After we had a balloon malfunction in one of our previous high-altitude missions to gather data above 18km, the KerBalloon Program engineers took a good look at what remained of the balloon when it was recovered out of the water. When a balloon bursts, most of it ends up in pieces that flutter down on their own and are not recovered, but a good portion stays attached to the cable strung out from the casing. Engineers eventually came to the conclusion that the material had become brittle during the flight and was not able to expand fully before bursting. They suspected ice and sent their data and hypothesis off to KerBalloon to get a second opinion from the balloon makers themselves.
We just heard back from KerBalloon today and they concur – icing on the balloon contracted the envelope and caused it to rupture well short of its intended altitude of 24.9km. This is, in retrospect, an event that everyone should have seen coming, but no one really did. We of course know high-altitude temperatures are very cold – by the time you get to 18km it can be a cold as -72°F (-57.8°C). It actually does start to get warmer the higher you go as you pass through 13km but that slow rise in temperature starts from around -95°F (-70.6°C) so it never gets above freezing. Any water vapor that happens to collect on the balloon on the way up will freeze and stay frozen, gradually tightening up on the envelope and preventing it from enlarging further as the gas inside expands due to the decrease of outside pressure.
Another sign of concern noted by KerBalloon is the ascent data did not record a significant drop in the balloon’s vertical speed, which means there wasn’t enough ice to weigh it down enough to affect its rate of climb. This low-tolerance to icing is a large disappointment for the new material that was developed for these high-altitude balloon models.
We have placed an order for additional units, as KerBalloon assures us they will not have a solution to this problem anytime soon, but they will be working on it. For future high-altitude balloon launches we will need clear skies and have to keep a careful eye out for high-altitude clouds that would signal enough water vapor in the upper-atmosphere to possibly have an icing effect on our balloon ascent. The low-altitude variants remain immune to this problem thanks to their much thicker and more robust envelope material, which also doesn’t have to expand as much.
