HERMES: A Data and Specimens Transporter from the Stratosphere to the Ground—The First Experimental Flight

Abstract

Large stratospheric balloons are the easiest access to near space. Large long duration balloons (LDBs) can float in the stratosphere for weeks collecting measurements (e.g., astrophysical or geophysical data) or samples (e.g., contaminants, volcanic ash, micrometeorites). The recovery of data media and samples is a common problem in this type of experiment because direct radio communication becomes useless when the balloon crosses the horizon, and satellite links are too slow and expensive. For this reason, physical recovery of the payload is mandatory to obtain experimental results, which is a difficult task, especially in polar regions. The goal of HERMES (HEmera Returning MESsenger) is to allow researchers to obtain experimental data prior to payload recovery. HERMES is a system equipped with an autonomous glider capable of physically transporting data and samples from the stratosphere to a recovery point on the ground. The glider is installed on the balloon payload via a remotely controlled release system and is connected to the main computer to store a copy of the scientific data and to receive the geographic coordinates of the recovery point. This allows scientists to obtain experimental results before recovering the payload. The article describes HERMES and the first experimental flight of the entire system, which was conducted at Esrange Space Center (Kiruna, Sweden) in July 2022.

1. Introduction

The glider we will describe here was originally intended for transferring scientific data from the gondola of the Olimpo [1,2] project to the ground. Like most LDB experiments, Olimpo is designed for use in polar regions. Polar regions (especially Antarctica) are very attractive for LDBs because, in summer, the sun is always present to power electronics, and batteries can be very small as they are needed only for impulsive power requests. In addition, stratospheric winds at certain times of year cause the balloon to follow quasi-circular paths around the poles [3,4,5,6], and, after separation, the balloon lands safely in an unpopulated area. Unfortunately, weather or terrain conditions may make recovery difficult, expensive, and delayed. Communication between the ground station (GS) and the balloon occurs via satellites. Direct radio communication can be used when the balloon is in line of sight, but it disappears from the horizon several hours after launch. Bandwidth and price limit the use of the satellite to house-keeping data and remote control, and the experimental data must await recovery in the payload’s mass storage. HERMES provides an affordable solution to this problem, allowing experimental results to be obtained before the LDB is terminated. The weight of the glider itself is about 3 kg, and its release does not significantly affect the attitude of a large payload. It is also possible to attach more than one glider to the same payload which can be released at a later time.

The launch of HERMES was part of the European project HEMERA [7,8], which was created with the goal of improving science activities in the stratosphere by offering researchers easy access to the space in individual or shared stratospheric payloads. In the case of HERMES, the launch campaign took place at the Swedish space base Esrange (Kiruna) and was carried out by the Swedish Space Corporation (SSC) in July 2022.

2. System Description

HERMES as a whole consists of the releasing platform, the glider, and the GS. It is designed to be attached to and powered by a host payload from which it expects to have some facilities (e.g., power and communications). If some facilities are not available, it can be configured to provide the missing facilities itself. Since the system is very modular, it is quite easy to configure it by adding the required modules. In the case of the test flight described, no facilities were available at all, and all modules were used to simulate a complete host payload. Figure 1 shows the overall mechanical view of the payload in a short-term (without solar power) experimental configuration.

Each block is a physical object that can be added or removed; most of them are equipped with a couple of parallel connectors and can be electrically connected in a chain. Adding a module is done simply by breaking the chain and inserting the new module using two parallel connectors. The connectors are used to transfer data (via the CAN (Controller Area Network) bus) and power. Some modules, such as the releaser, battery pack, and glider charger and junction box, are essential to the operation of the system and have a number of dedicated connectors for the main power supply and for servo control.

HERMES was born to be attached to stratospheric scientific platforms. However, during its first experimental flight to test the proper functioning of all modules, HERMES behaved like an autonomous payload. The presence of a host payload computer was simulated by the flight computer simulator (FCS), which generated synthetic data to be stored into a solid-state disk (SSD) aboard the glider.

Commands to HERMES are issued by GS via the Iridium satellite network, received by the Com and Control module, and transferred to the CAN bus to be executed by the interested modules. An example of a command is the release command that causes the glider to separate.

2.1. Payload Block Diagram

The modules that make up the system are listed with a brief description in Figure 2. The modules marked with ** are mandatory; those marked with * are optional.

