Saturday 21 September 2019

The TT7B1 Relaunch

Date: 2019-09-21
Time: 07:00 GMT (09:00 CEST)
Place: Roznov pod Radhostem, CZ
Tracking: http://aprs.fi

This is a relaunch of the TT7B1 tracker that ended up in treetops about a week ago under a new envelope.

Transmissions
Type APRS
Callsign OK7DMT-1
Frequency 144.39-145.57 MHz
Period 2 min
Transmit Power 12.5 mW

The packets are a combination of Base91 encoded position and custom telemetry data in the comment section of the packet. An online decoder and example packets can be found at this link. The tracker transmits on local APRS frequencies. In case of GPS failure, telemetry without position is transmitted. Packets also contain backlogged data from the past 7 days.

Flight Parameters
Lifting Gas hydrogen
Envelope Volume ~0.430 m3
Envelope Mass 116.0 g
Payload Mass 18.6 g
Free Lift 7.2 g
Neck Lift 25.8 g
Initial Ascent Rate 0.90 m/s
Expected Float Altitude 11930 m

The balloon on this flight was a Mylar balloon shaped envelope 1.35m in diameter (flat) from a 40μm multilayer (PE/PA/EVOH/PA/PE) film. The making of the envelope is in detail described in this blog post. A calculator was used to obtain the flight parameters based on previously measured envelope volume, mass and payload mass. Due to issues with the pressurizing rig and short time, the balloon volume was not measured. Since the envelope was made with the same process, a figure from the previous balloon was used. Latest atmospheric sounding data (Prostejov, 20190921 0:00Z) were used to calculate the expected float altitude.
Initial ascent trajectory was modeled with CUSF Landing Predictor, while the long term float predictions with HYSPLIT Trajectory Model. The HYSPLIT prediction contains three altitude levels. One 500m below and another 500m above the expected float altitude.
The design of the tracker is in detail described in this and this blog posts. This time, the final touches consisted of securing an Energizer Ultimate Lithium AA primary battery (estimated lifetime ~60 days) to the tracker. The battery was covered in black electrical tape, and leads were soldered directly to the battery's terminals. Based on in-flight low temperature behavior of TT7B2, another layer of Glob Top epoxy was deposited on the tracker. This time, covering the whole top side including the GPS module. An initial 5 minute satellite search period was removed from the code, and the maximum duration of GPS fix reacquisition was lowered to 37 seconds. Also, minor remnants of the previous balloon remained glued to the external sensors cable as they were impossible to remove.
A pressure sensor and a thermistor were sealed inside the envelope, and the gas inlet was glued shut - this time with additional layers of film. Two thin magnet wire antenna radials were fastened to the balloon's circumferential weld. The lead connecting the battery to the tracker was soldered just before the launch.
Day 1: The balloon ascended at an average rate of 0.9m/s (maximum 1.9m/s) to an initial altitude of 10800m, about a kilometer below expectation. The initial ascent was very slow at an average of 0.66m/s to 1000m. The previous mishap, though, didn't repeat.
The balloon flew over the countries of Slovakia, Hungary and Romania. It was received by a large number of stations as these areas are well covered. The satellite images didn't show any thunderstorms nor any other high altitude clouds in the area.
Having seen the TT7B2's data, it became apparent shortly after the balloon began floating that it would follow the same path and eventually descend due to hydrogen leaking from the envelope. The balloon still maintained the same altitude for hours before it started its inevitable descent at 15:00 GMT. Currently, the best guess is that the glue doesn't do a good enough job sealing the inlet given the conditions. Although, other issues arising from the manufacturing process of the envelope can't be ruled out at this point.

All the received APRS packets can be found in this text file: TT7B1_packets.txt
And all the decoded live and backlogged data can be found in this csv file: TT7B1_data.csv

Sunday 15 September 2019

The TT7B2 Flight

Date: 2019-09-15
Time: 07:00 GMT (09:00 CEST)
Place: Roznov pod Radhostem, CZ
Tracking: http://aprs.fi

Transmissions
Type APRS
Callsign OK7DMT-2
Frequency 144.39-145.57 MHz
Period 2 min
Transmit Power 12.5 mW

The packets are a combination of Base91 encoded position and custom telemetry data in the comment section of the packet. An online decoder and example packets can be found at this link. The tracker transmits on local APRS frequencies. In case of GPS failure, telemetry without position is transmitted. Packets also contain backlogged data from the past 7 days.

Flight Parameters
Lifting Gas hydrogen
Envelope Volume 1.045 m3
Envelope Mass 224.0 g
Payload Mass 18.5 g
Free Lift 10.0 g
Neck Lift 28.5 g
Initial Ascent Rate 0.88 m/s
Expected Float Altitude 13810 m

The balloon on this flight was an obround shaped envelope 2.35m in length (flat) from a 40μm multilayer (PE/PA/EVOH/PA/PE) film. The making of the envelope is in detail described in this blog post. A calculator was used to obtain the flight parameters based on previously measured envelope volume, mass and payload mass. Latest atmospheric sounding data (Prostejov, 20190915 0:00Z) were used to calculate the expected float altitude.
Initial ascent trajectory was modeled with CUSF Landing Predictor, while the long term float predictions with HYSPLIT Trajectory Model. The HYSPLIT prediction contains three altitude levels. One 500m below and another 500m above the expected float altitude.
The design of the tracker is in detail described in this and this blog posts. The final touches consisted of securing an Energizer Ultimate Lithium AA primary battery this time (estimated lifetime ~60 days) to the tracker. The battery was covered in black electrical tape, and leads were soldered directly to the battery's terminals. For more images of an identical tracker construction, except for a smaller battery (AAA), see TT7B1.
A pressure sensor and a thermistor were sealed inside the envelope, and the gas inlet was glued shut. Two thin magnet wire antenna radials were fastened to the balloon's circumferential weld. The lead connecting the battery to the tracker was soldered just before the launch. After the bad experience with TT7B1, this launch took place a couple hours later in the day, and the balloon and mainly hydrogen were left in the sun for a while to warm up.
Day 1: The balloon ascended at an average rate of 1.0m/s (maximum 2.3m/s) to an initial altitude of 13500m. It flew over the countries of Slovakia, Hungary, Croatia and Slovenia, and was received by a large number of stations along its way. The satellite images didn't show any thunderstorms nor any other high altitude clouds in the area.

