CANCELED Launch today - 5/12/10 14:00UTC from nr Worthing, Sussex. 434.075Mhz USB, 50 baud, 425shift, ASCII-8, no parity, 1.5 stop bits.
Test the concept developed by the Ivory series of flight in the states of using very low weight payload and under filled foil balloons to create a super-pressure balloon. The Ivory series used mylar(foil) party balloons with a payload based upon a programmed cell phone. Instead we'll use the Atlas flight computer but at its bare minimum; atmega168, voltage regulator, ntx2 radio and the spare sirfII GPS that I have left over, power will be a LiPo battery and the payload will be just wrapped in jiffy envelope.
Depending on how light I can make the payload we'll attempt to use a few mylar balloons, these will be underfilled to allow them to ascend. As the balloon rises the helium expands to take up the balloons volume. However unlike latex balloons the foil won't stretch, maintaining its volume. This results in an increase in pressure but not density - the balloon will stop ascending and float. The float altitude will depend upon the volume of helium used but will be approximately 5km, the lighter the payload the higher we can fly the balloon.
The limiting factor in a super-pressure balloon is the strength of the balloon material itself, as the pressure increases the balloon will slowly leak helium and will eventually lose buoyancy and descend. The other terminating factor is that the balloon bursts and the payload descends rapidly.
The plan will be to aim for a launch from Sussex on 5th December depending on the jetstream forecast - ideally would like it to head west and avoid major cities.
Currently doing general research for the project - have ordered a cheapy high precision digital scale from ebay as weight is going to be the key. Using the normal kitchen scales it seems that the energizer lithiums are a little to heavy for the payload and perhaps just a single 3.7v lipo would be more appropriate. That said to use this lower voltage powersupply I'd need to get a different LDO regulator to cope with the lower input voltage. Will have to wait for these scales before proceding so may be a short delay in any updates.
Had some further ideas to regarding the payload - instead of having a separate GPS module to use a GM862-GPS to provide both SMS and GPS (SIRF III will work nicely as its below 24km) and then have a Seeedfilm Arduino (1g) and an NTX2 Radio module. All this would run off a 2000mAh battery So:
The question is how much does it weigh! Will be able to make a close approximation before ordering as have many of the components already in some form. This setup would allow both radio comms and also GSM after landing to actually allow for recovery…
Due to a free day coming up the plan is now to fly a test flight on 5/12/10 from Sussex with the help of Ed Moore. With limited time the payload is relatively simple and while not weight optimised is cost optimised coming in at just under 100g in total.
Canceled - Purchased 45cm mylar balloons from Clintons and got them to fill them up - while they had some lift (enough to float up) when I measured the lift I found that it was 5g each! This is far less then what I'd expected which meant that there wasn't enough to lift the balloon and so I canceled the launch. I suspect that they don't provide pure helium instead a mix of He and air - enough that the balloon floats. Not all is lost though - the balloons are reusable and so I'll try to get my hands on a tank of Helium and try again.