# Applications of Digital Twin across Industries: A Review
## 2 Aerospace and Aeronautics  
Aerospace/aeronautics fields were the pioneer areas where DT was explored first by NASA and by the U.S. Air Force [40; 41; 42; 43]. The main applications of DTs in this industry include optimizing the performance and reliability of the space vehicle/aircraft, predicting and resolving maintenance issues, and making the missions safer for the crew. The main application of DT in this industry started with the intention to optimize the performance and reliability of the space vehicle/aircraft. According to the 2010 technological roadmap by NASA [44], the four applications of DT for them were:  
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2. Simulating the flight before the launch of the actual vehicle to maximize the mission success. 3. Continuously mirroring the actual flight and updating the conditions such as actual load, temperature, and other environmental factors to predict future scenarios. 4. Diagnosing damage caused to the vehicle. 5. Providing a platform to study the effects of modified parameters that were not considered during the design phase. Any damage detected by DT could be resolved by activating in situ repairs or recommending appropriate changes to the mission, resulting in a longer lifespan and a higher success rate of the mission [45]. By reducing the uncertainty of prognosis and maintenance intervals, DT improves the planning and reliability of future missions [46]. Besides securing the spacecraft and the mission, DT was also used to ensure the safety of the crew members operating in the remote and uncharted territories by testing various possible recovery scenarios in case of emergency [44]. Aside from performance and reliability, one of the benefits of using DT is that it can be used for predicting errors, which makes the maintenance of the spacecraft or aircraft easier and cheaper than the traditional scheduled maintenance. The U.S. Air Force Research Laboratory (AFRL) in 2020 announced that they would be creating a DT of one of their supersonic bombers, B-1B, for predictive maintenance, which will be done by 3D scanning each and every part of the aircraft down to the nuts and bolts [47; 48]. Through the scanning process, any structural faults or damages will be discovered which will help in creating a medical record of it. Then data from the aircraft will predict areas that are more likely to have structural issues. Over the aircraft's life cycle, layers of maintenance data, test/inspection results, and analysis tools will be superimposed with the digital model. AFRL has previously signed a contract with Northrop Grumman worth USD 20 million for the development of a DT to forecast the problems with the airframe of different kinds of Air Force aircrafts so that preventive maintenance can be performed [49]. Their experts are trying to improve predictive maintenance for the aircraft by improving flight load data, developing more realistic structural analysis models, and quantifying and reducing modeling uncertainty. An aircraft's life is highly dependent on the load it carries; the life of an aircraft can improve up to 200% with the reduction of just 20% weight [50]. With the help of DT technology, the load spectrum of an aircraft can be recorded and processed for determining its useful life. A DT for fan-blade reconditioning has been proposed by [PERSON] et al. [51] which incorporates the use of cameras and vision-based algorithms, along with a robotic arm for grinding the fan blade of the aircraft engine to the required specification in order to improve aerospace maintenance, repair, and overhaul services. [PERSON] et al. [52] proposed a DT for the machining processes of an aircraft's rudder which can mimic physical machining processes from three aspects: geometry, behavior, and context. NASA has even developed the DT of the rocket engine to predict real flight conditions and their impacts better on the engine start-up [53]. DT is the enabling aerospace sector to develop new products faster and cheaper while improving the production line quality and reducing the aircraft maintenance time [54].