Flight Failure - Part 1

[John Laidler]

I posted this on the picoballoons group this morning but thought I would share it on here.  Because of limitations with this site the message will have to be posted on more than one part.

I launched a picoballoon yesterday which sadly ended in failure soon after launch.  Before I come to describe what happened I thought it might be useful to give a quick review of the payload and the preparation of the balloon.

I decided, based on my experience of previous builds to go for a more more robust design and to fully support the 6 solar cells across the full length. Unsupported they are extremely prone to being broken. I also connected them using 1.6mm wide tabbing wire across the full width, previously I had only gone half way across. I hoped this would improve electrical performance.

The main platform was 20mm thick expanded polystyrene which I bevelled at the under edges to reduce a little weight. The U4B I decided to mount vertically on a small bracket off-centre with the antenna connections aligned with the middle of the payload.  Next time I will mount it flat underneath and not use a bracket.  The U4B was also wrapped in Kaptan tape. To keep it level 4 lengths of 6lb Dyneema were added in the middle of each side and joined to the main line about 20cm above. On the underside I added 2 more to help support the lower line. UHU POR glue was used throughout.

The antenna legs were 5.2m long and made from 0.1mm wire. I tied a single knot in the dyneema about every 40cm and on the first attempt put a drop of superglue on the knot straight away.  This was not successful as afterwards there were sections where the dyneema was slack and the wire under tension.  This had happened because the wire had moved before I had applied the glue.  For the second attempt I glued the first knot only then after tying all the rest I strung the antenna leg horizontally and while applying light tension to the Dyneema checked the wire was straight and without any kinks or under tension then worked along dropping a spot of glue on each knot.

The best way I found to tie the knots was to do it on the bench with the antenna wire wrapped around a cardboard tube and the Dyneema still on the reel it came on.  A second cardboard tube was on the other side and basically it was a matter of unrolling the wire a section at a time, tying the knot then rolling the now connected Dyneema and wire onto the second tube. The tubes themselves have to be turned to avoid introducing any twists in the wire which will lead to kinks.

It is just possible in one of the photos to see how I attached the antenna and line to the U4B.  A small length of 0.33mm copper wire was passed through each antenna connection point on the board and soldered in place. A loop, soldered shut,  was made in one end to which the Dyneema was tied (and glued) and the other end of the 0.33mm wire was folded back on itself to make a sort of hook.  The antenna wire was formed into another hook and the two fitted together and soldered. Because of the four upper and two lower stabilising "legs" I added in Dyneema the antenna connection points were not under tension but should they come under tension I was reasonably confident the thin antenna wire would be safe.

Total weight of the payload was 8g. 

The balloon used was a silver SAG balloon which I pre-stretched at 0.3psi in 70% RH and at a temperature of about 26C. I made sure the humidity was raised before inflating so the balloon would be filled with humid air as it seemed logical that the humidity should reach the interior surface.   After about 8 hours the wrinkles at the joints were down to about 10mm length at the longest.  Before stretching I reinforced the plastic attachment point found on these silver balloons with the 3M Tyvek tape recommended by Dave VE3KCL.  It was a good job I did as the attachment point detached itself during the inflation process, which you can see in one of the photographs. This might be because I was suspending the balloon from this so it was being subject to a load of the balloon plus the tubes used for inflation.  This probably added up to about 60g or 70g which isn't a lot but in future I will inflate this type of balloon with them resting on a sheet on the floor but the attachment point will still be reinforced with 3M tape. I also added a length of Dyneema from this main connecting point to the filling point, forming a loop at the end of this line which I passed other the filling neck then after filling and sealing the balloon the end of the neck was folded back over the loop and 3M tape used to hold everything in place. A photo of this is enclosed.

My plan was to fill the balloon with He the day before which I did, sealing the neck with 5 minute setting resin glue applied with a cotton bud. An hour or so later I double-checked everything and discovered I had made a massive error somewhere and the free lift was about 20g not the 7g I had planned for.  I'm still not sure how this happened.  It is possible that when I filled the balloon the He was still cold after the rapid expansion from the cylinder and by setting the free lift at that point the figure increased later when the gas warmed to ambient and expanded. Whatever the explanation there was no alternative to scrapping the balloon and starting again. This made for a late night and an early morning.  This time I only partially filled the balloon with gas and left it to warm up for 30 minutes before topping it up.

As an aside to remove the air from the balloon I use a vacuum cleaner with the pipe from the balloon inserted a little way down the vacuum cleaner hose leaving a large gap around it.  If you use a rigid tube you can clamp it to the vacuum cleaner hose which saves you having to hold it. It took about an hour to evacuate the balloon and it was essential to ensure the rigid tube going into the balloon was in far enough to be beyond the valve. After extracting the air I added a small amount to He, just enough so part of the balloon was trying to lift then after giving the balloon a gentle pummel to mix any remaining air with the He I extracted it.  the idea behind this "flushing" was to reduce the chance of moisture being left behind from the humid air used for inflation. It was interesting to see how much quicker the vacuum cleaner extracted the He compared to air.  It is a gas of very low viscosity.

As a further aside I found you need either a very sharp knife or very sharp scissors to cut Dyneema fishing line cleanly. The excellent 3M Tyvek tape can be cut with scissors but you need to clean the blades with alcohol after almost every cut as they get covered with adhesive and this makes a second cut difficult as the tape sticks to the blades.

I had originally planned for 7g free left but for this second balloon I reduced this to 5.5g.

The launch went smoothly but later than I had hoped.  There was a thin covering of cloud so I wasn't expecting the balloon to be spotted until it got above them but amazingly it was transmitting within 10 minutes of launch and being picked up on the other side of the Atlantic. At this point it was probably not more than 500 feet high.

I tracked it for about 30 minutes as it went East but for some reason there was no altitude data.  On Pedro's LU7AA site only the first packet was being received. It had worked in testing so my guess is that the sun through the clouds was too weak to transmit the second packet correctly. Not knowing the altitude became irrelevant after a while when for about 45 minutes the balloon kept sending the same grid.  I am reasonably convinced the balloon had a pinhole in it and gas was slowly escaping.  How this happened I don't know, either a fault or possibly the balloon brushed the grass during the launch.  This is the most likely explanation as the wind had picked up by this time.  Had I been able to launch at my planned time, which was missed by having to prepare a second balloon, conditions would have been perfect with as the air was totally still at 0700 local time.

Checking the wind speeds at different altitudes I think the balloon did not get above 500 feet and then slowly started to descend.  If it had gone higher it would have been swept south.  I can only guess it was caught in a tree or on wires as it was continuing to transmit and be picked up in Canada. Sadly it wasn't moving horizontally.

I have thought of going to look for it, it is an hour's drive away, but even with a 6 figure grid square it is a large area. It has also stopped transmitting so has either fallen to the ground or was spotted by someone. 

Very disappointing of course but I will try again with my second and last U4B but it won't be until nearer the end of the year as I will be off travelling soon until the middle of October.

Some images are enclosed, in no particular order, which might help with the description above and of course I would be glad to answer any questions.

[Kevin Walton]

Sorry to hear about the failure John, they happen to all of us, hopefully your looking forward to trying again!

Cheers
Kev

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Sent from my Android device. Please excuse my brevity.