On April 17, 2008, the U.S. Department of Defense Advanced Research Projects Agency (DARPA) announced that it had awarded one-year development contracts to Aurora Flight Sciences, Boeing and Lockheed Martin to start Carry out conceptual design work on a drone with a endurance of up to 5 years, which marks that the "Vulture" plan that has been brewing for many years has officially entered the implementation stage. The three companies will research and design their own UAV concepts in the first phase to address various technical challenges faced by ultra-long-endurance flight and meet the U.S. military’s urgent need for persistent surveillance capabilities.
The design goals are staggering
The "Vulture" project is an "unmanned reconnaissance unit with ultra-high altitude, ultra-endurance, and theater cruising" (Very-high altitude, Ultra -endurance, the English abbreviation of Loitering Theater Unmanned Reconnaissance Element), aims to develop an unmanned driving platform with a five-year endurance. The outside world also often refers to it as an "infinite endurance" drone. Strictly speaking, this is a technology exploration plan. The main purpose is to study the feasibility of achieving "infinite endurance" for drones and strive to promote the development and maturity of various key technologies, especially the development of reliability technologies for aircraft systems.
This plan is one of the "Blue Skies" plans proposed by DARPA in recent years, and is also a follow-up supplement to the "Integrated Sensors as Aircraft Structure" (ISIS) plan. When DARPA's Tactical Technology Office announced the plan on June 7, 2007, it emphasized that ultra-long endurance has great advantages in reconnaissance, intelligence, and surveillance (ISR) missions and can provide continuous and stable mission execution. Unprecedented capabilities. However, DARPA did not specify the reason for setting the endurance time to 5 years, nor did it specifically define the mission or payload type of the drone. It just hopes to push the flight boundaries of the drone to the limit.
The emergence of the "Condor" project caused great shock in the field of aircraft design. People's first feeling was like "Don Quixote is challenging a windmill." In terms of technical capabilities, the endurance time of most drones is currently measured in hours, and the "Condor" program strives to promote the development of key technologies for long-endurance drones and create flight records that are currently unimaginable. It is hoped that by This changes people's inherent concept of airplanes.
For potential military uses, DARPA has initially set the goals of the "Vulture" program as: being able to carry a payload of 450 kilograms and a rated power of 5 kilowatts; having sufficient cruising speed; and being able to operate without interruption Flying at an altitude of 20,000 to 27,000 meters for 5 years: the reliability reaches 99% and has a high mission success rate. After preliminary demonstrations, DARPA proposed three basic structural development ideas: one is an ultra-reliable independent system, equivalent to a satellite; one is a modular aircraft, each part can be flown back to the base for maintenance and repair; one is a Drones can serve for a long time while in the air and can be replaced in time.
It can be seen that the goal of the "Condor" program is essentially to develop a drone that works similar to a satellite, but will not be limited by orbital parameters and does not need to rely on foreign bases. Compared with satellites orbiting the earth, it can fly nearly 20,000 meters above the battlefield in the stratosphere for a long time, which will help significantly increase the resolution of airborne sensors and have greater flexibility.
DARPA divides the "Vulture" program into three phases: the first is the concept definition phase, which reduces risks by optimizing the concept; the second phase will A 1/6 scale drone will be manufactured for a three-month flight test; the third phase will manufacture, assemble and test-fly a full-scale verification aircraft, which will require a test flight of up to one year carrying the design payload. Currently, companies are conducting phase one analysis of military applications, developing operational concepts and defining target systems in detail.
Key technologies need to be solved urgently
With the "Condor" plan being made public, the many technical challenges faced by the "infinite endurance" drone concept have gradually attracted attention.
These challenges involve a series of issues such as environmental energy harvesting, high-density energy storage, efficient propulsion systems, increasing system reliability, efficient aircraft structural design, and mitigating the aging of materials and systems during long-term flights in the stratosphere. Among them, energy systems and reliability have become top priorities.
The first thing that must be solved is the propulsion system solution, the core of which is the method of energy acquisition and storage. For ultra-long endurance aircraft, there are currently three energy cycle solutions. One, nuclear power, was expressly prohibited from consideration. The second is refueling through the air, relying heavily on autonomous aerial refueling technology. However, compared to piston generators and gas turbine generators, the most interesting are fuel cells because of their higher efficiency and Higher reliability. The third option is solar energy.
