I’m Liam and I am a student of Miss Shulman’s Grade 12 Physics class. I am fortunate to have been given the opportunity to work on the Apollo ‘19 High-altitude Balloon and I will be sharing the scientific findings of our launch.
Aboard the payload of our balloon were several scientific instruments: two barometers, a compass, an accelerometer, two solar panels and two power assessment devices, all of which executed every 30 seconds to produce data.
There were two significant scientific objectives of this launch:
1) To test the effectiveness of a weather vane as a method of directional stability about the vertical axis, and;
2) To test the effectiveness of solar panels and to determine where they should be placed on the payload for optimal performance.
First, however, is the common data.
Common Scientific Data
Using the barometer, we were able to obtain common data about temperature, pressure, and height. We used two Adafruit BME280 I2C or SPI Temperature Humidity Pressure Sensor to deduce the temperature, pressure and height. Of course, there was no significant discoveries in regards to the temperature and pressure over time.
Of course, nothing overly fascinating here. However, there was a bit of a dilemma when calculating height. While we were able to successfully calculate the launch-site height, anything above was calculated incorrectly, reaching a maximum calculated height of about 2200 metres above sea level. Here is the equation utilised:
Where h is the height in metres above sea level, To is the initial reading temperature reading at the launch site, which was 23.6°C (though it should be converted to Kelvin), L is the atmospheric lapse rate, -0.0065 Kelivin/metre, Pa is the pressure reading taken every thirty seconds, Po is the initial pressure reading at the launch site, which was 98949 pascals, R is the specific gas constant, 287.053 Joules/Kilogram Kelvin, g is the acceleration due to gravity, 9.8065 m/s^2, and 197 is the elevation of the launch site in metres ASL.
We had thought that converting the temperature to Kelvin would do the trick. It did not, and now we are currently unsure of the issues behind this. To process our data in the meantime, we did find a barometric formula that we are currently using, but it reads, at most, 4000 metres apart the real-world data. For reference, the actual apogee of our balloon was about 33000 metres ASL, whereas the barometric formula we are currently using (until we sort out the issues with the other formula above), reads about 37000 metres ASL. As such, if anyone can spot the errors we cannot and help us calculate the height ASL appropriately, then that would be much appreciated.
Also, upon retrieval of the payload, it was discovered that the internal temperature was considerably high. It is assumed that the solar panels may have caused this (see ‘Solar Panel Effectiveness and Placement Test and Analysis’ for more details).
Weather Vane Testing and Analysis
Placed externally on the payload, the weather vane was designed to act as a stabilizer of the balloon’s rotation. It was made out of corrugated plastic (just like political campaign signs!) and was promptly fastened to a corner of the payload, held by a a couple electrical wires and a wire clothes hanger.
The long, flat and white ‘tail’ is the weather vane.
In theory, as horizontal wind is thrusted upon the payload, an imbalance is created on the weather vane as ample amounts of air hit one side of the device and leave the other side deprived. By simple aerodynamics, the weather vane is moved by the abundance of wind on one side and moves rotationally until there is an equal distribution of wind on each side of it, creating directional stability.
By using a compass, the BMM150 ( https://www.digikey.ca/product-detail/en/seeed-technology-co.,-ltd/101020492/1597-1618-ND/8558382?utm_adgroup=&mkwid=s7BTtmhKu&pcrid=310965931893&pkw=&pmt=&pdv=c&productid=8558382&slid=&gclid=EAIaIQobChMI6M6Zocq-4gIVlVqGCh093A31EAYYASABEgIRFvD_BwE ), to read the directional heading of the payload, clockwise from North and in degrees, we were able to determine the effectiveness of this device by seeing where the majority of the headings lied. To do this, the heading data was converted to trigonometric coordinates (sinx, cosx) and was plotted, with North, South, East and West being read like one normally would:
By this graph, it would appear that the weather vane did not work. However, a the compass also served as measuring the heading of the side-mounted solar panel. So, by plotting the voltage readings of the side panel with the heading:
We can see that most of the headings read are within the 250-350-degree range for our side-mounted solar panel (since the compass was oriented with it, not the weather vane), which is between North and West. So, the weather vane clearly did work in providing some stability.
So to say that the weather vane worked would be inaccurate, and for now the results remain inconclusive. Regardless, there was some trouble on descent. While there was no expectation for the payload to stabilize during freefall, we were certainly not expecting to lose the device.
Two lose orange wires can be seen as the weather vane (the flat white plastic object in the photo) loses its grip during descent.
We still have yet to recover the weather vane and probably won’t. However, it is clear the a much more secure fastening method may be needed in the future.
