HIGH-ALTITUDE BALLOONING 101
STEP 5 – LAUNCHING THE BALLOON
BEFORE LAUNCH DAY
We will continue to monitor the weather forecast and balloon flight prediction leading up to launch day. If either of these do not look favorable for a launch, we may be forced to postpone the launch until a later date. This decision will typically be made 24–48 hours before the launch.
The day before the launch, we like to meet with the teacher in person at the launch site, usually after school lets out. During this meeting, we ensure that the student payloads are ready for flight, check out the launch site, discuss launch and recovery logistics, and go over the launch procedures. It is better to do all of this the day before the launch instead of the morning of.
DAY OF LAUNCH
Setting Up Launch Site
Launch day can be quite hectic. To give us enough time to set up, we usually arrive at the launch site at least 90 minutes before the scheduled launch time and start by making sure the student payloads are ready to fly or completing any last-minute tweaks to their boxes. We then lay out tarps on the ground where the balloon will be inflated, move the helium cylinders into position, and attach the helium regulator valve to one of the cylinders. The payload items are then laid out in the order that they will hang below the balloon during flight.
The parachute is attached to the neck of the balloon with two looped strings. However, the neck must remain open while the balloon is being inflated. A good way to do this is to use a small piece of PVC pipe that has a similar or slightly smaller diameter than the neck of the balloon. We insert this PVC pipe into the neck and secure it there with several zip ties wrapped tightly around the outside of the neck and PVC pipe. As these zip ties are being attached, we run them through both parachute strings to attach the strings to the neck as well.
Finally, to prevent the balloon from escaping during the inflation process, we make sure to have a safety line tethering the neck of the balloon to something heavy on the ground, such as one of the helium tanks. We also attach another shorter safety line that includes an in-line digital scale to measure balloon lift as helium is being added.
Calculating Lift
How do we know how much helium to put in the balloon? During inflation, the in-line digital scale attached to the shorter safety line tells us how much the balloon is pulling upward at any given time. This measurement is called the neck lift.
How do we calculate what the neck lift should be? We know of two ways. The simplest way is to multiply the total weight of the payload train by 1.5. For example, a 10-lb payload train would yield a rough neck lift calculation of 15 lbs. When filling the balloon, once the in-line digital scale reads 15 lbs, we stop there and launch the balloon.
However, the more accurate method is to directly calculate the required neck lift using force balance and drag coefficient equations. We won’t delve into the details here since the math is actually quite advanced (see this document for the details). However, we’ve created an Excel spreadsheet that does this all for us. We input several parameters, such as the weather conditions at the launch site, the type of lifting gas and balloon we’re using, the total mass of the payload train, and our target ascent rate. The spreadsheet then calculates the neck lift. This is the method that we use on launch day.
Inflating the Balloon
About 45 minutes before the launch, we activate the SatCom which then begins transmitting data to the Mission Control page once every 15 minutes. We also turn on our backup SPOT GPS device at this time. About 30 minutes before launch time, we begin filling the balloon. To add helium to the balloon, we run the hose from the helium regulator through the PVC pipe in the neck of the balloon and open the regulator valve.
We typically like to inflate the balloon with enough helium to give it an ascent rate of about 5 m/s. However, inflating the balloon with more or less helium than “needed” can sometimes give us a little more control over how far the balloon will travel or where the payload train will land. All else being equal, an overinflated balloon will typically burst at a lower altitude but have a faster ascent rate and thus a shorter flight path. On the other hand, an underinflated balloon will typically burst at a higher altitude but have a slower ascent rate and thus a longer flight path.
The balloon inflation process usually takes about 20 minutes to complete under normal conditions. If it is breezy, we may ask several teachers or students to put on gloves and help us stabilize the balloon to prevent it from flopping around too much. Once the balloon is inflated with the desired amount of helium, we tie off the bottom of the neck of the balloon with zip ties to prevent the helium from escaping.
The Launch
After inflation, we finish assembling the payload train, ensuring that all payload items are securely attached to each other and to the balloon. We complete a final check of our GPS tracking devices to make sure they are still transmitting their location. We also turn on our onboard video cameras, mount them within their camera box, and attach the box to the payload train.
Next, we remove the safety lines from the balloon and walk the balloon and payload train to the official launch spot. We often ask the teacher to recruit some of their K-12 students to help us carry the payload train. Once we are ready to launch, we do a countdown and release the balloon. This is always an exciting moment. Often, if the weather is good and it isn’t too windy, we invite everyone to gather underneath the balloon during the launch and wave goodbye to the cameras.
WHAT EDUCATORS NEED TO KNOW
- The teacher/educator must make sure that the student payloads are finished and ready to go the day before the launch (we can help with this at the pre-launch meeting the day before).
- The morning of the launch, the teacher/educator is responsible for making sure the students behave and do not interfere with any of the launch operations.
- Students are welcome to come out to watch us set up and inflate the balloon. However, if other teachers are also planning to bring their students out just to watch the launch, we suggest that they show up at least 10 minutes before the scheduled launch time.
- If it is windy, we may recruit a few teachers or students to help us keep the balloon stable during inflation.
- Finally, we may ask the teacher/educator to choose a few students to help us launch the payload items.