# Feasibility of a Helium Closed-Cycle Gas Turbine for UAV Propulsion
## 1 Introduction  
In the context of this paper, the term unmanned aerial vehicles (UAV), referred to as uncrewed aerial vehicles, are aircraft that use aerodynamic forces to provide lift either directly or by forward motion, with no human on board and controlled autonomously or remotely. Hence, throughout this paper, the term uncrewed aerial vehicle is used interchangeably with remotely piloted vehicles (RPV) and uncrewed air system (UAS). UAV, UAS and RPV have been around since World War II, and there is growing interest in the use of uncrewed aircraft for special mission requirements [1], due to the advent of better technologies to support the intelligence of controlled UAVs. To this end, there are projections for significant growth in the use of UAVs in both civil and military applications due to the added economic and technological value related to them [2, 3]. Despite these projections, there are still many challenges facing the industry, with the salient ones being the choice of the propulsion system, the energy source, the conversion system, the fluid media, and the thrust converter. The considerations of energy source, conversion system, fluid media and thrust converter are linked to the choice of the propulsion system. The choice of the propulsion system is vital in terms of sustainability, usability, noise, controllability, reliability, and technology readiness level (TRL). Unlike the crewed aircraft vehicles, the design for UAVs will also need to meet long-endurance criteria, a duty cycle portrayed by heavyweight and high-altitude flight, compactness, and payload requirements. Hence, a propulsion system that does not meet these demands will not make any economic sense and will be less applicable for either military or civil usage. With sustainability, the goal will be to have a system that would not have a severe impact on the environment, as well as achieving the objective of a net-zero carbon footprint. The complexity of selecting and exploiting the propulsive system capability to meet all assumed requirements is not an easy task, but one that requires continuous optimisation. Within this context, several propulsion systems have been considered as part of ongoing innovative research programs. A broad spectrum of these initiatives includes open, closed and semi-closed propulsion systems such as the use of reciprocating engines (diesel engines), Rankine, Stirling, and Brayton cycles (Turbojet, Turboprop, Turbofan), as well as battery and fuel-cell, powered propulsion systems. For example, in the work of [PERSON], [4], the author proposed the use of a small turbofan or turbojet for UAV propulsion with the aim that a decrease in both the engine size and frontal area would decrease fuel consumption and improve engine affordability. The work addresses the drawback of remarkably high fuel consumption associated with the use of turbojet or turbofan. An example of a Turbojet UAV is the Bombardier CL-289 [1] developed by Dornier GmbH in Munich as a tri-national project between Canada, France and Germany with output thrust of 236 kN and the Rolls-Royce RA-4a Global Hawk [1] an example of Turbofan UAV. Similarly, [PERSON] et al. [5] discussed the modelling challenges for turbojet UAV using a one-dimensional correlation model to access the performance and operational requirements for a UAV mission. There are also several proposals to use fuel-cell and battery-powered propulsion recorded in the works of [PERSON] et al. [6] and [PERSON] et al. [7]. The stated advantages of the fuel cell-based system include higher efficiency and zero-emission compared to fossil fuel technology. However, the high cost of production and increased sensitivity to fuel contamination could create additional complexity. As the drive for a reduction in the cost of lithium is sustained and technology to reduce the mass-to-energy ratio to less than a quarter, battery-powered propulsion system could lead the UAV missions. Additionally, [PERSON] et al.