The mooring drone is an advanced UAV system that integrates a drone with a tethered cable, enabling stable and long-duration operations in various environments. Depending on the application scenario, the mooring system can be categorized into ground-based fixed, vehicle-mounted, or ship-borne mobile configurations. These three operational methods effectively cater to diverse working conditions. However, several core technical challenges remain unresolved, limiting the widespread adoption of this technology.
One of the key drivers for the development of tethered rotor drones comes from the need for airborne monitoring platforms. The tethered balloon lift platform system, for example, serves as an air-based solution for fixed-point electronic surveillance. TCOM Corporation, a leading U.S.-based manufacturer, has deployed such systems, including the sea-alert radar network along the West Coast, using captive balloons as airborne platforms.
Despite its usefulness, the tethered balloon suffers from significant limitations. It is bulky, difficult to transport, and lacks mobility. To address these issues, the industry has explored alternative solutions, such as inductively powered ground systems and motor-driven propellers instead of traditional balloons. However, early attempts were hindered by the lack of lightweight, high-performance rare-earth motors, which had poor power-to-weight ratios and could not support the development of practical tethered rotor systems.
In recent years, the rapid advancement of multi-rotor UAVs has provided the necessary technological foundation for the development of tethered rotor platforms. Many manufacturers have since introduced tethered rotor systems, commonly referred to as "tethered drones."
While promising, these systems face several critical technical challenges:
1. **Power-to-Weight Ratio of the Drive Motor**
The efficiency of a tethered rotor system largely depends on the motor's power-to-weight ratio—measured in kilowatts per kilogram. For a rotor aircraft, about 5–10 kg of lift is generated per kilowatt of power. A ratio below 1 kW/kg makes design extremely challenging. Importantly, the motor’s rated power—not peak power—is the key factor, and it must account for the weight of the motor, ESC, and cooling components. Unlike typical UAVs, tethered rotors operate continuously, requiring motors to function reliably within their rated power range.
2. **High-Voltage Power Supply System**
Unlike conventional drones, tethered rotors receive power through a long cable, making efficient power transmission crucial. High-voltage systems reduce current and power loss while allowing thinner cables, thus reducing overall weight. However, designing such systems involves trade-offs. Some systems use direct high-voltage motor drives, but this requires expensive high-power ESCs. Others use step-down power supplies, which add complexity and weight.
3. **Tether Design and Material**
The tether serves dual purposes: transmitting power and carrying optical fibers for data signals. This makes it a composite cable, requiring careful material selection. Modern tethers often use aluminum conductors instead of copper to reduce weight without compromising conductivity.
4. **Flight Control Challenges**
Although the tethered rotor typically hovers around a fixed point, flight control remains complex. The tether can cause oscillations under wind conditions, making it difficult to maintain stability. The system must withstand gusts up to 6–8 m/s, which poses significant control challenges. Robust flight algorithms are essential to manage these dynamic conditions.
5. **Aerodynamic Design**
The platform must operate in strong wind conditions, where aerodynamics play a critical role. At wind speeds of 10.8–20.7 m/s, the platform effectively flies at high speeds, necessitating efficient aerodynamic design. Some systems combine vertical take-off and landing (VTOL) with fixed-wing features, improving performance in adverse weather. This hybrid approach offers advantages in both hover and forward flight, enhancing the system’s versatility.
Although the mooring drone holds great potential, it still faces hurdles before becoming widely adopted. Continued innovation and collaboration across the industry will be essential to overcome these challenges and bring this technology to practical use.
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