• Battery pack**: It manages the battery that powers the system. It also acts as a power conditioner, buffer, and distributor, allowing the system to be powered from different power sources. The battery consists of seven pure lead elements. A microprocessor manages the battery correctly by charging it from the panels (using the maximum power point tracking (MPPT) algorithm) or from some other external dc source (between 18 V and 35 V), taking into account the temperature. The module powers the system and provides a dedicated power supply for the glider charger and junction box module.
• Com and control*: It is responsible for managing satellite and line-of-sight communications. A reduced version of this module is designed to be housed on board the glider so that commands can be sent via the satellite network.
• Inertial measurement unit (IMU)*: It contains the inertial sensors (three-axis accelerometer, three-axis gyroscope), a three-axis magnetic compass, a barometric sensor, and a global positioning system (GPS) receiver. The module, which is self-consistent for general use, allows knowledge of the payload’s attitude.
• Monitor and SD recorder*: This module provides the local log file capture of CAN traffic and provides an RS232-CAN interface which is useful for troubleshooting and testing. It also provides a possibility to communicate with the scientific host computer.
• Glider charger and junction box**: It manages the connection with the glider’s umbilical connector (connector J21 in Figure 2), allows data transfer from FCS to glider’s SSD, powers the SSD, charges the glider’s batteries, and maintains their temperature using a heater. The batteries are kept in storage mode and fully charged before release.
• Flight computer simulator*: It generates synthetic data with which it fills the SSD, simulating the presence of a flight computer. In fact, HERMES was designed to transport data produced by a balloon-borne experiment to the ground, and FCS, in the absence of real data, generating test data that are loaded onto the SSD in the aircraft via the umbilical cable. These data are the video taken by a camera (see Figure 1), active from the moment of launch, and the house-keeping data acquired by all the modules and transferred via the CAN bus.
• Releaser**: This module controls the servo motors that release the glider. The servo motors are controlled by position and velocity signals (pulse width modulation, PWM) and through the power supply, provided individually. The temperature of each servo and the overall current supplied are monitored. Four servo motors are used, two of which free the wings, one of which unplugs the umbilical connector, and one of which unscrews the secure bolt. The presence of the aircraft on the platform is detected by a proximity sensor installed near the rear motor. A manual switch can be used to operate the servo motors to install the aircraft on the release platform.

All modules place their house-keeping data on the CAN bus, to be used by interested modules or simply stored in the ‘Monitor and SD recorder’, to obtain a complete log of the flight.

2.2. About Releasing

The releaser and the glider charger and junction box are fundamental parts of HERMES. They are designed to:

• Keep the glider in place, allowing 10 g acceleration;
• Maintain the glider battery, keeping it warm and in storage mode until release;
• Supply to the battery the residual charge to pass from storage mode to flight mode;
• Receive the release command via the CAN bus or via a free contact;
• Operate all servos to release the glider.

The release command can be done by:

• A command via the CAN bus;
• A contact provided by logistics telemetry, which is activated in case of communication failure;
• A manual switch when installing the glider on the platform.

The release system is designed to withstand 10 g acceleration. In a smooth balloon flight, it is difficult to exceed 2 g acceleration. However, in the case of the glider still being attached to the releasing platform when the payload parachute deploys, this spec guarantees that accidental detachment of the glider does not take place. This function is accomplished by a safety screw that secures the fuselage to the releaser and is unscrewed and pulled out when the glide is released. To ensure high torque, a high current continuous rotation servo was used, requiring a power booster circuit (see Figure 1) to operate it.

Further safety is guaranteed by a pair of brackets that support the wings and may keep the glider in position in case of the safety screw breaking. An additional servo motor helps to detach the magnetic connector.

During balloon flight, the glider is secured to the release platform by the safety screw and wing brackets (Figure 3). The umbilical cable connector is plugged in. After receiving the release command, the following actions follow (Figure 4):

• The umbilical connector is unplugged by a servo arm;
• The wing’s brackets are loosened;
• The safety screw is unscrewed.

Figure 3. The glider is secured to the releaser.

Figure 3. The glider is secured to the releaser.

Figure 4. The release command arrives: the glider is released.

Figure 4. The release command arrives: the glider is released.