In contrast, the use of solar power for drones has shown good development prospects and seems to be a safer way, but it is still limited to use in summer when the sunshine hours are longer. In 2007, NASA announced a series of High Altitude Long Endurance (HALE) drone concepts. However, none of these concepts ultimately achieved the six-month endurance goal. Therefore, how to utilize solar energy more effectively and develop an effective energy storage technology to solve the problem of providing sufficient power at night has become a technical obstacle that ultra-long endurance drones must overcome.
The system reliability of "infinite endurance" drones has become a key consideration. According to DARPA, the reliability target of the "Condor" program is 200 times that of the current "Global Observer" drone. Currently, except for satellites, no one will consider the reliability of the aircraft's continuous flight for 5 years. To this end, DARPA emphasized that the design of the "Condor" program must be carried out in accordance with the standards of the aerospace industry, rather than the standards of the aviation industry, so that it can meet the demanding mission requirements.
Compared with satellites, the working environment of drones is obviously different. For a drone, the cycle of day and night is longer, so after being immersed in a very cold environment, it then enters an extremely hot environment. Compared with satellites operating in space, the radiation received by the drone is Less, but compared to the ground, it is still exposed to stronger ultraviolet radiation, which involves the aging of various materials.
Similarly, "infinite endurance" drones face the risk of excessive distortion in the design of the wing structure. In order to last for a long time in the stratosphere, UAVs must use ultra-large aspect ratio wings in the design to obtain solar energy. However, a feature of this layout is that it will produce great elasticity under the action of aerodynamic loads. Deformation ~ Especially after taking off from the ground, during the process of gradually ascending to the stratosphere, you will often encounter atmospheric turbulence in the troposphere. This can cause excessive distortion of the wing, which may even lead to disintegration in severe cases. Therefore, how to solve the contradiction between ultra-long endurance and ultra-large aspect ratio layout has become an important aspect of the exploration and innovation of the "Vulture" plan.
The "West Wind" program is beginning to take shape
The reason why DARPA launched the "Vulture" program was not out of nowhere, but to further promote the rapid development of high-altitude, long-endurance drones . Previously, DARPA has paid attention to the design of various high-altitude and long-endurance drones, such as the "Global Observer" drone powered by liquid hydrogen fuel cells. However, since the endurance of these solutions is still limited, DARPA has turned to the British company Qinetic's "Zewind" solar-powered drone.
The "West Wind" is a lightweight solar-powered drone with a wingspan of 16 meters but a weight of only 27 kilograms. The solar cell array covering the upper part of the wing provides power to the aircraft during the day and at the same time transfers energy to the aircraft. Stored in the battery pack to continue to provide sufficient power at night. Its design goal is to fly continuously for several months at an altitude above 15,240 meters. In July 2006, a Zephyr drone flew for 18 hours, including seven hours at night. In a series of test flights, in addition to confirming the expected flight performance, the "Zifeng" also conducted a payload test flight. Not only did it successfully conduct a communication relay demonstration for the first time, achieving beyond line of sight communication in mountainous terrain, but it also carried different The photoelectric/infrared sensor successfully acquired the photoelectric/infrared hybrid image.
After DARPA released the design goals of the "Vulture" program, Boeing began to cooperate with QinetiQ, using the "Zewind" UAV that has been test-flying for many years as a platform to build a platform for the "Vulture". The Eagle program provides proven technologies with a particular focus on energy storage requirements.
In 2007, Qinetic redesigned the Zephyr, increasing the wingspan to 18 meters, the weight to 30 kilograms, and adding a custom autopilot to make it closer to The most critical improvement of an actual combat system is the use of a new amorphous silicon solar cell array developed by Solar Effect Company. Its thickness is only equivalent to a few sheets of paper, which can efficiently absorb solar energy during the day. It is also equipped with SION A set of lithium-sulfur batteries and a new charger developed by the company significantly improved the storage efficiency of the power system. On September 10, the aircraft completed a 54-hour continuous flight carrying a surveillance payload, reaching a flight altitude of 17,780 meters. A new continuous flight record was set.
Less than a year later, "Zeifeng" once again broke the record for continuous flight time. On July 28 this year, the aircraft took off from the Yuma Proving Ground in Arizona, USA, and did not return to the ground until the 31st. It flew non-stop for 82 hours and 37 minutes. This time greatly exceeded the time currently maintained by the "Global Hawk" flight record of 30 hours and 24 minutes. According to the plan, Qinetic will further optimize various key technologies of the "Zeifeng" and strive to achieve the design goal of continuous flight for several months within two years.
