrene is very popular, but I was also thinking about filling the voids with expanding low density setting foam filler (sorry don't know the correct name). But then it occurred to me that something rigid with air inside might expand significantly as the pressure falls with altitude. Then again, doesn't expanded polystyrene contain air? From what I have read of the manufacturing process, it does. I'm now wondering why this doesn't cause any problems.

Has anyone considered using solar power (either primary or backup) so that if not found immediately, a payload could keep transmitting its position? Is this worthwhile exploring? Most projects I read about seem to locate the payload. I wonder if that's the norm, or if the lost payloads are simply not written up frequently!

As I understand it, the UK regulations mean that only 10mW 434Mhz transmitters are available (without special licence). I know they can work very well while in the air with good line of sight. What are they like once on the ground? I.e. if the exact location of the payload wasn't known, how likely would it be that you would be able to find it this was the only thing transmitting location data? Are there any options offering higher data rates, e.g. for live image broadcast?

For tracking, there seems to be three options:

- 10mW 434Mhz - very low weight and power, requires interfacing and programming, but a useful source of live data, especially while airborne

- mobile phone with GPS - cheap, but only really good for recovery as it's soon out of range when airborne

- SPOT type GPS tracker - uses sat phone technology, so available for the whole flight - low number of updates/expensive

Anything I have missed?

Thanks,
Philip

[Ed Moore]

On Tue, Jul 24, 2012 at 8:23 AM, PhilipM <phi...@philipm.com> wrote:
> Hi,
>
> I am new to HAB and am just thinking through what I'd like to design. Please
> forgive the noob questions!
>
> I was thinking about something very light, and consequentially very low
> powered. Therefore good insulation will be an important design
> consideration. Expanded polystyrene is very popular, but I was also thinking
> about filling the voids with expanding low density setting foam filler
> (sorry don't know the correct name). But then it occurred to me that
> something rigid with air inside might expand significantly as the pressure
> falls with altitude. Then again, doesn't expanded polystyrene contain air?
> From what I have read of the manufacturing process, it does. I'm now
> wondering why this doesn't cause any problems.

Usually the pressure within the polystyrene can equalise with the
atmosphere. However if you cover it in impermeable tape, like some
flavours of gaffa, then you will get a rather shrink-wrapped and
compacted payload case as it quickly falls back down.

As for expanding foam, I think it's one of those things where human
interface considerations outweigh technical ones, and so entombing
your flight computer forever in expanding foam will be very annoying
when you find you want to change a resistor. Also, radiation is an
important hermal transfer mechanism in hab which neither polystyrene
or expanding foam (other than by being white) address. You might
consider some aluminised mylar (thermal blanket) inside your payload
bay. Payload getting too cold is rarely a problem though. If you're
worried it's worth doing the actual sums and solving the problem iff
it actually looks like a problem.

>
> Has anyone considered using solar power (either primary or backup) so that
> if not found immediately, a payload could keep transmitting its position? Is
> this worthwhile exploring? Most projects I read about seem to locate the
> payload. I wonder if that's the norm, or if the lost payloads are simply not
> written up frequently!

Recovery is definitely the norm. If not physical recovery, identifying
which tree it is up anyway. It's also quite easy to fly enough
batteries to keep a low powered tracker going for several days. I'd
save solar for when you've done the sums that show you that you needed
(eg an ultra light weight multi-day floating payload).

>
> As I understand it, the UK regulations mean that only 10mW 434Mhz
> transmitters are available (without special licence). I know they can work
> very well while in the air with good line of sight. What are they like once
> on the ground? I.e. if the exact location of the payload wasn't known, how
> likely would it be that you would be able to find it this was the only thing
> transmitting location data? Are there any options offering higher data
> rates, e.g. for live image broadcast?

*It depends*

If you get a nice landing and the antenna is stuck facing up in the
air on a relatively high bit of ground, we've heard landed payloads
from >5km away. If it's in a ditch, you've got to be right on top of
it. If it's in a tree on a hill, you might get 30km. Throw a dar
somewhere into the space of outcomes for your answer.

Live TV - well again (pattern emerging) you'd have to do the sums. If
you have little radio experience then the answer is almost certianly
'No.' The Ofcom document to refer to is IR2030 [1]. If you know your
radio and you have a very impressive receiver high gain receiver (like
a big dish) and know your stuff, then there's a bit of room to
experiment with the notion of sending pictures down. But there's no
happy like wiki page that will give you some arduino code to plug an
hd camera in an NTX2HD Hero transmitter to get you live 3d images from
space.

>
> For tracking, there seems to be three options:
> - 10mW 434Mhz - very low weight and power, requires interfacing and
> programming, but a useful source of live data, especially while airborne
> - mobile phone with GPS - cheap, but only really good for recovery as it's
> soon out of range when airborne
> - SPOT type GPS tracker - uses sat phone technology, so available for the
> whole flight - low number of updates/expensive
> Anything I have missed?

Not that's common. I don't think any of the experienced ukhas members
would tell you anything other than to go with the radio option as
being head and shoulders above the other two. But by all means fly the
other two out of interest or as a backup. But having live telem gives
you access to the entire ukhas infrastructure, including live landing
prediction which will really help you maximise your chances of a
successful recovery.

