# Feasibility of a Helium Closed-Cycle Gas Turbine for UAV Propulsion
## 2 The Closed-Cycle UAV Propulsion System
### The Brayton Closed-Cycle Engines  
The Brayton closed-cycle engines have the same thermodynamic process (isothermal compression and expansion) as the Brayton open-cycle (turbojet and turbofan). The difference is that the working media is isolated from the environment within a control volume and the additional component (heat exchanger) needed to cool the working fluid before recirculating it through the compressor. The closed-cycle gas turbine could either be coupled directly or indirectly to its heat source, giving it that flexibility in terms of heat source choice selection. Similar to other closed-cycle engines, it offers compactness and high reliability as there is no moving part in the machine in high temperature and reduction of thermal stresses. Another advantage is the flexibility in terms of working fluid selection, adapting to a variety of high energy and be able to operate efficiently over a wide power range [21]. The Brayton closed-cycle provides a better advantage for small UAV than the open cycle as the efficiency at low-pressure ratio favours the closed Brayton cycle (CBC). The Brayton closed-cycle has a lightweight advantage and higher efficiency in large size compared to other closed cycle engines. Hence, the CBC engine has many characteristics that are required for UAV missions. The system is suitable for a wide variety of missions, such as lunar, deep space, and earth orbit missions. In the 1950s, a program for Aircraft Nuclear Propulsion (ANP) was initiated by the Atomic Energy Commission in the US to redesign B-36 which was later renamed NB-36 [22; 23; 24]. By the 1960s, progress was made with both direct and indirect closed-cycle engines tested, using air as the working fluid [23]. Similarly, after extensive experimentation with different engines and transfer systems, the Soviet Engineers developed the Tupolev (Tu) Tu-95 LAL and 34 more research flights completed between the 1950s and 1960s using the closed-cycle gas turbine technology [25]. Feasibility studies using closed-cycle gas turbine engines for UAVs have been documented; the study [15] evaluated the benefit of the closed-cycle gas turbine at high altitude performance, operation in polluted air environment using generic thermal heat sources. The NASA-Lewis Research Centre developed a radioisotope closed-cycle gas turbine power generation unit in the 1960s, which achieved an output power between 2 kW and 10 kW [26; 27]. Similarly, the NASA Space Station Freedom selected a solar-powered CBC for space power system application. To this end, there are several pieces of research and development programs been deployed to show the technology readiness of the closed-cycle gas turbine in aerospace application [15; 24; 28]. With the CBC, the layout configuration can be arranged as such that the performance and thermo-economics are taken into consideration to get the right balance between system performance and capital cost. Interestingly, the working fluid selection could have an influence on the physical layout of the CBC engine [21]. Due to the self-contained nature of the CBC, almost all permanent gaseous working fluid can be used, however, with UAVs, unique criteria are considered such as thermal stability, inflammability, component material compatibility, and behaviour of fluid at freezing conditions. Several working fluids that can be considered range from monoatomic inert gases and mixtures, however, this paper will focus on the use of helium. Helium was selected due to its thermophysical properties, allowing for compact small UAV design, and would demonstrate high efficiency at a low-pressure ratio to compete with other propulsion systems. The major challenge for the CBC design would be the heat source. As previously mentioned, this could be coupled directly or indirectly, and this can run on fuel ranging from nuclear fuel, natural gas, or solar. The most promising of these fuels are nuclear and solar; however, they both have their drawbacks. For this work, the authors have considered nuclear reactor coupled directly to the CBC.