# Feasibility of a Helium Closed-Cycle Gas Turbine for UAV Propulsion
## 3 The Helium Closed-Cycle Gas Turbine UAV Propulsion  
The helium closed-cycle gas turbine operates as a closed Brayton engine described in Section 2.3. The thermo-economic conceptual reason to use the helium closed-cycle gas is hinged on the need for an aircraft with a high payload, long-duration endurance, and unique missions for either low altitude high-speed penetrations or high-altitude mission. To this end, the closed-cycle gas turbine discussed in this paper is coupled directly to a small, very high-temperature reactor (VHTR) cooled by helium working fluid with a core outlet temperature simulated between 700 and 950 \({}^{\circ}\)C. With helium as the coolant and cycle working fluid, it brings several benefits such as chemical inertness, single-phase cooling and neutronic transparency, which makes reactivity effect with structural material low [29]. The choice of working fluid also has effects on the turbo set (compressor and turbine) in terms of the number of stages for the attainment of the required pressure ratio,high cycle efficiency and machine size. The conceptual system configuration shown in Figure 1 consists of a simple single-shaft arrangement connecting the turbo set, the heat sink (heat exchanger) and the VHTR. Understandably, the VHTR could pose several regulation issues due to radiation hazards; however, the use of a unit shield and an inert gas such as helium could reduce the radiation effect to satisfactory levels both on the ground and inflight, even under reactor malfunction conditions. Evolutionary advances in nuclear propulsion--specifically in reactor compactness and shielding--makes the application of a shielded closed system possible and payload that could compete effectively with conventional reciprocating piston engines and open-cycle gas turbines at ranges above 90,000 feet. The simple single-shaft cycle arrangement described in Figure 1 is different from the ERAST (Environmental Research Aircraft and Sensor Technology) semi-closed cycle configuration discussed in reference [30]. The altitude, flight Mach number, output thrust and payload requirement have been specified in Table 2 to establish a benchmark for the proposed UAV mission, The mission requirement for the UAV is defined in terms of operational altitude, range, endurance and payload. Although the cycle arrangement is still in the conceptual stage, the goal of this work is to ascertain the feasibility of using the helium closed-cycle gas turbine for the mission requirement. Table 1 provides details of existing UAVs characteristics, and Figure 2 provides the performance trend of the existing UAVs. \begin{table}
\begin{tabular}{c c} \hline \hline
**Description** & **Values** \\ \hline Cruise Altitude (m) & 30,500 \\ Cruise Mach no & 0.7 \\ Take-off Mach no & 0.4 \\ Compressor pressure ratio & 2–5 \\ Core outlet temperature (\({}^{\circ}\)C) & 700–950 \\ PC effectiveness (\%) & 87 \\ Compressor efficiency (\%) & 90 \\ Turbine efficiency (\%) & 92 \\ Propeller efficiency (\%) & 90 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Baseline performance data. Figure 1: Helium closed-cycle gas turbine configuration. Figure 2: Performance trend of existing unmanned aerial vehicles (UAVs) (**a**) Maximum take-off weight against power (**b**) flight range against endurance (**c**) Span against maximum take-off weight (**d**) endurance against maximum take-off weight [1; 12]. To this end, a pressure ratio of 4 was selected for the gas turbine to keep the turbo set weight and size within reasonable values. For a simple helium closed-cycle gas turbine, an increase in pressure ratio peaks between 4 and 6, from whence the cycle performance begins to retrogress. The core outlet temperature of 800 \({}^{\circ}\)C was chosen to manage the cooling requirement of the conceptual design and heat exchanger size. An unlimited range would offer an attractive and competitive UAVs requirement for today. However, there are constraints to achieving an unlimited range proposition which is linked to the fuel load and system weight.