# Feasibility of a Helium Closed-Cycle Gas Turbine for UAV Propulsion
## 1 Introduction
[8] compared the use of a semi-closed cycle to open Brayton cycles for medium-altitude UAV mission. The result from their work showed a slight performance improvement with the semi-closed cycle compared against the large open cycles. This is because the semi-closed cycle offered a compact and efficient design for medium-altitude UAV missions [9]. The use of reciprocating internal combustion engines (ICE) has also been documented in the works of [PERSON] and [PERSON] [10], [PERSON] [11] and [PERSON], [12], which is the most common form of the propulsion system used for the UAVs, ranging between 1-150 kW. The reciprocating engines are widely used because of weight and high power to weight ratio, but its disadvantage is hinged on its noisy operation, vibrational challenges at high revolution per minute and cooling requirement. The reciprocating piston engine has many pressurised seals that often fails, leading to significant loss of power or engine failure. An example of the UAVs with reciprocating ICE includes the General Atomics MQ-1 Predator developed by General Atomics in Poway, US. The UAV weighing 68 kg (engine weight) with 115 horsepower output [1]. Others are the Northrop Grumman RQ-5A Hunter developed in Fort Hood, Texas, Insitu ScanEagle A [13] and Honeywell Micro Air Vehicle (MAV). Table 1 provides an overview of the various UAV propulsion without mentioning the close-cycle options. The comparison of the closed cycle propulsion system is highlighted in Section 2 of this paper. Whilst each of these propulsion systems mentioned has its merits and demerits, which has been discussed extensively in references [6; 11; 14; 15], the focus of this paper is on the closed-cycle propulsion system. The reason for choosing the closed-cycle and especially the closed-Brayton cycle is because there is no valuable literature to the best of the author's knowledge that addresses the possibility of using a helium closed-cycle gas turbine for UAV application. In this paper, the authors give a brief performance comparison of the different closed-cycle engines with regards to UAV propulsion capability and also discusses the potentials of a closed-Brayton cycle, whilst highlighting various fluid media application and previous work by other researchers. The novelty of this paper is on the feasibility of helium closed-cycle gas turbine UAV propulsion, highlighting both the performance and component design considerations for high altitude and high payload mission requirement. This work did not consider complex Computational fluid dynamics analysis (_CFD_) of the system behaviour as this will be covered in a future publication. Additionally, the scope of this paper does not cover the transient behaviour of the system as this will be covered in future work. This paper is broken down into five sections, with Section 1 providing the background and rationale behind this work as well as the research novelty. Section 2 describes the closed-cycle UAV propulsion and performance comparison of the closed-cycle engines as well as brief limitations encountered in the use of the cycles for the propose UAV mission. Section 3 provides a conceptual overview of the helium closed-cycle gas turbine for UAV application whilst showing both performance and component design characteristics. Section 4 provides a discussion of some results presented. In contrast, Section 5 gives a conclusion of the work whilst highlighting some of the research limitation and proposal for future work.