After the sunset, the temperature measured by the onboard thermistor fell down to -63°C. The GPS module was the first to experience difficulties. The average active time needed for a valid fix increased. At times, the module failed completely and most likely communication with the module timed out. The MS5607 barometric pressure sensors eventually failed as well and only returned to outputting reasonable readings after the balloon descended to warmer altitudes (warmer than -35°C). The tracker never stopped functioning completely. The MCU kept working even during the lowest ambient temperatures.

After a gradual decrease in altitude to 13000m during the evening and night, at 0:50 GMT the balloon began descending at an average rate of -1.3m/s (maximum -1.9m/s) and landed in Croatia.
The top two charts contain data from the whole flight. The bottom two charts show data only until a point the pressure sensors began outputting nonsensical data due to extremely low temperature. In the last chart, the measured temperatures and pressures were used to calculate the resultant force acting on the balloon ($F_{lift}-F_{weight}$) if it contained the same amount of hydrogen as it was filled with. The curve shows positive values during the balloon's climb to altitude, then values around zero when it started floating. However, later the values begin to rise again, while the balloon's altitude stays the same or even decreases suggesting the envelope was leaking hydrogen which eventually resulted in the balloon's descent to ground.
Update: TT7B2 was recovered by Kruno 9A3SWO in northern Croatia near the town of Krapina lying on a field as can be seen in the images he provided. Kruno was kind enough to send me the tracker back, so I will probably re-launch it under a new envelope.

All the received APRS packets can be found in this text file: TT7B2_packets.txt
And all the decoded live and backlogged data can be found in this csv file: TT7B2_data.csv

Friday 13 September 2019

The TT7B1 Flight

Date: 2019-09-13
Time: 05:00 GMT (07:00 CEST)
Place: Roznov pod Radhostem, CZ
Tracking: http://aprs.fi

Transmissions
Type APRS
Callsign OK7DMT-1
Frequency 144.39-145.57 MHz
Period 2 min
Transmit Power 12.5 mW

The packets are a combination of Base91 encoded position and custom telemetry data in the comment section of the packet. An online decoder and example packets can be found at this link. The tracker transmits on local APRS frequencies. In case of GPS failure, telemetry without position is transmitted. Packets also contain backlogged data from the past 7 days.

Flight Parameters
Lifting Gas hydrogen
Envelope Volume 0.434 m3
Envelope Mass 116.0 g
Payload Mass 11.0 g
Free Lift 4.2 g
Neck Lift 15.2 g
Initial Ascent Rate 0.71 m/s
Expected Float Altitude 12410 m

The balloon on this flight was a Mylar balloon shaped envelope 1.35m in diameter (flat) from a 40μm multilayer (PE/PA/EVOH/PA/PE) film. The making of the envelope is in detail described in this blog post. A calculator was used to obtain the flight parameters based on previously measured envelope volume, mass and payload mass. Latest atmospheric sounding data (Prostejov, 20190913 0:00Z) were used to calculate the expected float altitude.
Initial ascent trajectory was modeled with CUSF Landing Predictor, while the long term float predictions with HYSPLIT Trajectory Model. The HYSPLIT prediction contains three altitude levels. One 500m below and another 500m above the expected float altitude.
The design of the tracker is in detail described in this and this blog posts. The final touches consisted of securing an Energizer Ultimate Lithium AAA primary battery (estimated lifetime ~22 days) to the tracker. The battery was covered in black electrical tape, and leads were soldered directly to the battery's terminals.
A 10L 200atm cylinder was rented from a local supplier. It contained 1.8m3 of usable gas - hydrogen. The cylinder was 100cm high, 45cm in circumference and weighted 18kg. The cost of the gas was 1921Kč (75€) plus a daily rent.
A pressure sensor and a thermistor were sealed inside the envelope, and the gas inlet was glued shut. Two thin magnet wire antenna radials were fastened to the balloon's circumferential weld. The lead connecting the battery to the tracker was soldered just before the launch.
Day 1: After an initial ascent, the balloon continued horizontally until it eventually descended into treetops about 350m from the launch site. Not sure about the source of the problem. The actual free lift after filling the balloon and removing excess inlet was slightly above the plan at 5.1g. The same calculation method and parameters were successfully used on the TT7F flights. The only difference that comes to mind was in that the TT7F trackers were solar powered, thus the balloon and gas were exposed to direct sunlight for a period of time before launch. Contrary to that, TT7B1 was launched earlier in the morning without direct irradiation.
As the balloon didn't reach high enough altitude, the only receiver was my own iGate. The signal eventually disappeared as rising wind combined with my recovery attempts pushed the balloon deeper into the trees and broke the antenna.
Update: TT7B1 was recovered during the afternoon. The envelope was destroyed and the tracker roughed up during the attempt. It kept working without any issues though, so it will be restored and prepared for another flight.