## 7 Challenges and Future Opportunities  
As presented in previous sections, IPTMSs have been studied widely in different research fields. Owing to increasing electrification, the design of aircraft systems requires a multidisciplinary approach. In this context, TMS designs necessitate integration with other aircraft systems [96], and current IPTMS designs require further enhancement to meet the requirements under various conditions [97]. As a result, it is necessary to discover the current challenges of IPTMSs and their future opportunities. This section initially presents the challenges and future opportunities for the IPTMS research topics mentioned in Sections 3-6 and explores the potential solutions for civil electrical propulsion aircraft (EPA) based on these discussions. Regarding the IPTMS architecture design with energy recovery, the system's high integration and multidisciplinary requirements add to its complexity. This complexity brings difficulties in terms of operating cost, reliability, and safety, and presents obstacles for various conceptual designs as well as control and power management system design. This IPTMS design is usually applied to military aircraft due to its ability to reduce overall weight and save volume for other subsystems, thus increasing the aircraft capability. Therefore, future research in this field should focus on integrating more functions into an IPTMS, while enhancing system stability and safety and developing feasible architectures for different aircraft conceptual designs. As presented in Section 3.2, an IPTMS approach is integrating EPS and TMS architecture designs. It has been found that the advanced EPS architecture design can reduce the requirements of the TMS and ultimately reduce the TMS's overall weight, power, and drag due to ram air. In this research field, the heat loads are usually determined by the efficiency-based method. However, the power required varies across the whole flight mission, which means the current is different as well at different operating points, so the efficiencies of electric components will change. To assess the performance of components and investigate the interactions between the TMS and PMS more accurately, other component modelling methods should be adopted, such as the experiment-based method and the physics-based method. The IPTMSs in this research field are usually designed for HEA. In this case, it is difficult to avoid the use of ram air. Thus, the future research should focus on discovering approaches to minimise ram air usage. The power management for an IPTMS manages the power and thermal energy flows at the aircraft level. However, its current stage still focuses on the conventional aircraft subsystem architecture. For example, in military aircraft, the power management optimisation is not based on the energy recovery architecture but focuses on the interactions between conventional aircraft subsystems. In civil aircraft, the power management mainly optimises battery performance. Usually, the battery has two power supply strategies: it provides propulsive power only, or provides propulsive power and all the cooling power of the propulsion system. However, there are limited papers that investigate the strategies of battery power supply. For example, the battery can satisfy the required propulsive power and provide from 10% or 20% to 100% of the TMS power needed for propulsion system cooling. Therefore, a potential solution is to design the power management strategies considering the power usage of the TMS to obtain the optimal battery power supply strategies across the aircraft's flight mission. As presented in Sections 5 and 6, various researchers have developed methods or tools for IPTMSs at the aircraft level, involving modelling, optimisation, and analysis. Although these methods were designed for different power conditions, they need to be more versatile to accommodate other conditions, such as subsonic/supersonic conditions and different aircraft platforms.