Balloon Meets Gum Tree

Today I attended the launch of Horus 38, a high altitude ballon flight carrying 4 payloads, one of which was the latest version of the SSDV system Mark and I have been working on.

Since the last launch, Mark and I have put a lot of work into carefully integrating a rate 0.8 LDPC code developed by Bill, VK5DSP. The coded 115 kbit/s system is now working error free on the bench down to -112dBm, and can transfer a new hi-res image in just a few seconds. With a tx power of 50mW, we estimate a line of site range of 100km. We are now out-performing commercial FSK telemetry chip sets using our open source system.

However disaster struck soon after launch at Mt Barker High School oval. High winds blew the payloads into a tree and three of them were chopped off, leaving the balloon and a lone payload to continue into the stratosphere. One of the payloads that hit the tree was our SSDV, tumbling into a neighboring back yard. Oh well, we’ll have another try in December.

Now I’ve been playing a lot of Kerbal Space Program lately. It’s got me thinking about vectors, for example in Kerbal I learned how to land two space craft at exactly the same point on the Mun (Moon) using vectors and some high school equations of motion. I’ve also taken up sailing – more vectors involved in how sails propel a ship.

The high altitude balloon consists of a latex, helium filled weather balloon a few meters in diameters. Strung out beneath that on 50m of fishing line are a series of “payloads”, our electronic gizmos in little foam boxes. The physical distance helps avoid interference between the radios in each box.

While the balloon was held near the ground, it was keeled over at an angle:

It’s tethered, and not moving, but is acted on by the force of the lift from the helium and drag from the wind. These forces pivot the balloon around an arc with a radius of the tether. If these forces were equal the balloon would be at 45 degrees. Today it was lower, perhaps 30 degrees.

When the balloon is released, it is accelerated by the wind until it reaches a horizontal velocity that matches the wind speed. The payloads will also reach wind speed and eventually hang vertically under the balloon due to the force of gravity. Likewise the lift accelerates the balloon upwards. This is balanced by drag to reach a vertical velocity (the ascent rate). The horizontal and vertical velocity components will vary over time, but lets assume they are roughly constant over the duration of our launch.

Now today the wind speed was 40 km/hr, just over 10 m/s. Mark suggested a typical balloon ascent rate of 5 m/s. The high school oval was 100m wide, so the balloon would take 100/10 = 10s to traverse the oval from one side to the gum tree. In 10 seconds the balloon would rise 5×10 = 50m, approximately the length of the payload string. Our gum tree, however, rises to a height of 30m, and reached out to snag the lower 3 payloads…..