Energy 33 (2008) 116–129 Exergy analysis of hypersonic propulsion systems: Performance comparison of two different scramjet configurations at cruise conditions V. Amati à , C. Bruno, D. Simone, E. Sciubba Department of Mechanical & Aeronautical Engineering, University of Roma 1 ‘‘La Sapienza’’, Via Eudossiana 18, 00184 Roma, Italy Received 10 December 2006 Abstract An exergy analysis of an advanced hypersonic vehicle, a scramjet, is presented and discussed with a twofold scope. First, to perfect the exergy approach to the design and optimization of aerospace propulsion systems: the exergy flow diagram can provide aircraft engineers and system designers with additional insight on the avoidable and unavoidable systemic losses, thus allowing for effective design improvements. Second, to explore limits and merits of two different fuelling solutions for a scramjet-powered aircraft. Two configurations are critically compared: one with a direct H 2 injection and one with an on-board kerosene reformer. The present study treats the scramjet-propelled plane as a Large Complex Energy System (‘‘LCES’’), and applies system balances (mass, energy, exergy) to calculate the relevant losses. The exergy analysis confirms that the introduction of an on-board reformer is advantageous from the point of view of the thrust efficiency (with a gain of 3 percentage points with respect to the H 2 -fuelled engine) and, more importantly, from the point of view of a more correct use of the available resources (the fuel in the tanks). Another advantage of the on-board reforming is that the higher value of the volumetric-specific impulse allows for reducing the fuel tank size. All calculations have been performed with CAMEL s , a modular simulator for energy conversion processes conceived and developed in the last decade by the Authors’ group at the Mechanical and Aeronautical Engineering Department of the University of Roma 1 ‘‘La Sapienza’’. Some additional component models have been studied and implemented, and a specific tool dedicated to the analysis of propulsion systems has been created and integrated in the simulation package. r 2007 Elsevier Ltd. All rights reserved. Keywords: Exergy analysis; Large Complex Energy System; Scramjet; Thrust efficiency 1. Introduction 1.1. General description of a scramjet A scramjet (acronym for Supersonic Combustion Ramjet) is a hypersonic airbreathing propulsion system resulting from the evolution of the ramjet engine. The basic principle of a ramjet is the same as that of a jet engine: intake, compression, combustion, and exhaust are the main operating processes, but the engine has no moving parts. The incoming air flow is compressed by means of the so-called ‘‘ram compression effect’’: the supersonic flow entering the intake is decelerated to subsonic velocities (comparable to those reached in a turbojet) by aerody- namic diffusion created by properly shaped inlet and diffuser. After the fuel injection and combustion, the expansion of the hot gases accelerates the exhaust air to a velocity higher than that at the inlet, producing positive thrust (see Fig. 1). The speed of the vehicle must be high enough to compress the inlet air, eliminating the need for turbocompressors. A scramjet differs from a ramjet in that the flow never decelerates to subsonic velocity, so that the combustion takes place in a supersonic air stream (see Fig. 2); compression at the intake is due to multiple oblique shock waves. A scramjet engine can operate only at hypersonic speeds (Mach numbers from 6 to about 24); thus scramjet-pro- pelled vehicles must be accelerated to supersonic velocity by other means; recent tests of prototypes [32] used a booster rocket. ARTICLE IN PRESS www.elsevier.com/locate/energy 0360-5442/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2007.08.012 à Corresponding author. Tel.: +39 06 44585272; fax: +39 06 44585249. E-mail address: valentina.amati@uniroma1.it (V. Amati).