2.3. Glider Block Diagram

Figure 5 shows the block diagram of the glider. J21 is the magnetic connector that connects the glider to the payload via the umbilical cable. Through this connector, the payload supplies the heater for the glider’s battery, keeps the battery charged, transfers backup data from FCS to the SSD, and allows communications with the glider to occur via the CAN bus.

The project envisions that the glider can host a COM and CONTROL module (the same as the payload) connected to the autopilot via CAN, of which it reads the positioning system, allowing the glider to be tracked when not in visual range, over the satellite network. It can load a new landing point on the autopilot and turn the motor on and off.

3. Glider

The custom-built glider was developed as part of the ABACHOS (Automatic BACk HOme System, [9]) project. It is built from foam reinforced with carbon fiber tubes. All parts are made by machining foam blocks with a numerically controlled (home-built) hot wire cutting machine (Figure 6a,b). The low weight of the glider (3 kg) allows experiments to be conducted with inexpensive latex balloons which are not suitable for long measurement campaigns in the stratosphere but are excellent for getting the glider aloft and studying its comportment.

In Figure 7, there is a mechanical drawing of the glider.

The glider is equipped with a commercial autopilot for navigation (Pixhawk). The system combines advanced sensors, including an accelerometer, gyroscope, magnetometer, barometer, and GPS, a powerful microcontroller, and a range of connectivity options. The open-source nature of this device allows users to access, modify, and adapt the source code to their own needs. The autopilot controls the aircraft via the left and right ailerons and receives commands from radio telemetry (linking to the glider control station) and, when in line of sight, from the radio remote control. The remote control was used to ensure a safe landing during the test. Usually when planning a mission, the home position is set as the location where the aircraft is armed. This mode was used by ABACHOS on 15 stratospheric flights (13 of which were successful) and also by HERMES in the Esrange test flight since the landing point coincided with the launch point. Alternatively, there is the possibility, provided by the glider control station, to use waypoints to set up an alternative landing point. The solution we are developing for the next glider configuration is to modify the code to allow the home position to be changed via the CAN interface.

Figure 8a,b show an example of an ABACHOS flight performed in Italy in Fragneto Monforte, the city of hot air balloons (https://en.wikipedia.org/wiki/Fragneto_Monforte (accessed on 1 April 2023)). The latex balloon with the glider was launched from the Fragneto Monforte sports field after the home position was set at the nearby Benevento-Olivola airport.

After a 100-min ascent at an altitude of 28 km, the latex balloon exploded and the glider free fell 11.5 km before entering flight mode at an altitude of 16.5 km. Thereafter, the flight proceeded smoothly and straightforward until reaching the airport. Once at the landing site, the glider’s altitude was lost in a spiral until it reached the airstrip. During the flight, the acceleration did not exceed 2 g and the speed reached 300 km/h. Since the glider was not equipped with a pitot tube and only GPS data were available, the speed relative to the air was evaluated using the balloon ascent to measure the wind speed, assuming that it remained the same during the glider’s flight. In this way, we obtained an estimate of the glider’s efficiency during the flight, which is shown in Figure 9.

A balloon flight is unpredictable in the long run. The flight must be constantly monitored by a ground team that decides if and when to release the glider. The glider is released when there is a need to recover data and there is an opportunity to land at a safely accessible location. In this case, the landing point is uploaded to the glider, and the release command is issued. Once released, the glider is capable of reaching the landing point on its own. In any case, in the final phase of landing, when the glider is visible to the operator, there is an opportunity to take control of the glider manually. This is not necessary for a real flight from a large LDB in polar regions, but it was very useful during the test to prevent the glider from being damaged by an incorrect automatic landing.

From a logistical standpoint, after executing the release command, the flight support team travels to the landing area (in the case that the return point is not the home position), waits until the glider is in sight, and then manually takes control of it by radio command to land it. To facilitate ground contact, a plastic carpet (about ten meters long) may be laid on the airstrip.

4. Launch Campaign Esrange 2022

The HERMES launch campaign took place in July 2022 at the Swedish Space Base in Esrange (Kiruna); it was carried out by the Swedish Space Corporation (SSC). In Figure 10a, we see the payload with the glider just before the launch; Figure 10b shows the launch method used by SSC, where a vehicle (Hercules) holds the gondola payload until the balloon launch.

