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.