Solar Panel Effectiveness and Placement Test and Analysis
As the second major scientific objective of the flight, we decided that, in light of the demand for electrical energy to power flight hardware, testing the effectiveness of solar panels and where to place one during flight would be another important idea for future mission considerations.
To conduct this test, two identical solar panels were placed on the payload; one on the lid and one on the side. Each was given the same set up, of which each consisted of a connection to an INA 219 Power Assessment Device which was further connected to a RaspberryPi for readings. The side panel, however, did consist of the compass, so that it could be seen as to which heading it generated the most voltage, as well as to assist in the analysis of the weather vane’s effectiveness.
Note that for half the ascent, it was particularly cloudy, with the Sun coming out in the later half. Also, given that we launched at about 10:30 AM, the Sun was positioned in the South-East area.
(For your consideration, the INA 219 is a small device designed to measure voltage, power and current. For more information, see the tutorial we used: https://www.rototron.info/raspberry-pi-ina219-tutorial/).
To conclude that a panel was successful, it must have had to generate a minimum of five volts for the majority of the ascent time. And, as seen by the following graphs, each panel performed exceptionally!
Effectiveness Test: Who Won?
Top Panel Side Panel
As seen, on ascent, each panel was capable of generating well over five volts, though the side panel did out outperform the top panel by generating, on average, 0.2107 volts more. This may have been due to the time of launch as well as our position in the Northern hemisphere, where the Sun may have delivered more solar radiation from an angle closer to the horizon than directly above. The balloon itself was also probably a factor in shading the top panel somewhat.
Current and power were also evaluated, though they are not an overly critical factor to consider when deciding the effectiveness of the solar panels, since the hardware is mostly dependant on the voltage input it receives.
In terms of the orientation of the side panel, it was found that most readings were grouped within the 250-350-degree range, which translates to a North-West heading, but the voltage generation never dropped below six volts regardless of the orientation.
Temperature and its Effect on the Efficiency of the Solar Panels
What is of particular interest in regard to voltage, though, is its output as the temperature changes:
Top Panel Side Panel
Peculiar enough, there is a strong linear correlation between these two variables. Not only that, but by comparing the ‘Voltage v. Time’ graphs with the ‘Temperature v. Time’ graph seen at the start of this post, one can identify that as the temperature dips at about 64 minutes, the voltage output reaches its maximum! Further research through the internet shows that the efficiency of solar panels tends to decrease as temperature rises and vice-versa. By regression analysis, it was also found that at a temperature of about 63.6°C is when the effectiveness of each panel diminishes beyond usability. So if we are to fly higher, we will need to take this into consideration.
After this discovery, two large graphs of all three electrical readings—voltage, current and power—were plotted with temperature to deduce a couple things:
As you can see, the power and voltage are directly affected by the change in temperature, and the current remains about the same throughout. Using the basic equation:
Where P is power, V is voltage and I is current, we can state, with fair confidence, that the internal heating of the payload is due to a rising voltage (caused by rising temperatures on ascent and in the final forms of descent) being multiplied by a constant current, of which all equals out to a higher power dissipation, P, which is essentially the heat. Wow!
Knowing this, we will begin seeking methods of temperature control for the payload so as to ensure no damage is done to the hardware in flight.
Interference of Tracking Signals
Another issue faced was the interference of one of our radios, specifically the satellite tracker. For significant periods of time during the flight, our satellite tracker would lose communication with one of the onboard radios. It is possible that the solar panels could have blocked out signals at those periods of time, of which the signal was regained after another satelittle was able to find an opening with no interference. This issue must be taken to consideration for future flights.
In general, for the purpose of daytime student-high-altitude-balloon launches, each panel is highly effective in their ability to generate an appropriate amount of voltage to power future electronic payloads and the placement of a panel, be it on the top or the bottom, is of little concern.
To conclude, the two main scientific objectives were achieved, along with the obtainment of common scientific data. We now know that a weather vane is viable option for directional stability if the payload and that solar panels can be used effectively, regardless of their placement or orientation, to produce a suitable amount of electricity to charge flight hardware.
A Final Word
I would like to thank Miss Shulman for her dedication to the students in the classroom. Very rarely are students awarded the opportunity to apply their knowledge in a practical scenario and to gain new skills that go beyond what a textbook can teach. Without her, this mission, and future ones, would not be possible.
It is the hope that others who choose to pursue the challenge of high-altitude ballooning will see this post and utilise the information presented as a means of learning and to carry on the efforts of improving the Global Space Balloon Challenge community. Because it is through error, as well as shared experiences, that allow us to better ourselves each and every day—even if it means blowing up a couple LEDs on the way!