Figure 11 (courtesy of SSC) shows the block diagram of the flight chain. Starting from the top, we find the zero-pressure balloon (ZPB, 3000 m³) which was used, the PTU sounding station (pressure, temperature, relative humidity, etc.), the termination system, the parachute (19 feet diameter, 26.4 m²), the SSC telemetry module (with transponder), the truck plate, and finally the payload.

Figure 12a shows the moments before the launch, then Figure 12b shows the balloon beginning to ascend at an ascent rate of about 4.1 m/s. After about 1 h and 40 min, the balloon reached an altitude of about 23 km and entered the floating phase.

Figure 13 shows the ground trajectories of the balloon (red track) and glider (blue track) after launch. The red and blue tracks overlap from the launch until their separation. Once the balloon entered Esrange’s Safety Zone B, the Iridium command was sent from GS to release the glider. The release system worked, properly releasing the glider, which then began its descent in free fall. Unfortunately (which remains to be investigated), the glider did not begin to glide until it reached about 6000 m. When in flight attitude, the glider worked properly, heading to the base (see the blue track in picture), and landed correctly several kilometers from the base. The aircraft was recovered unharmed by the recovery team.

The balloon flight lasted 4 h 45 m; the landing site was in Esrange’s zone B 53 km NE from the launch site.

Figure 14a,b show two plots describing the flight of the glider: the plots denote the altitude and distance from the base versus time. Figure 14a clearly shows the ascent phase, the floating phase, the free fall, and the beginning of the glide at about 5.500 m.

The release platform and glider were located using line-of-sight telemetry and Iridium SBD (Short Burst Data). The line-of-sight telemetry was set up using an 868 Mhz 500mW radio modem with a baud rate of 115,200 with a pin antenna. The ground station used a directional antenna that accompanied the aircraft throughout the flight. The lack of line-of- sight communication is shown in Figure 15, with the red trace indicating the presence of the telemetry signal.

The Iridium SBD enables a packet-based service for frequent short data transfers in quasi-real time. The device is lightweight (30 g) and small (41 × 45 × 13 mm³) and has been shown to work well in the stratosphere. Hermes was programmed to send data packets every 3 min. Moreover, 85% of the packets sent were correctly received by the ground station. The trigger command issued by the ground station was also received correctly by the releaser.

5. Discussion

The experimental results have shown that HERMES is a promising tool for collecting and transporting specimens and data, but they also suggest several improvements to optimize its performance. The goal of the system is to achieve the longest possible flight. A longer flight increases the possibility of choosing between more landing points. The duration of the flight depends on the efficiency, the height the glider enters in flight attitude, the energy stored in the engine battery, the efficiency of the propulsion system, or the wind. The efficiency of the glider (currently <10) can be improved by changing the design of the glider, which is feasible but will predictably lead to a more expensive prototype and require a long period of experimentation.

Experiments with lattice balloons have shown that the glider will settle into flight at various altitudes, from 15 to 22 km. The low altitude in the Kiruna test (5.5 km) represents a significant deviation from this range that requires further investigation. This issue can be rectified with an improved glider design or even by using an aerodynamic brake (such as an expandable little parachute) on the tail to facilitate the nose to stay in front at the beginning of the descent. With the actual configuration, a 100 Wh battery can increase the flight length to roughly 50 km, and this distance is proportional to the efficiency. The use of solar panels is encouraged by both the large wing surface (more than half m² per side) and in polar areas where the albedo allows the use of both sides of the wings to host solar panels. Moreover, the use of solar panels will keep the communication alive, allowing the glider to be tracked in case of delayed recovery. The flight length depends on the wind, and the autopilot must consider it to increase the flight length. The simplest way to do this is by increasing speed in case of frontal wind and by decreasing speed in case of tail wind. This can be done autonomously by the autopilot, using a pitot tube, by comparing the air speed to the ground speed. Actually, the autopilot relies on the magnetic compass. In polar areas, the horizontal component of the magnetic field is very weak, and magnetic data are unreliable. A modification of the autopilot algorithm will be necessary to overcome this issue.

Most of the experiments were conducted with inexpensive latex sounding balloons. The possibility to recover the payload of a latex balloon offers a new perspective in radiosounding. Usually, radiosounding payloads are disposable devices with their performance tied to their affordability. The possibility of recovering the payload may increase the quality of the instrumentation (no longer disposable), offering also the possibility of collecting samples during the flight and also to avoid dispersing probes in the environment.