Well its Tuesday 21/06/16 and the flight is due to take place tomorrow, so in order to finish things off I have been am spending the day making the final preparations. The last few jobs are:
- Make the filler tube
- Re-flash the SD card and install the PITS software
- Check the Pi camera and new v2 Pi Zero
- Build and test the payload antenna
- Build up the payload
- Clear the SD card
- Prep the parachute and payload lines to save time on the day
- Weigh the payload / parachute and string
- Calculate the neck fill and make a neck fill counter weight
- Pack up up the launch, mission control and tracking gear
Also in order to be sure I decided use six AA batteries instead of four. The PITS has an onboard 5v regulator so the extra batteries will ensure a longer transmission time. I had to order some extra Lithium Energizer batteries and some 6 x AA battery holders.
I picked up the Helium on Friday and it came with two fillers but to fill a large weather ballon a DIY filler assembly is required. This is the guide from the UKHAS Wiki which shows how to make the filler assembly.
The antennas that we have been using so far for testing are no practical for the flight so I had to make up a 1/4 wave ground pane antenna and then test it. This is the guide from the UKHAS Wiki which shows how to make the payload antenna. It turned out ok and when testing it worked brilliantly.
I made a hole in the bottom of the payload container for the antenna lead to fit through and once the antenna was in place this was sealed with Seam Sealer to prevent water ingress.
In order to ensure that the balloon follows the predicted path, reaches the desired altitude and ascends at the correct speed, the amount of helium is critical. To work out the desired amount of helium you need to calculate the neck life. This is done using a Neck Lift or Balloon Burst Calculator. You need to know the balloon type and how much the balloon needs to lift this will be the payload, parachute and string. When filling the fill assembly and tubing will affect the neck lift as such this needs to be taken into consideration. To work out the Neck Lift I used the balloon burst calculator on HabHub.
Building The Payload
Next I had to work out how to best place the Raspberry Pi Zero / PITS board and the batteries etc into the payload and secure them to ensure that nothing moved around during the flight.
Parachute and Rigging Lines
To save time we measured out the two correct amount of nylon cord for the rigging lines and tied one set to the top of the parachute and the other to the bottom. These were then rolled up and secured to prevent them tangling.
Weighing and Helium Calculation
Now that everything was ready, one of the last technical jobs was to weigh everything that the balloon would lift using a set of kitchen scales. This came to 425g. Once this figure was established using an online ‘Burst Calculator’ I was able to calculate the Neck Lift, this is a figure that tells you how much weight to hang off off the balloon when filling it to identify whether or not you have the correct amount of helium inside of it. One adjustment that needs to be made is this figure is that the weight of the filler assembly and part of the filler tube needs to be subtracted from the neck lift. The final figure then give you the counter weight to hang off of the filler assembly.
The last job was to pack up two plastic boxes, one containing all the gear for mission control and the other containing everything for the launch pad.
Everything was in place – just waiting for the CAA authorisation and good weather.