Different from the "Zifeng", the "infinite endurance" drone proposed by Boeing has doubled its wingspan, adopted an ultra-large aspect ratio design, and has electrically driven propellers accordingly. Increase. It is particularly worth noting that the designers did not use the original single tail brace + vertical tail + horizontal tail layout, but adopted a double tail brace + inverted V-shaped tail, trying to enhance the maneuverability by controlling the airfoil to avoid " The pitch oscillation that once occurred in the Helios drone.
However, the ultra-large aspect ratio design still has the potential danger of excessive wing distortion. In order to detect turbulence ahead in time, the plan installs atmospheric turbulence detection systems on the left and right wings and tail respectively. Once the drone flies into turbulence, the forward-extending detection tube will first measure the relevant data of the turbulence, and the onboard control system will immediately issue relevant instructions to adjust the flight status in a timely manner to avoid falling into an excessively distorted state. In order to solve this shortcoming, Boeing is trying to use more effective composite materials and further optimize the existing structural design in order to finally submit a more satisfactory solution.
"Odysseus" surfaced
Compared with the "West Wind" plan, Aurora Flight Sciences boldly broke through the traditional design concept and proposed an unexpected The "Odysseus" program has become a strong competitor to the "Vulture" program. From the perspective of energy acquisition methods, Aurora Company still uses solar energy and still faces various problems common to solar-powered drones. However, the novelty of the "Odysseus" drone is its modular design, which may better solve the technical problems faced in terms of structural distortion, energy acquisition, and reliability.
It can be seen from the design that the "Odysseus" drone is unique in that it is composed of three identical modular aircraft articulated into one. The modular aircraft uses straight wings with a wingspan of 50 meters and a weight of 1,350 kilograms. Three propellers are installed on the leading edge and are driven by solar cells. At the same time, it uses a cross-shaped tail that can rotate around the tail support axis, independently control the attitude change of the module aircraft, and change the overall configuration of the drone in flight.
From the perspective of use, each module can take off from the ground in sequence, fly to a predetermined high-altitude docking point, and then integrate into one with the help of automatic docking technology, thus forming a super-large aspect ratio drone. A man-machine with a wingspan of 150 meters. The modular design of "Odysseus" helps overcome the problems faced by the ultra-large aspect ratio layout during the take-off and ascent stage, and significantly enhances its ability to resist turbulence in the troposphere, thereby avoiding possible structural damage caused by excessive wing distortion. destroy.
The modular design of "Odysseus" also effectively improves solar energy absorption efficiency. Three modules can change the overall configuration according to flight and energy harvesting needs. When flying during the day, as the angle of sunlight changes, the drone can adjust the tilt angle of the three module aircraft through the cross-shaped tail, transforming the ultra-large aspect ratio wing into a folding wing.
In this way, the upper surfaces of the wings of at least two modules of the drone can receive sunlight to the greatest extent, so that the efficiency of solar cells in obtaining energy is increased by 4~ compared with the straight wing state. 5 times, which better ensures that the solar drone can fly normally throughout the year. At night, "Odysseus" can re-stretch into a straight wing with a large aspect ratio, allowing it to continue flying in a shape with the least resistance, effectively saving the electrical energy stored in the aircraft.
In view of the operational reliability required by the "Condor" program, Aurora Flight Sciences did not stick to the rules, but made full use of the advantages of modular structure to maintain the integrity rate of UAV aerial duty. , the modular aircraft can be replaced at any time. At present, the increasingly mature automatic aerial refueling technology has provided technical feasibility for the docking and detachment of modular aircraft. If any module aircraft fails during a mission flight, it can detach from the drone and return to the base alone for repairs. Even if there is no obvious failure of the entire UAV, Aurora Company plans to take turns to detach each module aircraft from the UAV after 1 to 2 years of flight and return to the base for routine maintenance. During this period, the new module will be docked and continued. flight mission.
As of now, Lockheed Martin, which is involved in the "Condor" program, has not disclosed its own UAV program, and Phantom Factory, which is responsible for the design task, is likely to come up with an unexpected The outstanding concept design is very worthy of attention. There is no doubt that as the "Condor" program continues to advance, DARPA will take an unprecedented step forward in aircraft design concepts.
Editor Qin Zhen