A High-Altitude Balloon Platform for Space Life Sciences Education

The HAB is composed of a 1.5 kg latex balloon (High Altitude Science), a 1.5 m diameter circular parachute (High Altitude Science) carrying two custom-built payloads, and a radar reflector (constructed with aluminum reflective tape and cardboard). These items were attached in series to a paracord Flightline. The diagram is shown in Figure 1.

Figure 1

The payload carried by the HABs consisted of two biological sample trays (BSTs), which can be used for the collection of microbial material in laminar airflow or exposure of organisms adhered to the coupons. The main payload also carried environmental sensors measuring ambient temperature, atmospheric pressure, and payload velocity; camera (GoPro Hero 5); and an embedded flight computer. A computer-aided design (CAD) of the assembled payload is shown in Figure 1. The flight computer system was controlled by an Arduino Uno based on the ATmega328P chip wired as depicted in Figure 2. An Altimeter Module MS5607 (Parallax) was used to assess ambient pressure, temperature, and altitude. The data logging system used a microSD card and a real-time clock for synchronization. These system communication protocols include serial peripheral interface (SPI) and inter-integrated circuit (I2C), which are dependent on the sensor. More information about the software used on the flight can be found in our online code repository: https://github.com/SLSTP-2018/shab.

Figure 2

To track the HAB in-flight, an MT-AIO (Byonics) module using a GPS5HAOEM GPS transmitted telemetry data at a frequency of 144.390 MHz with audio frequency-shift keying (AFSK) at 1,200 baud was used. AFSK packets were received by ground-based Automatic Packet Reporting System (APRS) digipeaters and relayed to a publicly available website (aprs.fi) for real-time position tracking. We also directly received and demodulated AFSK packets using an RTL-SDR USB dongle connected to a personal computer running open-source software-defined radio (SDR), GQRX (gqrx.dk). GQRX SDR includes an AFSK1200 decoder, which prints demodulated APRS telemetry packets. APRS was transmitted through an externally mounted half-wave V6 dipole antenna on path WIDE2-1. APRS transmissions were tagged with an appropriate FCC-issued amateur call sign. For all electronic components, consumer-grade lithium-ion batteries were used, as they are less likely to be affected by cold temperatures and are lighter than alkaline batteries.

Sample collection at altitude was accomplished with BSTs that were attached to two linear servos (Actuonix L12-R; 100 mm, 210:1, 6 V). The servos were used to move the BSTs from the shielded interior of the payload box (launch configuration) to the outside of the payload box (exposed sampling configuration) at target altitudes of 10,668 and 27,432 m above sea level. The altitude of the HAB was determined by an onboard altimeter, labeled “B” in Figure 2. When the HAB’s altimeter measured the first target altitude, one linear servo actuated a BST to the exterior of the payload, exposing the tray and allowing for atmospheric microbial sampling. A real-time clock tracked exposure time and retracted the BSTs after the programmed exposure time (900 s) had passed. When the second target altitude was reached, the process was repeated with the second linear servo and BST.

Extension and retraction of each BST were achieved using an L293D H-bridge, labeled “F” in Figure 2. The H-bridges reverse the polarity of the voltage delivered to the linear servos, serving as a switch to extend and retract the linear servos. For visual logging of the extension and retraction of the BSTs, a GoPro HERO5 was attached to the exterior of the payload and programmed to capture photos every 30 s for the duration of the flight.

As a preflight check of the payload hardware operations, four LEDs, labeled “E” in Figure 2, were added to the circuit to indicate errors in the hardware. The four colors corresponded to different errors in the wiring of specific components in the circuit.

Passive exposure of biological samples to high altitudes required no specialized hardware, except for an OptiCell Cell Culture Chamber (Cat#: 392-0603) that contained media and the organisms, which was taped to the side of the payload just before launch. Postflight analysis of the organisms required recovery of the samples.

