# Feasibility of a Helium Closed-Cycle Gas Turbine for UAV Propulsion
## 2 The Closed-Cycle UAV Propulsion System  
The concept of using the closed-cycle engines to power UAVs is not new, as there are several studies on the feasibility of these type of propulsion systems for aircraft and UAVs which developed in the second world war [10]. Some of the closed-cycle propulsion systems reportedly used for UAV mission include the Rankine, Stirling and Brayton cycle [15]. Experience from both experimental and prototype models has shown that the \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{1}{c}{**Power Plant**} & \multicolumn{1}{c}{**Altitude (km)**} & \multirow{2}{*}{**Speed**} & \multicolumn{1}{c}{**SFC @ Cruise (kg/kWh)**} & \multicolumn{1}{c}{**Power (or**} & \multicolumn{1}{c}{**P/W**} \\ \cline{3-6} \multicolumn{1}{c}{} & \multicolumn{1}{c}{Battery} & & & N/A & N/A & 0.1–1 \\ \cline{2-6} \multirow{3}{*}{Electric Motors} & Fuel Cell & 0–15 & \multirow{3}{*}{\(<\)Mach 0.6} & \multirow{3}{*}{N/A} & N/A & \multirow{3}{*}{\(<\)1} \\ \cline{2-6} \cline{7-7}  & Solar-powered & & & 0.090 (where the & & \\  & photovoltaic cell & & & reference fuel is H2) & N/A & N/A \\ \hline \multirow{3}{*}{Reciprocating Engines} & Two Stroke & & & 0.4–1.2 & & 0.8–2 \\ \cline{2-6}  & Four Stroke & 0–9 & \multirow{3}{*}{\(<\)Mach 0.6} & 0.3–0.4 & & 0.4–1 \\ \cline{2-6}  & Rotary & & & 0.35 & 15–70 & N/A \\ \hline \multirow{3}{*}{Gas Turbines} & Turbojet & & & &
\begin{tabular}{c} \(>\)4 kN \\ (0.13–4.45 kN \\ for target/cruise \\ missiles) \\ \end{tabular} & N/A \\ \cline{2-6}  & Turbofan & & & N/A & \\ \cline{2-6}  & Turboprop & 7–15 & \multirow{3}{*}{\(<\)Mach 0.6} & 0.3–0.5 & \multirow{3}{*}{\(>\)200} & 3.5–4.8 \\ \cline{2-6}  & Turboshaft & & N/A & & 4.3–9 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Unmanned aerial vehicle (UAV) propulsion systems overview [12]. closed-cycle engines are well suited for the UAV applications in several ways because of their advantage of containing fluid media, which eliminates the noise associated with the intake and exhaust systems. The high-power conversion efficiencies and power to weight ratio provided by the closed-cycle engines have demonstrated renewed interest in adapting the system for space power and UAV application. The outside air density does not have any significant effect on its output power/thrust requirement, and the operating pressure can be several times higher than the reciprocating engines which allow for a compact design. A better contender to the closed-cycle engines in terms of low noise is the fuel-cell or battery-powered propulsion system. The closed-cycle engine is also suitable for high altitude UAV missions, as its operation is isolated from the effect of pressure drop occurring outside the system. Hence, it could benefit from improved efficiency at high altitude because of the gain in the heat exchanger temperature difference across the system. Although, there could be some adverse effect in terms of heat transfer, in general, the temperature difference at high altitude compensates for this shortfall providing an improvement in the cycle efficiency. Other benefits of the closed cycle engines are identified based on the various types discussed in Section 2.1 to Section 2.3. In general, the major drawbacks to the use of the closed-cycle engines are its high heat rejection requirement and the additional weight from the heat exchangers and condenser, which could have its effect in a low altitude UAV mission. Other demerits have been highlighted for the individual closed-cycle propulsion systems. The differences between the Rankine, Stirling and Brayton cycles are their extra components, the cycle operations, and the fluid media. To select any of the closed-cycle engines for a UAV, it must prove to have a competitive advantage over the other internal combustion engines. It must also be safe and have good flexibility in terms of fuel and working media selection. The working fluid flexibility is important because of mission requirements, especially for freezing conditions which the system must be designed to withstand.