The mooring drone is an advanced UAV system that integrates a drone with a tethered cable, allowing it to remain airborne for extended periods. Depending on the application scenario, the mooring system can be classified into ground-based, vehicle-mounted, and ship-borne mobile configurations. These three working methods are highly effective in meeting the demands of various operational environments. However, several key technical challenges still need to be addressed before the technology can reach its full potential.
One of the main drivers of the market demand for tethered rotor drones comes from the mooring balloon lift platform system. This air-based platform is used for fixed-point monitoring of electronic equipment. TCOM Corporation, a well-known U.S. manufacturer, has deployed tethered balloon systems as part of the sea-alert radar network along the west coast of the United States.
Despite their utility, traditional tethered balloons are bulky, difficult to deploy, and lack mobility. To address these limitations, the industry has explored alternatives such as using inductive power from the ground and replacing the balloon with a motor-driven propeller for lift. However, during the last century, rare earth motors were not yet developed, and the weight-to-power ratio was insufficient to support the practical use of such systems.
In recent years, the rapid advancement of multi-rotor UAV technology has provided the necessary foundation for developing a tethered rotor lift platform. Many UAV manufacturers have since introduced tethered rotor systems, commonly referred to as "tethered drones."
There are several key technical challenges involved in the development of mooring rotor drones:
1. **Power-to-Weight Ratio of the Drive Motor**
The power-to-weight ratio of the motor is critical. It refers to the output power of the motor divided by its weight (in kW/kg). For a rotor aircraft, approximately 5–10 kg of lift is generated per kilowatt of power. Designing a system with a power-to-weight ratio below 1 kW/kg is particularly challenging. It's important to note that this refers to the rated power, not the peak power, and includes the motor, ESC, and cooling components. Since the tethered rotor must operate continuously, the motor must function within its rated power range, unlike typical multi-rotor UAVs that work intermittently.
2. **High-Voltage Power Supply System**
Unlike conventional drones, tethered rotors draw power from the ground through a long cable. A high-voltage power supply reduces current and thus minimizes energy loss and cable weight. There are two common approaches: either directly matching the voltage with a high-voltage motor or using a step-down power supply on the platform. Both options come with their own design and cost challenges, especially when dealing with high-power control devices.
3. **Tethered Cables**
The tether serves not only as a power line but also as a conduit for transmitting signals between the platform and the ground. This makes it a composite cable with both electrical and optical functions. To reduce weight, materials like aluminum are often used instead of copper. Additionally, placing antennas and equipment on the ground helps save lift capacity and increase the height and performance of the platform.
4. **Flight Control Function**
Although the tethered rotor mainly hovers around a fixed point, flight control remains complex. The tether itself can cause oscillations under wind conditions, which may lead to instability. Civilian tethered drones must be able to withstand at least a constant wind speed of 6 m/s and gusts up to 8 m/s. Controlling the platform in such conditions is technically demanding and requires robust flight control systems.
5. **Aerodynamic Design of the Rotor Platform**
Since the system operates in strong wind fields, aerodynamic efficiency is crucial. The platform behaves similarly to a plane flying against the wind, with wind speeds reaching 10.8–20.7 m/s (equivalent to about 75 km/h). A fixed-wing design offers better aerodynamics, and some vertical take-off and landing (VTOL) fixed-wing UAVs have been developed to combine the advantages of multi-rotor and fixed-wing systems. While not the most cost-effective solution, it provides a practical way to manage takeoff and landing in specific scenarios.
Though the mooring drone holds great promise for a wide range of applications, there are still significant hurdles to overcome before it becomes widely adopted. Continued innovation and collaboration across the industry will be essential to bring this technology to its full potential.
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