HAB flight planning was conducted using the online ASTRA High Altitude Balloon flight prediction software (http://astraplanner.soton.ac.uk/). The software accepts parameters of balloon mass, gas, current weather conditions, and parachute type to run up to 400 discrete flight simulations. ASTRA generates a map of possible flight paths and predicted landing zones, allowing balloon teams to determine optimal launch locations. Desired neck-lift (i.e., the vertical force due to the lift of helium) was calculated using the CUSF Flight predictor tool (http://predict.habhub.org/), which also predicts landing zones based on flight parameters and weather information. Flight trajectories, both actual and predicted, for both HAB flights are displayed in Figure 3.

Figure 3

A consumer-grade microcontroller (Arduino Uno) was used as an embedded flight computer. Each hardware component was tested for functionality, and a color-coordinated LED was lit if the hardware was dysfunctional. The linear actuators were tested before sterilization by extending and retracting each BST to ensure a successful deployment. The BSTs are shown in Figure 4. At 1-s intervals during flight, the barometric pressure sensor and thermometer collected temperature and pressure data, which were used to calculate altitude (Figure 5). In particular, we corrected the pressure reading for the temperature and compared it to an altitude table of known pressures per altitude to calculate the current altitude. This calculation is made possible by publicly available software (https://github.com/Schm1tz1/arduino-ms5xxx). Time of day, altitude, pressure, temperature, and BST position were logged onto the microSD card. The data logging and test software are described in further detail on the SHAB GitHub repository (https://github.com/SLSTP-2018/shab) and are freely available for download.

Figure 4

Figure 5

The payloads were housed in Styrofoam boxes and were internally reinforced with lightweight foam core. Each flight included two BSTs, each with nine acrylic coupons, which were laser cut with a dimension of 5.08 cm × 10.16 cm × 0.375 cm. The anterior faces of the coupons were covered with filter paper and screwed in place on the BSTs. The BSTs consisted of an acrylic platform (18.58 cm × 33.02 cm × 0.3175 cm) with pairs of holes for each coupon to be screwed into place. A protective shield was attached orthogonally to the exterior face of the BST to form a seal with the payload exterior when the BST was in the retracted position. The shield was created out of laser-cut acrylic (33.02 cm × 10.16 cm × 0.3175 cm). The linear actuator was fastened to the payload by rails, which were 3D-printed from the polylactic acid filament (LulzBot Taz 6). In our design, we positioned one linear actuator above the other in the payload box. To balance the payload’s center of mass, BSTs extended in opposite directions. All of the 3D models for our custom-made parts can be downloaded from Grabcad (https://grabcad.com/library/biological-sample-trays-bst-models-1).

Before launch, the BSTs, coupons, and filter paper were sterilized by 15 min of exposure to UV-C radiation in a biological safety cabinet. Immediately after sterilization, the coupons were covered with sterile aluminum foil. The BSTs were mounted to the linear actuators, and coupons were uncovered moments before launch. Thus, the coupons remained shielded within the payload until it reached its target altitude, at which point the flight software triggered linear actuator extension and coupon exposure.

Flight hardware for one balloon was acquired and constructed for $2,199.31, not including incidental retrieval expenses (i.e., food, gas, and lodging) with the payload weighing 1,645.7 g (full cost and mass budget in Appendix 1).

A 3.048 m × 3.048 m woven poly tarp was placed on the flat ground underneath the area where the balloon was filled with helium. Payloads were attached to the Flightline with 0.3175 cm paracord, and the balloon was inflated with a 3 cm weather balloon inflator (High Altitude Science). The pressure of the helium tank decreased from 204 to 5.4 atm during inflation, as measured by the tank’s valve gauge. As the balloon rose during inflation, three individuals steadied the balloon to ensure that it did not contact the ground. These individuals wore nitrile gloves to prevent oils and particles on their palms from degrading the balloon’s integrity. The pre-launch neck-lift was measured by a digital fish scale (Dr. Meter). When the desired neck-lift, as calculated by the CUSF Burst Calculator, was achieved, the balloon was sealed with duct tape and plastic zip ties, attached to the flight line, and released. About 48 h before launch, we notified the local Federal Aviation Authority of our launch plans.