Author: Jacob Smith -
Hello! I know it’s been a long time since my last blog post about my High-Altitude Balloon (HAB) project – but I promise you this is worth the wait! I left off in June having made the payload and successfully tested the tracker and cameras, with only the parachute and balloon to sort out before I could finally do the flight. Since then I’ve been on quite a journey with this project, with many things going right, and many things going wrong… read on to find out about it all!
Up and up, then falling with style
With the payload complete it was time to work out what size latex balloon I wanted and what size parachute I needed. As a quick recap, the balloon is filled with helium to a diameter of about 1.5m on the ground. The balloon is tied to the top of a parachute, and the bottom of the parachute to the payload. As the balloon rises it expands due to the lower atmospheric pressure, until it bursts at a diameter of about 8m! The parachute will open by itself as the payload falls towards the ground, slowing its descent to a ‘safe’ speed. There are dedicated online shops for buying balloons and parachutes for this purpose, as well as online calculators for determining which one suits your needs. First let’s see how I chose my parachute.
Parachute sizing is based on the mass of the payload and has two competing implications – a larger parachute means a slower landing speed and thus less chance of the payload being damaged, but also means a slower descent rate so the payload will drift further away. To find a balance between these factors it’s best to aim for a descent rate of between 4 and 5 m/s.
I estimated my final payload mass at 1.1 kg, which gave me a choice of two parachutes to be in the 4-5m/s range. A 42” parachute gave a descent rate of 4.47m/s (picture below) and a 48” parachute gave a rate of 4.37m/s. Not hugely different then, but I went for the slightly smaller 42” parachute as I wasn’t too concerned about the impact speed and it was 50g lighter, and cheaper, than the 48” parachute.
The online calculator I used to choose a parachute (Credit to Random Engineering Ltd.)
Choosing a balloon is a bit trickier because it depends on more parameters. Again we start with the mass, but this now includes the parachute as the balloon will have to lift this as well as the payload. Balloon sizes are measured by their mass in grams, with balloons between about 600g and 1200g typically used for my purpose. Using the calculator shown below, I chose an 800g balloon.
The online calculator I used to choose a balloon (Credit to Cambridge University Space Flight)
To find the right size balloon, I entered my payload + parachute mass and a target ascent rate of 5m/s (highly recommended), then chose different balloon masses to find the best balance between the results. A burst altitude of at least 25km is needed to get good pictures, but I wanted to aim for 28-30km. Bigger balloons can reach higher altitudes before bursting, but cost more and mean the flight is longer so could land a long way from the launch site. Also important is the volume of helium required, shown on the right of the image above. You don’t hire a specific volume of helium, but instead a helium cylinder - available in a few different sizes including 2.61m3 and 3.61m3. After doing some calculations it was clear I would need more than 2.61m3, so 3.61m3 seemed my best bet. All I had to do was make sure I chose a balloon size that didn’t require more than 3.61m3 of helium, since the next size cylinder up is a lot bigger and more expensive. In the end an 800g balloon fit the bill and meant I would have some spare helium if the payload mass increased.
Preparing for launch
Once I had the balloon and parachute there wasn’t much left to do before I was ready for launch! First I attached the parachute to the payload using braided nylon cord in such a way that the payload would be level in flight. A few weeks beforehand, I had applied to the Civil Aviation Authority for exemption from the 2005 ‘Air Navigation Order’, to get permission to launch my balloon. You apply for a time and date(s) for a specific launch location, and must do so at least 4 weeks before the proposed date. My permission eventually came through, but with two restrictions: 1. The wind cannot be in a northerly direction, and 2. The fight must take place before 08:00 due to a local glider competition. I wasn’t best pleased with this, but it was what it was – I could either do it then or wait another 4 weeks when the restrictions might be different. I didn’t want to wait any longer.
There’s a really good website that predicts where you balloon is going to land given your input parameters. In the days leading up to the flight I ran predictions to see where the landing would roughly be - you don’t want your payload landing in a densely populated area! Everything was looking good so I ran through my checklist, charged everything, put fresh batteries in the camera and tracker, and did a final test of everything in preparation for the big day.
Tool that predicts the flight path of the payload - here travelling south (Credit to Cambridge University Space Flight)
Alarms went off at 5:30 am and the whole family dragged themselves out of bed to drive to my Uncle Keith’s farm in Welford. We got there about 6:45am and it was a beautiful July day with clear blue skies – perfect! We unpacked the equipment and lay out a ground sheet for the balloon. With everything prepared, I began to set up the tracker. However, this is when things started to not go to plan. Having worked the day before, there was suddenly something wrong with the tracker. The signal I was picking up on my laptop was intermittent and not very strong – oh no! With pressure to launch by 8am we prepared everything else inside the payload and hoped the signal would improve. It did a bit, but it had to be working faultlessly since there’s no point launching if you can’t find the payload and retrieve the pictures! The clock ticked towards 8am and unfortunately we had to admit we wouldn’t launch before the deadline. 07:45 – launch aborted ☹
Of course I was quite annoyed at this and started to worry I would never manage to launch. But I thought about the many later successful rockets that didn’t work first or even second time, and I was determined to resolve whatever the problem was. I would not give up! Continue reading in my next post to find out how I got on…