# Data Downloaded via Parachute from a NASA Super-Pressure Balloon
## 2 Upgraded Hardware Design
### Thermal Management  
During the 2019 engineering flight, each DRs worked for only \(\sim\)15 min at a time before overheating and shutting down until it naturally cooled. Subsequent testing in a room temperature lab showed two main hot spots (see Figure 3). Assuming a thermal emissivity coefficient \(e=0.95\), the main CPU would reach 42.5 \({}^{\circ}\)C while executing a simple Python script (the Raspberry Pi itself was reporting a CPU temperature of 49.4 \({}^{\circ}\)C), and the usb sockets would reach 48.9 \({}^{\circ}\)C when a 5 GB file was written to an SD card in a small SD card reader. Achieving robust operation during the 2023 science flight required mitigations for both. To cool the main CPU, we milled a 53 g aluminium heat sink/radiator that sits between the Raspberry Pi and the PCB. It was held against the CPU by a spring glued to the PCB, and contact was ensured with thermal paste. The aluminium block extended through a 56 mm \(\times\) 14 mm hole in the plastic case, ending flush with that, and was covered with Mylar tape to increase its infrared emissivity. The heat was radiated to space through a hole in the foam shell that was cut by hand (using an indentation in the mould to align it). The ambient temperature in the stratosphere is \(-40\,^{\circ}\)C [8]. For what it is worth, we also stuck a thin, off-the-shelf copper radiator to the ram chip on the back. To reduce heat production at the usb sockets, we used two 6\" usb extension cables to relocate two sd card readers (they ended up sitting roughly behind the microcontroller). We also operated each drs on a 5 min on-20 min off rota: powering up each in turn, copying data, then powering down. During a typical cycle during the night (day), the cpu temperature would increase by 3.0 \(\pm\) 0.3 \({}^{\circ}\)C (3.7 \(\pm\) 1.0 \({}^{\circ}\)C) in 5 min (see Figure 4). On one occasion during night 13, drs1 was powered for two cycles, and its temperature rose 14.7 \({}^{\circ}\)C (from \(-11.6\)\({}^{\circ}\)C to 3.1 \({}^{\circ}\)C) in 30 min. It continued operating throughout this time and cooled half-way back to the same temperature as the others in approximately 2 h. We do not know why drs1 ran at \(-8.2\pm 8.6\)\({}^{\circ}\)C (\(4.7\pm 11.2\)\({}^{\circ}\)C) during the night (day), consistently a few degrees warmer than drs2 at \(-11.4\pm 8.1\)\({}^{\circ}\)C (\(1.0\pm 10.7\)\({}^{\circ}\)C) and drs5 at \(-11.9\pm 7.4\)\({}^{\circ}\)C (\(-1.2\pm 10.6\)\({}^{\circ}\)C). Before launch, four drs capsules were mounted underneath the rest of the payload (see Figure 5). They were oriented with their radiators towards the front of the telescope (which was never allowed to point towards the Sun) and enclosed by a loose skirt of Mylar, the inside of which was aluminised except for a \(\sim\)45\({}^{\circ}\) pleat near the radiator. On two sides, an additional sunshield of foam wrapped in aluminised Mylar also protected the drss from incidental knocks. drs1 was at the back left, drs2 at the back right, drs3 at the front right, and drs5 at the front left (drs4 was our flight spare). Power and Ethernet cables were routed from each drs up to SUPERBIT. Figure 3: Thermal imaging of the top and bottom of a Raspberry Pi, during testing in a room temperature laboratory. Thermal emission is interpreted as temperature (red, white, green numbers), assuming emissivity \(e=0.95\). (**a**) The main cpu is the hottest component while executing a simple Python script. Nothing is attached to the usb sockets in this image. (**b**) The underside of a Pi attached to a drs, oriented as in Figure 1. The heat sink and sd card containing the operating system are now cold at the top, but the usb sockets become hot during file transfer to sd cards in small readers mounted in the usb sockets. Figure 4: Self-reported cpu temperature of the Raspberry Pi on each drs during the superBIT mission for drs1 (red), drs2 (blue), and drs5 (green).