>
> Thanks,
> Philip
>
>
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[1] http://stakeholders.ofcom.org.uk/binaries/spectrum/spectrum-policy-area/spectrum-management/research-guidelines-tech-info/interface-requirements/IR_2030.pdf

[David Akerman]

I wouldn't use that setting foam filler stuff to fill the voids - it sticks to everything and it'll make a solid mess that as Ed says will then make it very very difficult to get to your tracker to change anything.  Also, having air inside the payload isn't a problem insulation-wise, so it isn't necessary to fill every void.  You do though want to make sure that the contents can't flap around and get damaged - initial descent can be very violent and of course you want the tracker still running after it hits the ground.

You mentioned "lightweight" so I'm assuming this is a tracker-only payload you're thinking of, without cameras and camcorder.  In that case you don#t need to worry about the cold as much as you probably are.  I've had one payload which was essentially in 2 halves and the gap wasn't sealed, resulting in an internal temperature of -53.  It stayed working.  That said I and others have trackers that stopped working at the coldest part (10km or so up) then started up again at higher altitudes (where it's a bit warmer) so don't go as minimalist as I did on that one!  For small payloads I use foam shapes - eggs or balls - available on ebay or at craft shops.  The smaller ones are solid and I cut them in half or slices with a foam cutter then cut out holes for the tracker board and batteries.  You can shake one of those about after and nothing will go wrong.  Actually that's a good test to do on yours!  The larger balls/eggs are 2-part hollow ones, and those need more care to make sure the contents are immovable.  Or you can glue or tape together 3 sheets of white foam sheet, with the tracker in the middle sheet.  Or .... lots of options really.  IMO 25mm of foam around your electronics and batteries is plenty.

I wouldn't bother with solar cells for a regular flight (for a deliberate "floater" that stays up for a couple of days or more, I would).  On my trackers I get about 15 hours out of 3 or 4 AAAs, and some power-saving on the GPS (as some have done) would increase that to about 20 hours.  That's plenty of time to get to where the payload has landed and locate it, if not recover it.  I did once land one in Belgium and was glad to have fitted AA's (40+ hours life) but that's unusual.

As Ed said, radio range from the tracker depends on where it lands.  Shortest I've had was when the aerial got bent flat under the payload, on flat ground, and that was decodable at half a mile or so.  The furthest was one that landed in a tree on a hill and that was picked up 15 miles away.  I had one on the ground, ner the top of a hill, and the signal was clear the chase car the other side of the valley.  For the most part, you in the chase car or the closest of the fixed trackers will get the payload down to 500m altitude or better, and if you drive to that location or the expected landing position you'll get a signal when you get there.  If that's not decodable then I get the yagi out and if still no decodes use that to direction-find till it is decodable.

The radio system is reliable enough that unless the payload has some expensive stuff inside I feel no need to add a backup tracker.  If I do, then I use a GSM module connected to the flight computer.  Or you can get GSM/GPS devices for £50.  SPOT is rather more expensive.

Dave

[PhilipM]

Wow, that was a quick and detailed reply, many thanks.

Usually the pressure within the polystyrene can equalise with the
atmosphere. However if you cover it in impermeable tape, like some
flavours of gaffa, then you will get a rather shrink-wrapped and
compacted payload case as it quickly falls back down.

Sounds almost cartoon like. Good to know!

 

As for expanding foam, I think it's one of those things where human
interface considerations outweigh technical ones, and so entombing
your flight computer forever in expanding foam will be very annoying
when you find you want to change a resistor. Also, radiation is an
important hermal transfer mechanism in hab which neither polystyrene
or expanding foam (other than by being white) address. You might
consider some aluminised mylar (thermal blanket) inside your payload
bay. Payload getting too cold is rarely a problem though. If you're
worried it's worth doing the actual sums and solving the problem iff
it actually looks like a problem.

Thanks, again good to know. I can't seem to find any projects that experienced temperature issues (other than a bit of radio frequency drift). Anything to watch out for?

 

Recovery is definitely the norm. If not physical recovery, identifying
which tree it is up anyway. It's also quite easy to fly enough
batteries to keep a low powered tracker going for several days. I'd
save solar for when you've done the sums that show you that you needed
(eg an ultra light weight multi-day floating payload).

Would an automatic line cutter (nichrome wire cut near the payload) help? I.e. do payloads tend to become entangled due to the lines/parachute? 

 

Not that's common. I don't think any of the experienced ukhas members
would tell you anything other than to go with the radio option as
being head and shoulders above the other two. But by all means fly the
other two out of interest or as a backup. But having live telem gives
you access to the entire ukhas infrastructure, including live landing
prediction which will really help you maximise your chances of a
successful recovery.

Thanks for the detail on 434MHz, recovery and live pictures. It now seems obvious that 434Mhz with a focus on successful recovery is the way to go. =)

[PhilipM]

Thanks for the reply Dave. On reflection and reading up a little more about lift and a desired climb rate of 5m/s, I think my payload may not be quite the lightweight I had in mind.

Here’s a quick calc for the sort of package I am now planning. I’ve approximated some of the weights, but I don’t think they’ll be too far off (tell me if I am dreaming!).

Flight Computer, Lithium AAs, antenna, wires, sensors, GPS, RFM22B – 60g target weight

Canon A1200 – 140g, plus another 30g for Lithium AAs – potential to remove non important items like the flash, LCD, some of the casing etc.

Foam/polystyrene/line/parachute – 100g? (Complete guess)

So I was hoping to put together something with around 350g of payload. So I guess I’m going to need an 800g balloon to get to 100k ft. Slightly more than I had hoped, but there you go.

I also read your post on the Even Tinier Tracker with great interest (http://www.daveakerman.com/?p=310). Very impressive use of 7.5g!